Development of Sustainable Thermoplastic Elastomers

Sustainable poly(styrene-b-(lauryl-co-stearyl acrylate)-b-styrene) (SAS) triblock copolymers containing a rubbery midblock derived from fatty acids were designed with targeted properties appropriate for thermoplastic elastomer applications. Well-defined polymers with desired compositions and controlled molecular weight distributions were successfully synthesized using reversible addition fragmentation chain transfer (RAFT) polymerization. SAS exhibited elastomeric behavior at room temperature and were processable above the order-disorder transition (ODT) temperature. The alkyl side-chain length of the polyacrylates (as well as the related midblock composition of SAS) was utilized as a convenient parameter for tuning the physical properties of SAS, including the melting temperature and viscosity of the midblock polyacrylate, melt viscosity of SAS above the ODT, and tensile properties.

The thermodynamic interactions between the components of SAS, polyacrylates and polystyrene, were explored through determination of the Flory-Huggins interaction parameter through cloud point measurements on binary polymer blends and measurement of the ODT of the triblock copolymers. It was shown that the Flory-Huggins interaction parameter was independent of the length of the alkyl side-chain of the polyacrylates, in the limit of large alkyl side-chains (i.e., more than 10 carbon atoms), in stark contrast to theoretical predictions using group contribution methods and solubility parameter theory.

The morphology of SAS was probed by transmission electron microscopy, showing the presence of spherical polystyrene domains in the polyacrylate matrix, as well as small angle X-ray scattering, indicating the presence of randomly oriented grains upon compression molding. Large amplitude oscillatory shear was employed at a temperature below ODT to align the randomly oriented grains. The predominant orientation was determined to be hexagonally close-packed spherical domains with the {0001} planes parallel to the shear plane, with a small population of face-centered cubic spherical domains with the {1-10} planes parallel to the shear plane.

Fully sustainable triblock copolymers with midblocks derived from fatty acids and endblocks derived from salicylic acid were also successfully synthesized with RAFT polymerization. The properties of the resulting poly(acetylsalicylic ethyl methacrylate-b-lauryl methacrylate-b-acetylsalicylic ethyl methacrylate) (ALA) triblock copolymers were examined for their utility as thermoplastic elastomers. Poly(acetylsalicylic ethyl methacrylate) was found to be a suitable replacement for polystyrene, with a glass transition temperature above room temperature. The morphology and mechanical properties of ALA were comparable to that observed in the partially sustainable SAS triblock copolymers.