High-Barrier Biobased Copolyesters with Targeted Glass Transition Temperatures as Renewable Alternatives for PET

A series of amorphous furan-based copolyesters were investigated with the goal of matching the glass transition temperature (Tg) of poly(ethylene terephthalate) (PET) while providing an enhanced O2 barrier. The biobased copolyesters were composed of dimethyl 5,5 '-sulfanediyldi(furan-2-carboxylate) (DM-SFA) with either dimethyl 2,5-furandicarboxylate (DM-FDCA) or dimethyl 2,2 '-bifuran-5,5 '-dicarboxylate (DM-BFDCA) as the main monomers with ethylene glycol as the diol component. The furfural-based monomers DM-SFA and DM-BFDCA in roughly equimolar ratios provided copolyesters with Tg in the range of PET, while also having low O2 and UV-light permeabilities. With the 5-hydroxymethylfurfural-based DM-FDCA monomer, DM-SFA was adjusted to be the minor comonomer in the feed in order to provide PET-like Tg for the copolyester. On the other hand, the O2 permeabilities of these copolyesters were substantially lower, although the DM-FDCA monomer lacked the UV-blocking benefits of DM-BFDCA. Finally, isosorbide (IS) was also investigated as a possible Tg-enhancing comonomer in conjunction with ethylene glycol and DM-SFA. However, thermal stability of the IS-containing copolyester was found to be lower, as indicated by thermogravimetric analysis. Incomplete dissolution after polycondensation was also observed, which was consistent with cross-linking under high-temperature conditions. Additionally, melt-pressed films did not completely yield at high temperatures during dynamic mechanical analysis, which contrasted with the behavior of the DM-FDCA and DM-BFDCA copolyesters. The DM-SFA/DM-BFDCA copolyesters were identified as especially interesting materials having relatively high glass transition temperatures while being completely amorphous and providing films with low O2 and UV-light permeabilities. They are also notable for having most of their carbon derived from the renewable platform chemical furfural.