Solid Cure of Poly(phenylene sulfide): Competition between Increasing Nucleation Density and Decreasing Growth Rate

Commercial cured poly(phenylene sulfide) (PPS) is produced by the thermal oxidation treatment of linear PPS in the solid state. Branching and oxidation reactions occur, and correspondingly, the crystallization behaviors of PPS change a lot. A linear pristine PPS and its acid-treated control PPS were carefully designed in this work, and crystallization behaviors of samples with different cure times were systematically investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). It was found that although primary nucleation density increases with cure time for both groups, Tc showed a decreasing trend for cured acid-treated PPS while an increasing trend for cured pristine PPS. The isothermal crystallization kinetics were quantitatively characterized based on the Avrami equation and Lauritzen–Hoffman secondary nucleation theory. It was shown that the spherulite growth dimension decreases with cure time and the growth rate decreases contrary to the rising secondary nucleation rate. The competitive results between increasing primary nucleation density and decreasing growth rate gave different isothermal crystallization rate versus cure time patterns for these two groups. Specifically, for cured acid-treated PPS, the crystallization rate first decreased, then increased, and finally decreased again, which is different from previous reports. Besides, the spherulites became gradually more irregular along with the growing nucleation density. We also found that enhanced melt memory accounts for the increasing nucleation density. It is proposed that lamellar crystals are cross-linked by curing reactions, making them more difficult to melt or fully relaxed. Thus, more local ordered structures are formed in premelting, which behave as nucleation sites.