Bands chain folding, extended chains

Boolaamphiphiles with two reactive head groups at both ends may bind in a linear or bent fashion to metal surfaces. The only example of a folded bolaamphiphiles on solid surfaces is provided by HOOC—(CH2)3o—COOH on silver. Ellipsometry yielded a surface film thickness of 20 2 A. In the reflection infrared spectra, the 2928 cm shoulder, as well as the lack of sharp bands between 1150 and 1350 cm for the progression of wagging modes indicated that the all-trans sequence length in the surface film should be much less than for the fully extended chain in the KBr pressed disk sample (Figure 6.23) . 

Figure 12. (F—E) Characteristic FTIR spectrum of 110 chain folds in solution-grown single crystals of n-alkane Ci98H3g8 (spectrum of extended-chain crystals subtracted from that of once-folded crystals) (PESC) solution-grown single crystals of polyethylene (bulk PE) melt-crystallized linear polyethylene. All bands are CH2 wagging defect modes except the CH3 band at 1378 cm-1. Spectra recorded at 110 K (from ref 68 by permission of Elsevier Science Publ.)...

A band of moderate intensity has been observed at 1080 cm-1 in the Raman spectrum of once-folded solution-crystallized n-alkane C168H338.72 This spectral region is associated with C—C stretching vibrations of methylene sequences containing gauche bonds. Extended-chain shorter alkanes (/v-CixHox, n-C72H146) did not show this band. The 1080 cm-1 band is also observed in semicrystalline polyethylene.73 Low-temperature spectra were not reported. 

Useful information about the stereoregularity and the conformation of the polyvinyl chloride molecule has been obtained from infrared spectra [5i]. For the key bands the C —Cl stretching bands are used. They are coupled with the skeletal deformation vibrations and are conformation sensitive. Fig. 1 shows the C —Cl stretching frequencies of syndiotactic polyvinyl chloride. The various conformations of the syndiotactic and atactic polyvinyl chlorides and their model compounds have their characteristic C —Cl stretching frequencies [5i]. These results clearly show that the syndiotactic PVC takes the extended zigzag structure and the atactic part causes the chain folding. The possibility of utilizing the backbone vibrations for the conformation study has not been explored for PVC. 

In the 1980s Keith and Padden proposed that chain tilt itself (not screw dislocations) results in axial twisting of growing lamellae that is manifested as banded spherulites. The reviews of Keith and Padden [44] and Lotz and Cheng [23] may be consulted for original references. Chain tilt is almost always found for chains crystallized in an extended (not helical) conformation. For orthorhombic polyethylene and a-poly(vinylidene fluoride), the chains are inclined to the basal lamellar surfaces to provide more volume for chain folding (Section 3.2.1). On the other hand, those aliphatic polyamides (nylons) and polyesters that crystallize with monoclinic... 

Before discussing the ROA band signatures and general spectral characteristics of the disordered types of structure found in unfolded proteins, it is helpful to review the ROA band signatures of cc-helix and /1-sheet together with those of loops, turns, and side chains, as shown by folded proteins containing significant amounts of extended secondary structure in order to demonstrate that ROA is able to discriminate adequately between ordered and disordered polypeptide sequences. Typical... 

FIGURE 11.42 Bloch diagram for (a) a staggered linear chain of [RH ] - ions where the unit cell includes two R ions (b) an eclipsed linear chain of [RHJ -ions having only one R per unit cell where the bands have been folded in half and then doubled back over. [Reproduced from Hoffmann, R. Solids and Surfaces A Chemist s View of Bonding in Extended Structures, Wiley-VCH New York, 1989. This material is reproduced with permission of John Wiley Sons, Inc.]... 

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