Network polyelectrolyte

Note 1 A polyelectrolyte network is sometimes called a cross-linked polyelectrolyte. Use of the latter term is not recommended unless the polyelectrolyte network is formed by the cross-linking of existing polyelectrolyte macromolecules rather than by nonlinear polymerization. (See the definition of a crosslink, definition 1.59, ref [4].)... 

Note 2 In contrast to a polyelectrolyte, a polyelectrolyte network is always insoluble, although swelling or contraction can occur when it is immersed in a solvent. 

Note 3 A polyelectrolyte network in contact with a solution of a salt is able to exchange counterions (cations or anions) with ionic species in the solution and act as an ion exchanger. Therefore, a polelectrolyte network is frequently described as an ion-exchange polymer. 

We present a review of theoretical and experimental results on the swelling behavior and collapse transition in polymer gels obtained by our group at Moscow State University. The main attention is paid to polyelectrolyte networks where the most important factor is additional osmotic pressure created by mobile counter ions. The influence of other factors such as condensation of counter ions, external mechanical force, the mixed nature of low-molecular solvents, interaction of network chains with linear macromolecules and surfactants etc. is also taken into account Experimental results demonstrate a good correlation with theoretical analysis. 

In this section, we will describe the theory of swelling of polyelectrolyte networks. The simplest problem of this type concerns a network sample swelling freely in an infinite solvent. The solvent may contain some low-molecular-weight salt. This problem will be considered in Sect. 2.1. 

Section 2.3 is devoted to the consideration of the conformational changes in polyelectrolyte networks under the action of external uniaxial extension or compression. 

Finally, in Sect. 2.5 the conformational changes in polyelectrolyte network swelling in the solutions of oppositely charged surfactants are discussed. 

Fig. 2. Dependence of the swelling parameter, a, on the quality of solvent, x, for polyelectrolyte networks (m = 86, a, = a = 11) swollen in solutions of lo w-molecular-weight salt of concentrations n,a3 = 0 (/), 0.001 (2), 0.004 (. ), 0.01 (4). The dashed line corresponds to an electroneutrai network. Reproduced from Ref. [27]...

The simplest way to consider the formation of ion pairs was discussed in Ref. [29]. It was shown that the concentration of ion pairs exponentially increases with the decrease of the dielectric constant of the solvent . This effect should be taken into account in the theory of collapse of polyelectrolyte networks, because effective values of in the collapsed state are usually much less than in the swollen state (e depends mainly on the water content which is much larger in swollen networks). This effect has not been taken into account in the theories developed so far. 

For polyelectrolyte networks (cr, < a2), these jumps exist only when T < 0. If the network is immersed in a solvent with t < tcr, the extension of sample can induce its decollapse. In contrast, the compression of swollen gels may lead to the collapse of the network. In the close vicinity of the transition point (t xcr), a very small applied force can induce the collapse of the gel or its jumpwise swelling (see curves 2, Fig. 5). 

In this section, we will consider the swelling of the network in the solution which is a mixture of two components. We will limit our consideration to the case of purely polyelectrolyte networks (ct = CTj) because in this case, the phenomenon of network collapse is most pronounced and it is this case that is usually studied experimentally. 

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... 

Fig. 10. Dependence of the swelling parameter, a. on the volume fraction, , of linear polymer in the solution for a polyelectrolyte network at p = 100, = Xps = 0...

Figure 11 shows the dependences of o on for polyelectrolyte networks swollen in the solution of incompatible polymer (yNp > 0). In this case, the network always deswells linear polymers do not penetrate inside the network... 

Polyelectrolyte Networks in a Solution of an Oppositely Charged Surfactant [38, 39]... 

The free energy of a polyelectrolyte network should be written as the sum of four terms (see Eq. (1)) ... 

In spite of the fact that the concentration of surfactants in the outer solution is assumed to be smaller than the critical micelle concentration, inside the network, micelles are supposed to be formed. The reason for this assumption is, first of all, intensive adsorption of surfactants on the network as a result of the ion exchange reaction. Moreover, in Refs. [38, 39], it was shown that critical concentration of micelles formation c c" within a polyelectrolyte network is much less than that in the solution of surfactant c° . Indeed, when a micelle is formed in solution immobilization of counter ions of surfactant molecules takes place, because these counter ions tend to neutralize the charge of micelles (see Fig. 13), whereas there is no immobilization of counter ions when the micelles are formed in the network the charge of micelles is neutralized by initially immobilized network charges which do not contribute to the translational entropy (Fig. 13). 

As mentioned above, it is possible to induce collapse of polyelectrolyte networks by an external mechanical action. Thus, the mechanical action allows us to increase the molar fraction of a good solvent within the gel and, thus, to perform partly a separation of the components of the solvent. The latter result can be of significant practical and theoretical interest, and is discussed in detail in one of the chapters of this volume. 

Interaction of Polyelectrolyte Networks with Oppositely Charged Surfactants... 

A detailed study of the structure of the aggregates of the ionic surfactants in polyelectrolyte networks was presented in Refs. [66,68]. The dynamics of the changes in the microenvironment of the fluorescent probe, pyrene, in slightly crosslinked networks of poly(diallyldimethylammonium bromide) (PDADMAB) during diffusion of sodium dodecyl sulfate (SDS) in the gel phase has been investigated by means of fluorescence spectroscopy. In Ref. [66], an analogous investigation was reported for complexes formal by the sodium salt of PMAA with cetyltrimethylammonium bromide (CTAB). 

Conformational Transitions in Polyelectrolyte Networks Induced by Electric Fields... 

Another effect of the influence of the electric field on the properties of charged networks was described in a recent publication by Osada. He discovered the phenomenon of contraction of polyelectrolyte networks under the influence of direct current in a good solvent [55, 76, 77]. 

These results have a natural explanation if we assume that contraction of polyelectrolyte networks in the course of the passage of the electric current is due to the electroosmotic transport of water. The phenomenon of electroosmosis can be described in the following way Suppose that the electric current passes through a finely porous medium with electrolytic solution inside the pores. In this case, the moving ions of electrolyte carry with them all molecules of water from the pores through which the current passes. 

Release of liposome-encapsulated CF from HA/PLL films has been observed at temperatures above the lipid transition temperature (Fig. 4f). Below this temperature, the vesicles were stable at least for a few hours. The polyelectrolyte network destabilizes the embedded vesicles, which show higher lipidic bilayer permeability upon heating than do vesicles in solution [84], No change in film properties upon heating has been reported as proof of the polyelectrolyte destabilization effect. 

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