Polymeric brush elastomers are an emerging class of synthetic materials that display unique nonlinear elastic character where the materials stiffen with deformation. The mechanical properties of these elastomers are best characterized by tensile stress−strain measurements to determine their softness (Young’s modulus, E₀) and firmness (β). The unique super-softness of brush-like elastomers is enabled by polymeric side chains densely grafted to the backbone, which dilute the network strands and hinder chain entanglements. By contrast, linear polymer-based elastomer networks fail to achieve comparable softness due to the inherent entanglement of network strands. Similar to linear polymer-based elastomers, the mechanical properties of brush elastomers can be tuned by varying the cross-link density (∼n_x⁻¹). However, side chains offer additional mechanical tunability by altering the degree of polymerization of the side chains (n_sc) and the grafting density (∼n_g⁻¹). The ratio between n_sc and n_g dictates the transition between the bottlebrush and comb regimes. The effects of every architectural parameter (n_sc, n_g, and n_x) on the stress−strain behavior of polymeric brush elastomers have been extensively studied for methacrylate backbone-based networks, but limited studies feature other classes of backbones. According to computational studies done by Dobrynin et al., mechanical properties of graft copolymers with chemically different backbone and side chains may change with stiffer backbones, but the effects of the backbone on brush networks have not been thoroughly examined. Here, we show that brush elastomers made by ring-opening metathesis polymerization (ROMP) of norborneneterminated poly(dimethylsiloxane) macromonomers are less extensible than brush elastomers with the methacrylate backbone yet not as extensible (λ_max) as predicted by the strain-stiffening parameter (β) derived from fitting the experimental stress−strain curves. The softness and firmness of the norbornenebased networks decrease with decreasing cross-link density as expected, but the λ_max significantly drops when the grafting density becomes low enough, which has not been previously observed in this class of materials.

Professor Wei You

Professor, University of North Carolina

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