Ethylene brassylate: Searching for new comonomers that enhance the ductility and biodegradability of polylactides

Abstract

Copolymers based on d,l-lactide (D,L-LA) and ethylene brassylate, with EB molar contents ranging from 3 to 39%, were synthesized by ring-opening polymerization at 130 °C using triphenyl bismuth. The poly (D,L-LA-co-EB) presented a random distribution of sequences (R→1), resulting in a fully amorphous structure with TgS between −17 and 43 °C that exhibited upgraded thermal stability and a final degradation temperature of ∼450°, which was approximately 100 °C higher than that observed for PDLLA. The incorporation of EB led to mechanical behaviour with improved flexibility and ductility, compared to polylactides. Moreover, the impact was greater than that provided by other monomers such as ε-caprolactone or δ-valerolactone. A macrolactone molar content of only 5% was enough to increase the elongation at break from 2 to 87% but its elastic modulus remained around 1 GPa, whereas the copolymer with 16% EB (Tg at 17 °C) displayed a clear elastomeric curve with secant modulus of 2.0 MPa and a tensile strength of 0.3 MPa at 1400% of strain (when the specimens did not break). Likewise, the disordered chain microstructure distribution of these copolymers with short sequence lengths of L-LA, D-LA and EB allowed high degradation rates (KMw) in the range of 0.055–0.062 per day, with a maximum KMw for the copolyester containing 9% of EB. In contrast, PDLLA and the 97:3 D,L-LA-co-EB copolymer, owing to their high resistance to water uptake and their packed and rigid amorphous phase, showed degradation velocities of only 0.012 and 0.021 per day, values that are slightly higher than that of poly (l-lactide) whose rate is 0.011 per day.