Modeling of Thin Layer Yielding in Polymers

It has been known for more than 20 years that, in crazes of polystyrene (PS), the craze fibrils show a very high ductility with elongations at break up to 300%. Later, a similarly high ductility was found in laminated multilayer tapes and in styrene‐butadiene‐styrene (SBS) block copolymers with an arrangement of very tiny PS lamellae. In both cases the widths of the highly ductile PS parts were in the range of 20–30 nm. This effect of “fibril yielding” or “thin layer yielding” is discussed in light of the entanglement network model of amorphous polymers, developed originally to explain the fibrillar structure of PS crazes. An analysis shows that the condition for homogeneous yielding of brittle polymers is fulfilled if the strands or layers have thicknesses less than two‐times the entanglement mesh size (30 nm in PS). In such structures, a coherent entanglement network exists only in the length direction and, if it is loaded, no constraints (due to lateral entanglements) can hinder stretching. Since, in this case, no hydrostatic tensile stresses favor cavitation or micro‐void formation, homogeneous yielding extends up to the maximum stretch ratio, λmax.