Enthalpy relaxation in polymer glasses: evaluation and interpretation of the Tool-Narayanaswamy parameter x for poly(vinyl chloride)

The enthalpy relaxation of poly(vinyl chloride) (PVC) in the glass-transition region has been investigated by differential scanning calorimetry (DSC), and the peak-shift method has been used to evaluate the Narayanaswamy parameter x, which controls the relative contributions of temperature and structure to the retardation times. The value of x = 0.27 +/- 0.05 which results from the present study is rather larger than the value of 0.11 found previously by Hodge for the same sample. Nevertheless, both values are considered to be small in comparison with those quoted for many other polymer glasses, which is consistent with its classification as a "fragile" system on the basis of Angell's concept of "strong" and "fragile" liquids.