Thermoreversion

From the colorless state it can be switched with light of short wavelength (A = 380 nm) via an electrocycHc ring opening and cis/trans rotation of one half of the molecule into a state with violet/purple color. The reverse reaction is effected by visible light (A = 580 nm). Since the system is metastable, one of the two reaction directions is matched by a rival thermal reaction, the thermoreversion. This progresses, however, in the case of benzospiropyran, at room temperature by a factor of 10 slower than the light-induced reaction. 

Less polysaccharide can also be used in products by taking advantage of the synergistic gelling of xanthan/galactomannan mixtures which forms thermoreversible gels at lower concentrations than if each is used separately. 

Sakurai S., Kawada H., Hashimoto T., and Fetters L.J. Thermoreversible morphology transition between spherical and cyUndrical microdomains of block copolymers. Macromolecules, 26, 5796, 1993. 

Polymers with a sizable number of ionic groups and a relatively nonpolar backbone are known as ionomers. The term was first used for copolymers of ethylene with carboxylated monomers (such as methacrylic acid) present as salts, and cross-linked thermoreversibly by divalent metal ions. Such polymers are useful as transparent packaging and coating materials. Their fluorinated forms have been made into very interesting ion-exchange membranes (considered further below). 

An interesting and potentially u.seful variant is deployment of thermoreversibility, a situation where the reaction occurs at relatively high temperature in a homogeneous phase, which becomes a two-pha.se system at lower temperatures, facilitating catalyst recovery. Here, tailored ligands have to be u.sed. Ethoxylated phosphines have been suggested by Jin, Fell, and co-workers (1996, 1997). 

Guenet JM (1992) Thermoreversible gelation of polymers and biopolymers. Academic, London... 

These physical cross-links are, in general, not permanent and may disappear on swelling or increase in temperature. The corresponding networks are referred to as "physical" or "thermoreversible" and are not considered in this chapter. The reader may refer to Burchard and Ross-Murphy [17-19] for further information on such materials. 

J. M. Guenet, Thermoreversible Gelation of Polymers and Biopolymers. Academic Press, 1992. 

Te Nijenhuis, K. (1997) Thermoreversible networks. Viscoelastic properties and structure of gels. Advances in Polymer Science, 130, 1-267. 

If the alkali step is removed a high acetyl gellan gum is produced. This type of gellan gum gives a more elastic thermoreversible gel similar to gelatine. This product might be more successful but it needs separate food approvals. 

The gel properties as well as the gelling conditions are radically different for the two types of pectin. High methoxyl pectins produce a gel that does not remelt, while some low methoxyl pectin gels are thermoreversible. 

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