Polyampholyte gels

Annaka and Tanaka recently found the presence of several phases in polymer gels between the fully swollen and shrunken phases [46]. This interesting phenomenon was observed in a polyampholyte gel, consisting of acrylic acid (anionic constituent, AAc) and methacryl-amido-propyl-trimethyl-ammonium-chloride (cationic, M APT AC). The chemical structures of these constituents are shown in Fig. 32. A series of copolymer gels were prepared by radical copolymerization, where the total molar concentration of the constituents was kept constant at 700 mM and the molar ratio of AAc/MAPTAC was varied systematically. 

The stable multi-phases are found not only in polyampholyte gels but also in a homopolymer gel, e.g. poly(/V-acryloyl-4-aminosalicyclic acid) gel [130], and... 

Collapse of Polyampholyte Gels in Salt-Free Solutions.152... 

In practice, the aqueous solution of proteins to be separated is injected by a syringe into the central section of the polyampholyte gel specimen. Then, the DC electric field is switched on, which induces the formation of a hnear pH gradient along the gel sample. In its turn, the generated pH gradient promotes the migration of protein molecules until they are gradually locahzed at their isoelectric pH. Protein molecules will be separated and concentrated if the isoelectric pH of the proteins coincides with appropriate pH zones of the amphoteric gel. 

Figure 35 combines the temperature-dependent and acetone-content-de-pendent behaviour of amphoteric gel APTAC-SA [155]. Equimolar polyampholyte gel undergoes a gradual volume change w ith an increase in acetone composition in an acetone/water mixture. When the content of the cationic groups increases, collapse of gel is observed. On the other hand, the same sample immersed in acetone/water mixtures of 60 and 65% acetone swells in a low... 

Synthetic polymer gels are known to exist in two phases, swollen and collapsed. Polyampholyte gels should show a re-entrant volume transition in response to pH [164]. Tanaka et al. [165] have reported the existence of more than two phases in randomly distributed polyampholytes of MAPTAC and AA. Let us consider the behaviour of a sample containing 65 mol% of AA and 35 mol% of MAPTAC. At neutral pH this sample has a diameter d=do that is referred to as phase 1 (d/do=l) according to the label in the original paper. At pH=8.5 the gel swells discontinuously to phase 2.7 (Fig. 38). If the pH is lowered from 8.5 to 7.0, the gel returns to phase 1 discontinuously. If instead the pH is increased further from 8.5 to 9.8 and goes back from this point the gel collapses into phase 1 at pH=6.4. The same experiments were done from the acidic side. NMR spectra... 

It is well known that polyelectrolyte gels swell, shrink or bend when DC electric current is applied [169]. These properties of gels are applicable for the construction of chemomechanical devices, artificial muscles, energy conversion systems etc. [170]. Osada and co-workers [171] have constructed an eel-like gel actuator on the basis of poly(2-acrylamido-2-methylpropane sulfonic acid) and studied its chemomechanical properties. Polyampholyte gel is bent to the cathode or anode side if it has predominantly negative or positive charges along the macromolecules (Fig. 39) [172]. As seen from Fig. 39, the amplitude of deflection is gradually decreased with the approach to the lEP. This is probably due to... 

Fig. 39. Dependence of amplitude of deflection on pH of the external solution for polyampholyte gel of the vinyl ether of monoethanolamine-acrylic acid (VEMEA-AA) with...

Fig. 40. Relative mass change of polyampholyte gels of the vinyl ether of monoeth-anolamine-acrylic acid (VEMEA-AA) under the influence of DC electric current at pH=3.0 (curve 1), 4.2 (lEP) (curves 2,4,5), 6.2. (curve 3) at p=M0 (curves 1-3), 1.5-10" (curve 4) and 2.5 10-2 M NaCl (curve 5)...

The formation of coordination and ionic bonds between functional groups of VEMEA-AA and metal ions is accompanied by the gradual shrinking of polyampholyte gel [193]. During the complexation process a shell layer is formed on the surface and moves to the core region of the gel as schematically illustrated below ... 

It should be noted that the concentration of absorbed copper reaches a constant value after 2 h while the volume contraction of gel continues to change for up to 10 h. This is probably due to the rearrangement of polyampholyte-copper complexes. The complexed layer will probably retard the further penetration of copper ions deep in the gel and the complex formation kinetics are diffusion limited. Figure 42 shows the kinetics of deswelling of polyampholyte gel and desorption of Cu + at various pH. The desorption of Cu + from the inside of the gel is accompanied by swelling of the gel. The degree of recovery of copper ions at the lEP of polyampholytes reaches up to 60-70%. 

Evidently, polyampholyte gels belong to multiphase systems due to the abiUty of macromolecules to adopt different stable conformations in response to changes in environmental conditions. Therefore, more extensive study is needed to identify the microscopic structure of multiphases. Stimuli of the responsive... 

Polyampholytic gels swell in addic and alkaline medium, but reside in a most contracted state at an intermediate pH couesponding to an eqrrimolar ratio of positively and negatively charged units (isoelectric point). Gel contraction at the isoelectric point is related both to a decrease in the osmotic pressrue of cormterions and to the Coulomb attraction of oppositely charged network units. 

A qualitatively different behavior is observed at the addition of low molecular mass salts to polyampholyte gels. Here, establishing the Donnan equilibrium results in higher salt concentration inside the gel than outside it. In the gel, the salt screens the attraction of the oppositely charged network units thus favoring the swellmg. ... 

Sutani, K., Kaetsu, L, Uchida, K. and Matsubara, Y. (2002). Stimulus responsive drug release from polymer gel Controlled release of ionic drug from polyampholyte gel. Radiation Physics and Chemistry, 64,331-336. 

Structure of Polyampholyte Gels and Their Behavior with Respect to Applied External DC Electric Field... 

Tab.3. Composition of monomers in the feed, composition of copolymers, swelling degree and the lEP of polyampholyte gels...

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