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

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 beta and x and volume of activation for structural relaxation time, Delta V*, 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, pf, with tage and (ii) of the memory (Kovacs) effect in V and pf of the polymer and (iii) provide generic plots of -(dV/dp)(T) against p for different combinations of beta, x, and Delta V*. 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.