(33c) Dynamic Rheology during Recycling of Photo-Responsive Star Polymer Networks

Polymeric materials are often designed to be permanent, such that single-use convenience supersedes considerations of recyclability. For hydrogels, this approach has resulted in materials comprising many components such as polymer precursors, crosslinking agents, and photo-initiators; such complex formulations preclude the chemical recovery of the original constituents. To simplify the formulation of and encode recyclability into hydrogel materials, we synthesized reversible, network-forming polymers based on multi-arm star polyethylene glycol with terminal anthracene groups (PEG-anthracene). Anthracenes form dimers under irradiation with ultraviolet (UV, 365 nm) light and subsequently dissociate with deep UV (265 nm) light, thus functioning as reversible crosslinks when appended to star polymers. Despite the simplicity of this photo-responsive polymer system, the crosslinking and uncrosslinking dynamics of these reversible polymer networks are unknown. Connecting molecular architecture of photo-responsive star polymers to macroscopic rheology will guide the optimum processing of photochemically recyclable materials.

The photo-triggered crosslinking and uncrosslinking kinetics of PEG-anthracene solutions were investigated using in situ dynamic rheology and UV-vis absorbance spectroscopy, enabling a connection between changes to macroscopic material properties and molecular-scale anthracene (un)dimerization. Upon UV exposure, PEG-anthracene solutions exhibited rapid gel formation as indicated by crossovers from liquid-like to solid-like behavior during in situ small-amplitude oscillatory shear rheology. The time required to form a sample-spanning gel was non-monotonic with polymer concentration and shorter for star polymers with more arms per molecule. Upon deep UV exposure, the extent and rate of uncrosslinking of the PEG-anthracene hydrogels to solutions were greater for star polymers with fewer arms, suggesting a polymer architecture-dependent tradeoff between crosslinking and reversibility. Additionally, qualitatively different uncrosslinking dynamics emerged, including monotonic softening, nonmonotonic softening and stiffening, or uncrosslinking into a liquid followed by re-crosslinking into a soft solid. These rheological dynamics were correlated with the extent of anthracene (un)dimerization measured by transient UV-vis absorbance spectroscopy, which revealed an equilibrium between free anthracene and anthracene dimers at long exposure times. Overall, these findings establish polymer architecture-dependent design rules for engineering photo-reversible hydrogel networks and highlight the importance of processing to achieve targeted material properties.