With the goal of
achieving high barrier with bio-based materials,
for example, for packaging applications, a series of novel furfural-based
polyesters bearing sulfide-bridged difuran dicarboxylic acid units
with high oxygen barrier properties were synthesized and characterized.
For the novel poly(alkylene sulfanediyldifuranoate)s, a 11.2–1.9×
higher barrier improvement factor compared to amorphous poly(ethylene
terephthalate) was observed which places the novel polyesters in the
top class among previously reported 2,5-furandicarboxylic acid (FDCA)
and 2,2′-bifuran-based polyesters. Titanium-catalyzed polycondensation
reactions between the novel synthesized monomer, dimethyl 5,5′-sulfanediyldi(furan-2-carboxylate),
and four different diols, ethylene glycol, 1,3-propanediol, 1,4-butanediol,
and 1,5-pentanediol, afforded difuran polyesters with high intrinsic
viscosities (0.76–0.90 dL/g). These polyesters had good thermal
stability, decomposing at 342–363 and 328–570 °C
under nitrogen and air, respectively, which allowed processing them
into free-standing films via melt-pressing. In tensile testing of
the film specimens, tensile moduli in the range of 0.4–2.6
GPa were recorded, with higher values observed for the polyesters
with shorter diol units. Interestingly, besides the low oxygen permeability,
the renewable sulfide-bridged furan monomer also endowed the polyesters
with slight UV shielding effect, with cutoff wavelengths of ca. 350
nm, in contrast to FDCA-based polyesters, which lack significant UV
light absorption at over 300 nm.
