| One cause of long-term dimensional changes in glassy polymers is the gradual evolution of the viscoelastic behavior through aging processes in the glassy state. Many applications of polymers involve changes in relative humidity (RH), under which the materials exhibit aging processes that may differ from those in constant RH conditions. In this work, results from a study of a glassy epoxy subjected to isothermal RH-jumps are reported. Similar to the temperature jump experiments of Kovacs, the volume recovery responses in different histories as intrinsic isopiestic (constant RH), memory effect and the asymmetry of approach are obtained. In addition, the effects of structural recovery on sorption and desorption of water and the physical aging responses of this epoxy resin are also measured. The experimental results qualitatively support the hypothesis that water has an effect similar to temperature on structure (volume, enthalpy) of the glass-forming material. However, quantitatively, at the same RH-temperature states, the glasses formed by RH jumps shows anomalous difference from those formed by temperature jumps. The conventional TNM-KAHR model is modified to fit the experimental results of isothermal RH jump experiments. Furthermore, application of the potential energy landscape theory to describe the glasses formed by relative humidity jumps, and especially to address the observed differences between temperature glass and concentration glass is discussed. |
