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.

Ordered nanoporous polymers from polystyrene-polylactide block copolymers.

Nanoporous polystyrene monoliths were prepared from polystyrene-polylactide (PS-PLA) block copolymers that form hexagonally packed nanocylinders of PLA in a PS matrix. A morphology diagram was developed to determine the range in composition and molecular weight over which this morphology existed. Macroscopic alignment of these materials gave anisotropic monoliths that were subjected to mild degradation conditions leading to the chemical etching of the PLA. The resulting nanoporous monoliths consisted of a polystyrene matrix containing hexagonally close-packed, oriented, and continuous nanoscopic channels (pore size was tunable through synthesis or blending) lined with chemically accessible hydroxyl functional groups. Both the precursors and the porous materials were analyzed moleculary (size-exclusion chromatography and proton nuclear magnetic resonance spectroscopy) and structurally (small-angle X-ray scattering, scanning electron microscopy, and differential scanning calorimetry). In addition, the surface area and pore size distribution of the nanoporous monoliths were characterized (N2 adsorption measurements). These nanoporous materials have remarkable potential as hosts for nanomaterial synthesis, size-selective catalyst supports, and advanced separations.