Effective cross-links, definition

Early work in this field was conducted prior to the availability of powerful radiation sources. In 1929, E. B. Newton "vulcanized" rubber sheets with cathode-rays (16). Several studies were carried out during and immediately after world war II in order to determine the damage caused by radiation to insulators and other plastic materials intended for use in radiation fields (17, 18, 19). M. Dole reported research carried out by Rose on the effect of reactor radiation on thin films of polyethylene irradiated either in air or under vacuum (20). However, worldwide interest in the radiation chemistry of polymers arose after Arthur Charlesby showed in 1952 that polyethylene was converted by irradiation into a non-soluble and non-melting cross-linked material (21). It should be emphasized, that in 1952, the only cross-linking process practiced in industry was the "vulcanization" of rubber. The fact that polyethylene, a paraffinic (and therefore by definition a chemically "inert") polymer could react under simple irradiation and become converted into a new material with improved properties looked like a "miracle" to many outsiders and even to experts in the art. More miracles were therefore expected from radiation sources which were hastily acquired by industry in the 1950 s. 

In the work [1], the dependences of PHE synthesis main characteristics, namely, reduced viscosity tired and conversion degree Q, on synthesis temperature T were studied. It was found out, that T rising up to the definite limits influences favorably on the indicated process a reaction rate rises, ti and Q increase. This effect can be observed in the narrow enough range of T=333-348 K. At T lower than 333 K PHE formation process decelerates sharply, that is due to insufficient activity of epoxy groups at low temperatures. At T > 353 K cross-linking processes proceed, which are due to the activity enhancement of secondary hydroxyls in polymer chain [1]. It is also supposed [1], that at the indicated temperatures of synthesis PHE branched chains formation is possible. 

Incorporation of multifunctional POSS into polymer systems has been investigated with different polymers [6,62-66]. In these cases, single-phase polymer networks with POSS molecularly dispersed are often formed. POSS acts as a polyhedral cross-link. But no definite effect of POSS on network properties has been established. Both a decrease [64,65] and no change in Tg [6] were reported. The rubbery modulus increases due to a high crosslink density, and thermal stability increases with POSS content. 

The introduction of chemical cross-links into an uncross-linked polymer converts it from a viscoelastic liquid to a viscoelastic solid in the sense of the definitions of Chapter 1 and the classification of Chapter 2 the viscosity becomes infinite and the material acquires an equilibrium modulus and compliance, so the properties in the plateau and terminal zones change profoundly. However, the properties in the transition zone may change very little. The effects of cross-linking are discussed in this chapter, as well as the effects in the plateau and terminal zones of incorporating fillers (finely divided particles, usually of high modulus) or other combinations of more than one phase. 

Polymer mechanical properties are one from the most important ones, since even for polymers of different special-purpose function a definite level of these properties always requires [20]. Besides, in Ref [48] it has been shown, that in epoxy polymers curing process formation of chemical network with its nodes different density results to final polymer molecular characteristics change, namely, characteristic ratio C, which is a polymer chain statistical flexibility indicator [23]. If such effect actually exists, then it should be reflected in the value of cross-linked epoxy polymers deformation-strength characteristics. Therefore, the authors of Ref [49] offered limiting properties (properties at fracture) prediction techniques, based on a methods of fractal analysis and cluster model of polymers amorphous state structure in reference to series of sulfur-containing epoxy polymers [50]. 

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