The mechanism of heat resistance improvement of polystyrene injection moldings by heat treatment was studied. The increase in the Heat Distortion Temperature（HDT） by heat treatment was confirmed. Meanwhile, in dynamic viscoelasticity measurement, the shoulder of the loss tangent（tanδ） appeared at 60 to 90°C in untreated specimen. In the heat-treated specimen, the shoulder at that temperature range disappeared. Further, when the frequency dependence of tanδ was evaluated, it was found that the mobility of molecular chain in the main dispersion did not change by the heat treatment, and the mobility in the temperature range where the tanδ shoulder appeared was reduced by the heat treatment. By differential scanning calorimetry, the progress of enthalpy relaxation by heat treatment was confirmed. Also, excellent correlation existed between the progress of enthalpy relaxation and the disappearance of the tanδ shoulder by heat treatment. Since the tanδ shoulder disappeared in the heat-treated specimen in which relaxation has progressed yet, it was considered that the tanδ shoulder was caused by molecular motion accompanying the relaxation phenomenon. Addisionally, as well as the disappearance of the tanδ shoulder, the increase in HDT correlated with the progress of enthalpy relaxation by heat treatment. In conclusion, with the progress of the relaxation by the heat treatment, the tanδ shoulder disappeared, and thereby the molecular mobility at 60 to 90°C decreasesed. Since the HDT of the untreated specimen was 78°C, the HDT increased due to the decrease in the molecular mobility below 78°C by heat treatment. Thus, it is considered that the increase in HDT correlated with the progress of enthalpy relaxation by heat treatement.
