Ultrasonic Study of High Pressure Phase in Polyethylene

Behaviors of transitions and the nature of high pressure phase in polyethylene are studied using methods of longitudinal and transverse ultrasonic waves, differential thermal analysis (DTA) and X-ray diffraction by PSPC X-ray system. Two anomalous absorption peaks of longitudinal wave agree with the normal phase to high pressure phase transition temperature and the high pressure phase to fused phase transition temperature as observed by DTA and X-ray diffraction, respectively. For the folded-chain crystal sample, the recrystallization phenomena by lamella thickening during the heating process has been discovered by the anomaly in longitudinal sound velocity. The result of transverse wave shows that the modulus of rigidity of high pressure phase is of the same order in magnitude as that of the fused state. Therefore, the high pressure phase should be a nematic liquid crystal-like state.