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

Physical aging of a glass decreases its volume, , entropy, and enthalpy, , 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, = (d/d), against 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 -(d/d) against , which resembles the thermal hysteresis observed in the against 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 and volume of activation for structural relaxation time, Δ*, of atactic poly(propylene) to investigate the effect of aging pressure, , of aging time, , and of the pressurizing rate on aging features in PSV scans. The scans show a post- feature on depressurizing before the -(d/d) overshoot peak appears. We provide quantitative plots (i) of the monotonic decrease of and increase of fictive pressure, , with and (ii) of the memory (Kovacs) effect in and of the polymer and (iii) provide generic plots of -(d/d) against for different combinations of β, , and Δ*. 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 computer-controlled, high-pressure equipment.