Effects of molecular weight on the structure of poly(phenylene sulfide) crystallized at low temperatures

We report a study of the effects of molecular weight on the structure and properties of poly(phenylene sulfide), PPS, crystallized from the rubbery amorphous state at temperatures just above the glass transition. PPS films were characterized using temperature-modulated differential scanning calorimetry (MDSC), small angle X-ray scattering (SAXS), and dynamic mechanical analysis (DMA). For samples crystallized so as to have the same half-time, we find (1) the degree of crystallinity, chi(e), is not dependent upon molecular weight and (2) as the molecular weight increases, the heat capacity increment at the glass transition increases. These results suggest that lower molecular weight PPS contains a greater fraction of the rigid amorphous phase, probably as a result of formation of taut tie molecules between crystals. It is found from DMA that molecular weight is the determining factor that affects the Young's modulus value at the higher temperature. A lower E' is observed for the higher molecular weight material. MDSC data suggest that higher molecular weight PPS has a slower crystallization rate and reorganization rate. The amount of crystals formed during heating is smaller in higher molecular weight PPS, and this may be the explanation for the reduced Young's modulus above T-g.