Heat capacity functions of polystyrene in glassy and in liquid amorphous state and glass transition

Abstract

The heat capacity or the specific heat is for any crystalline, partially amorphous or completely amorphous substance or material a significant thermodynamic property. The glass transition may be regarded as the melting point of amorphous substances and materials, a transition property of an outstanding technical importance. A crucial point is the fact that the presence of a glass transition is an unequivocal proof of an amorphous content of a material. Furthermore, the change of the specific heat at the glass transition temperature enables the quantitative determination of the amorphicity on a relative or absolute level of any substance or material. The absolute determination of the amorphicity affords a calibration with a reference corresponding to the material under investigation. The crystallinity for this reference substance must be known from the preparation and or by any independent analytical method. The literature data for the specific heat and the glass transition of polystyrene were collected and evaluated. Data were found for the specific heat in literature from 10 to 470 K. The data were unified for each of the reported temperature in a mean value and the corresponding standard deviation was determined. An excellent conformity was found in the glassy state of polystyrene with standard deviations lower than 0.7%. The standard deviations above the glass transition were considerably higher. All these literature data were transferred for each of the literature sets separately into linear specific heat functions in the vicinity of the glass transition. One set of our measurements performed with the DSC 204 and with polystyrene SRM 705a as sample material was additionally integrated in the mean of these functions for the glassy state and the liquid amorphous state respectively. The addition of our results gave practically no change of the mean coefficients and only a decrease of the standard deviations. In such a way, the data with the best statistical base for the specific heat of polystyrene are listed in this paper ( ‘Conclusions’). The glass transition as a transition in and out of a non-equilibrium state, the glassy state, is sensitive to all kind of influences such as thermal and mechanical treatments as well as to the selected experimental conditions. Therefore, certain standardized conditions procedures must be fulfilled to get reproducible data. The literature data for the glass transition temperature were also used to get a mean value. However, two values were omitted for the formation of the mean, because the authors reported values, which were too low on the base of impurities present. The mean value of the glass transition for polystyrene is according to the literature 369±2 K. A mean value of 370±2 K was extrapolated for an infinite molecular mass. The DSC and TMDSC measurements for the three thermodynamic properties reported in this paper, namely the specific heat, the glass transition temperature and the corresponding change of the specific heat gave results without significant differences compared with the literature values. Atactic polystyrene is a rather ideal polymer together with sapphire as calibration substance to elucidate and validate the DSC and TMDSC procedures for the determination of the specific heat and the glass transition.