Swelling of Networks and Responsive Gels

x is the number of repeat units in one network chain, /-q the number of solvent molecules, n2 the total number of network chains in the system, i the number of ionic groups on the chains, v the number of chains, and v20 the volume fraction of chains during the formation of the network. 

Equating the chemical potential to zero gives a relationship between the equilibrium degree of swelling and the molecular weight Mc. The relation for Mc ph is obtained for a tetrafunctional phantom network model as 

For the affine network model, the molecular weight between cross-links MC/af is obtained as 

When the network chains contain ionic groups, there will be additional forces that affect their swelling properties. Translational entropy of counterions, Coulomb interactions, and ion pair multiplets are forces that lead to interesting phenomena in ion-containing gels. These phenomena were studied in detail by Khokhlov and collaborators [74-77]. The free energy of the networks used by this group is 

FIGURE 4.5 A gel exuding solvent upon decrease in temperature, with the shrinkage (syneresis) generally described as gel collapse.  

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Based on the solution property of poly (DMAEMA-co-AAm) in response to temperature, the temperature dependence of equilibrium swelling of poly (DMAEMA-c6>-AAm) gel as a function of chemical composition was observed as shown in Figure 6. The transition temperature of copolymer gel between the shrunken and swollen state was shifted to the lower temperature with increases in AAm content in the gel network. This is attributed to the hydrogen bond in the copolymer gel network and its hydrophobic contribution to the LCST Copolymer II gel was selected as a model polymer network for permeation study because it showed the sharp swelling transition around 34°C. 

Before the phase transition was found, a shrinking and swelling effect of an electric field was recognized and studied by several researchers [40-43]. Tanaka and colleagues found the phase transition in hydrolyzed acrylamide gel in 50% acetone/water mixtures. Their original interpretation that the electrophoresis of the polymer network might be responsible for the phase transition does not seem correct [44]. The most important effect seems to be the migration and redistribution of counter and added ions within the gel [45]. 

In this article, we review recent advances in mechanical and swelling behavior of magnetic field-responsive soft materials, including flexible polymer networks and gels. 

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