Energy dissipation study of atactic polystyrene melts above Tg

Energy dissipation apparatus had been used to study atactic monodisperse polystyrene in this work, the plots of energy dissipation versus temperature had been presented for two kinds of polystyrene with different molecular weight (Mw): one was 21,167, which was below the critical molecular weight (Mc≈35,000) for chain entanglement; the other was 118,095, which was above Mc. An energy dissipation peak appeared above Tg for both of the polymers, which demonstrated the existence of the liquid-liquid transition above Tg. Furthermore, it was found that the appearance of the entanglement constrained the mobility of the polymer chain obviously and it decreased the variability of the toughness and the length of the mobile segments. On basis of the relaxation theory, the reason of the appearance of the energy dissipation peak was given and it was ascribed to the segment oscillation under given periodic stress.