Shrink-swell property

Of particular interest for chemical transport into a predominantly smectite medium is the shrink-swell property of the clay material. The swelling properties of smectites are explained by two concepts. The first one, developed by Sposito (1973), shows that smectite swelling is caused by the hydration and mobility of the cations, which in turn balance the negative charge of the layer silicates. The second concept, presented by Low (1981), emphasizes the direct interaction of water molecules with the silicate surface. Both viewpoints fit the common observation that smectite swells in a high-hydration environment and at low electrolyte concentrations and shrinks when water is lost and salt is added to the bulk solution. 

Klcinebudde P. Shrinkage and swelling properties of pellets containing microcrysullinc cellulose and low substituted hydroxypropyl cellulose I. Shrinking properties. Int J Riarm 1994 109 209-19. 

Gel-Type Polystyrenes. Gel-type polystyrenes can be synthesized by radical suspension polymerization of styrene and DVB as reported in 1946 by Hohenstein and Mark (9,32). When suspended in a solvent with good swelling properties, these white soft beads assume a gelatinous state their volume may increase by 6-8 times and they become optically transparent. This process of swelling, as well as the shrinking, occurs from the outside to the inside of the polymer network and the increment of volume is in correlation with its solvation. This plays an important role in chemistry, because it is through such processes that the reactants diffuse into the polymer network where 99% of the functional sites reside (33). 

Even when they have a partial crystallinity, conducting polymers swell and shrink, changing their volume in a reverse way during redox processes a relaxation of the polymeric structure has to occur, decreasing the crystallinity to zero percent after a new cycle. In the literature, different relaxation theories (Table 7) have been developed that include structural aspects at the molecular level magnetic or mechanical properties of the constituent materials at the macroscopic level or the depolarization currents of the materials. 

Indirect evidence for solvent exchange can also be gained by in-situ ellipsometry. This technique can provide information of film thickness and optical properties as a function of the electrode potential. Figure 2.19 shows that the film swells during oxidation and shrinks during reduction, in agreement with EQCM measurements. A decrease in the refractive index from 1.42 to 1.38 during oxidation also supports the uptake of water, which has a refractive index of 1.33. 

The relation between the response time and reciprocal of the heating rate of the gel is shown in Fig. 21. Hie response time at an infinite heating rate is estimated to be about 0.25 s and is very fast. These gels swell and shrink responding to temperature change isotropically. Mechanical properties of the gel are shown in Table 1. The mechanical strength increases remarkably by phase transition of... 

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