Unusually long polymers crosslinked by domains of physical bonds.

Polymers crosslinked by covalent bonds suffer from a conflict: dense covalent crosslinks increase modulus but decrease fatigue threshold. Polymers crosslinked by physical bonds commonly have large hysteresis. Here we simultaneously achieve high modulus, high fatigue threshold, and low hysteresis in a network of unusually long polymer chains crosslinked by domains of physical bonds. When the network without precrack is pulled by a moderate stress, chains in the domains slip negligibly, so that the domains function like hard particles, leading to high modulus and low hysteresis. When the network with a precrack is stretched, the chains in the domains at the crack tip slip but do not pull out. This enables high tension to transmit over long segments of chains, leading to a high fatigue threshold.