Small-Angle Neutron Scattering Analysis of Blends with Very Strong Intermolecular Interactions:  Polyamide/Ionomer Blends

Partially miscible blends of the lithium salt of sulfonated polystyrene ionomer (Li−SPS) and a methylated polyamide (mPA) were studied using small-angle neutron scattering. The blends exhibit lower critical solution temperature behavior (LCST), and the LCST increases with increasing sulfonation level of the ionomer. When the sulfonation level of the ionomer was below 20 mol %, a peak was observed in the scattering structure factor. The position of the peak moved to lower wavevector with increasing sulfonation level and with increasing temperature. The origin of the peak was a correlation hole due to the essential formation of a graft copolymer by the formation of an ion−dipole complex between the amide and sulfonate groups. The tendency of the unsulfonated polystyrene to phase separate from the polyamide was suppressed by the complexation, but it also produced large concentration fluctuations in the blends. The size and intensity of the concentration fluctuations increased with increasing temperature, i.e., as the LCST phase boundary was approached, and decreased with increasing sulfonation level as the length of the unsulfonated chain between sulfonate groups (and complex sites) decreased. The SANS data were successfully modeled with a modified form of the de Gennes model for scattering from a cross-linked polymer blend, which allowed for calculation of an interaction parameter, χ, for the system. In general, χ decreased with increasing sulfonation level and with increasing ionomer composition.