Melting behaviour of poly (phenylene sulphide): 2. Multiple stage melt crystallization

Abstract

Multiple stages of melt crystallization have been used in this study to investigate the origin of the multiple melting behaviour of poly(phenylene sulphide), PPS. In a companion study we reported results of single-stage melt crystallization of PPS and presented a model to explain the observation of multiple melting and crystallization temperature dependency of the melting peaks. In our model, the degree of undercooling determines whether the existence of preformed crystals or reorganization of imperfect crystals dominates the observed endothermic response. In this work, multiple sequences of crystallization and annealing from the melt have been used to vary the perfection of the crystals. At low crystallization temperature (high degree of undercooling), very imperfect crystals are formed which can reorganize during the d.s.c. scan. In Ryton V-1, triple melting peaks are observed, and the endothermic response depends strongly on annealing time and scan rate. Annealing at elevated temperature, which perfects the crystals, or increasing the scan rate, which reduces the chance for reorganization, both result in a decrease in the size of the uppermost endotherm. However, at high crystallization temperature (low degree of undercooling), the pre-existing crystals are much more perfect and less susceptible to reorganization. Studies of multiple stage crystallization indicate that it is possible to generate many distinct crystal populations by step-wise crystallization from the melt. The pre-existing crystals display their own melting points, each in response to a separate stage of crystallization. As many as seven melting points can be seen after five stages of melt crystallization, indicating that morphology (pre-existing crystals) dominates the appearance of the endothermic response after multiple stages of melt crystallization.