Structure-Property Relationships in Sustainable Thermoplastic Elastomers

Thermoplastic elastomers (TPEs), predominantly composed of ABA triblock copolymers containing glassy polystyrene endblocks and rubbery polydiene midblocks, are widely used in electronics, adhesives and automotive components. Sustainable TPEs derived from renewable resources can be attractive alternatives to petroleum-based TPEs when sustainability requirements are met without compromising material performance. Vegetable oils and their fatty acids are promising replacements for petroleum sources as polymer feedstocks due to their abundance, lack of toxicity and ease of functionalization. However, fatty acid-derived TPEs exhibit inferior mechanical properties to petroleum-based products due to lack of entanglements, stemming from the presence of long alkyl chains on the fatty acids. To facilitate the adoption of polymers derived from vegetable oils and fatty acids, we aim to develop approaches to enhance the mechanical properties of this class of materials. In this study, two approaches were evaluated as means to enhance the mechanical properties of sustainable TPEs with fatty acid-derived midblocks: incorporation of hydrogen bonds or ionic interactions. In the first approach, the hydrogen bond-containing comonomer acrylamide was incorporated into midblock. The triblock copolymers exhibited not only greatly improved mechanical properties, but also accessible processing temperatures. In the second approach, ionic interactions were introduced into midblock by neutralizing the comonomer methacrylic acid. The tensile strength and modulus were significantly enhanced by the incorporation of ionic interactions and the strain at break was improved at low degrees of neutralization. In addition, the fatty acid-derived triblock copolymers exhibited a closed packed spherical (CPS) morphology under oscillatory shear, attributed to the presence of the higher dispersity midblock. The presence of CPS morphology in the bulk block copolymers has rarely been reported, which motivated the investigation of the kinetics and mechanisms through in-situ small-angle X-ray scattering and Fourier transform-rheology. The orientation of CPS layers was found to be affected by the strain amplitudes. For the first time, we observed shear deordering at high strain amplitude for the sphere-forming block copolymers in the bulk. Finally, to expand the library of renewable building blocks for TPEs, a rosin-derived polymer possessing a high glass transition temperature was evaluated as the endblocks in sustainable TPEs. The triblock copolymers exhibited elastomeric behavior at room temperature and accessible order-disorder transitions, appropriate for thermoplastic elastomer applications.