Differential Scanning Calorimetry (DSC) Observation of the T ll Transition in Polystyrene

Abstract

The T ll liquid-liquid transition in atactic polystyrene can now be determined reproducibly on fused films of polymer in standard DSC equipment. Computer analysis of published adiabatic calorimeter data in polyisobutylene showed that the increase in C p above T g does not follow the quadratic usually assumed, but is much better fitted by three straight lines. We identify the first intersection with T ll , a liquid-liquid transition. By analogy, DSC thermograms on fused films of polystyrene should show an endothermic slope change above T g . For anionic polystyrenes T ll increases with increasing molecular weight but levels off asymptotically at a limiting value of around 435 K. This is called T ll (∞). The ratio T ll /T g is essentially constant with a range of 1.15 to 1.17 and an average of 1.16 for six specimens varying in molecular weight from 600 to 2,000,000. It does not change nature at M c , the critical weight for chain entanglement, as it does when measured by torsional braid analysis (TBA) and by DSC on powders. The latter involve melt flow, whereas DSC on fused film does not. The magnitude of the slope change at T ll is found to be dependent on the thermal history of the specimen. The greatest effect is observed when the specimen is annealed for 20 to 60 minutes about 5 to 10 K below T ll . The intensity of the T ll transition was also found to decrease as a function of crosslink density in a series of polystyrenes crosslinked with 0 to 5.5% divinylbenzene.