Cross-linking polymers main

Polycarbonates. Currently, all audio CDs (CD-AD), all CD-ROM, and the biggest fraction of substrate disks for WORM and EOD worldwide are manufactured from a modified bisphenol A—polycarbonate (BPA-PC) (3). In 1991, some 1.3 x 10 compact disks were produced, equivalent to an annual amount of about 35,000 t BPA-PC. WORM and EOD disks are manufactured mainly from BPA-PC for sizes of 5.25 in. and below, and glass for larger form factors (eg, 12 in.), partially also from BPA-PC, and in some cases from aluminum or from cross-linked polymers (epoxy resins) (190). 

Secondary bonds are considerably weaker than the primary covalent bonds. When a linear or branched polymer is heated, the dissociation energies of the secondary bonds are exceeded long before the primary covalent bonds are broken, freeing up the individual chains to flow under stress. When the material is cooled, the secondary bonds reform. Thus, linear and branched polymers are generally thermoplastic. On the other hand, cross-links contain primary covalent bonds like those that bond the atoms in the main chains. When a cross-linked polymer is heated sufficiently, these primary covalent bonds fail randomly, and the material degrades. Therefore, cross-linked polymers are thermosets. There are a few exceptions such as cellulose and polyacrylonitrile. Though linear, these polymers are not thermoplastic because the extensive secondary bonds make up for in quantity what they lack in quahty. 

In the lightly cross-linked polymers (e.g. the vulcanised rubbers) the main purpose of cross-linking is to prevent the material deforming indefinitely under load. The chains can no longer slide past each other, and flow, in the usual sense of the word, is not possible without rupture of covalent bonds. Between the crosslinks, however, the molecular segments remain flexible. Thus under appropriate conditions of temperature the polymer mass may be rubbery or it may be rigid. It may also be capable of ciystallisation in both the unstressed and the stressed state. 

In contrast to metals and semiconductors, the valence electrons in polymers are localized in covalent bonds.The small current that flows through polymers upon the application of an electric field arises mainly from structural defects and impurities. Additives, such as fillers, antioxidants, plasticizers, and processing aids of flame retardants, cause an increase of charge carriers, which results in a decrease of their volume resistivity. In radiation cross-linking electrons may produce radiation defects in the material the higher the absorbed dose, the greater the number of defects. As a result, the resistivity of a radiation cross-linked polymer may decrease. Volume resistivities and dielectric constants of some polymers used as insulations are in Table 8.3. It can be seen that the values of dielectric constants of cross-linked polymers are slightly lower than those of polymers not cross-linked. 

Figure 15. Preparation of branched or cross-linked polymer via the formation of main-chain polyrotaxanes 73.

Cross-Linked Polymers. In the 1980s, not only glass and BPA-PC but also uv-curable cross-linked polymers, eg, epoxy resins, were used as substrate material for optical mass storage disks with laige diameters (12 in., 14 in.) (219). The epoxy resins consisted of compounds containing one or several highly reactive epoxy or hydroxyl groups. The common epoxy resins (EP) mainly are reaction products of bisphenol A and epichlorohydrin ... 

Practical application of these materials for the separation of small molecules are somewhat limited due to the presence of microporosity. Gaps between cross-linked polymer chains are on the level of molecular size of low-molecular-weight analytes. These analytes could diffuse inside the body of a polymer-based packing material, which produce drastically different retention of a small portion of injected sample than the rest of it. At the same time, polymers are the main packing material for size-exclusion chromatography. 

P. T. Speakman (1961) obtained a linear relationship between set in borate solutions and [loss of cystine] Using Flory s (1956) expression for the isotropic length of a cross-linked polymer he concluded that lan-thionine cross-linkages determined the set length of the fibers. Two main assumptions were made (1) that cystine loss equals the lanthionine formed, and (2) that the fiber is an elastomer in hot water. Other attempts to investigate the formation of covalent bonds other than disulfides are equivocal (P. T. Speakman 1957,1958), as insufficient information is known concerning the chemical and physical effects of boiling bisulfite solutions on wool. 

The polymer indicated above has a linear skeletal structure that can be represented by a chain with two ends. There are also polymers with nonlinear skeletal structures of the type shown in Figures 1.1 and 1.2. Nonlinear polymers are called branched polymers when they have side chains or branches of significant length covalently finked to the main chain. These polymers are characterized according to the number and size of the branches. There are also nonlinear polymers, known as cross-linked polymers or polymer networks, that have three-dimensional structures in which each chain is finked to the others via a sequence of chemical bonds. 

Figure 2 Pass Ale types ofpolymers based upon catenane architecures A Linear mechanically-linked polymers B mechanically cross-linked polymers C stars with a flexible, potentially switchable, catenane core D combs with catenarw linkages E main chain catenanes with switchable surface properties F highly mechani-cally-crosslinked networks G poly- or [n]catenanes. Only examples of types A and F have thus far been prepared...

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). 

Molecular anchorages are of two main types crystalline regions and diemical cross-links (to which physical entanglements may also contribute in polym networks). Not surprisingly, then, most ESR studies of polymers under tensile stress have been carried out using crystalline fibres (nylon, PET, PE, PP for example) or cross-linked polymers ... 

Cross-linked elastomers (the other main class of polymers studied by ESR) can be pre-strained above Tg to any desired extent (up to fracture) and their orientation stabilised by cooling below Tg before testing. At high pre-strains, strain-induced crystallization may occur providing a morphology essentially similar to that of synthetic fibres Cross-linked polymers may therefore be used to explore in a systematic manner, the role of strain and orientation in rmlecular fracture. 

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