Reversible gel

Uses of gelatin are based on its combination of properties reversible gel-to-sol transition of aqueous solution viscosity of warm aqueous solutions abUity to act as a protective coUoid water permeabUity and insolubUity in cold water, but complete solubUity in hot water. It is also nutritious. These properties are utilized in the food, pharmaceutical, and photographic industries. In addition, gelatin forms strong, uniform, clear, moderately flexible coatings which readily sweU and absorb water and are ideal for the manufacture of photographic films and pharmaceutical capsules. 

Molded polyamide surfaces can be hardened by grafting with Ai,Ai-diallylacrylamide [3085-68-5] monomer under exposure to electron beam (159). AijAZ-DiaHyltartardiamide [58477-85-3] is a cross-linking agent for acrylamide reversible gels in electrophoresis. Such gels can be dissolved by a dilute periodic acid solution in order to recover protein fractions. 

Certain organic compounds form reversible gels with poly(vinyl alcohol). Congo red, for example, yields a red gel that melts sharply at about 40°C. Other organic compounds that form temperature-reversible complexes with PVA include a2o dyes, resorcinol, catechol, and gaUic acid (168—170). 

One of the simplest ways to prepare a chitin gel is to treat chitosan acetate salt solution with carbodiimide to restore acetamido groups. Thermally not reversible gels are obtained by AT-acylation of chitosans N-acetyl-, N-propionyl- and N-butyryl-chitosan gels are prepared using 10% aqueous acefic, propionic and bufyric acid as solvents for treatment with appropriate acyl anhydride. Both N- and 0-acylation are found, but the gelation also occurs by selective AT-acylation in the presence of organic solvents. 

It has been established 25) that a firm, thermally reversible gel of poly (vinyl alcohol) can be formed by mixing it with polyhydric phenols, 1-naphthols, and dihydric naphthols. Monohydric phenols have no action. Compounds such as catechol, phloroglucinol, and 1-naphthol are typical gelling agents. 

Thermally reversible gels can be prepared from poly (vinyl alcohol) and alkali metal salts of o-hydroxybenzal derivatives having benzenoid groups at both ends 24). Colored gels can be obtained, depending on the type of ketone used (Figure 4). 

The property of thermal, reversible gelation is obtained by the addition of water-soluble proteins and protein degradation products to an aqueous solution of poly (vinyl alcohol) 2). Protein products such as albumin, gelatin, glue, a-amino acids, and their condensation products—diketopiperazines—may be used. A typical formulation for the preparation of a thermally reversible gel is ... 

Reversible gels which are dispersible on heating and are soluble in excess. These have secondary valence or ionic bonds... 

The poly(vinylpyridine) and poly(tert-butyl methacrylate) copolymers can easily be converted to either cationic or anionic polyelectrolytes by protonation of the pyridine rings or by base hydrolysis of the tert-butyl ester units, respectively. The highly branched structure of the molecules, in combination with the polyelectrolyte effect, should confer useful properties to these materials in solution for applications such as pH-sensitive reversible gels. 

The features common to reversible polymer gels of many types are identified suid discussed. The nature of the gel state is carefully defined, and a novel classification scheme based on morphology, rather than chemical or mechanistic considerations, is proposed. The article also serves as an overview to some of the more commonly used techniques used in the study of gels, and as an introduction to some of the current trends in reversible gel research. Some speculations regarding future trends in reversible gel research are presented. 

The remainder of this chapter is devoted first to elaborating on the terminology 2uid common issues of reversible gel research, secondly to identifying and comparing some of the strategies that contributors from diverse fields have found successful, and lastly to speculation about future trends of research in reversible gels and related systems. 

Lack of steady flow of a liquid-bearing colloidal solution requires the existence of a space-filling, three-dimensional structure. As we might select a perfect crystal as a csuionical solid, or liquid argon as a prototypical liquid, we csui choose the covalently crosslinked network, without any entanglements, to represent the ideal gel state. Then an appropriate time scale for reversible gels would be the lifetime of a typical crosslink bond if subjected to conditions that would cause flow in a pure... 

Fishnet Gels. By emalogy with typical covalent gels, reversible gel structures are frequently discussed in terms of "crosslinks"... 

New methods for emalysis and their combined application to complex systems, such as reversible gels, is especially to be encouraged. An inspiring statement along these lines appears in the conclusion of Ref. 95. 

---