General Approach to Photo-Crosslink Bottlebrush Polymers*

Entanglement-free bottlebrush polymers can form exceptionally soft networks that are compelling in applications ranging from mimics of biological tissue to sensor skins. Controlled polymerizations provide a route to synthesize precisely defined bottlebrush polymers, but require crosslinking to form networks. We have developed a general strategy to photo-crosslink bottlebrush precursors with miscible bis-benzophenone-based additives in ambient conditions without solvent. We demonstrate that this approach is effective with a wide variety of different side-chain chemistries including acrylates, esters, and siloxanes. Current bottlebrush network models assume all crosslinks are elastically effective, which is inadequate for our randomly crosslinked networks formed with benzophenone-based additives. We present a modified Phantom network model that is capable of distinguishing elastically effective crosslinks from the ineffective ones. This model is validated by examining the moduli of networks formed from bottlebrush polymers with different molar masses and side-chain chemistries. We also experimentally verified the minimal amount of crosslinkers predicted to achieve the lowest network modulus and high gel fraction.