Crystallization kinetics and melting behavior of poly(butylene isophthalate/terephthalate) random copolyesters

The melting behavior of a series of random poly(butylene isophthalate/terephthalate) (PBIPBT) copolymers of various compositions was investigated. In the DSC scan performed after isothermal crystallization, multiple endotherms were experimentally evidenced in the case of the samples containing the highest amounts of butylene terephthalate units, due to melting and recrystallization processes. Wide-angle X-ray diffraction measurements permitted to identify the crystalline structure of PBT in all the copolymers, except in the case of the sample containing the lowest amount of PBT units, where the PBI lattice was detected. The heat of fusion (ΔHm) was evaluated and correlated to the specific heat increment (Δcp) for samples with different degree of crystallinity, in an attempt to calculate ΔHm∘ for PBI and PBT. The observed trend was interpreted on the basis of the existence, besides the crystal and amorphous phases, of an interphase due to polymeric amorphous chains linked to the crystal surface. As concern the crystallizaton kinetics, the Avrami equation was applied and the values of the exponent n, nonintegral and lower than 3 in all cases, indicate an irregular spherulitic growth, which was confirmed by optical microscopy investigations. The dependence of the crystallization half-time t1/2 on the crystallization temperature was found to be characterized by a minimum for the 40PBI60PBT copolymer, the crystallization kinetics being controlled by the nucleation or by the polymer chain diffusion, depending on the value of Tc adopted.