Trans-zigzag conformation

FIG. 15.8 Changes in UV-visible absorption spectrum of a poly(di-n-hexylsilane) spincast film in the course of crystallization to adopt an all-trans-zigzag conformation of the Si backbone at I5°C (a). Photocontrol of the crystallization rate as observed by the absorbance increases at 366 nm, corresponding to the increase of the all-trans-zigzag conformation of the Si chain (b). (From reference 39 copyright permission from the American Chemical Society.)... 

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. 

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. 

Fig. 17.5. A portion of a poly(inethylphenylsilane) chain with the all-trans-zigzag conformation. The phenyl rings are set perpendicular (type A) and coplanar (type B) to the Si—Si bond.

Figure 4.2 shows the IR speetra for two blends of different compositions. As is known, the informative stmetuie-sensitive band for PHB is that at 1228 cm [5]. Unfortunately, the intensity of this band cannot be clearly determined in the present case, beeause it eannot be separated from the EPC structural band at 1242 cm [3]. The bands used for this work were the band at 620 cm (PHB) and the band at 720 cm (EPC) [6], which correspond to vibrations of C-C bonds in methylene sequences (CH2), where n> 5, occurring in the trans-zigzag conformation. The ratios between the optical densities of the bands at 720 and 620 cm are transformed in the coordinates of the equation where (5 is the fraction of EPC and W is the quantity characterizing a change in the ratio between structural elements corresponding to regular methylene sequences in EPC and PHB. 

For polythiophene, Mo et al. [102], Bruckner and Porzio [135], and Yamamoto et al. [84] proposed a similar structure with lattice parameters around a = 7.8 A and 6 = 5.6 A (for both the orthorhombic and monoclinic lattices with negligible difference), a and b and their ratio (approximately 1.4) resemble those listed in Table 8.5, and so the presence of the herringbone structure is assumed, c (7.8 A) is consistent with the size of two repeated thiophene units, or the distance of C5-C4 (7.77 A) in DMQtT (see Figure 8.27). Using the Rietveld method [136], Bruckner and Porzio [135] have confirmed the chain planarity, or the fully-stretched S-trans zigzag conformation of the polythiophene backbone. 

From the above experimental results, it can be said that in the temperature range from room temperature to 80° C the long n-alkyl side chains of PBpT-012 do not take only the all-trans zigzag conformation in the immobile state, but the mobile state. At temperatures above 120° C, the all-trans zigzag form completely disappears. Such a behavior occurs also in the case of PG-18 with long n-octadecyl side chains, in which the main chain takes the a-helical conformation [21, 31-33]. 

CH2(A) and a-CH2(A)) are the same as those for Figure 7-21(b), the int-CH2 carbons in the amorphous phase are in the mobile state and undergo rapid exchange between trans and gauche conformations. In the crystalline phase, the side chain carbons take the all-trans zigzag conformation. 

In Figure 7-26 are shown CP-MAS NMR spectra of PG-12 as a function of temperature. Below -20° C, both of peaks for the side chains and the main chain are broad, and the mobility for them is in region A. At 0° C, the NMR chemical shift of the int-CHj carbons moves upfield and the intensity of the OCHj carbon becomes very weak. The side chains are in region B at this temperature. The NMR chemical shift of the int-CH2 carbons of PG-12 moves upfield by about 3 ppm as the temperature is raised from -20° C to 0° C. Above 0° C, the chemical shift value of ca. 30 ppm indicates that the side chains are in the amorphous phase. Below 0°C, the NMR chemical shift is about 32.9 ppm. Therefore, it shows that the int-CH2 carbons take the all-trans zigzag conformation in the immobile state. Therefore, the drastic change in the side chains between -20 and 0° C shows the melting of the side-chain crystallite. 

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