Fully bio-based poly(L-lactide)-b-poly(ricinoleic acid)-b-poly(L-lactide) triblock copolyesters: investigation of solid-state morphology and thermo-mechanical properties

In this work, a set of ABA triblock poly(L-lactide)-b-poly(ricinoleic acid)-b-poly(L-lactide) aliphatic copolyesters were prepared by consecutive AB type self-condensation and ring-opening polymerization. Condensation of methyl ricinoleate, produced from castor oil, in the presence of a small amount of 1,3-propanediol afforded alpha,omega-hydroxy-terminated poly(ricinoleic acid) with a molar mass of 11 kg mol(-1). Polymerization of L-lactide initiated from the terminal hydroxyl moieties of the alpha,omega-hydroxy-terminated poly(ricinoleic acid) led to triblock copolymers with a composition ranging from 35 to 83 wt% of PLLA. The block structure was confirmed by several techniques. The copolymers displayed a multi-step thermal degradation with a temperature corresponding to 5 wt% loss in the range 175-225 degrees C. DSC analyses showed that the PRic block had a moderate effect on PLLA melting behavior. The solid-state morphology of the so-formed copolymers was highly dependent on their chemical composition, as evidenced by SAXS and WAXD analyses. The high degree of separation of hard and soft phases was also confirmed by dynamic mechanical analysis as seen from the distinct alpha-relaxations. Finally, the tensile properties of these block copolymers ranged from thermoplastic to elastomeric depending on their composition.