Charged network

In addition to the experimental investigations, the phenomenon of the gel collapse was intensively studied theoretically. Tanaka and coworkers from Massachusetts Institute of Technology gave the first theoretical description of the collapse of charged networks in the absence of salt [7]. Further theoretical studies in this field were made by M. Ilavsky at the Institute of Macro-molecular Chemistry in Prague [18] and also at the Moscow State University [19, 20]. 

Furthermore, it has been found that the so-called reentrant jumpwise collapse can be realized when a charged network swells in a mixed solvent [31]. In this case, the network having a globular structure both in A or B one-component solvents decollapses sharply in some intermediate composition range or the network swollen in pure solvent shrinks abruptly in mixed solvent (Fig. 7). 

Collapse of Charged Networks in the Absence of Low-Molecular Weight Salts... 

First of all, we should point out that the majority of experimental findings on the collapse were made on the networks which contain only the charges of one sign. Before 1982, the phenomenon of the collapse was observed only for weakly charged networks on the basis of PAA in the mixtures of water with acetone. Then significant efforts were made to find out new systems, which exhibit the... 

The systematic investigation of the influence of the topological structure of weakly charged networks on the type of occurrence of the transition in the collapsed state was performed by Ilavsky and coworkers [42, 43]. In these papers, the authors studied the swelling and the elastic characteristics of PAA gels, containing a small number of anionic SMA groups. It has been shown that ... 

When our work started, the phase transitions was observed only for weakly charged networks of PAA gels swollen in the mixtures of water (good solvent) with acetone (precipitant) of different compositions. The first stage of our work was the investigation of the nature and polarity of a precipitant on the position and the amplitude of the phase transition. According to the results of theoretical consideration of Refs. [7,18,20], the transition point and the value of the jump of the volume are primarily determined by the network structure and by the parameter of polymer-solvent interaction Xns- By smoothly changing the composition of the binary solvent, it is possible to vary effective value of the Xns parameter and to convert the network to a collapsed state. In this case, the amplitude of phase transition should not depend on the nature of precipitant. 

As mentioned above, the discrete collapse of the gels is usually observed for charged networks. In Ref. [46], we showed that it is possible to obtain a jumpwise change of the dimensions for neutral networks by incorporation in a neutral gel of some charged linear macromolecules. In this case, the counter ions situated inside the network create there the same osmotic pressure as in the network, with some chemically connected charged groups. 

We have considered above the predictions of the general theory of the swelling and collapse of charged networks which contain both positive and negative charges. These predictions were first checked in Ref. [15]. The objects of the investigation were the copolymers of AA with SMA and 2-methyl-5-vinyl-pyridine, quatemized by dimethyl sulfate (MVPQ), crosslinked with BAA. The solvents were mixtures of water with ethanol. 

Phase Transitions in Charged Networks Induced by External Mechanical Force... 

The first quantitative theory of the reentrant collapse was developed in Ref. [49], The theory explained the phenomenon of the simple reentrant collapse which was observed in Refs. [14, 41]. A more general theory of swelling and collapse of charged networks in the binary solvent was developed in Ref. [31] and described in Sect. 2.4.1. We have seen that one of the most essential features of the swelling behavior in mixed solvents is a redistribution of solvent molecules within the network giving a different solvent composition in the gel and the external solution. This redistribution is more pronounced for the collapsed gel, because the probability of contacts of the molecules of the solvent with polymer links in the collapsed gel is higher than in the swollen gel. 

In addition to theoretical works, some experimental papers have been published recently which are devoted to the study of polyelectrolyte complexes formed by macroions with oppositely charged networks [50-52]. Also, in recent publications by Osada interpolymer complexes (I PC) were described which are formed by the networks of PMAA with polyethylene glycol) (PEG) and some other polymers [53-55]. 

PDADMABr gels was significantly higher than in the micelles of SDS in water. At the same time, for the more hydrophobic CTAB, the polarity of the microenvironment of the probe in micelles in the PMAA network is low in comparison to that of the micelles in an aqueous medium. Thus, the results obtained confirm the theoretical prediction that the CMC in charged networks is much lower than in the solution (see Sect. 2.5). At the same time, these results show that there is a significant difference between the structure of micelles which are formed in polyelectrolyte gels and in water. 

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]. 

The simplest example of a charged network based on trans-1,2-diaminocyclohex-ane is based on the carbon dioxide complex [51]. A forced carbonation of (R,R)-trans-1,2-diaminocyclohexane with carbon dioxide afforded the corresponding crystalline carbamate salt 41 (from ethanol-water), which was suitable for X-ray analysis (Scheme 18) [51,62],... 

In recent publications, it has been shown that the conformational state of polyelectrolyte networks can be controlled by external physical factors such as the electric field [69] and visible light [70,71], This fact opens new possibilities for the control of the properties of charged networks and may be of significant practical interest. The first experiments in this direction were made by Tanaka... 

Gitlin I, Carbeck ID, Whitesides GM. Why are proteins charged Networks of charge-charge interactions in proteins measured by charge ladders and capillary electrophoresis. Angew. Chem. Int. Ed. 2006 45 3022-3060. 

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