Abstract
Differential thermal analysis was performed at pressures from about 400 to 600 MPa to determine the phase diagram of a series of polyethylene from low (2×103) to ultrahigh molecular weight (2.5×106). The different phase diagram was drawn for the high pressure crystallized sample of as-polymerized powder and that of bulk kneaded in the melt at atmospheric pressure. For both samples, the phase transition temperature (Tt) from the extended-chain crystal (orthorhombic) to the high pressure phase (hexagonal) and the melting temperature (Tm) of high pressure phase increase with the molecular weight in the same manner up to the molecular weight about 105. Above this molecular weight Tm and Tt of the bulk sample decrease with increasing the molecular weight, while those for the powder sample increase slightly. This decrease of Tm and Tt in the bulk sample is most probably based on entanglements introduced during kneading at atmospheric pressure.
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Yasuniwa, M., Haraguchi, K., Nakafuku, C. et al. Effect of Molecular Weight on the Phase Diagram of Polyethylene under High Pressure. Polym J 17, 1209–1219 (1985). https://doi.org/10.1295/polymj.17.1209
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DOI: https://doi.org/10.1295/polymj.17.1209
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