Controlled swelling

Hydrogels. Controlled swelling of hydrophilic polymers, derived from the glossy/mbbery properties of polymers, is used to control the rate of dmg release from matrices. In the mbbery state, accompHshed by lowering the polymer s glass-transition temperature to an appropriate level, the dispersed dmg diffuses as the polymer swells in the presence of water. 

Since the equilibrium volume degree of swelling Q equals l 2, this equation shows that the parameters which control swelling are the polymer-solvent interaction parameter % and the effective cross-link density pt. Figure 2 shows that Q falls as either % or p) increases, although the influence of the cross-link density becomes negligible in poor solvents. 

Adrenocorticoid hormones are produced in the adrenal glands. They regulate a variety of metaholic processes. The most important mineralo-corticoid is aldosterone, an aldehyde as well as a ketone, which regulates the reahsorption of sodium and chloride ions in the kidney, and increases the loss of potassium ions. Aldosterone is secreted when hlood sodium ion levels are too low to cause the kidney to retain sodium ions. If sodium levels are elevated, aldosterone is not secreted, so some sodium will he lost in the urine and water. Aldosterone also controls swelling in the tissues. 

Harsh DC (1992) Controlling swelling characteristics of novel cellulose ether gels. Thesis, University of Cincinnati, Cincinnati OH... 

The first attempt at applying SSPGs in the field of functional immobilized biocatalysts was made by Park and Hoffman [63-65], who demonstrated the capability of thermosensitive gels for enhancing the diffusion of a substrate from the outside to the inside of a gel, which is based upon the possible absorption of the surrounding medium by the gel during a thermally controlled swelling process (see Fig. 2). 

Fig. 2. Schematic illustration of cyclic pumping of surrounding medium (represented by ) by thermosensitive gel during its thermally controlled swelling and collapse...

Norrish, K. and Quirk, J.P., 1954. Crystalline swelling of montmorillonite. Use of electrolytes to control swelling. Nature (London), 173 255—256. 

The kinetics of water transport has a major impact on many applications. Very rapid kinetics of fluid uptake is vital for products such as diapers and hygiene tissues, whereas controlled swelling and subsequent dissolution are... 

The origin of life in aqueous milieux and the necessity of the internal aqueous environment in living systems leads us to expect the hydration and swelling of biomolecules to be a central feature of their organization and function. Direct measurement of intermolecular forces in aqueous solutions now shows us new ways in which nature achieves the controlled swelling essential to living matter. 

Zhang Y, Won CY, Chu CC. Synthesis and characterization of biodegradable network hydrogels having both hydrophobic and hydrophilic components with controlled swelling behavior. J Polym Sci Part A. Polym Chem 1999 37 4554 569. 

Self-oscillatory patterns can be created by repeating swelling and de-swelling of the polymer network in a closed system, i.e., without any external stimuU. In comparison to stimuli-induced patterns (summarized above), self-oscillatory patterns have no on-off switch, and can repeat itself periodically, such as a beating heart (Fig. 9.9). The idea was based on Belousov-Zhabotinsky (BZ) type reaction [66] to induce spontaneous temporal change in the redox potential to control swelling... 

Another approach is to imbibe water-insoluble absorber dyes into latexes by controlled swelling. A latex is put into a solvent mixture which causes swelling and which also solubilizes the dye of interest. As the latex swells, the solubilized dye is imbibed into the particle. The solvent composition is then shifted to cause the latex particle to condense, with entrapped absorber dye. Provided that the latex polymer and dye are sufficiently miscible, the dye may be incorporated into any layer by adding a suspension of the loaded latex into the layer melt composition. 

Butler reported the chromatography of cationic polymers on aminated and quaternized polystryrene-divinylbenzene packings (52). This approach has apparently since been abandoned, possibly because of difficulty in controlling swelling and porosity. Talley and Bowman prepared a cationic CPG by deriva-... 

In addition, because of the presence of —OH groups, natural polysaccharides may be modified by controlled chemical reaction to give derivatives with new specific properties industrial derivatives are mainly obtained from cellulose (qv), starch (qv), and chitin and chitosan (qv). The presence of these —OH functional groups is also the origin of interaction with water molecules (hydrophilic character of oligo- and polysaccharides) and intra- and interchain H-bond network formation playing a role in reactivity control, swelling, or dissolution rate. 

These acids are also known to be very effective for controlled swelling of calf skins. This property is not only due to their dipole properties but is also due to their tendency to enter and form a hydrogen bond [108]. The physical properties of some of the organic acid are given in Table 12.10 (see Fig. 12.12). These properties are important from a thermal stability point of view. 

The pH of the fluid, and many other factors also control swelling. For example, hydrogel based systems exist for protecting acid-sensitive medications from the hostile environment of the stomach by shrinking when exposed to low pH conditions, but expand in the more alkaline environment of the intestines (83,84). 

Many types of polymers, both glassy and rubbery, are cross-linked to improve elastomer behavior or to control swelling. 

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