Loss of entanglement density during crazing

Abstract

The formation of fibril surface area during craze growth requires a loss of entangled strand density in the fibrils themselves. To demonstrate the decrease in entangled chain density, thin films of polystyrene are bonded to soft copper grids and strained in tension. This procedure produces crazed specimens in which the craze fibrils can be characterized by a well-defined draw ratio,λ ₀. The films are then exposed to electron irradiation. This produces chemical crosslinks between the molecules, thus forming a crosslinked network. Subsequent heating of the film aboveT _g results in the entanglement network trying to retract toλ=1. The crosslink network, however, tries to maintain theλ. of the craze fibrils atλ ₀. The craze fibrils thus retract to Ferry's “state of ease”,λ _S, where the tension of the entanglement network is balanced by the compression of the crosslink network. Measurements ofλ _s in crazes crosslinked and then healed confirm that a 25 to 50% loss of entanglement density in craze fibrils occurs, in agreement with theoretical predictions.