MICROHARDNESS AND STRUCTURE OF HIGH-PRESSURE CRYSTALLIZED POLY(ETHYLENE-TEREPHTHALATE)

The microindentation hardness (H) of PET samples crystallized isotherm ally at various temperatures from the melt under high pressure (4kbar) was determined in order to establish correlations with the thermal pr operties and microstructure. The results reveal that these materials s how density, enthalpy of fusion, and melting temperature values which are close to those corresponding to chain extended crystals. The high crystallinities obtained (alpha = 0.70-0.95) for high pressure crystal lized PET are shown to give rise to unprecedentedly high microhardness values, ranging between 300 and 400 MPa. The data are analysed on the basis of a two-phase model which allows an estimation of the influenc e of crystal thickness upon H. Finally, the temperature dependence of high pressure crystallized PET is also examined.