Sawtooth Tensile Response of Model Semiflexible and Block Copolymer Elastomers

We study via coarse-grained molecular modeling the elastic response of semiflexible elastomer networks with idealized diamond connectivity. Under strain-driven uniaxial deformation, these networks are found to exhibit first a perfect soft elastic behavior (including a buckling instability preceding hardening) up to moderate extension ratios (<2.9), followed by a sawtooth-shaped stress–strain curve for larger deformations at the nanoscale. This unique tensile response follows from a deformation mechanism that entails the successive creation and distortion of ordered (smectic CA) chain domains in concert with cross-link segregation and layering; such events arise from the proclivity of the chains to align (due to the chain stiffness) and to disintersperse (due to the network regularity and lack of trapped entanglements). Multiple segregation states hence occur for different strains, producing multiple minima in the stress. These regular networks also exhibit marked anisotropic elastic behavior and shape-memory effects. Additionally, we investigate the impact on the tensile behavior of chemical bidispersity by simulating end-linked semiflexible A–B–A triblock copolymer chains. A significant enhancement in toughness and modulus is observed in such networks due to the further stabilization of the (entropy-driven) smectic chain domains via the (enthalpy-driven) microphase separation of the different blocks.