MELTING BEHAVIOR OF POLY(PHENYLENE SULFIDE) .1. SINGLE-STAGE MELT CRYSTALLIZATION

We have studied the melting behaviour of poly(phenylene sulphide), PPS, that has been crystallized from the melt over a wide range of undercooling conditions. Two grades of PPS were used in the study: low molecular weight Ryton V-1 and an experimental medium molecular weight film grade. In this work we report the melting behaviour after a single stage of isothermal melt crystallization. In nearly all cases, dual or triple melting endotherms are seen, but the location and shape of the uppermost endotherm depend upon the degree of undercooling from the melt and on the prior thermal history. At a low degree of undercooling, for both materials, dual endotherms are observed and both melting points increase with the crystallization temperature. Using the immediate rescan technique, we show that the dual endotherms exist together very early in the crystallization process, when only a small fraction of crystals have formed. These results suggest that multiple crystal perfections form early on in the crystallization at low undercooling. As the degree of undercooling increases, the melting point of the uppermost endotherm becomes independent of the crystallization temperature. This result, and the appearance of a triple endothermic response for Ryton at the highest undercooling. indicate that now reorganization of imperfect crystals is dominating the observed endothermic response. We present a model in which the distribution of crystal perfections created during melt crystallization controls the multiple melting behaviour of PPS. At low undercooling conditions, a bimodal distribution of crystals can form which eventually may become two morphologies. At high undercooling conditions, a broad distribution of crystals can form, a part of which may melt and reorganize during the normal d.s.c. scan.