Fluoropolymers

4 Fluoropolymers: Properties and Structure 4.1 Introduction This chapter examines the reasons to exhibit extreme properties by fluoropolymers. It focuses on the significant impact of replacement of hydrogen with fluorine in hydrocarbon macromolecules on enhancement of thermal stability, chemical resistance, electrical properties, and coefficient of friction. Understanding of the role that introduction of fluorine plays in altering the properties of a polymer will contribute to a more in-depth appreciation and insight to the characteristics and attributes of fluorinated polymers. The discussion aspires to enable the readers to make more informed judgments about fluoropolymers and their applications. Design engineers routinely encounter questions about the choice of materials in their projects. A thorough understanding of the role of fluorine will hopefully aid in the decision to specify fluoropolymers for a given design scenario, and whether a perfluoropolymer or partially fluorinated polymer will meet the requirements of the application. Our starting point is polytetrafluoroethylene (PTFE), the predecessor of all fluorinated polymers. Polychlorotrifluoroethylene (PCTFE) came a bit earlier than PTFE.

which has replaced H with distortion of the geometry of the former. Polyethylene H

H

H

H

C

C

C

C

H

H

H

H

Polytetrafluoroethylene F

F

F

F

C

C

C

C

F

F

F

F

Let us compare the CeF and CeH bonds. Table 4.1 [1,2] summarizes the key differences arising from the differences in the electronic properties and sizes of F and H. On comparing fluorine and hydrogen, several relevant differences are noted: 1. Fluorine is the most electronegative of all elements 2. Fluorine has unshared electron pairs 3. F is more easily converted to its ionic F
4. Bond strength of CeF is higher than CeH 5. Fluorine is larger than H.

4.2 Impact of F and CeF Bond on the Properties of PTFE Fluorine is a highly reactive element with the highest electronegativity of all elements (4 Paulings on a relative scale of 0.7e4) [1]. The change in the properties of compounds where fluorine has replaced hydrogen can be attributed to the differences between CeF and CeH bonds. A simple way to frame the issues is to explore the differences between polyethylene (PE), which is linear, and PTFE. The two chemical structures appear similar on the paper yet in the latter, F

The electronegativity of carbon (2.5 Paulings) is somewhat higher than that of hydrogen (2.1 Paulings) and significantly lower than fluorine (4 Paulings). The consequence of the electronegativity values is that the polarity of the CeF bond is opposite to that of the CeH bond, and the CeF bond is more highly polarized (depicted in Figure 4.1). In other words, fluorine has a higher electron density because it pulls the shared pair of electrons closer to itself relative to the center point of the CeF bond. Conversely, in the CeH bond, the electron pair is closer to carbon, which has a higher electron density.

Fluoroplastics, Volume 2. http://dx.doi.org/10.1016/B978-1-4557-3197-8.00004-3 Copyright © 2015 Elsevier Inc. All rights reserved.

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Table 4.1 Electronic Properties of Hydrogen and Halogens [1,2] Element (Preferred Ionic Form)

Electronic Configuration

Electronegativity, Pauling

Ionization Energy, kcal/g atom X D D eL > X

Electron Affinity, kcal/g atom X D e L > XL

CeX Bond Energy in CX4, kcal/mol

CeX Bond Length in ˚ CX4, A

H (Hþ)

1s1

2.1

315.0

17.8

99.5

1.091

F (F
)

1s1 2s22p5

4.0

403.3

83.5

116

1.317

CI (CI)

1s1 2s22p5 3s23p53d0

3.0

300.3

87.3

78

1.766

X ¼ H, F, or Cl.

The difference in polarity of CeH and CeF bonds affects the relative stability of the conformations of the two polymer (PTFE and PE) chains. Crystallization of PE takes place in a planar and trans conformation.

Figure 4.1 Depiction of the comparative polarization of CeH and CeF bonds.

Figure 4.2 Phase diagram of polytetrafluoroethylene [3].

The crystal structure of PTFE, e(CF2)ne, is unusual in having a number of crystal forms (Figure 4.2) and also possess substantial molecular motion within the crystal well below the melting point. PTFE can only be forced into a planar conformation (form or phase III) at extremely high pressures [4]. PTFE, in contrast, at below 19  C, crystallizes as an incommensurate helix with approximately 1.69 nm per repeat distance [5]. It thus requires 13 carbon atoms for a 180 turn to be completed. At above 19  C, the repeat distance increases to 1.95 nm which means that 15 carbon atoms will be required for a 180 turn to be completed [6]. At above 19  C, the chains are capable of angular displacement, which increases at temperatures above 30  C until the melting point is reached (327  C). Substitution of F for H in the CeH bond increases the bond strength from 99.5 kcal/mol for CeH bond to 116 kcal/mol for the CeF bond, which is substantial. Consequently, thermal stability and chemical resistance of PTFE are much higher than PE because more energy is required to break the CeF bond. Additionally, the size of the F atom and the length of the CeF bond (Table 4.1) are such that the carbon backbone of PTFE is blanketed with fluorine atoms, thus rendering the CeF bond impervious to solvent attack. The polarity and strength of the CeF bond rule out an F atom abstraction mechanism for formation of chain branches in PTFE. Instead, fully and partially fluorinated comonomers with pendent groups are polymerized with tetrafluoroethylene (TFE) to produce copolymers. In contrast, highly branched PE (>8 branches per 100 carbon atoms) can be synthesized with relative ease [7]. The branching mechanism is a tool used to reduce the crystallinity of PE to produce polymers with differing properties.

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There are significant differences between most properties of PE and PTFE. Four properties that are vastly altered in PTFE are 1. PTFE has one of the lowest surface energies among the organic polymers 2. PTFE is the most chemically resistant organic polymer 3. PTFE is one of the most thermally stable among organic polymers 4. PTFE’s melting point and specific gravity are more than double PE’s. Properties of these PTFE and PE are listed in Table 4.2. Table 4.2 A Comparison of PTFE and PE Properties [8e10] Property

PTFE

PE

2.2e2.3

0.92e1

Melting temperature,  C

342 (first) 327 (second)

105e140

Dielectric constant (1 kHz)

2.0

2.3

Dynamic coefficient of friction

0.04

0.33

18

33

Excellent, no known solvent

Susceptible to hot hydrocarbons

505

404

0.000002

0.008

339

264

Density

Surface energy, dynes/g Resistance to solvents and chemicals Thermal stabilitya T1/2,  C k350, %/min Eact, kJ/mol b

Melt viscosity , Poise

10

10 e10

12 e

Refractive index

1.35

1.51

Chain branching propensity

No

Yes

PTFE, polytetrafluoroethylene; PE, polyethylene. a T1/2 is the temperature at which 50% of the polymer is lost after 30 min heating in vacuum; k350 is the rate of volatilization, that is, weight loss, at 350  C; Eact is the activation energy of thermal degradation. b Also called melt creep viscosity or MCV for PTFE at 380  C.

Commercial PE melts at 100e140  C, depending on the extent of branching, as compared to PTFE that melts at 327  C (first melting point 342  C). One could expect that weak intermolecular forces in PTFE should result in a lower melting point; or at most somewhat higher melting point because of the extremely high molecular weight of PTFE. On the contrary, PTFE melting point is significantly higher than PE. Why? The nature of the intermolecular forces, which are responsible for its high melting point, in PTFE is not fully understood. The answer may be in the differences between the molecular structure conformation and the crystalline structure of PE and PTFE. Fluorine atoms are much larger than hydrogen resulting in less PTFE chain mobility than PE. Steric repulsion due to the size of the fluorine atoms prevents the PTFE from forming a PE-like planar zigzag conformation. Instead, its conformation is helical in which steric repulsion is minimized. PTFE is insoluble in common solvents. The replacement of H with the highly electronegative F renders PTFE immiscible with protonated material. Conversely, PE can be plasticized and dissolved above its melting point much more easily than PTFE. It absorbs only small amounts of perhalogenated solvents such as perchloroethylene and carbon tetrachloride. The insolubility of PTFE in solvents is one of its most important characteristics in many applications such as lined pipe and other lined equipment for processing corrosive chemicals.

4.3 Copolymers of TFE TFE polymerization allows an overwhelming majority of the chains to crystallize, despite their very large molecular weight. This is important to the development of properties such as high modulus, low coefficient of friction, and high heat deflection temperature. Crystallinity of virgin PTFE (never melted) is in the range of 92e98% [11], consistent with an unbranched chain structure. In spite of the outstanding properties of PTFE, it is undoubtedly one of the most difficult plastics from the processing and manufacturing standpoint. The massively long chains of PTFE are required to obtain a polymer with commercially useful mechanical properties. Yet, that same chain results in high molecular weights and extremely high melts

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viscosities thus rendering the polymer impossible to process similarly to conventional thermoplastics. The standards polymer chemistry has, therefore, been applied to TFE that is copolymerizing other monomers to prepare polymer with lower molecular weight (melt viscosity). Two examples of TFE polymerization with hexafluoropropylene (HFP) and chlorotrifluoroethylene have been described in this chapter briefly. Commercially significant and important homo- and copolymers of TFE as well as those of vinylidene fluoride (VDF) and chlorotrifluoroethylene have been described in other chapters in this book.

4.3.1 Perfluorinated EthylenePropylene Copolymer Perfluorinated ethylene-propylene (FEP), a copolymer of TFE and HFP, contains a tertiary carbon at the branch point bonded to a pendent CF3. This carbon should have less thermal stability than primary carbons and, to a lesser extent, secondary carbons that constitute the rest of the backbone of the polymer chain. This is due to a steric effect in which the chain departs from a helix at the branch point. Figure 4.3 shows the results of thermogravimetric analysis of PTFE and FEP after 1 h of heating in the air. The lines in Figure 4.3 start at degradation rate of 0.02% weight loss/h at 300  C for FEP and 0.03% weight loss/h at 425  C for PTFE.

FEP

F

F

F

F

F

F

C

C

C

C

C

C

F

F

F F

F F

F

C F

Table 4.3 provides a comparison of the properties of FEP and PTFE. Melting point, processing temperature, degradation temperature, and upper continuous use temperature all have decreased significantly, the most important being the use temperature. The reason for lower thermal stability lies in the greater susceptibility, to oxidation, of the tertiary carbon bonded to the pendent perfluoromethyl group. FEP has about half the crystallinity of PTFE even though its molecular weight is an order of lower magnitude. CF3 side chains

25

Figure 4.3 Comparison of thermal degradation of perfluorinated ethylene-propylene (FEP) and polytetrafluoroethylene (PTFE) by thermogravimetric analysis [12].

disrupt the crystallization sufficiently to reduce the crystalline content. The melt viscosity of FEP is almost 100 million times lower than PTFE, which places it among the melt-processible thermoplastic polymers.

4.3.2 Polychlorotrifluoroethylene Chlorine has a larger atom than fluorine according to the electronic configuration data in Table 4.1. It disrupts the geometric perfection of the chain including its electronic balance because of chlorine’s lower electronegativity compared to Table 4.3 A Comparison of the Properties of FEP and PTFE [12] Property

FEP

PTFE

Melting point, C

265

327

Processing temperature,  C

360

400

TGA loss temperature of 1%/h,  C

380

465

Upper continuous use temperature,  C

200

260

MV (380  C), Poise

104e105

1011e1012

Crystallinity of virgin polymer, % wt

40e50

92e98



MV, melt viscosity (also called melt creep viscosity for PTFE); TGA, thermogravimetric analysis; PTFE, polytetrafluoroethylene; FEP, perfluorinated ethylene-propylene.

26

fluorine. This means that the symmetry of electron density is disrupted where chlorine has replaced fluorine (Figure 4.4). The polarity of the shared electron changes in favor of fluorine, therefore rendering the chlorine end of the CleCeF less negative. Chlorine in PCTFE is understood to be where attack on the molecule commences; CeCl bond is weaker than CeF bond. The various types of attacks include oxidation, branching, chain scission, cross-linking, solvent swelling, and partial or complete solubilization. In spite of presumed susceptibility of chlorine in PCTFE to chemical attacks, the same does not seem to be true about the impact of high energy radiation. Earlier studies reported chain scission [13e15] without identifying the permanent chemical changes to the polymer structure or formation of new structures. In a 2003 study [16], radiation chemistry of PCTFE at different temperatures was studied with an emphasis on detection of new chemical structure formation. PCTFE film was irradiated under vacuum at doses up to 1500 kGy. Three irradiation temperatures were 25, 50, and 220  C which represent ambient temperature, a temperature above the glass transition (Tg) and a temperature above the crystalline melting temperature. This study [16] concluded that the irradiation of PCTFE below the crystalline melt temperature yielded almost entirely new chain-end species only. The study found that main chain scission was the predominant process occurring upon radiolysis. At temperatures above the crystalline melt temperature, the radiolysis products include new chain ends and chain branches. The new chain branching points are formed through Y-linking. However, unlike PTFE, PCTFE does not form a network structure when irradiated above the crystalline melting point.

Figure 4.4 Comparative polarizations of CeCl and CeF bonds.

F LUOROPLASTICS , VOLUME 2

The authors also discovered [16] evidence of a preferential loss of fluorine atoms over chlorine atoms on radiolysis (Figure 4.5), even though the carbon fluorine bond has the higher bond energy. New peaks were observed in the FTIR spectra of the irradiated PCTFE samples, indicating formation of new chemical structures. Moreover, the relative intensities of the peaks CeF/CeCl in the FTIR spectrum changed with radiolysis dose. Figure 4.5 shows a plot of normalized ratio of the peak intensities to a value of 3 for the unirradiated polymer. The figure demonstrates that the CeF/CeCl ratio decreases with dose, indicating that there is a preferential loss of fluorine during the radiolysis. However, this preferential loss of fluorine appears to saturate at higher doses. The loss of fluorine relative to chlorine is unexpected [16]. It could arise because of the smaller size of the fluorine atom compared to chlorine, which may allow fluorine atoms to diffuse more readily from the cage before recombination than would the larger chlorine atoms. Alternatively, the preferential loss of fluorine could arise from the greater electronegativity of fluorine, which may result in a trapped electron leading to cleavage of the fluorine carbon bond to form a fluoride ion and a neutral carbon radical. This process has been proposed in the past (1953) to explain the observed trapping of FL in the presence of alkali and the increase in the electrical conductivity of irradiated PTFE.

Figure 4.5 Ratio of fluorine to chlorine atoms as a function of dose in polychlorotrifluoroethylene irradiated at 30  C [16].

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CF3 CF3 RF

C

C

CF3 CF3

r.t.

F

NaOR F113

1 NaOR

RF

C

C

OR + RFCH2COOR

3

4

-30 - - 40 °C

Perfluoroolefins, such as PTFE, in spite of broad chemical resistance, are generally more vulnerable to attack by nucleophiles than electrophiles which are the opposite of the case of hydrocarbon olefins. Nucleophilic attacks occur on the fluoroolefins by the scheme proposed in Figure 4.6. The nucleophile (Nuc) approaches the carbon side of the double bond (I) searching for a positive charge leading to the formation of a carbon ion (II). For example, if the nucleophilic compound was methoxy sodium, the CH3eOe side of the molecule would be approaching TFE. The carbon ion (II) is unstable and will give off a F
ion and generate reaction products. Which product is generated depends on the nature of reaction medium. In the example of methoxy sodium, in the absence of a proton donor such as water, F
would combine with Naþ to produce NaF and perfluorovinyl methyl ether (III). Reactions of TFE oligomers and nucleophiles have been reported such as the pentamer (1) of TFE with alkoxide nucleophiles (in Figure 4.7), sulfur containing nucleophiles and amines. The presence of mobile double bond in the pentamer molecule renders it susceptible to nucleophiles attack. It can either replace a fluorine atom at a vinyl position or attack the double bond causing rearrangement toward a terminal position. When the pentamer was reacted with alkoxide nucleophiles such as allylic alcohol, methanol, and ethanol at low temperatures (
30 to
40  C), kinetically controlled products (2) were obtained as the main products, whereas at room temperature, the main products were thermodynamically controlled (3) and accompanied by small amounts of degradation products (4) [17,18]. Generally, PTFE is not susceptible to nucleophilic attack because of the absence of double bonds. It is still susceptible to loss of fluorine by electrophilic attack particularly under heat and over long periods of exposure. Alkali metals, which are highly reactive

R = (a) CH2CH

CF2 CF3 RF

4.4 Reaction Mechanism

RF = C(C2F5)2CF3

r. Et t. 3N

F113

Figure 4.6 Proposed reaction scheme for nucleophilic attack on fluoroolefins [8]. TFE, tetrafluoroethylene.

C

C 2

CH2

= (b) C2H5

F

= (c) CH3

OR

r. t. = Room temperature Et3N: H3C

N

CH3

H 3C

Figure 4.7 Reaction of the pentamer of tetrafluoroethylene with alkoxide nucleophiles [17].

elements, such as cesium, potassium, sodium, and lithium, are among the most likely candidates for abstraction of fluorine from PTFE by an electrophilic mechanism. Certain other metals such as magnesium can attack PTFE if they are highly activated by etching or other means. Loss of fluorine destabilizes PTFE’s structure. As the fluorine-to-carbon ratio decreases, the color of PTFE changes from white to brown and then to black. The black layer is normally comprised of carbon, some oxygen, and small amounts of other elements.

4.5 Effect of Solvents on Fluoropolymers Earlier in this chapter, the structure of PTFE was likened to a carbon rod completely blanketed with fluorine atoms which render the CeF bond impervious to solvent attack. This postulate has been proven by testing the effect of almost all common solvents on this polymer. There are no known solvents for PTFE below its melting point. PTFE is attacked only by molten alkali metals, chlorine trifluoride, and gaseous fluorine. Attack by alkali metals results in defluorination and surface oxidation of PTFE parts which is a convenient route to render them adherable. Small molecules can penetrate the structure of fluoropolymers. Tables 4.4 and 4.5 provide a

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Table 4.4 Characteristics of Films in Sorption Studies [19]

Thickness, mm Preparation Crystallinity, %

PTFE

FEP

50

50

Cast from aqueous dispersion

Melt extruded

41

42

PTFE, polytetrafluoroethylene; FEP, perfluorinated ethylenepropylene.

summary of room temperature sorption of hydrogencontaining and nonhydrogenated solvents into films of PTFE and FEP. Table 4.4 describes the characteristics of the films used in these experiments. Most hydrogen-containing solvents are absorbed into PTFE and FEP at less than 1%. In their case, the extent of swelling does not depend on the solubility parameter. In contrast, halogenated nonhydrogenated solvents penetrate these polymers as a strong function of the solubility parameter. Maximum swelling (11%) takes place at a solubility parameter of 6, and it drops to less than 1% swelling at a solubility parameter of 10. A useful rule of thumb is that little hydrogencontaining solvent is taken up by perfluoropolymers, irrespective of the solubility parameter. The amount will increase with increasing temperature. For convenience, one can imagine that the solvent molecules are increasingly energized at higher temperatures and the polymer structure becomes more open. Both effects lead to more swelling. For nonhydrogencontaining solvents, swelling decreases when the solubility parameter of the solvent increases. More swelling occurs at higher temperatures, as with the hydrogen-containing solvents. “The more the solvent chemical structure resembles the fluoropolymer structure, the greater the swelling,” is the rule of thumb for this group.

related to the intermolecular forces of fluoropolymers and other materials. To help the reader, definitions of the forces are briefly discussed. Over a century ago (1879), Van der Waals postulated the existence of attractive intermolecular forces. His framework for the discussion of these forces was a modified form of the ideal gas law. Other researchers after Van der Waals have classified the intermolecular forces into four components: Dispersion (or nonpolar) force Dipoleedipole force Dipoleeinduced dipole (induction) force Hydrogen bonding These forces are referred to as Van der Waals forces [20,22e28]. The focus in this section is on short-range forces between two molecules which are fairly close to each other. Van der Waals forces can take place between any pair of molecules. A second class of repulsive forces acts in opposition to the Van der Waals forces. The net resultant of two forces is the actual repulsive force present between two molecules. All four forms of attractive energy are proportional to 1/r6 therefore allowing the Van der Waals forces to be expressed by Eqn (4.1). Repulsive energy for two neutral molecules, which get close to each other, is conventionally expressed in Eqn (4.2). Total energy between the two molecules is the sum of the attractive repulsive energies, shown in Eqn (4.3), known as Lennard-Jones potential [21]. We will focus on attractive forces in describing interactions between fluoropolymer molecules or between other molecules and fluoropolymers.  Ea ¼ A r 6  Er ¼ B r 12   Et ¼ A r 6 þ B r12

(4.1) (4.2)

4.6 Molecular Interaction of Fluoropolymers: Low Friction and Low Surface Energy

where r is the distance between two molecules, A and B are constants.

Friction and surface energy (critical surface tension) are very low for fluoropolymers (Table 4.6). Both characteristics are at the root of many applications of these plastics, such as bridge expansion bearings (low friction) and nonstick cookware (low surface energy). In this section, these properties are

PTFE molecules have little propensity for polarization or ionization, which minimizes the nonpolar energy or force between PTFE molecules and between PTFE and other molecules. There are also no permanent dipoles in its structure, which is not the case for polymers such as PCTFE and

(4.3)

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Table 4.5 Sorption of Various Compounds by Perfluorocarbon Polymers at Room Temperature [19] Wt Gain% Solubility Parameter (cal/cm3)1/2

Compound

PTFE

FEP Resin

Compounds containing hydrogen Isooctane

6.85

0.8

0.4

n-Hexane

7.3

0.7

0.5

Diethyl ether

7.4

0.8

0.6

n-Octane

7.55

1.2

0.5

Cyclohexane

8.2

1.1

0.4

Toluene

8.9

0.4

0.3

1,1-Dichloroethane

9.1

1.5

0.6

Benzene

9.15

0.4

0.3

CHCI3

9.3

1.4

1.4

CH2Cl2

9.7

0.5

0.6

1,2-Dichloroethane

9.8

0.8

0.4

CHBr3

10.5

0.5

0.2

Average

0.8

0.5

Standard deviation

0.4

0.3

10.6

11.0

Compounds without hydrogen FC-75a Perfluorokerosene

6.2

11.2

6.1

Perfluorodimethylcyclohexane

6.1

10.1

10.4

C6F12b

9.1

8.4

1,2-Br2TFE

6.5

7.2

SiC14

7.6

5.2

3.6

CC14

8.6

2.4

1.8

SnC14

8.7

3.4

2.0

TiC14

9.0

2.2

1.3

CCI2]CCl2

9.3

1.9

1.4

CS2

10.0

0.4

0.2

Br2

11.5

0.7

0.7

FEP, perfluorinated ethylene-propylene; PTFE, polytetrafluoroethylene. CF2

CF2 a

Structure:

CF

CF2

CF2

CF2CF2

CF3

O

. b

Cyclic dimer of hexafluoropropylene:

F2 F2

F2 F2

F2 F2

.

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F LUOROPLASTICS , VOLUME 2

Table 4.6 Coefficient of Friction and Surface Energy of Unfilled Fluoropolymers

Formula

Coefficient of Friction (Dynamic)

Critical Surface Tension [20], dyn/cm

Surface Tension [21] (Harmonic-Mean Method), dyn/cm

Polyethylene

ech2ech2e

0.33

31

36.1

Polyvinyl fluoride

echfech2e

0.3

28

38.4

Polyvinylidene fluoride

ecf2ech2e

0.3

25

33.2

Polytrifluoroethylene

ecf2echfe

0.3

22

e

Polytetrafluoroethylene

ecf2ecfe

0.04

18

22.5

Polyvinylchloride

echc1ech2e

0.5

39

41.9

Polyvinylidene chloride

ecci2ech2e

0.9

40

45.4

Fluoropolymers

polyvinylfluoride, minimizing dipoleedipole energy and force. Low polarizability coefficient minimizes dipoleeinduced dipole energy. The neutral electronic state of PTFE and its symmetric geometry rule out hydrogen bonding. Consequently, PTFE is very soft and easily abraded. The molecules slip by and slide against each other [29]. The absence of any branches or side chains eliminates any steric hindrance, which could constrain the slipping of PTFE molecules past each other. In PTFE (and fluoropolymers in general), relative to engineering polymers, these characteristics give rise to properties like Low coefficient of friction Low surface energy High elongation Low tensile strength High cold flow The electronic balance and neutrality of the molecule of PTFE result in High chemical resistance Low polarizability Low dielectric constant Low dissipation factor High volume and surface resistance These properties serve as the foundation of the applications for this plastic.

4.7 Conformations and Transitions of PTFE The special size and electronic relationship of fluorine and carbon atoms set apart the conformational and transitional arrangement of PTFE from seemingly similar molecules such as PE. Polymerization of TFE produces a linear molecule without branches or side groups. Branching would require removal of fluorine from CeF bonds, which does not occur during the polymerization. The linear chain of PTFE does not have a planar zigzag conformation, as is the case with PE. Only under extreme pressure (5000 atm) does the chain adopt a zigzag conformation [30e32]. The chain assumes a helical conformation to accommodate the large atoms of fluorine. In 1956, Clark et al. presented an unusual room temperature transition for PTFE that occurs at 19  C between forms II and IV as seen in Figure 4.2 [33]. It was interpreted as an untwisting in the helical conformation of the molecule from a 13/6 conformation to a 15/7 conformation. Below 19  C, a helix forms with a 13.8 angle of rotation around each carbonecarbon bond. At this angle, repeat units of 13 CF2 are required to complete a 180 twist of the helix. At above 19  C, the number of CF2 groups to complete a 180 twist increases to 15. The crystalline structure of PTFE changes at 19  C, which is significant due to its closeness to the ambient temperature: the repeat distance is 1.69 nm and the separation of chain axes is 0.562 nm [34]. Above 19  C, the repeat distance increases to 1.95 nm and separation of chain axes decreases to 0.555 nm. In the phase III (zigzag) crystal state, at a pressure of 12 kbar,

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c.4–9 Å 100 µm Crystalline ‘slice’

Crystalline

1 µm

Disordered region

c.5–7 Å 200 Å

Figure 4.8 Crystalline structure of polytetrafluoroethylene [35] (figure was redrawn for Chapter 4, Volume 1).

density increases to 2.74 g/cm3 and crystal dimensions are a ¼ 0.959 nm, b ¼ 0.505 nm, c ¼ 0.262 nm, and g ¼ 105.5 [34]. The helical conformation of the linear PTFE molecules causes the chains to resemble rod-like cylinders [29] which are rigid and fully extended. The crystallization of PTFE molecules occurs in a banded structure depicted in Figure 4.8. The length of the bands is in the range of 10e100 mm while the range of the bandwidth is 0.2e1 mm, depending on the rate of the cooling of the molten polymer [36]. Slowing the cooling rates generates larger crystal bandwidths. There are striations on the bandwidths that correspond to crystalline slices, which are produced by the folding over or stacking of the crystalline segments. These segments are separated by amorphous polymer at the bending point. The thickness of a crystalline slice is 20e30 nm [35]. PTFE has several first- and second-order transition temperatures ranging from
110 to 140  C (Table 4.7) [11]. The actual quantity of minor transitions is somewhat dependent on the experimental method used. From a practical standpoint, the two

first-order transitions that occur at 19 and 30  C are most important. Figure 4.2 shows the phase diagram of PTFE. It can be seen from this figure that the only phase, which cannot be present at the atmospheric pressure, is phase III. It requires elevated pressures under which the polymer molecule assumes a zigzag conformation. Below 19  C, the crystalline system of PTFE is a nearly perfect triclinic. Above 19  C, the unit cell changes to hexagonal. In the range of 19e30  C, the chain segments become increasing disorderly and the preferred crystallographic direction disappears. Between 19 and 30  C, there is a large expansion in the specific volume of PTFE approaching 1.8% [37] which must be considered in measuring the dimensions of articles made from these plastics.

4.7.1 Images of PTFE Molecule There has been an interest in studying the characteristics of unidirectionally oriented PTFE chain. Samples of PTFE transferred to the glass surface have been studied by atomic force microscopy (AFM). AFM

Table 4.7 Transitions of Polytetrafluoroethylene [11] Temperature, °C

Order

Region Affected

Type of Disorder

19

First order

Crystalline

Angular displacement

30

First order

Crystalline

Crystal disorder

90

First order

Crystalline


90

Second order

Amorphous


30

Second order

Amorphous

130

Second order

Amorphous

Onset of rotational motion around CeC bond

32

is used as a powerful scanning probe technique for surface analysis of a variety of materials with nanometer scale. AFM is a powerful tool for the analysis on nonmetallic materials. The technique does not require special interactions between the probe tip and the analyzed material surface such as conducting current, tunneling current, magnetic forces, etc. Therefore, AFM investigations of thin films and crystals of polymers and polymer-related compounds have been conducted successfully [38e40]. AFM studies of PTFE film thickness and molecular structure [41e43] have yielded important results. The image resolution from these studies, however, was insufficient to distinguish clearly the individual fluorine atoms from the PTFE macromolecular chains. A study by National Aeronautics and Space Administration (NASA) in 2000 provided the first direct observations of individual fluorine atoms, and the measurement of the fluorine-helix and carbon-helix radii from highly oriented PTFE films using AFM [44]. A thin PTFE film was mechanically deposited onto a smooth glass substrate at specific temperatures by a friction-transfer technique. Atomic resolution images of these films show that the chain-like helical structures of the PTFE macromolecules are aligned parallel to each other with an ˚ (Figure 4.9 and intermolecular spacing of 5.72 A 4.10), and individual fluorine atoms are clearly observed along these twisted molecular chains with ˚ . Furthermore, the an interatomic spacing of 2.75 A first direct AFM measurements for the radius of the fluorine helix and of the carbon helix in the sub˚, angstrom scale are reported as 1.7 and 0.54 A respectively (Table 4.8).

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Figure 4.9 Atomic resolution image taken with a 50A˚ field of view shows the chain-like structure of the polytetrafluoroethylene macromolecules with an ˚ [44]. intermolecular spacing of 5.72 A

The degree and kind of crystallinity may be controlled by its thermal history, especially the cooling speed during processing. In general, its range may be approximately from 40% to 80%, but it is never completely crystalline or amorphous. Molded PCTFE with high crystallinity is a dense material which has high mechanical strength and low elongation. On the other hand, the amorphous-rich PCTFE moldings are optically clear, more elastic, and have a lower density [51]. Density of the polymer depends on its degree of crystallinity. Hoffman and Weeks have estimated the

4.8 Conformations and Transitions of PCTFE The crystal structure of PCTFE has been determined by X-ray diffraction and accepted to be pseudohexagonal with lattice parameters of a ¼ 0.644 nm and c ¼ 4.15 nm [48,49]. The polymer chain is helical with an average of 16.8 monomer units in one turn of the helix. The skeletal angles of the CF2 and CFCl groups differ by 5e7 , which is accommodated by the helical structure of the polymer chain. Crystal microstructure in PCTFE is spherulitic, and the crystallites consist of folded polymer chains [50].

Figure 4.10 Image is taken with a 30-A˚ field of view, showing the unique twisted character of the polytetrafluoroethylene macromolecules [44].

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Table 4.8 X-ray Diffraction and AFM Measurement Comparison for PTFE Molecules [44]

X-ray [45] ˚ Diffraction, A

AFM Data NASA Study, ˚ A

PTFE intermodular spacing

5.54

5.72

5.80

5.30

Bragg spacing along chain axis

1.29

1.43

e

e

Fluorine atomic spacing

2.60

2.75

e

e

Period length (13-atom chain)

16.8

11.4

e

PTFE Molecular Configuration

CF2 group helix spacing

16.9

AFM Data [46], AFM Data [47], ˚ ˚ A A

2.0e2.4

2.36

e

e

Fluorine-helix radius

1.64

1.70

e

e

Carbon-helix radius

0.42

0.54

e

e

AFM, atomic force microscopy; PTFE, polytetrafluoroethylene.

density of completely amorphous and crystalline PCTFE [52]. The values of 2.077 and 2.187 g/cm3 for entirely amorphous and crystalline phases can be used to estimate the degree of crystallinity of fabricated parts from Eqn (4.4). The degree of crystallinity of an article is affected by the molecular weight of PCTFE and the cooling rate from melt to solidified state. q ¼

d30
2:077 2:187
2:077

(4.4)

Note that q is the degree of crystallinity at 30  C and d30 is the density of resin at 30  C. There is conflicting information about the glass transition temperature of PCTFE in a review article, placing the temperature in the range of 45e60  C [53]. Three transition temperatures at 150, 90, and
37  C have been reported. Another article reported a temperature of 52  C. There is general agreement that the glass transition temperature of PCTFE is about 95  C [54,55]. A more recent study has proposed a glass transition temperature of 75  2  C, based on extensive measurements using dynamic mechanical analysis, thermomechanical analysis, and differential scanning calorimetry [56].

4.9 Conformations and Transitions of FEP Copolymers FEP is a copolymer of TFE and HFP. The crystallinity of FEP as polymerized is about 70% compared to 95e98% for as-polymerized PTFE [57].

FEP has a significantly lower crystallinity than PTFE because of the replacement of a fluorine atom with pendentdCF3 groups that prevent efficient chain packing. The presence of this group distorts the structure of PTFE, resulting in a higher proportion of amorphous region. Crystalline FEP has a lamellar morphology. FEP manifests a single first-order transition that is its melting point. Similarly to PTFE, FEP’s melting point increases with pressure (Table 4.9). The presence of the pendent group causes a lowering of melting point relative to PTFE. Melting of commercial FEP results in an 8% increase in volume. In the presence of the HFP comonomer in the TFE chain, intramolecular distance increases as a result of crystal distortion, which suppresses the melting point [58]. Crystallinity of parts after processing (i.e., melting) depends on the cooling rate of the melt. Relaxation temperature of FEP increases with HFP content of the copolymer. FEP has a dielectric transition at
150  C which is unaffected by the monomer composition or crystallinity (specific gravity [58]). Relaxation temperatures of FEP are shown in Table 4.10.

Table 4.9 Effect of Pressure on Perfluorinated EthyleneePropylene Melting Point [57,58] Pressure

Rate of Melting Point Increase, °C/MPa

Low

1.74

High

0.725

34

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Table 4.10 Relaxation Temperatures of Perfluorinated EthyleneePropylene [58] Transition Type

Temperature, °C

a (glass I)

157

g (glass II)


5 to 29

The chain conformation and crystal structure of a series of HFPeTFE copolymers containing up to 50 mol% of HFP have been studied [59]. Increasing HFP content leads to significant departures from the highly ordered crystalline structure of PTFE. The helical conformation of the chain (TFE) relaxes and untwists to accommodate the CF3 pendant group in the HFP unit [60].

4.10 Conformations and Transitions of Perfluoroalkoxy Polymer Perfluoroalkoxy polymer (PFA) is a copolymer of TFE and perfluoroalkyl vinyl ether (PAVE). The melting point of PFA is lower than PTFE; it ranges from 305 to 315  C for commercial resin depending on the PAVE content of the copolymer. The crystalline content of as-polymerized PFA is about 70% (5%) because of the pendent groups that PAVE comonomer impart to the chain. A small amount of PAVE comonomers is required to reduce crystallinity and obtain a tough polymer. PAVEs are more effective in reducing crystallinity than HFP because they possess longer side chains. PFA tends to crystallize in spherulitic morphology. Crystallinity and specific gravity of PFA parts decrease when the cooling rate of the molten polymer is increased. The lowest crystallinity obtained by quenching molten PFA in ice was 48% (specific gravity 2.123). PFA exhibits one first-order transition at
5  C in contrast to two temperatures for PTFE at 19 and 30  C. It has three second-order transitions at
100,
30, and
90  C [61].

4.11 Conformations and Transitions of Polyvinylidene Fluoride Polyvinylidene fluoride (PVDF) chains form smaller amorphous molecular dimensions than PTFE

because of strong interaction between eCH2e and eCF2e dipoles along the chain [62]. Crystallinity of PVDF is 35e70% depending on the polymerization method and polymer finishing history. The characteristics of PVDF depend on the molecular weight, molecular weight distribution, the amount of chain irregularity, side chains, and its crystalline regime. In an alternating chain, head (eCF2e)-to-tail (eCH2e) addition dominates. There are occasional reversed head-to-head and tail-to-tail additions resulting in defects, the extent of which depends on the polymerization conditions, particularly temperature [63]. The amount of the defects is determined by 19F NMR and other techniques. Emulsion polymerization produces more head-to-head defects that are not followed by tail-to-tail links than suspension polymerization [64,65]. PVDF has several polymorphs including four known chain conformations and a fifth suggested one [66]. The most common crystalline phase (density of 1.92 g/cm3) is the trans-gauche conformation in which hydrogen and fluorine atoms are alternately located on each side of the chain [67,68]. It forms both during polymerization and during cooling of the molten polymer. The b crystalline form has a density of 1.97 g/cm3 and is obtained when PVDF is mechanically deformed, for example, stretched, near its melting point. It has an all-trans chain conformation positioning the fluorine atoms on one side and hydrogen atoms on the other side of the chain. The g crystals are less usual and are obtained from ultrahigh molecular weight PVDF [69,70]. The d crystals are generated by the distortion of one of the other crystalline forms [71]. Amorphous PVDF has a density of 1.68 g/cm3 suggesting that a typical part with a density of 1.75e1.78 g/cm3 has a crystallinity of 40% [72]. PVDF has four relaxation temperatures at 100  C (a0 ), 50  C (a00 ),
38  C (b), and
70  C (g). PVDF processing techniques influence phase quantities in the polymer. For example, injection molding produces more amorphous phase while compression molding leads to more crystalline phase. This difference is due to the principles of the two techniques. The plastic melt is cooled rapidly in the mold during injection molding thus leading to incomplete crystallization and thus more amorphous phase. Compression molded plastics are cooled more slowly from the melt toward room temperature, allowing more crystallization and formation of larger crystals.

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The crystalline structure of the polymer depends strongly on the conditions under which it is formed. A high or moderate cooling rate generally produces the a conformation [73]. The latter can be transformed to the b conformation by stretching. This conversion is strongly influenced by the stretching temperature; a maximum is obtained with a temperature around 353 K [74,75]. The melting temperature of a and b conformations is around 440 K, albeit the latter has a broader melting peak. However, the melting peak of the g conformation is around 456 K, which is substantially higher than those of a and b conformations. PVDF has a propensity to liberate Hydrofluoric acid causing it to be susceptible to attack by nucleophiles such as strong bases. PVDF is soluble in polar solvents such as esters, acetone, and tetrahydrofuran. Solubility of PVDF allows film casting from the solutions.

4.12 Conformations and Transitions of Ethylenee Tetrafluoroethylene Copolymer Commercial ethyleneetetrafluoroethylene (ETFE) is a nearly equimolar copolymer of ethylene and TFE (1:1 ratio) and is isomeric with PVDF. ETFE has a higher melting point than PVDF and a lower dissipation factor [76] because of its special chain conformation. Crystalline density was 1.9 g/cm3 for a polymer containing 12% head-to-head defect [77]. The unit cell of the crystal is expected to be orthorhombic or monoclinic with cell dimensions of a ¼ 0.96 nm, b ¼ 0.925 nm, c ¼ 0.50 nm, and g ¼ 96 . ETFE with a degree of alteration of 100% does not exist. Typically, commercial ETFE has a degree of alternation of 92% [76,78]. An equimolar polymer has been confirmed to comprise of perfectly alternating comonomer units that result in the highest amount of crystalline phase and maximum melting point and glass transition temperature (Figure 4.11) [80,81]. As comonomer ratio departs from 1:1, crystalline content is reduced and melting point decreases. For example, a nearly 100% alternating ETFE would have a melting point of 300  C versus 270  C for an 88% alternating polymer. The melting point of ETFE reaches a minimum at 65e70 mol% TFE in the copolymer. ETFE has a molecular conformation in which extended zigzag chains are packed in orthorhombic cells. Each molecule is adjacent to four other

35

Figure 4.11 Tetrafluoroethylene (TFE) content dependence of Tm (melting point) and Tg (glass transition temperature) observed for a series of ethylenetetrafluoroethylene copolymers [79].

molecules in which the eCH2e groups of a chain are positioned next to the eCF2e groups of the next chain [82]. In essence, the bulky eCF2e groups nestle into the space above the smaller eCH2e groups of an adjacent chain [80], thus interlocking the chains. This structure is responsible for ETFE properties including its unique behaviors of melting point and glass transition temperature as a function of TFE content of the copolymer as contrasted with other TFE copolymers such as TFE and VDF copolymers (Figure 4.12). It also affects other properties including low cold flow, high tensile strength, and high modulus. A reduction in the degree of alternation reduces the interlocking of the chains.

Figure 4.12 Tetrafluoroethylene (TFE) content dependence of Tm of vinylidene fluoride (VDF)eTFE random copolymers in comparison with that of ethylene-tetrafluoroethylene (ETFE) copolymers [81].

36

Intermolecular forces among the chains preserve the properties of ETFE until temperature reaches a transition value of 110  C, above which physical properties decline [80]. Commercial ETFE (with w1:1 comonomer ratio) has two other transitions, b at
25  C and g at
120  C. ETFE is more resistant to strong bases than PVDF, nonetheless susceptible at elevated temperatures. ETFE can be dissolved in diisobutyl adipate at 230  C to measure the molecular weight by laser light scattering. Commercial ETFE has molecular weight in the range of 0.5 to just over 1 million [80].

4.13 Conformations and Transitions of Ethylenee Chlorotrifluoroethylene Copolymer Commercial ethyleneechlorotrifluoroethylene (ECTFE) copolymer is an alternating polymer with 1:1 ratio of comonomers. It is partially crystalline (50e55%) and melts at 240  C [83]. ECTFE exhibits three second-order transitions at 140  C (a), 90  C (b), and at
65  C (g). The first and last transitions are ascribed to the motion of chain segments in the crystalline phase, whereas the b transition is thought to relate to the motion of chain segments in the amorphous phase. The glass transition temperature of ECTFE is 85  C. The unit cell of alternating copolymers of ECTFE is hexagonal with a chain repeat distance of 0.502 nm [84,85]. The unit cell contains three molecules and occupies a volume of 0.324 nm3 [86]. The preferred morphology of ECTFE chains is similar to that of ETFE where chlorotrifluoroethylene units line up opposite ethylene in the adjacent chain. This structure has been credited for the relatively high melting point and physical properties of equimolar ECTFE. ECTFE is resistant to organic and inorganic chemicals and does not dissolve in any solvents. It absorbs hot polar and chlorinated solvents.

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[56] Khanna YP, Kumar R. The glass transition and molecular motions of polychlorotrifluoroethylene. Polymer 1991;32(11). [57] Gangal SV. Perfluorinated ethylene-propylene copolymer. In: KirkeOthmer encyclopedia of chemical technology. 4th ed., 11. New York: John Wiley & Sons; 1994. p. 644e56. [58] Eby RK. J Appl Phys 1963;34:2442. [59] White ML, et al. J Polym Sci Part B: Polym Phys 1998;36:2811e9. [60] Gangal SV, Brothers PD. Perfluorinated polymers, perfluorinated ethylene-propylene copolymers. In: Ency of polymer sci and tech. Wiley; June 15, 2010. [61] Gangal SV. Tetrafluoroethylene-perfluorovinyl ether copolymer. In: KirkeOthmer encyclopedia of chemical technology. 4th ed., 11. New York: John Wiley & Sons; 1994. p. 671e83. [62] Lovinger AJ. In: Bassett GC, editor. Developments in crystalline polymers, vol. 1. Barking, UK: Elsevier Applied Science Publishers, Ltd; 1982. p. 195e273. [63] Gorlitz M, Minke R, Trautvetter W, Weisgerber G. Angew Makromol Chem 1973;29/30:137. [64] Lutringer G, Weill G. Polymer 1991;32(5):877. [65] Lutringer G, Meurer B, Weill G. Polymer 1991;32(5):884. [66] Lovinger AJ. Macromolecules 1982;15:40. [67] Herschinger J, Schaefer D, Spiess HW, Lovinger AJ. Macromolecules 1991;24:2428. [68] Bachmann MA, Lando JB. Macromolecules 1981;14:40. [69] Weinhold S, Litt MH, Lando JB. J Polym Sci Polym Lett Ed 1979;17:585. [70] Litt MH, Lando JB. J Polym Sci Polym Phys Ed 1982;20:535e52. [71] Loufakis K, Miller KJ, Wunderlich B. Macromolecules 1986:1271. [72] Dohany JE, Humphrey JS. Vinylidene fluoride polymers. In: Encyclopedia of polymer science and engineering. 2nd ed., 17. New York: John Wiley & Sons; 1989. p. 532e48. [73] Gregorio Jr R, Cestari M. J Polym Sci Part B Polym Phys 1994;32:859e70.

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[74] Richardson A, Hope PS, Ward IM. J Polym Sci Polym Phys Ed 1983;21:2525e41. [75] Sajkiewicz P, Wasiak A, Goclowski Z. Eur Polym J 1999;35:423e4299. [76] Starkweather HW. J Polym Sci Part A-2 1973;11:587. [77] Wilson FC, Starkweather HW. J Polym Sci Part A-2 1973;11:919. [78] Modena M, Garbuglio C, Ragazzini M. J Polym Sci Polym Lett 1972;10:153. [79] Aimi K, Ando S. Conformation analysis and molecular mobility of ethylene and tetrafluoroethylene copolymer using solid-state 19F MAS and 1Heˆ19F CP/MAS NMR spectroscopy. Magn Reson Chem July 2004;42(7):577e88. [80] Kerbow DL. Ethylene/tetrafluoroethylene copolymer resins. In: Scheirs J, editor. Modern fluoropolymers. New York: Wiley Series in Polymer Science, John Wiley & Sons; 1997. [81] Arai K, Funaki A, Phongtamrug S, Tashiro K. Influence of alternating sequential fraction on the melting and glass transition temperatures of ethylene-tetrafluoroethylene copolymer. Polymer 2010;51:4831e5. [82] Gangal SV. Tetrafluoroethylene-ethylene copolymers. In: KirkeOthmer encyclopedia of chemical technology. 4th ed., 11. New York: John Wiley & Sons; 1994. p. 657e71. [83] Miller WA. Chlorotrifluoroethylene-ethylene copolymers. In: Encyclopedia of polymer science and engineering. 2nd ed., 3. New York: John Wiley & Sons; 1989. p. 480e91. [84] Sibilia JP, Roldan L, Chandrasekaran S. J Polym Sci 1972;10:549. [85] Sibilia JP, Schaffhauser JP, Roldan L. J Polym Sci 1976;14. [86] Mannon PJ. Nucleonics 1964;22(9):72.

Further Reading [1] Zissman WA. Influence of const. On Adh. Ind Eng Chem October 1963:18e38. [2] Eby RK, Wilson FC. J Appl Phys 1962;33:2951.