Crazing in glassy homo-and hetero-polymers

Crazing requires a stress field that must have at least one tensile component where in unoriented homo-polymers crazes form and grow normal to the maximum principal tensile stress. Craze microstructures in homo-polymers that have been widely studied consist of drawn polymer fibrils a few nanometers in diameter and have extension ratios of 2-4 that bridge the two faces of a craze and result in a density reduction of up to 0.75 locally. 

Finally, a different and quite complementary form of crazing occurs on a much larger microstructural scale in diblock copolymers composed of phase-separated blocks of compliant rubbery components such as PB in spherical morphologies or randomly dispersed spheres or wavy rods in a majority component of a stiff polymer such as PS. In such hetero-polymers, under stress the compliant component can be made to cavitate, which triggers plastic expansion of the remaining carcass of the stiff polymer to form a very regular kind of cellular craze matter over substantial parts of the volume and can result in a very tough response. 

In the following sections we start by presenting much experimental phenomenology of crazing both in homo- and in hetero-polymers and develop associated predictive meehanistic models for the initiation and growth of crazes that eompare well with the experimental findings. 

Ideally, crazing is a form of plastic deformation of a linear-chain glassy polymer whereby under a tensile stress a slender polymer layer undergoes a uniaxial planar dilatational transformation ej producing a uniaxial strain c in the direction of the tensile stress, in proportion to the volume fraction c of transformed polymer, i.e., 

This response, which is unique to glassy polymers, is possible, in spite of the very substantial magnitudes of ej of the order of 3 4, because of the entangled nature of polymer molecules in the initial precursor state where the uniaxially dilated polymer matter, with substantially reduced density, remains fully load-bearing, in the form of stretched elastomer nano-fibrils. 

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