(558a) Influence of Polymer Architecture on the Reversible Photocrosslinking of Hydrogel 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 photoinitiators; 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 365 nm ultraviolet (UV) light and subsequently dissociate with deep UV light (265 nm). The crosslinking and uncrosslinking kinetics of reversible, network-forming polymer solutions were investigated using UV-vis absorbance spectroscopy and in situ dynamic rheology. Upon 365 nm 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. The extent and rate of uncrosslinking of the polymer hydrogels to solutions were greater for star polymers with fewer arms, suggesting a polymer architecture-dependent tradeoff between crosslinking and reversibility. Overall, these findings establish polymer architecture-dependent design rules for engineering photoreversible hydrogel networks.