Solid-state structures and thermal properties of aliphatic-aromatic poly(butylene adipate-co-butylene terephthalate) copolyesters

Aliphatic-aromatic copolyesters of poly(butylene adipate-co-butylene terephthalate) [P(BA-co-BT)] with wide copolymer compositions were synthesized by melt polycondensation. Solution C-13 NMR analyses show that the copolyesters are statistically random copolymers of BA and BT units. Their solid-state microstructures and thermal properties were investigated by wide-angle X-ray diffraction (WAXD), solid-state C-13 NMR, differential scanning calorimetry (DSC) and atomic force microscopy (AFM). The WAXD shows that the copolyesters with 10-25 mol% BT contain PBA crystals, while those with 27.5-80 mol% BT contain PBT crystals. In contrast, solid-state C-13 NMR analyses demonstrate that the copolyesters with 20-30 mol% BT units contain both PBA and PBT crystals, but the minor one cannot be detected by X-ray diffraction due to the small sizes and amounts. Both the melting temperature and crystallinity of copolyesters show minimum values at around 25 mol% BT content which is the transition point from PBA crystal structure to PBT crystal structure. The composition dependences of melting temperature, crystallinity, crystallization rate and spherulite morphology are discussed and correlated to the biodegradability of the P(BA-co-BT) copolyesters. (C) 2003 Elsevier Ltd. All rights reserved.