Polymer particle-reinforced polymers

When applied to particle reinforced polymer composites, micromechanics models usually follow such basic assumptions as ... 

Tavman Ismail Hakki. Thermal conductivity of particle reinforced polymer composites. In Nanoengineered nanofibrous materials book series NATO science series ii, mathematics, physics and chemistry, Guceri Selcuk, Gogotsi Yuri G., Kuznetsov Vladimir (eds.), pp. 451—458. Dordrecht Kluwer Academic Publishers, 2003. 

Depending on the reinforcing materiai, composites are divided into two main categories, normally referred to as particle-reinforced polymer composites and fiber-reinforced polymer composites. 

Fibers and anisotropic particles reinforce polymers, and the effect increases with the anisotropy of the particle. In fact, fillers and reinforcements are very often differentiated by their degree of anisotropy (aspect ratio). Plate-like fillers, like talc and mica, reinforce polymers more than spherical fillers and the influence of glass fibers is even stronger [17]. Anisotropic particles orientate during proeessing, and the reinforcing effect depends very much also on orientation distribution. 

When applied to particle reinforced polymer composites, micromechanics models usually follow such basic assumptions as (i) linear elasticity of fillers and polymer matrix (ii) the fillers are axisymmetric, identical in shape and size, and can be characterized by parameters such as aspect ratio (iii) well-bonded filler-polymer interface and the ignorance of interfacial slip, filler-polymer debonding or matrix cracking. The first concept is the linear elasticity, that is, the linear relationship between the total stress and infinitesimal strain tensors for the filler and matrix as expressed by the following constitutive equations ... 

Transition from liquid behavior to solid behavior has been reported with fine particle suspensions with increased filler content in both Newtonian and non-Newtonian liquids. Industrially important classes are rubber-modified polymer melts (small rubber particles embedded in a polymer melt), e.g. ABS (acrylo-nitrile-butadiene-styrene) or HIPS (high-impact polystyrene) and fiber-reinforced polymers. Another interesting suspension is present in plasticized polyvinylchloride (PVC) at low temperatures, when suspended PVC particles are formed in the melt [96], The transition becomes evident in the following... 

An important feature of filled elastomers is the stress softening whereby an elastomer exhibits lower tensile properties at extensions less than those previously applied. As a result of this effect, a hysteresis loop on the stress-strain curve is observed. This effect is irreversible it is not connected with relaxation processes but the internal structure changes during stress softening. The reinforcement results from the polymer-filler interaction which include both physical and chemical bonds. Thus, deforma-tional properties and strength of filled rubbers are closely connected with the polymer-particle interactions and the ability of these bonds to become reformed under stress. 

It is also possible to reinforce polymers with metallic particles. D. T. Turner and one of his students observed that good electrical conductivity can be measured even at very low fillings, such as only 6% by volume. Microscopic examinations showed that the metallic particles formed continuous chains segregated around zones of unpenetrated polymer. 

Suspension Rheology. Particles suspended in a material, such as in filled or reinforced polymers, have a direct effect on the properties of the final article and on the viscosity during... 

ABS and HIPS. The yield stress vs. W/t curves of ABS and HIPS are very similar. They are somewhat surprising because the yield stresses reach their respective maximum values near the W/t (or W/b) where plane strain predominates. This behavior is not predicted by either the von Mises-type or the Tresca-type yield criteria. This also appears to be typical of grafted-rubber reinforced polymer systems. A plausible explanation is that the rubber particles have created stress concentrations and constraints in such a way that even in very narrow specimens plane strain (or some stress state approaching it) already exists around these particles. Consequently, when plane strain is imposed on the specimen as a whole, these local stress state are not significantly affected. This may account for the similarity in the appearance of fracture surface electron micrographs (Figures 13a, 13b, 14a, and 14b), but the yield stress variation is still unexplained. 

In general, polymers have low stiffness and strength in comparison with other materials, e.g., metals and ceramics, and consequently these materials present serious difficulties in structural applications. To improve their mechanical properties, polymers are reinforced by the addition of rigid particles or fibers to form composite materials (1). Thus, polymer matrix composite materials are made up of a low modulus phase, the polymer matrix, and a high modulus phase, the reinforcement, which is usually carbon or glass. The modulus of the composite is higher than that of the polymer matrix, and the increment is proportional to the volume fraction of the reinforcement. In general, the properties of the composite depend not... 

Polymer-based multicomponent systems are abundant in many applications. The properties and performance of particulate-filled systems, such as elastomers and impact modified polymers, and also polymer blends, block copolymers, and fiber reinforced systems, depend to a large extent on the distribution of the components. Hence the local analysis of these distributions down to sub-100 nm length scales (dictated, e.g., by the size of primary filler particles) is of considerable significance. Materials contrast in several AFM approaches offers the possibility to address these issues directly at the surface of specimens or on bulk samples that have been prepared correspondingly. 

Table 8.4 shows that substantial gains can be obtained by filling crystalline polymers but amoiphous polymers are not much affected by reinforcement. Also, particulate fillers are substantially less effective than fibrous fillers. Glass fiber is the most useful filler in this application. Figure 8.55 shows the effect of two grades of particulate fillers on the heat deflection temperature of polypropylene." Small changes are obseiwed at smaller additions followed by a rapid increase in HDT above a 30% filler content. The particle size has only small difference. 

In industrial practice it may be important to use mixtures of filler particles not only of spheroidal shape (as discussed above) but also of different shapes, e.g., filling and reinforcing polymer with CaCOj particles and glass fibers. The theoretical basis for optimization of such systems was developed by Wieckowski and Strek... 

This category mainly comprises fiUed and reinforced polymer melts. There are numerous reviews on the topic [Chaffey, 1983 Goetder, 1984 Metzner, 1985 Utracki, 1987, 1988 Utracki and Vu-JQianh, 1992], There is particularly strong interest in flow of polymeric composites filled with anisometric, reinforcing, particles, with properties that strongly depend on the flow induced morphology and distribution of residual stresses. 

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