Molecular entanglements in crazing

Since then the research activity may even have increased. Central themes were the molecular mechanisms in craze initiation, the influence of molecular weight and presence of entanglements, the nature of plastic instabilities, the role of crazes as precursors to cracks, and last but certainly not least, the formation of crazes in semicrystalline and multiphase polymers. Although considerable progress has been made in the above mentioned fields, some important questions are still open today. 

A. M. Donald and E. J. Kramer, Effect of Molecular Entanglements on Craze Microstructure in Glassy Polymers , J. Polym. Sci., Polym. Phys. 20, 899-909 (1982). 

Figure 13.12 Bright-field TEM image of craze (a) formed in PMMA and its corresponding iow-angie eiectron diffraction pattern (b) an ideaiised representation of the craze microstructure is shown in c. (Reproduced from Berger, LL (1989) Relationship between craze microstructure and molecular entanglements in giassy-poiymers. Macromoiecuies, 22,3162. Copyright (1989) American Chemical Society.)...

Berger, L.L. (1989) Relationship between craze microstructure and molecular entanglements in glassy-polymers. Macromolecules, 22 3162. 

Donald AM, Kramer EJ. Effect of molecular entanglements on craze microstructure in glassy polymers. J Polym Sci Polym Phys Ed 1982 20(5) 899-909. 

As mentioned, the molecular weight between entanglements, Me, or equivalently the entanglement density, ve, is involved in crazing, in craze fibril stability and, thus, in crack formation and propagation. 

Fig. 37. Effect of the diluent molecular weight in blends with 1,800,000 molecular weight PS (X = 0.3) on craze fibril stability (e — ej. The samples were unfiltered and the strain rate used was 5 X 10 s Mc(as2Me) is the critical molecular weight for entanglement effects on the zero-shear-rate viscosity (From Ref. courtesy Macromolecules (ACS))...

We note from the outset that crazing, which is a form of cavitational localization of deformation, can be viewed as a form of transformation plasticity made possible by the long chain molecular nature of the material and the natural molecular entanglements that give rise to well-defined cavitational transformation strains. Therefore, we have called craze plasticity also dilatational plasticity. Thus, if well managed to avoid fracture in the fibrilated craze matter, crazing can be an attractive mechanism of inelastic deformation and a source of toughness. 

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