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

Abstract

Poly(ethylene terephthalate) (PET); 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 Tg for the repeated melt quenched blends remains invariant with WPET. The melting point (T_m); crystallization temperature (T_c) of PET decrease significantly with decrease in WPET in the blend; but the T_m; 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 decrease is larger for the former. The equilibrium melting points (T_m⁰s) of PET in the blends are determined by the extrapolation procedures using (i) T_m-T_c method for 5% crystallinity; (ii) T_m-T_a method; where T_m; T_c; T_a are melting; crystallization; annealing temperatures; respectively. The data of both the methods indicate a large depression of T_m⁰ of PET with increase in PVF ²concentration. The χ₁₂ values determined from both the data are essentially the same, -0.4. This negative value of χ₁₂ 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.