NMR (Nuclear Magnetic Response) Study of Polyethylene Crystallization Kinetics under High Pressure

摘要

Crystallization of polyethylene under hydrostatic pressures of 1 - 4.5 kbar is directly observed using pulsed proton NMR. The rate of growth of extended-chain polyethylene crystals is measured over this pressure range and to a maximum temperature of 227 C. The observed crystallization isotherms are superimposable on a log time scale; this implies a consistent mechanism for extended-chain growth over this pressure range. Avrami coefficients for high-pressure extended-chain crystallization are determined to be 1.3 - 1.7. A decrease of crystal nucleus surface free energies with increasing pressure is indicated. Findings are consistent with Wunderlich's model of initial folded-chain crystallization followed immediately by chain extension. Future applications of this NMR technique are briefly considered.