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 β 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 γ‐peak at around °50°C, while the hydrostatically extruded samples and the extended samples have no γ‐peak. Consequently, the hydrostatic pressure suppresses the formation of the molecular structure relevant to the γ relaxation.