Synthesis And Properties Of Thermo-reversibly Cross-linked Polyester Elastomers Via Diels-Alder Reaction

Generally, chemical cross-linking structure and thermoplasticity are contradictory or incompatible for polymers. Chemical cross-linking thermoset polymers have excellent thermo-mechanical properties, but the chemical crosslinking causes them difficult to be recycled. Thermoplastics have good thermoplasticity so that they can be easily recycled, but their thermo-mechanical properties are often not as good as thermoset. If thermally reversible reactive groups are introduced into the polymer, it is possible to simultaneously obtain chemical crosslinking structure to have good performance and thermoplasticity for recyclability. Thermally reversible Diels-Alder (DA) reactions between furan and maleimide derivatives have attracted particular attention probably because of the fact that these reactions can take place under mild conditions with high chemo-selectivity. In this article, a functional monomer containing furan side group was designed and synthesized, and several copolyesters with furan groups were synthesized through polycondesation with small amounts of functional monomer. The resulting copolyesters were blended with bismaleimide to prepare thermal-reversibly covalent crosslinked copolyesters and their thermo-mechanical properties, thermoplastic and recyclability were investigated and discussed in detail.Firstly, the functional monomer diethyl 2-(furan-2-ylmethyl)thio) succinate (DESF) was synthesized from furfuryl mercaptan (FM) and diethyl fumarate (DEF) (molar ratio 1:1) with potassium carbonate (K2CO3,10 mol% relative to FM) as catalyst via Michael addition reaction at room temperature. The final product was obtained in purity higher than 99.0% by filtration of reaction mixture with 0.45 um microfiltration membrane to remove the catalyst.Secondly, the furan modified PBA-Fn copolyester was designed and prepared as a model system to test and verify the concept of thermal-reversibly covalent crosslinked copolyesters via Diels-Alder reaction. The PBA-Fn copolyester was synthesized via direct esterification followed by melt polycondensation of adipic acid (AA),1,4-butanediol (BDO) and functional monomer DESF in the present of tetrabutyl titanate (TBT) catalyst. The sample code of PBA-Fn symbolizes that the molar feed ratio of DESF/AA is n/(100-n).PBA-Fn and 1,1’-(methylenedi-4,1-phenylene) bismaleimide (BMI) (0.5 equiv based on furan content in PBA-Fn) were mixed and then compression-molded. The resulting films were postcured at 50℃ for 48 h to get the thermal-reversibly cross-linked PBA-Fn-BMI which can be de-crosslinked at 120℃ by retro Diels-Alder (rDA) reaction. The crosslinked copolyesters had good thermal stability and mechanical properties, while maintaining the performance of thermal processing. What’s more, the mechanical properties of PBA-Fn-BMI did not show clear decline within several crosslinking-decrosslinked cycles.Thirdly, the furan modified PPSSe50-Fn copolyester was designed to prepare thermal-reversibly covalent crosslinked elastomer PPSSe50-Fn-BMI, and to investigate the influence of the furan content on their mechanical properties. The PPSSe50-Fn was synthesized via direct esterification followed by melt polycondensation of succinic acid (SA), sebacic acid (Se),1,5-pentanediol (PDO) and functional monomer DESF in the present of TBT catalyst. The sample code of PPSSe50-Fn symbolizes that the molar feed ratio of DESF/(SA+Se) is n/(100-n). PPSSe50-Fn and BMI (0.5 equiv based on furan content in PPSSe50-Fn) were mixed and then compression-molded. The resulting films were postcured at 50℃ for 48 h to get the thermal-reversibly cross-linked PPSSe50-Fn-BMI which can be de-crosslinked at 120℃ by rDA reaction. The crosslinked copolyesters PPSSe50-Fn-BMI had good thermal stability and mechanical properties, while maintaining the performance of thermal processing and good recyclability. What’s more, PPSSe50-F5-BMI behaved as thermoplastic plastic whose elastic modulus, tensile strength and elongation at break respectively is 136 MPa,17 MPa and 637%, while PPSSe50-F10-BMI performed as elastomer whose elastic modulus, tensile strength and elongation at break is 4.5 MPa,7.8 MPa and 408%, respectively. Similarly, the mechanical properties of PPSSe50-F5-BMI did not show clear decline within several crosslinking-decrosslinked cycles. Therefore, it is feasible to prepare the high performance thermoplastics or recyclable thermoplastic elastomers by adjusting the composition and the content of furan groups in the flexible copolyester.Finally, the furan modified PBAT45-F5 alipha-aromatic copolyester was synthesized to prepare thermal-reversibly covalent crosslinked elastomer PBAT45-F5-BMI with high performance. PBAT45-F5 was synthesized via direct esterification followed by melt polycondensation of adipic acid (AA), terephthalic acid (TPA),1,4-butanediol (BDO) and functional monomer DESF in the present of TBT. The sample code of PBAT45-F5 symbolizes that the molar feed ratio of DESF/(AA+TPA) is 5/95 and the molar feed ratio of AA/TPA is 55:45. PBAT45-F5 and BMI (0.5 equiv based on furan content in PBAT45-F5) were mixed and then compression-molded. The resulting films were postcured at 50℃ for 48 h to get the thermal-reversibly cross-linked PBAT45-F5-BMI which can be de-crosslinked at 120℃ by rDA reaction and recycled by both compression molding and injection molding. PBAT45-F5-BMI behaves as elastomer with high performance whose elastic modulus, tensile strength and elongation at break are 68 MPa,30 MPa and 544%, respectively. The crosslinked copolyesters PBAT45-F5-BMI had good thermal stability and mechanical properties, while maintaining the performance of thermal processing and good recyclability.