THERMODYNAMICS OF AGGREGATION IN ASSOCIATING IONOMER SOLUTIONS

Small angle neutron scattering has been used to study the aggregation behaviour of a sodium sulfonated polystyrene ionomer (SPS) with a mole cular weight of 10(5) g mol(-1) and a sulfonation level of 0.95 mol% i n dilute p-xylene solution. The variation with concentration of both t he average aggregate molecular weight and the average aggregate radius of gyration at temperatures ranging from 25 to 60 degrees C were obta ined. All the results observed can be quantitatively interpreted by us e of a closed association model in which single chains are assumed to be in equilibrium with aggregates of one size. At concentrations below 0.1 g dl(-1) this ionomer exists in solution mainly as single collaps ed chains with an average radius of gyration of 78 Angstrom. By compar ison, the single chain dimension of the unsulfonated polystyrene at in finite dilution is 130 Angstrom in xylene. As the ionomer concentratio n is raised, an increasing fraction of the single chains associates to form small compact aggregates with a radius of gyration of 150 Angstr om and consisting of three chains. Both the single chain and aggregate dimensions are temperature independent, as is the extent of aggregati on at a given concentration. This shows that the free energy change on forming aggregates from single collapsed chains is primarily entropic in nature. (i.e. there is no enthalpy change on aggregation). Moreove r, no enthalpic contribution to the foe energy of mixing for the ionom er aggregates with the solvent is observed. A contrast matching method was used to probe the aggregate internal structure. Within an aggrega te the individual chains were found to expand from their collapsed sta te to 115 Angstrom. The combined results suggest that the aggregates a nd single chains consist of a compact core which is primarily ionic in nature surrounded by solvent swollen polystyrene chains. In this pict ure the aggregates would form because of the increase in both configur ational entropy of the polymer chains and the greater mixing of the po lystyrene chains with the solvent on single chain expansion within the aggregates. Copyright (C) 1996 Elsevier Science Ltd.