The crack growth dynamics of the carbon-black (CB) filled elastomers is studied experimentally and analyzed while focusing on both kinetics and crack tip profiles. The CB amounts are varied to change the mechanical properties of the elastomers. Static crack growth measurements simultaneously reveal the discontinuous-like transition of the crack growth rate v between the "slow mode" (v≈10^{-5}-10^{-3} m/s) and "fast mode" (v≈10^{-1}-10^{2} m/s) in a narrow range of the input tearing energy Γ and the accompanying changes in the crack tip profiles from blunt to sharp shapes. The crack tip profiles are characterized by two specific parameters, i.e., the deviation δ from the parabolic profile and the opening displacement a in the loading direction. The analysis based on the linear and weakly nonlinear elasticity theories of fracture dynamics demonstrates that the Γ dependence of δ and a is simply classified into three groups depending on the mode (slow or fast) and the magnitudes of δ, independent of CB volume fractions. The theories well explain the results in the slow and fast modes with small magnitudes of δ, while they fail to describe the data in the fast mode with large magnitudes of δ, where the contributions of the strong nonlinearity and/or energy dissipation become significant. The correlation between a power-law relationship Γ∼v^{α} observed in the fast mode and the linear viscoelasticity spectrum is also discussed. The correlation in elastomers with low CB volume fractions is quantitatively explained by the theory of Persson and Brener [Phys. Rev. E 71, 036123 (2005)PLEEE81539-375510.1103/PhysRevE.71.036123], whereas the deviation from the theory becomes appreciable for elastomers with higher CB volume fractions which exhibit strong nonlinear viscoelasticity.

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Amorphous cellulose-based tough double-network (DN) elastomers were successfully fabricated. These elastomers comprise interpenetrated poly(ethyl acrylate) (PEA) network as the soft matrix and the amorphous cellulose network as the brittle component. Unlike carbonblack-filled conventional rubbers, the obtained cellulose/PEA DN elastomers are transparent and can be dyed without any color limitation. Although the cellulose network in the DN elastomer comprises only 2.55 wt%, such cellulose network efficiently reinforces in toughness (10 times), stiffness (28 times), strength (8 times), and durability of the DN elastomer compared to the PEA elastomer. The structure and toughening mechanism of the cellulose/PEA DN elastomers are different from previously reported cellulose composites, in which cellulose nanocrystals were used simply as fillers. Upon deformation, the brittle cellulose network in the DN elastomer is ruptured sacrificially to dissipate energy, which effectively prevents crack propagation. The damaged cellulose network recovers its original structure to show recoverable mechanical properties by thermal annealing.