Glassy polymers craze formation

Fracture processes are associated with deformations. In glassy thermoplastics craze formation is the most frequent pre-failure deformation process. Just ahead of the crack tip, where the stresses are particularly concentrated, molecular chains of the polymer are drawn out of their amorphous arrangement in the bulk material into fibrils (see Fig. 1.1 and Chapter 1) under the action of the principal tensile stress... 

Cavitation is often a precursor to craze formation [20], an example of which is shown in Fig. 5 for bulk HDPE deformed at room temperature. It may be inferred from the micrograph that interlamellar cavitation occurs ahead of the craze tip, followed by simultaneous breakdown of the interlamellar material and separation and stretching of fibrils emanating from the dominant lamellae visible in the undeformed regions. The result is an interconnected network of cavities and craze fibrils with diameters of the order of 10 nm. This is at odds with the notion that craze fibrils in semicrystalline polymers deformed above Tg are coarser than in glassy polymers [20, 28], as well as with models for craze formation in which lamellar fragmentation constitutes an intermediate step [20, 29] but, as will be seen, it is difficult to generalise and a variety of mechanisms and structures is possible. 

Craze formation is a dominant mechanism in the toughening of glassy polymers by elastomers in polyblends. Examples are high-impact polystyrene (HIPS), impact poly(vinyl chloride), and ABS (acrylonitrile-butadiene-styrene) polymers. Polystyrene and styrene-acrylonitrile (SAN) copolymers fracture at strains of 10 , whereas rubber-modified grades of these polymers (e.g., HIPS and ABS) form many crazes before breaking at strains around 0.5. Rubbery particles in... 

Glassy polymer-diluent mixtures deformed in a temperature range close to are susceptible to exhibit a cavitational mode of plasticity at hi stresses and strains. Activation of this mechanism in mixtures of polycarbonate with esters of the phthalic acid results in extensive fibrillation and stress whitening of the material. There is strong evidence that the diluent plays an important role in enhancing chain slippage, which is required for the formation of craze fibrils. One of the most fundamental problems which is still unsolved is the elucidation of the molecular mechanism by which diluents become active. 

Brittle fracture in glassy polymers occurs by the prior formation of crazes which then fracture. In the craze, fibrils of highly oriented polymer are produced and it is the fracture of these fibrils that almost certainly produces the observed radicals. Even in this case, then, the signal derives ultimately from a fibre in which... 

S. Rabinowitz and P. Beardmore, Craze formation and fracture in glassy polymers, Crit. Rev. Macromol. Sci. 1, 1 (1972). 

Kambour, R. P. and Bernier, G. A. (1968) Craze formation yield stress and the so-called ductile-brittle transition in glassy polymers, Macromolecules, 1, 190-191. 

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