Multiple melting and crystal annealing of poly(ethylene terephthalate) in its blends with poly(ether imide)

The melting behaviour and crystal annealing of poly(ethylene terephthalate) (PET) in its blends with poly(ether imide) (PEI) have been investigated by differential scanning calorimetry (d.s.c.). The multiple melting endotherms of PET were considered to arise from the occurrences of initial melting, recrystallization, and remelting processes in the melting region. The effects of blend composition, heating rate and crystallization duration on the multiple melting behaviour of PET/PEI blends were examined. It was found that the extent of recrystallization during d.s.c. heating decreased with increasing PEI composition and crystallization duration. The extent of recrystallization was suggested to be controlled by the degree of undercooling, T-^○ _m T_m, and the remixing after initial melting. The reduction in extent of recrystallization of PET upon blending with PEI was attributed to the remixing between PEI and PET after the initial melting. The effect of blend composition on the annealing rate of PET crystals was also evaluated from the change of the onset of melting with annealing time. The results showed that the PET crystals exhibited a slower annealing in the blends than in the pure state. This observation was correlated with the molecular morphology in the crystalline/amorphous blends.