Gauche conformation crystalline structures

The crystalline structure of the stereocomplex has not yet been elucidated. Liquali et al.301) proposed a model for the stereocomplex (see Eq. (13)) involving two flat syndiotactic chains with the same number of trans- and gauche-conformations which may lie in the grooves of one isotactic 5rhelix. More recently, Tadokoro et al.302) found that iso-PMMA forms a lOj-helix. Further-... 

Based on high-resolution solid-state NMR and TEM observation, the mechanism for the crystallisation of nylon 46 is summarised in Figure 27.69 When the temperature of the melt sample decreased to the crystallisation temperature, hydrogen bond is formed with the nearby chains. At this stage, the size of the crystalline phase is small and the crystal structure is not energetically stable. The amorphous phase is mobile and the conformation of the amorphous phase contains the gauche conformation around the bond. As the crystallisation proceeds, the crystal... 

The true conformation in the crystalline structure of a polymer is more complicated, the backbone chain often taking a helical shape in which alternate chain bonds take trans and gauche positions, such that the carbon chain is not in a plane. For example. 

The chemical shift is determined by the relatively local electronic structure. One of the most important parameters which affect chemical shift is conformation. As mentioned in the section about crystalline and amorphous phases, a typical example for the conformational effect on the chemical shift is the chemical shift difference between the crystalline and amorphous phases of polyethylene. In the crystalline phase, polyethylene takes the all trans-zigzag conformation, while, in the amorphous phase, a rapid transition between the trans and gauche conformations takes place. As a result, the chemical shift of the amorphous phase is the average of the trans and gauche conformations. 

For all the temperatures studied, the simulations lead to crystalline monolayers with strong orientational ordering of the chains in a distorted hexagonal structure even in the absence of an applied pressure. The chains are tilted at an angle of 30° to the surface normal. Most of the gauche defects are found near the ends of the chains, as observed in crystals of n-alkanes, but even at 400 K the average number of gauche conformations per chain is only about one. 

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