Mechanisms for the creation of hierarchically structured block copolymer hydrogels via nonsolvent induced phase separation*

Inspired by the extracellular matrix in biological tissues, researchers have envisioned a new generation of hydrogels that enable technologies like self-healing artificial tissues, soft bioelectronics, and bioresponsive actuators. However, replicating the complexity of biomaterials in synthetic systems requires control over both molecular structure and a hierarchical microstructure. Recently, our experimental collaborators have shown that nonequilibrium processing of hydrophobic-hydrophilic-hydrophobic ABA triblock copolymers creates hydrogels with diverse multiscale architectures. In this work, we strive to elucidate the theoretical mechanisms underpinning the formation of these microstructures. The task is not trivial, due to the presence of multiple dynamic modes across a wide range of length and time scales. We use multiple theoretical tools, including phase diagrams generated by a random phase approximation, 1D transport models, and phase-field models that incorporate Ohta-Kawasaki free energy functionals, to investigate the dynamics that leads to the nonequilibrium formation of hierarchical structures. Specifically, we find that the thermodynamic driving forces for microphase and macrophase separation in combination with mass transport are key elements to consider.