ISSN:
0098-1273
Keywords:
Physics
;
Polymer and Materials Science
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
,
Physics
Notes:
A position-sensitive proportional counter (PSPC) x-ray measuring system is employed to observe directly phase transition processes of polyethylene at high temperature and high pressure. X-ray diffraction measurements reveal important new experimental data. First, an irreversible crystal transition from the hexagonal to the orthorhombic structures occurs in the critical region where the hexagonal structure begins to appear at a pressure of 350 MPa. That is, the (100) hexagonal reflection is observed only on cooling at 350 MPa. At pressures above about 400 MPa, however, the hexagonal phase is stable and the phase transitions melt ↔ hexagonal ↔ orthorhombic occur reversibly. Second, during cooling at pressures above 400 MPa, the (100) hexagonal reflection can be observed at temperatures below the hexagonal ↔ orthorhombic transition temperature. This behavior suggests that all the crystal morphologies of polyethylene, from “highly-extended-chain” crystals to crystals with a low melting point, are formed by the transitions melt → hexagonal → orthorhombic. Third, in heating at elevated pressures above 500 MPa, a shoulder in the peak intensity versus temperature plot for the (100) hexagonal reflection is observed at a higher temperature than the large maximum which occurs immediately after the crystal transition. This behavior indicates melting in two stages of hexagonal structures with different thermal stabilities, and the shoulder at higher temperature may be due to the fusion of the hexagonal phase annealed either below or above the transition point.
Additional Material:
11 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/pol.1981.180190903
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