Fully-renewable and degradable thermoplastic elastomers.

Abstract

The most common polymers derived from renewable feedstocks, poly(3-hydroxybutyrate), polyglycolide, and polylactide (PLA), have high stiffness and tensile strength, but are inherently brittle, thus limiting the potential for these polymers to replace elastic and ductile polymers derived from fossil fuels. The work described in this thesis was directed toward expanding the properties of renewable resource polymers through the investigation of completely-biorenewable thermoplastic elastomers. Polymenthide (PM), a soft biorenewable polymer derived from (-)-menthol, is immiscible with PLA and was utilized as the middle block in a PLA-containing ABA triblock copolymer. Tensile measurements demonstrated impressive elongations and elastomeric properties typical of thermoplastic elastomers, however, the materials were relatively weak. The tensile properties of the polymers were found to be highly dependent on the molecular weight and crystallinity of the polylactide blocks. Substituting the amorphous PLA with semi-crystalline PLLA or PDLA significantly improved the strength of the material. Blends of the enantiomeric triblock copolymers further increased the strength through stereocomplexation of the enantiomeric polylactide segments. These results led to the investigation of stereocomplexed micelles as nucleating agents for PLLA. Quantifiable improvements in the nucleation efficiency of PLLA were observed when blending PLLA with PDLA-containing triblock copolymers. Finally, potential applications of these all-biorenewable triblock copolymers were investigated through hydrolytic degradation and adhesion studies. During hydrolytic degradation, the triblock copolymers were able to maintain a significant amount of their mechanical properties for many weeks.