Enthalpy Relaxation Study of Poly(Vinyl Chloride) Films Based on the Time-Temperature Superposition Principle

In the enthalpy relaxation of poly(vinyl chloride), a decrease in enthalpy upon the isothermal ageing was measured using the differential scanning calorimetry method as a function of ageing time (tA) and ageing temperature. The range of the ageing temperature was from 56 °C (Tg − 25 °C) to 72 °C (Tg − 9 °C) where Tg denotes the glass transition temperature. The limiting value of the decrease in enthalpy was determined by applying a stretched exponential function to the measured enthalpy data. The relaxation function (ϕ) was derived from the measured enthalpy and the construction of a master curve was tried by shifting the ϕ − tA curves of the respective ageing temperatures horizontally. Although there was no agreement between the shift factors (aT) and the relaxation times of the ϕ − tA curves, the superposition was successfully constructed and the aT values obtained for the poly(vinyl chloride) sample were found to be comparable to those reported for viscoelastic experiments over a broad temperature range above and below Tg carried out for different polymers. The origin of the decrease in enthalpy was briefly discussed in terms of the chain dynamics in the isothermal condition.