In this study, we investigated the relationship between the amorphous structural changes associated with enthalpy relaxation and the Heat distortion temperature (HDT) in polystyrene injection moldings.
It was clarified that the longer heat treatment time, the more enthalpy relaxation progressed. Also, an extrapolation glass transition temperature (Tig) increased by heat treatment. In addition, long-time heat treatment caused the high-temperature shift of the storage modulus (E') decrease temperature near glass transition temperature (Tg). Since the increase in Tig and the high-temperature shift of E' decrease temperature correlated with the enthalpy relaxation, they can be closely related to the enthalpy relaxation. Thus, we considered that the segment motion of polymer chain was suppressed as the enthalpy relaxation progressed.
At long-term heat treatment, the enthalpy relaxation and the HDT were correlated. Furthermore, density increased by the heat treatment, and the increase in density also correlated with the progress of the enthalpy relaxation. Therefore, we considered that the polymer chains densely packed along with the enthalpy relaxation. As the result of polymer chains being densely packed, the segment motion of polymer chains near Tg can be suppressed and the HDT increased.
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