Molecular breakdown

It is commonly found that polymers are less stable particularly to molecular breakdown at elevated temperatures than low molecular weight materials containing similar groupings. In part this may be due to the constant repetition of groups along a chain as discussed above, but more frequently it is due to the presence of weak links along the chain. These may be at the end of the chain (terminal) arising from specific mechanisms of chain initiation and/or termination, or non-terminal and due to such factors as impurities which becomes built into the chain, a momentary aberration in the modus operandi of the polymerisation process, or perhaps, to branch points. 

Chemical Degradation A chemical action involving the molecular breakdown of the material due to contact with a chemical. The action may cause the personal protective equipment to swell, shrink, blister, discolor, become brittle, sticky, soft or to deteriorate. These changes permit chemicals to get through the suit more rapidly or to increase the probability of permeation. 

Direct dyes of the disazo diarylurea type, such as Cl Direct Orange 26 (3.5) and Cl Direct Red 79 (3.21), are particularly prone to decomposition in high-temperature dyeing because molecular breakdown can occur by hydrolysis as well as by the reductive mechanism under alkaline conditions. Thus the ureido linkage in this red dye may be broken by hydrolysis to give two monoazo dye fragments (3.22), or the azo groups can be reduced to yield a diaminodiphenylurea (3.23) and two molecules of H acid (Scheme 3.3). 

Falloff in tensile strength, modulus and elongation as the temperature rises is due to accelerated degradation of the rubber. Irrespective of the type of base rubber used, the physical properties of rubber fall as the temperature is increased to above 120°C due to molecular breakdown. Since rubbers lose their physical properties at higher temperature they become weak and as such susceptible to chemical attack. 

A number of optical photoion-photoelectron coincidence experiments have been reported107 giving qualitative information on molecular breakdown. However, in these experiments little or no attempt has been made to correct for either the transmission efficiency of the photoelectron spectrometer or the ion kinetic-energy discrimination. Fragment ions usually... 

The organic hydroperoxides formed are relatively unstable, and the decomposition of intermediates often leads to molecular breakdown and formation of subsequent intermediates with lower carbon numbers. These reactions continue rapidly until the formation of acetic or formic acid. Both acids will be converted into carbon dioxide and water as final products (Gloyna and Li, 1993). 

The basic process whereby radicals are formed remains obscure. The energy available per quantum of radiation is very large and in many ways it is surprising that molecular breakdown, as detected by electron spin resonance, is both simple and remarkably selective. 

The proposed reaction schemes (Figures 4-17 and 4-18) show the molecular breakdown commencing at the extremities of both molecules to leave a polar core that eventually separates from the liquid reaction medium. In the case of the decomposition of the amphoteric molecule, thermal degradation could just as easily commence at the aliphatic carbon sulfur bonds followed by thermal scission of the alkyl moieties from the aromatic systems. If the removal of the... 

The C60 molecules are surprisingly stable, but heating to above 1000 K or compression to above 40 GPa [ 135], or a combination of similarly extreme conditions, give permanent changes to the material which must be interpreted in terms of molecular breakdown. 

Starch acetate solutions exhibit viscosity effects characteristic of dissolved high polymers. Since the viscosity of a starch solution varies in relation to the size of the starch molecules, viscosity measurements have afforded a rapid means for following molecular breakdown, and, in addition, have provided information on the molecular weight and shape of the dissolved molecules. 

These are used to increase mastication efficiency by increasing the rate of molecular breakdown, particularly in natural rubber. Normally less than 0.5 phr are used, but higher dosages are required for synthetic rubber. As sulfur inhibits their action, these are normally added before mastication. Zinc salt of pentachlorothiophenol and di-o-benzamidophenyl disulfide are commonly used as peptisers in lining compounds. 

These reactions may be stimulated to provide 80% decomposition by using a plasma process to excite the methane molecule. In this case, the molecular breakdown may occur in the plasma to produce charged species. Hydrocarbons other than methane may be used as the feedstock. The usual operating sequence involves flushing and evacuation, heating to temperature under the inert atmosphere, carburizing for a predetermined time followed by a diffusion anneal in a carbon-free atmosphere. This cycle is designed to provide the optimum surface carbon content and carburized depth. - ... 

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