Toughening mechanisms dispersed systems

The aim of this work is to study the influence of particle size, interparticle distance, particle volume content, and local stress state on the toughening mechanism in several dispersed systems. The systems consist of a matrix of an amorphous or semicrystalline thermoplastic (see Figure 1). It is necessary to determine whether the particle diameter or the interparticle distance is of primary importance. But it is difficult to check the influence of both parameters because there is an interrelation between D, the average minimum value of A, and the particle volume content, t>P ... 

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. 

Figure 19. Schematic representation of the three different toughening mechanisms in dispersed systems, where the assumed loading direction is vertical (a) induced formation of fibrillated crazes (i.e., with microvoids in them) at the equatorial zones of rubber particles (b) induced formation of homogeneous crazes at cavitated particles and (c) induced formation of shear deformation between cavitated particles.

Use has been made of the miscibility of polycarbonate and PMMA to assist in toughening poly(buiylaie terephthalate) with core-shell particles which have a PMMA shell [138]. By addition of a small amoimt 10 wt%) of polycarbonate, the immiscibility of PMMA and poly(butylene terephthalate) is overcome and good particle dispersions are achieved. The polycarbonate simply acts as an interfacial agent. The mechanism of toughening in this system has been studied... 

A variety of mechanisms related to the crack/inclusion interaction can be exploited to increase the toughness of dispersion-reinforced glass matrix composites. In fact, toughening rather than strengthening has frequently been the goal when developing this kind of composite systems. Toughening mechanisms are also affected by the presence of residual stresses in the material. 

Compositions of these adhesives are suggested in a number of recent patents (5- )- All of these reactive adhesive patents indicate essentially the same concept an elastomer is colloidally dispersed in a monomer, or a monomer/oligomer/polymer solution. The system is then polymerized using a free radical mechanism. What occurs is a rapid, "in situ" polymerization of a (typically) methyl methacrylate system, toughened by elastomeric domains which have beer, incorporated into the structure by grafting. 

The second means of transforming a liquid adhesive entirely into a solid without the loss of a solvent or dispersion medium is to produce solidification by a chemical change rather than a physical one. Such reactive adhesives may be single-part materials that generally require heating or exposure to electron beam or UV or visible radiation (see Radiation-cured adhesives) to perform the reaction, and which may be solids (that must be melted before application), liquids or pastes. The alternative two-part systems require the reactants to be stored separately and mixed only shortly before application. The former class is exemplified by the fusible, but ultimately reactive, epoxide film adhesives and the latter by the two-pack Epoxide adhesives and Polyurethane adhesives and by the Toughened acrylic adhesives that cure by a free-radical Chain polymerization mechanism. 

Unlike for PS, very little work has been published on the fracture of block copolymers. There is a substantial amount of literature on PS toughened by blending with block copolymers. In those systems, the block copolymer is dispersed, and provided the proper particle size and modulus are obtained, substantial improvements in toughness are obtained, as described above for HIPS. For the neat block copolymers, however, fracture mechanisms are fundamentally different than for PS. 

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