Cross-linked polymers, definition

Section 3 is concerned with the methods of the preparation of polymers with covalently bonded LM that meet the requirements of the PL method and are located in definite parts of macromolecules (in the main chain, at its ends, in side chains of various structure or in certain structural fragments of branched chain and cross-linked polymers). 

FIGURE 5 Definition of linear, branched, and cross-linked polymers. 

A polymer, by conventional definition, is a macromolecule made up of multiple equivalents of one or more monomers linked together by covalent bonds (e.g., carbon-carbon, amide, ester, or ether bonds) [1]. These conventional polymers come in many configurations for example, linear homopolymers, linear copolymers, block copolymers, cross-linked polymers, dendritic polymers. 

The polymers physical aging represents itself the structure and properties change in time and is the reflection of the indicated materials thermodynamically nonequilibriiun nature [61, 62], As a rule, the physical aging results to polymer materials brittleness enhancement and therefore, the ability of structural characteristics in due course prediction is important for the period of estimation of pol5mier products safe exploitation. For cross-linked polymers the quantitative estimation of structure and properties changes in physical aging process was conducted in Refs. [63, 64] within the frameworks of fracture analysis [65] and cluster model of polymers amorphous state structure [7, 66]. The authors of Ref. [67] use the indicated theoretical models for the description of PC physical aging. Besides, for PC behavior closer definition in the indicated process such theoretical notions were drawn as structure quasiequilibrium state [68] and the thermal cluster model [69], which is one from variants of percolation theory. 

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. 

Cross-linked versions of water-soluble polymers swollen in aqueous media are broadly referred to as hydrogels (52) and have a growing commercial utility in such apphcations as oxygen-permeable soft contact lenses (qv) (53) (Table 4) and controUed-release pharmaceutical dmg deflvery devices (54). Cross-linked PVP and selected copolymers fit this definition and are of interest because of the following stmcture/performance characteristics ... 

A substantial proportion of cross-linked material was obtained with 2 x 107 r, clearly indicating that ring participation, probably following scission43, is important. The infrared spectra of these polymers were not very informative, but definitely showed carbonyl and C=C bands, obviously coming from cleaved rings. 

Polymer films that are sensitive to light, x-rays, or electrons— known as photoresists—are nsed extensively to transfer the pattern of an electronic circuit onto a semiconductor surface. Such films must adhere to the semiconductor surface, cross-link or decompose on exposure to radiation, and nndergo development in a solvent to achieve pattern definition. Virtually all aspects of photoresist processing involve surface and interfacial phenomena, and there are many outstanding problems where these phenomena mnst be controlled. For example, the fabrication of multilayer circuits requires that photoresist films of about 1-pm thickness be laid down over a semiconductor surface that has already been patterned in preceding steps. 

---