Recycling/Upcycling of Physically Aged Poly(butylene succinate) into PBS Vitrimers through Zn(II)-Catalyzed Melt Vitrimerization

Biobased and/or potentially biodegradable polymers have penetrated the market over the last years as sustainable alternatives to their fossil-based equivalents, driven by the community's increasing sensitivity to environmental issues. Nonetheless, replacing fossil-based polymers with bio-ones will not just resolve all problems, and sustainable recycling/upcycling approaches should already be explored. The current work proposes vitrimerization as an alternative route for upcycling low molecular weight polyesters that may derive from end-of-life waste, (chemical) recycling residues, or hydrolyzed in intense storage conditions. Utilizing a hydrolyzed model polyester, poly(butylene succinate) (PBS), two sets of semi-crystalline PBS vitrimers were prepared, cross-linked by either diglycidyl ether of bisphenol A (DGEBA) or glycerol in the presence of Zn(II) transesterification catalyst. By tuning the cross–linker type and loading (0–10% mol with respect to PBS repeating unit), tailor–made cross–linked materials were prepared with high insolubility (gel fraction up to 92%) and lower swelling (up to 900%) compared to conventionally cross–linked PBS, as well as enhanced melt strength (4 orders of magnitude increase in complex viscosity). The characteristic vitrimer behavior was established by oscillatory frequency sweep experiments (up to 10,000% increase of storage modulus at 120 °C) and rapid stress relaxation (e.g., 54–243 s at 160 °C). Interestingly, the reaction temperature was found to only influence the cross-link density attained, while the molar ratio of Zn(II) and actual cross-links dominated the ability for network rearrangement. In addition, the as-prepared vitrimers exhibited improved thermal properties and stability compared to the aged polymer, while they were reprocessed three times by compression molding without significant loss of cross-link density or mechanical performance (tensile strength, Young's modulus).