Kinetic and Mechanistic Aspects of Plasma Polymerization

CHAPTE R

6

Kineti c and Mechanisti c Aspect s of Plasm a Polymerizatio n

As pointed out in Chapter 5, the level of vacuum generally used in plasma polymerization ( l O ^ - l O 1 torr) is too low to form polymers in a practical manner by ordinary polymerization such as the addition polymerization of olefinic monomers. At this point, the most important thing to recognize is that plasma polymerization is an entirely different type or category of poly­ merization than the polymerizations discussed in Section 5.2. In this chapter, the kinetic and mechanistic aspects of plasma polymerization are discussed as an extension of the comparisons presented in Section 5.4. Most studies dealing with the kinetic and mechanistic aspects of plasma polymerization have been based on an a priori concept of polymerization, such as (addition type) chain-growth polymerization, which is, according to the considerations discussed in Section 5.4, very unlikely to play a significant role in plasma polymerization or any polymerization that occurs in a vacuum. Another prevailing trend in studies of the kinetics or mechanisms of plasma polymerization has been the direct application of theories of, or data on, the ionization of gases (the majority of data having been obtained with nonreactive gases such as monoatomic inert gases and relatively simple diatomic gases including N 2 , 0 2 , etc.) to polymerizable monomers such as vinyl monomers. In this sense, perhaps too much emphasis has been placed on the ionization aspects of plasma, and too little attention has been paid to the elementary steps of polymer formation (the growth mechanism of polymerization). Among the polymerizations described in Chapter 5, the closest to plasma polymerization are radiation polymerization and Parylene polymerization. The former involves the ionization of monomers as the primary step of the 72

6.1. ROL E O F I O N I Z A T I O N I N P L A S M A P O L Y M E R I Z A T I O N

73

initiation of polymerization, and the latter involves the polymerization mechanism by which polymer depositions in a vacuum occur. Radiation polymerization does not yield polymers in a vacuum, and Parylene poly­ merization does not involve the ionization process. Therefore, neither is a true model of plasma polymerization; however, plasma polymerization can be visualized as a hybrid of these two polymerization mechanisms. In this sense, it is worth examining how plasma polymerization differs from radiation and Parylene polymerizations to gain insight into the mechanisms of plasma polymerization.

6.1

Role of Ionizatio n in Plasm a Polymerizatio n

The ionization of a molecule by collision with an accelerated electron is an essential process for creating plasma of a m o n o m e r (with or without carrier gas). It is premature to assume, however, that the ionization of molecules is the first elementary step of plasma polymerization. Here, we must recognize the difference between the ionization of atoms and that of molecules, particularly relatively complex organic molecules. With atoms, the ionization can occur only by the elimination of an electron from an electron orbital, and the process requires a relatively high energy (e.g., 13-25 eV for inert gas atoms). Therefore, in the ionization of inert gases, electrons having energies lower than the ionization potential or off-centered collisions that do not result in the transfer of the entire energy of an electron to an a t o m d o not contribute to the ionization. In the ionization of molecules, particularly organic molecules, this is not a complete picture of the ionization step in plasma. First of all, the ionization energy of greater than 10 eV is far above the bond energies of primary bonds involved in organic compounds. Typical bond energies are given in Table 6.1. In Table 6.2, the dissociation energy, metastable energy, and ionization energy for noble gases and diatomic gases are compared. The low-energy electrons a n d / o r off-centered collisions that cannot ionize molecules can break bonds in organic molecules or create excited species, which can trigger chemical reactions. These side reactions associated with ionization are absent in the ionization of atoms. Thus, the ionization of an organic molecule is far more complex than the ionization of an atom, and one can easily estimate the extent of the side reactions associated with the ionization by comparing the energies necessary for the side reactions and the ionization energy of a molecule. The following examples provide some indication of the energies involved in the side reactions that occur in the glow discharge of organic molecules [the

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

74 Tabl e 6.1

TYPICA L B O N D E N E R G I E S Dissociatio n energ y (eV)

Bon d C

C

3.61 6.35 (2.74 for % bond ) 4.30 3.17 9.26 3.74 7.78 5.35 3.52 4.04 4.83 1.52

c=c

C H C N C=N

c o c=o

C C N O O

F CI H H O

Tabl e 6.2 D I S S O C I A T I O N E N E R G Y , METASTABL E E N E R G Y , A N D IONIZATIO N ENERG Y FO R NOBL E AND D I A T O M I C GASE S

Ga s H Ne A Kr Xe H2 N2

o2

Dissociatio n energ y (eV)

— — — —

4.5 9.8 5.1

Metastabl e energ y (eV)

Ionizatio n energ y (eV)

19.8 16.6 11.5 9.9 8.32

24.6 21.6 15.8 14.0 12.1

— — —

15.6 15.5 12.5

enthalpies of reaction (7) are given in units of electron volts]: AH (eV) e~ + C 2 H 4

e~ + C 2H 3C 1

e~ + C 2H 3F

> C 2H 2 + H 2 + e~

1.8

C 2H 2 + 2 H . + £"

6.3

> C 2H 2 + HC 1 + e~

1.1

C 2H 2 + H - + CI - + e~

5.6

C 2H 2 + H F + e~

0.8

> C 2H 2 + H - + F - + e~

6.6

6.1. ROL E O F I O N I Z A T I O N I N P L A S M A P O L Y M E R I Z A T I O N

75

Whereas most ionizations require energy greater than 10 eV, the dissociation of a molecule requires much less energy. It seems noteworthy that the dehydrogenation, dehydrochlorination, dehydrofluorination, and so on, of an organic molecule require very little energy in comparison with the ionization energy. The relative ease with which such dissociation reactions occur should depend on the activation energy associated with each reaction. However, a rough estimate by the magnitude of the heats of reaction seems to provide reasonably accurate comparisons between the dissociation and ionization of a molecule (7). Ionization is the essential step in creating and sustaining plasma but is not necessarily the primary step in initiating plasma polymerization. The scission of bonds occurs with a far greater frequency than the formation of ions. Bell estimated that the concentration of free radicals in plasma is usually five to six orders of magnitude higher than that of ions (1). In other words, the scission of bonds does not occur as the consequence of the ionization of molecules but rather occurs simultaneously with ionization. In this respect, there is a significant difference between radiation poly­ merization and plasma polymerization. Namely, in radiation polymerization (Section 5.3) the formation of the chain-carrying species such as the cation, anion, or free radical is a consecutive process to the ionization of a monomer, whereas in plasma polymerization, the reactive species are not necessarily formed as the consequence of the ionization. Thus, ironically, ions play a much less important role in plasma polymerization than in radiation polymer­ ization. Nevertheless, the role of ions in plasma polymerization should be examined. In comparing the effects of ions in plasma polymerization with their effects in radiation polymerization, one must focus attention on the following factors. 6.1.1

REACTIVIT Y O F

CARBOCATION S

Carbocations are generally very unstable, and it was only after procedures for the rigorous drying of monomers had been applied that the role of cationic polymerization in radiation polymerization became evident. U n d e r ordinary conditions of plasma polymerization, the quantity of impurities is far greater than that in the superdried m o n o m e r used in radiation polymerization. The ordinary plasma reactor contains a large quantity of impurities with respect to the polymerization. At the level of 1 0 ~ 3 torr or higher without addition of a monomer, an ample a m o u n t of water vapor exists in the system, particularly in relation to the a m o u n t of m o n o m e r to be introduced. Furthermore, as soon as plasma is initiated, a large a m o u n t of by-product gases is formed, and there are many species that can react with cations,

76

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

including the free electrons in plasma. Therefore, it is very unlikely that the cations produced in the first step of ionization would play any significant role in plasma polymerization.

6.1.2

CHEMICA L NATUR E

OF

MONOMER S

Although cations are formed by the ionization of a monomer, cationic polymerization can occur only with a limited group of monomers in which the substituent groups to the olefinic double bond are nucleophilic and, con­ sequently, the electron density at the double bond is high. Olefinic monomers with electrophilic substituents cannot polymerize by cationic polymerization (see Table 5.4). Conventional polymerizations are therefore highly dependent on the structure of the monomer, and a particular m o n o m e r can be polymerized only by limited polymerization mechanisms dictated by the monomer structure. This implies that, without considering the m o n o m e r structure, one cannot automatically assume that the cations formed by the ionization are the chaincarrying species. In plasma polymerization, in contrast to this restriction, not only m o n o ­ mers that can be polymerized by addition polymerization, but also any organic compound without a polymerizable structure such as a double bond, polymerize. Furthermore, the rates at which monomers polymerize are by and large similar regardless of the structures of the monomers (e.g., styrene versus ethylbenzene, ethylene versus ethane, etc.). It should be emphasized that the chemical reactions involved in con­ ventional polymerizations are very specific to the chemical structure and mechanisms. As shown in Fig. 5.4, the rate constant of propagation kp for styrene varies by seven orders of magnitude, depending on the mechanisms of the polymerization. In other words, when we refer to fast polymerization or slow polymerization, the difference corresponds to several orders of mag­ nitude in the rate constants. It should also be emphasized that such a large difference in the polymerization rates of a m o n o m e r (e.g., styrene) is caused by the difference in the polymerization mechanisms. The study of polymerization kinetics is commonly employed to elucidate polymerization mechanisms. With this background, a comparison of the polymer formation rates of various monomers by plasma polymerization would provide an overview of the kind of reaction mechanism responsible for the plasma polymerization. The variation in the plasma polymer deposition rates for vinyl monomers versus saturated vinyl monomers is only within an order of magnitude (2) (Table 6.3).

77

6.1. ROL E O F I O N I Z A T I O N I N P L A S M A P O L Y M E R I Z A T I O N Tabl e 6.3 P O L Y M E R I Z A T I O N PARAMETER : V I N Y L V E R S U S S A T U R A T E D VINY L C O M P O U N D S Saturate d vinyl compound s

Viny l compound s

7.59

^—CH=CH 2

CK„;

CH ,

H

<^

H

^

3

CH

CH

CH=CH

4.05

5.33

5.65

2

C - H ^ ^ - C H = C H

N

4.72

2

7.65

H

3

C H ^ ^

N

2

CH

- CH

2

CH

3

2

CH

7.55

3

7.38

3.76 O

H 2C = C H

C= N

H 2C = C ^

5.71

H 3C

CH

5.47

H 3C

CI CH X C1

2.86

H 3C

CH

CI H 2C = C H a

CH

2

NH2

2

2

C=N

4.49 2.98

CH

2

NH2

2.52

Paramete r k is expresse d in unit s of c m - 2 x 1 0 4; r = fcF w, wher e r is th e rat e of polyme r depositio n (g/cm 2«min ) an d F w th e weight-base d monome r flow rat e (g/min) .

It should be reiterated that the polymerization rates of vinyl and saturated vinyl monomers polymerized by the conventional process are different in orders of magnitude (i.e., polymerization and no polymerization). These data indicate that (1) plasma polymerization is nonspecific, and (2) not all the polymerization mechanisms discussed in Chapter 5 seem to apply to plasma polymerization. Thus, the direct initiation of polymerization by a cation formed by the ionization of an organic molecule is very unlikely, and cationic (chain-growth) polymerization can be ruled out as a possible plasma polymerization mechanism.

78 6.1.3

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

DOSE-RAT E

(INITIATION-RATE )

DEPENDENC E

As another important comparison to radiation polymerization, the doserate dependence of polymerization should be considered for plasma poly­ merization. As mentioned in Section 5.3, in radiation polymerization by the free radical mechanism (in the presence of impurities), the rate of poly­ merization is proportional to the square root of the dose rate, and the degree of polymerization is inversely proportional to the square root of the dose rate. In plasma polymerization, the empirical deposition rate of polymer is dependent on energy input (e.g., discharge wattage); however, there are two distinctly different cases insofar as the dependence of polymer deposition rate on the discharge power is concerned, and the dependence in both cases differs significantly from that in radiation polymerization. Plasma polymerization can be generally divided into two cases according to the rate-determining process dictated by the experimental conditions. These are (1) the flow-rate-controlled case and (2) the discharge-power-controlled case. In the former case, ample electric power is provided, and the ratedetermining factor of the process is the m o n o m e r feed-in rate, and in the latter case, ample m o n o m e r is available but the power input rate is the ratedetermining factor. In the former case, the polymer deposition rate is independent of the discharge power, that is, Polymer deposition rate oc (initiation rate) 0, and in the latter case, the polymer deposition rate is proportional to the discharge power, that is, Polymer deposition rate oc (initiation rate) 1. These dose-rate dependencies indicate again that the basic mechanisms of polymer formation are completely different from those applicable to radiation polymerization. Another important implication of the dose-rate dependence of poly­ merization can be pointed out. W h a t kind of polymer could be formed by plasma polymerization if one intuitively assumed that the same principle that is applicable to radiation polymerization applied to plasma polymer­ ization? Westwood pointed out, on the basis of the radiation yield (G value) of polymerization, that the dose rate employed in typical plasma poly­ merization is 1 0 6 times greater than the typical dose rate used in radiation polymerization (3). If one applies this figure to the dose-rate dependence of the degree of polymerization [i.e., (dose r a t e ) - 1 2/ dependence], the degree of polymerization is found to be of the order of one one-thousandth that obtained by radiation polymerization in nearly all practical cases. This turns out to be less than 1,

6.2. G R O W T H M E C H A N I S M O F P L A S M A P O L Y M E R I Z A T I O N

because the degree of polymerization obtainable by radiation polymerization is usually smaller than 1000. Thus, even if chain-growth radiation poly­ merization were possible, the polymer could not be formed at such a high dose rate, the recombination of primary free radicals would predominate. In reality, at such a high dose rate, polymer would be formed by different mechanisms, as evidenced by plasma polymerization. In this sense, the study of plasma polymerization would lead to a prediction of what one could expect if the dose rate in radiation polymerization were increased by a few orders of magnitude. Namely, in such a case, the square root dependence on the dose rate would not be obeyed, and polymers would be formed by the rapid stepgrowth mechanism.

6.2

Growt h Mechanis m of Plasm a Polymerizatio n

The discussions presented so far suggest that chain-growth polymerization, represented by M*+ M

> M * +1

where M * is the reactive chain-carrying species and M the m o n o m e r molecule, would not play a significant role in plasma polymerization and also that the growth mechanism of plasma polymerization would very likely be the rapid step-growth reaction, [M J + MJ

> M

M J+

x N

where N represents the number of repetitions of similar reactions. This reaction mechanism differs from conventional step-growth polymerization, in which the reaction occurs between molecules. In case of Parylene polymerization, * M * is a difunctional reactive species and the overall polymerization can be represented by n*M*

> *(M ) *

that is, if we take the diradical form of p-xylylene,

the reaction can be written as

80

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

(The exact mechanism of the propagation of p-xylylene is not well elucidated, and this mechanism is a model representing the rapid step-growth mechanism based on the difunctional reactive species.) Thus, the polymer formed should contain a considerable quantity of unreacted free radicals. It was found that as-polymerized polymers of Parylene indeed contain a measurable quantity of free radicals. If the reactive species represented by M * is monofunctional, such as a free radical R-, the reaction given by M m- + M „ - - > M m +n is essentially a ter­ mination process that occurs in free radical polymerization a n d does not contribute to growth without additional elementary steps. In the case of monofunctional reactive species, a single elementary step is indeed a termination process; in plasma polymerization, however, the product of the reaction is not immune to the activation process that produced the active species considered in the reaction. In other words, the reactivation of the product of an elementary reaction is bound to occur in plasma, as pointed out by Yasuda and Lamaze in 1973 (2). The overall polymerization mechanism based on the rapid step-growth principle is shown in Fig. 6.1. There M x refers to a neutral species that can be the original m o n o m e r molecule or any of the dissociation products including some atoms such as hydrogen, chlorine, fluorine, a n d others. The activated species that are capable of participating in the chemical reaction to create a covalent bond are given by M-. Difunctional activated species are shown by •M«. The subscripts i, y, a n d k merely indicate the difference in the size of

Cycle I

(6-4)

Cycle II

Fig . 6.1 Schemati c polymerization .

representatio n

of

bicycli c

step-growt h

mechanis m

of

plasm a

6.2. G R O W T H M E C H A N I S M O F P L A S M A

POLYMERIZATIO N

81

species involved {i = j is possible); thus, i = j = 1 corresponds to the original monomer. Although the activated species are represented by the symbol for a free radical, for simplicity and reasons to be explained later, any activated species can be considered in the reactions shown in Fig. 6.1. F o r instance, the opening of a double bond by ionization produces an ion-radical as shown in Sec­

tion 5.3. The ionization of styrene yields a cation-radical, represented by the symbol -M«, indicating that the species has two reactive sites. Thus, the opening of a double b o n d and of a cyclic structure in plasma (not by the addition of a reactive species) produces the difunctional reactive intermediate As shown in Fig. 6.1, the overall reaction contains two major routes of rapid step growth. Cycle I is via the repeated activation of the reaction products from monofunctional activated species, and cycle II is via difunctional or multifunctional activated species similar to what is found in Parylene polymerization. As shown in later sections, plasma diagnostic analysis indi­ cates that ample difunctional species such as '0-12• and 'CF2* are found in plasmas of organic compounds. Therefore, it seems likely that cycle I and cycle II may play equally significant roles. In both cases, however, the growth mechanism of polymerization is rapid step-growth polymerization (RSGP). Chain-growth polymerization carried by the activated species, particularly free radicals, of certain monomers is possible under certain conditions. F o r instance, if the plasma polymerization of a vinyl monomer such as styrene were carried out at high vapor pressure and low substrate temperature (low enough to condense the liquid monomer), addition polymerization by the activated species created in plasma would occur. The infrared (IR) spectrum of the plasma polymer of styrene carried out under such conditions indicates that the polymer is almost identical to conventional polystyrene. Reactions (6-1) and (6-4) in Fig. 6.1 are essentially the same as the first step of propagation by the addition mechanism. Therefore, those reactions require a chemical structure that allows for the addition of F o r the reasons mentioned in Chapter 5, consecutive addition to a sizable kinetic chain length does not occur in a vacuum. Reaction (6-2) is essentially a termination by the recombination mechanism. Reaction (6-3) is similar to reaction (6-2), but one reactant is bifunctional. The loss of a reactive site due to disproportionation in free radical polymerization can be included in reactions (6-2) and (6-3). Reaction (6-4) is the same as reaction (6-1), except that a reactant is

82

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

bifunctional. Reaction (6-5) is a combination of bifunctional intermediates, which may play a major role in certain monomers under certain conditions. This reaction represents the principal growth mechanism in the Parylene polymerization discussed in Section 5.4.2. Some of the reactions involved may be identical to those in free radical addition polymerization; a major distinction, however, is the absence of the chain-growth mechanism. If we examine these mechanisms further, the types of activated species, M«, that can contribute to this kind of R S G P become clear. Namely, ionic species can contribute in this scheme via the reaction between a cation and an anion, that is, M f + + Mk~ Mi+k, because M + cannot react with M + , and M~ cannot react with M ~ , due to the similar charge they carry. As discussed in Chapter 4, in most plasma, the number of positive ions, n+, is equal to the number of electrons, n e. Therefore, such a contribution of ionic species would be possible only in plasma that contained a considerable quantity of negative ions (i.e., plasma containing negative ions of halogens a n d / o r oxygen). Thus, it seems quite probable that plasma polymerization proceeds mainly via reactions of neutral species (4), although ions do indeed exist in plasma.

6.3

Reactiv e Species in Plasm a Polymerizatio n

Although plasma cannot be created without ionizing monomer molecules used for plasma polymerization, it is more appropriate to consider plasma polymerization as a side reaction of the ionization process. Without recognition of this situation, some plasma diagnostic data cannot be appropriately interpreted. Namely, one falls into the situation in which everything found in a plasma must be correlated with the plasma poly­ merization. It may be, however, that many species in the plasma do not contribute to the plasma polymerization but rather contribute to the ablation. Therefore, it is important to recognize the regime of the plasma poly­ merization within the overall plasma that exists in the reactor. Among the possible neutral activated species that can participate in the R S G P mechanism given by Fig. 6.1, the free radical is the most probable and important species. Direct support for this assumption is the fact that most polymers formed in plasma contain a high concentration of free radicals (i.e., 1 0 1 7- 1 0 2 0 spins per cubic centimeter). Although a number of possible alternative mechanisms can be written for the presence of free radicals in plasma polymers, the most probable one is based on this polymer formation mechanism. Direct proof that the residual free radicals involved in the R S G P mechanism remain in the polymer as trapped free radicals can be seen in Parylene polymerization, in which an alternative mechanism such as the

6.3. REACTIV E SPECIE S I N P L A S M A P O L Y M E R I Z A T I O N

83

postirradiation of polymer is completely absent. The principle for the existence of the remaining free radicals in the polymer is exactly the same as that for the "living polymer" formed by anionic polymerization; that is, the reactive species are not extinguished on the completion of polymer formation. Considering the free radical as the activated species in the R S G P mechanism, it seems appropriate to review some of the mechanisms or primary processes that have been suggested to explain free radical formation in the radiolysis of hydrocarbons. These processes include the following: 1. 2. 3. 4. 6.3.1

Dissociation of excited molecules Dissociation of ions Neutralization of radical-ions Ion-molecule reactions DISSOCIATIO N O F

EXCITE D

MOLECULE S

This process can be represented by two types of reaction: Molecular detachment (R 1

R 2) *

R ^ + R 2-

Hydrogen detachment (RH) *

>R . + H -

Hydrogen detachment seems to play a very important role in plasma polymerization. Yasuda et al. (5) examined the gas phase of a closed system after a known a m o u n t of a hydrocarbon was subjected to plasma poly­ merization conditions. According to the results, nearly all hydrocarbons were converted to polymers, with the yield varying from 85 to more than 9 9 % in a relatively short time under the conditions used, and the gas phase after the polymerization (excluding unreacted organic vapor, which is 0 - 1 5 % of the m o n o m e r depending on the polymer yield) consisted mainly of hydrogen. The hydrogen production expressed as the hydrogen yield per m o n o m e r molecule (number of hydrogen molecules evolved when a m o n o m e r par­ ticipates in polymer formation) increases with the increasing number of hydrogen atoms in a hydrocarbon (Fig. 6.2). In order to distinguish the role of double bond, triple bond, cyclic structure, and aromatic structure, the hydrogen yield is plotted against the parameter (number of hydrogen atoms in a molecule)/(number of structures) in Fig. 6.2. F o r instance, in the case of cyclohexene, the total number of hydrogens, 10, is divided by two structures (i.e., one cyclic structure and one double bond). Although there are clear separations of curves depending on the types of monomer structure, there is a strikingly regular dependence of the hydrogen

84

6. K I N E T I C A N D M E C H A N I S T I C

ASPECT S

C H 3( C H 2) 4C H 3 group

n

2.0 r-

H3C -CH2 CH3 GROUP n

o 1.0 H,C-CH^CH

H,C=CH.

0.5

H 2C = CH-QH-CH 2

HC= C H - C H 2- C H 3

^ H , C - C = C-CH3

HC - CH

2

H 3C H ^ - C H 3

3-

© - C H 2- C H 3

GROUP r

I 5

10

15

Number of hydrogens per multiple bond and/or cyclic structure or carbon Fig . 6.2 Numbe r of hydroge n molecule s evolved per molecul e of startin g materia l whe n hydrocarbon s polymeriz e (hydroge n yield) as a functio n of chemica l structure . Adapte d fro m Yasud a et al. (5).

yield on the number of hydrogen atoms in a molecule (within a group of monomers). This smooth and regular dependence strongly indicates that every C — H bond in hydrocarbon molecules has an equal probability for hydrogen detachment. According to Smolinsky and Vasile's diagnostic data (6) of C H 4 plasma, the ratio H 2 / C H 4 in plasma varies from 1.5 to 4.7, depending on the flow rate of C H 4 at a fixed discharge wattage. This means that the gas phase of C H 4 plasma contains generally more H 2 than C H 4 (because polymermic species formed by C H 4 plasma leave the gas phase).

85

6.3. REACTIV E SPECIE S I N P L A S M A P O L Y M E R I Z A T I O N Tabl e 6.4 H Y D R O G E N / C A R B O N RATIO S I N P L A S M A P O L Y M E R S A N D CORRESPONDIN G MONOMERS 0 H/ C (polymer )

Monome r

H/ C (monomer )

H/ C (polymer )

H / C (monomer )

Acetylen e Ethylen e Propylen e Isobutylen e ds-2-Buten e Butadien e Methan e Ethan e Propan e

1.0 2.0 2.0 2.0 2.0 1.50 4.00 3.00 2.67

0.95 1.49 1.40 1.44 1.34 1.33 2.40 1.55 1.58

0.95 0.75 0.70 0.72 0.67 0.88 0.60 0.52 0.59

a

Fro m Kabayash i et al (7).

Hydrogen detachment can be also correlated with the deficiency of hydrogen in the plasma polymer compared with the corresponding monomer. It has been reported by a number of investigators that the H / C ratio in a plasma polymer is significantly lower than the corresponding H / C ratio in the monomer. Typical data for the change in H / C ratio on plasma polymerization are shown in Table 6.4 (7). The actual value of H / C in a plasma polymer is highly dependent on the conditions of polymerization, and therefore the absolute values shown in Table 6.4 cannot be taken as values specific to any particular m o n o m e r in the general case.

6.3.2

DISSOCIATIO N O F ION S

The formation of free radicals by this process can be illustrated by the following example: CH 3

I

H 3C

C

I

+

CH 3

CH

3

CH3 . + H 3 C

C

+

CH 3

CH 3

In the radiolysis of neopentane, in which the yield of methyl radical is far greater than that of pentyl radical, the elimination of a methyl group from the molecular ion, yielding a tert-pentyl carbocation-radical as in the example given, has been suggested. Subsequent neutralization of the ion-radical may lead to a tert-bu\y\ radical or a hydrogen atom and an isobutene.

86

6. K I N E T I C A N D M E C H A N I S T I C

6.3.3

NEUTRALIZATIO N OF

ASPECT S

ION-RADICAL S

This process can be explained by using the same reaction given in the preceding section: H 3 C - C + - C H 3 + e-

6.3.4

H >

ION-MOLECUL E

H 3C

C

CH 3

or

H- +

C / C = C H 2

REACTIO N

This process can be generally expressed by RH + + RH

» . R H 2+ +

R.

Those reactions indicate that a variety of reactions involving the various kinds of species that exist in the plasma of a m o n o m e r could lead to the production of free radicals. Furthermore, as mentioned earlier, free radical formation by molecular detachment and by hydrogen detachment, which requires much less energy than the ionization of a molecule, does not occur as a consequence of the ionization. These considerations support the suggestion that free radicals are the most likely reactive species in the R S G P mechanism.

6.4

Fre e Radical s in Plasm a Polymer s

O n e of the most important features of plasma polymers is that a large quantity of free radicals are often trapped in the polymer. Although the amount varies with the type of m o n o m e r and the conditions of the plasma polymerization, it is safe to consider that plasma polymers contain a certain amount of trapped free radicals. Therefore, the presence of trapped free radicals and the reasons that such high concentrations of free radicals are trapped in plasma polymers are extremely important factors in the con­ sideration of the mechanisms or kinetics of polymer formation in plasma. 6.4.1

GROWT H

MECHANIS M

A N D POLYMERI C FRE E

RADICAL S

It should be recalled that ordinary free radical addition polymerizations d o not yield polymers that contain trapped free radicals. All free radicals are quenched in the termination reaction, which is the final step in forming a polymer. According to the R S G P mechanism, in which the reactions between activated species and the reactivation of the products play key roles, the formation of trapped free radicals in the polymer is expected from the reaction

6.4. F R E E RADICAL S I N P L A S M A P O L Y M E R I Z A T I O N

87

mechanisms if it is assumed that the activated species are free radicals. As discussed in the preceding section, free radicals seem to be the most logical activated species for polymer formation in plasma. It should be reiterated that, in the reaction scheme shown in Fig. 6.1, the formation and dissipation of the activated species are not necessarily sequential and, consequently, not balanced quantitatively. In other words, the formation and the dissipation of the activated species by growth reactions are dependent on different factors, and the steady-state assumption with respect to the concentration of activated species cannot be applied to plasma polymerization. F o r the sake of discussion, let us consider that M - shown in reaction (6-1) is a free radical. In the cycles shown in Fig. 6.1, free radicals are formed independently from the dissipation of the formed free radicals. In this respect, plasma polymerization differs significantly from Parylene polymerization. In the latter, a stoichiometric quantity of free radicals is formed, and each polymer molecule has two free radicals (one each o n both ends of a molecule). Therefore, the number of free radicals can be correlated with the molecular weight of a Parylene polymer. Thus, on the basis of the kinetic mechanisms of polymer formation, the presence of trapped free radicals appears to be quite reasonable, and it is worth examining these species in detail. Dealing with free radicals trapped in plasma polymers, one must also recognize the fact that unpaired spins have been observed on polymer surfaces treated with plasma that form n o deposit. The formation of free radicals has been ascribed by Hansen and Schonhorn (8) to the impingement of energetic particles on the surface, and by Hudis (9) to ultraviolet (UV) radiation from the plasma. Both mechanisms would contribute to the free radical formation in the substrate polymer subjected to a plasma containing either polymerizable or nonpolymerizable species as well as in the polymer layer formed on the substrate by plasma polymerization. This m o d e of free radical formation can be termed free radical formation by irradiation in plasma, in order to distinguish it from another mode in which the residual free radicals are formed in the polymer formation processes discussed earlier. A comparison of the free radicals formed by irradiation and those formed in the polymer formation process will provide important information not only on the mechanistic aspects of plasma polymerization, but also on the overall characteristics of this process, in which the irradiation aspect of plasma plays an important role, as will be discussed in detail in this section. Because the energy levels of the electrons and ions involved in plasma polymerization are much lower than those involved in other irradiation processes such as y rays and high-energy electron beams (plasma has a high dose rate, but the energy levels of electrons are low), it is thought that these energetic species have less penetrating power; that is, plasma irradiation is

88

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

restricted to the surface, compared with the more penetrating radiations. This situation enables us to separate the irradiation effect and the free radicals formed in the polymerization process. 6.4.2

FRE E RADICAL S IN PLASM A A N D IN TH E

POLYMER S

SUBSTRAT E

By means of electron spin resonance (ESR) spectroscopy, Morosoff et al. (10) studied the free radicals in plasma polymers deposited on a glass tube. The free spin signals observed with a plasma-polymer-coated glass tube consist of the free spin signal of the glass and that of the plasma polymer. A typical ESR signal observed with a glass rod exposed to N 2 plasma (non-polymer-forming plasma) together with the background signal observed with an untreated glass tube is shown in Fig. 6.3. Some typical ESR signals (10,11) observed with a plasma-polymer-coated glass tube are shown in Figs. 6.4 and 6.5. By removing plasma polymer coating from the surface of a glass tube, it is possible to examine quantitatively both the free spins in the plasma polymer and the free spins in the substrate glass (by subtracting the glass signal from the composite signal observed with the

20G Fig . 6.3 (A) ES R spectru m of radical s forme d by exposur e of a glas s tub e t o N 2 plasm a at an initia l N 2 pressur e of 12 fim Hg , 30 W power , for 5 mi n wit h subsequen t exposur e of th e tub e t o air . (B) "Background " fro m untreate d glas s tube . Relativ e ordinat e scale : 1:1 . Adapte d fro m Morosof f et al. (10).

Fig . 6.4 ES R signa l of plasm a polyme r of cyclohexane . (A) Polyme r signa l superimpose d on glass signal . (B) Polyme r signa l afte r subtractin g glass signal . Adapte d fro m Yasud a an d Hs u (11).

Fig . 6.5 ES R signa l of plasm a polyme r of tetrafluoroethylen e (by pulse d discharge) . (A) Polyme r signa l superimpose d on glas s signal . (B) Polyme r signa l afte r subtractin g glas s signal . Adapte d fro m Yasud a an d Hs u (77).

90

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

coated tube). It is clear that the ESR signals of the substrate glass tube are due to the irradiation effect of (polymer-forming) plasma. However, the ESR signals observed with plasma polymers could, in principle, be due to the residual free radicals (from the polymer formation process) and also to the free radicals formed by the irradiation effect of plasma (onto once-formed plasma polymer). One can examine this situation by changing the type of monomers to be used. If the ESR signal of the plasma polymer (polymer signal) is due mainly to the irradiation effect, the polymer signal should be proportional to the glass signal, because the polymer signal should come from the irradiation effect of plasma from which the polymer is deposited, and the glass signal is the direct measure of the effect. If the polymer signal is due mainly to the residual free radicals, however, one cannot expect direct proportionality between the polymer signal and the glass signal. The experimental results showed not only that there is n o direct proportion­ ality between the polymer signal and the glass signal, but also that there is an inverse proportionality between them; that is, the higher the polymer signal, the lower is the glass signal. This means that the polymer signal is due mainly to the residual free radicals. The data of Morosoff et al. (10) obtained with 4-picoline, ethylene, and acetylene, used as pure monomer and also in combination with gases that are nonpolymerizable by themselves but are copolymerizable with other monomers, are shown in Table 6.5. The change in the steady-state system pressure due to glow discharge is expressed in Table 6.5 by S = pjpm, where pg is the steady-state system pressure in a glow discharge and pm is that observed before a glow discharge is initiated. It is important to note that in no system studied did the glass signal increase with time. The free spins in the glass are evidently generated in the first few minutes (i.e, in a time period smaller than 20 min), and the further formation of free spins in the substrate is prevented by the coating deposited on it. The formation of free spins in the glass observed in the study is caused by U V irradiation. This interpretation is supported by the following experiments dealing with the free spins in the glass. The first is the examination of the ESR signals of the glass rods, which are coated with a relatively thick layer of polyethylene and exposed to N 2 plasma. The situation is schematically represented in Fig. 6.6, and the results are summarized in Table 6.6 (11a). If the formation of free radicals is due to the impinging electrons and ions (which have low penetrating power at the energy level employed), it should be limited to the polymer-plasma interface; that is, all free radicals should be at the surface of the polyethylene coating. The results shown in Table 6.6 indicate that (1) not many free radicals are found in the polyethylene coating, and (2) free radicals are formed in the glass, which did not make contact with the plasma.

Tabl e 6.5 U N P A I R E D SPIN S DETECTE D I N D E P O S I T E D FIL M A N D SUBSTRAT E AFTE R G L O W DISCHARG E TREATMENT S FO R < 1 H R AT 30 W POWER "

Component s an d p m (mtorr ) of component s 4-Picolin e (30),

Tota l p m (mtorr ) 60

N 2 (30) N 2 (30), 4-picolin e (20) 4-Picoline(16) ,

50

32

Duratio n of glow discharg e treatmen t (min )

film ( m g / c m 2)

Spi n concen › tratio n in polyme r film [(spins/g ) x 1 0 " 1 ]9

linewidt h of ES R first-derivativ e signa l (G )

20 40

0.05 0.11

0.23 0.20

16 17

60

0.18

0.18

16

0.4

0.15

20 40 60

0.03 0.06 0.09

0.20 0.14

17

0.11

60

0.03

0.17

Yield o f polyme r

(mtorr )

Cod e on Fig . 6.10

—

—

—

15 15

0.5 0.5 0.5

0.14 0.11 0.11

7 6 6

— —

A

17

0.75

0.12

3.8

C D

Spin s nea r surfac e of glas s substrat e [ ( s p i n s / c m 2) x 10~ 1 ] 5

—

P%

S =

Pg/Pm

9

B

N 2( 1 6 ) 4-Picolin e (50)

50

60

0.10

0.20

16

0.4

0.16

8

4-Picolin e (49),

98

20

0.16

0.21

15

0.1

0.61

60

60

20 40 60

0.05 0.06 0.14

0.19 0.21 0.24

14

0.7 0.7

0.33

20 20 20

20 40 60

0.008 0.016 0.05

0.41

20 40

0.03 0.06

60

0.07

— —

N 2 (49) 4-Picolin e (25), N 2 (25), H 2 0 (25) (10) N 2 (30),

60

ethylen e oxid e (30) N 2 (20), acetylen e (30), H 2O ( 1 0 ) a b

60

Fro m Morosof f et al. (10). Rat e o f film depositio n per hou r obtaine d fro m 20-mi n value .

0.18

16 19

0.8

0.33 0.33

— —

2.5 2.5 2.8

1.0 1.0

—

3.1 3.0

0.32 0.32

19 19

16

3.3

0.32

19

17

1.0

60 60 60

— — F

— — — —

—

92

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

Fig . 6.6 Schemati c representatio n of ES R prob e use d for th e investigatio n of th e locatio n of fre e radical s forme d by plasma . Adapte d fro m Yasud a (11a).

The small quantity of free radicals in the polyethylene coating may in part be due to the quick quenching of free radicals caused by the high mobility of polyethylene molecules at the ambient temperature or the quick reaction of the surface free radicals with 0 2 after the sample is exposed to air before the ESR measurement. Nevertheless, the fact that the strong ESR signal comes from the free spins in the glass, which did not make contact with the plasma, strongly indicates that free radicals are formed by some energetic species that have a stronger penetrating capability than the electrons and ions in plasma. These observations strongly suggest that the U V emission from plasma may be the primary cause of free radical formation by plasma, as suggested by Hudis (9). This scheme of free radical formation by plasma is supported by a second type of experiment in which a glass rod (ESR probe) is encased in a sealed quartz or glass envelope (Fig. 6.7). The entire sealed envelope is placed in a plasma reactor (in a vacuum) and exposed to N 2 plasma. The results of these experiments are shown in Table 6.7. Ultraviolet absorption characteristics of media are summarized in Fig. 6.8. With media that d o not absorb U V radiation of wavelength greater than 1650 A (i.e., quartz, vacuum, and N 2 ) , the substantial free spin concentrations in the glass, whereas the presence of glass or 0 2 (UV-absorbing medium) between the

Tabl e 6.6 ES R SIGNAL S O B S E R V E D WIT H P O L Y E T H Y L E N E - C O A T E D GLAS S T U B E S E X P O S E D T O N 2 G L O W D I S C H A R G E (120 jim Hg) fl

P E coatin g mas s (mg)

P E coatin g thicknes s (/mi)

Plasm a ga s

4.6

2.6

N2

Wip e off polyethylen e 6.2 Wip e off polyethylen e 12 17.3 14.4 15.2 No PE a b

Fro m Yasud a (11a). Arbitrar y units .

coatin g 3.5 coatin g 6.8 9.3 7.8 8.2

Plasm a powe r (W ) 75

Tim e of exposur e t o plasm a (hr ) 1

— N2

75

N2 N2 N2 H2 H2

75 75 75 100 100

1 0.17 1 1 0.33 0.33

Tim e of subsequen t exposur e t o air (hr )

ES R signa l intensit y (a.u.) >

0.25 20 20 0.17 0.33 0.17 0.33 0.17 0.25 0.17

5.2 4.9 4.1 2.2 1.5 0 1.9 1.4 1.5 32

94

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

Fig . 6.7 Schemati c representatio n of ES R prob e (glas s rod ) use d t o investigat e th e effect of ultraviole t emissio n on th e formatio n of fre e radicals . Adapte d fro m Yasud a (11a).

plasma and the glass rod leads to a substantial reduction (with 0 2 ) or com­ plete absence (with glass) of free spins in the glass rod. Other supporting evidence for the interpretation that the free spins in the glass are due entirely to the irradiation effect was obtained by using a side-arm attachment to a plasma polymerization reactor (Fig. 6.9). There was no Tabl e 6.7 G E N E R A T I O N O F S P I N S O N A GLAS S T U B E E N C L O S E D I N A G R A D E D SEAL , C L O S E D AT B O T H E N D S , A N D S U B J E C T E D T O N 2 PLASMA "

En d of grade d seal Quart z Glas s Quart z Glas s Quart z Quart z Control , bar e glas s tub e expose d directl y t o plasm a a b

Fro m Morosof f et al. (10). Arbitrar y units .

Ga s an d pressur e insid e grade d seal

Tim e of plasm a treatmen t (hr )

Signa l intensit y (a.u.) b

Vacuum , 5 x 1 0 ~ 6 tor r Vacuum , 10 ~ 6 tor r Air , 1 at m Air , 1 at m N 2, 6 6 4 tor r 0 2, 6 7 8 tor r

3 3 3 3 3 3 1

4.4+1 0 1.0 0 5.2 1.3 8.7

95

6.4. F R E E RADICAL S I N P L A S M A P O L Y M E R I Z A T I O N —

N2

—

Quartz

o2 0 3

1 1000

1

1 2000

Glass

1

1 3000

1 4000

Wavelength ( A ) Fig . 6.8 Ultraviole t absorptio n characteristic s of variou s media : 0 2 , absorb s at 2000 A an d below , an d form s 0 3 ; 0 3 , absorb s strongl y in th e rang e 2 0 0 0 - 3 0 0 0 A ; N 2, transparen t abov e 1250 A ; quartz , absorb s belo w 1650 A ; glass , absorb s belo w 3900 A .

physical barrier between the glass tube (ESR probe) in the side arm a n d the plasma, but the plasma did not penetrate into the side arm, as indicated by the absence of glow therein a n d by the fact that n o polymer was deposited in the side arm even after 5 hr of operation, which left a heavy brown deposi­ tion on the wall of the main reactor. The glass signal increased with time of exposure in a manner similar t o that observed with non-polymer-forming (e.g., N 2 ) plasma (details of this aspect are discussed in Section 9.9). In this side-arm experiment with a prolonged exposure to the polymerforming plasma, the highest mean free spin surface concentration obtained was 5 x l O 1 5 spins per square centimeter, well above the limiting free spin surface concentration, 3 x 1 0 1 5 spins per square centimeter, observed with the polymer deposition onto the substrate (Table 6.5). It should be noted that the free spins in the plasma polymer are expressed as spin concentration (spins per cubic centimeter) by assuming the uniform distribution of free spins in the polymer, b u t the free spins in the glass are expressed as surface concentration (spins per square centimeter) due to a lack of knowledge of the depth profile of the free spins in the glass. Monomer

Vacuum

if 3£

\

Pyrex tubes for ESR

-Gas

Coil

Fig . 6.9 Reactio n tub e of plasm a apparatu s wit h sid e ar m for exposur e of Pyre x tub e t o ultraviole t radiatio n only . Pyre x tube s ar e show n in th e sid e ar m a s well a s in th e norma l position , tha t is, in direc t contac t wit h th e plasma . Adapte d fro m Morosof f et al. (10).

96

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

The concept that the free spins in the glass are created at an early stage and the plasma polymer deposited on the surface prevents the further formation of glass free spins (by protecting the glass from further U V irradiation) is also supported by the dependence of the glass signal on the rate of polymer deposition. The decrease in the total pressure (16 mtorr, 4-picoline; 16 mtorr, N 2 ) leads to a slower rate of polymer deposition and an increase in the glass free spins; the increase in the total pressure (49 mtorr, 4-picoline; 49 mtorr, N 2 ) shows the opposite effect (see Table 6.5). The effect can be quantitatively expressed as follows. Assuming that U V radiation is the sole cause of the formation of free spins in the glass substrate under the conditions of the experiments, the U V intensity of a monochromatic light penetrating through the plasma polymer deposit can be expressed by Beer's law. The number of free spins in the substrate (glass rod) can be correlated with the deposition rate of polymer using the following assumptions: 1. The intensity of U V radiation after passing through a polymer film of thickness / can be given by

where I0 is the intensity of incident light, and A and a are proportionality constants. 2. The number of free spins, S, is proportional to the exposure time, dS = ki dt, where k is the quantum yield of the spin formation. 3. The thickness of polymer deposition is proportional to the deposition time (i.e., / = rt, where r is the polymer deposition rate constant). Then, the total number of free spins, 5, after the plasma polymerization time t has elapsed can be given by S = (kI0A/ar)(l

- e~art).

(6-6)

This relationship indicates that (1) S is proportional to the intentisty I0 of U V emission, (2) S is inversely proportional to the rate of polymer deposition r, and (3) S approaches a constant value as the deposition time increases. Namely, when art is very large, Eq. (6-6) reduces to = kI0A/ar9

(6-7)

indicating that, after a certain thickness of coating is built up, no radiation reaches the substrate. In the case of a polychromatic radiation source with a constant spectral composition, it would be expected that the quantity 5 f would be expressed by a relationship somewhat more complex than Eq. (6-7), but it can be shown that

97

6.4. F R E E R A D I C A L S I N P L A S M A P O L Y M E R I Z A T I O N

can be expressed as (6-8)

S « = ( / 0/ r ) C

where I0 is the total intensity of the U V radiation emitted by the polymerforming plasma, and C a constant. To relate this expression to the measured quantities, we may use the empirical relationship that I0 increases with the system pressure p g in a glow discharge in the range of low values of p g . Assuming that I0 is directly proportional to p g , we may write (6-9) Clearly, the quantity is that given for glass signals in Table 6.5. N o n e of the glass signals change as a function of time. A plot of the number of free spins induced in the glass tube against the quantity pg/r is shown in Fig. 6.10 for all 4picoline systems described in Table 6.5. Thus, Eq. (6-9) appears to describe reasonably well the generation of free spins in the glass substrate used in plasma polymerization. Point E in Fig. 6.10 represents the case in which pg is so high that we may expect that I0 is no longer proportional to pg due to the insufficient energy input in a fixed-wattage experiment scheme. Similar data obtained with the glow discharges of a variety of monomers to coat a Pyrex glass tube for a period of 1 hr at 30 W discharge power are shown in Table 6.8. It is important to note that there are clear trends that have O F

08r-

50

100

150

Pg/r (mtorr • cmi 2: • hr/g) Fig . 6.10 Spi n surfac e concentratio n of glas s spin s obtaine d afte r plasm a treatmen t wit h all 4-picolin e system s given in Tabl e 6.5 plotte d agains t pjr; pg is th e tota l pressur e durin g plasm a treatmen t an d r th e rat e of film deposition . Adapte d fro m Morosof f et al. (10).

oo

Tabl e 6.8 U N P A I R E D SPIN S D E T E C T E D I N P O L Y M E R F I L M S A N D SUBSTRATE S AFTE R G L O W D I S C H A R G E T R E A T M E N T S F O R 1 H R AT 30 W POWER

0

Component s an d p m (mtorr ) of component s Acetylen e (81) Acetylen e (60) Acetylen e (40) N 2 (30), acetylen e (30) Acetylen e (30), H 2 0 (20) N 2 (20), acetylen e (30), H 2 0 (10) C O (20), acetylen e (30) C O (20), acetylen e (30), H 20 ( 1 5 ) Allene (40)

(mtorr )

Rat e of polyme r depositio n (mg/cm 2*hr )

Tota l Pm

Spi n concen › tratio n in polyme r film [(spins/g ) x 1 0 - 1 ]9

Linewidt h of ES R first-derivative signa l (G)

Spi n nea r surfac e of glas s substrat e [(spins/cm 2) x 1 0 - 1 ] 5

= Pg/Pm

(mtorr )

81

0.09

4.8

16

—

0.10

60

0.04

7.4

18

—

0.12

7

40

0.02

6.5

16

0.6

0.10

4

60

0.02

3.3

16

0.3

0.08

5

50

0.05

3.8

0.79

40

60

0.07

2.7

0.37

22

0.18

16

0.10

50

0.19*

4.1

16

65

0.14*

0.29

12

2.1

1.3

40

0.05

2.0

20

1.1

0.20

8

5 84 8

99

N 2 (30), allen e (30) Allene (30), H 2 0 (20) N 3 (20), allen e (30), H 20 ( 1 5 ) N 2 (20), allen e (30), H 2 0 (10) Ethylen e (40) Benzene(40 ) Tetrafluoroethylen e (40) Propioni c acid (40) Ethylen e oxid e (40) N 2, hexamethyl disiloxan e (30) Tetramethyl disiloxan e (40) Divinyltetramethy l disiloxan e (40) a b

60 50 65

0.09 0.04 0.03

0.18

19

1.1

60

0.08

40 40 40 40 40 60

0.025 0.066 0.011 0.013 0.089 0.22

0.04

40

0.12

40

0.38

18

0.8 3.6 2.0

0.12 0.90 0.50

7 45 32

0.21

18

1.3

0.25

15

0.7 2.2

21 19

3.7 0.6 8.7 6.9 7.2

6

0.55 0.10 0.90 2.30 1.5 1.1

22 4 36 92 50 66

0.07

7

1.15

46

0.06

6

0.70

28

—

Fro m Morosof f et al, (10). Th e abnormall y hig h polyme r depositio n rat e observe d for thi s sampl e is no t a characteristi c of th e monome r an d ga s use d bu t cause d b y th e fact tha t thi s sampl e wa s ru n on a slightl y differen t apparatus .

100

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

significant implications in the consideration of the kinetic and mechanistic aspects of plasma polymerization. First, the irradiation effect of the polymerforming plasma is minimal with m o n o m e r s that yield polymers with the highest level of trapped free radicals, which implies that the formation of free radicals in plasma polymers is not caused by the irradiation effect. Second, the quantity of free radicals trapped in a plasma polymer is related to the chemical structure of the monomer; that is, the quantity of trapped free-radicals depends on the types of monomers described in Fig. 6.2, which indicates the extent of hydrogen detachment for various monomers (hydrogen yield of the plasma polymerization). G r o u p I monomers, which contain a triple bond or aromatic or heteroaromatic structures, form plasma polymers with the highest level of trapped free radicals and have the least irradiation effect. G r o u p III monomers, saturated aliphatic hydrocarbons, yield the lowest quantity of free radicals trapped in the plasma polymer but the highest level of irradiation effect. These trends are in accordance with the intensity of glow observed in the plasma polymerization of monomers. The relationship between the irradia­ tion effect and the a m o u n t of trapped free radicals can be explained as follows. Let us first consider a nonreactive plasma, such as argon, which is not consumed in polymerization. In plasma of argon, atoms gain energy by collisions with electrons (as the first step). The energy levels of the ground state and an excited state are schematically shown in Fig. 6.11. When an excitedstate argon atom returns to the ground state, the energy is released as photons (/iv), the energy or the frequency of which is dependent on the energy level difference (EA* — EA) [see Eq. (4-11)]. N o w let us consider a similar situation for a monomer molecule M in plasma polymerization (see Fig. 6.12). The excited m o n o m e r molecule M * has the following choices. It can (1) dissipate the excess energy by emitting photons as it returns to a lower energy level and eventually to the ground state or (2) form reactive species by the opening of a double bond, a triple bond, or a cyclic structure (leading to the formation of free diradicals) or by the detach­ ment of a hydrogen or an organic ligand. In this process, the excess energy is dissipated in the chemical reactions, and no emission of photons occurs. Figure 6.12 depicts these choices.

Fig . 6.11 ato m A*.

Schemati c representatio n of th e dissipatio n of th e excess energ y of an excite d

101

6.4. FRE E RADICAL S I N P L A S M A P O L Y M E R I Z A T I O N (II )

(•M*

EM *

- j s / ^^

Fig . 6.12

or probability

probability

#

1 - <*>

Schemati c representatio n of th e dissipatio n of th e excess energ y of a n excite d

molecul e M* .

If we take the probability of M * following the second process (chemical reactions to form reactive species) to be , the probability of p h o t o n emission is given by 1 — 0. Then the intensity I0 of p h o t o n emission can be given by J 0 = (hv)n(l -) = n(l - <£)(£* - £),

(6-10)

where n is the number of m o n o m e r molecules excited to M * in unit time. Thus, the intensity of p h o t o n emission and the consequential irradiation effect are greater when the excited-state energy is larger, the number of excited molecules is larger, a n d the probability of free radical formation by the dissociation is smaller. Assuming that n is proportional t o p in the gas phase, according t o the empirical relationship used t o derive Eq. (6-9), I0 = KAEp(l-(t>l

(6-11)

where K is a proportionality constant a n d AE the energy level difference between a n excited state and the ground state of a monomer, that is, AE = E* -E. Because the quantity of free radicals trapped in the plasma polymer is proportional t o the total quantity of free radicals produced, it should be proportional t o (j). When increases, 1 — decreases; thus, when the free radicals trapped in the plasma polymer increase, the free radicals formed in the substrate decrease. 6.4.3

STRUCTUR E O F MONOMER S A N D FRE E RADICAL S I N PLASM A POLYMER S

The structure of a m o n o m e r plays an important role in determining the quantity of free radicals trapped in the plasma polymer. F o r instance, when group III monomers polymerize, producing free radicals mainly by hydrogen detachment, the number of species in the gas phase that d o n o t yield free radicals increases, which leads to a lower overall probability (j). Therefore,

Tabl e 6.9 ES R SPI N C O N C E N T R A T I O N I N P L A S M A P O L Y M E R S A N D GLAS S S U B S T R A T E S

0

C s x 1 ( T 19 (spins/cm 3)

C g x 1 ( T 15 (spins/cm 2)

Monome r

Continuou s

Pulse d

Change *

Continuou s

Pulse d

Change

C 6F 6 Styren e C 2H 4 C 2F 4 Cyclohexan e Ethylen e oxid e Acryli c acid Propioni c acid Vinyl acetat e Methy l acrylat e Hexamethyldisilan e Tetramethyldisiloxan e Hexamethyldisiloxan e Divinyltetramethyldisiloxan e

8.6 3.2 7.4 3.8 1.36 13.0 0.84 0.75 0.76 1.0 0.42 0.31 0.5 0.49 0.21 0.15

15.6 1.6 5.4 0.54 14.5 8.4 0 0.5 1.85 1.0 0.33 0.15 0.24 0.05 0 0.05

7(81) - 1 . 6 ( - -50) - 2 . 0 ( - 27) -3.26 ( -86) 13.1 (970) - 4 . 6 ( - 35) -0.84 ( -100) -0.25 ( -33) 1.09 (140) 0(0) -0.09 ( -21) -0.16 ( -52) -0.26 ( -52) -0.44 ( -90) -0.21 ( -100) -0.10 ( -67)

0 0 0 0 4.0 11.2 1.1 6.6 4.4 6.3 6.1 6.4 0 0 0 0

0 0 0 0 0.85 1.8 0 1.6 0 1.6 1.8 1.5 0 0 0 0

0 0 0 0 3.15 (-- 7 9 ) - 9 . 4 ( - 84) - 1 . 1 ( - 100) - 5 . 0 ( - 76) - 4 . 4 ( - 100) - 4 . 7 ( - 75) - 4 . 3 ( - 71) - 4 . 9 ( - 77) 0 0 0 0

C 2H 2 C 6H 6

a b

Fro m Yasud a an d Hs u (7/). Change s ar e base d on value s of continuou s discharge . Number s in parenthese s ar e percentages .

6

6.4. F R E E RADICAL S I N P L A S M A P O L Y M E R I Z A T I O N

103

group III monomers yield plasma polymers containing the least number of free radicals, but the substrate contains the highest concentration of free radicals; that is, the irradiation effect associated with the plasma poly­ merization is the highest. The formation of free radicals from monomers containing a triple bond, a double bond, or a cyclic structure proceeds by the cleavage of a covalent bond, which (1) forms a diradical and (2) does not increase the number of gas molecules (no H 2 is produced). Once diradicals are formed, polymer formation can proceed via cycle II in Fig. 6.1, which leads to the free radical "living polymer" without complete quenching of free radicals. Thus, it can be easily understood that group I monomers yield plasma polymers with the highest level of trapped free radicals, and the irradiation effect associated with the plasma polymerization of these m o n o m e r s is the lowest. G r o u p II monomers yield plasma polymers with a moderate level of trapped free radicals, and the irradiation effect is also moderate. This overall relationship is supported by experimental results obtained by using a pulsed radio frequency (rf) discharge (77). When similar experiments with glass substrates, as described earlier, were carried out by using a pulsed rf (100 psec on, 900 /zsec off) power source, the glass signal always decreased by a factor somewhat close to the duty cycle of the pulse (Table 6.9). With some monomers (e.g., ethylene, acetylene, acrylic acid, vinyl fluoride, and vinylidene fluoride), however, the trapped free radicals increased significantly when pulsed plasma was used. The reason for such a large increase in trapped free radicals is not quite clear; however, it provides strong proof that the free radicals trapped in plasma polymers are not produced by the irradiation mechanism. The increase in trapped free radicals due to the pulsed discharge found for double-bond-containing monomers can be explained, at least in part, by the following principle. Unlike the free radical chain-growth polymerization of vinyl monomers, the formation and dissipation (by recombination) of free radicals in plasma are not sequential, and they can be considered to be independent processes, which is a key principle in the R S G P mechanism for plasma polymerization discussed in the previous section. The effect of a pulsed rf discharge is discussed in more detail in the following section. 6.4.4

EFFEC T OF PULSE D

RADI O

FREQUENC Y GLO W

DISCHARG E

As shown in the previous section, a large quantity of the free radicals found in plasma polymers are due primarily to the mechanisms of polymer formation. According to the growth mechanisms of plasma polymerization shown in Fig. 6.1, there are two major cycles; cycle I consists of reactions of

104

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

monoradicals, and cycle II consists of reactions of diradicals. Because both kinds of radicals are formed simultaneously, reactions between monoradicals and diradicals also occur depending on the concentrations of both radicals; that is, reaction (6-3) is the cross-cycle reaction. The results of ESR studies show that the presence of a triple bond, a double bond, and an aromatic structure in the m o n o m e r results in a large number of free radicals in the plasma polymer. Because the homolytic scission of the n bond of a double or a triple bond yields a diradical, polymer formation via diradicals would play an important, if not dominant, role in the plasma polymerization of such monomers. The reactions in cycle II d o not terminate free radicals, and the cycle itself leads to a free radical "living polymer" that contains a large number of free radicals. In contrast to those in cycle II, the reactions in cycle I are essentially the recombination reactions of free radicals, which dissipate free radicals and yield species that have no free spins. O n the other hand, the completion of the termination reactions and the formation of free radicals by excitation and reexcitation occur independently. Therefore, the complete recombination of free radicals formed would not be realized even in a case in which only cycle I could take place; consequently, there would still be some free radicals in the plasma polymer. It is anticipated, however, that the concentration of free radicals would be much smaller than in the case where cycle II predominates. The effect of a pulsed rf discharge can be reviewed in terms of this growth mechanism. O n the basis of the anticipated contribution of cycle I and cycle II, three representative cases might be chosen, that is, (1) primarily cycle I, (2) nearly equal contributions of cycle I and cycle II, and (3) predominantly cycle II with a small contribution of cycle I. G r o u p I monomers are expected to belong to case (3), group II to case (2), and group III to case (1). When a pulsed rf discharge is employed, cycle I ceases while the discharge is "off," whereas cycle II can keep going during the "off" period. Furthermore, during the "off" period, the cross-cycle reaction [(6-3)] also ceases, leading to a reduction in the dissipation of diradicals by the cross-cycle reaction. O n the basis of the contribution of the cross-cycle reaction, the effect of the pulsed rf discharge would be most dramatic if the contributions of cycle I and cycle II were comparable. This is indeed the case, as shown by the experimental results in Table 6.9; that is, free radicals in the plasma polymer of ethylene increased nearly 10-fold when a pulsed rf discharge was employed. In the case of acetylene, which makes a relatively small contribution to cycle I (based on the detachment of hydrogen), the increase of free radicals in the plasma polymer is less pronounced ( 8 1 % increase). In the case of tetramethyldisiloxane (pri­ marily cycle I), the pulsed rf discharge decreases the free radical concentra­ tion in the plasma polymer, which can be explained by the more complete coupling of free radicals before new free radicals are formed by the subse­ quent "on" cycle of the pulsed rf discharge.

105

6.4. F R E E RADICAL S I N P L A S M A P O L Y M E R I Z A T I O N

The unexpected and conspicuous increase of free radicals in plasma polymers of some monomers can be thus explained on the basis of the rapid step-growth mechanism. Examining why not all group I monomers showed an increase in free radicals by the pulsed rf discharge may provide further insight into the growth mechanism of plasma polymerization. A notable difference can be detected for monomers that contain certain aromatic structure (e.g., benzene and styrene showed a substantial decrease of free radicals in the plasma polymers by pulsed rf discharge). 6.4.4.1

AROMATI C

STRUCTUR E

Acetylene ( H C = C H ) and benzene (
Analysis of the gas phase in both closed and flow systems is given in Tables 6.10 and 6.11. The most remarkable difference in these data is found Tabl e 6.10 EFFEC T O F P U L S E D RADI O FREQUENC Y O N ACETYLEN E A N D BENZEN E I N A C L O S E D SYSTEM "

Polymerizatio n paramete r t112 (sec) Monome r typ e paramete r yb Fractio n of residua l vapo r Xc Hydroge n yield yd Polyme r yield Z = 1- X a

Benzen e

Acetylen e Continuou s

Pulse d

Continuou s

Pulse d

1.0 0.17

5.0 0.16

2.5 0.15

18.5 0.15

0.023

0.021

0.014

0.010

0.15 0.98

0.14 0.98

0.13 0.99

0.14 0.99

Fro m Yasud a an d Hs u (11). Monome r typ e paramete r y == P^/PQ, wher e p 0 is initia l monome r pressur e in a closed syste m an d p ^ th e pressur e afte r glow discharge . c Fractio n of residua l vapo r X = (p^ /? H2)/Po> wher e p Hl is th e pressur e afte r th e cold finger is surrounde d by liqui d N 2 . d Hydroge n yield y = pHJp0. b

106

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

Tabl e 6.11 V A P O R - P H A S E ANALYSI S I N A F L O W SYSTE M A T STEAD Y STATE " Acetylen e Paramete r

Continuou s

Tota l vapo r pressur e at stead y stat e in glow discharg e (/m i Hg ) Vapo r pressur e afte r condensatio n wit h liqui d N 2 ( H 2 pressure ) (urn Hg )

Pulse d

Continuou s

Pulse d

5.1

10.7

5.8

12.4

3.4

4.8

3.8

6.3

68 32

H 2 (% ) Condensabl e vapo r (% ) a

Benzen e

45 55

66 34

51 49

Fro m Yasud a an d Hs u (11).

in the dissipation rates of m o n o m e r given by tlf2 indicating that acetylene polymerizes (disappears from the gas phase) ~ 2 . 5 times faster than ben­ zene. This difference is not very surprising if we recognize that we are mea­ suring the rate of disappearance of the number of gas molecules. As far as the effect of pulsed rf discharge (on the gas-phase kinetic data) is concerned, these data indicate that the effect is nearly the same for acetylene and benzene. In contrast, the concentration of ESR spins in plasma-polymerized polymers showed noteworthy differences between acetylene and benzene. Spin concentrations given in Table 6.9 are cited here for a comparison of acetylene, benzene, styrene, and ethylene. Cs x 1 0 " Compoun d

Continuou s

HC=C H

8.6

19

(spins/cm 3) Pulse d 15.6

Q H 2C = C H - ^ H 2C = C H

2

^

3.8 1.4

0.54 14.5

The effects of a pulsed rf discharge on the free radicals trapped in the plasma polymers of these monomers are most pronounced. When pulsed plasma is employed, the concentration of the trapped free radicals changes from that in continuous plasma. The ratio (free radicals by pulsed

6.4. FRE E RADICAL S I N P L A S M A P O L Y M E R I Z A T I O N

107

plasma)/(free radicals by continuous plasma) for acetylene is 1.8, and the corresponding ratios for benzene and styrene are 0.5 and 0.14, respectively. Namely, an increase in trapped free radicals by the use of pulsed plasma is observed only with acetylene, and a decrease is observed with benzene and styrene. Consequently, the difference between acetylene and styrene becomes nearly 30 times that of the concentration in styrene, whereas this ratio for continuous plasma is only about 2.0. It is important to note that all of these monomers, excluding ethylene, are group I monomers, and their behavior in plasma polymerization, except under pulsed plasma conditions, is very similar to that described in previous sections and chapters. These conspicuous differences observed as a con­ sequence of the use of pulsed plasma may be attributed to the behavior of the aromatic structure under the conditions of plasma polymerization. Although numerous kinds of reactions could occur in plasma, as far as the dissipation of vapor-phase molecules (plasma polymerization) is concerned, one benzene molecule behaves as three acetylene molecules. Consequently, the final polymers formed (from acetylene and benzene) under the condition of relatively high W/FM [discharge-power-independent region; where W is discharge wattage, F volume or molar flow rate, and M molecular weight of m o n o m e r (details of this composite factor are described in Section 9.11)] are very similar. The transport characteristics of ultrathin films of plasma polymers and copolymers (with N 2 a n d / o r H 2 0 ) of acetylene and benzene are nearly identical. However, with respect to the kinetics of polymer formation, benzene is one step behind acetylene, and in its original form (without the dissociation of the molecule in plasma) lacks reactions (6-1) and (6-4) of Fig. 6.1. Pulsed rf discharge highlights the difference in a dramatic way. Namely, the diradicals formed by plasma can react with acetylene during the "off" period of the pulsed discharge, although the addition is not a chain reaction with a long kinetic chain length, as discussed in Chapter 5. In contrast, the diradicals cannot react with benzene. Thus, although the olefinic double bonds or triple bonds may conserve the overall concentration of radicals that have built up during the " o n " period by the addition onto these bonds during the "off" period of the pulsed discharge, the conjugated double bonds in the aromatic structure evidently do not behave in the same manner. In other words, the diradicals can be conserved by the reaction with the m o n o m e r during the "off" period in the case of acetylene and partially in the case of styrene; in the case of aromatic compounds, however, cycle II, the reactive species of which might be identical to those of acetylene, tends to cease during the "off" period because the diradicals cannot react with the original monomer. Styrene can be viewed as a m o n o m e r that combines the effects of a double bond and an aromatic structure. In a continuous discharge, the dominating

108

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

factor is that of the phenyl group (styrene is a group I monomer). A large concentration of free radicals in the plasma polymer is due to this phenyl group. With a pulsed rf discharge, the dissociation of the phenyl group ceases during the "off" period, but the contribution of the vinyl double bond is much smaller than that of the triple bond of acetylene, which leads to the net loss. The large increase found with ethylene is due largely to the low value of trapped free radical concentration by the continuous discharge owing to the dissipation of free radicals via reaction (6-3). Hydrocarbons that show a conspicuous increase of free radicals in plasma polymers upon the application of a pulsed rf glow discharge can be categorized as compounds having a double or triple bond but not an aromatic structure. It is important to note that, even with those monomers, the deposition rates of polymer generally decrease with the pulsed discharge, although the reduction is not as great as one might expect on the basis of the duty cycle of the pulsed discharge. Tabl e 6.12 P R E S S U R E PARAMETE R S O F V A R I O U S M O N O M E R S I N C O N T I N U O U S A N D PULSE D RADI O FREQUENC Y DISCHARGES 0

Monome r

Continuou s

Pulse d

Change ’

C«F 6 Styren e C 2H 4 C 2F 4 Cyclohexan e Ethylen e oxid e Acryli c acid Propioni c aci d Vinyl acetat e Methy l acrylat e Hexamethyldisilan e Tetramethyldisiloxan e Hexamethyldisiloxan e Divinyltetramethyldisiloxan e

0.10 0.13 0.10 0.15 0.63 0.65 1.03 1.45 2.00 2.30 2.25 2.25 1.50 1.15 1.50 0.73

0.30 0.33 0.15 0.28 0.75 0.58 1.25 1.38 1.43 1.88 2.13 1.93 1.20 1.05 1.15 0.75

0.2 (200) 0.2 (150) 0.05 (50) 0.13 (87) 0.12 (19) -0.07 (-11) 0.22 (21) -0.07 (-5) -0.57 (-29) -0.42 (-18) -0.12 (-5) -0.32 (-14) -0.30 (-20) -0.10 (-9) -0.35 (-23) 0.02 (3)

C 2H 2 C 6H 6

a

Fro m Yasud a an d Hs u (77). 3 = pjpm, wher e pg is th e pressur e of a steady-stat e flow in glow discharg e an d pm th e pressur e of monome r befor e th e discharge . c Change s ar e base d on value s of continuou s discharge . Number s in parenthese s ar e percentages . b

6.4. FRE E RADICAL S I N P L A S M A P O L Y M E R I Z A T I O N

109

Tabl e 6.13 D E P O S I T I O N RATE S O F V A R I O U S M O N O M E R S I N C O N T I N U O U S A N D P U L S E D R A D I O F R E Q U E N C Y DISCHARGES " Depositio n rat e x 1 0 8 (g/cm 2«min ) Monome r

Continuou s

Pulse d

Styren e C 2H 4 C 2F 4 Cyclohexan e Ethylen e oxid e Acryli c acid Propioni c acid Vinyl acetat e Methy l acrylat e Hexamethyldisilan e Tetramethyldisiloxan e Hexamethyldisiloxan e Divinyltetramethyldisiloxan e

31 110 190 173 42 18 92 15 28 7 31 32 251 191 233 641

24 101 149 145 43 37 9 14 61 15 16 33 65 102 43 277

C2H 2

C 6H 6 C 6F 6

Change * - 7 (-23) - 9 (-8) - 4 1 (-22) - 2 8 (-16) 1(2) 19(110) - 8 3 (-90) -l(-7) 33 (120) 8(110) - 1 5 (-48) M3 ) -186 (-74) - 8 9 (-47) -190 (-82) -364 (-57)

a

Fro m Yasud a an d Hs u (11).

b

Change s ar e base d on value s of continuou s discharge . Number s in parenthese s ar e percentages .

Notable exceptions to this observation on deposition rates are found for acrylic acid and tetrafluoroethylene. In order to visualize the overall effect of a pulsed discharge, one should refer to the data given in the following tables: pressure parameters in Table 6.12, deposition rates of polymers in Table 6.13, characteristics of ESR spin signals in Table 6.14, and contact angles of water in Table 6.15. 6.4.4.2

POISONIN G

EFFEC T

OF

OXYGEN-CONTAININ G

FUNCTION S

The increase in the deposition rate of acrylic acid by a pulsed rf discharge is due to the additional effect of oxygen-containing monomers, which is absent in the case of the other monomers. This effect can be visualized as a poisoning of the growth mechanism of cycle II (diradical cycles in Fig. 6.1), which is apparent from the data of polymer free radicals and glass free spin signals in Table 6.8. The plasma polymerizations of acetylene and allene are characterized by a high concentration of free radicals trapped in the polymers and very low glass

Tabl e 6.14 CHARACTERISTIC S O F SPI N S I G N A L S 0 Normalize d pea k heigh t (a.u.) ft Monome r C 2H 2 QH 6 C 6F 6 Styren e C 2H 4 C 2F 4 Cyclohexan e Ethylen e oxid e Acryli c acid Propioni c acid Vinyl acetat e Methy l acrylat e Hexamethyldisilan e Tetramethyldisiloxan e Hexamethyldisiloxan e Divinyltetramethyl disiloxan e

b c

Fro m Yasud a an d Hs u (77). Arbitrar y units . Signa l no t detected .

Half-lif e of initia l deca y (min )

Pea k widt h (G )

Continuou s

Pulse d

Continuou s

Pulse d

Continuou s

Pulse d

20.6 4.9 2.2 5.8 2.3 7.4 1.6 1.2 1.6 1.9 0.7 0.5 11.3 5.2 3.6 1.6

56.5 2.7 2.3 1.5 20.1 2.4

15.7 19.6 44.1 19.6 18.6 52.9 17.6 18.6 16.7 17.6 18.6 18.6 5.9 7.5 5.9 7.5

12.7 18.6 37.2 14.7 20.6 45.1

60 32 60 30 17 40 36 65 83 13 26 100 15 28 20 19

90 15 64 55 18 25

c

1.1 3.5 2.4 0.7 0.3 4.1 1.1 c

0.5

c

16.7 17.6 15.7 16.7 17.6 5.9 5.1 c

7.5

c

40 115 10 25 110 14 20 c

21

6.4. FRE E RADICAL S I N P L A S M A P O L Y M E R I Z A T I O N

111

Tabl e 6.15 C O N T A C T A N G L E S O F WATE R WIT H P L A S M A P O L Y M E R S " c o s 0 ( H 2O ) Monome r

Continuou s

Pulse d

Chang e

C 2H 2 C 6H 6 C 6F 6 Styren e C 2H 4 C 2F 4 Cyclohexan e Ethylen e oxid e Acryli c acid Propioni c acid Viny l acetat e Methy l acrylat e Hexamethyldisilan e Tetramethyldisiloxan e Hexamethyldisiloxan e Divinyltetramethyldisiloxan e

0.82 0.94 0.05 0.29 0.15 -0.22 0.01 0.44 0.50 0.46 0.39 0.39 -0.04 0.23 -0.18 0.03

0.81 0.18 0.01 0.20 0.06 -0.23 -0.08 0.34 0.99 0.99 0.36 0.37 -0.12 -0.05 -0.29 0.03

-0.01 -0.76 -0.04 -0.09 -0.09 -0.01 -0.09 -0.10 0.49 0.53 -0.03 -0.02 -0.08 -0.28 -0.11 0

a

Fro m Yasud a an d Hs u (77).

spin signals, which are typical features of group I monomers. When these monomers are copolymerized with H 2 0 , the characteristics of the plasma polymer based on the polymer free radicals change from those of group I to those of group III (i.e., low polymer signals and high glass signals). A more detailed discussion of copolymerization of H 2 0 is given in Section 6.5; only the effect on the trapped free radicals is discussed here. The change in the plasma polymerization characteristics of acetylene with the addition of H 2 0 is shown in Fig. 6.13, which depicts the pressure change due to the addition of the comonomer ( H 2 0 ) , and in Fig. 6.14, which shows the decrease in trapped free radical concentration as a function of the added comonomers ( H 2 0 and N 2 ) . This change due to the addition of H 2 0 can be explained by the relative contributions of cycle I and cycle II to the growth mechanisms of the plasma polymerization of the monomers. In the absence of H 2 0 , the polymer formation from these monomers depends heavily on cylce II (diradicals). The reversal of free radical characteristics by the addition of H 2 0 indicates that polymer formation in the presence of H 2 0 must depend on cycle I (based on monoradicals), because plasma-activated species of H 2 0 evidently block the diradical cycle. Because of the poisoning effect, most organic compounds with

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

112

Discharge power (W) Fig . 6.13 Dependenc e of flow pressur e in th e discharg e of a c e t y l e n e / H 20 mixture s on discharg e power . Adapte d fro m Yasud a an d Hirots u (14).

CO

02

04

0J6

08

1^0

1L2

1^4

Mole ratio Fig . 6.14 Chang e in concentratio n of fre e radical s trappe d in plasm a polymer s by copolymerizatio n of acetylen e with N 2 or H 2 0 . Adapte d fro m Yasud a an d Hirots u (14).

6.4.

FRE E RADICAL S IN PLASM A

POLYMERIZATIO N

113

oxygen-containing groups such as — C O O H , — C O — , — O C O — , — O H , and — O — , are generally reluctant to form a polymer, and the plasma polymers rarely contain the original oxygen-containing groups. Because of the poisoning effect of oxygen-containing groups, acrylic acid shows a lower deposition rate than one would expect from the molecular weight of the monomer, and the plasma polymer is rather hydrophobic, as seen in Table 6.15. In a pulsed glow discharge, the poisoning effect is also reduced due to the low duty cycle of the pulse, but once-formed, the radicals can be conserved by the vinyl double b o n d during the "off" period of the pulse. Consequently, the net effects of a pulsed rf discharge on acrylic acid are the following: 1. Remarkable increase in the polymer deposition rate (Table 6.13) 2. Increase in the concentration of free radicals trapped in the plasma polymer (Table 6.9) 3. Decrease in the substrate glass signals (Table 6.9) 4. Significant increase in the wettability by water of the plasma polymer (Table 6.15). F o r m o n o m e r s with growth mechanisms in which cycle II mechanisms have a minor role, a pulsed glow discharge generally decreases the concentration of polymer signals, but the reduction is much less than what one might expect from the duty cycle of the pulsed discharge. The reduction of polymer signal in such cases can be explained by the more complete coupling of free radicals before new free radicals are created by the subsequent " o n " cycle of the pulsed glow discharge. 6.4.4.3

PERFLUOROCARBON S

The effects of a pulsed rf discharge on the plasma polymerization of perfluorocarbons are more complicated due to the complex behavior of the detached fluorine under plasma conditions. Some aspects of the behavior of fluorine plasma are discussed in Chapter 7. In general, the concentrations of free radicals in the plasma polymers of fluorine-containing monomers are higher than those of other organic compounds, with the exception of acetylene and ethylene. The free spin concentrations observed with glass substrates for both fluorine-containing monomers and hydrocarbons, however, are by and large the same. The effect of a pulsed rf discharge is generally a reduction in the polymer signals, with the exception of perfluoromethylcyclohexane, which exhibits a small increase (22%). In view of the fact that the largest increase in polymer signal is observed with ethylene and that fluorohydrocarbons (e.g., vinyl fluoride and vinylidene

114

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

fluoride) show a conspicous increase in polymer signal by a pulsed discharge, the decrease in polymer signal observed with tetrafluoroethylene and the generally observed decrease in polymer signal with most perfluorocarbons may indicate a significant difference between the plasma polymerization of hydrocarbons and that of perfluorocarbons. This difference can be seen in the behavior of hydrogen and of fluorine in plasma. As evidenced by investigations with pulsed rf discharge, the detachment of hydrogen plays a significant role in the overall growth mechanisms of the plasma polymerization of ethylene. The detachment of fluorine, however, has a completely different effect on plasma polymerization. This situation can be visualized by a comparison of bond energies of C — H and C — F and the dissociation energies of H — H and C — F . Energ y (kcal/mol ) Bon d

C

H

C

98.8

C

116 H

Dissociatio n

F

H

104.2

C 83

F

F

37.7

The detachment of hydrogen to free H 2 is energetically favored, but the detachment of fluorine to free F 2 is not. In order for a polymeric molecule to be formed by plasma polymerization, a covalent bond must be broken to create a new covalent bond. In the case of hydrocarbons, the scission of C — H would provide an efficient route for building a polymer, because the scission of C — C is generally degradative. In the case of perfluorocarbons, however, the scission of C — C becomes the major route for building a large molecule because of the energetics just mentioned. Furthermore, if fluorine is detached from a perfluorocarbon under plasma conditions, because of the extremely high reactivity (evidenced by the very low dissociation energy of F — F ) it participates in various chemical reactions much more actively than the detached hydrogen in the plasma of hydrocarbons. This means that the contribution of fluorine detachment to the growth mechanisms of plasma polymerization is considerably less than that of hydrogen detachment in hydrocarbon plasma. Thus, the plasma polymerization of perfluorocarbons depends heavily on cycle II growth mechanisms, whereas cycle I plays a significant role in the plasma polymerization of hydrocarbons. The rather heavy dependence on cycle II means that diradical-forming structures such as a double bond and a cyclic structure are needed for the efficient plasma polymerization of perfluorocarbons. The addition of H 2 , however, changes this situation significantly, as discussed in Chapter 7. The effects of a pulsed rf discharge on the plasma polymerization of some monomers representing different types of growth reaction are summarized in Table 6.16 (pp. 116-117).

6.5. N O N P O L Y M E R I Z A B L E GASE S I N P L A S M A P O L Y M E R S

6.5

115

Incorporatio n of Nonpolymerizabl e Gase s in Plasm a Polymer s

Another important and unique feature of plasma polymerization is the incorporation of gases that d o not form polymer or solid deposits in plasma by themselves during the process of polymer formation of organic molecules in plasma. This incorporation of gases is plasma copolymerization and not the trapping of gas molecules in plasma polymers. There is a significant difference between copolymerization and codeposition. An example of codeposition is the simultaneous deposition of Parylene or a plasma polymer and an evaporated metal, in which each component can be deposited regardless of whether or not the other component is deposited. In contrast, plasma copolymerization of gases occurs only in the presence of polymer-forming plasma. In this context, the plasma polymerization of more than one organic monomer may be more appropriately recognized as plasma codeposition rather than as plasma copolymerization. An example of plasma copolymerization of gases is the incorporation of N 2 in the plasma polymer of styrene. Yasuda and Lamaze (12) observed that N 2 mixed with styrene was consumed in plasma polymerization. In a closedsystem experiment, pressure measurement is a very useful tool for investigat­ ing plasma polymerization, particularly when the m o n o m e r used does not produce gaseous by-products. The pressure changes observed in a closedsystem plasma reactor with mixtures of N 2 and styrene are shown in Fig. 6.15. The partial pressure of N 2 in each experiment is shown as a horizontal line crossing the pressure decay curve. As can be seen, the system pressure decreases beyond the partial pressure of N 2 in a mixture, indicating that N 2 is incorporated into the plasma polymer (and thus disappears from the gas phase). Such a copolymerization of an unusual m o n o m e r has been observed for N 2 , C O , and H 2 0 and is particularly efficient with monomers containing triple bond(s), double bond(s), or aromatic structure(s) (group I and group II monomers discussed in Sections 6.3 and 6.4). Yasuda et al. (13, 14) studied the plasma copolymerization of N 2 , C O , and H 2 0 with acetylene, and the results, in light of R S G P mechanisms (Section 6.2), are summarized as follows. The a m o u n t of N 2 incorporated in the polymer formed from acetylene/N 2 mixtures in plasma is shown in Fig. 6.16 as a plot of ( N 2 / C 2 H 2 ) p o l y rm versus ( N 2 / C 2 H 2 ) g a ,s the values of which are e estimated by an analysis of system pressure. In these experiments, partial system pressures of acetylene of 10, 20, and 40 m t o r r were employed, and a constant discharge power of 30 W (13.56-MHz rf, inductively coupled tube reactor) was used. Thus, the total system pressure increases as the partial N 2 pressure is increased. Figure 6.16 indicates that, at lower N 2 / C 2 H 2 ratios,

116

Tabl e 6.16 EFFEC T O F P U L S I N G O N PLASM A P O L Y M E R S O F S O M E M O N O M E R S

Monome r

Type s of reactio n involved "

Cycle s in plasm a poly › merizatio n mechanis m

Change s du e t o pulse d discharge 6

S

Depositio n rat e

COS 0

( H 20 )

(-9)

(-47)

(-90)

0

-0.28

I I an d poisonin g

(-18)

(-110)

0

(-81)

0.53

I I an d poisonin g II

(-29)

(120)

(140)

(-100)

0.49

I II

(21)

(-90)

(-100)

(-100)

-0.09

Detachmen t of H Openin g of doubl e bon d

I II

(19)

(2)

(970)

(-79)

-0.09

Acetylen e

(Detachmen t of H ) Openin g of tripl e bon d

(I) II

(200)

(-23)

(81)

0

-0.01

Benzen e

Detachmen t of H Aromati c rin g openin g

I II

(150)

(-8)

(-50)

0

-0.76

Tetramethyldisiloxan e

Detachmen t of H Detachmen t of C H 3

Propioni c acid

Detachmen t of H Detachmen t of C O O H

Acryli c acid

Detachmen t of H Detachmen t of C O O H Openin g of doubl e bon d

Cyclohexan e

Detachmen t of H Rin g openin g

Ethylen e

I I

1

Detachmen t of H Aromati c rin g openin g Openin g of doubl e bon d

Vinyl acetat e

Detachmen t of H Detachmen t of O C O C H Openin g of doubl e bon d

117

Styren e

I II II

3

(87)

(-16)

(-86)

0

-0.09

I I an d poisonin g II

(-5)

(-48)

(-21)

(-71)

-0.03

(-5)

(-7)

(-33)

(-76)

-0.09 -0.01

Ethylen e oxid e

Detachmen t of H Rin g openin g

I II an d poisonin g

Tetrafluoroethylen e

Detachmen t of F Openin g of doubl e bon d

I an d ablatio n II

(-11)

(106)

(-35)

(-84)

Hexafluorobenzen e

Detachmen t of F Aromati c rin g openin g

I an d ablatio n II

(50)

(-22)

(-27)

0

Vinyl fluoride

Detachmen t of H Detachmen t of F Openin g of doubl e bon d

I I an d ablatio n II

(47)

(-45)

(480)

(-100)

0.07

Vinyliden e fluoride

Detachmen t of H Detachmen t of F Openin g of doubl e bon d

I I an d ablatio n II

(-6)

(-47)

(100)

(-64)

-0.36

a b

-0.045

Possibl e reaction s in continuou s plasma . Number s in parenthese s ar e percentage s base d on value s of continuou s plasma ; S = pg/pm; Cf , spi n concentratio n in plasm a polymer ; C g, spin concentratio n in glass substrate ; 6, contac t angl e of water . A negativ e valu e for cos 6 mean s tha t th e polyme r become s mor e hydrophobic ; a positiv e valu e indicate s tha t it is mor e hydrophilic .

118

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

30

40

50

60

70

T (sec) Fig . 6.15 Pressur e chang e in a closed syste m containin g (1) 200 /m i H g styrene , (2) 200 /mi H g styren e an d 100 /mi H g N 2, an d (3) 200 /mi H g styren e an d 200 /m i H g N 2, with tim e of glow discharg e at 60 W . Adapte d fro m Yasud a an d Lamaz e (12).

nearly all N 2 is incorporated into the plasma polymer, and the N 2 / C 2 H 2 ratio in the polymer is nearly identical to that in the gas phase. Similar experiments were also carried out with C O as the comonomer of acetylene (Fig. 6.17). The trends found in Fig. 6.17 are strikingly similar to those in Fig. 6.16. Here again, the incorporation of C O is directly proportional to the a m o u n t of C O added to acetylene at the lower region of C O / C 2 H 2 ratios. Because in those experiments the partial system pressure of N 2 or C O was used to express the N 2 / C 2 H 2 ratio, respectively, rather than the flow rates

6.5. N O N P O L Y M E R I Z A B L E GASE S I N P L A S M A P O L Y M E R S

119

3r

Gas phase ( N 2/ C 2H 2)

Fig . 6.16 Amoun t of N 2 incorporate d int o th e plasm a polyme r of acetylen e an d N 2 as a functio n of th e N 2/ a c e t y l e n e ratio s in th e ga s phas e at differen t partia l pressure s p C 2 z H(/mi Hg ) of acetylene : , 10; # , 20; O , 40. Adapte d fro m Yasud a et al. (13).

of each component gas, the absolute ratios of N 2 to C 2 H 2 and C O to C 2 H 2 are questionable, as pointed out later by Yasuda and Hirotsu (75). The same ratios, however, are used both in the polymer composition and in the gasphase composition, and the dependencies shown in Figs. 6.16 and 6.17 seem to be valid. In other words, whether ~ 2.5 times as much N 2 and nearly 3.0 times as much C O as acetylene can be incorporated into acetylene plasma polymer is questionable, and it is very likely that only a fraction of these values are actually incorporated. Nevertheless, it can be said that the direct porportionality between the polymer composition and the gas-phase composition exists

Gas phase ( C O / C 2 H 2 )

Fig . 6.17 Amoun t of C O incorporate d int o th e plasm a polyme r of acetylen e an d C O a s a functio n of th e CO/acetylen e ratio s in th e ga s phas e at differen t partia l pressure s p (/mi Hg ) of acetylene : , 10; , 20; O , 40. Adapte d fro m Yasud a et al (13).

120

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

up to a limiting concentration of the comonomer, which depends on the partial pressure of acetylene (and consequently the total pressure of the mixture) when a fixed discharge power is employed. A deviation from linear dependence is seen in these figures, particularly at higher partial pressures of acetylene, which is due partly to the experimental conditions in that a fixed discharge power was used. According to the concept of the W/FM parameter for plasma polymerization (described in Section 9.10), plasma polymerization conditions employing a fixed wattage with a variable quantity of monomer always show the apparent maximum in the a m o u n t of polymer formed due to the transition from the case in which the flow rate is the rate-determining step to the case in which the rate of energy input of discharge is the rate-determining step. It is anticipated that if the wattage were increased as the total flow rate in the system increased, the linear dependence would be extended to a wider range. The incorporation of N 2 , C O , and H 2 0 in the plasma polymer of acetylene is evident from the results of elemental analysis and also from IR spectra obtained with the copolymers. Table 6.17 shows the results of elemental analysis of plasma polymers of acetylene copolymerized with N 2 , C O , and H 2 0 . The incorporation of N 2 is clearly evident; the incorporation of C O a n d / o r H 2 0 is not as clear, however, because nearly all plasma polymers contain oxygen due to the postpolymerization reaction of residual free radicals with atmospheric 0 2 a n d / o r H 2 0 , and because elemental analysis does not distinguish between these two types of oxygen atoms. However, a major portion of the oxygen in the copolymers with H 2 0 and with H 2 0 / N 2 seems to result from the copolymerization of H 2 0 but not from the postpolymerization reaction of free radicals with atmospheric 0 2 , because the addition of H 2 0 to the m o n o m e r mixture dramatically reduces the quantity of free radicals trapped in the plasma polymer of acetylene, as discussed in Section 6.4. The complex nature of plasma polymers makes the precise interpretation of their IR spectra difficult. However, much useful information concerning the general nature of polymers can be obtained, especially when other data, such as free radical concentrations and elemental analysis, are available. The IR spectra of the six plasma polymers considered in the study are shown in Figs. 6.18 and 6.19, and a summary is presented in Table 6.18. The plasma copolymers of acetylene and the unusual comonomers are though to consist of r a n d o m and highly branched hydrocarbon chains with various amounts of hydroxyl, carbonyl, and nitrogen-containing functions randomly distributed throughout the network. The highly branched nature is evident from the absence of strong absorption in the region 720-770 c m - 1 characteristic of the straight chains of four or more methylene groups. The hydrocarbon nature of these chains produces the C - H stretching bands near

Tabl e 6.17 E L E M E N T A L ANALYSI S O F G L O W D I S C H A R G E P O L Y M E R S O F A C E T Y L E N E WIT H N 2, CO , A N D H 2O fl Monome r (pressure , fig Hg ) Acetylen e (50) Acetylene/N 2 (50/33) A c e t y l e n e / H 20 (40/20) A c e t y l e n e / N 2/ H 20 (30/20/15) Acetylene/C O (30/20) A c e t y l e n e / C O / H 20 (30/20/15) "Fro m Yasud a et al. (13).

C(% )

H(% )

79.5

5.4

64.0

5.8

66.5

7.6

53.2

6.5

82.6 72.0

N(% )

0(% )

Empirica l formul a

Colo r

15.1

C 2H t O 6

13.5

C 2H 2N 0. 5O 0. 3

Dar k brow n

25.9

C 2H 2 O 7

Off-whit e

24.6

C 2 H 2 - N9 0 O5 0

6.9

10.5

C 2H X O 8

8.4

19.6

C 2H 2. 5 O 0

16.7

15.7

0

0

0

Dar k brow n

3

4

2

4

7-

Brow n Dar k brow n Ligh t brow n

122

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

Fig . 6.18 Infrare d spectr a (4000-1250 c m - 1) of plasm a copolymer s of acetylene : A, acetylene ; B, a c e t y l e n e / H 20 ; C, acetylene/CO ; D , acetylene/N 2; E, a c e t y l e n e / C O / H 20 ; F , a c e t y l e n e / N 2/ H 20 . Adapte d fro m Yasud a et al. (13).

2900 c m " 1 observed in each polymer. Except for the a c e t y l e n e / N 2 / H 2 0 system, the polymers also produce C - H bending modes around 1400 c m - 1. Both ESCA and elemental analysis indicate the presence of oxygen in all plasma polymers investigated. It is most reasonable to attribute this to the postplasma reaction of trapped free radicals with atmospheric 0 2 a n d / o r

123

6.5. N O N P O L Y M E R I Z A B L E GASE S I N P L A S M A P O L Y M E R S

I

5.0

6.0

1

7.0

1

8.0

1

9.0

I

10.0

I

I

IIX)

12.0

I

13.0

I

»4.0

I

I

15.0

I

16.0

Fig . 6.19 Infrare d spectr a (2000-625 c m - 1) of plasm a copolymer s of acetylene : A, acety › lene; B, a c e t y l e n e / H 20 ; C , acetylene/CO ; D , acetylene/N 2; E, a c e t y l e n e / C O / H 20 ; F, a c e t y l e n e / N 2/ H 20 . Adapte d fro m Yasud a et al. (13).

H 2 0 . Such an explanation is especially appealing because of the high concentrations of free radicals found in most plasma polymers. There also seems to be a correlation between the free radical concentration and the subsequent increase in the carbonyl and possibly the hydroxyl groups as a function of time. The free radicals in the plasma polymer may capture

Tabl e 6.18 I N F R A R E D A B S O R P T I O N CHARACTERISTIC S O F E L E C T R O D E L E S S G L O W D I S C H A R G E P O L Y M E R S

0

Monome r syste m Absorptio n regio n ( c m - 1) 1370-1380 1325-1440 1430-1470 1445-1485 1490-1580 1515-1570 1560-1640 1630-1680 1630-1670 1665-1685 1680-1705 1705-1725 1710-1740 2843-2863 2916-2936 2862-2882 2952-2972 3070-3100 3140-3180 3270-3370 3310-3350 3400-3500 3200-3600 a b

Sourc e C H symmetri c bend , methy l C C aldehyd e C H asymmetri c bend , methy l C H asymmetri c bend , methylen e N H bend , secondar y amin e N H bend , secondar y amid e N H bend , primar y amin e C = 0 stretch , secondar y amid e C = 0 stretch , tertiar y amid e C = 0 stretch , a,/?-unsaturate d keton e C = 0 stretch , a,j9-unsaturate d aldehyd e C = 0 stretch , saturate d keton e C = 0 stretch , saturate d aldehyd e C H symmetri c stretch , methylen e C H asymmetri c stretch , methylen e C H symmetri c stretch , methy l C H asymmetri c stretch , methy l N H stretch , secondar y amid e bonde d N H , cis or tran s N H stretch , secondar y amid e bonde d N H , cis N H stretch , secondar y amid e bonde d N H , tran s N H stretch , dialkylamin e N H stretch , primar y amin e O H stretch , bonde d hydroxy l

Acetylen e

Acetylene / H 20

Acetylene / CO

M W

w w w w

W W

Acetylene / N2

W W w

Acetylene / C O / H 20

Acetylene / N 2/ H 20

M M M M W

s

M S S S

S S M

M M W

S S S M

S S S s

s s s s

S s M S S S S

S s s s

M M M M W M S

S

Give n as peaks : stron g (S), mediu m (M), an d wea k (W). Fro m Yasud a et al. (13). Possibl y carbony l overtone .

M"

w

s M M

w

M

6.5. N O N P O L Y M E R I Z A B L E GASE S I N P L A S M A P O L Y M E R S

molecular 0

2

125

or H 2 0 , producing hydroxyl and carbonyl groups: C

+ H 20

C

D a t a on the free radical concentrations from the ESR measurements can be found in Table 6.19. Let us first consider the change in the IR spectra with the time of exposure to the ambient conditions for the plasma polymer of acetylene (without a comonomer). Figure 6.20 clearly indicates an increase with the time of the signal in the carbonyl region (ketones and aldehydes absorb generally in the region 1665-1740 c m - 1) . There is also an apparent increase in the bonded hydroxyl O - H stretch band (3200-3600 c m " 1 ) relative to the C - H stretching signal ( - 2 9 0 0 c m " 1 ) . Concurrent with the carbonyl formation observed in tthe IR spectra, there is a decrease in the concentration of free radicals trapped in the polymer. Over a Tabl e 6.19 FRE E RADICAL S I N P L A S M A P O L Y M E R S O F A C E T Y L E N E WIT H H 2 0 , N 2, A N D C O a Monome r (pressure , fim Hg ) Acetylen e Acetylene/N 2 (30/30) A c e t y l e n e / N 2/ H 20 (30/10/20) (30/20/10) A c e t y l e n e / H 20 (30/20) Acetylene/C O (30/20) A c e t y l e n e / C O / H 20 (30/20/15) a

Fro m Yasud a et al. (13).

Spi n concentratio n [(spins/cm 3) x 1 0 " 1 ]8 280 180

9 9 0 217 1.5

126

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

4000

3000

I

'

2900

2000 1

'

1600

n

1400

i

Fig . 6.20 Infrare d spectr a of th e plasm a polyme r of acetylen e take n at variou s time s afte r th e polymerization . Adapte d fro m Yasud a et al. (13).

15-month period, the free radical concentration as measured by ESR drops by 87%. The loss of free radicals is a very slow process, as is the oxidation of the polymer film. This indicates a rather high stability of the free radicals in the polymer network a n d / o r the very low accessibility of those free radicals to oxygen. The resonance stabilization of free radicals could be expected in a very highly branched and highly cross-linked polymer matrix. In addition to the absence of methylene chain signals, the IR spectra give other indications of a branched polymer. The strong, broad O - H stretching absorption shifts down from the high to middle and lower 3000 c m - 1 region, which results from the intramolecular hydrogen bonding fully expected in a branched hydrocarbon polymer. The plasma polymer of acetylene can now be described as a highly branched and highly cross-linked hydrocarbon network with a fairly large number of free radicals distributed throughout. Subsequent exposure to the atmosphere results in the incorporation of carbonyl and hydroxyl groups through the reaction of the free radicals. A plasma of acetylene/N 2 mixture produces a polymer that shows "aminelike" characteristics with N - H stretching and bending of both primary amine

6.5. N O N P O L Y M E R I Z A B L E GASE S I N P L A S M A P O L Y M E R S

127

and dialkylamine. Because the hydroxyl and carbonyl stretchings are found in the same general region as the N - H stretching and N - H bending, respectively, and because both regions produce broad and fairly strong signals, the nature of the oxygen incorporated into the plasma polymer cannot be directly inferred from the IR spectra. The a m o u n t of oxygen in the polymer as determined by elemental analysis is the same as in the acetylene-only polymer and is similarly best explained by atmospheric oxidation. Prolonged exposure to the atmosphere unquestionably intensifies the absorption in the carbonyl region (1665-1740 c m - 1; see Fig. 6.21). Again, the effects on the hydroxyl absorption are less clear. This is due primarily to the coincidence of the N - H and O - H stretching regions of absorption, a problem complicated, as before, by the effects of hydrogen bonding. In the a c e t y l e n e / N 2 / H 2 0 case, the C - H stretching modes are attenuated, and the bending modes undetected. The polymer displays more polyamide than hydrocarbon character (see Figs. 6.18, 6.19, and 6.22). The N - H stretching at 3300 c m " 1 , the C - O stretching at 1650 c m " 1 , and the N - H deformation at 1515 to 1570 c m " 1 provide strong indications of a second­ ary amide. Furthermore, the N - H stretching at 3300 c m " 1 rather than at

4000

3000

2500

2000

1600

Fig . 6.21 Infrare d spectr a of th e plasm a polyme r of acetylen e an d N 2 take n at variou s time s afte r th e polymerization . Adapte d fro m Yasud a et al. (13).

128

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

Fig . 6.22 Infrare d spectr a of th e plasm a polyme r of acetylene , N 2, an d H 2 0 take n at variou s time s afte r th e polymerization . Adapte d fro m Yasud a et al. (13).

3400 c m " 1 implies intramolecular hydrogen bonding, as might be expected in a highly branched polymer. O n e cannot rule out the possibility of the hydroxyl or carbonyl groups coexisting with the amides, however. Indeed, the absorption bands are quite broad and ill defined, and the superimposing of peaks to some extent seems quite likely. Given the fairly equal incorporation of nitrogen and oxygen from the elemental analysis, it seems best to term this plasma polymer "amide-like." The duration of contact with the atmosphere seems to have little effect on the a c e t y l e n e / N 2 H 2 0 polymer (see Fig. 6.22). This is consistent with the ESR results, which indicate that the free radical concentration in the newly produced polymer is several orders of magnitude lower than that in the acetylene or acetylene/N 2 polymer. Hasty conclusions, however, should be avoided, because the newly formed polymer produces fairly broad and strong hydroxyl and carbonyl signals, making the increases difficult to detect. It is not unreasonable to suggest, however, that the plasma polymer of a c e t y l e n e / N 2 / H 2 0 is less susceptible to atmospheric oxidation due to a lower initial free radical concentration.

6.5. N O N P O L Y M E R I Z A B L E GASE S I N PLASM A P O L Y M E R S

129

The presence of H 2 0 in acetylene plasma seems to have two major effects on the resultant polymer: (1) a marked decrease in the number of free radicals in the polymer and (2) the incorporation of carbonyl groups. An additional effect of the added H 2 0 , very important as far as the growth mechanism of plasma polymerization is concerned, is a remarkable increase in the irradiation effect of the plasma, which appears in the substrate but not in the plasma polymer. Whether the inclusion of H 2 0 in the plasma contributes to the oxygen incorporation in the hydroxyl form cannot be determined, because hydroxyl absorption is a generally observed characteristic of plasma polymers. Thus, the plasma polymer of acetylene H 2 0 seems to be a highly branched and highly cross-linked hydrocarbon network with hydroxyl and carbonyl groups that exhibit a fair degree of stability against oxidative reactions in the atmosphere. The addition of C O to acetylene plasma apparently results in the incorporation of the carbonyls groups into the polymers but n o appreciable decrease in the free radical concentration (see Figs. 6.18 and 6.19). The primary difference, then, between the a c e t y l e n e / H 20 and the acetylene/CO polymers lies in their stability. Conclusive evidence of this, however, has yet to be obtained. The a c e t y l e n e / C O / H 20 polymer seems a likely enough hybrid of the preceding two (see Fig. 6.19 and Table 6.19). The hydrocarbon C - H stretching and bending signals are quite strong. The hydroxyl signal is very weak. It can be concluded that the addition of H 2 0 , N 2 , C O , or various combinations of these comonomers to a plasma polymerization of acetylene produces chemically distinct polymers. The characteristics of these polymers are summarized in Table 6.20. The copolymerization of H 2 0 reduces the quantity of free radicals trapped in the plasma polymers in a remarkable manner and enhances the stability of the polymers. Tabl e 6.20 G E N E R A L CHARACTE R O F D I S C H A R G E P O L Y M E R S F R O M I N F R A R E D DATA " Monome r syste m

Functiona l characte r of polyme r

Acetylen e A c e t y l e n e / H 20 Acetylene/C O Acetylene/N 2 A c e t y l e n e / C O / H 20 A c e t y l e n e / N 2/ H 20

Hydroxy l Hydroxyl-carbony l Carbony l Amin e Carbony l Amid e

a

Fro m Yasud a et al. (13).

130

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

The significance of copolymerization described here is that the comonomers are not monomers of polymerization in the ordinary sense, and these monomers can copolymerize only by plasma polymerization. The fact that these comonomers copolymerize with acetylene is in accord with the R S G P mechanism. Conversely, the R S G P mechanism can best explain the copolyme­ rization of such unusual comonomers. If we assume, according to the justification discussed in Section 6.3, that the free radical is the major reactive species involved in the R S G P mechanism, the comonomers in consideration act as radical scavengers. In free radical addition polymerization, such scavengers act as inhibitors of the poly­ merization. In contrast, in plasma polymerization, the incorporation of such comonomers does not affect the overall polymer formation, but reduces the concentration of the residual free radicals trapped in the plasma polymers. The latter aspect certainly reflects the radical scavenger role of those comonomers. In discussions of the mechanism of plasma polymerization appearing in the literature, polymerization, particularly the growth mechanism of polymer formation, is dealt with in a somewhat vague manner without any clear distinction between mechanism of polymerization and mechanism of polymer deposition. F o r instance, the hypothesis that plasma polymerization occurs via the polymerization of adsorbed m o n o m e r on the surface invokes a mechanism of polymer deposition rather than one of polymerization; that is, the mechanism of polymerization, whatever that would be, is intuitively or a priori assumed. Nevertheless, such a hypothesis constitutes an important school of thought in dealing with the polymerization mechanism. It must be reiterated that the subject of this chapter is the mechanism of polymer formation, not the mechanism of polymer deposition, which is discussed in Chapter 8. The results of copolymerization of unusual comonomers provide some important implications for our discussion of the polymer formation mecha­ nism. The following aspects are worth mentioning. If the polymerization of adsorbed m o n o m e r (without specifying what kind of polymerization) played an important role, the copolymerization of N 2 a n d / o r H 2 0 would not occur. The adsorption of N 2 at nearly room temperature is negligible; this principle is used in the calculation of the surface area in a sorptometer, in which adsorbed N 2 at liquid-N 2 temperature is allowed to desorb at room temperature for the gas analysis. Adsorbed H 2 0 certainly would not copolymerize in the adsorbed monomer, because reinitiation would take place to a much smaller extent (if at all) than in the plasma phase. Nitrogen a n d / o r H 2 0 are active only in the gas phase (plasma). The incorporation or copolymerization of N 2 and H 2 0 must involve the activated species of these comonomers, which means that reactions that occur in the gas phase (plasma) are essential to plasma polymerization.

6.6. I N T E R P R E T A T I O N O F P L A S M A D I A G N O S T I C D A T A

131

It has been suggested by Kobayashi et al (16) that the formation of acetylene and its subsequent polymerization are the major route of the plasma polymerization of hydrocarbons (including ethylene). Such a polymerization mechanism requires the polymerization of acetylene by a chain-addition mechanism, although these authors either did not specifically mention it or did not distinguish the plasma polymerization mechanism from the plasma polymer deposition mechanism. It would be difficult, by invoking a poly­ merization mechanism that relies on addition polymerization, explain the copolymerization of N 2 a n d / o r H 2 0 with acetylene and other hydrocarbons. These discussions d o not necessarily rule out nor contradict the possibility that the adsorbed m o n o m e r polymerizes under the influence of plasma or that acetylene is formed in plasma of hydrocarbons. If the m o n o m e r used has a chemical structure that is capable of polymerization, such a polymerization would certainly occur under certain conditions, particularly at the beginning of plasma polymerization, and acetylene is certainly one of the most probable reaction products in plasma of hydrocarbons. The issue is the extent to which such mechanisms contribute to the overall polymerization and their applica­ bility to the general case. The copolymerization of N 2 a n d / o r H 2 0 with acetylene seems to add to the unique characteristic features of plasma polymerization, which should be considered in any attempt to construct a generalized picture of plasma polymerization. As far as the growth mechanism is concerned, it is important to recognize the following points: 1. Nitrogen and C O have similar electron structures and evidently participate in the chemical reactions of cycle II. Consequently, the copolymerizations of these gases do not decrease the concentration of free radicals trapped in the plasma polymers. 2. Water is a blocking agent of cycle II, and an excess a m o u n t of H 2 0 added to a m o n o m e r inhibits plasma polymerization. Because H 2 0 acts as an efficient modifier of the growth mechanism, which shifts the major growth path (if it is applicable) from cycle II to cycle I, the addition of H 2 0 decreases the concentration of free radicals trapped in the plasma polymers. Figure 6.14 clearly demonstrates the characteristic difference of these two types of comonomers.

6.6

Interpretatio n of Plasm a Diagnosti c Dat a

O n e of the important aspects of the R S G P mechanism in plasma is that polymer formation is not the primary and direct consequence of ionization, which is the essential process for creating and maintaining plasma under plasma polymerization conditions. A great number of combinations of

132

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

consecutive and competitive reactions lead to a variety of species that did not exist in the original m o n o m e r or m o n o m e r mixtures. Diagnostic measurements of the gas phase in plasma such as by mass spectroscopy, emission spectroscopy, and gas analysis of the effluent provide important information concerning the kinds of chemical species that exist in the plasma. Such diagnostic data are extremely important when one is dealing with the elementary plasma chemistry of molecules under plasma conditions. The major issue in this section is the way in which this important information might be related to the kinetic and mechanistic aspects of plasma polymerization, which is, unfortunately, not as straightforward as one would wish it to be. In dealing with data of diagnostic measurements such as mass spectroscopic analysis of the gas phase in a plasma polymerization system and chemical analysis of the effluent gas from the system, there seem to exsit two major underlying concepts. The first is the search for a "precursor" structure that satisfies an a priori concept of the polymerization mechanism. The second is the concept that the chemical structures found by such a diagnostic measurement might offer a clue to the kinds of reactive species or intermediate species that are involved in the plasma polymerization. Namely, the first is a search for the plasma-synthesized monomers, and the second is identification of the reactive intermediates of the polymerization. The important points here are that the R S G P mechanism, probably based on the free radical as the activated species, does not require any special "precursor" structure and also that even if a preferred "precursor" structure were found in plasma, it would not polymerize in plasma (in a vacuum) for the reasons discussed throughout this chapter. Therefore, the interpretation of diagnostic data is very much dependent on the a priori concept of poly­ merization or the philosophical attitude of the interpreter. This situation can be explained by an analogy dealing with the empirical aspects of plasma polymerization. Plasma polymerization can be visualized as a composite process consisting of three different kinds of "discharge" processes. These are (1) the controlled discharge of a m o n o m e r (from a gas cylinder to a vacuum pump), (2) the electric discharge characterized as glow discharge, and (3) glow discharge polymerization (synonymous with plasma polymerization for all practical purposes) of the monomer. Before the electric discharge is initiated, a steady-state flow of the monomer is generally established by controlled discharge of the monomer. When the electric discharge is initiated, it is generally assumed, intuitively, that glow discharge polymerization occurs. It is not difficult to prove, in many, but not all cases, that plasma polymerization indeed occurs. However, it is important

6.6. I N T E R P R E T A T I O N O F P L A S M A D I A G N O S T I C D A T A

133

to recognize that (1) the presence of the electric discharge does not unequivocably mean the occurrence of plasma polymerization, and (2) glow discharge polymerization, if it occurs, does not necessarily replace the controlled discharge of the m o n o m e r established as the first step of the procedure. In many cases, perhaps in the majority, the three discharge processes continue to occur simultaneously under the conditions of plasma polymerization. Under typical conditions of plasma polymerization in a bell-jar-type reactor, in which the plasma volume is only a small fraction of the total value of the reaction, the conversion ratio of a m o n o m e r to a polymer is often less than a few percent, which means that the majority of m o n o m e r is merely discharged (dumped) through the reactor. In a strict sense, the major process under such conditions should still be considered controlled discharge (dumping) of the monomer. However, the major process is generally described as plasma polymerization on the basis of the minor process, which constitutes only a few percent of the entire process. Another example is the plasma polymerization of ethylene oxide in an inductively coupled tube reactor in which the entire m o n o m e r flow passes through the portion under the inductive coil with a sufficiently high discharge power input. In this case, nearly 100% of the m o n o m e r is subjected to glow discharge, and the simple controlled discharge of the m o n o m e r is completely replaced by the glow discharge process. However, in this case, little polymer deposition occurs, indicating that the complete glow discharge of a m o n o m e r does not necessarily mean that plasma polymerization of the m o n o m e r has occurred. An analogous situation seems to exist in the interpretation of plasma diagnostic data used for the speculation or discussion of the polymerization mechanism. Although the diagnostic data cited in this section might have been interpreted differently in the original references by the respective investigators, there do not seem to be any diagnostic data in the literature that do not support the R S G P mechanism or that seriously contradict the mechanism. The precursor concept relies on unaddressed, unspecified "polymerization." It merely identifies a certain "precursor" structure, and the basic questions of whether such precursors indeed polymerize in plasma conditions, and if they do, by what mechanism, have not been raised or answered. The precursor concept in the literature can be divided into two categories. One is based on the structures that can polymerize by the addition mechanism. The other is the "black box" approach, in which the growth mechanism is not taken into consideration, and the precursor species are picked on the basis of the abundancy of species in the diagnostic data and on the structure that seems to be sufficiently reactive.

134

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

The diagnostic data identify the chemical species represented by M, or possibly M f - in Fig. 6.1, which certainly should be different depending on the starting materials, but d o not provide, by themselves, information pertinent to the growth mechanism (polymerization mechanism) we are discussing in this chapter. The lifetime of the reactive intermediate species of the poly­ merization, such as the free radical in free radical polymerication and the cation in cationic polymerization, is generally too short to be detected by the methods conventionally used in the diagnostic analysis of plasma poly­ merization. Therefore, such diagnostic data certainly provide information on the state of plasma in which the plasma polymerization takes place but do not necessarily provide information directly pertinent to plasma polymerization. According to the R S G P mechanism of plasma polymerization, each species found by mass spectroscopic analysis of the effluent gas from the plasma polymerization system represents the end product of numerous consecutive and competitive reactions, whereas with the "precursor" concept one looks at the same data as a result of plasma synthesis of a polymerizable structure that should somehow have polymerized in plasma but did not (and remained in the effluent). In dealing with analytical data of the effluent gas, one must also consider the following relationship between the effluent gas and the polymer deposit. In order to simplify the discussion, let us first consider a hypothetical case in which a m o n o m e r A in the gas phase in a closed system polymerizes by a molecular process and forms a polymer deposit. If we express the fraction of monomer polymerized in a given time by a (a is proportional to the rate of polymerization), the fraction of m o n o m e r remaining in the gas phase is given by 1 — a. Because a is proportional to the rate of polymerization RP, the amount of monomer remaining in the gas phase is 1 — RP. Thus, the higher the rate of polymerization, the smaller is the a m o u n t of m o n o m e r remaining in the gas phase. Extending this relationship to a mixture of m o n o m e r A and m o n o m e r B (1:1 mole ratio) in which m o n o m e r A polymerizes faster than monomer B, that is, (RP)A > (RP)B> and the monomers d o not copolymerize each other, gasphase analysis would tell us that the a m o u n t of m o n o m e r B remaining in the gas phase is greater than that of m o n o m e r A. Thus, the more a b u n d a n t species in the gas phase is the one that did not polymerize well: This relationship also applies to a flow system in which a portion of a monomer may bypass the polymerization process. Thus, if polymerization occurs as a molecular process, the most a b u n d a n t species found in the effluent is the least polymerizable species. In other words, the most probable precursor, if polymerization of such precursors occurs, should not be the predominant species found in the effluent unless the yield of polymerization is negligible (no polymerization).

6.6. I N T E R P R E T A T I O N O F P L A S M A D I A G N O S T I C D A T A

135

Mass spectroscopy can be applied to the analysis of the effluent of a plasma polymerization system by the following modes. The first is the in situ application of a mass spectrometer to a plasma polymerization reactor, and the second is the use of a mass spectrometer for the analysis of the effluent gas collected from a plasma polymerization reactor. The in situ application can be divided into (1) direct mass analysis of the ions that effuse through a pinhole in the discharge (without the ionization device) and (2) the analysis of effluent gas (collected at nondischarge zones of a reactor) by the normal operational m o d e of the mass spectrometer. The pinhole used to collect samples in m o d e (1) must be heated to prevent the deposition of polymer, which plugs the pinhole. The deposition of polymer on the pinhole is less troublesome in m o d e (2), but because of this factor, the measurement is primarily the analysis of neutral nonpolymerizing species (at the time of sample collection). M o d e (3), the use of a separate mass spectrometer, is essentially the same as m o d e (2), although mode (2) has an advantage in that measurement can be performed while the discharge is in progress. In both modes (2) and (3), neutral species are subjected to ionization in the mass spectrometer, and the ionization energy has a great influence on the cracking pattern of the original species injected into the mass spectrometer. The higher the ionization energy, the more extensive is the breakdown of the original species in the effluent gas. Because of the cracking pattern of a sample in the mass spectrometer, the data obtained with a multicomponent sample can be dealt with only in a semiquantitative manner. The complex cracking pattern observed with the m o n o m e r itself is a good indication of the kinds of reactions that occur when the m o n o m e r is subjected to ionization in plasma, particularly if low ionization energy, close to the electron energy in plasma polymerization, is used in the mass spectroscopic analysis. The mass spectroscopic data of the effluent gas obtained by Kobayashi et al. (16) via mode (3) are given in Table 6.21. The data clearly show the following important general trends: 1. A considerable degree of breakdown of the molecule occurs when a m o n o m e r is subjected to ionization in a mass spectrometer. 2. The cracking patterns of the m o n o m e r and the corresponding effluent of the plasma polymerization are by and large the same. 3. Polymerizable c o m p o u n d s such as acetylene and ethylene remain the predominant species in the mass spectra of both m o n o m e r and the effluent. These observations indicate that monomers such as acetylene and ethylene d o not polymerize when they are subjected to ionization in a vacuum, which is essentially the same process as plasma polymerization. If polymerization occurs, acetylene or ethylene should not be found in the effluent, regardless of

136

Tabl e 6.21 MAS S SPECTRA L D A T A F O R E F F L U E N T G A S 0 A. Mas s Spectr a of Effluen t Ga s C 2H 2

C 2H 4

m/e

Monome r

0.5 torr , 40 c m 3/ m i n , 50 W

22 23 24 25 26 27 28 29 30

4.0 13.8 65.5 1.5 0.2 0.2 0.1

3.2 11.0 59.8 1.6 0.5 0.2 0.1

C 2H 6

C 4H 6

Monome r

2.0 torr , 40 c m 3/ m i n , 100 W

Monome r

2.0 torr , 40 c m 3/ m i n , 100 W

0.6 2.0 10.4 11.3 51.3 0.9 0.6

0.9 3.9 18.1 12.4 21.5 3.8 0.6

0.3 1.5 9.3 13.3 44.8 11.4 15.9

0.3 1.4 9.1 12.4 41.7 12.4 14.9

Monome r

0.1 0.9 4.9 11.0 9.6 0.2

2.0 torr , 40 c m 3/ m i n , 100 W

0.3 0.9 6.6 7.9 4.8 1.5 0.4

137

31 32 33 34 35 36-47 48-59 >60

0.1

0.4

0.4

0.1

0.1

0.1 0.1

0.1 1.4 1.2

1.1

17.6 9.1 8.4

2.9 10.5 3.2

24.7 18.6*

4.6 0.7 0.4

23.7 13.6" 10.5

B. Compariso n of C 2H 2 Mas s Spectra l Intensitie s an d Polyme r Depositio n Rate s

a b c

Paramete r

C 2H 2

C 2H 4

C 2H 6

C 4H 6

C 2H 2 content 0 (% ) Depositio n rat e (mg/cm 2«hr ) Polyme r deposited/monome r fed x 100

59.8 3.95 51

13.7 1.29 15

0.4 0.053 0.6

3.1 1.14 7.2

Fro m Kobayash i et al. (16). Copyrigh t 1974 America n Chemica l Society . m/e = 54. Define d as IClH2 for C 2H 2 an d IC2li2 — / mo n ( ^ C 2 H 2/ ^ m o n ) f r all othe r monomers , wher e J designate s th e intensit y of th e mas s spectra l pea k in th e effluen t ga s an d 1° th e pea k intensit y for th e monomer .

138

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

whether or not higher molecular weight oligomers or polymers could be found by mass spectroscopy. This is in accordance with the thermodynamic limitation of polymerization in a vacuum discussed in Section 5.4. In part B of Table 6.21, the C 2 H 2 content is compared with the deposition rates. O n the basis of the apparent correlation between the C 2 H 2 content and the polymer deposition rates, Kobayashi et al. proposed a mechanism in which the initial decomposition of the monomers by electron impact to form free radicals (the chain-initiating species) and the eventual formation of acetylene (the monomer) are the key steps in determining the rates of plasma polymerization. However, the direct analysis of species in plasma [mode (1)] by Smolinsky and Vasile (6) for the plasma polymerization of methane showed that the rate of polymerization is correlated only with one- and two-carbon ions, especially C 2 H 3 + , C 2 H 2 + , C H 3 + , C H 2 + , and C H + , but the neutral molecules ethylene and acetylene do not influence the polymerization rate. They concluded that ions arriving on a surface are more important than neutral molecules in determining the rate of polymerization. An important aspect of this direct measurement of species in plasma is that the type of ionic and neutral species found in plasma is dependent on the location of sample collection and on the conditions of discharge, as shown in Tables 6.22 through 6.24. The symbols used in these tables are explained in Fig. 6.23 (16a). Similar experiments carried out with benzene/argon plasma by Niinomi and Yanagihara (17) are shown in Figs. 6.24 and 6.25 for ions and in Figs. 6.26 Tabl e 6.22 P E R C E N T A G E O F Q I O N S P R E S E N T I N D I F F E R E N T R E G I O N S O F A 150-V M E T H A N E D I S C H A R G E AT SEVERA L PRESSURES "

Pressur e (torr )

Region 5

CH

0.8

R G W

0.9

2.2 0.3

0.5

R G W

2.6 0.45

5.5 1.3

R G W

3.7 0.9

0.3

a b

Ion(% ) +

— —

— —

CH

2

+

—

— 7.8 2.6

—

Fro m Smolinsk y an d Vasil e (6). See Fig . 6.23 for an explanatio n of th e symbols .

+

CH

15.1 10.3 0.75

0.3

15.8 14 3.1

0.25 0.2

18.4 16.2 5

0.5 0.3 0.5

CH

3

4

— —

—

+

CH

5

+

I C /

1.6 6.3 1.8

20.1 16.9 2.6

1

25.2 18.7 4.9

2.7 1.8 1 1.9 2.9

31.4 21 8.4

139

6.6. I N T E R P R E T A T I O N O F P L A S M A D I A G N O S T I C D A T A Tabl e 6.23 P E R C E N T A G E O F C 2 I O N S P R E S E N T I N D I F F E R E N T R E G I O N S O F A 150-V M E T H A N E D I S C H A R G E A T SEVERA L PRESSURES "

Pressur e (torr )

a b

Ion(% ) Region 5

C 2H 2+

0.8

R G W

11.5 3.8 0.4

40 49 3.1

2.2 2.1 1.9

5.8 11 16.5

59.5 65.9 33.3

0.5

R G W

17 9 0.5

34.7 3.3 12.5

2.1 2.5 1.8

3.4 6 14.3

57.2 2.7 29.1

0.3

R G W

15.5 10.9 1

28.5 38.6 1.9

2.1 2.3 4.9

2.8 5.1 17.6

48.9 56.9 40.9

C 2H

3

+

C 2H 4+

C 2H

5

+

IC

2

+

Fro m Smolinsk y an d Vasil e (6). See Fig . 6.23 for an explanatio n of th e symbols .

and 6.27 for neutral species. These data also show the trends found in Smolinsky and Vasile's data; that is, (1) the types of ionic and neutral species found in plasma are dependent on the conditions of plasma polymerization, and (2) there is considerable fragmentation of the original monomer. Unlike Tabl e 6.24 M O L E F R A C T I O N O F N E U T R A L SPECIE S P R E S E N T I N D I F F E R E N T R E G I O N S O F A 150-V M E T H A N E D I S C H A R G E AT SEVERA L PRESSURES "

Pressur e (torr )

a b

Mol e fractio n C 2H 2

C 2H 4

C 2H 6

Rati o H 2C H 4

0.37 0.56 0.78

0.021 0.022 0.012

0.017 0.015 0.014

0.050 0.063 0.047

1.5 0.61 0.18

0.63 0.41 0.22

0.30 0.49 0.69

0.018 0.023 0.017

0.013 0.012 0.017

0.040 0.061 0.058

2.1 0.84 0.32

0.80 0.54 0.46

0.17 0.38 0.44

0.011 0.022 0.023

0.008 0.014 0.016

0.018 0.040 0.062

4.7 1.4 1.0

Region *

H2

CH

0.8

R G W

0.55 0.34 0.14

0.5

R G W

0.3

R G W

4

Fro m Smolinsk y an d Vasil e (6). See Fig . 6.23 for a n explanatio n of th e symbols .

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

140

® m

<6>

Fig . 6.23 Schemati c representatio n of a capacitivel y couple d radi o frequenc y (rf ) discharg e in C H 4. Th e letter s R, W, an d G denot e th e dar k spac e betwee n th e luminou s plasm a bul k an d th e rf electrode , containe r wall , an d groun d electrode , respectively . Adapte d fro m Smolinsk y an d Vasil e (6).

the case of C H 4 , however, numerous oligomers that can be considered the reaction products of various combinations of the fragments as well as the original m o n o m e r are found. Under conditions of rapid polymer formation, which yields deposition of powders rather than a coherent film, the total amount of species detected by mass spectroscopy (ions in Fig. 6.25 and neutral

L 10;8

(

40

40

1

M 1 0 A8

|

52

64

78

91

M 1 0 A8

102115

m

154

1

2 01

1

H 1 0 A9

254

279

340

H 1 0 A9

430

IE

Fig . 6.24 Mas s spectru m of positiv e plasm a ion s unde r condition s of 0.5 torr , flow rat e Q (benzene ) = 400 c m f X /Pm i n , Q (argon ) = 300 c m f T P/ m i n , 40 W . Fro m Niinom i an d Yanagihar a (77). Copyrigh t 1979 America n Chemica l Society .

1

m 10:

• i to:

M 10"

H10 -

hio: 500

40

I

51

63

1 II 1 I

78

90

103

1J_ L

156

128

_dj _

m/ e Fig . 6.25 Mas s spectru m of positiv e plasm a ion s unde r condition s of 0.4 torr , flow rat e Q (benzene ) = 200 c m f T /Pm i n , Q (argon ) = 300 c m | X /Pm i n , 40 W . Fro m Niinom i an d Yanagihar a (77). Copyrigh t 1979 America n Chemica l Society .

L

10A

M

10A

• M 1 0 A-

H 10A

-H

io;

OVER

i

10

J

78

L

m/e Fig . 6.26 Mas s spectru m of neutra l specie s in plasm a unde r condition s of 0.5 torr , flow rat e Q (benzene ) = 500 c m f X /Pm i n , Q (argon ) = 150 c m | Tp / m i n , film formed . Fro m Niinom i an d Yanagihar a (77). Copyrigh t 1979 America n Chemica l Society .

142

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

L

1 0 A5

M 10; •

M

1 0 A'

H

10; 8-

H 1 0 A9

10

2

41

40

78

102

154

m/e Fig . 6.27 Mas s spectru m of neutra l specie s in plasm a unde r condition s of 0.2 torr , flow rat e Q (benzene ) = 1 5 0 c m 3/ m i n , Q (argon ) = 1 5 0 c m 3/ m i n , powde r formed . Fro m Niinom i an d Yanagihar a (17). Copyrigh t 1979 America n Chemica l Society .

molecules in Fig. 6.27) was much less than that under conditions in which polymer is yielded slowly in the form of a coherent film. This seems to be a good demonstration of the relationship between the gas phase and the deposition rate discussed earlier. The data from mass spectroscopic analysis of the effluent obtained by mode ds-FHC=CHF, (2) for H 2 C = C H 2 , H 2 C = C H F , trans-FHC=CHF, H 2 C = C F 2 , F H C = C H 2 , and F 2 C = C F 2 by Dilks and Kay (18) are shown in Figs. 6.28 through 6.30. The c o m m o n denominator aspects of these data are the following: 1. N o specific structure polymerizes in a straightforward manner in plasma. 2. Significant fragmentation and rearrangement of atoms in the original starting material occur. 3. Structures of monomers largely determine the fragmentation pattern that occurs in plasma; however, the fragmentation pattern is also dependent on the conditions of plasma polymerization, including the location of sample collecting within a reactor. Consequently, what one can find by mass spectroscopic analysis is dependent on where the samples are collected (the experimental setup of the analysis) and how the discharge is created (the conditions of the plasma polymerization).

6.7. I N T E R P R E T A T I O N O F P O L Y M E R P R O P E R T I E S

143

(amu) Fig . 6.28 Mas s spectr a take n wit h a n ionizatio n energ y of ~ 15 eV for th e neutra l gas-phas e product s of plasma s excite d in (a) ethylen e an d (b) fluoroethylene. Adapte d fro m Dilk s an d Ka y (18). Copyrigh t 1980 America n Chemica l Society .

Thus, the interpretation of complex diagnostic data tends to depend largely on the underlying concept of "polymerization" of the investigator who inter­ prets the data.

6.7

Interpretatio n of Polymer Propertie s

Most plasma polymers, which are distinguished from conventional polymers in their characteristics as a new kind of material, exist in the form of a highly cross-linked and highly branched three-dimensional network. Because of the insoluble and infusible nature of such a network, the characterization of

144

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

10(34)

(34)

(420)

(c)

(50)

W 5-

C H

6J

CO/N.

W Wt

Wj

CF,

ill

I til

1

I

10-

(32)

(34)

5-

W Wa Wl

CO/N. _

(b)

(420) (60)

M20 CF,

i 10^

2

(30)

CjMF

W2

(32)

(390)

iliJ

L (a)

5CO/Nj

C6 H e

W

Wa

C2HF5 C 4H/ 2

II

il III 50

111

100

150

(amu) Fig . 6.29 Mas s spectr a take n wit h an ionizatio n energ y of ~ 15 eV for th e neutra l gas-phas e product s of plasma s excite d in (a) 1,1-difluoroethylene , (b) cis- 1,2-difluoroethylene , an d (c) trans1,2-difluoroethylene . Adapte d fro m Dilk s an d Ka y (18). Copyrigh t 1980 America n Chemica l Society .

145

6.7. I N T E R P R E T A T I O N O F P O L Y M E R PROPERTIE S

10-

(a)

(180)

5C0/N2 M-0

CF, C

CF

f*

AL

10-

C*F,ft

C F J

L_X

C2H2F2| (17)

(b)

(36)

(330)

C?HF

C.HF3 Q^F4 C3MF3

5C 2H,, C O2/ N Hp

_

Ws CF,

l i

r

o

W

50

150

100

200

250

(amu) Fig . 6.30 Mas s spectr a take n wit h an ionizatio n energ y of ~ 15 eV for th e neutra l gas-phas e product s of plasma s excite d in (a) trifluoroethylen e an d (b) tetrafluoroethylene . Adapte d fro m Dilk s an d Ka y (18). Copyrigh t 1980 America n Chemica l Society .

polymer molecules by conventional analytical tools is greatly hampered. F o r instance, if one were to form an ideal polymer coating by plasma poly­ merization (e.g., on a metal surface) and an ultrathin layer of less than 1000 A were tenaciously bonded to the substrate surface, which would provide excellent protection against chemicals that would damage the substrate material, many physical methods, such as X-ray, UV, and IR spectroscopy, would fail to provide information relevant to the coating due to the thinness of the coating (extremely small mass ratio of the coating to the substrate). Because of excellent bonding, the collection of coating material would also be virtually impossible (from such a thin coating), and analysis by chemical means would be practically useless because of the insolubility and unusual chemical inertness of the coating. Of course, such an ideal situation does not

146

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

represent many of the plasma polymers we have dealt with to date; the difficulty of characterization, however, is not much different from that described here. Furthermore, an inherent difficulty lies in the fact that the more successful one is in obtaining good materials, the more difficult becomes the characterization of the films. O n the other hand, as far as the kinetic a n d / o r mechanistic aspects of plasma-state polymerization are concerned, the information obtained from plasma polymers has more direct implications than that obtained from species in plasma or effluent gas. F o r instance, as mentioned in Section 6.4, the significant quantity of free radicals found in a plasma polymer must be considered to be the consequence of polymer formation mechanisms. The high degree of branching and cross-linking is the direct result of polymerization kinetics. There are two major practical approaches to dealing with these difficulties. The first is to select conditions of plasma polymerization such that the product polymers can be handled by conventional analytical tools. In this case, the information obtained would be less relevant than that needed in the ideal case mentioned earlier; however, significant information with respect to the characteristic nature of plasma polymers could be obtained. The second approach is to rely on surface analysis, which provides direct information about the top surface layer (e.g., less than 50 A ) of the coating without being hampered by the massive a m o u n t of substrate under the plasma polymer. Significant progress has been made by this approach. In the interpretation of the data obtained by the first approach, one must keep in mind the following aspects of plasma polymerization: 1. The broad meaning of plasma polymerization covers a wide range of polymer formation principles—from the almost complete molecular polymerization that occurs in the distillation of a vinyl m o n o m e r under the influence of plasma to the nearly complete atomic polymerization to form graphitic or diamondlike carbon films from an organic vapor. 2. The unique polymerization mechanism under consideration refers to plasma-state polymerization. 3. The greater the extent of manipulation of operational factors to produce polymers suitable for the analysis (e.g., to form soluble oily or powdery products rather than coherent films or to achieve a thick deposition that can be easily removed), the less relevant becomes the information obtained to plasma-state polymerization. 4. The plasma polymers obtained from a particular m o n o m e r may differ significantly dependent on the conditions of polymerization, although within a set of experimental conditions in a particular reactor the variation may appear to be subtle rather than significant.

147

6.7. I N T E R P R E T A T I O N O F P O L Y M E R P R O P E R T I E S

6.7.1

ELEMENTA L ANALYSI S

In molecular polymerization such as the addition polymerization of a vinyl monomer, elemental analysis provides no particularly significant information because the elemental ratios of the polymer must be identical to those of the monomer. In plasma polymerization, however, this simple procedure can provide meaningful information because a significant deficiency of elements is generally found in the polymers. Some typical data (7, 79, 20) are shown in Tables 6.4 and 6.25 through 6.27. The following two trends are generally seen in the plasma polymers prepared from a variety of organic c o m p o u n d s using considerably different reactors and methods. These are (1) the deficiency in the polymer of hydrogen and halogens, which are attached to carbon in the monomer, and (2) the inclusion of oxygen in the polymers even though the m o n o m e r s d o not contain oxygen. The first aspect strongly indicates that the detachment of hydrogen or halogens (i.e., breaking of C — H or C — X bonds), irrespective of the actual and detailed reaction mechanisms, constitutes a significant step Tabl e 6.25 RESULT S O F E L E M E N T A L ANALYSI S O F S O M E P L A S M A P O L Y M E R S

Plasm a polyme r

C(% )

H(% )

N(% )

0(% )

Acrylonitril e Propionitril e Propylamin e Allylamin e Ethylen e Ethylene/N 2 Allen e A l l e n e / H 20 Allene/N 2 A l l e n e / N 2/ H 20 Acetylen e Acetylene/N 2 A c e t y l e n e / H 20 A c e t y l e n e / N 2/ H 20 Ethylen e oxid e Hexamethyl disiloxan e Tetrafluoro ethylen e

59.16 55.19 63.23 60.48 74.13 49.38 78.26 73.15 62.68 63.77 79.5 64.0 66.5 53.2 72.6 30.0

5.61 7.3 8.89 7.86 8.11 6.26 8.13 8.63 6.71 7.86 5.4 5.8 7.6 6.5 9.0 7.5

24.68 18.56 18.02 18.43

10.55 18.95 9.86 13.23 17.77 25.85 13.61 18.22 12.88 17.58 15.1 13.5 25.9 24.6 18.4 22.2

a

27.8

Fro m Yasud a et al. (19).

—

18.51

— —

17.73 10.79 16.7 15.7

—

5.28

Other s (% )

0

Empirica l formul a of repeatin g uni t

—

C 3H 3 N , O 0. 4 CsH^N.Oo. , C 3H 5N 1O 04

— —

C 2H 2. 6 ^ 0 A C 2H 3N 0 O 6 0 C3H3

O70

8 4

C3H4..2O 0 6 C3H3

N80

O07

C3H4_4.N0.4 O 5 0 6< C 2 H 1 6O 0 3 C2H2.2N0 .5O0.3

— —

40.2 (Si) 66.92 (F )

C2H2.7O0.6

C2H2 .9N0.5O0.7 C 2H 2 O90. 4 C3.5H10.5S12O2

C2F3O0.3

5

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

148 Tabl e 6.26

C H E M I C A L ANALYSI S O F G L O W D I S C H A R G E P O L Y M E R S Polyme r

0

Discharg e condition s

Empirica l formul a

Polyviny l chlorid e Polyviny l chlorid e (vinyl chloride/argon ) Polyviny l fluoride Polyviny l fluoride Polyvinyliden e fluoride Polyvinyliden e fluoride (powde r form )

1 1 6 V / c m , 1.75 m A / c m 2, 0 . 7 6 tor r 240 V/cm , 2.95 m A / c m 2, 1.14 tor r

C 2H 2.o6Clo.5iOo.4o C2Ul97C\0i51 O0 8

tor r tor r tor r tor r

C2H2.32F0.3tO0.i6 C 2H 2. 2 F5 0. 2 4O o . 3 7 C 2H j 4 5 F 0 . 9 5 O 0 . i 5 C 2H i . 4 F 6 0. 9 8O 0. i 6

Polyvinyliden e

109 V/cm , 1.85 m A / c m 2, 0 . 9 5 tor r

C2Hj.69F0.99O0.07

fluoride

148 113 100 100

V/cm , V/cm , V/cm , V/cm ,

2.25 1.95 1.72 1.72

m A / c m 2, 1.52 m A / c m 2, 0 . 5 7 m A / c m 2, 0.88 m A / c m 2, 0 . 8 8

F r o m Westwoo d (20).

Tabl e 6.27 V A R I A T I O N O F C H E M I C A L ANALYSI S WIT H T E M P E R A T U R E ’ Polyme r

Discharg e condition s

Tem p ( C)

Empirica l formul a

Polyviny l chlorid e

2.50 m A / c m 2, 1.10 tor r 2.50 m A / c m 2, 1 . 1 0 tor r 2.50 m A / c m 2, 1.10 tor r

-10 20 40

C 2H 2.14CI0.68O0.24

Polyviny l fluoride

2

2.05 m A / c m , 1 . 1 2 tor r 2.05 m A / c m 2, 1.12 tor r 2.05 m A / c m 2, 1.12 tor r

-10 20 40

C2H1.97CI0.56O0.27 C2Hi.85Clo.57Oo.33

C 2H 2

6F 30

O2 03. 4 5

C 2 H 2. i 4F 0. 29 O 0. 3 2 C2H2.09^0.28^0.28

F r o m Westwoo d (20).

Tabl e 6.28 E L E M E N T A L ANALYSI S O F H Y D R O C A R B O N OIL S P R O D U C E D BY P L A S M A P O L Y M E R I Z A T I O N

0

Monome r

H / C (monomer )

H / C (oil)

H / C (propose d structure )

Ethylen e Ethylene/acetylen e Butadien e Benzen e

2.0 1.9 1.50 1.0

1.41 1.40 1.16 0.92

1.62 1.62 1.24 1.07

0

Fro m Tibbit t et al. (21).

2

6.7. I N T E R P R E T A T I O N O F P O L Y M E R P R O P E R T I E S

149

in plasma polymerization. A similar trend is found with plasma poly­ merization products in the form of oils (soluble in CC1 4) (27), as shown in Table 6.28. The inclusion of oxygen is generally considered to be a consequence of the postplasma reaction of trapped free radicals with ambient 0 2 . (This aspect is discussed in Section 6.4.) 6.7.2

INFRARE D

SPECTR A

Reflecting the variable extent of fragmentation a n d / o r rearrangement of atoms and ligands during the process of polymer formation in plasma, IR spectra also vary with the conditions of the plasma polymerization. In general, the IR spectrum of a plasma polymer (e.g., the plasma polymer of styrene), in comparison with the conventional polymer of the m o n o m e r (e.g., polystyrene), may contain most major peaks characteristic of the conventional polymer, but not always nor in a quantitative manner. Sharp peaks in the spectrum of the conventional polymer generally become less resolved broader bands, and some peaks are significantly reduced. These changes are dependent on the conditions of the plasma polymerization, particularly on the energy input level. Indeed, one can roughly estimate what kind of conditions were used by examining the IR spectra of plasma polymers of a particular m o n o m e r (with knowledge of the IR spectrum of the conventional polymer or that of the monomer). A series of Fourier transform infrared (FTIR) spectra for plasmapolymerized 2-vinylpyridine obtained by Bieg and Ottesen (22) illustrates the general trends described here. The F T I R transmission spectrum of con­ ventional linear poly (2-vinylpyridine) is shown in Fig. 6.31. The b r o a d feature at 3450 c m - 1 is due to water absorbed in the K B r pellet. The F T I R reflectance spectrum for the plasma-polymerized film prepared under conditions in which the reactor was operated near the minimum power below which the glow would extinguish (2-vinylpyridine pressure, 0.23 torr; power, 10 W; film thickness, 28,000 A ) is shown in Fig. 6.32. This spectrum is quite similar to that of the linear polymer. The presence of well-resolved intense C - C and C - N ring stretching bands at 1590 and 1570 c m - 1, respectively and the rather strong ring vibration at 1430 c m " 1 indicate a substantial retention of the aromaticity in the plasma polymer. A number of important differences are also apparent, however. Additional relatively strong absorptions are at 2200 and 2165 c m - 1, which are assigned to a nitride group, possibly an amino- or iminonitrile group. Thus, cyano- or nitrogen-containing fragments in the discharge may be incorporated into the polymer. Also noteworthy in the spectrum of the plasma polymer is strong new band at 2965 c m " 1 and a rather weak band at 1370 c m " 1 . These absorptions may be assigned to the C H 3 stretching and deformation vibrations, suggesting a significant a m o u n t of branching in the

150

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

4000

3000

2000

1000

1 Wavenumber (cm - )

Fig . 6.31 FTI R transmissio n spectru m of conventiona l poly(2-vinylpyridine ) in a KB r pellet . Fro m Bieg an d Otteso n (22). Copyrigh t 1979 America n Chemica l Society .

4000

3000

2000 Wavenumber (cm"1)

1000

Fig . 6.32 FTI R reflectanc e spectru m of plasma-polymerize d 2-vinylpyridine . Power , 10 W ; voltage , 250 V; 2-VP pressure , 0.23 torr ; thickness , 28,000 A. Fro m Bieg an d Otteso n (22). Copyrigh t 1979 America n Chemica l Society .

151

6.7. I N T E R P R E T A T I O N O F P O L Y M E R PROPERTIE S

plasma polymer. A very small carbonyl shoulder is present near 1700 c m - 1, which was observed to increase in intensity with exposure to dry air, indicative of the postoxidation of trapped free radicals. Figure 6.33 shows the F T I R spectrum of a film prepared at a high rf power level (2-VP pressure, 0.03 torr with 0.20 torr argon; power, 30 W; film thickness, 9000 A). It is clear from the presence of only a few broad absorption bands that a significant extent of fragmentation of the m o n o m e r occurred under those conditions. In particular, the ring C — H stretching is no longer apparent, indicating nearly complete breakup of the pyridine ring. Also, the previously well-resolved features at 1550 to 1600 c m " 1 are manifested as a b r o a d band at 1630 c m - 1, assignable primarily to the C = C and C = N vibrations. The remaining features are the C H 2 and C H 3 deformation bands at 1450 and 1370 c m - 1, respectively. This structure is one of a highly branched, unsaturated aliphatic polymer. In 1966 Jesch, Bloor, and Kronick (23) studied the IR spectra of plasma polymers formed from pentane, ethylene, butadiene, benzene, styrene, and naphthalene and summarized the structures of the plasma polymers (Ta­ ble 6.29). O n the basis of these structures, they summarized the following

• 4000

i-

1 3000

1

1 2000 1 Wavenumber (cm - )

i

i 1000

Fig . 6.33 FTI R reflectanc e spectru m of plasma-polymerize d 2-vinylpyridine . Power , 40 W ; voltage , 250 V; 2-VP pressure , 0.03 torr ; argo n pressure , 0.20 torr ; thickness , 9000 A. Fro m Bieg an d Otteso n (22). Copyrigh t 1979 America n Chemica l Society .

152

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S Tabl e 6.29 STRUCTURE S I N GLO W DISCHARG E POLYMER S FRO M HYDROCARBONS 0 Startin g vapo r Pentan e Ethylen e Butadien e

Benzen e

Styren e Naphthalen e

Functiona l group s in polymer s Branche s at eac h pentan e molecule , methy l chai n ends , ( C H = C H ) ( C H 2 C H 2 C H 2 ) , methy l chai n ends , ( C H = C H ) , cross-link s at saturate d carbon s ( C H 2 C H 2 C H 2 ) , ( C H = C H ) , methy l chai n ends , an d cross-link s at saturate d an d unsaturate d carbon s ( C H 2 C H 2 C H 2 ) , ( C H = C H ) , ( C = C ) , or ( C = C = C ) , methy l chai n ends , an d pheny l side group s Sam e as benzene , ( C 6H 6) C H 2 ( C H 2 C H 2 C H 2 ) , ( C H = C H ) , ( C = C ) , or ( C = C = C ) , methy l chai n ends , an d CH 2

^Y^V

or <^ y~ / CH 2

\

CHl

CH 2

r^Yi

C H 2

~\ ff~

r H2C

CH2

/ CH 2

Fro m Jesc h et al (23).

aspects of plasma polymerization of the monomers. Whether the monomer is aromatic, olefinic (conjugated, unconjugated), or fully saturated, the plasma polymer is highly branched and cross-linked and contains identifiable unsaturation in the form of both olefinic bonds and free valances. Although aromaticity is not produced in the reaction, it is partially preserved in the products formed from aromatic compounds. F o r instance, n-pentene, ethylene, and butadiene yield similar polymers, with some small but repro­ ducible variation due to the monomer structure. Benzene, styrene, and naphthalene produce polymers exhibiting the features of nonaromatic condensates plus acetylene groups and the aromatic function of each starting material. This summary, along with the additional factor of the extent of fragmentation, which is a function of the plasma energy density (often parallel to the energy input level), seems to cover all trends observed with various glow

6.7. INTERPRETATION OF POLYMER PROPERTIES

153

discharge polymers formed under a variety of plasma polymerization conditions by many investigators. Tibbitt et al (27) studied the structure of plasma-polymerized oils and film from ethylene, ethylene/acetylene, butadiene, and benzene. With the oils, it is possible to obtain the IR spectra of the CC14 solution (Fig. 6.34) and also to examine the NMR spectra of the plasma-formed products. With the help of NMR data (in the case of oily products), the hypothetical structures of the plasma polymers are constructed as shown in Fig. 6.35 for the oils (a) and for the film (b). These hypothetical structures are good representations of the complexity of structures of plasma polymers, which is the consequence of the polymerization mechanism. Thus, the structure of a plasma polymer cannot be uniquely related to the structure of the monomer.

Fig. 6.34 Infrared spectra of (I) CC14 solutions of oils and (II) films produced in a plasma. Ethylene (a); ethylene/acetylene (b); butadiene (c); benzene (d). Adapted from Tibbitt et al (21).

154

6. KINETIC AND MECHANISTIC ASPECTS

(a)

(b) Fig. 6.35 Model structure of plasma polymers of ethylene. (a) Oil; (b) film. Adapted from Tibbittef a/.(27).

155

6.7. I N T E R P R E T A T I O N O F P O L Y M E R P R O P E R T I E S

6.7.3

NUCLEA R MAGNETI C RESONANC E

SPECTR A

Because most plasma polymers are insoluble, conventional N M R , which requires a solution of a polymer, cannot be directly applied to the analysis of the structure of plasma polymers. Tibbitt et al. (21) used N M R for the analysis of oils prepared by the glow discharge of hydrocarbons mentioned in the preceding section. The N M R spectra of ~ 4 wt % C C 1 4 solutions of the oils are shown in Fig. 6.36. The assignments of the N M R peaks are given in Table 6.30. The aromatic proton absorption (3 ~ 7.04) is clearly seen in the spectra of all

Fig . 6.36 N M R . spectr a of CC1 4 solution s of oils produce d in plasma . Ethylen e (a); ethylene/acetylen e (b); butadien e (c); benzen e (d). Adapte d fro m Tibbit t et al. (21).

156

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S Tabl e 6.30 ASSIGNMEN T O F NM R ABSORPTIO N BANDS" Chemica l shift 6 (ppm )

0.96 1.26 1.61 1.91

2.31 5.01] 5.26 [ 5.41 J 6.14 7.04

Assignmen t R-CH 3 R CH 2 R R 2 CH R C=C CH H5C5

R

2

CH 2

R

Variou s

>=< ’

H H

c=c

11 H H 5C 6- H

X

c=c

"Fro m Tibbit t et al (21).

four plasma-polymerized oils and is extremely strong for the one derived from benzene. 3 Dilks et al. (24) applied solid-state 1 C N M R , which offers the advantage of enabling the investigator to distinguish saturated and unsaturated environ­ ments for each carbon type. By employing the delayed decoupling technique to suppress protonated carbon peaks, along with difference spectroscopy, one can distinguish five resolved spectral bands. These bands are assigned to (I) unsaturated nonprotonated, (II) unsaturated C H and C H 2 , (III) quaternary, (IV) methine and methylene, and (V) methyl carbons by comparison with the 3 standard 1 C shifts compiled for organic materials. The relative quantities of these structures in the polymers are shown in Table 6.31. The unsaturated character of the polymers increases from ethane (19%) to ethylene (24%) to acetylene (38%), and the methyl carbons decrease from 1 in 5 carbons for the plasma polymers of ethane and ethylene to 1 in 16 carbons for the plasma polymer of acetylene. For the polymers shown Table 6.31, 50 + 5% of the unsaturated carbons and 21 + 5% of the saturated carbons have no directly bonded hydrogen, suggesting a high degree of branching and cross-linking. Qualitative information on branching and cross-linking is not readily obtained from an IR analysis, and therefore the N M R technique offers more quantitative information on the molecular structures of plasma polymers.

157

6.7. I N T E R P R E T A T I O N O F P O L Y M E R P R O P E R T I E S Tabl e 6.31 RELATIV E AREA S I N T H E

1 3

C - N M R SPECTR A O F P L A S M A - P O L Y M E R I Z E D C 2

HYDROCARBONS " Pea k

Injecte d ga s

=<

Ethan e Ethylen e Acetylen e

9 12 17

a

II I

II

I

/

=C H + =CH

2

X

10 12 21

17 12 16

V

IV

^ C H + ^:CH 47 43 40

2

-CH

3

17 21 6

Dat a reproducibility , – 15% . Fro m Dilk s et al. (24).

6.1 A

ELECTRO N

SPECTROSCOP Y

FO R CHEMICA L

ANALYSI S

ESCA offers a distinct advantage in the study of the structure of plasma polymers, because the method provides information pertinent to the top surface of a material (e.g., 2 0 - 5 0 A ) and hence is free of the overwhelming signals from the substrate material, which greatly hamper many other physical analytical methods applied to an ultrathin film. ESCA is used primarily for the analysis of elements, however, and its application to the determination of structural features of an organic material is limited. Considerable effort has been expanded to obtain structural information on plasma-polymerized materials by means of ESCA. In this respect, the outstanding contributions of Clark and co-workers in this field are note­ worthy. Clark extended the use of ESCA to obtain structural information on polymers of fluorine-containing compounds and polymers with aromatic structure. Because fluorine is the most electronegative element in the periodic table, it causes a sufficiently large shift in the binding energy of carbons bonded to a fluorine atom or atoms. ESCA has been used to study the structure of plasma polymers in the following manner (25-30): 1. The relative overall intensity ratios of F l s to C l s levels in a fluoropolymer film can be used to derive the stoichiometry after sensitivity factors for a particular instrumental configuration have been established. 2. An analysis of the overall C l s line profiles with component peaks that correspond to distinctive structural features (e.g., C F 3 groups) also allows an independent determination of stoichiometry and provides information on the structural features present.

Tabl e 6.32 STOICHIOMETRIE S D E T E R M I N E D BY ESC A F O R P O L Y M E R F I L M S P R O D U C E D F R O M I N D U C T I V E L Y C O U P L E D R A D I O F R E Q U E N C Y PLASMA S EXCITE D I N P E R F L U O R O B E N Z E N E " 0.05 tor r

Stoichiometrie s

Incl . H / C " C/ F 2C

C/F\b

0.10 tor r

Excl . H / C C/ F 1

C/ F 2C

F l /s F 2s

Incl . H / C

0.2 tor r

Excl . H / C

C / F l"

C/F2 C

C/ F l b

C/ F 2C

F l /s F 2s

Incl . H / C C/F\b

C/F 2C

Excl . H / C C/F

lb

C/ F 2C

F 1 (S F 2s

5.0 W Glo w 30 Glo w 70 N o n g l o w 30 N o n g l o w 70

1.0 0.80

0.86

1.06

0.92

8.92

0.70 1.02

0.83

7.57

0.96 0.92

0.90 0.71

1.01 1.04

0.79

8.29 6.94

1.11 0.98

11.55

1.33

1.60

1.63

1.83

0.98

8.7

1.04

0.79 0.62

0.92

0.79

1.0

1.13 0.92

0.9 0.77

1.25 1.15

0.99 0.97

0.98 0.98

0.85

1.05 1.07 1.14

1.12

0.95

1.18

0.94

0.78

0.72

0.92

0.85

8.4

0.61

0.86

0.81 0.79

1.10 0.99

0.70 0.95 0.98

8.71

17.27

0.75 0.94 0.8

0.90

7.72

0.87

0.67

0.96

0.74

7.64

0.79

7.33 7.96

0.82

0.65

1.10 1.0

0.95 0.79

1.01 1.21 1.17

0.79 1.04

7.93 12.09

0.91

8.76

8.38

11.8 12.6

0.0 W Glo w 30 Glo w 70 N o n g l o w 30 N o n g l o w 70

b c

1.37

1.24

1.59

1.44

13.13

Fro m Clar k an d Shuttlewort h (31). C / F is th e stoichiometr y obtaine d fro m th e C l s profile . C / F 2 is th e stoichiometr y obtaine d fro m th e C / F ratios .

7.95

6.7. I N T E R P R E T A T I O N O F P O L Y M E R P R O P E R T I E S

159

3. Angular dependence studies in which data are recorded as a function of electron takeoff angle with respect to the normal to the sample surface, coupled with investigation of intensity ratios for photoemission from different levels of the same element (e.g., F l s and F 2 s) , provide direct information on the vertical homogeneity of the polymer films. With an M g k a 2l p h o t o n source (hv = 1253.7 eV), typical sampling depths for levels of interest (three times the typical mean free paths) are F l s, ~ 2 5 A; C l s, ~ 4 5 A; and F 2 s, - 9 0 A. 4. Monitoring of O l 5 and C l s core levels at 285 eV (CH) provides direct information on the low level of extraneous surface contamination or oxidation of the surface. Combined with an angular dependence study, the depth profile of oxidation can be studied. In many studies, it has been shown that the oxygen incorporation occurs at the surface. The large chemical shift caused by the attached fluorine atom(s) can be used to determine some structural features of plasma polymers formed from fluorine-containing organic compounds. Chemical shifts observed with fluorine-containing structures (57) are shown in Table 6.32. Typical examples of C l s spectra obtained with the plasma polymer of perfluorocyclohexane (31) are shown in Fig. 6.37 for polymers formed in the glow region and in Figure 6.38 for polymers formed in the nonglow region. The stoichiometrics of the plasma polymers based on these ESCA data are summarized in Table 6.33. When the conditions of plasma polymerization are varied widely or the polymer samples are collected in various sections of a plasma polymerization reactor, ESCA C l s spectra reveal the system-dependent nature of the R S G P mechanism. F o r example, O ' K a n e and Rice (32) used the plasma poly­ merization reactor shown in Fig. 6.39 in which an inductively coupled rf (13.56 MHz) power source was used to create plasma. Substrates were placed immediately before the rf coil (A), inside the rf coil (B), immediately after the rf coil (C), in the deposition chamber (0.41 m diameter x 0.21 m) (D), and at the exit of the deposition chamber (E). Figure 6.40 shows the ESCA C l s spectra for the corresponding films deposited in the plasma regions as compared with conventional polytetrafluoroethylene. This figure clearly indicates that a certain structural feature (e.g., — C F 3 ) is incorporated into the polymer in the preferred region. In other words, considerable fragmentation of the m o n o m e r or rearrangement of atoms occurs in plasma, and the resultant fragments behave according to their gas-phase kinetic characteristics. Because of this aspect, the kind of polymer that is deposited and the portion within a reactor on which it is deposited are dependent on the size (cross-sectional area) of a tube or a reactor. A similar experiment using a relatively small glass tube (33) ( ~ 1 4 mm) shows a completely different pattern of deposition of structural features.

160

297 293

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

289

285

Fig . 6.37 Th e C l s levels of plasma-polymerize d C 6 F 12 prepare d in th e glow region . Adapte d fro m Clar k an d Shuttlewort h (31).

Characteristic shapes of ESCA C l s peaks are shown in Figs. 6.41 and 6.42. At the lower energy input level, the more fluorinated polymer is deposited before the rf coil and very few fluorine atoms are found in the polymer that is deposited on the downstream side of the rf coil in the nonglow region. At the higher level of energy input, the polymers found at both locations contain very few fluorine atoms, indicating that the preferred location of deposition for — C F 3 , > C F 2 , and others shifted beyond the region of the reactor where samples were collected. In a system in which the flow does not pass through the energy input zone (rf coil) and plasma polymerization is carried out in the tail-flame portion of the glow discharge with a very low W/FM level (in a relatively larger tube, e.g., 5 cm i.d.), the polymer formed in the downstream nonglow region (33) is

6.7. I N T E R P R E T A T I O N O F P O L Y M E R P R O P E R T I E S

297

293

289

161

285

Fig . 6.38 Th e C l s levels of plasma-polymerize d C 6 F 12 prepare d in th e nonglo w region . Adapte d fro m Clar k an d Shuttlewort h (31).

completely different from that formed in the downstream nonglow region of a small-tube reactor (Fig. 6.43). In this case, the polymer formed in the nonglow region is nearly identical to conventional polytetrafluoroethylene. Thus, ESCA reveals that plasma polymers formed from the same m o n o m e r but in different sections of a reactor, by different reactors, or under different conditions are significantly different. Clark and Abrahman (34) used ESCA to measure the initial deposition rate, together with ESCA spectrum analysis, for the plasma polymerization of perfluoro-2-butyltetrahydrofuran, and the result is a good illustration of this aspect of plasma polymerization. Because of the presence of another electronegative element, oxygen, and because of the

Tabl e 6.33 STOICHIOMETRIE S D E T E R M I N E D BY ESC A F O R P O L Y M E R F I L M S P R O D U C E D F R O M I N D U C T I V E L Y C O U P L E D R A D I O F R E Q U E N C Y PLASMA S EXCITE D I N P E R F L U O R O C Y C L O H E X A N E * 0.05 tor r

Incl.H/ C

Stoichiometric s 5.0 W G l o w 30 G l o w 70

C/ F 1"

C/ F

1.23 1.12 1.4

1.30

T

Excl. H / C

Inc h H / C C/ F \b

C/ F 2C

1.13

1.18

8.8

0.83 0.68

1.22 1.22

1.06 0.95

1.33 1.24

8.2 8.94

1.19 1.05

1.79

10.2

0.86

10.7

0.86

1.08 1.27

1.68

Fis/

1.28 1.29

1.58

8.4

1.27

1.52 1.38

1.49 1.27

8.22 10.24

G l o w 30

1.29

1.24

1.37

G l o w 70

1.14

1.04

1.36

N o n g l o w 30

1.46

1.63

N o n g l o w 70

1.31

1.37

1.50 1.57

Excl. H / C C/ F 2C

C/ F 2C

1.12

0.2 tor r

C/ F 1"

C/ F \b

1.11 1.37 1.04

N o n g l o w 30 N o n g l o w 70

0.1 tor r

F 2s

Inc h H / C

Excl. H / C

F 2s

C/ F 1"

1.33 1.27

6.7

1.21

1.13

8.5

1.31 1.37

1.68 1.86

—

1.19 0.9

1.16 1.3

1.24

1.21

1.27

8.7

1.08

1.08

1.19 1.47

1.21 1.86

7.89

0.78 0.32

1.46

2.17

11.54

Eis/

—

C/ F 1"

C/ F 2C

1.26 1.26

1.24 1.24

1.20

1.73

8.57 19.17

1.08

1.26

1.27

9.57

0.83

1.18 0.91

1.24

8.75 —

(

F

2C

—

—

Fi,/

F 2s

8.4

—

10.0 W

a b c

Fro m Clar k an d Shuttlewort h (31). C / F 1 is th e stoichiometr y obtaine d fro m th e C l s profile . C / F 2 is th e stoichiometr y obtaine d fro m th e C / F are a ratios .

0.39 0.27

—

1.13

—

—

163

6.7. I N T E R P R E T A T I O N O F P O L Y M E R P R O P E R T I E S Radio Frequency Generator Plasma Zone

Heated Window

r — , . Rotating H M o tr o Samples

B

Flexible Coupling

Pressure Gauge

Quartz Crystal Thickness Sensor Photodetector Recorder Polymer Deposition Chamber

Monomer Supply

Metering Valve

Argon Vacuum Pump

Fig . 6.39 Rice (32).

Glo w discharg e thi n polyme r film depositio n system . Adapte d fro m O’Kan e an d

relatively large molecular weight of the monomer, the plasma polymerization of this m o n o m e r is very sensitive to variations of plasma polymerization conditions. An important aspect found in a comparative study of perfluoro-2butyltetrahydrofuran and perfluorocyclohexane is that the ether oxygen atom in the furan ring is much more sensitive to ionization than the fluorine atoms in the rest of the perfluorocarbon structure. The ionization potential for the oxygen lone pair is significantly lower (AE « 3 eV) than that for perflu­ orocyclohexane, and consequently the oxygen atom becomes the unbuttoning site, which evidently occurs at a relatively low level of W/FM (e.g., 1 0 " 7 J/kg). ESCA results show a dramatic decrease of oxygen in the polymer as the power input level (W/FM) increases (Fig. 6.44). The ring opening, or breaking up, of the cyclic structure occurs in the plasma polymerization of both perfluorocyclohexane and perfluoro-2butyltetrahydrofuran, but because of the preferential unbuttoning site (ether oxygen), the loss of fluorine in the plasma polymer of perfluoro-2butyltetrahydrofuran is significantly less than that in the plasma polymer of perfluorocyclohexane. Even in this case, however, the F / C ratio is significantly lower than that for the monomer, and conversion of C F 2 sites to both C F 3 and C F sites take place. The relative percent contribution of C F 3 / C F 2 / C F structural features in the polymer film of 33/42/21 may be compared with corresponding figures of 12.2/75/12.5 for the m o n o m e r perfluoro-2butyltetrahydrofuran.

164

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

I

l

I 295

I

I

I

i_J

i

1

I

290 Binding energy (eV)

1

1

1

1

285

Fig . 6.40 Th e C l s X-ra y photoelectro n spectr a of plasma-deposite d fluorocarbo n polyme r films fro m differen t region s of th e plasm a syste m (PTFE , polytetrafluoroethylene ; cts, counts) . Adapte d fro m O’Kan e an d Rice (32).

6.7. I N T E R P R E T A T I O N O F P O L Y M E R P R O P E R T I E S

165

Fig . 6.41 Dependenc e of th e ESC A C l s peak s of glow discharg e polymer s of tetraflu oroethylen e on discharg e condition s an d locatio n of polyme r deposition . Polyme r depositio n occurre d at tw o location s in th e reacto r show n in th e inset : (A) befor e th e radi o frequenc y coil an d (B) afte r th e radi o frequenc y coil. Discharg e powe r level, 1.9 x 1 0 7 J/kg . Adapte d fro m Yasud a (33).

Thus, in plasma polymerization, no single dominant mechanism can be found, and many simultaneous and competitive reactions occur. The com­ petitive nature of reactions is highly dependent on the conditions of plasma polymerization, in particular the energy input level. These characteristic aspects of plasma polymerization have been clearly demonstrated by ESCA studies.

166

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

Glow

Fig . 6.42 ESC A C l s peak s of glow discharg e polymer s of tetrafluoroethylen e in th e sam e reacto r show n in Fig . 6.41, bu t at th e highe r discharg e powe r level of 7.7 x 1 0 8 J/kg . Adapte d fro m Yasud a (33).

6.8

Atomic (Nonmolecular ) Natur e of Plasm a Polymerizatio n

The plasma polymerization so far discussed is characterized by (1) the extensive fragmentation a n d / o r rearrangement of a structural moiety or atoms in the starting material and (2) simultaneously occuring reactions involving many kinds of chemical species. These phenomena are seldom found in conventional polymerizations, which can be recognized as molecular reactions. In other words, in con­ ventional polymerizations, molecules of monomers are linked together with a minimum alteration of chemical structure of the monomer, and the structure of a polymer can be well predicted from the structure of the monomer. In

167

6.8. A T O M I C N A T U R E O F P L A S M A P O L Y M E R I Z A T I O N

300

290 ev

300

290

280

ev

Fig . 6.43 ES C A C l s peak s of glow discharg e polymer s of tetrafluoroethylen e prepare d in a reacto r show n in th e inset : (C) at th e en d of th e glow region ; (D) at th e en d of th e tub e in th e nonglo w region . Adapte d fro m Yasud a (33).

contrast, the structure of a polymer formed by plasma polymerization cannot be predicted in this way. Perhaps the only sure prediction one could m a k e would concern the elements constituting the polymer and, to a lesser extent, the elemental ratios. This situation becomes clearer once we recognize the fact that plasma polymerization is not a regular "molecular polymerization." Such a "nonmolecular polymerization" can be expressed by the term atomic poly­ merization. The word atomic might be misleading in a general sense; however, insofar as we are discussing the growth mechanisms of polymer formation, perhaps atomic polymerization is the most adequate and accurate expression of what is occurring in plasma polymerization.

168

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

I 0

I

4

I

I

I

I

8 12 16 20 W / F M ( x 1 0 " 7J / k g )

L24

Fig . 6.44 Dependenc e of oxygen conten t in th e plasm a polyme r of perfluoro-2 butyltetrahydrofura n on W/FM. Key : O , reacto r B; reacto r C. Adapte d fro m Clar k an d Abraha m (34).

In a molecular polymerization, the arrangement of atoms to form a special molecule of monomer is carried out in the organic synthesis of the monomer, and the rearrangement of atoms and chemical ligands seldom occurs during the process of polymerization. Therefore, the two independent chemical reactions—the synthesis of monomer structure and the polymerization—can be clearly distinguished. In "atomic" polymerization, the structures prepared during monomer synthesis may be largely destroyed: on the other hand, polymers may be formed from simple molecules such as C H 4 and C S 2 that are not the monomers of conventional polymerizations. "Atomic" polymerization does not imply that the deposition of atoms is a process of plasma polymer formation similar to the formation of a film of metal in the sputter-coating process. It simply means that the building blocks are no longer molecules. After all, one of the most important aspects of plasma polymerization in comparison with other film deposition processes is the formation of covalently bonded materials. In the process of forming covalently bonded materials, conventional (molecular) polymerization uses molecules as the building blocks, but in plasma polymerization this is not the case. In this way, graphitic or diamondlike carbon films can be prepared from benzene by "atomic" polymerization.

6.8. A T O M I C N A T U R E O F P L A S M A P O L Y M E R I Z A T I O N

169

The atomic aspect of plasma polymerization is evident in the incorporation of nonpolymerizable gases such as C O , H 2 0 , and N 2 discussed in Section 6.5. Namely, these gases are incorporated into the polymer structure not as molecules, but as atoms (from a gas). The ionization characteristics of a gas determine its plasma copolymerization characteristics. The important aspect is that active species created by the ionization of a gas d o participate in the polymer-forming reactions. Because of the atomic nature of the growth mechanism of polymer formation, plasma polymerization is highly system dependent. The frag­ mentation of molecules of a starting material or the rearrangment of molecular structure as visualized by diagnostic data based on the fragmenta­ tion products depends on the structure of the starting material and the conditions of plasma polymerization. Therefore, the selection of a m o n o m e r for an expected polymer requires a knowledge of the plasma polymerization conditions and of the atomic nature of the process under the conditions. This situation can be illustrated by the plasma polymerization of tetrafluoroethylene versus that of hexafluoroethane in the presence of H 2 gas. The details of this comparison are presented in Chapter 7, where ablation in plasma and the competitive nature of ablation and polymer formation are discussed. As far as the importance of the atomic nature of plasma polymerization and its significance in the selection of m o n o m e r are concerned, the fact that essentially the same polymer can be formed from tetrafluoroethylene and from hexafluoroethane under certain conditions of plasma polymerization should be recognized. There is a significant difference in the costs of these monomers. Under certain conditions, however, the precious structure, C = C , contributes little to the overall polymer formation mechanism and the results of IR and ESCA indicate that the polymers are almost identical. The extent of "atomic" polymerization that takes place in a particular set of plasma polymerizations is dependent on parameters specific to the reactor; perhaps the most important factor is the energy input level manifested by W/FM, that is, energy input per mass of m o n o m e r given in joules per kilogram. Because of the difficulty of determining the effective flow rate in a reactor, the value of W/FM contains some degree of uncertainty; within a given reactor, however, the value of W/FM can be used as the major parameter of plasma polymerization. It is important to note that plasma polymerization manifested by polymer deposition rates for various kinds of monomers (carried out in the same reactor) can be reduced to the master relationship (35) shown in Fig. 6.45, where the deposition rate is expressed by polymer deposition rate divided by m o n o m e r mass flow rate, and the energy input is expressed by W/FM. In the figure, 16 different monomers from four different groups (each group

170

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

W/FM (J/kg ) Fig . 6.45 Depositio n rate/F M (depositio n yield) relationship s versu s W/FM for four group s of monomer s studied . Key : O , sulfur-containin g compounds ; , organosilicones ; V , fluorocarbons ; A , hydrocarbons . Adapte d fro m Gazick i an d Yasud a (35).

containing different atoms, e.g., carbon, sulfur, fluorine, and silicon are compared. The deposition rate/mass flow rate represents the value pro­ portional to the conversion yield of m o n o m e r to polymer in the system. The following aspects are important, as far as the growth mechanism is concerned, for the contribution of "atomic" polymerization: 1. Despite orders of magnitude differences among deposition rates of monomers at various discharge conditions, the data plotted in this manner show rather small differences among monomers, indicating that all monomers polymerize, by and large, in a similar fashion. 2. There is a clear difference between hydrocarbons and other types of monomers investigated, indicating that the type of atoms contained in the m o n o m e r structure plays an important role in plasma polymerization. 3. The differences among monomers within a group, such as hydrocarbons, are greater at a lower energy input level (W/FM), but these differences tend to diminish as the value of W/FM increases, and at very high W/FM regions, all monomers polymerize in an almost identical manner. (This aspect is not clearly seen in Fig. 6.45 due to the compressed scale of the plot.) The fact that we can make such a master diagram for plasma polymerization has several important implications for the growth mechanism of polymer

6.8. A T O M I C N A T U R E O F P L A S M A P O L Y M E R I Z A T I O N

171

formation in plasma. First, if plasma polymerization occurred mainly by true molecular polymerization, which depends on the chemical reactivity, con­ densability, adsorption, and other characteristics of the monomer, such a master diagram would be beyond the realm of scientific expectation. Second, even in the region where plasma polymerization is considered least atomic or most molecular, the polymer deposition rate is determined largely by the mass of monomer, which indicates that the characteristic rate by which m o n o m e r molecules are linked together (to form a polymer) is almost independent of the chemical properties of monomer. Another "atomic" feature of plasma polymerization is evident from the plasma codeposition of more than one monomer. In conventional molecular polymerization, copolymerization is governed by the m o n o m e r reactivity ratio, and a mixture of m o n o m e r A and m o n o m e r B in a 1:1 ratio does not produce a 1:1 copolymer of A and B, except in the special case of azeotropic copolymerization. This is due to the facts that the reactivities of two monomers differ depending on the chemical structures and that polymerization is very specific to the m o n o m e r structure and to the chain-carrying species. In contrast, the plasma polymerization of a mixture of two monomers nearly always yields polymers corresponding to azeotropic copolymerization, in­ dicating the absence of specificity of monomers toward plasma poly­ merization. Because of this, plasma codeposition rather than plasma copolymerization was used to describe the plasma polymerization of more than one m o n o m e r when the plasma copolymerization of nonpolymerizable gases was discussed earlier (Section 6.5). As the extent of the atomic nature of polymerization increases, the extent of fragmentation of the starting material becomes greater, and this straightfor­ ward dependence on the mass is less evident. However, in the extreme case of maximum atomic polymerization (in the context discussed in this chapter), the deposition of material is governed by the feed-in rate of atoms that form the backbone or constituent network of material (deposit), and the generalized relationship becomes evident again. Thus, one cannot discuss plasma polymerization, particularly the poly­ merization mechanism, in general terms without specifying the domain of plasma polymerization conditions which is discussed in detail in Chapter 9. In conjunction with the major domains of plasma polymerization conditions, we must also pay attention to the location of the polymer deposition. Because there is a very important factor involved in plasma polymerization that cannot be separated from the deposition aspect, detailed discussions of the deposition mechanisms are presented in Chapter 8. In this section, only an important aspect relevant to the growth mechanism is pointed out. The polymer formation that takes place at or near the electrode surface can be significantly different from that in the rest of the glow. An example of this

172

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

aspect can be seen in a comparison of a polymer deposited on the electrode surface and one deposited on a substrate suspended in the plasma phase. ESCA C l s spectra of polymers obtained by the plasma polymerization of tetrafluoroethylene (36) are compared in Fig. 6.46. F r o m the figure, it is quite evident that the polymer formation at the electrode surface and that in the plasma phase could be significantly different. The important point here is that these differences (i.e., the properties of polymers deposited onto substrates at different locations within a reactor a n d / o r onto the electrode surface, etc.) are due to the deposition mechanism rather than the growth mechanism of polymerization. Therefore, it is premature to draw a conclusive picture of plasma polymerization based on a limited case of experimental conditions. Apparently, contradictory findings reported in the literature could often be attributed to this point. The atomic nature of plasma polymerization can also be seen in a comparison of the properties of plasma polymers of tetrafluoroethylene and of hexafluoroethane. With a double bond in the monomer, F 2 C = C F 2 , the formation of a new C — C bond without the detachment of fluorine (molecular polymerization) is possible, whereas plasma polymerization of hexafluoroethane cannot proceed without the detachment of fluorine atoms 284.6

(a)

(b)

Fig . 6.46 ESC A C l s spectr a of plasm a polymer s of tetrafluoroethylene . (a) Polyme r deposite d on th e electrod e surface ; (b) polyme r deposite d on a substrat e suspende d in plasma . Adapte d fro m Morosofl f et al. (36).

173

6.8. A T O M I C N A T U R E O F P L A S M A P O L Y M E R I Z A T I O N

from the monomer. However, because of simultaneous "atomic" poly­ merization, which occurs in the plasma polymerization of tetrafluoroethylene and increases with increasing energy input, the contribution of the olefinic double bond is relatively small, as evidenced by the IR spectra of the polymers (37, 38). Infrared and ESCA spectra of the plasma polymers of hexafluoroethane prepared in the presence of H 2 ( H 2 / C 2 F 6 = 1.0) are essentially identical to those of the plasma polymer of tetrafluoroethylene polymerized under conditions of low W/FM (see Figs. 6.47 and 6.48). Furthermore, IR and ESCA data for the plasma polymer of tetrafluoroethylene polymerized under conditions of high W/FM, in which "atomic" polymerization dominates, are again virtually identical to those for the plasma polymerization of hexafluoroethane (without H 2) . Acetylene has been considered a main precursor of plasma polymerization by many investigators because of its relatively high rate of polymer deposition and its a b u n d a n t presence in plasma of hydrocarbons based on plasma diagnostic data. (The precursor concept of the first category discussed in Section 6.6 relies on the intuitive assumption that plasma polymerization proceeds via the chain-growth polymerization of the double or triple bond.) If we recognize the difference in the mass of the starting materials and the

3000

1

1

1400

1200

1000

800

600 500

Wavenumber ( c m - 3) Fig . 6.47 Infrare d spectr a of variou s plasm a polymer s prepare d unde r variou s H 2 / C 2 F 6 rati o condition s an d als o fro m C H 4 plasm a polymerizatio n for reference : 30 W ; F ( C 2F 6) = 0.93 c m 3/ m i n ; H 2 / C 2 F 6 = 0. Adapte d fro m Masuok a an d Yasud a (37).

174

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

IOW

1400

1200

1000

800

600

500

400

Wavenumber (cm ) Fig . 6.48 Infrare d spectr a of plasma-treate d sodiu m chlorid e powde r unde r variou s wattag e condition s (F = 0.155 c m 3/ m i n ) . Adapte d fro m Masuok a an d Yasud a (38).

difference in the mode of polymer collection, however, the specific rates by which polymers deposit are not so different. Most studies that utilize polymer deposition onto the electrode surface are carried out under conditions of relatively high flow rate and low discharge wattage. Thus, the domain of the plasma polymerization conditions is, in most cases, in the power-deficient region, where the atomic nature of the polymerization is less pronounced. In contrast, Inagaki and Yasuda's data (39) obtained by collecting polymer onto substrates located between electrodes in the monomer-deficient, or the energy-saturated, region obtained by a magnetron glow discharge powered by 10 k H z show very little difference among the deposition rates of polymer from acetylene, ethylene, and methane (Fig. 6.49). If we consider the difference in the mass of the monomers, we come to a different conclusion, namely, that the specific rates by which methane and acetylene polymerize in plasma are very close under these conditions. If one argues that the plasma polymerizations of acetylene in these cases are different, then the entire subject of plasma polymerization becomes meaning­ less. The growth mechanism with which we are dealing in this chapter is a basic

6.8. A T O M I C N A T U R E O F P L A S M A P O L Y M E R I Z A T I O N

175

Reaction time (min) Fig . 6.49

Depositio n rat e a s a functio n of use d gas . Depositio n rate s (mg/cm 2»hr ) ar e a s

follows: O ( C H 4) , 1.6 x 1 0 " 2;

( C H 2C H 2) , 2.5;

(CHCH) , 3.6. Adapte d fro m Inagak i an d

Yasud a (39).

mechanism that can be applied to all cases. As discussed in more detail in Chapter 8, this problem cannot be thoroughly understood unless the growth mechanism and the polymer deposition mechanism are separated conceptually. Thus, "atomic" polymerization, in the context described in this section, is a very important aspect of plasma polymerization. Although the degree to which plasma polymerization is atomic in nature is dependent on the chemical structure of the monomers, the ionization characteristics, and the conditions of plasma polymerization, the complete elimination of "atomic" poly­ merization from plasma polymerization by manipulating the conditions and the structure of the monomers seems to be nearly impossible. If one could succeed in completely eliminating the contribution of "atomic" polymeriza­ tion, all advantageous features of plasma polymerization would be lost. In summary, the most important aspects of the growth mechanism of plasma polymerization are as follows: 1. The chain-growth mechanism does not play a significant role in the vapor depositions of polymers in a vacuum. Addition reactions to the multiple bonds can and would occur in a vacuum; however, the kinetic chain length is too short to be recognized as polymerization in the conventional sense. 2. The major growth mechanism is the rapid step-growth mechanism in which the reactions between the activated species (in contrast to an activated species and a molecule) predominate. Such a reaction might be termed polyrecombination, that is, the consecutive recombination of the activated species. The mechanism shown in Fig. 6.1, however, contains reactions that are not polyrecombination.

176

6. K I N E T I C A N D M E C H A N I S T I C ASPECT S

3. Consequently, any single species or a certain number of dominating species cannot be responsible for the plasma polymerization of a monomer. Furthermore, the nature and number of activated species formed from a m o n o m e r are dependent on the conditions of plasma polymerization.

Reference s H. Kobayashi , M . Shen , an d A. T. Bell, J. Macromol. Sci., Chem. A8, 1354 (1974). H. Yasud a an d C. E. Lamaze , J. Appl. Polym. Sci. 17, 1533 (1973). A. R. Westwood , Eur. Polym. J. 7, 363 (1971). A. T. Bell, Top. Curr. Chem. 94, 43 (1980). H. Yasuda , M . O . Bumgarner , an d J. J. Hillman , J. Appl. Polym. Sci. 19, 531 (1975). G. Smolinsk y an d M . J. Vasile , J. Macromol. Sci., Chem. A10, 473 (1976). H . Kobayashi , A. T. Bell, an d M . Shen , Macromolecules 7, 277 (1974). R. H . Hanse n an d H . Schonhorn , J. Polym. Sci., Part B 4, 203 (1966). M . Hudis , J. Appl. Polym. Sci. 16, 2397 (1972). N. Morosoff , B. Crist , M . Bumgarner , T. Hsu , an d H. Yasuda , J. Macromol. Sci., Chem. A10, 451 (1976). 11. H . Yasud a an d T. Hsu , J. Polym. Sci., Polym. Chem. Ed. 15, 81 (1977). 11a. H . Yasuda , J. Macromol Sci., Chem. A10, 383 (1976). 12. H . Yasud a an d C. E. Lamaze , J. Appl. Polym. Sci. 17, 1519 (1973). 13. H . Yasuda , H . C. Marsh , M . O . Bumgarner , an d N. Morosoff , J. Appl. Polym. Sci. 19, 2845 (1975). 14. H . Yasud a an d T. Hirotsu , J. Polym. Sci., Polym. Chem. Ed. 15, 2749 (1977). 75. H . Yasud a an d T. Hirotsu , J. Appl. Polym. Sci. 22, 1195 (1978). 16. H. Kobayashi , A. T. Bell, an d M . Shen , Macromolecules 7, 277 (1974). 17. M . Niinom i an d K. Yanagihara , Plasm a diagnostic s of polymerizin g benzen e plasma , in "Plasm a Polymerization " (M . She n an d A. T. Bell, eds.), p. 87. ACS Symp. Ser. 108. Am . Chem . S o c , Washington , D . C , 1979. 18. A. Dilk s an d E. Kay , Macromolecules 14, 855 (1980). 19. H. Yasuda , M . O . Bumgarner , H . C. Marsh , an d N. Morosoff , J. Polym. Sci., Polym. Chem. Ed. 14, 195 (1976). 20. A. R. Westwood , Eur. Polym. J. 7, 377 (1971). 21. J. M . Tibbitt , M . Shen , an d A. T. Bell, J. Macromol. Sci., Chem. A10, 1623 (1976). 22. K. W. Bieg an d D . K. Ottesen , Fourie r transfor m infrare d analysi s of plasma-polymerize d 2vinylpyridin e thi n films, in "Plasm a Polymerization " (M . She n an d A. T. Bell, eds.), p. 87. ACS Symp. Ser. 108. Am . Chem . S o c , Washington , D . C , 1979. 23. K. Jesch . J. E. Bloor , an d P. C. Kronick , J. Polym. Sci., Part A-l 4, 1487 (1966). 24. A. Dilks , S. Kaplan , an d A. VanLaeken , J. Polym. Sci., Polym. Chem. Ed. 19, 2987 (1981). 25. D. T. Clar k an d D. Shuttleworth , J. Polym. Sci., Polym. Chem. Ed. 16, 1093 (1978). 26. D. T. Clark , A. Dilks , an d D . Shuttleworth , in "Polyme r Surface " (D. T. Clar k an d W . J. Feast , eds.). Wiley , London , 1978. 27. D. T. Clar k an d D. Shuttleworth , Eur. Polym. J. 15, 265 (1979). 28. D. T. Clar k an d D. Shuttleworth , J. Polym. Sci., Polym. Chem. Ed. 17, 1317 (1979). 29. D. T. Clar k an d D. Shuttleworth , J. Polym. Sci., Polym. Chem. Ed. 18, 407 (1980). 30. D. T. Clar k an d M . Z. Abrahman , J. Polym. Sci., Polym. Chem. Ed. 19, 2129 (1981). 31. D. T. Clar k an d D. Shuttleworth , J. Polym. Sci., Polym. Chem. Ed. 18, 27 (1980). 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

REFERENCE S 32. 33. 34. 35. 36. 37. 38. 39.

177

D. F. O’Kan e an d D . W . Rice, J. Macromol. Sci., Chem. A10, 567 (1976). H . Yasuda , J. Polym. Sci., Macromol. Rev. 16, 199 (1981). D . T. Clar k an d M . Z. Abrahman , J. Polym. Sci., Polym. Chem. Ed. 20, 691 (1982). M . Gazick i an d H . Yasuda , J. Appl. Polym. Sci., Appl. Polym. Symp. 38, 35 (1984). N. Morosoff , H . Yasuda , E. S. Brandt , an d C. N. Reilley, J. Appl. Polym. Sci. 23, 3449 (1979). T. Masuok a an d H . Yasuda , J. Polym. Sci., Polym. Chem. Ed. 20, 2633 (1982). T. Masuok a an d H . Yasuda , J. Polym. Sci., Polym. Chem. Ed. 19, 2937 (1981). N. Inagak i an d H . Yasuda , J. Appl. Polym. Sci. 26, 3425 (1981).