Sorption, diffusion and conduction in polyamide-penetrant systems: 1. Sorption phenomena

Sorption, diffusion and conduction in polyamide-penetrant systems: 1. Sorption phenomena G. Skirrow and K. R. Young* Department of Inorganic, Physical and Industrial Chemistry, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK (Received 20 February 1974) Isotherms for the uptake of water, methanol, ethanol, propan-l-ol and propan-2-ol by nylon-6 have been determined by gravimetric methods (quartz spiral spring and electronic recording microbalance) over wide temperature and concentration ranges. The isotherm shapes and the partial molar free energy, enthalpy and entropy changes associated with the uptake are consistent with sorption at specific sites. The data have been used to explain the order of the equilibrium uptake: propan-l-ol > water> propan-2-ol > ethanol > methanol. Some isotherms showed an upward sweep at high penetrant concentrations, and examination by the Zimm-Lundberg method showed clustering to become apparent in this region. INTRODUCTION In recent years a number of authors 1-5 have reported sorption investigations of penetrant-polyamide systems during studies of penetrant-polymer interactions and the consequent change in polymer structure. Generally, results have been restricted to a limited number of penetrants, to a small temperature range or both. This paper outlines an investigation of the uptake of a series of penetrants (water and alcohols) by nylon-6 over a wide temperature range.

EXPERIMENTAL Nylon-6 film (2.8 × 10-3cm thick) was obtained in the form of tubular sheet from British Cellophane Ltd. Before use each sample cut from the sheet was soaked in distilled water for 48 h. The penetrants used were distilled water and Analar grade alcohols. Most of the sorption isotherms were obtained using a calibrated quartz spiral spring 6 (92.2 cm extension/g) which was mounted in a Pyrex jacket connected to a conventional high vacuum system, a penetrant storage system and a mercury manometer. The portion of the apparatus containing the sample was mounted in a thermostat, the temperature of which could be controlled to +0"2°C, and the upper portion of the spring (jacket) was maintained at a temperature close to that of the sample so as to minimize convection. The manometer used to measure the applied penetrant pressure and all connecting lines were heated so as to prevent condensation of vapour, and care was taken to prevent the distillation of mercury vapour onto the * Present address: The Queen's School, City Walls Road, Chester, CH1 2NN, UK.

spring or sample by isolating the manometer except when readings were taken. The extension of the spring was measured by means of a precision cathetometer to + 10-acm. A vacuum electronic recording microbalance (Combustion Instruments Microforce Balance, Mark 2 model B) was used to obtain absorption/desorption isotherms (and also to follow the kinetics of the vapour uptake by a semi-automatic method 7) for a number of systems. When using this method, the sample was suspended in a jacket, the temperature of which was controlled to +0.2°C by a forced-draught thermostat. Particular care was taken to minimize vibrations by isolating the jacket from mechanical contact with the thermostat by mounting it on a vibration-damping support. The entire microbalance system was mounted on vibration resisting support. The balance was calibrated against the spring by comparison of equivalent isotherms. Samples were degassed for 6 h until a stable zero was attained, a pre-determined pressure of penetrant (measured using a wide-bore mercury manometer and cathetometer) introduced from a reservoir and the weight change to equilibrium followed. RESULTS AND DISCUSSION Isotherms were obtained using as penetrants water (36, 40, 43.9, 48.0, 52.0, 55-0, 59.0, 70.0, 80.0 and 90-0°C), methanol (32.0, 36.0, 40.1, 44.1, 47.9, 52.0, 59.0, 70.0, 80.0 and 90.0°C), ethanol (48.0, 59.0, 70.0, 80.0 and 90.0°C), propan-l-ol (70.0, 80.0 and 90.0°C) and propan-2-ol (80.0, 90.0 and 100.0°C). Those for water and methanol penetrants are shown in Figures 1 and 2, the concentration being expressed in cm a penetrant vapour (at STP) taken up by 1 cm a of the dry polymer. Occasionally, slight hysteresis was observed, particularly

POLYMER, 1974, Vol 15, December

771

Sorption, diffusion and conduction in polyamide-penetrant systems (1): G. Skirrow and K. R. Young for large uptake changes. The magnitude of this hysteresis was similar to that observed previously in a number of other polyamide-water systems 1. Absorption isotherms are considered in this paper. IOO

Isotherm shape

¢_,

E

'-6

I.cO 0

0..

>o O3

E

50

U

jJ

E;

0

I

I

!

50

I00

150

Penetront pressure (mmHg) Figure 1 Isotherms: nylon-6/water system. ©, 36.0; A, 40.0; Y, 43.9; O, 48.0; I-], 52.0; x , 55.0; A , 59.0; V, 70"0; -k, 80"0; II, 90"0°C

IOC t_

E -6

O.,

%

Comparison of isotherms

o Ct. o

For a particular penetrant pressure, the equilibrium uptake sequence by nylon-6 is:

E 50 u

propan-l-ol > water > propan-2-ol > ethanol > methanol

o

0

I00

2O0

300

P e n e t r a n t p r e s s u r e (mmHg)

Figure 2 Isotherms: nylon-6/methanol systems. O, 32.0; A, 36.0; I', 40.1; O, 44.1; 17, 47.9; x , 52.0; A, 59.0; V, 70.0; + , 80.0; II, 90'0°C

772

For small amounts of penetrant uptake, most of the isotherms were convex to the pressure axis; at high applied penetrant vapour pressure an upward sweep was apparent. The general shape of the isotherms is characteristic of nylon-water systems and has often been noted for various polyamide-penetrant systems2, 3, 8-12. The initial Langmuir-type behaviour suggests the presence of a limited number of sorption sites which are immediately available to the penetrant molecules, and the eventual upward sweep is probably indicative of clustering or multi-layer formation9 at the higher penetrant concentrations. Although the sorption process in nylons is complex, it is generally considered that the penetrant molecules do not readily enter the crystalline regions but that they are confined to the amorphous portions or to the crystalline surfaces (see, however, Campbell13). Since in the unperturbed polymer a small fraction (~6%) of the imide groups in the amorphous regions are not hydrogen bonded to other chains 14, the initial uptake of water probably occurs on these 'free' sites. This primary monolayer formation is likely to be complete at about 2wt% (~20cm 3 (STP)/cm 3) uptake, a value consistent with the position of the inflection of the isotherms for water shown in Figure 1. Studies of the effect of water on the narrow line n.m.r, intensity for the nylon-6/water system at 25°C by Kawasaki and Sekita 3 indicated a further region of low water content over which the penetrant molecules are relatively immobile. In this region the uptake is accompanied by the breaking of intramolecular hydrogen bonds. Above about 4 wt % the water molecules are much more mobile, and it is in this region (~40cm 3 (STP)/cm 3) that clustering in nylon-6 is apparent (see below). For these higher uptakes when there are no longer available singly occupied primary sites, penetrant molecules must attach to secondary sites or, where suitable voids in the polymer matrix exist or can be created, associate with other water molecules in clusters.

POLYMER, 1974, Vol 15, December

(I)

although at low pressures the uptake for ethanol is closely similar to that for methanol. Clearly, neither polarity nor molecular size can be used as simple criteria for predicting the relative uptakes. Thus, steric effects, which might be expected to restrict access of propan-2-ol to sites do not appear to be critical. The partial molar free energy, enthalpy and entropy changes for uptake at a concentration c' of penetrant can be defined on the basis of one or other of the three processes: (i) 1 mole of liquid penetrant

+

infinite amount of polymer-penetrant ~mixture mixture (conc. c')

Sorption, diffusion and conduction in polyamide-penetrant systems (1): G. Skirrow and K. R. Young (ii) 1 mole of penetrant infinite amount of vapour at saturated+polymer-penetrant --*mixture vapour pressure (ps) mixture (conc. c')

Table I

Enthalpy changes for polyamide-penetrant systems Concentration (cm 3 vapour (STP)/ - A/-Tii -- AHi cm 3 polymer) (kJ/mol) (kJ/mol)

System

(iii) 1 mole of penetrant infinite amount of vapour at reference+polymer-penetrant --*mixture pressure Pr mixture (conc. c')

nylon-6/water (AHc= - 42.05 kd/mol)

For each polymer/penetrant system at a given temperature, provided that the vapour can be regarded as a perfect gas, the partial molar enthalpy changes in these three processes are inter-related by:

nylon-6/methanol (~Hc= -34.84kd/mol)

A/t7ii ----A/tTiii= A/~i -- AHc

(1)

where AHe is the molar enthalpy of condensation of the penetrant. Also: AGi = AGii = ACiii - RTln(ps/pr)

(2)

where R and T have their usual meanings. The appropriate quantities can be obtained by making use of standard relationships 15. For example: (Olnp~

A/Tu

O-T-]c-RT 2

.

,

lnp = R T + constant

(4)

Typical isosteres are shown in Figure 3, and values for A/-Tii and A/-Ti (derived from the isostere slopes) are given in Table 1. Mean values 16 for AHe for the relevant temperature range were used to evaluate

A/-Ti.

~ 2-©

30

Figure 3

nylon-6/propan-l-ol (AHc= - 43.30 kJ/mol)

(3)

where p and c are the penetrant pressure and concentration respectively, and if A/~ii is temperature independent:

afiii

nylon-6/ethanol (AHc= -- 39" 91 kd/mol)

3"2 I / r x l O 3 ( K -I )

34

Isosteres: nylon-6/methanol systems. O, 15; O, 25; x , 35; 17, 45; _A, 55; II, 70cm 3 (STP)/cm 3

nylon-6/propan-2-ol (AHe=-40.08kd/mol)

20 30 40 70 100

46.61 44.85 44.02 44-22 42.43

4"56 2.80 1.97 2.18 0.38

15 25 35 45 55 70

38.65 37.97 36.19 35.62 33.32 35.47

3'80 3'12 1-35 0.77 0.47 0.63

15 25 35 45 55

38.20 37.50 38.05 36.22 36-07

-1'71 -2'41 - 1"87 -3-69 -3.85

15 25 35

28.62 33.51 35.86

-- 16.78 --11.88 --9"54

15 20 25

35"56 32"76 30"59

--4"51 --7"32 --9"49

With the exception of the nylon-6/propan-l-ol system (for which the data were not sufficiently precise to allow a trend to be detected), both A/Tii and A/Ti became less exotherrnic with increasing concentration of penetrant. Process (ii) is exothermic, but as the concentration increased, the exothermicity decreased. Process (i) incorporating as it does penetrant vaporization, is sometimes endothermic (see e.g. the values for the uptakes of ethanol and propanol). For the nylon-6/water systems, A/tTii (and A/Ti) become progressively less exothermic by about 4.2 kJ/mol over the concentration range 20-100cm 3 (STP)/ cm 3 (Figure 4a). Calorimetrically determined values for A/li for this system show a similar fall at least over part of the range 1. Thus, Nekryach and Samchenko 17 found the heat of wetting ( - A / I i ) for the nylon-6/water system at 20°C to be constant (~9.2 k J/tool, exothermic) up to 5 w t ~ , but beyond this uptake it became progressively less exothermic until at 9Wt~o it was almost zero and constant. The first region of constant A/ti was attributed to sorption on sites, and the second to clustering or multi-layer formation. Similar behaviour was noted by Muller and Hellmuth l° (using the data obtained by Bull 18) for the nylon-6,6/water system. For the nylon-6/water system they found A/-Ti to be -12.9 kJ/mol at a concentration of 9.66 cm 3 (STP)/cm3; this value should be compared with that of -4.56kJ/mol found in this work for a concentration of 20 cm 3 (STP)/cm 3 (Table 1). Isosteric determinations of A/-Ti (and possibly also of A/Tii) are generally regarded as not being sufficiently precise compared with calorimetrically determined values to allow computation of meaningful A~¢i values (cf. Puffr and Sebendal). Both A/~i and A~¢i are very sensitive to isostere slope. This limitation on the accuracy of ASi must be borne in mind when examining the estimates for it given in Table 2 and Figure 4b.

POLYMER,

1974, V o l 15, D e c e m b e r

773

Sorption, diffusion and conduction in polyamide-penetrant systems (1): G. Skirrow and K. R. Young b

c

+2.C

-4"0

-~ I0

0

E

-)

"~

~-2'0

%

I

I

50

I00

0

"7..=

-2'0 o

2'o

Co

4'o

5'0

I 0' 0

Penetrent concentretion (cm 3 ve pour (5TP)/cm3 polymer) Figure 4 AHi,

A~i, ~(~ii

for nylon-6/water systems. (c): @, 40; ©,

Table 2 Thermodynamic data, nylon-6/water system at 80°C: AGi; ARi (kd/mol); ASi (d mol-ZK-z)

90°C Table 3 Thermodynamic data (kJ/mol), nylon-6/penetrant systems at 8O°C; pr=100mmHg; c=10cm 8 vapour (STP)/cm 3 polymer

cma vapour (STP)/cm s polymer HzO -AGi - ARi --A,~i

15

20

25

30

40

5.19 6- 02 2"38

4.31 4.56 0.71

3.64 3.56 --0.25

3.10 2" 80 --0.84

2-30 1.97 --0.96

A~d -- AG'il -- AGiii -- ARii -- TA,,%i

3"72 6"53 2"81 46" 41 43" 80

methanol ethanol propan-l-ol propan-2-ol 7"66 6"74 --0'92 38" 66 39" 58

6"15 5"44 --0"71 38" 20 38" 91

3'81 6"99 3"18 28" 62 25" 44

5'65 6"02 0"37 35" 56 35" 19

over the range 36-59°C was - 6 . 3 k J / m o l for a similar concentration to that used by previous workers ( ~ 1 0 cm a (STP)/cma). For the nylon-6/alcohol systems at 80°C (Figure 5), - A d u ( = - A G t ) for processes (i) and (ii) is in the order: O

propan-1-ol > methanol > water > propan-2-ol > ethanol

E

4

for low uptakes, and

I

propan-l-ol > methanol > propan-2-ol > water > ethanol

0

210

4'0

610

Penetront concentrotion (cm3vapour(STP~m3polymer)

Figure 5 Free energy changes for nylon-6/penetrant systems at

8O°C. @, water; x, methanol; ©, ethanol; m, propan-l-ol; F1, propan-2-ol

Thus, the apparent tendency for A ~ to become more positive with increasing water concentration is surprising if with increasing concentration the likelihood of a water molecule taking part in more than one hydrogen bond simultaneously increases. ACii ( = AG0--concentration curves (based on ps values taken from the literature 19-zl) for a number of systems are shown in Figures 4c and 5. For all the systems examined, AGu is negative, the numerical value decreasing with increasing penetrant concentration. The effect of temperature on AGu for water uptake is small, but there is a tendency for it to become more negative at higher temperatures. Muller and Hellmuth 11 found ACu for the nylon-6/water system to fall from - 6 - 3 kJ/mol at 14°C to - 5 . 4 k J / m o l at 59°C. In the present work the mean value for AGu

7"/4

(II)

POLYMER, 1974, Vol 15, December

(III)

for higher uptakes. These orders are, of course, immediately evident if the isotherms obtained (e.g. Figures 1 and 2) are replotted showing the uptakes as a function of P/Ps. In the above discussion of the partial molar quantities, AGlt corresponds to the uptake of one mole of penetrant (either as a liquid or as the vapour at the saturated vapour pressure) by an infinite amount of the penetrantpolymer mixture at the reference concentration. However, AGui (see above) and the corresponding value of A~lu are, unlike AGi ( = AGii), A,.~I and ASii, uncomplicated by the process of vaporization of the penetrant, and the acceptance of a common reference pressure (pr) as the standard state for the pure penetrant means that direct comparison of the changes of the thermodynamic parameters for the different systems is more realistic. The quantities ACa [ = RTln(ps/pr)], AGii, AGiil, A/tli and TASui for the various nylon-6/penetrant systems at 80°C and a penetrant concentration of 10 cm a (STP)/ cm 8 are given in Table 3. The following sequences are evident: - AGui; propan-l-ol > water > propan-2-ol > ethanol_methanol (IV)

Sorption, diffusion and conduction in polyamide-penetrant systems (1): G. Skirrow and K. R. Young -

-

A/~ii ; water > methanol > ethanol > propan-2-ol > propan-l-ol

- TASm; water > methanol > ethanol > propan-2-ol > propan-l-ol

• .0

Clustering When the polymer and penetrant differ in polarity, a tendency for penetrant clustering might be expected, particularly at high concentrations. This tendency is usually examined by the method proposed by Zimm and Lundberg 2~, 24. These authors defined a cluster integral Gag, viz. :

rd(aA/vA)]

where vA and vB are the volume fractions of penetrant and polymer respectively, aA is the penetrant activity and ffA is the partial molar volume of the penetrant. According to this treatment, when GAA/ITA>--I, clustering occurs whereas when GAA/P'A<- 1 sorption occurs on sites with little tendency for clustering.

20I 15

D

13

X 4X

-I-~i

~II

o ~llt~¢

X +v

-

X m+I A o • ~ A • o 0 ~..m'~'m~.~.~l~k

D+. o 0 A m 06O

S

~P'-+= ~ -

0

o

IO

o

o

,, I

0"5

A

I.O

aA Figure 6 Clustering in the nylon-6/water system. ©, 36.0; 0 , 40"0; + , 48-0; I , 52-0; ~ , 59"0; D, 70"0; x , 80.0°C

n A

nn •

X xx×

n • nA

,~;o~4Z--~,*"T-2-...~ •

(VI)

The equilibrium uptake sequence (I) is, of course, the same as that for AtTlii (sequence IV). Thus, although water uptake shows the highest exothermicity (as expected because of its high polarity), the effect of this on AGm is offset by the large value for --TASiii and consequently the uptake of water is, relative to the other penetrants, lower than might otherwise have been expected. The - T A g m values indicate that the nylon-6/water system is more ordered than the corresponding nylon/ alcohol systems. This is understandable if the water molecules are more tightly bound and are capable of multiple hydrogen bonding in a comparatively ordered structure. It is known that each oxygen atom in liquid water may cooperatively form four hydrogen bonds with its neighbours and thereby give an ordered structure whereas the evidence available suggests that for alcohols the maximum number of hydrogen bonds which can normally be formed is two2L It seems likely that the relative facilities of water and alcohols for hydrogen bond formation may be preserved to some extent when they enter the polymer system.

GAA

7I

(V)

, •

--< 5

°l~x 4!

O

0.5 eA

I.O

Figure 7 Clustering in the nylon-6/methanol system. &, 36.0; C], 40.1; +, 44.1; O, 47.9; 0, 52.0; ×, 70.0; I , 80'0°C

When using this approach, it is customary to take the activity of the penetrant to be PiPs and to plot aa/vA against aA. Typically, for a system in which clustering develops as the concentration increases a curve with a maximum is obtained; the concentration corresponding to the activity at which the maximum occurs is that for the onset of clustering. Plots of this sort derived from the basic data for nylon-6/water and nylon-6/methanol isotherms are shown in Figures 6 and 7. There is some scatter, particularly for the points corresponding to low uptakes where the measurement precision is low, but it is apparent that clustering is evident in both the methanol and water systems. Within the precision of the measurements there is no detectable shift with temperature of the position for the onset of clustering, and for both systems this position falls within the range 26 to 40cm 3 (STP)/cm 3, the majority being in the range 28 to 32cm 3 (STP)/cm a. Evidence for clustering during the uptakes of ethanol and the propanols is less conclusive, partly because of some scatter in the basic data. However, it is possible that the greater bulk of these molecules in relation to their polarity may make their localized aggregation in a relatively inflexible polymer network more difficult. This is not inconsistent with the smaller decrease in A~qiii for uptake of these molecules compared with that of water. Further examination of the uptakes of these penetrants is desirable. The agreement between the amounts of water and methanol taken up at the onset of clustering implies that at this point there is one penetrant molecule per accessible sorption site. If it is assumed that in the polymer network there is one polar site per polymer unit, then for a polymer of density 1.16g/cm 3 the potential molar concentration of sites is 9.4× 10-3 mol/cmL A penetrant uptake of 30cm a (STP)/cm 3 is equivalent to a concentration of i.35× 10-3mol/cm 3. Evidently only some 15% or so of the polar sites are /o, a large proportion accessible. Of the remaining 85 °~ will constitute the crystalline regions and for this reason will be unavailable. ACKNOWLEDGEMENTS The authors wish to thank British Cellophane for the provision of samples and D. L. Dare for technical assistance. One of us (K. R. Y.) would also like to

POLYMER, 1974, Vol 15, December

775

Sorption, diffusion and conduction in polyamide-penetrant systems (1): G. Skirrow and K. R. Young t h a n k the Science R e s e a r c h C o u n c i l for the p r o v i s i o n o f a R e s e a r c h Studentship. REFERENCES 1 2 3 4 5 6 7 8 9 10

Puffr, R. and Sebenda, J. J. Polym. Sci. (C) 1966, 16, 79 Puffr, R. Kolloid-Z. 1968, 222, 130 Kawasaki, K. and Sekita, Y. J. Polym. Sci. (A) 1964, 2, 2437 Ash, R., Barter, R. M. and Palmer, D. G. Polymer •970, 11, 421 Dodding, R. A. and Skirrow, G. to be published McBain, J. W. and Bakr, A. M. J. Am. Chem. Soe. 1926, 48, 690 Skirrow, G. and Young, K. R. to be published Sebenda, J. and Puffr, R. Colin. Czech. Chem. Commun. 1964, 29, 60 Kawasaki, K., Sekita, Y. and Kanou, K. J. Colloid Sci. 1962, 17, 865 Muller, F. H. and Hellmuth, E. Kolloid-Z. 1961, 177, 1

776 POLYMER, 1974, Vol 15, December

11 Ramsden, D. K., Wood, F. and King, G. J. Appl. Polym. Sci. 1966, 10, 1191 12 Rogers, C. E. in 'Engineering Design of Plastics', (Ed. E. Baer), Reinhold, New York, 1964 13 Campbell, G. A. J. Polym. Sci. (B) 1969, 7, 629 14 Pauling, L. J. Am. Chem. Soc. 1945, 67, 555 15 Young, K. R. PhD Thesis University of Liverpool (1970) 16 Washburn, E. H. (Ed.) 'International Critical Tables', Nat. Res. Council, Washington, D.C., 1926, Vol 5, p 138 17 Nekryach, E. F. and Samchenko, Z. A., Zolloidn Zh. 1960, 22, 288 18 Bull, H. B. J. Am. Chem. Soc. 1944, 66, 1499 19 Stall, D. R. Ind. Eng. Chem. 1947, 39, 517 20 Young, S. Sci. Proc. R. Dublin Soc. 1910, 12, 374 21 Weast, R. C. (Ed.) 'Handbook of Chemistry and Physics', 49th edn, Chemical Rubber Co., Cleveland, 1968 22 Franks, F. and Ives, D. J. G. Q. Rev. Chem. Soc. 1966, 20, 1 23 Zimm, B. H. and Lundberg, J. L. J. Phys. Chem. 1956, 60, 425 24 Lundberg, J. L. J. Macromol. ScL (B) 1969, 3, 393