EXTENSION BEHAVIOR AND MORPHOLOGY IN POLYPROPYLENE EXTENDED UNDER HYDROSTATIC-PRESSURE

The effect of hydrostatic pressure on molecular deformation of polypropylene extended under hydrostatic pressure is investigated by using internal friction measurements, wide angle X-ray diffraction, and polarizing microscopy. A homogeneously transparent straight part is obtained by extending samples in the pressure range from 78 to 128 MPa. The overall mechanical properties of a sample extended under P = 102 MPa is significantly different from that of a sample extended under P = 128 MPa. The difference is related to the magnitude of hydrostatic pressure, the extension ratio, and the extent of transparency. The observed results on the beta-relaxation in the extended samples is found to be related to the morphological reorganization from a coarse spherulitic to a fine spherulitic structure. The cold-drawn sample at atmospheric pressure has the gamma-peak at around -50-degrees-C, while the hydrostatically extruded samples and the extended samples have no gamma-peak. Consequently, the hydrostatic pressure suppresses the formation of the molecular structure relevant to the gamma-relaxation.