Novel fully biobased poly(butylene 2,5-furanoate/diglycolate) copolymers containing ether linkages: Structure-property relationships

Novel fully biobased aliphatic-aromatic poly(butylene 2,5-furanoate/diglycolate) random copolyesters were successfully synthesized from 2,5-furandicarboxylic acid, diglycolic acid and 1,4-butanediol through two-stage melt polycondensation using titanium tetrabutoxide and titanium tetraisopropoxide as catalysts. The synthesized polymers were characterized in terms of molecular and solid-state properties, among these, barrier properties to different gases. In addition, biodegradability studies in compost have been conducted. All the polymers showed a good thermal stability, even higher than that of the thermally stable homopolymer poly(butylene furanoate), and at room temperature appeared as semicrystalline materials. The main effect of copolymerization was a lowering, up to 60 °C, in Tm respect to the homopolymer. The dependence of Tm on composition for copolymers was well described by Baur's equation. Diffractometric measurements indicated that only the poly(butylene furanoate) crystalline phase is present in the copolymers under investigation. Amorphous samples showed a monotonic decrement of Tg, up to 30 °C, as the content of butylene diglycolate units is increased and this can be explained on the basis of the higher flexibility induced in the polymer chain by the ether-oxygen atoms. A Wood-type equation was found to fit the Tg data of completely amorphous samples. Final properties and biodegradation rate of the materials under study were strictly dependent on the copolymer composition and the crystallinity degree. As a matter of fact, hydrophilicity regularly increased with the increasing of butylene diglycolate mol%, due to the highly electronegative ether-oxygen atoms present in the co-units (water contact angle decreased of almost 15°). The elastic modulus decreased till one order of magnitude and the elongation to break increased, up to 400%, as co-unit content was increased. Regarding the gas barrier properties, copolymerization did not imply a worsening with respect to the homopolymer. A slight increment of the gas transmission rate was recorded just for the copolymer containing the highest amount of butylene diglycolate units, as a consequence of the increasing flexibility of the macromolecular chain due to the ether-oxygen atoms. This behavior was more evident with CO2 gas test than with O2, N2 and C2H4 ones.