Consequences of polylactide stereochemistry on the properties of polylactide-polymenthide-polylactide thermoplastic elastomers.

A series of polylactide-polymenthide-polylactide triblock copolymers containing either amorphous poly(D,L-lactide) or semicrystalline, enantiopure poly(L-lactide) or poly(D-lactide) end segments were synthesized. Small-angle X-ray scattering and differential scanning calorimetry data were consistent with microphase separation of these materials. The Young's moduli and ultimate tensile strengths of the semicrystalline triblock copolymers were 2- and 3-fold greater, respectively, than their amorphous analogs. Symmetric (50:50) and asymmetric (95:5) blends of the triblock copolymers containing two different enantomeric forms of the polylactide segments formed stereocomplex crystallites, as revealed by wide-angle X-ray scattering and differential scanning calorimetry. Compared to the enantiopure analogs, these blends exhibited similar ultimate elongations and tensile strengths, but significantly increased Young's moduli. Collectively, these results demonstrate that the properties of these new biorenewable thermoplastic elastomers can be systematically modulated by changing the stereochemistry of the polylactide end blocks.

Hydrolytic degradation behavior of a renewable thermoplastic elastomer.

The hydrolytic degradation of polylactide-polymenthide-polylactide triblock copolymers (37 degrees C, pH 7.4) is compared to that of the component homopolymers. In addition to mass loss and water uptake measurements, size exclusion chromatography (SEC), 1H NMR spectroscopy, differential scanning calorimetry (DSC), and mechanical testing were used to monitor property changes during degradation. The rate of copolymer degradation was significantly influenced by the molecular weight of the polylactide end blocks. Mass loss of the polylactide homopolymer and the copolymer samples was observed once a decrease in the total molecular weight of the samples of 20% occurred. 1H NMR spectroscopy and DSC analysis of the copolymers during degradation revealed that the released oligomers contained mostly polylactide. After initial water uptake in which the mechanical properties were compromised to an extent, the Young's modulus and elongation at break of the biorenewable copolymers remained relatively unperturbed for up to 16 weeks, with significant retention of thermoplastic elastomeric properties for up to 21 weeks.

Renewable-resource thermoplastic elastomers based on polylactide and polymenthide.

An alpha,omega-functionalized polymenthide was synthesized by the ring-opening polymerization of menthide in the presence of diethylene glycol with diethyl zinc as the catalyst. Termination with water afforded the dihydroxy polymenthide. The reaction of this telechelic polymer with triethylaluminum formed the corresponding aluminum alkoxide macroinitiator that was used for the controlled polymerization of lactide to yield biorenewable polylactide-b-polymenthide-b-polylactide triblock copolymers. The molecular weight and chemical composition were easily adjusted by the monomer-to-initiator ratios. Microphase separation in these triblock copolymers was confirmed by small-angle X-ray scattering and differential scanning calorimetry. A representative triblock was prepared with a hexagonally packed cylindrical morphology as determined by small-angle X-ray scattering, and tensile testing was employed to assess the mechanical behavior. On the basis of the ultimate elongations and elastic recovery, these triblock copolymers behaved as thermoplastic elastomers.