Weakly Ionized Gels

In Sect. 2.2, it was shown that the transition temperature at which a volume-phase transition occurs is very sensitive to the fraction of ionization, f. In the case of these NIPA gels, an addition of a slight amount of acrylic acid (AAc) as a comonomer increases the transition temperature quite significantly as shown in Fig. 6. What happens in the microscopic structure of there gels Shibayama 

The appearance of the scattering maximum in SANS was observed by several workers particularly for polyelectrolyte solutions [83]. The mean field theories used to predict such scattering curves were presented by Borne et al. [84] and Joanny et al. [85]. Recently, such a peak was also observed in weakly ionized polymer gels as well by Schosseler et al. [78, 79]. Let us discuss the origin of the peak at high temperatures. 

According to the Borue and Erukhimovich theory for polyelectrolyte solutions [84], the scattered intensity function for ionic gels may be given by the 

Volume Phase Transition and Related Phenomena of Polymer Gels 

The reduced scattering vector is given by using the characteristic scale screening of Coulombic interaction by ideal Gaussian chains, r0, as follows 

The appearance of the 2ittering maximum in SANS was observed by several worlters partioilariy for pol lectrolyte solutions [83]. The mean field theori u to predict sudi scattering oirves were presented by Borne et ak 

Joanny and Leibler obtained the same result to Eqs. (4.11), (4.13), and (4.18) for weakly charged polyelectrolyte solutions. The long spacing D of the concentration fluctuation is given by 

---

When an ionizable polymer is chemically cross-linked to form a three-dimensional network, an increase in the ionization of the network brings about extensive swelling of the gel, which can be observed visually on a macroscopic level. The expansion of the conformation is due to an increase in the electrostatic potential appearing on the macromolecules. On the basis of swelling and contraction of a weak polyelectrolyte gel, Katchalsky, Kuhn, and co-workers [25-27J proposed a muscle model referred to as mechanochemical or later as a che-momechanical system [1,28]. 

The reverse process, i.e., mechanically induced changes in shape of a cross-linked weak polyelectrolyte, should cause a change in its ionization state. When a piece of weak polyelectrolyte gel was placed in a cell and compressed from the vertical direction using a glass piston (Fig. 8a), the pH of the gel changed as shown in Fig. 9. When the gel was unloaded, the pH quickly returned to its original value with some overshoot. 

The mechanically induced ionization was observed only for weak polyelectrolyte gels, which can sensitively change their ionization state with respect to their conformation, and not for the fully ionized strong polyelectrolyte gels. 

In fact, if a weak electrolyte gel such as a poly(acrylic acid) is deformed, macroscopic dielectric polarization is observed. This results from the stretching of polymer chains by the deformation and resultant automatic acceleration of ionization. Utilizing this phenomenon, it is possible to develop a piezoelement that converts deformational stress into electrical energy. A pressure sensor whose diode emits light when the gel is pressed and an artificial contact sensing device also have been proposed [20]. 

Recently, an attempt was made to design a polyelectrolyte gel as a mechanoelectrical conversion system based on the spontaneous ionization induced by mechanical deformation that produces an electric potential of several millivolts. Actually, the generation of an electromotive force has been confirmed for the deformation of a weak polyelectrolyte gel such as PAA [67]. When a piece of gel was compressed, the pH of the gel changed, and when the gel was unloaded the pH value quickly recovered to the original value. Because there was no water outlet in the experimental course of the deformation, the pH change should be associated with an enhanced ionization under deformation. On the basis of this phenomenon, a soft pressure-... 

The potentiometric titration curves of gels, which relate the pH of the exterior solution to the degree of ionization of the gel, resemble the titration curves of monofunctional acids or bases. However, the dissociation constants differ, often by two orders of magnitude, from the expected value for the functional group, and the slope of the curves is not the usual one. Addition of neutral salt changes the picture markedly and brings the curves closer to expectation. In the case of weak or medium... 

Han and Bae have demonstrated that an aqueous solution of a copolymer of 2-5mol% acrylic acid and NIPAAm exhibits reversible gelation without significant hysteresis (8). The critical gel concentration (CGC) in this case was 4% by weight. Gelation, rather than precipitation, was attributed to polymer chain entanglements and to the weak physical association of polymer precipitates while hydration was maintained by an ionized segment of AAc (7,8). 

An unfavorable property of silica is its solubiHty at high pH, which, however, is rarely applied with the mobile phases used in NPLC. Silica gel has weak cation-exchange properties (the silanols on the silica gel surface may be ionized to SiO groups), whereas the bonded amine stationary phases behave as weak anion exchangers. This mixed retention mechanism may cause excessive retention and tailing or even split peaks of ionic or ionizable compounds. 

---