Molecule-microstructure-property relation of reversible soft materials self-assembled by bottlebrush-based triblock copolymers

Linear-bottlebrush-linear (LBBL) triblock copolymers can self-assemble to reversible, unentangled, and solvent-free networks with elasticity orders of magnitude lower than that of conventional elastomers. The self-assembly depends on molecular architecture but remains poorly explored. We synthesize poly(benzyl methacrylate)-bottlebrush poly(dimethylsiloxane)-poly(benzyl methacrylate) (PBnMA-bbPDMS-PBnMA) with controlled PBnMA weight fraction f and bottlebrush flexibility κ. At f<0.06, LBBL polymers form a disordered sphere phase regardless of the bottlebrush flexibility; this is in stark contrast to classical stiff rod-flexible linear block copolymers that are prone to form highly ordered nanostructures such as lamellae. For a semiflexible bottlebrush with κ~1, the self-assembled microstructure transitions from sphere to cylinder to lamellae as f increases from 0.04 to 0.48, but with the crossover f values much smaller than existing theoretical predictions. Remarkably, the network shear modulus increases exponentially with f for the cylinder phase but becomes saturated around 100 kPa for the lamellae phase. Our results establish the previously unexplored molecule-microstructure-property relation of bottlebrush-based triblock copolymers.