Networks collapsed

In the case of a neutral gel, the values a and Q change smoothly as Qo increases, while in the case of a polyelectrolyte network the jumpwise collapse takes place. Note that the composition of the mixture in the swollen network practically coincides with Q0, whereas a significant difference between solvent compositions in the collapsed network and solution exists. The enrichment of the sample by good solvent can be very considerable. An analysis shows that this redistribution increases with the growth of interaction parameter Xab °f solvent components. The reason for this is the following with an increase of Xab. the tendency to phase separation becomes stronger and preferential solvation of... 

In case of a porous gel, the collapsed network chains form a thicker layer. It acts as a barrier and prevents shrinking processes. The shrinkage barrier separates the inner swollen part of the gel from the outer part. The shown distribution of solvent inside the sponge-like gel was measured three days after immersion it into water/methanol. 

The collapse of polymer networks has recently attracted a lot of attention. This boom is partially due to some important applications, which all stem from the fact that you need only slightly change the quality of the solvent to make the network collapse rapidly. It is especially useful that the collapse is very sensitive to the presence of charged monomers and counterions in the solution. Thus collapsing networks can be adapt to detect small ion impurities in a solution, as well as to clear the impurities away. Besides all this, the collapse of networks can also serve as a good model for some other processes in biology (e.g., in the vitreous body in the eye). 

Figure 1 SEM images of fractured xwogels of 1 in cyclohexane (a) random repartition of vray long and rigid fibras (b) detail showing the presence of thinner and more flexible fibrils (c) collapsed network of the phase-separated solid of 1/heptane gel, showing fibras onanating from a central point (d) thick bundles of fibras (e and f) high orientation degree of fibers. (Reproduced from Ref. 8. American Chemical Society, 1998.)...

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

An unstabilized high surface area alumina siaters severely upon exposure to temperatures over 900°C. Sintering is a process by which the small internal pores ia the particles coalesce and lose large fractions of the total surface area. This process is to be avoided because it occludes some of the precious metal catalyst sites. The network of small pores and passages for gas transfer collapses and restricts free gas exchange iato and out of the activated catalyst layer resulting ia thermal deactivation of the catalyst. 

The parameters which characterize the thermodynamic equilibrium of the gel, viz. the swelling degree, swelling pressure, as well as other characteristics of the gel like the elastic modulus, can be substantially changed due to changes in external conditions, i.e., temperature, composition of the solution, pressure and some other factors. The changes in the state of the gel which are visually observed as volume changes can be both continuous and discontinuous [96], In principle, the latter is a transition between the phases of different concentration of the network polymer one of which corresponds to the swollen gel and the other to the collapsed one. 

The swelling behavior of hydrogels in solutions of multivalent ions capable to associate with the network-fixed charges, e.g., Cu2+, substantially differs from that described above, viz. the collapse of gels takes place [107]. As a result of this... 

The salt attack is also an important factor determining the SAH efficiency in the soil medium. In terms of Eq. (4.3), it is manifested by a sharp decrease of the coefficients y and B. The hydrogel structure prediction for specific application conditions requires to take into account universal (ionic strength) and specific (collapse) suppression phenomena and, therefore, a rather delicate balancing in search for a compromise between swelling gains due to the network density (n ) and the ionicity ((3). 

The increase in density on melting is assumed to arise from two competing effects that occur as water is heated. First, increasing translational freedom for the water molecules weakens the hydrogen-bonded network that exists in ice I. This network thus collapses, and reduces the volume. Second, increased vibrational energy for the molecules causes an effective increase in the volume occupied by any one molecule, thus enlarging the overall volume of the liquid. The first effect is considered to predominate below 4 °C, the second above 4 °C. 

M Ilavsky. Phase transition in swollen gels. 2. Effect of charge concentration on the collapse and mechanical behavior of polyacrylamide networks. Macromolecules 15 782-786, 1982. 

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