The effect of film thicknesses on craze microstructure

Abstract

Transmission electron microscopy of the fibrillar microstructure of air crazes grown in polystyrene (PS) films thicker than 150 nm, shows this microstructure to consist of fine fibrils 4 to 10 nm in diameter with a mean value of 6 nm, in excellent agreement with recent small angle X-ray scattering measurements on crazes in bulk PS. For films 100 nm and thinner, the crazes have a much coarser microstructure, often resembling a perforated sheet more than a set of discrete fibrils. Where fibrils exist they are much larger in diameter (up to 150 nm) than those in thick film crazes. This change in craze micro-structure with decreasing film thickness is attributed to the absence of plastic constraint in the direction of film thickness. Even in much thicker films the absence of plastic constraint at the film surface gives rise to a surface plastic zone producing a surface groove of depth ∼ 25 nm which can extend up to 1000 nm ahead of the craze tip. The absence of lateral stresses in these films increases the wavelength of the meniscus instability (the mechanism of craze tip advance and fibril formation) until for a film thickness less than this wavelength a true fibril structure can no longer form.