Crack Healing in Polymers (Fracture, Reptation, Tack)

Abstract

The mechanical property development associated with the healing of polymer-polymer interfaces was studied using theoretical and experimental approaches. The healing of crazes in atactic polystyrene (PS) and microvoids in styrene-isoprene-styrene block copolymers was described phenomenologically, from dark-field optical microscopy and light transmission measurements. For the healing of bulk polymer surfaces, molecular-level models were coupled with experiments to investigate the effects of healing time t, temperature T, and molecular weight M. The reptation theory of self-diffusion in entangled polymers was used to describe chain motions and interpenetration at a polymer-polymer interface. Various dynamic quantities, including the average segment interpenetration depth (gamma), were related to the fracture strength (sigma) and fracture energy E through microscopic fracture criteria. For a polybutadiene elastomer, (sigma)(,o) is the surface wetting contribution, K is a constant, and t(,(INFIN)) is the time to achieve full healing. A reanalysis of literature data for natural rubber showed (sigma) (TURN) M('- 1/4) for a constant t 2.5 x 10('5). The M(' 1/2) behavior is consistent with chain pullout, and M('o) is consistent with chain fracture.