Polarizers, infrared

Q are the absorbance and wavenumber, respectively, at the peak (center) of the band, p is the wavenumber, and y is the half width of the band at half height. Liquid band positions ate usually shifted slightly downward from vapor positions. Both band positions and widths of solute spectra are affected by solute—solvent interactions. Spectra of soHd-phase samples are similar to those of Hquids, but intermolecular interactions in soHds can be nonisotropic. In spectra of crystalline samples, vibrational bands tend to be sharper and may spHt in two, and new bands may also appear. If polarized infrared radiation is used, both crystalline samples and stressed amorphous samples (such as a stretched polymer film) show directional effects (28,29). 

A powerful characteristic of RAIR spectroscopy is that the technique can be used to determine the orientation of surface species. The reason for this is as follows. When parallel polarized infrared radiation is specularly reflected off of a substrate at a large angle of incidence, the incident and reflected waves combine to form a standing wave that has its electric field vector (E) perpendicular to the substrate surface. Since the intensity of an infrared absorption band is proportional to / ( M), where M is the transition moment , it can be seen that the intensity of a band is maximum when E and M are parallel (i.e., both perpendicular to the surface). / is a minimum when M is parallel to the surface (as stated above, E is always perpendicular to the surface in RAIR spectroscopy). 

Novotny et al. [41] used p-polarized reflection and modulated polarization infrared spectroscopy to examine the conformation of 1 -1,000 nm thick liquid polyperfluoropropy-lene oxide (PPFPO) on various solid surfaces, such as gold, silver, and silica surfaces. They found that the peak frequencies and relative intensities in the vibration spectra from thin polymer films were different from those from the bulk, suggesting that the molecular arrangement in the polymer hlms deviated from the bulk conformation. A two-layer model has been proposed where the hlms are composed of interfacial and bulk layers. The interfacial layer, with a thickness of 1-2 monolayers, has the molecular chains preferentially extended along the surface while the second layer above exhibits a normal bulk polymer conformation. 

Figure 5 Polarized infrared spectra (s on top, p on bottom) recorded for an oriented 80/20 PS/PPO miscible blend using normal incidence transmission. Reproduced with permission from Lefebvre et al. [23]. Copyright Elsevier 1981.

In the normal-incident transmission measurements of LB films deposited on transparent substrates, the electric vector of the infrared beam is parallel to the film surface (Figure 5A). Therefore, only absorption bands which have the transition moments parallel to the film surface can be detected by this method. On the other hand, in the above-mentioned RA measurements, in which the p-polarized infrared beam is incident upon the LB film prepared on Ag-evaporated substrates at a large angle of incidence, we have a strong electric field perpendicular to the film surface as shown in Figure 5B. Therefore, in this case, only absorption bands which have the transition moments perpendicular to the film surface can be detected with a large intensity enhancement. Thus, if the molecules are highly oriented in the LB films, the peak intensities of particular bands should be different between the transmission and RA spectra. 

The polarized infrared spectra of crystalline q3-C3HsFe(CO)2(NO) and n3C3H5Co(CO)3... 

The differently produced conductive polymer structures described above all have enhanced conductivity, which can be employed in microelectronics [44] and as sensors using immobilized enzymes [46, 47[. Martin and coworkers used polarized infrared absorption spectroscopy to access the alignment of the polymer fibers on the outer surface of the nanotubes [48[. The study showed that the enhancement of the conductivity is due to the alignment of the polymer fibers on the outer surface of the tubes. 

Sakamoto A, Nakamura O, Tasumi M (2008) Picosecond time-resolved polarized infrared spectroscopic study of photoexcited states and their dynamics in oriented poly(p-phenylene-vinylene). J Phys Chem B 112 16437... 

A STUDY WITH PLANE-POLARIZED INFRARED RADIATION OF SOME CRYSTALLINE MODIFICATIONS OF CELLULOSE ... 

Mechanical deformation induces orientation into the polymer samples and polarized infrared can be used to characterize this orientation either by direct measurement of the dichroic ratio 287,289) or by spectral subtraction 286), three dimensional sample tilting 68,286), or internal reflection spectroscopy 130). 

Evidence that the proton lies midway between the fluorine atoms in the crystal KHF has been provided by entropy measurements,28 study of the polarized infrared spectrum,29 neutron diffraction,80 and nuclear spin magnetic resonance.81 The uncertainty in the location of the proton at the midpoint between the fluorine atoms is reported to be 0.10 A for the neutron diffraction study and 0.06 A for the nuclear magnetic resonance study. 

What is the nature of the insoluble forms of the prion protein They are hard to study because of the extreme insolubility, but the conversion of a helix to (3 sheet seems to be fundamental to the process and has been confirmed for the yeast prion by X-ray diffraction.11 It has been known since the 1950s that many soluble a-helix-rich proteins can be transformed easily into a fibrillar form in which the polypeptide chains are thought to form a P sheet. The chains are probably folded into hairpin loops that form an antiparallel P sheet (see Fig. 2-ll).ii-11 For example, by heating at pH 2 insulin can be converted to fibrils, whose polarized infrared spectrum (Fig. 23-3A) indicates a cross-P structure with strands lying perpendicular to the fibril axis >mm Many other proteins are also able to undergo similar transformation. Most biophysical evidence is consistent with the cross-P structure for the fibrils, which typically have diameters of 7-12 rnn."-11 These may be formed by association of thinner 2 to 5 nm fibrils.00 However, P-helical structures have been proposed for some amyloid fibrils 3 and polyproline II helices for others. 1 11... 

Spectra. Infrared, Raman, and NMR spectra of PTHF have been reported. Imada et al. (84) have reported the Raman and polarized infrared spectra (Fig. 33). They made a tentative assignment of the bands. 

Fig. 33. (a) Raman spectrum and (b) polarized infrared spectra of PTHF. The solid (or broken) line in (b) represents the spectrum measured with the electric vector perpendicular (or parallel) to the orientation... 

The study of oriented polymers with polarized infrared radiation is an equally important tool in the detailed analysis of the vibrational spectrum, since it permits us, within certain restrictions, to determine the orientation (with respect to the molecular structure) of the transition moment for a given normal mode. This makes it possible, as we shall see, not only to classify bands in the spectrum but to establish their origin. Although polarizers are available with commercial spectrometers, their use has not yet become as general as would be desirable. Some comments... 

We noted earlier (Section I. 1.) that the intensity of an absorption band is proportional to the square of the changing dipole moment in the molecule (i. e., transition moment) during the corresponding normal vibration. The intensity also depends upon the direction that the electric vector in the incident radiation makes with the transition moment. In particular, the intensity is proportional to the square of the scalar product of the transition moment and electric field vectors. This implies, for example, that if the electric field vector is perpendicular to the transition moment vector no absorption will occur. This fundamental relationship is the basis for the utilization of polarized infrared radiation as a powerful tool in the study of the spectra and structure of oriented polymers. We consider below some aspects of this technique. 

A polarized infrared spectrum of a stretched sample of polyethylene is shown in Fig. 4 [Krimm, Liang, and Sutherland (102)]. This specimen was of moderate crystallinity and had a fairly... 

Polarized infrared spectra of oriented specimens of isotactic polypropylene are shown in Fig. 14 [Peraldo (167)]. The band positions, polarizations, and relative intensities are given in Table 14, together with Raman data [Tobin 231)] and information on the phase of origin of the bands [Natta [149) Pokrovsky and Volken-shtein 178) Abe and Yanagisawa (/)]. Some preliminary data on partially deuterated polypropylenes are available [Peraldo 167) Natta 130)], but no detailed spectra have as yet been published. 

Miyachi K, Matsushima J, Ishikawa K, Takezoe H, Fukuda A (1995) Spontaneous polarization parallel to the tilt plane in the antiferroelectric chiral smectic-CA phase of liquid-crystals as observed by polarized infrared-spectroscopy. Phys Rev E 52 R2153-R2156... 

Other optical and spectroscopic techniques are also important, particularly with regard to this segmental orientation. Some examples are fluorescence polarization, deuterium nuclear magnetic resonance (NMR), and polarized infrared spectroscopy.92,256,304... 

Figure 9 shows the change of the polarized infrared spectra by adsorption of diethyl ketone. Several bands appear by crystallization especially at 858 cm-. The bands due to diethyl ketone itself are not observed. The essential features of the x-ray diagrams and infrared spectra are the same irrespective of the kinds of solvents. From these results it may be considered that formation of a complex of syndiotactic PMMA and solvent molecules is not reasonable, and the adsorbed molecules contribute to the stabilization of crystallites of syndiotactic PMMA. 

The refined structure had residuals R=0.244, R"=0.274. The amide groups are approximately perpendicular to the chain axis and form hydrogen bonds N-H...0=C, as indicated by the polarized infrared spectrum. The 06-H group is close to the gt conformation but is not hydrogen bonded and has a short 06 05 contact. Elimination of the short contact with a non-bonded constraint increases the residuals to R=0.250, R"=0.288, an insignificant change. However the 06-H groups remain unbonded, contrary to infrared indications. 

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