Temperature-dependent sol-gel transitions

The sol-gel transition has been determined visually, with calcium and copper, for different pectins under different external conditions. As shown in Figure 5 for sample C44 the homogeneous gel phase is situated between the two transition lines. The extension of this phase was found to depend mainly on the DE, temperature and nature of the cation. With calcium the amount of cation required to get a gel increased with the degree of esterification and above 50% it became impossible to get a gel [8]. 

When the helix amount increases the medium changes from a viscous liquid (sol) to an elastic solid (gel). The kinetics of gelation depends strongly on the quenching temperature. The rheological measurements that we performed are particularly focused on the sol-gel transition and on the definition of the "gel point". The greatest difficulty encountered is due to the weakness of the bonds which can easily be destroyed by the mechanical stress. 

Thermosensitive polyphosphazenes with (o-methoxy poly(ethylene glycol) in combination with glycine ethyl ester and depsipeptide ethyl ester or al-kylamines as substituents have been synthesized and characterized. All polymers exhibit LCST properties. Thermosensitive polymers have also been prepared by the introduction of a-amino-co-methoxy- poly(ethylene glycol) and L-isoleucine ethyl ester substituents to the phosphazene backbone. Polymer (230) shows reversible sol-gel transitions in aqueous solution depending on the temperature of the solution. ... 

An interesting property of many polymers is their ability to undergo a temperature dependent transition from a primarily liquidlike state (sol) to a gel possessing elasticity. The temperature at which this transition occurs is termed the sol-gel transition. Given the relevance of this transition to both the clinical and nonclinical performance of pharmaceutical systems, various authors have examined this phenomenon using thermoanalytical methods, including thermal oscillatory analysis. Examples of these are now presented. 

Synthesis of a thermosensitive, biodegradable hydrogel consisting of blocks of poly(ethylene oxide) and poly(L-lactic acid) was carried out (4). Aqueous solutions of these copolymers exhibited temperature-dependent reversible gel-sol transitions. The hydrogel can be loaded with bioactive molecules in an aqueous phase at an elevated temperature (around 45°C), where they form a solution. In this form, the polymer is injectable. On subcutaneous injection and subsequent rapid cooling to body temperature, the loaded copolymer forms a gel that can act as a sustained release matrix for drugs (4). 

Aqueous solutions of high-PEG block copolymers possess a unique temperature-dependent reversible gel-sol transition. This allows hydrogels to be loaded with bioactive molecules, such as insulin, in an aqueous phase above the gel-sol transition temperature, usually around 45°C. As a sol, the insulin-loaded copolymer system is injectable. Upon exposme to the subcutaneous... 

It has already been stated that, in thermoreversible gels, the sol-gel transition temperature is no precisely defined constant and that it depends on the rate of cooling, and further that hysteresis phenomena occur. An 0.5% ar-agar solution sets to a gel when cooled to 35°. When heated, it liquefies again, but not until a temperature of 90° is reached. Below 90° the gel does not liquefy, however long it may be maintained at the particular temperature. Consequently, it is possible to obtain this system either in a sol or a gel condition at any temperature between 35° and 90°. This is an extreme case, but most of the thermoreversible sol-gel transformations show hysteresis to such a degree, that the sol or the gel state can be obtained within a temperature of 10 to 20 degrees. ... 

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