Environmental effects on the structural recovery responses of an epoxy resin after carbon dioxide pressure jumps: Intrinsic isopiestics, asymmetry of approach, and memory effect

Novel volume measurements on an epoxy resin subjected to carbon dioxide pressure jumps (PCO₂) are used to investigate the structural recovery of glassy polymers after plasticizer jumps. Previously, we had investigated an epoxy resin after plasticizer concentration jumps using relative humidity (RH) and CO₂ and evidenced structural recovery and physical aging responses.^1-6 In that work, two things were demonstrated: 1(a) qualitatively, the physical aging (after CO₂ and RH jumps) and structural recovery (after RHjumps) responses are similar to those observed after temperature jumps, and 1(b) quantitatively, the responses are not the same and, moreover, they exhibit anomalous behaviors. The purpose of the present work is to further investigate the nature of the anomalous behaviors by investigating the structural recovery response of the same epoxy when subjected to PCO₂-jump conditions. We provide new data using CO ₂-jumps and show intrinsic isopiestics, asymmetry of approach, and memory effect and then compare these with similar data for temperature-jump histories. We show that the extended Kovacs-Aklonis-Hutchinson-Ramos does not adequately describe the structural recovery of the epoxy resin after PCO ₂-jumps. This is consistent with what was previously observed in our laboratory onmodeling the structural recovery data of the epoxy resin subsequent to relative humidity changes. Finally, we show a further anomaly in that the volume in CO₂-jump experiments seems to evolve toward a different final state²³ than it does in temperature-jump experiments, suggestive of a metastable glassy state that differs from that of the temperature-jump created glass.