Miscibility and crystallization behavior of poly(ethylene terephthalate)/poly(vinylidene fluoride) blends

Poly(ethylene terephthalate) (PET) and poly(vinylidene fluoride) (PVF2) are miscible in the melt-state for the whole composition range. The glass transition temperature (T-g) of the solvent cast film decreases with the decrease in W-PET (weight fraction of PET) in the blend, however, the T-g for the repeated melt quenched blends remains invariant with W-PET. The melting point (T-m) and crystallization temperature (T-c) of PET decrease significantly with decrease in W-PET in the blend, but the T-m and T-c of PVF2 decrease slightly with increase in W-PET. The crystallinity of both PET and PVF2 decreases with increasing concentration of the other component in the blend, however, the decreasng is larger for the former. The equilibrium melting points (T-m(0)'s) of PET in the blends are determined by the extrapolation procedures using (i) T-m-T-c method for 5% crystallinity and (ii) T-m-T-a method, where T-m, T-c and T-a are melting, crystallization and annealing temperatures, respectively. The data of both the methods indicate a large depression of T-m(0) of PET with increase in PVF2 concentration. The chi(12) values determined from both the data are essentially the same, -0.14. This negative value of chi(12) indicates that the two polymers are miscible in the melt-state, however, they are not miscible in the crystalline state. The onset of degradation of PET increases with increase in PVF2 concentration in the blend.