Characterization of Polylactide-b-polyisoprene-b-polylactide Thermoplastic Elastomers

Model polylactide-b-polyisoprene-b-polylactide (PLA-PI-PLA) triblock copolymers were prepared by an efficient protocol starting with alpha,omega-dihydroxy polyisoprene (HO-PI-OH). Using a moderately electrophilic Al(O-i-Pr)(3) catalyst and carefully controlling the ratio of Al to HO-PI-OH avoided gel formation and resulted in acceptable lactide polymerization rates. The triblock copolymers were free of homopolymer or diblock contaminants as determined by chromatographic and spectroscopic analyses. Three representative PLA-PI-PLA materials were prepared with spherical, cylindrical, and lamellar morphologies as confirmed by small-angle X-ray scattering and transmission electron microscopy. We employed dynamic mechanical analysis and tensile testing to assess the viscoelastic and mechanical behavior. The morphology largely determined the tensile properties of these materials, with the Young's modulus and ultimate tensile strength following predicted trends. Excellent elongations were achieved especially for the PLA-PI-PLA sample with the cylindrical morphology, and the PLA-PI-PLA sample with the spherical morphology showed the best elastomeric recovery. Microphase alignment and pull rate significantly influenced the resultant tensile properties.