Molecular mobility in amorphous biobased copolyesters obtained with 2,5- and 2,4-furandicarboxylate acid

Abstract

Poly(ethylene 2,5-furandicarboxylate) (2,5-PEF) is one of the most credible biobased alternative to poly (ethylene terephthalate) (PET). The Henkel disproportionation reaction that leads to furandicarboxylic acid (FDCA) provides three position isomers: 2,5-FDCA (obtained with the highest yield), 2,4-FDCA (so far considered as a by-product), and 3,4-FDCA (traces). The copolymerization of the two main isomers of FDCA with a diol, e.g. ethylene glycol (EG), is an interesting approach to obtain a family of furan-based biopolymers with adjusted physical properties. This work investigates the molecular mobility of three copolymers obtained with EG and ratios of 2,5-FDCA and 2,4-FDCA ensuring the complete disruption of crystallization (90:10, 85:15 and 50:50 mol % of 2,5:2,4 FDCA), as compared to the homopolymers 2,5-PEF and 2,4-PEF. The molecular mobility was investigated by cross-comparing the results obtained by Modulated-Temperature Differential Scanning Calorimetry (MT-DSC), Dielectric Relaxation Spectroscopy (DRS) and Thermo-Stimulated Depolarization Currents (TSDC), with the aim of evaluating the local and segmental molecular mobilities, their activation energies, as well as the temperature dependence of the relaxation time and of the cooperatively rearranging regions at the glass transition. The furan ring in 2,5-FDCA (2,5-PEF) has a rotation axis that is less linear compared to the benzene ring in terephthalic acid (PET), with consequences on the ring-flipping mechanisms. 2,5-FDCA and 2,4-FDCA differ by the position of the carbonyl groups on the furan ring, which adds asymmetry to non-linearity. The incorporation of 2,4-FDCA-based units into a polymer backbone mainly constituted of 2,5-FDCA-based repeating units is responsible for longer relaxation times associated with the local β relaxation processes, no striking effects on the kinetic fragility index m, no obvious effects on cooperativity (a slightly increase in the cooperativity length is observed in the liquid state), no effects on the activation energy for the segmental α relaxation in the liquid state, and a decrease in the activation energy in the glassy state.