Examples of gels

Branched polymers are produced by the polycondensation of multifunctional molecules whose functionality is greater than or equal to 3. If the reaction proceeds to the stage where the products grow as large as the space dimensions of the entire system, a three-dimensional network whose parts are connected by covalent bonds is formed. This is the gel point. The reaction continues after this gel point is passed. The polycondensation reaction is irreversible under ordinary conditions, so that the gelation of chemical gels is irreversible. 

Let us use the symbol R A/ for a monomer unit carrying the number / of A functional groups, R AB/ i for a monomer carrying one A functional group and / - 1 of B functional groups, etc. 

Physical cross-linking often makes complex junctions. The cross-links are not strictly localized but extend in the form of junction zones. 

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A simple example of gel formation is provided by chitosan tripolyphosphate and chitosan polyphosphate gel beads the pH-responsive swelling abihty, drug-release characteristics, and morphology of the gel bead depend on polyelectrolyte complexation mechanism and the molecular weight. The chitosan beads gelled in pentasodium tripolyphosphate or polyphosphoric acid solution by ionotropic cross-hnking or interpolymer complexation, respectively. 

AMsopropylacrylamide (NIPA) gel is a typical example of gels which shows a thermoshrinking type of phase transition in aqueous solutions [2]. As shown in Fig. 1, an increase in the temperature causes this particular gel to shrink by one order of magnitude. 

Statistically defined structures may also arise from the formation of crosslinks in a melt the resulting gels are described within a percolation framework which predicts the existence of definite meshes [7, 8]. Contact-lenses, jellies or even jellyfish are common examples of gels. Latex beads with specific functionalities attached, such as antigens, are used in biodiagnostics. 

Visualize and photograph the agarose gel stained with ethidium bromide under transillumination at 300 nm (UV light). The nucleic acid-lipoplex should remain inside the well, while the free or weakly bound nucleic acid should run in the gel. An example of gel retardation assay is shown in Fig. 2. 

Pectin, guar and glucomannans are examples of gel-forming fibres, whereas cellulose, lignins and seed coats are examples of nonsoluble fibres. The non-soluble fibres bind water and swell and keep water in the intestine, while the gel-forming fibres are degraded, which make them unsuitable as laxatives. 

This method is a description of the use of gel filtration for desalting or buffer exchange, while Subheading 3.2.2 describes the use of gel filtration for purification of some classes of antibodies. Desalting and buffer exchange of protein samples are particular examples of gel filtration in which the size difference of the molecules to be separated is large (at least a factor of 10). 

Table 7.2 Composition and electrochemical properties at 25 °C of some selected examples of gel-type polymer electrolytes (average thickness = 100 mm) ...

Figure 2 shows a typical example of gel-filtration (Sepharose 4B) for isolating multilamellar (fr. 9-13) and single-walled... 

Fig. 2. A typical example of gel-filtration of liposomes coated with FITC(0.54)-OPP-50(1.8) from the free FITC-OPP developed with 200 mM Tris-HCl (pH 8.0) as an eluant. Liposomes were detected by turbidity at 220 nm (— —) and fluorescence intensity of FITC moiety at 520 nm (—O —) Coating with the polysaccharide was perfomed by incubating 3.0 ml of a sonicated liposome suspension prepared from 30 mg of egg lecithin with 1 ml of 200 mM Tris-HCl containing 5 mg of the FITC-OPP at 25 C (see text).

Sols that become viscous and semisolid are called gels. In these colloids, the solid dispersed phase has a very high affinity for the dispersing medium. The gel sets by forming a three-dimensional network of solid and dispersing medium. Other examples of gels are fruit jellies and canned heat (jellied alcohol). 

Numerous examples of gels based on polymers may be identified Gels produced by overlap or entanglement of polymer chains (physical gels), gels produced by association of polymer chains ( associative thickeners ) and gels produced by physical or chemical cross-linking of polymer chains. 

The gel films formed at the interface between hexane and ammonia water did not shrink upon drying and remained intact through out the process until completely dried on a silica glass substrate. They were translucent or white in appearance depending on the pH of ammonia water and the concentration of E-40 in the precursor solutions. Some examples of gel film obtained by the ammonia-catalyzed reaction of E-40 are shown in Figure 16-6. 

Table 1 Characteristics, functions and application examples of gels.

Biomembranes perform simple and/or eomplex functions of filtration, permeation, and transport. These fimctions work either in series or parallel. They also function selectively. To imitate these functions, sensitivity in gels can be developed Ity varying crosslink density. Selective interaction such as compatibility or solubility with a penetrant is also required. An example of gels in vdiich permeability is important is soft contact lenses [9]. The oxygen permeability of contact lenses is known to increase as the water content of the gel increases. For contact lenses, other parameters, such as biocompatibility, arc important. For further details, readers are referred to Chapter 4, Volume 3, Applications. 

The number of application studies for gels to be used in DDS is increasing as the numbers from the Pharmaceutical Society, DDS, Society, and Association of Gel Research reveal. Table 3 summarizes the typical examples of gels for DDS. 

A search of patent literature yields examples of gel development or application to sporting goods, even if they are not yet commercialized. Of those examples found, the one that interests the author is selected and... 

Tabic 1. Bectrodiemical properties at 25 C of soroe selected examples of gel-type polymer dectrolytes (average thickness 100 m). Reproduced from reference 28 with permission from Elsevier Science. 

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