Poly(ethylene furanoate-co-ethylene terephthalate) biobased copolymers: Synthesis, thermal properties and cocrystallization behavior

A series of poly(ethylene furanoate-co-terephthalate) (PEFT) copolymers, with compositions ranging from neat poly(ethylene furanoate) (PEF) to poly(ethylene terephthalate) (PET), was synthesized by melt and solid state polycondensation (SSP). ¹H NMR spectra revealed that the copolymers were random, while the WAXD patterns of the copolyesters indicated isodimorphic cocrystallization. A minimum was observed in the plot of the melting temperature (T_m) vs composition while the glass transition temperatures (T_g) varied almost linearly with increasing ET units. The crystallization rates and degree of crystallinity decreased with comonomer content. Several thermodynamic models were applied for the analysis of the melting point depression. A small portion of the comonomer units was found to be introduced into the homopolymer crystals. It was also realized that it is easier to incorporate the EF units into the PET crystal than the opposite. PLM was used to observe the spherulitic morphologies formed during isothermal melt crystallization. Thermogravimetric analysis (TGA) indicated that the thermal stability of PEFTs decreases slightly with increasing furanoate content. Finally, the mechanism of decomposition was evaluated via Py-GC/MS, which consisted of mostly heterolytic scission and less of homolytic scission reactions.