Gels, physical

In the process ia the center of Figure 17, complete hydrolysis is allowed to occur. Bases or acids are added to break up the precipitate iato small particles. Various reactions based on electrostatic iateractions at the surface of the particles take place the result is a colloidal solution. Organic binders are added to the solution and a physical gel is formed. The gel is then heat treated as before to form the ceramic membrane. 

As the quinone stabilizer is consumed, the peroxy radicals initiate the addition chain propagation reactions through the formation of styryl radicals. In dilute solutions, the reaction between styrene and fumarate ester foUows an alternating sequence. However, in concentrated resin solutions, the alternating addition reaction is impeded at the onset of the physical gel. The Hquid resin forms an intractable gel when only 2% of the fumarate unsaturation is cross-linked with styrene. The gel is initiated through small micelles (12) that form the nuclei for the expansion of the cross-linked network. 

Deficiency of cross-linker molecules (off-balancing of stoichoimetry) was found to increase the relaxation exponent value [7, 65, 66]. The gel becomes more lossy , and stress relaxation is accelerated. Adding of a non-reacting low molecular weight solvent also increases the relaxation exponent [58, 65], even in physical gels [67]. Both effects have been attributed to screening [44, 65],... 

On the other hand, bulky crosslinks as developed during the crystallization of polymer melts (no solvent) lower the relaxation exponent. The lowest values of n which we have been able to generate so far occurred with physical gels in which the crosslinks consisted of large crystalline regions [68, 69]. 

The critical gel equation is expected to predict material functions in any small-strain viscoelastic experiment. The definition of small varies from material to material. Venkataraman and Winter [71] explored the strain limit for crosslinking polydimethylsiloxanes and found an upper shear strain of about 2, beyond which the gel started to rupture. For percolating suspensions and physical gels which form a stiff skeleton structure, this strain limit would be orders of magnitude smaller. 

Strictly speaking, the physical gel at the gel point is still a liquid when observed at experimental times tp which exceeds Xpg. We therefore define a new dimensionless group, the gel number iVg... 

Physical gels, as exemplified by gelatin gels, exhibit many common features with chemical gels. Among them, we found the topological disorder of the network formed by the polymer chains, and the formal similarity of the process of gelation with a percolation problem. 

Figure 26-36 (a) A chemical gel contains covalent cross-links between different polymer chains, (b) A physical gel is not cross-linked but derives its properties from physical entanglement of the polymers. 

De Nobili, M., Bragato, G., and Mori, A. (1999). Capillary electrophoretic behaviour of humic substances in physical gels. J. Chromatogr. A 863,195-204. 

The term gels refers to a range of materials which are grouped under the general definition A dilute mixture of two or more components which form a separate uninterrupted phase throughout the system [147,148]. This classification includes chemical gels—where covalent bonds create the continuous network—and physical gels which are maintained by non-covalent interactions, which concern us here. 

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