Structural recovery and physical aging in polymer glasses in plasticizing environments
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Small molecule plasticizers impart significant effects on the viscoelastic and mechanical responses of polymer glasses. The effects of plasticization on the structural recovery and mechanical responses of polymer glasses have been investigated using carbon dioxide as a small molecule plasticizer and an epoxy resin and polystyrene as model polymer glasses. This work reports the first physical aging results on a polymer glass subsequent to carbon dioxide pressure jumps. Also reported are the first structural recovery results of a polymer glass subsequent to small molecule plasticizer jumps. Consistent with the hypothesis that for a polymer glass, a change in plasticizer concentration is similar to a change in temperature, the three signatures of structural recovery; intrinsic isopiestics, asymmetry of approach, and the memory effect were constructed and shown to be qualitatively similar to those observed after temperature jumps. However, quantitative and anomalous differences were also observed. This work was modeled and illustrated the limitations of current phenomenological models. Subsequent work showed that, in fact, concentration glasses are fundamentally different from temperature-hyperquenched glasses. An attempt to investigate the real-time monitoring of mass change during the structural evolution of a polymer glass was made. The results report serious fundamental errors in the use of piezoelectric devices for the investigation of mass changes in compliant materials. Since these devices have been used as sensitive mass sensing devices, these results exposed their limitations. A semi-quantitative model of errors induced is also presented. The anomalous differences reported above led to the motivation to complete the thermodynamic surface of polymer glasses by studying the calorimetric response after carbon dioxide pressure jumps. To date there are no results reported in the literature on a direct measurement of the enthalpy recovery for glassy polymers subsequent to changes in temperature or small molecule concentration. This work reports the first measurements of the enthalpy recovery responses of polymer glasses subsequent to plasticizer concentration changes by measuring the heat flow under isothermal conditions as the structure evolves. The first intrinsic isopiestics, asymmetry of approach, and memory effects in enthalpy recovery after carbon dioxide jumps for polystyrene and the epoxy resin are reported.