Definition of a gel

Gels are three-dimensional networks made up of molecules, polymers, particles, colloids, etc., that are connected with each other by the specific parts on them such as functional groups and associative groups. The connected parts are called cross-links. Gels usually contain many solvent molecules inside their networks, and hence they are close to liquid in composition, but show solid-like mechanical properties due to the existence of the cross-links [1 ]. 

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Corrected from [2], where the definition is a repetition of the incorrect definition of a gel followed by an inexplicit reference to the porosity of the structure. 

Any formal definition of a gel has been avoided, because its formulation would be difficult on the basis of external properties. By any criterion, a 1 % aqueous solution of agar does form a gel when it is cooled from 95 to 20°, whereas a 1 % solution of sucrose does not but properties may be so continuous between these extremes that any dividing line would be arbitrary. Most of this Chapter is concerned with fairly permanent network structures formed from polymer solutions. As a definition of gels, this description is incomplete. Some cellulose gels, and pastes of gelatinized, starch granules, would be excluded, because they are formed by limited dispersion of solids. In other words, an arbitrary choice will be made to focus on those gels that are dilute with respect to polymer, because the information available about them at the molecular level is more precise than for others. 

Though it seems to have been shown that in molten glass upon cooling transitionary structures occur which fit the definition of a gel (see page 655), the solidified glass should, hence, not be termed a xerogel. 

Although gels must have been used since prehistoric times, the definition of a gel is still a matter of discussion [3]. In 1926 Jordan Lloyd [4] made her famous comment that ... 

Kramer et al. [3] in the opening of the first issue of Polymer Gels and Networks in 1993, reviewed the term Gel and gave their opinion of a phenomenological definition of a gel as ... 

This gel point uses the definition of a gel based on the connectivity of the system [10, 11, 12, 1]. The gel point is the point at which the reacting system is geometrically percolated by the connected objects. The appearance of a macroscopic object in the products is called gelation, or sol-gel transition. In chemical gels, the transition is irreversible, while in physical gels it is generally thermoreversible. 

Salt Effects. The definition of a capacity factor k in hydrophobic interaction chromatography is analogous to the distribution coefficient, in gel permeation chromatography ... 

When a bead of a gel-type CFP material is in the dry state, for practical purposes it can be considered as a solid material (thus possessing a mass, a volume and a shape). When in the swollen state, a CFP material can be still considered as a solid from the practical point of view, but this circumstance is now an authentic approximation. In fact, physico-chemical analysis reveals that under some circumstances this alleged solid material is rather a very viscous liquid. More precisely, it is a suspension of interconnected polymer chains in the swelling agent. Each swollen CFP bead can be considered as a drop, which can retain a definite shape owing to the existence of the polymer framework. 

Both Qv(x) and Q°(x) decrease as the polymerization proceeds and, after a definite conversion Qv(x) may reach the value of Qx(x). Since the dilution of a gel cannot be greater than its equilibrium degree of swelling, the excess of solvent should separate from the gel phase resulting in the syneresis, i.e. in phase separation. The condition for incipient phase separation during copolymerization of divinyl/vinyl monomers is given by [107]... 

For small microgels, like crosslinked particles of a microemulsion (diameter below 102 nm), the definition of the gel point is no longer clear. For instance, a... 

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. 

We do have to be careful in the way we apply the definition of a phase to the n-butylammonium vermiculite system. According to Gibbs [13], a phase is any homogeneous and physically distinct part of a system that is separated from other parts of the system by definite boundary surfaces. Because the gel can be lifted out of the supernatant fluid on a spatula, it clearly justifies description as a phase in the latter sense, but it is inhomogeneous on the nanometer-to-micron (colloidal) length scale. It can only be defined as homogeneous on the macroscopic length scale. The same considerations apply to the tactoid phase. 

Both Qy(x) Qv(x) decrease as the polymerization proceeds and, after a definite conversion reach the value of Since the dilution of a gel can-... 

ECHA Guidance for monomers and polymers (2008) states The preferred method to determine whether a substance falls under the definition of a polymer is gel permeation chromatography (GPC). The ECHA guidance makes reference to Organisation for Economic Co-operation and Development (OECD) 118 [5] as a particular technical guideline to follow, which is entitled Determination of the number-average Mw and the Mw distribution of polymers using GPC. 

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