All-trans planar zigzag conformation

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.

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 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. 

The result of the factors above is that the most likely (lowest-energy) conformation for many polymers that have all-carbon backbones is the all-trans planar zigzag, whereas for many others it is some form of helix. The simplest helix is the 3i helix for which trans and gauche bonds alternate along the backbone (fig. 3.5(b)). This helix is described in more detail in section 4.1.5. 

Syndiotactic polystyrene, s-PS, is a highly stereoregular, semicrystalline vinyl polymer that normally melts at 270 °C [45,46], S-PS shows a large number of crystalline polymorphs [47-50] obtained by melt crystallization and solvent-exposure techniques. However, s-PS assumes only two distinct crystalline conformations [all trans, planar zigzag (... tttttttt...) and 2 -helical (... ttggttgg. ..) t= trans and g = gauche], which are characterized by fiber repeats of... 

In these connections, it is useful to consider the behavior of the chemical shift of the CH2 carbons of paraffins and polyethylene in the crystalline and noncrystalline components. It is known that the CH2 carbons in paraffinic chains appear at lower frequency by 4-6 ppm if a carbon atom three bonds away is in a gaMc/ie-conformation rather than in a frans-confor-mation (y-effect) [7]. In fact, cyclic paraffins which crystallize in a conformations are characterized by two parallel all-fran5-planar zigzag strands connected by two GGTGG loops, it is found that the CH2 carbons with a y-effect resonates at a lower frequency by about 6.5 ppm as compared with those with no y-effect [8, 9]. Furthermore, it is found that the CH2 carbons of cyclic paraffins, and n-paraffins in the noncrystalline state, appear at lower frequency by 2-3 ppm more than those in the crystalline state. In the crystalline state, the CH2 carbons assume the all-trans-zigzag conformation, which is fixed because motion is frozen, but in the noncrystalline state a rapid transition between the trans- and gawc/ie-conformations occurs [11]. Weeding et al. [12] obtained the same results on the noncrystalline state. 

The use of rotated Fischer projections has been retained in the present edition in order to provide a link with, and an explanation of, the bulk of existing published polymer literature, although the present common practice [4] is to depict main-chain bonds in planar, extended zigzag (all-trans) conformations, together with a stereochemical representation of side-groups at tetrahedrally-bonded atoms. 

Syndiotactic poly(styrene) displays a complex polymorphic behavior that reflects the specific role played by solvents. Four crystalline forms have been reported.(289,290) The a and p forms can be obtained from the melt (or glass), depending on the crystallization conditions.(291) Both structures comprise planar zigzag chains that have the same identity period of 5.1 A. The a form has a trigonal unit cell while the p form is orthorhombic. The P form can also be produced by crys-taflization from solution.(292,293) The y and 8 structures develop after interaction with solvent. In contrast to the all trans bond orientation of the a and p structures, the chains in the y and 8 crystals adopt a ttggttgg sequence of bond orientation. Thus a helical ordered structure evolves. This structure is similar to the crystalline chain conformation of syndiotactic poly(propylene).(294) The difference between the y and the 8 polymorphs is that in the former the sample is completely dried, while the solvent is included in the 8 form. It therefore represents a clathrate type structure. The formation of these structures is, thus, solvent specific.(292,293,295,296) The... 

---