Because of its special qualities, elastomers—a type of polymer with a low modulus and high elasticity—can rapidly return to their initial form following the removal of an external force. Because of these qualities, they are indispensable in both daily life and the economy [1–4]. Elastomers have been the subject of ongoing research and development to improve their strength, stability, aging resistance, and wear resistance in order to fulfill the ever-increasing performance requirements. However, these advantages make it harder to dispose of the waste products made from conventional elastomers, which pollutes the environment over time [5, 6]. However, the majority of elastomers are derived from poorly sustainable non-renewable fossil fuels. The creation of novel elastomers that are not dependent on fossil fuels has become highly demanded in recent times due to the immense strain imposed by carbon emissions. Bio-based elastomers are a great way to reduce carbon emissions and conserve resources in the elastomer sector since they may be made from natural plants directly or by synthesizing them from bio-based monomers produced from biomass resources.
Due to excellent biodegradability, aliphatic polyester became a prominent area of research for environmentally-friendly products. Linear polyesters, such as poly(lactic acid) (PLA), polyglycolide acid (PGA), polycaprolactone (PCL), and their copolymers, have been extensively studied in the field of plastic for many years. And their industrial production is seeing tremendous growth [7–9]. Polyesters having a three-dimensional network have gained increasing interest, motivated by these linear polymers [10–11]. Like vulcanized rubber, the cross-linked network is formed by the incorporation of "star" monomers to create multi-branched chains, enabling the material to rapidly regain its shape when subjected to substantial deformation. Poly (glycerol sebacate) (PGS), polyoctyl citrate (POC), and their copolymers are the primary biodegradable polyester elastomers that have undergone substantial research. The pioneering work is the synthesis of PGS, an in-situ cross-linked polyester elastomer with a three-dimensional network structure and exceptional robustness, conducted by Langer et al. successfully synthesized in 2002 [10, 12]. In 2011, Zhang et al. firstly introduced chemically crosslinked polyester elastomer [13]. Subsequently, many other bio-based elastomers have been created [14, 15].
It is well known that monomers of longer chain are easier to be synthesized to be polyester with higher stability, due to the lower density of ester linkage. For example, PCL and PBS are commercially mature materials, which employ 4- or 6- carbon chain monomer [16], while Polyglycolide acid (PGA) with 2-carbon monomer is unstable to hydrolysis and its commercial application is very limited [17]. However, on the other hand, when the goal of our research is to obtain elastomers, the high crystallinity and rigidity due to longer chains becomes a disadvantage. Hereby a technical difficulty arises, how to construct biodegradable polyester with decent stability, cross-linkage network, and sufficient elasticity? That is, we probably need longer chain monomers, multi-functional "star" monomer, and some component to lower the crystallinity.
In chemically crosslinked polyester elastomers, the glass transition temperature can be decreased and crystallization can be hindered by employing multicomponent copolymerization and including monomers with side groups, which in turn improves the elasticity [18, 19]. Particular monomers with side group, large volume and/or rigid segment can play a crucial role in decrystallization. Gao et al. [20] designed and synthesized biobased poly(1,4-butanediol/2,3-butanediol/succinic acid/iconocarboxylic acid) co-polyesters (PBBSI) with varying 2,3-butanediol content. The co-polyesters underwent a transformation from stiff plastics to flexible elastomers due to the alteration in the amount of 2,3-butanediol. Hu et al. [21] synthesized the elastomer poly(2,3-butanediol-co-1,4-butanediol-co-succinate) (PBBS) by introducing 2,3-butanedioic acid into PBS, whose elasticity can be thereafter improved with the decrystallization effect.
Isosorbide, as a bulky, rigid, and food and environment safe monomer, can be introduced into the copolyester for tuning the crystallization and thermal properties. Zhang et al. [22] reported a bio-based shape memory polymer for biomedical applications, which employs butanediol, isosorbide, sebacate, and itaconate to achieve a series of cross-linked copolyesters with excellent shape-recovery property. However, this series of (PBISI) copolyesters were not elastomers in the room temperature. On the other hand, our group has focused on the development of biodegradable elastomers using the dual-crosslinking property of sebacate [23]. Thus, it is inherently captivating to us that this system shall also possess exceptional potential to function as an elastomer with significant industrial utility. The absence of longer chain monomers with greater flexibility may be the crucial factor for elasticity.
In this work, we select butanediol, hexanediol and itaconate as “long chain” monomers, and take sebacate as the chemically crosslinking site to obtain a stable copolyester Poly(butanediol-hexanediol-itaconate-sebacate) PBHIS. Isosorbide is then introduced to the copolyester to prepare the biodegradable elastomer as PBHIIS. Linear chains of this two copolyesters are synthesized as so-called pre-PBHI(I)S, and a final thermal curing with initiator benzoyl peroxide (BPO) vulcanized the linear chains to be structural network. The scheme of this work is illustrated in Fig. 1. The chemical structure, thermal and mechanical properties, and biodegradability of the final products were characterized.