Glass transition kinetics of amorphous polymers
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The glass transition temperature (Tg), an important characterizing parameter for amorphous materials, is correctly measured only on cooling, whereas the limiting fictive temperature (Tf') is measured on heating. Both parameters depend on the rate of cooling. In this work a comparison of the values of Tg measured on cooling and Tf' measured on heating is performed for a polystyrene sample using both capillary dilatometry and DSC. The results indicate that Tg is systematically lower than Tf', presumably due to the breadth of relaxation on cooling. The Tool-Narayanaswamy-Moynihan (TNM) model is used to fit the experimental data in order to ascertain the origins of higher value of Tg compared to Tf'. The values of Tg and Tf' are used to examine the relationship between the timescales of volume and enthalpy relaxation. The analysis in this study suggests that both properties exhibit similar timescales at temperatures above and below the nominal Tg. The divergence of times required to reach equilibrium noted in the literature at temperatures several degrees below than nominal Tg is attributed to the effect of nonlinearity. Experimental results are presented that corroborate this hypothesis. The data from dilatometry and DSC measurements is also used to apply an isoconversion analysis to determine the variation of activation energy throughout the glass transition. Although isoconversion methods have been used in the literature to determine the variation of activation energy with conversion through the glass transition using DSC heating curves, the results in this work demonstrate that the method should be applied on cooling rather than heating. In addition, it is shown that the conversion dependence is simply due to the non-Arrhenius temperature dependence known to be exhibited by glass-forming materials near Tg. Finally, the ability of the TNM model to describe the enthalpy relaxation data obtained at low heating rates for polycarbonate blends is examined in order to test the sensitivity of the model parameters to changes in chemical composition.