Cross-linking-chain scission theories

Flory ( 9) has treated the interesting case of subsequent removal of the first stage cross-links without chain scission. Even after complete removal, i.e. =0, there is still a certain memory of the structure of the first network since the composite network strands were physically part of both networks. According to Flory s theory, the resulting network may be treated as if a certain fraction, 0, of the strands of the second network were effectively converted into strands of the first network, >ie. 

One further topic merits discussion in this section in view of its success in dealing with the mechanical properties of oriented fibres, which are after all anisotropic polymers. That is the theory of kinetic fracture, developed mainly by Zhurkov and co-workers. Evidence has been presented from electron spin resonance (e.s.r.X " mass spectrometry, and infra-red spectroscopy that when highly oriented fibres or heavily cross-linked rubbers experience a tensile stress (along the axis for fibres) an appreciable fraction of main-chain bonds are broken by the applied stress. These scission events are observed to occur more or less homogeneously throughout the fibre and are not localised in the fracture plane. Many sets of data show that the lifetime tb of a fibre under stress is described approximately by the following equation... 

That epoxide groups are also present in oxidized natural rubber has been demonstrated by Golub et al (1975) by the use of H NMR and NMR spectroscopy. The presence of occasional epoxide groups is not believed to have a great effect on the properties of natural rubber but it may be noted in passing that, in theory, it provides sites for cross-linking by such materials as amines and acid anhydrides which are well known as epoxide resin hardeners. At the present time the above reaction sequence is of more interest in that it provides a route for converting peroxy radicals into alkoxy radicals and is believed by some workers to play an important role in chain scission due to autoxidation. 

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