Main-chain rupture

Reference to Tables 1 and 2 shows that the primary radicals invariably arise from main chain rupture, when radicals are produced by mechanical degradation. When irradiation is employed, primary radicals can also arise from the removal of side groups attached to the main chain. It is evident, therefore, that primary radical formation in mechanical degradation is always attributable to the polymer network being stretched sufficiently to rupture the chains, rather than molecules sliding over each other to strip off side groups. 

Interestingly, polyacrylonitrile, poly (methyl acrylate), and polystyrene behave differently in the rigid state and in dilute solution. This may be explained in terms of lateral macroradicals being generated upon the release of side groups in a primary step. The combination of these radicals competes with decomposition through main-chain rupture. In dilute solution, where radical encounters are much less probable than in the rigid state, main-chain rupture predomi-... 

Various theoretical approaches have been developed to describe mechanical degradation. One of the earlier studies was made by Frenkel [39] and Kauzmann and Eyring [40]. They proposed that linear macromolecules are extended in a shear field in the direction of motion. The strain of the molecules is primarily concentrated at the middle of the chain. No degradation is expected when the degree of polymerization is below a certain critical value. Bueche [41] predicts that entanglements produce preferential tension in the mid-section of macromolecules. Thus, chain scission is more likely to occur in the center of the chain. He also predicts that main chain rupture increases dramatically with increasing molecular weight. 

It should be noted that, depending on the location of the radiochemical attack, graft and block copolymers are obtained, rupture of the main chain giving block copolymers (213). 

The oxidation chemistry of small, partially-oxygenated fuels is of great interest in combustion chemistry as these are important intermediates in the combustion of virtually all commercial hydrocarbon fuels. Fuels with long carbon backbones react in their early stages mainly through a sequence of reactions that cause chain rupture, yielding smaller hydrocarbon fragments such as radicals. These then typically react with O2 to produce precursors of aldehydes, ketones etc. Not all of the features of acetaldehyde chemistry are completely representative of hydrocarbon oxidation, but this point is developed in the next chapter. 

In this method the amount of wool capable of being dissolved with a solution, which rupture the disulphide cross-links, is determined. Essentially it is a test which detects main chain breakage caused, most often, by action of acids on wool [21,22]. 

Much of the chemical behavior of cellulose fiber can be attributed to cellulose structure. Since cellulose is a highly crystalline polymer, it can absorb mechanical energy efficiently for mechanical stress reaction ( 5, 19). The mechanically activated thermal energy, in addition to rupture of main chains, may alter morphology or microstructure of cotton cellulose. Accordingly, the crystallinity and accessibility of cotton fiber may be influenced. 

The structure of such a polymer is characterized by four features. First, the main-chain bonds attached to a given C5 ring always have a cis relationship this is a consequence of the fact that ring-opening occurs by complete rupture of the double bond. 

A polymer is thermally stable untill the decomposition process starts. Two (main) types of thermal decomposition processes are usually recognised for polymers chain depolymerisation and random decomposition. Chain depolymerisation is the release of monomer units from a chain end or at a weak link and is essentially the reverse process of polymerisation. It is often called depropagation or unzipping. Random degradation occurs by chain rupture at random points along the chain, giving a disperse mixture of fragments. These two different processes may occur separately or in combination the latter case is rather normal [2]. Both processes cause sample mass losses which can be measured with a TGA. 

Scissions of main-chains by the mechanical destruction of the polymers are experimentally proved by the analyses of the observed ESR spectra for the various pdy-mers PE, PTFE, PB, PP and PMMA. A pair formation of the radicals, (mechano-radicals), after the milling is clearly demonstrated and this pair formation is believed to be the direct evidence for tl mechano-radicals formed primarily by the medianical actions. A model for chain rupture in an amorphous pdymer was proposed. Excess electrons produced by the triboelectricity due to the friction, diich is always accompanied with the mechanical fracture, play an important role, with coexistence of oxygen, in the thermal conversion of the mechano-radicals. The characteristic behaviors of the mechano-radicals, the hi er reactivity with oxygen, complete photoconversion of the peroxy radical, indicate that the mechano-radicak are formed and trapped on the fresh surfaces produced by cleavage in the solid polymer. The polymerizations initiated at the low temperatures by the PTFE mechano-radicals were reported and the copol5mierization is experimentally evidenced. 

Very recently the first synthesis of a main chain LCE has been described [46]. It turns out, as was to be expected due to the completely different location of the mesogenic units, that the stress strain properties of these novel materials are rather different from what is known from side chain LCEs. For example, the main chain LCEs require a stretching by a factor of about 5 to get macroscopic alignment of the director. Such an extension is not even an option for side chain LCEs, which typically rupture when elongated by a factor between about 2 and 3. Thus in many ways the main chain LCEs might resemble more closely a classical rubber than the sidechain LCEs. 

As has been pointed out in Section 7.1.2, polymers commonly undergo different kinds of bond ruptures simultaneously upon exposure to light, i.e. bond cleavage processes occur both in side chains and in the main chain of linear polymers. Bond rupture in side chains results in the formation of lateral macroradi-... 

Campbell and Peterlin and Peterlin concluded from e.s.r. measurements on isotropic and highly drawn nylon 6 and 6.6 fibres that no detectable free radicals were formed in the isotropic state, whereas approximately 1 chain in 250 was fractured in a fibre under high axial tension at failure. These fractured chains were later identified with the tie molecules linking adjacent crystallites together in the fibre direction. Quantitative theories have since been developed by Kausch et and more recently by DeVries et alP which attempt to correlate creep, creep-rupture, and stress-relaxation in fibres in terms of the measured main chain scission. 

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