Cross-linked polymerization definition

Whea there are reactants with three or more functionahties participating ia the polymerization, branching and the formation of iatermolecular linkages, ie, cross-linking of the polymer chains, become definite possibiUties. If extensive cross-linking occurs in a polymer system to form network stmctures, the mobiUty of the polymer chains is greatiy restricted. Then the system loses its fluidity and transforms from a moderately viscous Hquid to a gelled material with infinite viscosity. The experimental results of several such reaction systems are collected in Table 6. 

Note 1 A polyelectrolyte network is sometimes called a cross-linked polyelectrolyte. Use of the latter term is not recommended unless the polyelectrolyte network is formed by the cross-linking of existing polyelectrolyte macromolecules rather than by nonlinear polymerization. (See the definition of a crosslink, definition 1.59, ref [4].)... 

Thermoset polymers assume a permanent shape or set once cured. Once set, they cannot be reshaped. They are formed by a large amoimt of cross-linking of linear prepolymers (a small amount of cross-linking will produce elastomers) or by direct formation of networks by the reaction of two monomers. The latter is the more prominent of the two methods. It is a stepwise or condensation method that has been defined as "the reaction of two monomers to produce a third plus a by-product, usually water or alcohol." Because in some cases a by-product is not produced, this definition is no longer exactly correct. The reaction is now referred to as a "stepwise" polymerization. When the reaction results in a by-product, it is called a "condensation reaction." Table 3.1 lists the principal thermoset polymers. 

Macroscopic amounts of natural and synthetic polymers comprise a huge number of macromolecules (unless they consist of a completely cross-linked network). The individual polymeric molecules are different in the degree of polymerization, i. e. in molecular mass, as another principal difference from the low-molecular materials having a definite molecular mass common to all molecules. A polymer can be characterized by an average degree of polymerization or average molecular mass on a number or mass basis. 

Pre-cross-linked Latex Blends. In these materials the individual latexes are cross-linked during synthesis and then blended, and a film is formed. Because of limited deformation and/or interdiffiision capabilities, such films tend to be weak and only used for special purposes (Zosel and I y 1993 Lesko and Sperry 1997). However, light cross-linking, as occurs in SBR latexes, may be tolerated. Pre-cross-Unked latex blend materials are actually not IPNs, because the definition requires that at least one of the polymers be polymerized and/or cross-linked in the immediate presence of the other. An application of pre-cross-linked suspension-polymerized blends, in animiic and cationic form, is as ion-exchange resins. In suspensions, the particles are larger, usually of the order of 10-200 tm. 

Latex IPNs, by definition, have their origin in emulsion polymerization. Several types of latex IPNs exist. If one blends two kinds of latex particles, followed by film formation and cross-linking of both polymers, the material is called an interpenetrating elastomeric network, TEN. Usually, lENs form a three-dimensional mosaic structure (69,70). 

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