Ethylene extended chain conformations

Poly (ethylene terephthalate) can be grown to extended chain crystals similar to the nylons 164). No special high pressure polymorph has been suggested. But there may be, as in the nylons, the possibility that a continuous increase in mobility exists in the crystal phase at higher pressure and temperature. This mobility would have to be based on conformational changes, i.e. a condis crystal phase. 

Accordingly, the influence of MW on the crystallization behaviors of semicrystalline polymers has been studied in various articles. For example, linear crystal growth rates of poly(ethylene oxide) and poly(ethylene succinate) (PES) reach a minimum value at a critical MW. This value is related to the crystallization transition from an extended chain to a folded chain conformation [96,97], suggesting that high MW polymers require sufficient reconformation time to achieve an ordered structure. As evidence of this MW dependence of the semicrystalline polymer on... 

The DSC technique was used by Li et al [42] to investigate a series of segmented PUs with different HS flexibilities, based on the aliphatic diisocyanate HDI or aromatic MDI, as the HS showed a folded-chain conformation. The chain extenders were the diol BDO or 4,4 -diaminodiphenyl ether (DDE) the SS macrodiols were PTMO or poly(ethylene/propylene adipate) (PES) with a molar mass 2 000 g/mol. The materials were prepared by a solution polymerization. 

While conformation II (Fig. 2.34) of Uke-y -amino acids is found in the 2.614-helical structure, conformation I, which similarly does not suffer from sy -pen-tane interaction, should be an appropriate alternative for the construction of sheet-like structures. However, sheet-like arrangement have not been reported so far for y-peptides composed of acyclic y " -amino acid residues. Nevertheless, other conformational biases (such as a,/9-unsaturation, cyclization between C(a) and C(y)) have been introduced into the y-amino acid backbone to restrict rotation around ethylene bonds and to promote extended conformation with formation of sheets in model peptides. Examples of such short chain y-peptides forming antiparallel (e.g. 152 [208]) and parallel (e.g. 153-155 [205, 208]) sheet-hke structures are shown in Fig. 2.38. 

This paper describes a continuation of the above work, firstly to apply conformational analysis to predict the conformation of the poly(MDI-butandiol) chain, and secondly to extend these x-ray diffraction and model building techniques to investigate the structures formed with other chain extenders, notably propandiol and ethylene glycol. More detailed accounts of this work have been published elsewhere. (11,12)... 

Poly(ethylene tephthalate) (PET) is a crystalline polymer characterized by a triclinic unit cell. The chain axis is 10,7 A and corresponds to chains in fully extended conformation, with all dihedral bonds close to the trans-planar conformation (tf symmetry) [258], As shown in Fig. 30A, in this conformation, the terephthalic residues are all coplanar. 

Celluloses are described by the type of substituent carried by the cellulose backbone and the viscosity. They can separate DNAs at low concentrations (<2%). Hydroxyethylcellulose (HEC) is a linear derivative with bulky ethylene oxide side chains terminating in hydroxyl groups. In aqueous solutions, it has a stiff, extended conformation. It has relatively high viscosities at the entanglement concentration. A mixture of HEC of different sizes offers a wide range of DNA separation from 100 bp to 23 kbp. Hydroxypropylcellulose (HPC) solution provides similar separation ability, and it has 2-3 times lower viscosity than HEC solution. 

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