Brittle matrix polymers

Brittle fibers and brittle matrix polymers can be synergistic, leading to tough materials, operating by a combination of mechanisms to keep cracks small, isolated, blunted, and dissipate energy. 

Thermoplastic polymers, such as poly(styrene) may be filled with soft elastomeric particles in order to improve their impact resistance. The elastomer of choice is usually butadiene-styrene, and the presence of common chemical groups in the matrix and the filler leads to improved adhesion between them. In a typical filled system, the presence of elastomeric particles at a level of 50% by volume improves the impact strength of a brittle glassy polymer by a factor of between 5 and 10. 

Ac can be approximated to Cf if Cm is neglected in brittle matrix composites (Harris, 1980). It is shown that Rd( contributes substantially to the total fracture toughness of glass fiber-polymer matrix composites (Harris et al, 1975 Kirk et al., 1978 Beaumont and Anstice, 1980 Munro and Lai, 1988). 

Figure 5.91 Schematic illustration of tensile strength versus fiber volume fraction for continuous, unidirectional ductile fibers in a brittle matrix. Adapted from N. G. McCrum, C. P. Buckley, and C. B. Bucknall, Principles of Polymer Engineering, 2nd ed., p. 269. Copyright 1997 by Oxford University Press.

Data regarding the effects of cyclic loading and creep on the life of brittle matrix composites are limited. The concepts to be developed thus draw upon knowledge and experience gained with other composite systems, such as metal matrix composites (MMCs) and polymer matrix composites (PMCs). The overall philosophy is depicted in Fig. 1.7. 

For electrostatic and steric stabilization, the particles can be viewed effectively as colloids consisting of a soft and deformable corona surrounding a rigid core. Colloidal particles with bulk elastomeric properties are also available. These particles, which are generally of submicron size, are developed and used as reinforcement additives to improve the Impact resistance of various polymer matrices [28-30]. The rubber of choice is often a styrene/butadiene copolymer. The presence of chemical groups at the matrix-filler interface leads to improved adhesion between them. Typically, the addition of about 30% by volume of these elastomeric particles increases the impact strength of a brittle glassy polymer like polystyrene by up to a factor of 10. For some applications, particles with more complex architecture have been... 

Distinct improvement of MC in toughness was accomplished by employing the block copolymers. The typical examples are the blends of PPTA-b-nylon 6, 66/nylon 6, 66 [18] and PPTA-b-butadiene rubber (BDR)/ABS resin [19]. In the latter case, only 2.5 wt % of PPTA in MC improved the energy-to-fracture by a factor of four in comparison with that of unmodified ABS resin, while the blend employing homopolymer of PPTA was brittle. Thus, the block copolymerization of PPTA with flexible matrix polymer blocks has advantages of the removal of defects associated with heterogeneous texture in MC and the increase in the degree of dispersion of PPTA block in MC. 

The ceramic matrix composites (CMCs) contain brittle fibers and a brittle matrix. This combination ends up in a damage tolerant material. CMCs are of interest to thermostmctural applications. They consist of ceramics or carbon reinforced with continuous ceramic or carbon fibers. Their mechanical behavior displays several typical features that differentiate them from the other composites (such as polymer matrix composites, metal matrix composites, etc. .. ) and from the homogeneous (monolithic) materials. 

Type IV. Once in a while, the polymer blend may exhibit properties greater than either of the individual polymers, a major synergistic improvement in practical utility. The leading example of this phenomenon is the use of finely dispersed rubbery domains to increase the impact strength of a brittle glassy matrix polymer. Commodity examples are... 

An key requirement of an impact modifier is its ability to bond, either mechanically or, more recently, chemically, with the matrix polymer. It is important, however, to differentiate between impact modification and reinforcement. In some polymer matrices, reinforcement such as glass fibre actually makes the matrix more brittle (and an impact modifier has to be included). 

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