Time-temperature superposition in the enthalpy relaxation study of polystyrene

Abstract

Enthalpy recovery measurement was used to study the physical aging behavior of a polystyrene. The aging experiments at temperatures (TA) near Tg were conducted with aging times (tA) ranging from several minutes to several days. The recovered enthalpy, ΔH(tA, TA), increased with tA, and then a plateau value of ΔHplate appeared in the long time aging experiment. The master curve was successfully constructed concerning the normalized relaxation isotherm of ϕ for the temperature range of Tg−16 < TA < Tg−8 if ΔHplate was used to calculate the normalized term. This implies that the enthalpy relaxation evolves with the same molecular dynamics in respective TA. The shift factor in the master curve was interpreted in terms of the WLF (Williams–Landel–Ferry) equation to determine the coefficients for the superposition of ϕ–tA curves. A smaller value was found in c1 in comparison with those reported in the literature on polystyrene viscoelastic data, which was discussed in terms of temperature dependence of the relaxation rate in the enthalpy decay. POLYM. ENG. SCI., 56:561–565, 2016. © 2016 Society of Plastics Engineers