Crystallization Kinetics as a Probe of the Dynamic Network in Lightly Sulfonated Syndiotactic Polystyrene Ionomers

The effect of alkali metal counterion type on the crystallization kinetics of sulfonated syndiotactic polystyrene with ion contents of 1.4 mol % was investigated. For crystallization temperatures less than 180 °C, the rate of crystallization was independent of counterion type. In contrast, for temperatures greater than 180 °C, the rate of crystallization increased with increasing counterion size. Since the glass transition and melting temperatures were found to be constant for all counterion types, the variations in crystallization behavior were attributed to differences in chain diffusion within the dynamic network of electrostatic cross-links. At low temperatures, the rate of ion-hopping is slow for all of the counterion forms relative to the rate of crystallization, and thus crystal growth occurred in the presence of kinetically stable cross-links. At high temperatures, chain diffusion is controlled by the ion-hopping process, and the kinetics of crystallization become influenced by the strengths of ionic interactions. Between 180 and 215 °C, the rate of crystallization was found to be inversely proportional to the ionic radii of the counterions. As the activation energy for ion-hopping increased with decreasing counterion size, longer periods of time were required to achieve the same degree of crystallinity.