Cross-linking effect mechanism

ENR/EVA blends. Irradiation produced the initiating free radicals that then reacted with a bismaleimide or a multifunctional acrylate molecule through unsaturation to produce cross-linking. The mechanism is given in Figure 9.7. The effective cross-linking by HVA-2 or TMPTA was confirmed by dynamic mechanical analyzer (DMA). 

In all cases, the films were obtained by oxidative electropolymerization of the cited substituted complexes from organic or aqueous solutions. The mechanism of metalloporphyrin Him formation was suggested to be a radical-cation induced polymerization of the substituents on the periphery of the macrocycle. As it was reported for the case of polypyrrole-based materials ", cyclic voltammetry and UV-visible spectroscopy with optically transparent electrodes were extensively used to provide information on the polymeric films (electroactivity, photometric properties, chemical stability, conductivity, etc.). Based on the available data, it appears that the electrochemical polymerization of the substituted complexes leads to well-structured multilayer films. It also appears that the low conductivity of the formed films, combined with the cross-linking effects due to the steric hindrance induced by the macrocyclic Ugand, confers to these materials a certain number of limitations such as the limited continuous growth of the polymers due to the absence of electronic conductivity of the films. Indeed, the charge transport in many of these films acts only by electron-hopping process between porphyrin sites. 

The EB treatment of the GTR containing LDPE and EVA matrix blends showed significant benefits. The 200 kGy EB absorbed dose (in air) resulted in a better tensile strength and increased elongation at break, without changing the tensile modulus, which provides more rubber-Uke properties [142]. The modest change in hardness proves the cross-linking effect caused by the EB treatment in all cases. The dynamic mechanical analysis confirmed the compatibilization effect of EVA and... 

PVA is generally quite tolerant to different ranges of pH and heat. However, metal ions and certain metal salts, particularly salts of boric acid, cause cross-linking of the polymer chains. This is the mechanism for the development of the well-known commercial product called slime (Fig. 17) (40,41). The gelation of PVA by borate, a particularly well-known example of this cross-linking effect, is pH dependent and occurs chiefly in alkaline solutions. 

If a modest number of cross-links between the polymer backbone are introduced, the polymer Hquid crystal takes on elastomeric properties. The useflilness of these materials probably Hes in the coupling of mechanical and optical effects. 

In contrast, antioxidants can have an opposite effect when peroxide curing. Because peroxide cross-linking involves a free-radical mechanism, and antioxidants are designed to scavenge free radicals, it is obvious that peroxide efficiency can be compromised by the addition of antioxidants. Thus the decomposition products of the ppds were acting as accelerators (29). 

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