Molecular chains interrupted

Figure 6-3. Schematic view of photo-induced electron or hole transport through the DNA-based molecular chain. Reversible random walk of the charge along the chain is interrupted by irreversible chemical degradation ( quenching ) at given sites. Charge i.e. electron or hole) injection at the terminals corresponds to interfacial charge transfer in the in situ STM or nanogap electrode configurations to be discussed in Section 4.

When we say a polymer is crystalline, it does not mean the polymer has periodically translational atomic or molecular structure as small molecules. In fact, the crystalline regions in polymer where chains are folded and arranged regularly, mingles with amorphous regions where the randomness of chains interrupts the ordering (Figs. 3.1A and B). 

Double bond oxidation interrupts the polyene sequences and thus reduces yellowing or reverses it completely. Water-sensitive molecules are also created that can be easily removed from the surface. Oxidation also results in molecular chain cleavage. 

An inhibitor is a substance that retards a reaction. An inhibitor is also present in "catalytic" or sub-stoichiometric amounts. In a radical chain reaction an inhibitor may be a radical scavenger that interrupts the chain. In a metal catalysed reaction an inhibitor could be a substance that adsorbs onto the metal making it less active or blocking the site for substrate co-ordination. We also talk about a poison, a substance that stops the catalytic reaction. A poison may kill the catalyst. The catalyst dies, we say, after which it has to be regenerated wherever possible. We will often see the word co-catalyst, a substance that forms part of the catalyst itself or plays another role somewhere in the catalytic cycle. We inherited a florid language from our predecessors to whom catalysis was black magic. Naturally, these words are rather imprecise for a description of catalysis at the molecular level. 

Unless the molecular weight of the star arm is rather low, this period of relaxation by fluctuation is generally interrupted by the reptation of the linear polymers. This happens when the centre of mass diffusion of the linear chains is able to renew the tube of all (central) portions of the chain not relaxed by the star-like modes. So the reptation time is given by... 

In order for a solid to burn it must be volatilized, because combustion is almost exclusively a gas-phase phenomenon. In the case of a polymer, this means that decomposition must occur. The decomposition begins in the solid phase and may continue in the liquid Illicit) and gas phases. Decomposition produces low molecular weight chemical compounds that eventually enter the gas phase. Heat from combustion causes further decomposition and volatilization and. therefore, further comhusiion. Thus Ihe bunting of a solid is like a chain reaction. For a compound to function as a flame retardant it must interrupt this cycle in sonic way. There are several mechanistic descriptions by which flame retardants modify flammability inen gas dilution, thermal quenching, protective coatings, physical dilution, and chemical interaction. 

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