Multiple craze formation

Electron microscope studies have shown that the toughness of ABS polymers is caused largely by multiple craze formation (1,2). The rubber particles appear both to initiate and to control craze formation, so that impact energy is dissipated in the production of numerous small crazes (3). However, this theory does not exclude the possibility of contributions from other mechanisms. The observation that many ABS polymers tend to neck during a tensile test suggests that shear mechanisms are also significant. 

The role of elastomeric tougheners is intimately related to the fracture mechanism in operation. In matrices that are inherently shear yielding, impact modifiers act as stress concentrators where shear bands are initiated. When the polymeric matrix tends to craze, the toughening particles induce multiple craze formation and their elastomeric nature prevents the growth of large crazes that could develop into unstable cracks. 

It is well documented [2-4] that the precursor to fracture in PE is the failure of the craze structure ahead of the crack tip during SCO, The formation of the craze and the mechanism that leads to craze breakdown have been described frequently. The craze nucleation is characterised by the formation of a highly localised zone ahead of the crack tip which consists of multiple voids. Their growth and subsequent coalescence leads to the formation of a fibrous structure. Depending on the stability of the craze structure, the craze may widen by drawing material from the craze-bulk interface into the craze fibrils and eventually rupture at the midribs, or fail at the craze-bulk interface with little or no signs of material fibrillation [5],... 

Figure 8. Three-stage mechanism of multiple crazing (a) stage 1 stress concentration and craze initiation at rubber particles (b) stage 2 superposition of stress fields (small interparticle distance, high rubber volume content) and formation of broad craze bands and (c) stage 3 limitation of crack length and crack stopping at rubber particles.

Bucknall and Smith 17 Multiple Mechanisms (Crazing and Fibril Formation)... 

Fig. 4). Semibrittle materials show a small volume of plastically stretched polymeric material ahead of a crack tip on a micrometer scale, eg, in the form of crazes or shear bands. Macroscopic fracture is quasibrittle or semiductile, occasionally with necking. Larger plastic deformation involves large volumes on a mesoscopic scale (10-/um scale), including formation of larger or multiple crazes or shear bands. In this sequence (from top to bottom in Fig. 4), the plastic deformation involves larger volumes, yielding an increase in the amount of energy absorption (an increase of toughness).

PLA-CToss-Iinked HBP blend. The domain size of cross-linked HBP particles in the PLA matrix was less than lOOnm as obtained from TEM. The presence of cross-linked HBP in the PLA matrix exhibited 570% and 847% improvement in the toughness and elongation at break, respectively, as compared to unmodified PLA. The increase in the ductility of modified PLA was related to stress whitening and multiple crazing initiated in the presence of cross-linked HBP particles. Formation of a networked interface as revealed by rheological data was associated with enhanced compatibility of the PLA-cross-linked HBP blend as compared to the PLA-HBP blend. The cross-linking reaction of HBP with PA was confirmed with the help of Fourier transform infrared spectroscopy and low-temperature dynamical mechanical thermal analysis (DMTA). 

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