Gelation dynamics during photocrosslinking of polymer nanocomposite hydrogels

Polymer nanocomposite hydrogels exhibit mechanical properties that depend on material composition and processing route. The gelation kinetics of network-forming polymer solutions in the absence and presence of silica-coated nanocapsules were investigated using in situ dynamic rheology measurements. Network-forming polymers comprised either 4-arm or 8-arm star polyethylene glycol (PEG) with terminal anthracene groups, which dimerize under irradiation with ultraviolet (UV) light. Upon UV exposure, 4- and 8-arm 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. Adding nanocapsules to PEG-anthracene solutions resulted in faster gelation than nanocapsule-free PEG-anthracene solutions with equivalent effective polymer concentrations. The final elastic modulus of nanocomposite hydrogels increased with nanocapsule volume fraction, signifying synergistic mechanical reinforcement by nanocapsules. Overall, these findings quantify the impact of nanocapsule addition on the gelation kinetics and mechanical properties of polymer nanocomposite hydrogels, which are promising materials for applications in optoelectronics, biotechnology, and additive manufacturing.