Morphological studies of ionic random graft copolymers based on scattering techniques

In this work, we have studied the morphology of an ionic random graft copolymer, partially sulfonated poly([vinylidene difluoride-co-chlorotrifluoroethylene]-g-styrene) [P(VDF-co-CTFE)-g-sPS], using small angle X-ray and neutron scattering (SAXS and SANS), and wide angle X-ray scattering (WAXS) techniques. SAXS and SANS measurements were used to investigate the phase separation and morphology of the polymer at nano-scale lengthscales, and WAXS measurements were used to determine the degree of the crystallinity of the polymer. We have developed a self-consistent morphological model for the studied system. In this model, dry samples consist of quasi-spherical fluorous domains embedded in a continuous phase of partially sulfonated polystyrene. We were able to characterize the size and spacing of the domains and also quantify the degree of phase separation. Our analysis shows that crystallinity acts as a hindering factor in the phase separation of the fluorous domains. Moreover, we find that, generally, partially sulfonated samples possess a less-developed phase separation than the unsulfonated or fully sulfonated samples. The morphology of the wet samples consists of fluorous domains as well as quasi-spherical water-rich domains. We find that samples swell homogeneously atthe nano-scale level and that the swelling is consistent with water content measurements for these samples, as long as the water content is < 80% vol. We have also characterized the spherical water-rich domains of the samples and argue that these water-rich domains are connected with nano-channels of water containing ionic acid groups. Our results suggest that conductivity of the membrane is proportional to the size of the spherical water-rich domains and to the water volume associated with each domain, but is inversely proportional to the spacing between them.