An investigation into the synthesis, properties, and biodegradation characteristics of copolyesters derived from 2,5-thiophenedicarboxylic acid

Abstract

The advancement of biodegradable polymers offers a promising alternative to traditional polyolefins. However, many biodegradable plastics, primarily aliphatic, exhibit inadequate mechanical and gas barrier properties for packaging. Recently, copolyesters with rigid aromatic components, such as 2,5-furandicarboxylic acid (2,5-FDCA), have gained attention for their exceptional gas barrier qualities due to the unique asymmetric structure of furan. Similarly, 2,5-thiophenedicarboxylic acid (TDCA), a bio-derived aromatic diacid akin to 2,5-FDCA, shows potential in creating biodegradable copolyesters. TDCA-based homopolyesters exhibit outstanding gas barrier properties, suitable for high-barrier packaging. This study investigates the effects of aliphatic monomers with varying chain lengths on the synthesis, properties, and biodegradation of TDCA-derived copolyesters, providing a foundation for developing viable alternatives to commercial packaging polymers.
High molecular weight copolyesters composed of TDCA and various aliphatic diols were synthesized utilizing a two-step melt polycondensation technique. The crystallization behaviour of TDCA-based copolyesters distinguishes them from those derived from 2,5-FDCA and terephthalic acid (TPA). Particularly, the copolyester employing 1,3-propanediol (PDO) demonstrates superior crystallization propensity compared to its counterparts utilizing 1,4-butanediol (BDO). Poly(propylene adipate-co-2,5-thiophenedicarboxylate) (PPATh) showcases a well-balanced array of characteristics, featuring a melting point of 88.5 ℃, a tensile strength of 6.2 MPa, and gas barrier attributes exceeding those of commodity poly(butylene adipate-co-terephthalate) (PBAT) by over threefold.
In order to prepare copolyesters based on PDO and TDCA with better overall properties, the type and content of diacid components were further adjusted. The synthesis of poly(propylene succinate/adipate-co-2,5-thiophenedicarboxylate) (PPXTh) copolyesters with varying levels of TDCA and succinic acid (SA)/adipic acid (AA) resulted in notable properties. PPATh70 exhibited the highest melting temperature of 144.8 ℃, while crystallization kinetics revealed dominance of diol-TDCA segments in crystalline phases, aided by long aliphatic AA units. PPXThs with over 50 mol% TDCA showed superior tensile modulus (102 to 734 MPa) compared to PBAT (100 MPa), along with outstanding gas barrier properties, the highest surpassing PBAT by over 500 times. Dynamic mechanical analysis suggested enhanced gas barrier due to suppressed localized chain motions and reduced free volumes. By comparing their residual weights after four weeks of hydrolysis, those containing TDCA of below 50 mol% underwent more significant degradation with lower residual weights (96.7% to 98.3%) than PBAT (98.4%).
The incorporation of sebacic acid (SeA) into copolyesters based on PDO and TDCA was explored to enhance crystallization properties and ductility without significantly affecting melting points. Results indicate efficient promotion of crystallization with SeA units, maintaining acceptable melting points (5 to 10 ℃ lower to those of PPXThs). These copolyesters exhibit improved ductility compared to PPXThs, with elongation at break of up to 1100%. While gas barrier properties decrease with increasing SeA content, copolyesters with TDCA content above 60 mol% still demonstrate comparable or better gas barrier properties than typical packaging materials. Moreover, their biodegradability surpasses that of PPXThs, with SeA units promoting both hydrolysis and enzymatic degradation processes.

Date of Award	15 Oct 2024
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Di Hu (Supervisor), Lionel O'Young (Supervisor) & Jin Zhu (Supervisor)