Polymers toughening mechanisms

Wu [234] reviewed the basic principles of optical and electron microscopes and their applications to characterization and investigation of polymer toughening mechanisms. Multiphase polymers are prepared for SEM by methods such as fractography, etching, and extraction and for OM and TEM by thin sectioning methods. The dispersed phase in a... 

The matrix polymers can be divided into brittie or ductile categories, each having specific requirements for achieving toughness (Table 3). Numerous variations are possible. For instance, often mbber particles that vary in both size and kind are desirable for optimum performance. In these cases, the requirements of the mbber phase and the toughening mechanisms are complex. 

Toughening of BMIs with thermoplastics is a promising approach however, more information is required about the toughening mechanism involved in order to select the most promising polymers in terms of backbone chemistry, molecular weight, and reactive groups. 

Fig. 8.1. Toughening mechanisms in rubber-modified polymers (1) shear band formation near rubber particles (2) fracture of rubber particles after cavitation (3) stretching, (4) debonding and (5) tearing of rubber particles (6) transparticle fracture (7) debonding of hard particles (8) crack deflection by hard particles (9) voided/cavitated rubber particles (10) crazing (II) plastic zone at craze tip (12) diffuse shear yielding (13) shear band/craze interaction. After Garg and Mai (1988a).

Two families of transparent polycarbonate-silicone multiblock polymers based on the polycarbonates of bisphenol acetone (BPA) and bisphenol fluorenone (BPF) were synthesized. Incorporation of a 25% silicone block in BPA polycarbonate lowers by 100°C the ductile-brittle transition temperature of notched specimens at all strain rates silicone block incorporation also converts BPF polycarbonate into a ductile plastic. At the ductile-brittle transition two competing failure modes are balanced—shear yielding and craze fracture. The yield stress in each family decreases with silicone content. The ability of rubber to sustain hydrostatic stress appears responsible for the fact that craze resistance is not lowered in proportion to shear resistance. Thus, the shear biasing effects of rubber domains should be a general toughening mechanism applicable to many plastics. 

The modification of polymer deformation mechanisms in the localised area between mineral particles is another toughening contribution. In our case, the ligament thickness A, is about the same size as the mean particle diameter ... 

Summary of Different Toughening Mechanisms. In toughened polymers with a dispersed modifier phase (i.e., in the dispersed systems), the three mechanisms sketched in Figure 19 may, in general, be distinguished. The characteristics of these different mechanisms are as follows. 

It is believed that this particular two phase morphology is the key to the toughening mechanism of the host matrix (4-9). The success of the epoxy toughening principle by RLP depends vitally on the interaction of the rubber with the epoxy matrix, and on the phase separation mechanism. Control of these two major variables is complex and only partially understood (8,9). Experimental results of the mechanical properties of RLP modified epoxy resins are frequently conflicting, and depend on the preparation techniques of the polymer composites (9,10,11). 

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