Craze matrix fibril interface

Usually, the molecular strands are coiled in the glassy polymer. They become stretched when a crack arrives and starts to build up the deformation zone. Presumably, strain softened polymer molecules from the bulk material are drawn into the deformation zone. This microscopic surface drawing mechanism may be considered to be analogous to that observed in lateral craze growth or in necking of thermoplastics. Chan, Donald and Kramer [87] observed by transmission electron microscopy how polymer chains were drawn into the fibrils at the craze-matrix-interface in PS films [92]. One explanation, the hypothesis of devitrification by Gent and Thomas [89] was set forth as early as 1972. 

As with block copolymers, the important parameters are the surface density and length of the copolymer chains. Toughening of the interface may occurs as a result of pull-out or scission of the connector chains, or of fibril or craze formation in matrix. This last mechanism gives the highest fracture toughness, F, and tends to occur at high surface density of chains. 

A transmission electron micrograph of a craze in a thin film of poly(styrene-acrylo-nitrile), shown in Fig. 1 a, will serve to introduce the principal microstructural features of crazes. The direction of the tensile stress is marked and it can be seen that the craze grows with the primary direction of its fibrils parallel to this tensUe stress and with the interfaces between the craze and the nearly undeformed polymer matrix normal to the stress. Since the overwhelming portion of the experiments to be reviewed here rely on the use of thin film deformation and transmission electron microscopy techniques, a brief review of the general methods of these experiments is in order. 

When the PEELS measurement was conducted, an abrupt drop in density was observed at the interface between the matrix and the craze bands (Figure 4). In addition, a drop of approximately 50% in density was found at the base of the already unloaded craze band. This observation implies that an extension ratio of at least 2 exists for the craze fibrils. This phenomenon is not uncommon for thermoplastic crazes (5, 10). To ensure that the PEELS method gives reasonable results, the density of the craze band inside a polystyrene tensile specimen was measured (Figure 5) using the same sample-preparation procedures described in the section Experimental Details. The measured density of the craze band in the unloaded polystyrene was found to be about 0.62 g/cm3, which is in good agreement with the number reported in the literature (5,10, 24). 

Case a stress-induced formation of fibrillated crazes. The weak rubber particles act as stress concentrators. Crazes are formed starting from the particle-matrix interface around the equatorial region of particles. The voids inside the crazes initiate a stress concentration at the craze tip, which propagates together with the propagating craze therefore, the crazes reproduce the stress state necessary for their propagation. Cavitation inside the rubber particles is not necessary, but it enables a higher stress concentration and easier deformation of the particles. 

Case b stress-induced formation of homogeneous crazes. The stress concentration at the particles causes homogeneous crazes to start at the particle-matrix interfaces. Propagation of these crazes into the matrix is accomplished by an increase of volume, which arises from cavitation inside the particles (the possible mechanism of cavitation inside the originally homogeneous crazes is unlikely). Therefore, these crazes are closely connected to the cavitated rubber particles—they cannot propagate for distances as long as those of the fibrillated crazes—and appear mainly between particles. 

The structure of the diffuse weld interface resembles a box of width X, with fractal edges containing a gradient of interdiffused chains as shown by Wool and Long. When the local stress at a crack tip exceeds the yield stress, the deformation zone forms and the oriented craze fibrils consist of mixtures of fully entangled matrix chains and partially interpenetrated minor chains. 

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