The influence of pressure on relaxations in glassy-state polymers

Glass-forming high polymers have been densified by application of high hydrostatic pressure (≈ 1.5 kbar) in the melt, followed by cooling under pressure to ambient. A density increase of about 1% was induced in each of the following polymers: polyvinyl chloride, polymethyl methacrylate, polystyrene, poly-4-chlorostyrene, poly-3-chlorostyrene, poly-4-methoxystyrene and poly-4-phenoxystyrene. Differential thermal analysis (DSC) and volume relaxation techniques were used to study the reversion of the densified glass to a more normal glass at a temperature ≈Tg–15K in general. Enthalpy relaxation (a change from glass I to glass II) in this region gives a peak or diffuse hump on the DSC scan prior to a normal glass transition temperature. It is considered that although the densified glasses may become thermodynamically stable at a sufficiently low temperature they are inherently unstable at ambient. Reversion to a more normal class is kinetically too slow to measure at ambient in all cases studied except polymethyl methacrylate. [Continues.]