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

The melting behavior of a series of random poly(butylene isophthalate/terep hthalate) (PBIPBT) copolymers of various compositions was investigated. In the DSC scan performed after isothermal crystallization, multiple endotherm s were experimentally evidenced in the case of the samples containing the h ighest amounts of butylene terephthalate units, due to melting and recrysta llization 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 th e PBI lattice was detected. The heat of fusion (DeltaH(m)) was evaluated an d correlated to the specific heat increment (Deltac(p)) for samples with di fferent degree of crystallinity, in an attempt to calculate DeltaH(m)degree s for PBI and PBT. The observed trend was interpreted on the basis of the e xistence, besides the crystal and amorphous phases. of an interphase due to polymeric amorphous chains linked to the crystal surface. As concern the c rystallizaton kinetics, the Avrami equation was applied and the values of t he exponent n, nonintegral and lower than 3 in all cases, indicate an irreg ular spherulitic growth, which was confirmed by optical microscopy investig ations. The dependence of the crystallization half-time t(1/2) on the cryst allization temperature was found to be characterized by a minimum for the 4 0PB160PBT copolymer, the crystallization kinetics being controlled by the n ucleation or by the polymer chain diffusion, depending on the value of T-c adopted. (C) 2001 Elsevier Science Ltd. All rights reserved.