Crystalline Gelled

All of the eommereial alkyl eyanoaerylate monomers are low-viseosity liquids, and for some applications this can be an advantage. However, there are instances where a viseous liquid or a gel adhesive would be preferred, sueh as for application to a vertical surface or on porous substrates. A variety of viscosity control agents, depending upon the desired properties, have been added to increase the viscosity of instant adhesives [21]. The materials, which have been utilized, include polymethyl methacrylate, hydrophobic silica, hydrophobic alumina, treated quartz, polyethyl cyanoacrylate, cellulose esters, polycarbonates, and carbon black. For example, the addition of 5-10% of amorphous, non-crystalline, fumed silica to ethyl cyanoacrylate changes the monomer viscosity from a 2-cps liquid to a gelled material [22]. Because of the sensitivity of cyanoacrylate esters to basic materials, some additives require treatment with an acid to prevent premature gelation of the product. 

The nature of the solidification process in these cements has received little attention. Rather, attention has focussed on the crystalline components that form in cements which have been allowed to equilibrate for some considerable time the nature of such phases is now quite well understood. Gelation is reasonably rapid for these cements and occurs within a significantly shorter time than does development of crystalline phases. The conclusion may be drawn that initial cementition is not the same as crystallization, but must occur with the development of an essentially amorphous phase. Reactions can continue in the amorphous gelled phase, but are presumably limited in speed by the low diffusion rates possible through such a structure. However, reactions are able to proceed substantially to completion, since in many cases X-ray diffraction has demonstrated almost quantitative conversion of the parent compounds to complex crystalline mixed salts, though several days or weeks of equilibration are required to bring this about. 

A change in the perception of their mechanism of action came in the sixties when Lawrence (7) pointed out that short chain surfactants would delay the gelling to a liquid crystalline phase which takes place at high surfactant concentrations. Friberg and Rydhag (8) showed that hydrotropes, in addition, prevent the formation of lamellar liquid crystals in combinations of surfactants with hydrophobic amphlphiles, such as long chain carboxylic acids and alcohols. The importance of this finding for laundry action was evident. 

Addition of the dicarboxylic acid to a water/ detergent combination prevents (B) the gelling caused by a model dirt (octanol, CgOH) below the cmc (A). A The model dirt octanol (CgOH) forms a liquid crystalline phase with water and sodium octanoiate (CgOOH at pH 10) in area 3 and 4 Partial substitution of the sodium octanoate with the diacid soap (pH 10) leads to an increase of solubilization of the octanol (B). 1 aqueous... 

According to Tarascon and co-workers, the swelling of PVdF—HFP by liquid electrolytes was never complete due to the semicrystalline nature of the copolymer, which tends to microphase-separate after the activation by electrolyte. On the other hand, it is those crystalline domains in the gelled PVdF—HFP that provide mechanical integrity for the resultant GPE. Thus, a dual phase structure was proposed for the Bellcore GPE by some authors, wherein the amorphous domain swollen by a liquid electrolyte serves as the ion conduction phase, while tiny crystallites act as dimensional stabilizer. 

Since organogelators are in crystalline or lyotropic aggregate states in a gel. the nature of their intermolecular interactions becomes a factor of paramount importance in determining the nature and the intensity of emitted radiation from a gelled sample. Molecular proximity opens possible reactive channels for the excited states that are not available in dispersed solutions. For example, it has been shown that CAB dimerizes in its neat solid, liquid-crystalline, and gelled (fiber) states when exposed to UV radiation [47,48]. (See Structure I.) In dilute isotropic solutions, no photoreaction is observed because the time required for an... 

Comparing the solubility behavior of a- and p-chitins (although the later exists in a crystalline-hydrated structure, which is much looser than that of the ot-chitin), p-chitin shows lower solubility due to the penetration of wafer between the chains of the lattice. Based upon chitin molecule-solvent conformation and solubility mechanisms, p-chitin starts gelling at a lower concentration than a-chitin. Table 2.6 illustrates the solubility of chifin and structurally related compounds in a saturated CaCl2 2H20-methanol solvent system. 

At relatively high concentrations (>20%), poloxa-mers form thermoreversible gels however, they gel on heating rather than cooling The amphiphilic nature supports the gelling mechanism of poloxamers, where micelle-like junction zones form at or above room temperature. The junction zones consist of large populations of micelle-like structures, which apparently form a viscous, liquid crystalline phase. Poloxamers can also form gels in dilute hydroalcoholic solutions. 

In a commercial context, the end use dictates the properties desired. For solution applications, the rheology of the polysaccharide, its ability to retain water, and its gelling tendency are often the most important. For solids applications, the thermal properties (e.g., and T ), the mechanical properties (e.g., stiffness, tensile, texture, and adhesion), and other features such as water content, crystallinity, and spatial heterogeneity are relevant. Many polysaccharides are used in interfacial applications, in which case the surface-active properties of the polysaccharide are important. 

Sol-gel synthesis of LiA102 involves an alcohol-alkoxide route. Different alcohols and alkoxide combinations can be used. The alkoxide and alcohols are mixed and hydrolyzed by addition of pure water. The mixture is then gelled by heating to 60°C and is subjected to hydrothermal treatment in an autoclave. The morphology of the crystalline phases is dependent on the length of the alkoxy groups used in the alkoxide-alcohol mixing step. For example, rod-like crystals are produced with butoxide and propoxide mixtures. 

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