Gelation point

Gelation characteristics Gelation point Gelcap Gel coats Geldan aldehyde Geldanamycin... 

Fig. 8. (a) Cross section of cellulose acetate isotropic porous hoUow-fiber dry-jet wet spun at incipient gelation point of dope mixture (b) magnification at... 

A primary sol particle in an acid-cataly2ed sol has radius between 1 and 2 nm (3). The secondary fractal particle has a radius, R, of 5 to 20 nm as seen from saxs (3). For the TMOS-based sols investigated by saxs, ( increases with time, as does the Guinier radius, R. The stmcture reaches a fractal dimension around 2.3 at the gelation point. 

The structure and properties of a network polymer are determined by the relation between the inter- and intramolecular reactions of functional groups. The latter gives rise to ineffective cycles, and has been studied fairly well3-5 7°.71-8i-85.i°8 u2 Their occurrence in the system results in the gelation point being shifted towards higher conversions, higher sol fraction, fewer elastically active chains, and smaller equilibrium modulus of the network. 

Gelation. A sol becomes a gel when it can support a stress elastically, defined as the gelation point or gelation time. tg. A sharp increase in viscosity accompanies gelation. A sol freezes in a particular polymer structure at the gelation point. This frozen-in structure may change appreciably with time, depending on the temperature, solvent, and pH conditions or on removal of solvent. 

As the polymeric network extends throughout the total volume, the sol thickens to a gel.6 The sol-to-gel conversion is a gradual process, which is easily observed qualitatively, but difficult to measure analytically. The gelation point tge, is defined as the point where elastic stress is supported.13 The t is not an intrinsic property of a sol. It is influenced by the size of the container, the solution pH, the nature of the salt concentration, the anion and solvent, the type of initial alkoxy group and the amount of water. 

It is claimed that commercially-available ultrasound equipment can measure the following quality parameters of dairy products levels of solids, solids non-fat (SNF), protein, water and fat solid fat content (SFC), colloidal stability, gelation point, adulteration with oil, particle size, particle size distribution, oil composition, protein denaturation and fat oxidation. This incomplete list represents an impressive contribution towards the solution of food quality measurement although the present authors are slightly skeptical regarding some of these claims. In this review only those applications will be addressed which are regarded as robust. 

The formation of a polymer networks starts with an increase in molecular weight and formation of branched structures. At a typical extent of reaction, the gelation point is that point at which a network is first formed. The extent of reaction as well as the cross-linking density in radiation-induced cross-linking processes is determined by the radiation dose. The term dose means the quantity of radiation applied to or absorbed accidentally by a given volume or mass of sample. The absorbed dose is measured in Gray (Gy), 1 Gy = 1J kg-1. Therefore, the formation of a polymeric network needs a certain dose, the gelation dose Dg, which can be determined by sol-gel analysis. ... 

Boltom H40 dendritic molecules were covalently linked in this work to make a network with aliphatic 1,6-hexamethylene diisocyanate (HDI). The molar NCO/OH ratio was varied for the reactants from 10 to 50% to prepare networks with different degrees of connectivity of dendritic units. The network samples are designated as H40/Z where Z stands for NCO/OH ratio expressed as a percentage. The network formation reaction of H40 with HDI was carried out in N,N-dimethylformamide (DMF) of 99.8% purity at 90 C. No catalyst was added. Isocyanates react readily with moisture to form urea linkages, therefore special precautionary measures were implemented to prevent moisture uptake either by HDI or DMF. Observation of the reaction vessel was maintained over the course of the reaction to monitor the viscosity of the solution. As viscosity of the solution increased to the desired level, suggesting that the gelation point was near, the solution was cast onto a glass plate which was immediately placed... 

Dimethacrylate monomers and polymethacrylate monomers must be discussed separately from other methacrylates because of their ability to cross-link under normal free-radical polymerization conditions. Even at very low conversion, less than 1%, they produce completely cross-linked polymers that cannot be solvent-swollen and are insoluble. CCT agents reduce molecular weight and thereby move the gelation point to a much higher degree of conversion, though CCT cannot prevent gelation completely.320... 

Much more detailed procedures are described in Prime (1997a). Ramis et al, (2003) use shrinkage measurements on a TMA to determine the gelation point of polyester-based thermoset coatings. As shown in Figure 3.55, the gel point is defined as the point at which the rate of change of shrinkage abruptly slows down. 

Thermal cure reactions were also carried out as thin films sandwiched between salt plates and the edges sealed with Teflon, or on a single plate with the surface exposed to the atmosphere. This permitted an easy method of monitoring the extent of cure beyond the gelation point of I and determination of the i r characteristics of highly cured samples. 

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