Filled polymers filler fraction, effect

Interfacial structure is known to be different from bulk structure, and in polymers filled with nanofillers possessing extremely high specific surface areas, most of the polymers is present near the interface, in spite of the small weight fraction of filler. This is one of the reasons why the nature of the reinforcement is different in nanocomposites and is manifested even at very low filler loadings (<10 wt%). Crucial parameters in determining the effect of fillers on the properties of composites are filler size, shape, aspect ratio, and filler-matrix interactions [2-5]. In the case of nanocomposites, the properties of the material are more tied to the interface. Thus, the control and manipulation of microstructural evolution is essential for the growth of a strong polymer-filler interface in such nanocomposites. 

Various fillers were dispersed in different pol)mier matrices [21,22] and their relative viscosity vs. volume fraction was plotted as shown in Figure 6.3(a). It is seen that the relative viscosity of the filled polymer system adjudged at the same volume fraction varies with the properties of the filler such as shape, size, size-distribution, surface appearance, etc. It is quite obvious what the effect of the physical nature of the filler surface would be on the steady shear viscous properties of the filled polymer systems. The higher the surface roughness the greater the resistance to flow deformation and hence the viscosity of filled system... 

It is interesting that there is a qualitatively similar character in the plots of the dependence on Tg, and also on the fraction of polymer in the surface layer, v, on the filler concentration. The absence of a linear relation between Tg, in the filled system, and the amount of filler is usually explained by the fact that with an increase in the fraction of solid phase there is an aggregation of the solid particles and reduction in the effective surface of contact with the polymer phase. At the same time, we may expect there is a certain linear dependence of the properties of filled polymer (e.g., Tg and Vh/vi, o) on the fraction of polymer in the surface layer. Plots of these dependencies are shown in Figure 3.12. 

All the works cited above do not take into consideration the contribution of the change of polymer properties in the interface in relation to the dependence of the modulus of elasticity of the filled polymer on the filler concentration yet the separation of those effects is essential. This was attempted in the woric (104) in the analysis of dynamic mechanical properties of polyurethanacrylates, filled with quartz powder, with one and the same volume fraction of the filler and various sizes of its particles. The dependence of E and fg 6 on the filler concentration has been analyzed with particles of sizes that permit the contribution of the surface layers to be neglected. Then, studying the properties of the filled polymer with various sized particles, the effect of the filler associated with its own volume can be excluded, and the effects determined by the surface layers of the polymer may be isolated. In the filler low-concentration region the dependence of the modulus on the filler content is well depicted from the empirical correlation (i05) ... 

To illustrate how the effect of the adsorption on the modulus of the filled gel may be modelled we consider the interaction of the same HEUR polymer as described above but in this case filled with poly(ethylmetha-crylate) latex particles. In this case the particle surface is not so hydrophobic but adsorption of the poly (ethylene oxide) backbone is possible. Note that if a terminal hydrophobe of a chain is detached from a micellar cluster and is adsorbed onto the surface, there is no net change in the number of network links and hence the only change in modulus would be due to the volume fraction of the filler. It is only if the backbone is adsorbed that an increase in the number density of network links is produced. As the particles are relatively large compared to the chain dimensions, each adsorption site leads to one additional link. The situation is shown schematically in Figure 2.13. If the number density of additional network links is JVL, we may now write the relative modulus Gr — G/Gf as... 

It was found that the total fraction of the free-volume in the system increases with increasing concentration of the polymeric filler. The temperature dependence of fg for the epoxy matrix was calculated on the supposition that free-volume is an additive value of the constituent components and using the temperature dependence of the fractional free-volume of polystyrene. It was found that with increasing filler concentration the fractional free-volume becomes greater than for pure epoxy resin. Since the fraction of the free-volume increases with increasing total surface area of the filler, it may be supposed that this effect is associated with the surface layers of polymer. It was found that the rate of free-volume expansion in a filled system is higher than in an unfilled one, which means that the expansivity of the free-volume... 

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