Sorption of water by aliphatic polyamides. Review

Polymer Science U.S.S.R. Vol. 27, No, 4, pp. 75[-768. 1985 Prirtted in Poland

0032-3950/85 $10.00+.00 (t') 1986 Pergamon Press Ltd.

S O R P T I O N OF WATER BY ALIPHATIC POLYAMIDES. REVIEW* L. P. RAZUMOVSK1KV. S. MARKIN and G. YE. ZAIKOV Institute of Chemical Physics, U.S.S.R. Academy of Sciences (Receiced 27 October 1983)

The published findings on the diffusion and solubility of water in aliphatic polyamides are systematized. Attention is focused on the influence of the chemical nature of the polyamide and its supramolecular structure on the sorption capacity and the diffusion and swelling coefficients. From perusal of the existing ways of predicting the values of solubility S and diffusion D coefficients in polyamides the authors propose a scheme for calculating S and D on t~e basis of only the structural formula of the polymer chain. The results may be used in study of the hydrolytic stability of the polyamides and prediction of their pilysiochemical properties en sorption of water. RECEN'ILY with increase in the total output of aliphatic potyamides (PA) there has bezn increase in the specific contribution of new materials based on mixed PA and P ' \ with high (CHz)2 • ( C O N H ) ratio in the structural unit. Mixed PAr are widely u~-:d in view of their high. rupture strength and are employed, for example, in the ai~craft industry for sealing alumirtium. The low melting point of and insignificant ~aoisture absorption by PAr with. a high (CH)2 : (CONH) ratio allow th.,em to be used as protective coats for metal products. The working characteristics of a PA depend on a host of factors, the most important of which, is its ability to absorb water. The sorbed moisture chartges the melting and glass tra~sition points of the polymer [1,2] (Fig. 1) and influences the strength. [3, 4] (Table 1) a~d electrical [5-7] (Table 2) properties. The action of moisture on the polymer may be of complex nature, for example, th.e introductiort of small amounts of water (to 2.5 g~av. ~o) in PA-6 artd PA-12 sharply reduces the growth, rate of cracks under mechanical loads while large quantities ( > 8 . 5 grav. %) raise it as compared with a dry polymer [8]. The aim of the present review is to systematize the published data on the diffusion zmd solubility of water in PA, a necessary step in the creation of polymer materials ~ irk predetermined properties. Within the scope of this work it is not possible to analyse in detail the whole set (~f physicochemical properties of PA-water systems and, therefore, we shall only go iazo the most important factors: the influence of the nature of th.e PA and its supramolecular structure on the sorption capacity; the state of water in the polymer matrix;

* Vysokomol. soyed. 27: No. 4, 675-688, 1985 751

L . P . RAZUMOVSKII et al.

752

swelling; and the method of evaluatings the solubility and diffusion coefficients of water in PA. Degree of crystallinity and accessibility of a polymer. Since in solid polymers anisotropy in structural properties is always present, in calculating the concentration of dissolved matter it is necessary to know that fraction of the volume of the sample in which dissolution take place. It has now been firmly established that the sorbed water molecules penetrate only into the amorphous region of a polymer*. One way to allow for this is to use the degree of crystallinity of the polymer which is determined from the formula

[10] C-Ca n = C o - Ca'

(1)

where C is the measured index of a certain property of the polymer; Ca and Cc are respectively the values of this index for purely amorphous and purely crystalline polymers. The parameter C may be the density of the polymer [11, 12], the intensity of certain absorption bands in the IR spectrum [13, 14] or the reflexions in the diffraction pattern [15, 16], transparency, etc. [17, 18]. The values of the degree of crystaUinity given by

rg,K

"Imp, t(

550

cl

I "-.. 7

350 1 •

b

qso 250

~'"""".... 3

I

5

/5

I

5 15 SH2o, g/t 0o g

FIG. 1. Trap (a) a n d Tg (b) as a function of water content for different polyamides [2]: 1 - PA-66; 2 - PA-6; 3 - PA-6/66. TARLE 1. VALUES OF YOUNG'S MODULUS E AND RUPTURE STRENGTH a t OF P A AS A fUNCTION O r RELATIVE ATMOSPHERIC HUMIDITY [4] (25°C)

E x 10 -7, N / m z

Polymer PA-6 PA.610

Copolymer PA-66/610

a , x 10- 6, N / m 2

Dry sample

Sample for 100 relative humidity

Dry sample

Sample for 100 relative humidity

5.5 4'5 5'0

1-4 2"7 1'2

4-6 4.4 6.0

3-3 3.7 4.3

(50 : 50) * I n reference [9] an assumption on the penetration of insignificant amounts of water into the crystalline phase is made.

Sorption of water by aliphatic polyamides

753

TABLE2. Sv~cn~c VOLUMETRIC RESISTIVITY ..Q AS A FUNCTION OF RELATIVE

ATMOSPH ERIC

HUMIDITY

(ASTM test 257 [5-7], 25°(2) I2 (f2.m) for polyamide series Relative humidity, %

PA-6

PA-66

0

10ta

l0 ta

50

10 i t

10 t t

100

10 6

107

PA-6 + 33 PA-610 PA-612 PA-11 PA-12 P A - 1 3 PA-1313 glass fibre 10ta l0 ta 10la 5 x 10t2 8'6 x 10t2 10la 10 t l 1"5× 1012 -l'6x 10la 8'5x 1012 - x 1012 ]09

109

l 0 tl

8xl0

t°

1 ' 5 × 1 0 ta

l'lxI0

t2

107

different physical methods often do not agree so that the term "degree of crystallinity'" is an ambiguous characterization of the structure of the sample. Nevertheless, as a primary estimate of the structural features of a polymer this characterization may be useful. Thus, change in the degree of crystallinity made it possible to explain the difference in the equilibrium moisture absorption of oriented and non-oriented polycaproamide fibres [19]; with rise in the degree of drawing the degree of crystallinity rises and the solubility of water in the fibre diminishes. However, the solubility of water in oriented and non-oriented fibres is the same if the calculations are made only for the amorphous phase. The low degree of crystallinity of the polymer (owing to irregular alternation of the functional groups in the chain [20]) explains the higher moisture absorption of mixed PAs as compared with homoaliphatic [21]. The concept "accessibility" is also used to evaluate the structural inhomogeneity o f the polymer matrix. The definition of accessibility f is based on the difference in the rates of certain chemical reactions (hydrolysis [22], acetylation [23]) occurring in the amorphous and crystalline phases. Since the reaction rate constant in the amorphous phase is much greater than in the crystalline, apparent transgression of the reaction rate is observed for a certain degree of conversion. The fraction of functional groups reacting by this moment of time is called accessibility. For polymers with mobile hydrogen atoms (such as PA) a convenient method of determination of f is the reaction o f isotope H--*D exchange [24-27]. The use of the concept of accessibility has given good results in describing the hydrolytic degradation of the PA series [25, 28, 29] and also methylcellulose and its derivatives [22]. Let us take a closer look at the difference in the concepts of the degree of crystallinity and accessibility. Depending on the chemical nature and size of the sorbate molecule the real volume in which the sorbate is dissolved may be greater than the amorphous phase (if during sorption there is partial dissolution of the crystalline regions) or less than it. The dependence o f f on the size of the sorbed molecules is indicated in Table 3. It will be seen that with increase in the size of the sorbate molecules its value drops. For example, for PA-6 on passing from water to ethyl alcohol it falls ~ 2 times.

754

L. P. RAZUMOVSKII etal. TABLE 3. ACCESSIBILITY OF A NUMBER OF POLYAMIDES FOR WATER Accessibility o f p o l y m e r s T°

25 25 25 40 60 80 100 120 150

PA-4 [28]

PA-6 [29, 30]

0.46 0"49 0.51 0.57 0"72

0"60 0.52* 0.32t 0.66 0"73 0'77 0.79

0-89 1.00

1 "00

PA-610

0'40

0'60 0'65

PA-12

[25] 0.70 0"63* 0.45t 0.80 0.88 0-92 1-00

PA-12

[251 0.57 0"51" 0.32 t 0-68 0"76 0"83

1-00

* Accessibility of PA for methanol. t For ethanol.

The accessibility of a polymer depends not only on its supramolecular structure and the chemical nature of the sorbent but also on temperature. Table 3 indicates ehaage in f f o r the PA series in relation to water in the range 25-100°C. It is clear that with rise in temperature the values of f for all PAs rise. From this it follows that the concept of accessibility is more valuable for calculating the solubility of the sorbate than is the degree of crystallinity. Solubility of water in PA. It has been established that the solubility of water in aliphatic PAs is a certain function of the content of the CONH-groups in the macromoleeule: the higher the (CONH) : (CH)2 ratio for the structural unit the greater the moisture absorption [21, 31, 32]. This statement holds for all relative vapour pressures (Fig. 2). The sorption isotherms have a concave character relative to the abscissa (Fig. 3). This indicates that in the polymer the water-water interaction is stronger than the water-polymer. A number of authors [33-37] note the S-shape character of the isotherms for low P/Po which weakens with fall in the ratio (COHN) : (CH)2. This is particularly noticeable on passing from collagen to PA [38]. The view is taken [39] that the S-shape is connected not so much with the value of the (CONH) : (CH)2 ratio in the macromolecule as with the structure of the polymer: on sorption of water by the initially annealed PA-6, PA-12 and PA-610 samples the S-shape pattern in the sorption isotherms is absent. The observed difference in the sorption and desorption isotherms (hysteresis) in the oriented fibres [40, 41] may also be probably explained by structural rearrangements in the polymer matrix during sorption. In fact, it suffices to run several sorption-desorption cycles for hysteresis to disappear almost completely [42, 43]. In work with unstretched fibres [44] and films [38] hysteresis was not detected even in the first cycle. It is interesting to note that the form of the sorption isotherms of water in PAy6 barely changed both with fall in the course of hydrolysis of the number average mass from 9000 to 3000 [45] and with increase in the times of storage of the polymer from 3 months to 4 years [46].

755

Sorption of water by aiiphatic polyamides

Analysis of the data on the solubility of water in PA is usually based on the theories of polymolecular adsorptiort [47], of Flory-Huggins [10] and of Zimm-Lunberg [48]. To describe the sorption isotherm of water by PA, Bull [43] was the first to use an equation stemming from the theory of polymolecular adsorption

P/Po 1 c- l k- P/Po, a(1--p/po ) ca., ca,.

(2)

where a is the concentration of sorbed substance; a,, is the concentration of substance in a continuous monomolecular layer; c is a constant associated with the heat of sorption; P/Po is the relative pressure of the water vapours. A similar approach was used in most later work [34-36, 38, 49-51]. In the coordinates of equation (2) (BET) linearity of the sorption isotherms to P/Po=0-5-0.6 is observed. This allows one, despite the formal description of the process of sorption, to determine the "inner surface of the sorbent"

S=a,,,ogNA x S,8/ioog 10-

10 -7

(3)

S. F/;CC,3

eok t

8-

2~Ib~_~

~

23-

x.z ?. a3 o#

t

I

/ i0

-I T

I

7

I

[:;{2] :[CONU] FIo. 2

I

II 0.2

&J

;:,'p:.:

FIG. 3

FIo. 2. Equilibrium moisture absorption by PAs as a function of the ratio (CH2) • (CONH) in structural unit for vapour activities of 0-5 (1) and 1'0 (2) [5]. FtG. 3. Sorption isotherms of water by polyamides PA-4 (1), PA-6 (2), PA-8 (3) and PA-I 2 (4) at 25°C [33]. and to calculate the amount of sorbed water. In equation (3) oJ is the area occupied by one sorbate molecule; NA is the Avogaadro's number. With rise in the hydrophilicity of PA the amount of water in the monolayer increases [38]. The BET equation does not have predictive force and, in addition, disregards the specific properties of a solid body apart from those arising from the assumption on the constancy of the energy of interaction in uniform portions on sorption of the first layer.

756

L.P. RAZUMOVSKIIet al.

The adsorption of a substance in a solid body may be restricted, for example, by the elastic forces appearing on swelling. As a result the possible formation of sorption layers is limited to the finite number n. The modified BET equation has the form [52] amep/po . a = 1 - p/p~

1-(n+

l)(p/po) n

1 - ( c - 1) ( P / P o ) - c ( p / p o ) n + 1

(4)

and adds up to the equation (2) at n--, oo. Dole [53] showed that equation (4) describes the sorption isotherm of water by collagen (a natural compound closest to the synthetic polyamides) to the relative pressures p / p o = 0 . 8 (n=7). As Table 4 shows, this statement also holds for PA (n=6--7). Unfortunately, equation (4) also does not possess full predictive force since the values am and c axe calculated from the experimental data with the aid of expression (3) in the portion of the sorption isotherm to P/Po =0.5--0.6. Good data for describing the sorption isotherms are obtained from the FioryHuggins theory according to which at P'2>>~'x change in the chemical potential of the solvent in the system A I t / R T = l n ( p / p o ) =ln(1 - tp2) + qh +7.1 q~2,

(5)

where X1 is the dimensionless Flory-Huggins parameter; tp2 is the volumetric fraction of the polymer; ~'1 and ~'2 are the partial molar volumes of the sorbate and polymer. Figure 4 presents the experimental and calculated values of the solubility of water in PA-66 and PA-610 [39]. Thus, equation (5) may be used to predict the solubility of water in PA if the value Zl is known. The joint application of the Zimm-Lundberg and polymolecular adsorption theories found reflexion in the studies of the state of water in the polymer matrix [21, 38]. In reference [54] from study of the influence of water vapour on the mechanical properties of the polyamides it was concluded that sorbed water is usually of two types: TABLE4. EXPERIMENTALAIxDCALCULATED(FROMEQUATION(4)) VALUESOFThe SOLUBILITYOFWATER IN PA-6 (ATn----7) ANDPA-12 (AT n=6) (Temperature 25°C)

p/po 0.1 0"2 0.3 0-4 0.5 0.6 0.7 0.8 0-9

Sn2o, g/100 g polymer PA-6 [59] PA-12 [24] calculation experiment calculation experiment 0.071 0"46 0"47 0"072 0.140 0"93 0"91 0-142 1 "41 0"214 0.214 1"37 1 "90 1"89 0.289 0-290 2"41 0.39 2"50 0"372 0.49 2"99 3"23 0"472 3"75 4-05 0.569 0.61 4"70 0.72 4'92 0"683 5"83 5-80

Sorption of water by aliphatic polyamides

757

interacting with the free amide groups (i.e. groups not bound by hydrogen bonds) and rupturing the hydrogen bonds between amide groups. Such a view was developed in studies [21, 38, 55, 56]. Most investigators are now inclined to consider that as well as these two types, water also exists in the polymer in the form of clusters. Water forms hydrogen bonds with free amide groups only in that region of the sorption isotherm described by the BET equation. At this stage there is complete filling of the monolayer. For PA-6, for example, the first stage of sorption ends when there is one water molecule per two amide groups and is accompanied by release of a large quantity of heat. With increase in the amount of water in the polymer matrix swelling occurs and some of the hydrogen bonds between the chains of the macromolecule weaken owing to the removal of the amide groups over large distances. This makes possible the hydration of new active centres and also the formation of water clusters. The process of sorption completely ends when to two amide groups (in the accessible region) there are three water molecules. If the first molecule of water forms hydrogen bonds with two carbonyl groups, then the two remaining water molecules form hydrogen bonds between the carbonyl of one amide group and the - N H of the other [55].

~too9

HzO -[0 9 DC= 0 , [crnZ/sec] 7'5'-

8

I

2

8"5

0.2

0.6

P/Po

t'O

9"6

FIG. 4

I

I.. 5

I

I lO [CONH],mole/l.

FIG. 5

FIG. 4. Experimental and calculated (from equation (5)) sorption isotherms of water for PA-66 (1) and PA-610 (2) at 23°C [39]. Points, experimental. For PA-66 X1=1.46; for PA-610 Z1=2.18. H|O FIG. 5. Diffusion coefficient Dc=o as a function of the concentration of amide groups in sample.

However, Schroeder and Cooper [57] studying the influence of the hydrogen bonds on the strength properties of PA concluded that in dry PA-6 ,-~99 ~o of the amide groups are bound by hydrogen bonds i.e. the BET equation ought to be observed only to P/Po <0.1 (this corresponds to a concentration of water ~ 5 x 10 -2 mole/l.) and not to 0.54).6. One may establish from the Zimm-Lundberg ratio the start of cluster formation and evaluate the size o f the aggregates formed

~Gll=(1--tpl)(Oal/q01~ --1, Vz \ Oaz /v,r

(6)

758

L.P. RAZUMOVSKIIet al.

w h e r e Gll is the integral of cluster formation; a I and (/91 are respectively the activity and volumetric fraction of water in the polymer. According to the theory, the value GII/~'I characterizes the intensity of aggregation of the sorbate molecules the start of which corresponds to GII/~'~=-1. The values of GII/~'I>-I indicate the preferential interaction of the sorbed molecules between themselves. The size of the clusters is judged from the value tpl GI 1 ~'1 equal to the excess of the mean number of sorbate molecules close to the given molecule. The start of cluster formation of the water molecules (on conversion to the amorphous phase) lies for all PAs in the interval ¢I = 1.92.5 x 10 -2 somewhat increasing with rise in the hydrophilicity of the polyamide [38]. The results obtained on the basis of the Zimm-Lundberg theory also indicate that in PA-6 cluster formation begins when to two amide groups there is one water molecule [58]. The presence of an acid in the initial polymer or increase in temperature leads to shift of the start of cluster formation to the region of higher values of tpi [59]. The tendency for water to aggregate is high for hydrophobic PA-12 (GII/F'I=100) and falls greatly with increase in the hydrophilicity of the polyamide, for collagen GI ~/F'I does not exceed 2 [38]. Increase in the water content of the polymer with rise in P/Po is not necessarily accompanied by increase in the number of clusters. The number of dusters may fall if the newly adsorbed water forms hydrogen bridges with the already existing clusters. Such a phenomenon was observed by Starkweather on sorption of water in PA-66 [56]. Predicting the solubility of water in PA. To evaluate the solubility of water in PA three main approaches exist: the method of group contributions, the empirical method and analyses of the data on the basis of the Flory-Huggins theory of polymer solutions. In the group contribution method to each functional group of the macromolecule is ascribed the ability to bind a certain number of water molecules (Table 5, [60]) and therefore from the structural formula of the polymer one may calculate its moisture absorption for different P/Po. From Table 6 it will be seen that for PA-6 the calculated values of solubility exceed the experimental ~ 2 times and for PA-12 6-7 times. Thus, the group contribution method predicts moisture absorption very crudely. An example of the empirical approach to the calculations of the hygroscopicity of PA is afforded by work [61]. From analysis of a large number of experimental data the authors established that for a 100 ~o humidity the relation S=0.81 x 10-45c~10"6

(7)

TABLE 5. SOME CHARACTERISTIC PARAMETERS OF THE FUNCTIONAL GROUPS OF MACROMOLECULES

Group ~H2 ZOO ZOOH ZONH NH2

No. of moles of water per functional group of P/Po ] 0-9 0.3 I 0.5 I 07 l'5x10 -5 2"5x10 -st 3"3x10 -5 4"5 x 10-5 0-14 0.025 0"05 0.075 !1"0 0.2 0.3 0.6 1"5 0-35 0'5 0-75 0"35 0"5 0"75 1"5

[60]

for values I

1"0 5.0× 10-s I 0.2 1"3 2"0 2"0

k J/mole

cm3/mole

4.93 14-3

15-85 18"25 j

85-93

24"9

759'

Sorption of water by aliphatic polyamides

o/ and the paran~eter exists between the content of water in the polymers (Sago, gray./o) of solubility of the polymer (62) at 32 = 19.4 X 103 (J/m3) °'5 . Although equation (7) gives good results for evaluating the solubility of wa~cr in PA-6, PA-12 and PA-548 it cannot be applied to the other PAs since from the work it is not clear how the values of ~52 were chosen: there are no generally accepted xalues for 32 in the literature. The magnitude 02 depends both on the mode of calculation and the values of the cohesion energy E e used, the molar attraction constant F and the partial molar volume Vof the individual groups.For example, for PA-6, according to various literature sources, the magnitude ~Jz varies from 23 x 10 a to 31 x i0 a (J/m3) °'s [60, 62-64]. The arbitrary choice of 62 in the range indicated leads to a large error in calculating S owing to the power character of the dependence of S on din. To the extreme possible values of ~2 correspond the magnitudes of solubility 14.5 and 300 7/0 (experimental value 15-i 7 %).

TABLE 6. EXPERIMENTAL AND CALCULATED (GROUP CONTRIBUTIONS METHOD) DATA ON I~HE SOLUBILITY OF WATER In P A - 6 AND P A - 1 2 (Temperature 25°C)

Sx 102, g/g* PA-6

P/po 0.3 0"5 0'7 0.9

I

experiment [59] 2-5 4"3 68 10.5

PA-12 calculation

experiment [25]

calculat k'n

5.6 8-0 12.0 23.9

0.4 0.7 1.1 -

3"2 4.6 6.9 -

* Here and in Tables 7 and 8 the solubility is given with reference to the accessibility or the degree ~(crystallmity.

The task of predicting moisture absorption from the Flory-Huggins theory as noted adds up to determining the parameter Z1 and of calculating from equation (5) the volumetric fraction of water in the polymer for different P/Po. While the contribution of the non-combinatorial mixing entropy may be ignored (Z, = 0), the parameter Z, characterizing the affinity of the polymer for the solvent is calculated from the equation

~(61 -a2) 2

Z1 ----ZH--

18)

RT

where 61 is the parameter of solubility of water equal to 47.94 x 103 (j/m3) °'5. G o o d agreement is observed between Seep and S=~c (Table 7) if in the calculation one uses the values of E c and P in Table 5. The error in determining solubility does not exceed the scatter between the data of different authors. For example for PA-6 and PA-12 (relative humidity 1.0) the values of solubility move in the intervals 0.1443.17 and 0-020.04 g/g and the calculated values are 0" 15 and 0-02 g/g.

"760

L.P. RAZUMOVSKIIet al.

"TABLE 7. EXPERIMENTAL AND CALCULATED DATA ON THE SOLUBILITY OF WATER IN ALIPHATIC P A s FOR RELATIVE HUMIDITY 0 . 6 AND 1"0 (25 "4- 5 ° C )

Polymer PA-4 PA based on ~t-pyrollidone '[CONH(CH2)a .717.3COO PA-6

'A-66

~548 A-7 PA-8 'A-610

•1 talc

(J/ma) °'s

Degree of crystallinity,

%

38580 36050

0"64 1.03

51 44*

33620

1.49

39-42 56t 42t 34--42t 30--39? 65 34--39 55 27 56t 38 38 36 46 57t 37t 39--41 18* 33 46 44 32 47? 27 42t 35 39 19 30 33 35__.5t 25 9 25 24* 21 40-55 t 17 34 26 26

33620

1"49

33010 31970

1"62 1"85

30640

2"17

30640

2"17

PA-9

29540

2"46

PA-613 PA-10

29060 28620

2"59 2"71

PA-11

27860

2"93

A-12

27200

3"13

A-13 'A-1313

26600 26600

3'31 3"31

Solubility* (g/100 g), at P/Po 1.0 0.6 14.9/19-8~ 8.6/10.4 -/36.2 7.1/6.3 6.1 7.0 7.3 6.6 4.9 8.5 5.1 7-3 6-9/7.0* 7.1t 6.9t 7"7/7-0* 6.1/6-3 5.5 5"1/5-4 4"2/4-6* 4.3/4.2

3.5/2.9 2.5/3.2 2.9/2.9 2.4/2.4§ 3.4/3-2 t 2.3/2.1 3.8/2.5 * 2.1/1.5§ 1.8/1.6 2-2/1.8 * 1-7/1.4 t 1.2/1.3 1.1/1.1§ 1.5~ 1-2/1.1 1.0 2.1 1.0/1.1: 2-or 0.5/0.7§ 0.4/0.7§

15.6/15.1 16.4

Literature [65] [66] [67] [68] [121

[50] 15.4 17.3

[21] [691 [701 [32] [711 [191

[721 16.9

14.0 -/12.3

[731 [651 [65] [391 [741 [701 [681

-/8.8

[651

16.7/15-1

-/5-7

5-8/5"8 4"2

-/4-0

-/3.5 -/3.0 --/2.3 1.8

--/1.9 2.0 4.0

1.4/1.6 1.0/1.6

[32] [321 [651 [39] [7] [65] [32] [651 [7] [32] [65] [75] [321 [71 [65] [68] [69] [76] [73]

[65] [7] [7]

* Numerator, experimental; denominator, calculated data. t These values of the degree of cxystalllnity are given in the work quoted; the other values calculated by us from the ,dene~y of the polymers. * The experimental and calculated values of S are given for PIPeffi0-65. § For pipe=0.50.

Sorption of water by aliphatic polyamides

761

It should be noted that in comparing Scale and Soxp (assigned to the unit of accessible volume) the best results are given by the use of Eo = 85 kJ/mole (Table 8).* Evaluation of the solubility of water in PAs on introducing a modifier into them is made in reference [61]. The authors propose that the parameter J2 be calculated by the additive scheme 62 = f t

C~M+f2 6PA,

(9)

where f l and f2 are the weight fractions of the modifier and polymer and then S be calculated from equation (7). From equation (9) it will be seen that for any dM
AH 1=RT•I ~22,

(10)

amounts to 8.3 kJ/mole and the partial mixing enthalpy found from the sorption isosters 5-5-7.5 kJ/mole (depending on the content of water in the polymer). We would recall that the concepts "partial heat of mixing" and the "partial enthalpy of mixing" are equivalent if the volume changes by the additive scheme. Generally speaking, the Flory-Huggins theory gives better results in describing the sorption isotherms than in calculating separately the entropy or enthalpy of the process [39]. One possible explanation is that the total volume of the system differs from the volume calculated by the additive scheme [77]. In fact, for the more hygroscopic PA-66 the calculated and experimental values of AH: differ. This difference may be removed by introducing into equation (10) an additional term allowing for change in the volume on mixing [56]

AH1 =RTz1 q~ + T(a/fl)AV1,

(11)

where a is the thermal expansion coefficient; fl is the isothermal compressibility coefficient; A V~ is the partial molar change in the volume of the polymer-water system. In conclusion, we would note that most of published data on the values of AHt for PA [38, 39, 78-80] are not quite correct since the dependence of accessibility on temperature was disregarded in calculating them. * The value E. = 93 k J/mole was obtained by us.

762

L.P. RAZUMOVSKIIet al.

:Swelling ofpolyamides. Since some polyamides sorb considerable quantities of moisture (Table 7) simultaneous with diffusion there is swelling of the polymer the limiting value of which is determined by the partial pressure of the water vapours, temperature and the pre-history of the sample. An interesting feature of the process of sorption o f water by a polyamide is the non-isotropicity of swelling of oriented and non-oriented samples. Reference [81] gives the following data:

Sample Moisture absorption, ~ Longitudinal swelling, Yo Transverse swelling, ~

Stretched f i b r e 8"6 2.7 2"6

Unstretchedfibre 10"6 6-9 1.9

A difference also exists in the rates of the kinetics of sorption and swelling. Thus, in reference [12] it is noted that the sorption and swelling of PA-6 films ends simultaneously while for PA-6 threads elongation occurs more rapidly than sorption [82]. The authors of reference [82] explain this by the fact that in the initial stages of sorption in the main the sample is elongated while for heavy moisture absorption the diameter of the sample also begins to increase. The facts presented indicate that the attempts to calculate the diffusion coefficient of water from the kinetics of swelling of the sample may give incorrect results if at the same time change in all the geometric dimensions of the object is not checked. In addition, it is noted that the crave of swelling of PA has a S-shaped character while the sorption curve is of the normal type [12, 83]. Kunzman [83] attributed this difference to the slow relaxation of the polymer chains on sorption of moisture. The authors of [12] consider this explanation unconvincing since according to their data the sorptiort curves of water by films of different thicknesses (from 27 to 83/~m), in the coordinates presented, match. The match of the sorption curves excludes the existence of slow relaxational processes. The difference in the rates of swelling and sorption may be explained by assuming that the relaxation of the polymer chains occurs instantly on sorption of moisture while slowing of the process of swelling is caused by the presence of the pressure gradient of swelling over the diffusion coordinate. Calculation based on this model gave good agreement of the experimental and calculated curves for the process of sorption and swelling for different moisture contents. It should be noted that the data on swelling show absence of additivity of the volume of water and polymer on sorption of moisture (for moisture absorption 8-I0~o). In reference [43] it was found that for PA-66 relative change in the volume of the sample is only 80 ~ of the expected. A similar value on sorption of water by the polyamide resin P-548 ~ 70 ~o. Diffusion o f water into polyamides. As compared with the investigations of solubility of water in PA comparatively little work has been done oft the diffusion characteristics in the PA-water system [26, 67, 71, 74, 76, 84-89]. It mainly concerns diffusion of water in PA-6 and PA-12. While the solubility of the sorbate in the polymer usually changes proportionally

Sorption of water by aliphatic p01yamides

763

to ti~e content o f the accessible (amorphous) region, the diffusion coefficient changes in m o r e complex fashion. In a n u m b e r o f cases the dependence o f D on the degree o f crystallinity o f the polymer n is represented by the following f o r m u l a [60, 87]:

D = D . ( I - n)'",

(12)

where m is a constant; D. is the diffusion coefficient of the sorbate in a purely amorphous polymer. However, equation (12) is not always fulfilled since m often depends on the prehistory o f the sample; D with rise in n m a y change even by a curve with a minimum. The experimental data on the dependence o f E a o n the degree o f crystallinity also conflict. For some polymers E a remains constant on passing f r o m the a m o r p h o u s to the crystalline state but increases for others and for still others falls [90]. Finally, in reterence [9!] the spectrum o f the values o f the activation energies for films based on polyolefines wa~, observed. TABLE

8.

COMPARISON

OF T H E I I X P E R I M E N T A L A N D C A L C U L A T E D POLYAMIDES

t~'f;,;

PA- 6 (Za = 1.74)

-0.Z 0.:~ 0.6 0-3 l 09 i

1.7/1.3 3.4/2.8 5-4/4.7 8-5/8.7 10"5/8"5

copolymer 6/11 (85 : 15) (Z ~= 2-02) o.8/1.o 1.7/2.1 2-7/3.4 4.0/5.0 4.8/5.8

V A L U E S OF S O L U B I L I T Y OF W A T E R

IN

(25 °)

S x 102, g/g copolymer PA-69 PA-610 PA-I 1 PA-12 6/12 (80 : 20) (Z1 =2" 17) (Z~ =2.28) (Zt=2.43) (Z,=3.16) (Z~=3.36) 0.8/0.8 1.7/1.8 2.7/2.9 3' 8/4-1

0-8/0.9 1.6/1.6 2.4/2.5 3.5/3-6 4.2/4.3

0.7/0.7 1-4/1.4 2.2/'2.2 3.1,,'3.1 _

0.3,,'0.3]0.3/0.3 0.6/0.6 [ 0.5/0.5 1.0/l.0 ! 0-9/0.8 1.4/'1.4 iI 1-2/1-1 i

_

[ i

._

Note. H e r e a n d in T a b l e 10, n u m e r a t o r is t h e f o u n d a n d d e n o m i n a t o r t h e c a l c u l a t e d figure,

The polyamides as Table 9 shows are no exception in this context: orientation influence the diffusion coefficient ot water in PA-6; on passing t h r o u g h the glass transition point L:, for PA-6 practically does not change but changes fairly substantially for the PA-548 resin. As for the dependence o f D on the concentration o f water in a polymer, tbr PA-6 D rises with increase in the water concentration [68, 71, 59]* but remains practically constant for PA-I 2 [25, 67, 76]. Let us consider this difference between PA-6 and PA-12 more closely. Two factors acting in opposite fashion greatly influence the diffusional coefficient of water in PA. Increase irt the concentration o f the polar groups ( - C Q N H - ) in the matrix leads to fall in D owing to the interaction with these groups o f the water molecules; increase in the concentration of water in the polymer, i.e. its plasticization, leads to rise in D. In line with this, for C ~ 0 the effect o f plasticization is absent and the value of the diffusional coefficient as a first approximation must depend only on the * An exception is work [87] reporting fall in the diffusional coefficient with rise ill P'PO.

764

L. P. RAZUMOVSKII et al. HjO

TABLE9. VALtmOVDc=o AND E. ON SORPTIONOF WATERBY PA (25°C)

Polymer PA-6

PA-548 PA-12

Fraction of amorphous phase (accessibility) 0.44

Ea, kJ ¢mole

D~*~° x 10 9, cm2/s~

0"73 0"6--0"7 (0"6)

0"38 0"35-0"70* 0"95 0"4-1 "3* 0"7

0"82 0"76 0"45-0"6 (0"57-0"7)

1"0 1"0 2"7 2"5 3"6-4"4*

T< T, 83"5

T> T, 83"5

38

38

59"5±8"5 102 36"5 46"0 63 58"5 46"5 57"5±8"5

59"5 + 8"5 102 36-5 58"0 115 71 46"5 57"5+8"5

Literature [681 [86] [71, 76] [21] [591 [921 [93] [89] [681 [68] [76] [25]

* The range of values of D is associated with the prehistory of the sample.

concentration of the amide groups. In fact, as follows from reference [68] a linear relation exists between D C=O H2° and the concentration of the amide groups in logarithmic coor dinates (Fig. 5) log Dcn~° = log D o + bCcoNn,

(13)

where Do is the diffusion coefficient of water in PE (1.9 x 10 - s cm2/sec) [68]; b is a constant for the polyamide-water systems ( - 0 . 1 4 2 1./mole). When the solubility of water is 2-3 gray. % the plasticizing effect becomes manifest. Analysis shows t h a t the dependence of r#2O •-'c=o on the concentration of water in PA m a y be expressed in the f o r m DH2O=

DH20

2

C=O(I+~CH~o),

(14)

w h e r e • = 7" 1 x 10-+ dm~/g 2 and ~c=onH2°is calculated from equation (13). The calculated and experimental values of D u:° for the PA series are presented in effect on all aliphatic PAs. Thus, f r o m the structural formula of a PA using equations (13) and (14) one m a y with satisfactory precision calculate the dependence of D n2° on the concetration of water in the polymer. As an example let us look at the calculation of the solubility and the diffusion coefficient in the PA-6/66/610 copolymer (weight ratio 30 : 40 : 30) at 25°C and relative humidity 0.9 ( [ C O N H ] = 8.8 mole/1.) F r o m Table 5 one m a y calculate the magnitude J2 =(~E,/~,, ~ ) ½ = 31,460 (J/ma) °'s i

i

and then f r o m equation (8) obtain Z1 = 1.97 and use the latter to calculate the solubility of water in the polymer (equation (5)): ~1 =6.25 x 10 -2 and Sc,jc=71 g/dm 3. K n o w i n g the solubility of water in a polymer and successively applying equations (13) a n d (14) let us find log Dc= o = - 9 . 0 9 and Dn~o=3-1 × 10 -9 cm2/sec. The experimental value o f D H a O is 3"5 × 10 - 9 cm2/sec [35].

Sorption of water by aliphatic polyamides

765~,

TABLE 10. EXPERIMENTAL AND CALCULATED VALUES OF THE DIFFUSIONAL COEFFICIENTS OF

WATER

INTOPOLYAMIDES(25°) Concentration of H20 in polymer, mole/l. 0 0"5 1"0 1"5 2.0 2"5

D X 10 9,

cm2/sec

PA-11

PA-610

PA-69

2"7/3"2 2"7/3-4 3"1/4"0

1.7/1.7 2.0/1.6 2.3/2.1 2.8/2.6 3.7/3.3

1.2/1.5 1-5/1.6

2-0/1.8 2.5/2.3 3.1/2.9 3.8/3.7

copolymer PA-6/12 1.5/1.4 1.6/1.4 2.1/1.7 2.8/2.6 3.7/2.6 5.1/3.3

eopolymer PA-6/11 1.0/1.1 1.1/1.2 1.6/1-4 2-5/1-7 3.6/2.2 5.0/2.8

Thus, examination of the main published data on the sorption of water by aliphatic polyamides shows that the polymers containing water differ from the initial ones in a number of important physicochemical properties. Change in the solubility and diffusion coefficient of water in the course of sorption has been most fully explored; relatively little information exists on the detailed picture of sorption (structure of sorbed water, swelling, heat of sorption, etc.). The parameters of the utmost interest for predicting the stability of polymer products are the solubility of water S and the diffusion coefficient D. Although the calculations of S and D from existing theories are of a formal and approximate character, the solubility and diffusion coefficient may be satisfactory evaluated from equations (5), (12), (13) and (14), knowledge ordy of the structural form of the polyamide being necessary for such an evaluation. As follows from the published data, solubility is calculated most precisely from the Flory-Huggins theory, the diffusion coefficient is found from empirical equations. The insufficiency of the experimental data and their contradictory nature still do not allow one to propose for the whole class of polyamides a scheme for calculating S and D similar to that presented above. Nevertheless, the unflagging interest of researchers in the problem of the sorption of water by polymer materials makes one hopeful that in the not too distant future information will be obtained on the influence of water on the physicochemical properties of PA and sorption models devised helping to predict change in these properties. Translated by A. CROZY REFERENCES

1. 2. 3. 4. 5. 6. 7. 8.

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Polymer ScienceU.S.S.R. Vol. 27, No. 4, pp. 768-775, 1985 Printed in Poland

0032-39~0/85 $10.00+.00 $) 1986 Pergamon Press Ltd.

NEW TYPE OF METAL-CONTAINING P O L Y M E R S m METALLIC CLUSTERS IN POLYMER MATRICES* I. D. KOSOBUDSKll, L. V. KASHKINA, S. P. GUBIN, G. A. PETRAKOVSKII, V. P. PISKORSKII and N . M. SV1RSKAYA Institute of Inorganic Chemistry, Siberian Division, U.S.S.R. Academy of Sciences Krasnodar State University Kirenskii Institute of Physics, Siberian Division, U.S.S.R. Academy of Sciences

(Received 10 June 1983) The results of study of metallopolymers of a new type are outlined-metallic clusters in PE and PTFE polymer matrices obtained on thermal degradation of the organic salts of the metals: iron, nickel and chromium or their carbonyls in the polymer melt. The N M R (continuous method) was used to study on the 1H and 19F nuclei, the mechanism of penetration of the clusters of iron, nickel and chromium metals into the polymer matrices. It is shown that the subatomic particles of the metals (clusters) reduce the mobility of the molecular chains, raising the thermal stability of the polymer compositions. The method of small angle X-ray scatter is used to determine the size distribution of the metallic clusters and the periodicity of their disposition in the matrix. OF MUCH interest are p o l y m e r c o m p o s i t i o n s c o n t a i n i n g s u b a t o m i c m et al particles (metallic clusters) the size o f w h i c h is c o m p a r a b l e w i t h t h a t o f the m a c r o m o l e c u l e s ; these m e t a l particles m a y be o b t a i n e d in a p o l y m e r m e d i u m on t h er m al b r e a k d o w n o f o r g a n i c salts o r o r g a n o m e t a l l i c c o m p o u n d s . * Vysokomol. soyed. A27: No. 4, 689-695, 1985.