Effect of polymer architecture on gelation and reversibility in photocrosslinkable polymer hydrogels

Polymer hydrogels exhibit mechanical properties that depend on material composition, molecular architecture, and processing route. The gelation and uncrosslinking kinetics of reversible, network-forming polymer solutions were investigated using in situ dynamic rheology measurements. Reversible, network-forming polymers comprised either 3-, 4-, or 8-arm star polyethylene glycol (PEG) with terminal anthracene groups, which dimerize under irradiation with 365 nm ultraviolet (UV) light and undimerize with deep UV light (265 nm). Upon 365 nm UV exposure, PEG-anthracene solutions exhibited rapid gel formation as indicated by the crossover 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. Reversion of the polymer hydrogels to uncrosslinked polymer solutions was monitored by time-dependent changes to the viscoelastic moduli upon exposure to 265 nm deep UV light. Overall, these findings quantify the effects of polymer molecular weight, number of arms, and material composition on the (un)crosslinking kinetics and mechanical properties of multi-arm star polymer hydrogels.