Unzippering

In other instances the reactions appear to occur in sequence down the chain, for example in the depolymerisation reaction of polyformaldehyde (polyacetal) and polymethyl methacrylate which are referred to as zippering or sometimes unzippering reactions. In other cases cyclisation reactions can occur such as on heating polyacrylonitrile ... 

Fig. 33. Unzippering of a staggered helical state according to model II,48)...

The composition of thermal degradation products was previously shown to be greatly affected by the processing temperature (34)-PVC is inherently unstable because of the presence of allylic chlorine atoms throughout the polymer. These chlorine atoms are easily removed by exposure to minimal heat and/or light which results in the well-known unzippering reaction (35). 

Unified theory of crystallisation processes, 706 Unit of a physical quantity, 53 Unperturbed random-coil macromolecule, 248 Unperturbed state, 246 Unperturbed viscosity coefficient, 791 Unstable flow, 578 Unzippering, 769... 

The next step in the process consists of removing the unstable terminal hemiacetal and formal groups responsible of depolymerization reactions which may easily occur by unzippering mechanism (—CH2CH2O—) - (-CH2-0-) H. 

Figure 18 shows the LAMMA 1000 negative spectrum for Poly Alpha Methyl Styrene, an unzippable polymer (meaning that it breaks apart sequentially as monomers). Again, an EI mass spectroscoplst would recognize the material. 

Figure 18. Poly alpha methyl styrene, an unzippable polymer—by LAMMA. (Reproduced with permission from Ref. Copyright 1983 International Conference on X-ray Optics and Microscopy.)...

Monosulfide polymer 19 is the most stable of the polymers discussed, not only in its resistance to base, oxidizing acids, and light but also in thermal stability. Degradation is slow at 300°C, but at 350°C chain scission results in an unzippering to evolve tetrafluoroethylene and di-thietane 6 (n = 2). The latter gives rise to polydisulfide 16 and may cycloadd to tetrafluoroethylene to form dithiane 7, the third major product. More likely under these conditions, however, is direct formation of 7 by a backbiting mechanism as illustrated below. 

The general sulfur atoms in 22 render it less stable than 19, so that, while quite stable toward acids, amines degrade it rapidly. Thermal decomposition occurs above about 175 °C by unzippering to regenerate monomer. 

We construct the two extreme cases of three-dimensional free energy surface (or potentials of mean force [PMF]) felt by the tethered head by exhaustively sampling the kinesin configurations on the MT surface by modeling the neck-linker of the MT-bound head either in the ordered (zippered) or in the disordered (unzippered) state. 

Zoravkovic, S. and Sataric, M.V., Single molecule unzippering experiments on DNA and Peyrard-Bishop-Dauxois model. Submitted to J. Mol. Biol. 

After several minutes, the unstable end-groups were removed and a stable polymer resulted. More recently (1981), this technique has been refined by researchers at Mitsubishi Gas Chemical Co. who performed this controlled thermal unzippering in a vented extruder[10]. By either approach, this must be carried out in the presence of thermal stabilizers, especially antioxidants, to prevent random oxidative attack of the polymer chain. 

One such factor is the presence of weak links which can seriously affect the heat resistance. One polymer that is particularly affected in this way is polyvinyl chloride where stepwise dehydrochlorination can occur by means of an unzippering reaction initiated from a weak link. In rubbers the weak link, as has been mentioned, is rather more important in causing degradation under the more common oxidative conditions to be dealt with later. 

This reaction is sometimes called unzippering and is thought to originate at certain structural defects within the polymer [23]. 

When the PVC unzippers, it discolors and becomes brittle. To prevent this dehydrochlorination reaction, heat stabilizers are added. Lead stabilizers are effective and a major type of heat stabilizer. Data from 2003 [24] shows lead stabilizers to represent more than 50% of the world PVC stabilizers, but less than 20% in the U.S. where tin stabilizers predominate. The use of lead stabilizers is on the decline and being phased out due to concerns about the health effects of lead. Nonetheless, the news occasionally reports that lead has been found in a toy or other PVC article imported from a country such as China. The source of lead is likely due to the stabilizer. Another type of heat stabilizers is the mixed metal packages which are based upon mixtures of different... 

Use of surfactants is an effective way for dispersing CNTs [39]. Reports show that the outer most nanotubes in a bundle are treated more than the innermost tubes and the nanotube remains predominantly btmdled even after surfactant treatment. But mechanical methods like ultrasonication can debundle the nanotubes by steric or electrostatic repulsions [40]. On sonication the high local shear will unravel the outer carbon nanotubes in a bundle and expose other sites for additional surfactant adsorption, thus the surfactant molecules gradually exfoliate the bundle in an unzippering mechanism [41]. Some of the common surfactants used for the dispersion of carbon nanotubes are sodium dodecyl benzene sulfonate (SDBS) [42], dodecyl trimethyl ammonium bromide (DTAB) [43], hexadecyl trimethyl ammonium bromide (CTAB) [44], octylphenol ethoxylate (Triton X-100) [45] and sodium dodecyl sulfate (SDS) [46]. Covalent modification is another way to solubilize the CNTs in different solvents and to improve the interaction with the matrix in composites [47]. 

FASTENERS NEEDED TO PROVIDE FAIL SAFETY AND TO PREVENT CATASTROPHIC UNZIPPPING OF BOND)... 

Adhesive Weaker than the Adherends (Fasteners Needed to Provide Fail Safety and b to Prevent Catastrophic Unzippping of Bond)... 

Depolymerization is characterized by the scission of the main polymer chain backbone. A polymer molecule can be broken down into fragments by (i) random degradation, where bonds in the chain are broken at random (2) depolymerization, where chain scission is the reverse of addition polymerization and monomer units are released successively from a chain end or at a weak link (often called depropagation, unzippering, or unzipping) and (3) weak-hnk degradation, where chain scission occurs at weak links distributed at random along the chain, which are more easily ruptured than normal bonds (Table 1). These types of reactions may occur separately or in conjunction with each other and are initiated thermally, photochemically, by ultrasonic irradiation, mechanical action, or some other method. 

Chain reaction (depropagation, unzipping, unzippering, chain depolymerization). Stepwise reaction. 

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