On specific factors affecting the crystallization of PET: role of carboxyl terminal groups and residual catalysts on the crystallization rate

The non-isothermal crystallization rate of poly(ethylene terephthalate) (PET) has been studied in differential scanning calorimetry experiments, by cooling from the molten state of samples of various molecular weights, prepared using various catalysts (mainly Ti(OBu)4). The crystallization temperature, Tc, was obviously influenced by the molecular weight, but also by the carboxyl group content, by the type of residual catalyst, by dissolution-reprecipitation treatments and by re-melting phenomena. The observed effects were interpreted assuming that interactions of terminal groups of the PET chains (mutual or with residual catalyst) led to an apparent increase in molecular weight, which in turn determined a decrease in the chain mobility and was responsible for a decrease in the rate of crystallization. Thus, a higher concentration of carboxyl terminal groups as well as interactions of PET terminal groups with the catalyst decreased Tc. Any factor which was able to destroy these interactions (treatments of dissolution-reprecipitation, phosphorous derivatives, aging) led to an increase of the overall crystallization rate.