5
Solubility
Parameter
of
Perfluorosulfonated
Polymer
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RICHARD S. YEO The Continental Group, Incorporated, Energy Systems Laboratory, 10432 North Tantau Avenue, Cupertino, CA 95014 The solubility parameter, δ , defined as the square root of the cohesive energy density is used considerably in the field of polymer science (1,2). The cohesive energy density is the ratio of the molar energy of vaporization minus RT, the work of expansion on vaporization, to the molar volume. It is not defined in this manner for polymers which cannot be vaporized. However, for various theoretical reasons, the cohesive energy density of materials which cannot be vaporized equals that of vaporizable solvents in which they dissolve athermally. The solubility parameter of a material is a measure of the inter molecular forces in a given substance and is a fundamental property of a l l matter. A knowledge of intermolecular forces in polymers would enable a better understanding of their physical and chemical properties on a molecular basis. It is convenient to express the cohesive energy density in cel/cc units and to refer to the solubility parameter by the symbol Hb (Hildebrand). The solubility parameter concept has been used to correlate many physical phenomena. Miscibility of solvents with polymers, diffusion of solvents within polymers, effects of intermolecular forces on the glass transition temperature and interfacial i n teractions within copolymer materials would be included, just to mention a few examples. In many cases, meaningful interpreta tion of results was facilitated with the use of the solubility parameter. The solubility parameter of Nafion membranes has been determined experimentally in a recent study (3). The samples which have been studied have an equivalent weight (EW) of either 1100 or 1200 (weight of polymer per sulfonic acid group). Since the samples are not soluble, the solubility parameter of the polymer can be determined only from the swelling technique (4). 2
Determination of δ from Swelling Measurement The degree of swelling of the membrane in solvent is related to the closeness between the solubility parameters of 0097-6156/82/0180-0065$05.00/0 © 1982 American Chemical Society In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
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66
PERFLUORINATED IONOMER M E M B R A N E S
TABLE I SOLUBILITY PARAMETERS AND MOLAR VOLUMES OF SOLVENTS AND SOLVENT UPTAKE BY NAFION ( 3 ) .
% Increase i n weight Solvent
δ . 1/2 (cal/cm )
T r i e t h y l Amine D i e t h y l Amine 2 - E t h y l Hexanol n-Amyl A l c o h o l Cyclohexanol n-Butanol 2-Propanol 1-Propanol Ethanol Methanol Ethylene G l y c o l Glycerol Formamide Water
a.
7.4 8.0 9.5 10.9 11.4 11.4 11.5 11.9 12.7 14.5 14.6 16.5 19.2 23.4
,Vi cm /mole 139.4 103.2 158.0 109.0 106.0 91.5 76.8 75.2 58.5 40.7 55.8 73.3 39.8 18.0
a 1100 22 21
— 73
— 74 58 55 50 54 66 56 56 21
a 1200 24 40 77 59 64 65 50 40 32 37 44 40 37 17
E q u i v a l e n t Weight
In Perfluorinated Ionomer Membranes; Eisenberg, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
5.
YEO
Solubility
67
Parameter
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t h e p o l y m e r and t h e s o l v e n t (4^), a s w e l l a s t o t h e bonding c a p a b i l i t y of the s o l v e n t ( 2 ) .
hydrogen
S w e l l i n g i n Pure S o l v e n t s . T a b l e I shows t h e s o l v e n t u p t a k e by N a f i o n - H . F i g u r e 1, a p l o t o f t h e s o l v e n t u p t a k e f o r t h e 1200 EW s a m p l e a g a i n s t t h e s o l v e n t s o l u b i l i t y p a r a m e t e r , i v a l u e s f o r t h e i o n i c m a t e r i a l s w o u l d be h i g h e r t h a n t h a t f o r t h e n o n - i o n i c materials. T a b l e I I I shows t h e t r e n d i n
