Chain folding mechanism

At this level of styrene, the chain-folding mechanism of ethylene sequences is completely interrupted by the phenyl branch, thus eliminating ethylene-containing crystalline regions from the solid lattice. 

Such sequence structures are required to reduce the crystallinity of the polyethylene sample as the comonomer n-butyl branch from incorporation of 1 -hexene into the polymer backbone interrupts the chain-folding mechanism responsible for the crystallinity in polyethylene. Long units of ethylene along the polymer backbone are able to crystallize the polymer sample and are undesirable in polyethylene products that require a large degree of elasticity for particular applications. 

Although the cyclic oligomers of ethylene oxide have been prepared, they have not been used to study chain folding mechanisms. 

In contrast, for flexible-chain polymers, the transition into the ordered state is possible only if the flexibility can be decreased to values below fcr (in the absence of external deformational fields, the crystallization of flexible-chain polymers occurs by the mechanism of chain folding). 

Usually, crystallization of flexible-chain polymers from undeformed solutions and melts involves chain folding. Spherulite structures without a preferred orientation are generally formed. The structure of the sample as a whole is isotropic it is a system with a large number of folded-chain crystals distributed in an amorphous matrix and connected by a small number of tie chains (and an even smaller number of strained chains called loaded chains). In this case, the mechanical properties of polymer materials are determined by the small number of these ties and, hence, the tensile strength and elastic moduli of these polymers are not high. 

For density values g > 0.92 g/cm3 the deformation modes of the crystals predominate. The hard elements are the lamellae. The mechanical properties are primarily determined by the large anisotropy of molecular forces. The mosaic structure of blocks introduces a specific weakness element which permits chain slip to proceed faster at the block boundaries than inside the blocks. The weakest element of the solid is the surface layer between adjacent lamellae, containing chain folds, free chain ends, tie molecules, etc. 

Figure 14.16 Mechanism by which poly(ether ester)s function as crystallization promoters. Such materials internally lubricate and plasticize the PET molecular chains, thus allowing reptation (i.e. chain folding) to occur more quickly...

Comparison within the family of carotenoid oxygenases show that the four histidines are strictly conserved as well as their close environment. Presumably, all members of this family, including the protein catalyzing central cleavage of P-carotene, share a common chain fold, possess similar active centers and follow a similar reaction mechanism, vide infra. 

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