Evolution of nonmonotonic viscous moduli during the formation of polymer hydrogels

Polymer hydrogels are versatile materials used in consumer products, biomedical materials, and sustainable agriculture. Gelation involves a crosslinking process, during which liquid polymer solutions transition into solid polymer networks. This transition is monitored using in situ rheology, which measures the dynamic elastic (G') and viscous (G") moduli. Typically, both moduli increase monotonically during gelation: G" initially exceeds G' (liquid-like), then a crossover leads to G' surpassing G" (solid-like), and both moduli plateau as the system approaches maximum crosslinking. In some gelation processes, the viscous modulus overshoots its final plateau value shortly after the crossover, before decreasing to its plateau value. We hypothesize that the G" overshoot signals polymer cluster growth after percolated network formation, followed by cluster incorporation into the network. Using star polymers as a model system, we observe that more polymer arms and higher polymer concentrations enhance the G" overshoot. Time-resolved frequency sweeps reveal the G" overshoot is enhanced in high-frequency measurements, consistent with faster relaxation of clusters in the sol than the overall polymer gel. This work improves understanding of polymer network gelation, providing valuable insights to guide network development for numerous applications.