MSE Seminar: “Hierarchically Ordered Block Copolymer Materials via Nonequilibrium Processing”

The diversity and vastness in the types of properties of living systems, including enhanced mechanical properties of skin and bone, or responsive optical properties derived from structural coloration, are a result of the multiscale, hierarchical structure of the materials. The field of materials chemistry has leveraged equilibrium concepts to create complex materials seen in nature, yet achieving the remarkable properties present in living systems requires moving beyond this formalism by utilizing nonequilibrium processes to create new and exciting materials. Here, the presentation will describe a new method to create hierarchically ordered, physically crosslinked hydrogels, and recent developments in further processing the hydrogel materials to create linear and rotary actuators. Specifically, we have explored a modified nonsolvent-induced phase separation method termed rapid injection processing to produce hierarchically ordered hydrogels with structures and mechanical properties resembling those of living biomaterials. The hydrogel fabrication process entails injecting a triblock copolymer, such as poly(styrene)-poly(ethylene oxide)-poly(styrene) (SOS), solution into a coagulating liquid (i.e., water), driving the hydrophobic polymer domains to organize at the nano and microscale and forming bulk hydrogels. We have established a universal and quantitative method for fabricating and controlling physically crosslinked hydrogels exhibiting hierarchical ordering by controlling the initial pre-injection triblock copolymer solution concentration and water-miscible organic solvent. Additionally, water-swollen hydrogel materials are easily processed to create high-performance linear and rotary actuators via strain-programmed hydrogel crystallization. The crystallized fibers display enhanced mechanical properties due to the aligned alternating amorphous and crystalline domains, and actuation is triggered using either water or heat. The work presented here highlights that by harnessing nonequilibrium methods, it is possible to create materials with tunable physical properties via controlling the structure from the nanometer to the micrometer.