Linking of Polymer Chains

Cross-Linking of Polymer Chains.—Formation of chemical bonds between linear polymer molecules, commonly referred to as cross-linking, also may lead to the formation of infinite networks. Vulcanization of rubber is the most prominent example of a process of this sort. Through the action of sulfur, accelerators, and other ingredients present in the vulcanization recipe, sulfide cross-linkages are created by a mechanism not fully understood (see Chap. XI). Vulcanized rubbers, being typical network structures, are insoluble in all solvents which do not disrupt the chemical structure, and they do not undergo appreciable plastic, or viscous, flow. 

Hydroxyl containing polymers may be cross-linked with diisocyanates. Fordyce and Ferry cross-linked styrene-maleic anhydride copolymers through the action of glycols. The copolymerization of divinyl with vinyl monomers may be looked upon as a method of cross-linking chain polymers. The cross-linkages are introduced simultaneously with the growth of the linear polymer chains, rather than afterwards, but this difference is secondary. 

Clearly, if e l, each generation will tend to be smaller than the one preceding, and indefinite continuation of the structure without limit is impossible. Only if e l may indefinite continuation sometimes occur. Hence the critical condition for incipient formation of infinite structures is =1, or  

If the primary molecules are large, the proportion of cross-linkages required for gelation becomes remarkably small a ratio of one cross-linkage for two primary molecules is sufficient for onset of formation of infinite structures (gel) according to Eq. (11). 

The relation (13) should hold regardless of the primary molecular weight distribution, provided only that the cross-linking proceeds between units at random. The number of cross-linked units per primary molecule, which has been called the cross-linking indexequals pyn-For a homogeneous primary polymer — 2/, and the critical value 

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Polymer-matrix materials include a wide range of specific materials. Perhaps the most commonly used polymer is epoxy. Other polymers include vinyl ester and polyester. Polymers can be either of the thermoset type, where cross-linking of polymer chains is irreversible, or of the thermoplastic type, where cross-linking does not take place but the matrix only hardens and can be softened and hardened repeatedly. For example, thermoplastics can be heated and reheated, as is essential to any injection-molding process. In contrast, thermosets do not melt upon reheating, so they cannot be injection molded. Polyimides have a higher temperature limit than epoxies (650°F versus 250°F or 350°F) (343°C versus 121°C or 177°C), but are much more brittle and considerably harder to process. 

Organic insulating materials are generally subject to temperature dependent deterioration during their operational lifetime. The relevant chemical processes are complicated and may be described (with a certain simplification) as slow oxidation which introduces acid groups into the polymeric insulation, the loss of plasticizer, oxygen cross-linking of polymer chains and internally catalysed depolymerization of plastic insulation. 

Inducing the cross-linking of polymer chains in the absence of monomers or required cross-linker molecules... 

The microencapsulation and cross-linking of polymer chains converted Pd(PPh,)4 to... 

A FIGURE 12.41 Cross-linking of polymer chains. The cross-linking groups (red) constrain the relative motions of the polymer chains, making the materiai harder and less flexible than when the cross-links are not present. 

Zotti, G., R. Salmaso, M.C. Gallazzi, and R.A. Marin. 1997. In situ conductivity of a polythiophene from a branched alkoxy-substituted tetrathiophene. Enhancement of conductivity by conjugated cross-linking of polymers chains. Chem Mater 9 791-795. 

Michael addition reaaion was frequently used for the cross-linking of polymer chains and formation of miaogds. Hydroxypropylcellulose (HPC) nanogds were sjmthesized in aqueous solution by the assodation of HPC above its lower critical solution temperature (LCST) ( 41 °C) to metastable aggregates followed by cross-linking after addition of divinyl-sulfone (DVS). - ... 

Basically, four different types of reactions are possible (Table 4) (I) recombination of radicals, (II) cross-linking of polymer chains, (111) addition of radicals from the reactive atmosphere, and (IV) addition of bi-functional molecules with... 

The question of the effect on the permeability of cross-linking of polymer chains by sulphur or oxygen still remains uncertain. Edwards and Pickering (6) studied the influence of ageing and of vulcanisation of rubber upon its permeability, using as membranes rubber-coated balloon fabrics. It was observed ... 

Numerous examples of gels based on polymers may be identified Gels produced by overlap or entanglement of polymer chains (physical gels), gels produced by association of polymer chains ( associative thickeners ) and gels produced by physical or chemical cross-linking of polymer chains. 

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