Pressure Scanning Volumetry of Physically Aged Polymer Glasses, Fictive Pressure, and Memory Effect

Abstract

Physical aging of a glass decreases its volume, V, entropy, and enthalpy, H, toward the equilibrium state values. For glasses usually formed by cooling a melt, the effect is modeled in terms of non-exponential, nonlinear structural relaxation by using a plot of the heat capacity, C p = (dH/dT) p , against T obtained from differential scanning calorimetry (DSC) cooling and heating scans. A melt becomes glass also on isothermal pressurizing and the glass formed becomes liquid on depressurizing, showing a hysteresis of the sigmoid-shape plot of −(dV/dp) T against p, which resembles the thermal hysteresis observed in the C p against T plots. By analogy with DSC, it was named pressure scanning volumetry (PSV). Here, we use the known values of non-exponential and nonlinearity parameters β and x and volume of activation for structural relaxation time, ΔV*, of atactic poly(propylene) to investigate the effect of aging pressure, p age , of aging time, t age , and of the pressurizing rate on aging features in PSV scans. The scans show a post-p g l feature on depressurizing before the −(dV/dp) T overshoot peak appears. We provide quantitative plots (i) of the monotonic decrease of V and increase of fictive pressure, p f , with t age and (ii) of the memory (Kovacs) effect in V and p f of the polymer and (iii) provide generic plots of −(dV/dp) T against p for different combinations of β, x, and ΔV*. The study is of academic significance because PSV scans show a change in the density fluctuation response. It is of technological significance in polymer-extrusion processing and it may stimulate the commercial development of computercontrolled, high-pressure equipment.