Polybutadiene (PB) has a low glass temperature Tg and exhibits rubbery behavior during mechanical perturbation. The corresponding PB-based polyurea (PU) has a higher Tg and fails in a brittle mode for high strain rates. However, unlike in glasses, this brittle failure is accompanied by large energy dissipation. Dielectric relaxation measurements demonstrate that whereas the PB segmental dynamics are faster than the strain rate during impact loading, for PU these motions are on the order of the strain rate, 105s1. Consequently, impact induces a transition to the glassy state, with the accompanying response markedly different from that of a rubber.

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