New insight into the mechanism for the excellent gas properties of poly (ethylene 2,5-furandicarboxylate) (PEF): Role of furan ring's polarity

Compared with poly(ethylene terephthalate) (PET), the better gas barrier properties of bio-based poly(ethylene2,5-furandicarboxylate) (PEF) is usually attributed to the geometric characteristics of asymmetric furan ring. In order to figure out the role of furan ring's polarity in influencing the gas permeability of PEF, poly(ethylene-2,5-thiophenedicarboxylate) (PETH), which possessed the very similar skeleton structure to PEF, was synthesized from the analogue of 2,5-furandicarboxylic acid (FDCA), 2,5-thiophenedicarboxylic acid (THcA). After chemical structure identification and basic thermal properties study, their gas barrier properties in terms of CO2 solubility/diffusion coefficient as well as apparent activation energies were investigated. Results indicate that the barrier of PEF to CO2 is much higher than that of PETH, in spite of the higher free volume fractions and larger free volume diameters of PEF determined by rheological analysis and Positron Annihilation Lifetime Spectrum (PALS). Considering the geometrical similarity between PEF and PETH, the better barrier property of PEF is attributed to the higher dipole moment of furan ring, caused by the higher electronegativity of oxygen compared to the sulfur atom in thiophene ring. Results reported in this work give us more deep insights into the mechanism for the excellent gas barrier properties of PEF, which is benefit for the design and synthesis of new high gas barrier polymers.