Crystallization behavior and spherulite morphology of hard and soft segments in butylene terephthalate-?-caprolactone copolyesters

The crystallization behaviors and spherulite morphology of a series of butylene terephthalate–ϵ-caprolactone (BCL) copolyesters were explored with differential scanning calorimetry, polarized light microscopy, and wide-angle X-ray diffraction. The crystallization characteristics reflecting the segmented properties of BCL copolyesters are discussed. For BCL copolyesters with low or high hard-segment contents, the ϵ-caprolactone segments or butylene terephthalate segments are long enough to crystallize and even grow spherulites under appropriate conditions, and the crystallizability strengthens with increases in the corresponding segment sequence length. In BCL copolyesters, the crystallization of the soft segments requires a considerably long sequence length, whereas the hard segments even containing only one structural unit can still crystallize and even grow spherulites. The hard segments can grow the usual spherulites at a higher temperature like the poly(butylene terephthalate) homopolymer. The crystallizabilities of hard segments and soft segments for BCL and ethylene terephthalate–ϵ-caprolactone copolyesters are compared and discussed.