Kinetics of Polymer Gel Formation Cause Deviation from Percolation Theory in the Dilute Regime

Gelation has long been conceptualized and modelled as a percolation process, in which bond formation or destruction events are random. Percolation assumes that connections are created or destroyed randomly, such that the critical point should occur at the same point when approached from either direction. Here, the gel point of an end-linked poly(ethylene glycol) gel was measured during forward (bond forming) and reverse (bond breaking) gelation and de-gelation processes to interrogate how gel point scales with synthesis concentration, where decreased concentration leads to an increased prevalence of inelastic loops. Forward gel points were identical to results generated from a kinetic Monte Carlo (KMC) simulation, demonstrating the expected gel point suppression as concentration decreased. Reverse gel points were within error of forward gel points at high concentration but displayed a lesser degree of suppression as concentration decreased. This deviation between forward and reverse gel points at low concentration was qualitatively reproduced in the KMC simulation. These experiments and simulations show that forward and reverse gel points diverge as the gel system becomes more dilute, suggesting that kinetic effects cause a departure from percolation behavior in defect-rich gels.