Volume changes, gels

The parameters which characterize the thermodynamic equilibrium of the gel, viz. the swelling degree, swelling pressure, as well as other characteristics of the gel like the elastic modulus, can be substantially changed due to changes in external conditions, i.e., temperature, composition of the solution, pressure and some other factors. The changes in the state of the gel which are visually observed as volume changes can be both continuous and discontinuous [96], In principle, the latter is a transition between the phases of different concentration of the network polymer one of which corresponds to the swollen gel and the other to the collapsed one. 

Gels Stress 0.1-0.3 Mpa Shows large volume change Slow response times, transport... 

A procedure for characterizing the rates of the volume change of gels has not been uniformly adopted. Often, the kinetics are simply presented as empirical sorption/desorption curves without quantitative analysis. In other cases, only the time required for a sample of given dimensions to reach a certain percentage of equilibrium is cited. One means of reducing sorption/desorption curves to empirical parameters is to fit the first 60% of the sorption curve to the empirical expression [119,141]... 

M Marchetti, S Prager, EL Cussler. Thermodynamic predictions of volume changes in temperature-sensitive gels. 1. Theory. Macromolecules 23 1760-1765, 1990. 

BG Kabra, MK Akhtar, SH Gehrke. Volume change kinetics of temperature-sensitive poly(vinylmethylether) gel. Polymer 33 990-995, 1992. 

J Singh, ME Weber. Kinetics of one-dimensional gel swelling and collapse for large volume change. Chem Eng Sci 51 4499-4508, 1996. 

The volume change occurring on dehydration of a typical silica gel is shown in the following curve ... 

As discussed in this section, the contribution of the shear relaxation is not trivial. This is due to the fact that the diffusion occurs in all three dimensions for the spherical gel, two dimensions (radial direction) for the cylindrical gel, and only one dimension (thickness direction) for a slab gel. Because of the existence of shear modulus, the volume change caused by diffusion is shared by the remaining dimensions through the shear relaxation process. A detailed discussion is given by Li and Tanaka [93],... 

Fig. 24a-c. a. Equilibrium radius of a NIPA gel sphere as a function of temperature. At lower temperatures the gel is swollen and at higher temperatures it is shrunken. At about 34 °C the swelling curve becomes infinitely sharp, which corresponds to the critical point, b. Relaxation time of gel volume change in response to a temperature jump, as a function of temperature, c. Thermal expansion coefficient, the relative radius change per temperature increment, also diverges at the critical point... 

Fig. 25a, b. a. Collective diffusion coefficient D of a NIPA gel as determined by the kinetics of volume change, as a function of temperature. It diminishes at the critical point, b. collective diffusion coefficient as determined from the density fluctuations by use of photon correlation spectroscopy. The agreement between the results obtained from dynamics of microscopic fluctuations and from kinetics of macroscopic volume change is excellent considering the difficulty in the dynamic experiments... 

The second example used visible light absorption that increased the temperature locally within the thermosensitive gel [39]. The gel consisted of a covalently cross-linked copolymer network of N-isopropylacrylamide and chloro-phyllin, a combination of a thermo-sensitive gel and a chromophore. In the absence of light, the gel volume changed sharply but continuously as the temperature was varied. Upon illumination the transition temperature was lowered, and beyond a certain irradiation threshold the volume transition became discontinuous. The phase transition was presumably induced by local heating of polymer chains due to the absorption and subsequent thermal dissipation of light energy by the chromophore. The details will be discussed in a later section. 

Fig. 31. Temperature dependence for equilibrated volumes of NIPA gel including the Con A-DDS complex (DSS-gel, open circles), MP (MP-gel, filled circles), and free of both DSS and MP squares). The latter was prepared as a control sample. Hysteresis was observed in the volume changes of DSS-gel and the free-Con A gel on heating and cooling, indicating a discontinuous phase transition. The diameter of each gel in the collapsed state, determined at 50 °C, was do = 0.074 mm the volume of this gel is denoted by V0. The concentration of dry matter in the collapsed state was estimated from the preparation recipe to be 90wt%.

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