Gelation Dynamics in Polydimethylsiloxane Bottlebrush Elastomers

Network formation in elastomers with grafted side chains is investigated to understand the entanglement-free nature of the bottlebrush architecture. Competition between elastically effective cross-linkers and dangling side chains creates a unique environment where reaction kinetics and steric effects dictate network percolation. The evolving viscoelasticity of linear and bottlebrush networks with an equivalent number of cross-linkers to backbones is measured during a catalytic curing reaction using time-resolved rheology. The impact of reaction kinetics on network formation is addressed through the sol-gel transition by tuning catalyst concentration. Solidification falls into a rate-limited regime where the modulus growth rate increases with increasing catalyst. The network formation process remains independent of the cure rate in the bottlebrush and linear systems. Side chains significantly decrease the fractal dimension of the critical gel cluster despite a comparable number of cross-links. Time-cure superposition is applied to quantify dynamics in the pre- and post-gel states. The divergence of the shift factors around the gel point is independent of cure kinetics. The collapse of shift factors in the post-gel region further suggests the universality of the network formation process. The approach to understanding internal structure development during processing will be critical as bottlebrush elastomers are utilized among a wide range of applications.