Morphological Evolution of Ionomer/Plasticizer Mixtures during a Transition from Ionomer to Polyelectrolyte

X-ray scattering data were used to examine the morphological evolution that accompanies the ionomer-to-polyelectrolyte transition. Bulk random ionomer based on methacryl diglyme side chains and sodium sulfonated styrene exhibits an amorphous halo, a backbone correlation peak, and an ionic aggregate spacing peak in X-ray scattering. The ionic peak intensifies as either polymer content or temperature increases due to enhanced aggregation, since the dielectric constant of the polar liquid decreases as temperature is raised. Addition of polar plasticizer into the ionomer softens the ionic interactions by raising the dielectric constant, which weakens the ionic peak, leading to more polyelectrolyte character at higher plasticizer content, with many dissociated Na counterions. There is a wide range of ion content and dielectric constant (gray region in graphic) within which these materials transition from ionomer to polyelectrolyte as the polar plasticizer is added, and both the ionic aggregation peak of the ionomer and the correlation length (interchain spacing) of polyelectrolyte solutions can be resolved. The complete transition to polyelectrolyte occurs when the average distance between the ions becomes larger than the Bjerrum length, so that many Na counterions dissociate, and the ionic aggregate spacing peak disappears.