Crosslinks, H-type

The product shown on the right of equation 11 is called an H-type crosslink. A number of alternative mechanisms of cross-linking have been proposed. 

The crosslinks formed in irradiated polymers are usually attributed to combination of radicals on different molecules in close proximity to each other. This proximity may result from C-H scission, followed by H abstraction from the adjacent molecule, or by migration of radical sites along the polymer backbone, e.g. by a hydrogen-hopping mechanism. This conventional and widely accepted mechanism of crosslinking produces a H-type crosslink. The reactive species may also be ions or radical-ions. 

The yield, G(X), of H-type crosslinks can be obtained from a plot of s + sV2 versus 1/dose, where s is the soluble fraction. 

High resolution NMR in solution can be used to measure the type and number of crosslinks in polymers [13]. The C-NMR techniques possesses the required sensitivity to detect concentrations of long-chain branches (Y-type) and H-type crosslinks as low as approximately 0.5 structural units per 10,000 carbon atoms [14]. The limitation of the NMR technique is the limiting solubility arising from gel for-... 

A related study has been the elucidation of the crosslink structures induced within polyethylene by high energy radiation. The secondary carbon radicals thus produced by C—H bond scission may diffuse by hydrogen atom abstraction. They have been shown to combine in pairs to form H type junctions, and to create Y type junctions by reactions with the vinyl end groups of the chains and with primary carbon radicals produced by main chain scission. In each case the shifts characteristic of the new structure were identified [32], The shifts of the H junctions are distinct, being 41.1, 31.9 and 28.7 ppm respectively at the (CH) junction and the first and second linked carbons, as is shown in Scheme 1, but the shifts of the Y junctions coincide with those at the roots of long branches, and their formation is recognised only when a careful comparison has been made of the areas of these shifts before and after irradiation. 

Figure 5.5 Calculated dependence of weight fractions of various substructures in the crosslinking system of H3 + I2 type on the initial molar ratio of H-groups to l-groups, ah (a polyurethane system) DC - dangling chains, BC - backbone chains, S - sol (backbone chains are elastically active network chains without dangling chains)...

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