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

Abstract

The Tll 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 i olyisobutylene bowed that the increase in Cp above Tg does not follow the quadratic usually assumed, but is much better fitted by three straight lines. We identity the tirst intersection with Tu a liquid-liquid transition. By analogy, DSC thermograms on fused films of polystyrene should show an endothermic slope.change above T9. For anionic polystyrenes T11 increases with increasing molecular weight but levels off asymptotically at a limitin1 C of around 435 K. This is called T11 (-) he rati , T111T. is essentially constant with a range of 1.15 to ~k~ 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 T11 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 T11. The intensity of the T11 transition was also found to decrease as a function of crosslink density in a series of polystyrenes crosslinked with 0 to 5.5% divinylbenzene. hin1vue 1.1 0 2,~;~~ reoes not change naC,1 ar aseeaso ecii is C T11 alled Ar 0 .0, s w t C C rid ~43"m' u 5 v ra 1.17an average o . oF iix- specimens va.~ing in molecular weight from 600 I oe, t d _ 'Iban e 2,e ~ccr~iica weig or c ain entanglement, as it does when measure The discontinuity in dCP1dT is characteristic of n apparent third oilder~ltransiti:.n. >possible physical interpretation in terms of a third order traT11 is interpreted as a basic molecular process related to the breaking of weak secondary bonds, with an accompanying drop in the dynamic elastic modulus. A brief survey of T11 observations in several other polymers is presented and the relevance of our results to recent DSC studies of polystyrene is discussed.