Small angle neutron scattering from ionomer gels

Small angle neutron scattering intensities for sols and gels of the physically associating ionomer 1.39 mol% sodium sulphonated polystyrene with molecular weight 10(5) g/mole in xylene have been obtained over a broad wavevector (q) and concentration range. In the low q and concentration range the scattering behaviour of this ionomer/solvent system can quite readily be interpreted using the open association aggregation model. In more concentrated solutions and at higher q, however, interpretation of the scattering behaviour for polymers associating via an open association mechanism (OAM) is more difficult, particularly if, as in this investigation, the single chains and aggregates have varying densities and fractal parameters. In this study various methods have been developed to interpret the low and high q scattering from systems whose extent of aggregation can be modelled using the OAM. Using these methods it has been possible to confirm that the open association model can be used to interpret the extent of aggregation of the above ionomer in xylene even after the solutions appear to be gelled. Single ionomer chains within both the dilute solutions and gels were found via modelling to have a radius of gyration of 60 Angstrom, which compares with dimensions of 25 Angstrom and 93 Angstrom calculated for a solid sphere of polystyrene or an unperturbed polystyrene Gaussian coil, respectively. The aggregates, however, all had radii of gyration comparable with what would be expected for polystyrene of the aggregate molecular weight in an unperturbed state. These results suggest that gelation of ionomer solutions at particular concentration thresholds is not due to an abrupt change in the aggregate structures at some critical concentration but occurs as a result of interactions between the very large aggregates that the OAM predicts should gradually form as the ionomer concentration increases. (C) 1998 Elsevier Science Ltd. All rights reserved.