The Physical Aging and Structural Recovery Responses of a Polymer Glass after CO_2 Pressure Jumps

摘要

When a non-crystallizable polymer is removed from its equilibrium in the liquidrnstate by a sudden decrease in temperature to below the glass transition temperature, itrnreaches a new glassy, non-equilibrium state. This state has a specific volume and anrnenthalpy higher than the underlying equilibrium one, so that the volume, under isothermalrnconditions and in the absence of any external stimulus, decreases over time towards thernequilibrium value. This phenomenon is known as structural recovery. Structural recoveryrneffects on other material properties, such as optical or mechanical properties, are knownrnas physical aging. It is also well known that small molecules affect the viscoelasticrnresponse of polymers by, for example, depressing the glass transition temperature of thernmaterial. Therefore, one can anticipate that a rapid change in plasticizer content such asrnCO_2 or H_2O would result in a change in the volume and viscoelastic response of thernmaterial. Here, we describe previously physical aging results for epoxy films subjectedrnto sequential creep tests after carbon dioxide pressure down-jumps at constantrntemperature and after temperature down-jumps at constant carbon dioxide pressure. Thernaging rate for the CO_2-jump was slightly lower than that for T-jump and the retardationrntime for PCO_2-jump experiments was up to 6.3 times longer than observed in T-jumpsrn(1). The work is extended here by volume recovery measurements on the same epoxyrnsubjected to isothermal PCO_2 jumps. Similar to the temperature jump experimentsrnperformed by Kovacs (2), we show the volume recovery responses in different historiesrnas intrinsic isopiestics (constant PCO_2), asymmetry of approach and the memory effect.rnIn addition, we present preliminary results on the sorption/desorption of CO_2 byrnpoly(vinyl acetate) (PVAc). The experiments were performed at 24℃. The desorption ofrnCO_2 by the epoxy showed a glass transition in mass.