Craze initiation and growth in high‐impact polystyrene

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Abstract
Quantitative transmission electron microscopy and optical microscopy is used to study craze initiation and growth in thin films of high‐impact polystyrene (HIPS). Dilution of the HIPS with unmodified polystyrene reduces the craze–craze interactions, permitting equilibrium growth and craze micromechanics to be studied. It is found that the equilibrium craze length depends on the size of the nucleating rubber particle, but not the internal structure; no short crazes less than a particle diameter are observed. The long crazes can be adequately modelled by the Dugdale model for crazes grown from crack tips. The effects of particle size and particle internal occlusion structure on craze nucleation have been separated. Craze nucleation is only slightly enhanced at highly occluded particles relative to craze nucleation at solid rubber particles of the same size. There is a strong size effect, however, which is independent of particle internal structure. Crazes are rarely nucleated from particles smaller than ∼1 μm in diameter, even though these make up about half the total number. These craze nucleation and growth effects may be understood in terms of two hypotheses for craze nucleation: (1) the initial elastic stress enhancement at the rubber particle must exceed the stress concentration at a static craze tip and (2) the region of this enhanced stress must extend at least three fibril spacings from the particle into the glassy matrix. Since the spatial extent of the stress enhancement scales with the particle diameter, the second hypothesis accounts in a natural way for the inability of small rubber particles to nucleate crazes.