Investigating The Dynamics of Photo-Reversible Polymer Hydrogels using In Situ Shear Rheology

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 photoinitiators; such complex formulations preclude the chemical recovery of the original constituents. To simplify the formulation of and encode recyclability into hydrogel materials, we report photo-reversible, network-forming polymers that are crosslinked and un-crosslinked using different wavelengths of light. Light-driven un-crosslinking has potential as a low-cost, low-energy, on-demand recycling technology. Photo-reversible polymer hydrogels comprise multi-arm star polyethylene glycol with terminal anthracene groups (PEG-anthracene). PEG-anthracene undergoes photo-crosslinking upon irradiation with ultraviolet light (UV, 365 nm) and un-crosslinking upon irradiation with deeper UV light (265 nm). The photo-reversibility of PEG-anthracene with 3, 4, 6, or 8 arms (5 kg/mol per arm) was compared using UV-vis absorbance spectroscopy and <i>in situ</i> dynamic rheology. Upon 365 nm UV exposure, PEG-anthracene solutions exhibited rapid gel formation indicated by crossovers from liquid-like to solid-like behavior during <i>in situ</i> small-amplitude oscillatory shear rheology. During photo-crosslinking, more arms generally led to the quicker formation of stiffer materials. In contrast, polymers with fewer arms underwent un-crosslinking more readily, indicated by changes from solid-like to liquid-like rheological responses. PEG-anthracene with fewer arms demonstrated liquid-like “recycling windows” that allow for facile polymer solution handling prior to re-crosslinking. These findings demonstrate opportunities for on-demand recycling of photo-reversible polymers, as well as polymer structure-dependent tradeoffs between crosslinking and reversibility. Further understanding of the interplay between polymer structure, material composition, and dynamic processing will advance the use of these materials in optoelectronics, biotechnology, and sustainable additive manufacturing.