RELATING MICROHARDNESS OF POLY(ETHYLENE-TEREPHTHALATE) TO MICROSTRUCTURE

The microhardness (H) of poly (ethylene terephthalate) (PET) containing catalysts, as well as of PET without catalysts has been investigated. Two types of morphologies have been examined: (a) structures where spherulitic growth is incomplete, resulting from a primary crystallization from the glassy state and (b) samples in which spherulitic crystallization is completed. It is shown that for the former materials, H is an increasing linear function of the volume of the spherulites and depends on annealing time and catalyst content. For the latter materials, H is nearly constant with increasing annealing temperature (T(A)). Results are discussed in the light of three principal structural factors which determine the microhardness behavior: (1) the volume content of spherulites within the material; (2) the value of crystallinity within the stacks of the lamella, which turns out to be a constant; (3) the average thickness of the crystals, which increases slightly with T(A) after crystallization is completed. An expression which takes into account the above parameters and offers a description of the H of polyethylene terephthalate is proposed.