Filled networks

Elastomers, particularly those which caimot undergo strain-induced crystallization, are generally compounded with a reinforcing filler [9]. The two most important examples are the addition of carbon black to natural rubber and to some synthetic elastomers [164,165] and silica to polysiloxane rubbers [166,167]. The advantages obtained include improvements in abrasion resistance, tear strength, and tensile strength. Disadvantages include increases in hysteresis (and thus heat build up) and compression set (permanent deformation). 

The mechanism of the reinforcement is only poorly understood. Some elucidation might be obtained by precipitating reinforcing fillers into network structures rather 

In the simplest approach to obtaining elastomer reinforcement, some of the organometallic material is absorbed into the cross-linked network, and the swollen sample placed into water containing the catalyst, typically a volatile base such as ammonia or ethylamine. Hydrolysis to form the desired silica-like particles proceeds rapidly at room temperature to yield of the order of 50 wt% filler in less than an hour [9, 22, 168,169]. 

If the hydrolyses in organosilicate-polymer systems are carried out with increased amounts of the silicate, bicontinuous phases can be obtained (with the silica and polymer phases interpenetrating one another) [61]. At still-higher concentrations of the silicate, the silica generated becomes the continuous phase, with the polymer 

Reinforcing fillers can be deformed from their usual approximately spherical shapes in a number of ways. For example, if the particles are made of a glassy polymer 

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The surface energy of silicones, the liquid nature of the silicone polymers, the mechanical properties of the filled networks, the relative insensitivity to temperature variations from well below zero to very high, and the inherent or added reactivity towards specific substrates, are among the properties that have contributed to the success of silicone materials as adhesives, sealants, coatings, encapsulants, etc. 

Table 1. Preparation and properties of the silica-filled networks... 

So far, we have not introduced a specific model of the polymer network chains. This problem can be rigorously solved for cross-linked polymer networks consisting of phantom chains [13], or even in the more general case of filled networks where the chains interact, additionally, with spherical hard filler particles [15]. 

Monte Carlo computer simulations were also carried out on filled networks [50,61-63] in an attempt to obtain a better molecular interpretation of how such dispersed fillers reinforce elastomeric materials. The approach taken enabled estimation of the effect of the excluded volume of the filler particles on the network chains and on the elastic properties of the networks. In the first step, distribution functions for the end-to-end vectors of the chains were obtained by applying Monte Carlo methods to rotational isomeric state representations of the chains [64], Conformations of chains that overlapped with any filler particle during the simulation were rejected. The resulting perturbed distributions were then used in the three-chain elasticity model [16] to obtain the desired stress-strain isotherms in elongation. 

See also Axial dispersion aerosols, 1 774-775 aqueous, 18 292 behavior of, 15 685-690 chemical processing aids, 8 705-711 chromatic, 11 134 classification, 8 698-699 colorants for plastics, 7 360-361 donor-acceptor interactions, 8 707-708 electrostatic repulsion, 8 732-734 in filled networks, 22 572 of filled polymers, 11 307-308 flow, 8 726-730 flushing, 8 711... 

Eimco High-Capacity thickener, 22 66 Einsteinium (Es), 1 463-491, 464t electronic configuration, l 474t ion type and color, l 477t metal properties of, l 482t Einstein relation, 22 238. See also Einstein s viscosity equation filled networks and, 22 571, 572 Einstein s coefficient, 14 662 Einstein s equation, 7 280 21 716 23 99 Einstein s law, 19 108 Einstein s viscosity equation, 22 54. 

Nonnegative least squares (NNLS), 6 63 Non-Newtonian behavior of filled networks, 22 572 of silicone fluids, 22 575 versus Newtonian behavior,... 

Process aids, for filled networks, 22 571 Process analysis, Raman scattering in, 22 328... 

Thixotropic agents, 70 4, 430 Thixotropic behavior, of filled networks, 22 572... 

The data presented in Figure 4.14a are consistent with the following mechanism. The dispersion that emerges from the blender is fundamentally unstable with respect to coagulation and coagulates rapidly to form a volume-filling network throughout the continuous phase. Except for the size and structure of the chains, the situation is comparable to a cross-linked polymer swollen by solvent. In both, the liquid is essentially immobilized by the network of chains, and the system behaves as an elastic solid under low stress. The term gel is used to describe such systems whether the dispersed particles are lyophilic or lyophobic. 

As the force applied to the surface of the gel is increased, however, a point is ultimately reached —the yield value —at which the network begins to break apart and the system begins to flow (curve 1 in Fig. 4.4a). Increasing the rate of shear may result in further deflocculation, in which case the apparent viscosity would decrease further with increased shear. Highly asymmetrical particles can form volume-filling networks at low concentrations and are thus especially well suited to display these phenomena. 

It was shown that the stress-induced orientational order is larger in a filled network than in an unfilled one [78]. Two effects explain this observation first, adsorption of network chains on filler particles leads to an increase of the effective crosslink density, and secondly, the microscopic deformation ratio differs from the macroscopic one, since part of the volume is occupied by solid filler particles. An important question for understanding the elastic properties of filled elastomeric systems, is to know to what extent the adsorption layer is affected by an external stress. Tong-time elastic relaxation and/or non-linearity in the elastic behaviour (Mullins effect, Payne effect) may be related to this question [79]. Just above the melting temperature Tm, it has been shown that local chain mobility in the adsorption layer decreases under stress, which may allow some elastic energy to be dissipated, (i.e., to relax). This may provide a mechanism for the reinforcement of filled PDMS networks [78]. 

Gel structures are ubiquitous in foods and responsible for many of their physical properties. The space-filling network of polymers or aggregates provides solidlike properties in the presence of an enormous amormt of water. They are a form of solid water at ambient temperature and in fact they are used to immobilize free water in dietetic products. Gels have been extensively used as model systems to study strue-ture-property relationships due to their simple biphasic nature and the faet that the kinetics of structural changes can be continuously followed by oseiUatory rheometry. 

This space-filling network of percolating and interacting fumed silica particles may result in an enormous high viscosity or even a yield point. The thickened liquid gets a gel-like consistence and will resists shear stress until the shear stress overcomes the strength of the particle-particle interactions and... 

Structural models emerge from the notion of membrane as a heterogenous porous medium characterized by a radius distribution of water-filled pores. This structural concept of a water-filled network embedded in the polymer host has already formed the basis for the discussion of proton conductivity mechanisms in previous sections. Its foundations have been discussed in Sect. 8.2.2.1. Clearly, this concept promotes hydraulic permeation (D Arcy flow [80]) as a vital mechanism of water transport, in addition to diffusion. Since larger water contents result in an increased number of pores used for water transport and in larger mean radii of these pores, corresponding D Arcy coefficients are expected to exhibit strong dependencies on w. 

The response of unvulcanized black-filled polymers (in the rubbery zone) to oscillating shear strains (151) is characterized by a strong dependence of the dynamic storage modulus, G, on the strain amplitude or the strain work (product of stress and strain amplitudes). The same behavior is observed in cross-linked rubbers and will be discussed in more detail in connection with the dynamic response of filled networks. It is clearly established that the manyfold drop of G, which occurs between double strain amplitudes of ca. 0.001 and 0.5, is due to the breakdown of secondary (Van der Waals) filler aggregation. In fact, as Payne (102) has shown, in the limit of low strain amplitudes a storage modulus of the order of 10 dynes/cm2 is obtained with concentrated (30 parts by volume and higher) carbon black dispersions made up from low molecular liquids or polymers alike. Carbon black pastes from low molecular liquids also show a very similar functional relationship between G and the strain amplitude. At lower black concentrations the contribution due to secondary aggregation becomes much smaller and, in polymers, it is always sensitive to the state of filler dispersion. 

Gels. These are systems that consist mainly of solvent (mostly water), with the solid character being provided by a space-filling network. Some idealized types of network are provided by (a) long and... 

Gel formation involves a number of consecutive reactions (1) protein molecules become denatured (2) denatured molecules aggregate to form (roughly spherical, or elongated) particles and (3) these particles then aggregate further to form a space-filling network. After a little while, all of these reactions proceed at the same time, unless the temperature is much higher than the denaturation temperature of the protein, when denaturation can be a very fast reaction. 

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