Trans-planar zigzag conformation

The results listed in Table VI illustrate to what degree the band structure of polyacetylene (corresponding to Xl - X2 H in the Table VI) is affected by periodic substitution of the pendant H atoms along the chain. While substituting a pendant H by a OU appears to have a negligible effect on the values of E and BW, substitution instead by SiH or Si(CH,)2 increases E and decreases BW (suggesting poorer conductivity). A trend appears"to exist for increased E values (and decreased BW values) with an increase in the steric Bulk of the substituent X. Since in these immediate calculations all chains were considered in their trans planar zigzag conformation, it would not seem that the steric bulk of the substituent would have a major effect on the calculated band structures. 

Fig. 10.1 The orthorhombic unit cell of polyethylene (a) in the ab plane, (b) in the ac plane showing the all-trans planar zigzag conformation of crystalline polyethylene. The 2i helix (one complete turn for each two CH2 units) is also apparent.

X-Ray diffraction observations of E-V powders and fibers showed an expansion of the unit-cell basal plane, mainly in the a-direction, and a constant fiber period, respectively, as their chlorine content increased. Together these observations indicate that at least some V units, with their attendant Cl atoms, are incorporated into the crystals, resulting in an increase in interchain separation, but no alteration in the M-trans, planar zigzag conformation found for crystalline PE. 

PVDF has at least five crystalline phases (a-, P-, y-, 5-, and s-phase), arrd the a- and P-phases are the most common. Because of the all trans planar zigzag conformation (TTTT), the P-phase has the largest spontaneous polarization, and many applications take advantage of the properties of this polar phase. Therefore obtaining the polar P-phase and suppressing the non-polar a-phase (thermodynamically favored) is the major goal of PVDF processing. ... 

The ordered structures of some polymers are governed by the influence of specific diluents. This involves a specific type of polymorphism, the more general aspects of which will be discussed in the chapter concerning thermodynamic quantities. Syndiotactic poly(styrene) is a polymer that is rich in compound formation with solvent mediated polymorphic behavior.( 126-130) The polymer can crystallize in four major crystalline modifications that involve two different chain conformations. In the a and p modifications the chains adopt an all trans planar zigzag conformation. These two modifications are formed by crystallization from the melt and, under special conditions, from solution. In contrast the y and 5 modifications are characterized by a helical conformation. The 5 polymorph can only be prepared in the presence of solvent. Its exact crystal structure depends on the nature of the solvent. Compound formation between the 5 form of the polymer and the solvent has been demonstrated. Complete elimination of the solvent results in the pure, helical y form. 

C-C bond length / and degree of polymerization n) was calculated to be ca. 30. appreciably higher than the value 7 of PE coil [2]. In the case of PE, on the other hand, the all-trans planar-zigzag conformation is the most stable, as observed predominantly in the... 

The above method for calculating 8 v) was applied to the R v) spectrum in Figure 8.4c. Then, the n(v) and k v) spectra were calculated by the use of Equations (8.4a) and (8.4b), and the results obtained are shown in Figure 8.5. The k(v) spectrum has the shape of an absorption spectrum, from which band positions can be accurately determined. The bands observed at 2919 and 2850 cm are, respectively, due to the CH2 antisymmetric and symmetric stretching vibrations characteristic of the all-trans planar zigzag conformation of the -alkane chain. The doublet bands at 1471 and 1462 cm are assigned to the CH2... 

The most stable a and p forms are characterized by chains in trans-planar (zigzag) conformation, whereas y and 8 forms are characterized by chains in helical conformation. The a and P forms can be obtained principally by melt crystallization or by annealing at a proper temperature, whereas the Y and 8 forms can be obtained through solvent treatments of the amorphous or a form. 

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