Sustainable thermoplastic elastomers with a transient network

Abstract

Vegetable oils and their fatty acids are convenient sources for polymers due to their wide availability, ease of functionalization, and lack of toxicity; however, the long alkyl chains of the fatty acids have a large impact on the resulting polymer properties. Polymers with bulky constituents, such as the long alkyl side-chains of fatty acid-derived polymers, typically exhibit poor mechanical performance due to lack of entanglements. In this study, hydrogen bonding moieties were incorporated into the fatty acid-based midblock of a thermoplastic elastomeric triblock copolymer as a means to improve its mechanical behavior. Poly(styrene-b-(lauryl acrylate-co-acrylamide)-b-styrene), containing lauryl acrylate (a derivative of lauric acid) and the hydrogen bonding comonomer acrylamide in the midblock, was synthesized via reversible addition-fragmentation chain transfer polymerization. The chemical and physical properties of triblock copolymers of varying composition were explored. Quantitative FTIR analysis confirmed the formation of a transient network, which exhibited a reduction in crosslink density with increasing temperature, beneficial for high temperature melt processing. The triblock copolymers exhibited spherical morphologies lacking long-range order at room temperature, which were unaffected by the presence of acrylamide. Moreover, the order-disorder transition temperature reduced with increasing acrylamide content, due to a reduction in the Flory-Huggins interaction parameter. Importantly, incorporation of acrylamide into the midblock greatly improved both the tensile strength and strain at break. Incorporation of a transient network into the midblock is therefore an effective method of improving the mechanical properties of triblock copolymer-based thermoplastic elastomers containing bulky constituents.