The cold crystallization of PET and its blends with polystyrene (PS) was investigated by differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMA), scanning electron microscopy (SEM) and mechanical properties. PET/PS blends form separate phases, as confirmed by SEM and DMA, showing distinct phases and two glass transition temperatures. On the other hand, the determination of the equilibrium melting temperature of PET indicated that this parameter decreased when PS was added, suggesting that a limited solubility should exist. The presence of non crystallizable molecules like polystyrene, partially soluble in the PET crystallizable phase, reduces the driving force for crystallization. In the current study this effect was observed as a shifting of the cold crystallization DSC peaks to higher temperatures and also by the reduction in the rate of cold crystallization. The approach developed by Avrami was applied to study the kinetics of cold crystallization under isothermal conditions. It was shown that the crystallization occurs in two stages and that the rate constant K decreased significantly when PS was present. The blends containing only 1% of PS had the same magnitude of reduction in the rate of crystallization as the blends with higher PS content. This behaviour has a high practical significance since in low concentrations of PS the mechanical properties of PET are not affected, as also shown in this study.
Isothermal cold crystallization; PET; polystyrene; blends
