Shape-changing gels

An outstanding feature of the adsorption of water vapour on silica is its sensitivity to the course and subsequent treatment of the silica sample, in particular the temperature to which it has been heated. Figure 5.15 shows the strong dependence of the isotherm for a particular silica gel on the temperature of its heat treatment the isotherm is progressively lowered as the temperature increases, especially above 400°C, and the shape changes from Type II for the lower temperatures to Type III for 600°C, 800°C and 1000°C. 

As discussed in the previous section, the molecular interactions rule the mam)-scopic size and shape of gels. Since these interactions are functions of temperature, polymer concentration, solvent composition (if a mixture of solvents is used), and pH and salt concentration (for geb capable erf ionization), the volume phase transition can be induced by controlling one or some of these parameters. Before the phase transition was found in gels, various researchers had developed gels that change their degree of swelling when a stimulus b applied to them. This article, however, will describe only the systems that use the phase transition phenomenon. 

It seems possible to amplify the photostimulated conformational changes in solution at the molecular level into shape changes of polymer gels or solids at the visible macro level. The first proposal to use the structural changes at the molecular level for direct conversion of photon energy into mechanical work has been made by Merian (13.) in 1966. Since then, many materials, most of which contained azobenzene chromophores, have been reported to show photostimulated deformation(JM). Till now, however, the reported deformations were limited to less than 10%. In addition, Matejka et. al. have pointed out that in many cases photo-heating effect instead of photochemical reaction plays a dominant role in the deformation(15,16). 

Figure 4 shows the size and shape changes of the gel before and after photoirradiation. When the whole gel was irradiated with ultraviolet light, both the gel length and diameter expanded by as much as two times(Figure 4B). When the irradiation beam was localized to one side of the rod shaped gel, the gel showed bending motion (Figure 4D).

Figure 4. Photostimulated size and shape changes of polyacrylamide gels having triphenylmethane leucocyanide groups (3.1 mole %) (A) before photoirradiation, (B) the whole gel being irradiated with ultraviolet light, (C)before photoirradiation, (D) upper side of the rod shaped gel being irradiated.

Fig. 10 Illustration of the helix-to-coil transition, anisotropic-to-isotropic transition, and shape change of the uniaxial PHEG gel. Reprinted with permission from [94], Copyright 2012 American Chemical Society...

Figure 4 Shadowgraph snapshots of the dynamical shape changes in the oscillatory regime. Arrows point in the direction of the main swellingdeswelling dynamics. Snapshots from (a) to (f) at 0, 20, 30, 40, 44, 00 minutes, cover about one period of oscillation of the mid-heigth point of the gel Experimental conditions d=0.5mm, [OH ]o=4-25xlO M. White scale bar in (a)=3mm.

When a water-swollen cross-linked polyelectrolyte gel is inserted between a pair of electrodes and a DC voltage is applied, it undergoes anisotropic contraction and concomitant fluid (water) exudation [13,20]. Figure 4 shows a schematic view of the shape change in an anionic gel with time in an electric field [21]. It is found that... 

FIGURE 4 Schematic view of shape change of an anionic gel with time in an electric field. (From Ref. 21.)... 

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