Effect of chain length of α,ω-aliphatic diacid on melt crystallization and optical, mechanical and gas barrier properties of aliphatic-aromatic PBXT copolyesters

Two series of high molecular weight aliphatic-aromatic random copolyesters: PBXT, where X represents the carbon atom number of the aliphatic α,ω-diacids, PBXT₄₈ and PBXT_50w (X = 4–14, even), with molar and mass copolymer compositions (48 mol% and 50 wt%) equal to those of commercial poly(butylene adipate-co-terephthalate) (PBAT) were synthesized from butanediol, terephthalic acid and various aliphatic α,ω-diacids. They were characterized with ¹H NMR, and their melt crystallization, optical, mechanical and gas barrier properties were assessed. In these copolyesters, the melt crystallization of butylene terephthalate (BT) sequence is dominant, while the butylene adipate (BX) sequences can also crystallize from melt when the BX content is high enough, the chain length of α,ω-diacid unit or the crystallization time is long enough. Long enough chain length of α,ω-diacid unit (X ≥ 6) endows PBXT with sufficient chain flexibility/mobility and, therefore, excellent melt crystallizability at rapid cooling. For PBXTs with X ≥ 6, the effect of X on BT crystallization is not monotonic. In particular, both PB6T and PB12T displayed a higher melt crystallization temperature than the adjacent PBXTs. Compared to PB6T, PB4T has higher transparency and gas barrier properties; with increasing X from 6 to 14, the PBXTs show increased transparency and water vapor barrier, but decreased oxygen barrier performance. The tensile modulus and strength of PBXT decrease, and the elongation at break increases with increasing X. But the effect of X is clearly weakened for PBXTs with the same mass composition of 50 wt%.