Abstract

A series of amorphous furan-based copolyesters were investigated with the goal of matching the glass transition temperature (T_g) 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 T_g 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 T_g 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 T_g-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.