Highly Entangled Bottlebrush Polymer Networks

Entanglements are topological constraints that govern the dynamic mechanical behavior of polymer networks. Linear polymers inevitably entangle at relatively low molecular weights, whereas bottlebrush polymers-consisting of a long backbone densely grafted with many relatively short side chains-suppress entanglements, enabling solvent-free networks with tissue-like softness. Yet, the same steric crowding pre-strains the backbone and renders such networks brittle. Here, we report highly entangled bottlebrush elastomers that combine extreme softness and toughness. Using short polyethylene glycol side chains, we synthesize high molecular weight bottlebrush polymers (>3 × 10⁶ g/mol) that remain amorphous at room temperature. We identify an entanglement threshold of 2.4 × 10⁶ g/mol with an entanglement modulus of ∼1.3 kPa, nearly 1000 times lower than that of linear counterparts. While unentangled bottlebrush networks exhibit strain-stiffening, entangled bottlebrush networks display pronounced strain-softening followed by delayed stiffening due to entanglement slippage. Despite their low modulus (∼1 kPa), these elastomers stretch up to ∼1800% and show a fatigue threshold of ∼63 J/m², comparable to natural rubber. Their intrinsic fatigue strength-fatigue threshold normalized by modulus-surpasses that of highly entangled linear polymer networks by >40-fold. These results establish a new class of soft yet tough polymer networks and provide a model system for understanding nonlinear mechanics in architecturally complex polymers.