Long-chain branched poly(butylene succinate-co-terephthalate) copolyesters: Impact of (reactive) synthesis strategies on melt strength properties

Highly biobased poly(butylene succinate-co-terephthalate) (PBST) with processing temperatures close to those of commodity polymers (160-180 °C) and long-chain branched architectures (LCB) are synthesized by different strategies.Their rheological properties are investigated, in particular their melt strength properties.A two-step synthesis route is first proposed based on linear LCBs produced by polycondensation followed by reactive extrusion with an epoxy-based multifunctional agent Joncryl ® at concentrations up to 2 wt%.A one-step synthesis strategy is also developed using glycerol as a branching agent, introduced at a low concentration (0.5 wt%) directly during the PBST polycondensation process.The molecular weights, LCB structures, and thermal properties are determined by triple detection size exclusion chromatography and differential scanning calorimetry.For PBSTs synthesized in two steps, gelation takes place simultaneously with the branching reactions.However, a concentration of Joncryl ® close to 2 wt% is required to improve the melt strength properties, with strain hardening effects under elongation conditions.Interestingly, PBSTs synthesized by in-situ addition of glycerol show remarkable melt strength and a high melt stabilization process.Dynamic rheology investigations allow attributing these effects to statistical/homogeneous gel-free LCB architectures obtained during reactive extrusion without any additional post-processing.The effectiveness of approaches to easily improve the melt strength of highly biobased aliphatic-aromatic copolyesters (theoretical biobased content up to 85%) and to eliminate extrusion defects/instabilities in PBSTs is thus demonstrated, allowing the possibility of expanding the industrial application domains of these polymers in packaging and sustainable applications.