The influence of electrostatic interactions on the crystallization of sulfonated syndiotactic polystyrene ionomers
Sulfonated syndiotactic polystyrene (SsPS) was utilized as a model system to understand the fundamental link between ionic aggregation and crystallization in semicrystalline ionomers. Syndiotactic polystyrene was sulfonated in a postpolymerization sulfonation reaction using acyl sulfate complexes in solutions of 1,2,4 triclorobenzene, 1,1,2 trichloroethane and chloroform. The sulfonation efficiency of the acyl sulfate increased significantly when anhydrides containing long aliphatic groups were used to complex sulfuric acid. The increased sulfonation efficiencies were attributed to the increased solubility of the longer-chained hydrocarbon complexes in the nonpolar solvents. The incorporation of small quantities (less than 3.4 mol %) of ionic groups onto the syndiotactic polystyrene backbone was found to have little effect on the glass transition temperature but had a significant effect on the crystallization of these new materials. The melting point, degree of crystallinity, and the rate of crystallization decreased with increasing ion content. In addition, the strength of the electrostatic interactions associated with the neutralizing counterion influenced the crystallization of these ionomers. For crystallizations at temperatures greater than 180$\sp\circ$C, the rate of SsPS crystallization was observed to decrease with decreasing alkali metal counterion size. In contrast, isothermal crystallizations below 180$\sp\circ$C showed no influence of the counterion size on crystallization rate. These crystallization kinetics data suggest that, at temperatures greater than 180$\sp\circ$C the strength of the ionic interactions plays an important role in governing the rate of crystallization, whereas at lower temperatures, the ionic groups act as simple multifunctional cross-links. In addition to the effect of ionic interactions on the rate of crystallization, the polymorphic crystal structures of sPS were strongly influenced by the incorporation of small quantities of ionic groups along the polymer backbone. In contrast to syndiotactic polystyrene homopolymer which favors the $\alpha$ crystal form, the SsPS ionomers were found to preferentially organize into the $\beta$-form crystal structure. Since strong electrostatic interactions between sodium sulfonate groups along the polymer backbone significantly inhibit the crystallization of these materials from the melt, the addition of small amounts of the sodium salt of dodecylbenzene sulfonic acid was used to selectively plasticize the ionic domains of SsPS ionomers. This preferential plasticization was found to increase the molecular mobility of the polymer chains which enhances the rate of crystallization and the resulting degree of crystallinity.