Chemistry three-dimensional cross-linking

Thermosets cure into nonmelting, insoluble polymers. Frequently, the curing needs heat, pressure, or catalyst to proceed. Often the final cure, which is nothing more than completion of the cross-linking, takes place in the fabrication or molding operations. The chemistry is about the same as you saw in the thermoplastics, but there are more reactive sites per monomer. (They are polyfunctional.) Consequently, more three-dimensional cross-linking takes place. 

Oxides with layered stmcture or those whose structures contain large tunnels or cavities may display abnormal ion movement or serve as templates for heterogeneous catalysis (see Ionic Conductors Intercalation Chemistry Oxide Catalysts in Solid-state Chemistry andZeolites). Many oxides are stabilized by the formation of structures that are highly defective nature and have similar properties to those listed above (see Defects in Solids). The strong bonds, which result in three-dimensional cross-linked structures, give rise to inert, refractory materials that have a variety of uses (see Section 5.3.6 and Ceramics). 

In the previous chapter we talked about linear polymers and have mentioned the concept of cross-linking only in passing. Linear polymers are usually thermoplastic they soften or melt when heated and will dissolve in suitable solvents. They can be remelted and shaped into their finished product with no further chemical reactions. Thermoset resins, those having elaborately cross-linked three-dimensional structures, set or harden by undergoing a chemical reaction during the manufacture of finished products. They decompose on heating and are infusible and insoluble. Their chemistry and physical properties are quite different from thermoplastic polymers. The important ones are now discussed. 

In designing a prepolymer for a specific use, this technique provides for more control of the chemistry. For elastomers, for instance, including some three-dimensional character changes the physical properties of the polymer. More cross-linking lowers elongation and increases strength. Cross-linking is not necessary for elastomers, but it is required for foams. 

Fig. 7 Node-strut topology for series of CMPs networks produced by Sonagashira-Hagihara cross-coupling chemistry [19]. The benzene nodes are shown in red. The total number of ethyne plus benzene links per strut increases in increments of one from two (CMP-0) to six (CMP-5) in this series of samples. It should be stressed that these are simple representations of the repeat unit structures for the networks. The actual three-dimensional materials have more complex structures and include both terminal alkyne and halogen end groups (see also molecular simulations, Fig. 9)...

Knowledge of the chemical stnictin-e of the monomer and the distribution of chain lengths allows prediction of the method used for the production of that polymer. If the monomer is effectively difimctional, then the polsrmer will have a linear backbone structure and is a thermoplastic. Such polymers can be heated to an elevated temperature and shaped. If the monomer has functionality greater than two, then a cross-linked three-dimensional matrix can be created and the material is a thermoset, which cannot be reshaped by heating to an elevated temperature. Thermoplastics and thermosets (qv) can be created using similar chemistry, the only difference being the number of functions associated with the primary monomer unit. 

The above simple description of sol-gel chemistry identifies two key ideas. First, a gel forms because of the condensation of partially hydrolyzed species into a three-dimensional polymeric network. With time the colloidal particles and condensed species link together to become a 3-D network. The physical characteristics of the gel network depend greatly upon the size of the particles and extent of cross-linking prior to gelation. At gelation, viscosity increases sharply, and solid object results in the shape of the mold. 

Point of view of the chemistry lignin is a natural amorphous cross-linked resin that has an aromatic three-dimensional polymer structure containing a number of functional groups such as phenolic, hydroxyl, carboxyl, benzyl alcohol, methoxyl, and aldehyde as shown in Figure 5.1, which make lignin potentially useful as an adsorbent material for removal of heavy metal ions from water [52]. 

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