Cross-linking, of polymers

The mineral talc is extremely soft (Mohs hardness = 1), has good sHp, a density of 2.7 to 2.8 g/cm, and a refractive index of 1.58. It is relatively inert and nonreactive with conventional acids and bases. It is soluble in hydroduoric acid. Although it has a pH in water of 9.0 to 9.5, talc has Lewis acid sites on its surface and at elevated temperatures is a mild catalyst for oxidation, depolymerization, and cross-linking of polymers. 

Uses. /-Butyl hypochlorite has been found useful in upgrading vegetable oils (273) and in the preparation of a-substituted acryflc acid esters (274) and esters of isoprene halohydrins (275). Numerous patents describe its use in cross-linking of polymers (qv) (276), in surface treatment of mbber (qv) (277), and in odor control of polymer latexes (278). It is used in the preparation of propylene oxide (qv) in high yield with Httle or no by-products (269,279). Fluoroalkyl hypochlorites are useful as insecticides, initiators for polymerizations, and bleaching and chlorinating agents (280). 

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. 

From a practical stand point almost any of the lower-molecular weight vinyl monomers, cross-linking oligomers, and polymers can be blended with a high-molecular weight thermoplastic polymer to enhance cross-link density at lower dose rates [39]. The influence of various other additives on the efficiency of cross-linking of polymers will be highlighted in the subsequent sections. 

Cross-linking of polymers is done routinely by radiation technology. The factors influencing the cross-linking or the efficiency of cross-linking of polymeric materials are ... 

Ic. 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. 

The standard approach to reducing scorch (e.g. discoloration resulting from oxidation) involves addition of one or more antioxidants. Scorch retardants prevent premature decomposition of peroxides and cross-linking of polymers (pre-vulcanisation). 

The ability of nitrile oxides to undergo addition and cycloaddition reactions makes it possible to use them in polymer chemistry and technology. Major trends might be synthesis, modification, cross-linking of polymers, addition of nucleophiles, and 1,3-dipolar cycloaddition of nitrile oxides. Taking into account the scarcity of reviews devoted to this topic, not only recent but also previous references will be cited in this subsection. 

I.4.2.2. Cross-linldng of Polymers Among applications of nitrile oxides, cross-linking of polymers is of main importance. Both nucleophilic addition and 1,3-dipolar cycloaddition are the pertinent reactions. 

Photochemically-generated radicals are encountered as reactive intermediates in many important systems, being a major driving force in the photochemistry of ozone in the upper atmosphere (stratosphere) and the polluted lower atmosphere (troposphere). The photochemistry of organic carbonyl compounds is dominated by radical chemistry (Chapter 9). Photoinitiators are used to form radicals used as intermediates in the chain growth and cross-linking of polymers involved in the production of electronic circuitry and in dental treatment. 

The rate of cross-linking of polymers, such as NR, is increased through the use of accelerators, often misnamed as catalysts. 

This chapter describes many of the important reactions of polymers. Synthesis and curing (cross-linking) of polymers and telomerization are chemical reactions of polymers that have been discussed in previous chapters. 

In this chapter we will focus on side chain functionalized supramolecular polymers as well as main chain noncovalent functionalized polymers, which are the two main areas of supramolecular polymers. We will initially discuss the design principles and methodology of side chain functionalization, in particular, multifunctionalization. In the later part of the chapter, we will discuss in detail two important applications of side chain functionalized supramolecular polymers. The first application involves the use of noncovalent interactions to yield highly functionalized materials, whereas the second application involves the reversible noncovalent cross-linking of polymers to yield responsive materials. 

The difference between these polymers lies in the degree of polymerization or cross-linking. Thus LP-2 and LP-32 are cross-linked polymers. Of these two types the cross-linking of polymer LP-32 is weaker, therefore the modulus of elasticity of LP-32 (after curing) is smaller. The bursting stress of cured LP-32 is twice that of cured LP-2. 

The radical-accepting ability of blacks used as stabilizers can interfere with the free-radical cross-linking of polymers. 

Thus the density and the density distribution and consequently the chemical composition and heterogeneity and also the cross-linking of polymer latex particles can be determined in a few minutes by rapid density gradient centrifugation. 

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