Aging of oriented polymer glasses

Dimensional (D) and enthalpy relaxation (ΔH) of oriented polymer glasses (PS and PC) have been studied as function of temperature, between Tg and Tg−20 °C, and aging time t, ranging to several weeks. The dimensional relaxation (shrinkage) and enthalpy relaxation curves verify the logarithm law D(t) ∼ H(t) ∼ log t, between an incubation τi and a final relaxation time τf. The time τf to reach the equilibrium (D and ΔH) follows the Vogel–Tamann–Fulcher (VFT) law. Enthalpy relaxation and shrinkage exhibit important differences. Enthalpy relaxation of oriented and isotropic polymers follows the same logarithm law, independent of the draw ratio λ and the mode of deformation, the relaxation time τf coincides with the relaxation time of the α segmental motions. Shrinkage depends on λ and the mode of deformation, the relaxation time τf is attributed to the normal mode, the relaxation time of the whole chain. Finally the shrinkages of PS and PC show some differences. PC at short aging times presents another type of dimensional relaxation which would be due to the β motions. This would be in close connection with the ductile (PC) and fragile (PS) behavior of these two polymers far below Tg.