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Two dimensional correlation spectroscopy applications in vibratioal and optical spectroscopy - Isao N.

Isao N. Two dimensional correlation spectroscopy applications in vibratioal and optical spectroscopy - Wiley publishing , 2004. - 312 p.
ISBN 0-471-62391-1
Download (direct link): twodimensionalcorrela2004.pdf
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2D IR correlation spectra based on the spectral changes induced by the static compression of LLDPE at 60 °C are shown (Figure 10.8). The negative synchronous cross peaks (Figure 10.8(A)) between two crystalline bands at 1473 and 1463 cm-1 indicate that the intensity of a-axis band is increasing while that of the b-axis band is decreasing. The increase in the intensity of the lower side of the amorphous band below 1455 cm-1 is also positively correlated with the increasing a-axis band at 1473 cm-1. The asynchronous cross peaks (Figure 10.8(B)) indicate that the compression-induced melting of crystallites occurs after the flattening of the semicrystalline superstructure via the uncoiling of the crystal lamellae.
Figure 10.9 schematically summarizes the proposed mechanism of deformation based on the 2D IR correlation analysis of compression-induced changes in the IR spectra of LLDPE. Initially, the LLDPE sample consists of a complex superstructure of crystalline lamellae oriented in many different directions embedded within an amorphous matrix. Upon compression, the overall crystalline
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Generalized 2D Correlation Studies of Polymers and Liquid Crystals
Figure 10.8 2RIR correlation spectra based on the spectral changes induced by the static compression of LLDPE at 60 °C: (A) synchronous spectrum and (B) asynchronous spectrum. (Reprinted from Vibrational Spectroscopy, 19, I. Noda, G. M. Story, and C. Marcott, p. 461, Copyright (1999), with permission from Elsevier.)
Figure 10.9 A possible deformation mechanism of a semicrystalline LLDPE film under compression. (Reprinted from Vibrational Spectroscopy, 19, I. Noda, G. M. Story, and C. Marcott, p. 461, Copyright (1999), with permission from Elsevier.)
Reorientation of Nematic Liquid Crystals by an Electric Field
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superstructure flattens to accommodate the dimensional change. This flattening is achieved mainly by the local rotation of crystalline lamellae around the growth direction (b-axis). Such rotation of lamellae results in the preferred reorientation of the crystalline a-axis in the direction normal to the compression. However, the reorientation of lamellae b-axis in the direction normal to the compression was not observed. Further compression of the sample results in the disintegration of crystal lamellae and production of more disordered amorphous components.
We thus demonstrated that 2D IR spectroscopy applied to a LLDPE sample under temperature change and compression reveals surprising details of the complex transition mechanisms of this polymer system.
10.2 REORIENTATION OF NEMATIC LIQUID CRYSTALS BY AN ELECTRIC FIELD
The study of time-dependent reorientation processes of a liquid crystalline system under external perturbations, such as an electric field, is of great industrial importance in many optical display applications. The applied external field usually varies with time in a nonsinusoidal manner, most often as a step function corresponding to the switch-on or switch-off process. The subsequent dynamic reorientation responses of liquid crystals are also nonsinusoidal and often exhibit highly nonlinear waveforms with respect to time. Generalized 2D correlation analysis will be ideally suited to the spectroscopic study of such complex transient systems.
We choose here the electric field-induced dynamic reorientation of a simple nematic liquid crystal system of 4-pentyl-4'-cyanobiphenyl (also known as 5CB) as a model. 5CB consists of a rigid cyanobiphenyl head group with a flexible pentyl aliphatic tail (Figure 10.10). The large dipole moment associated with the cyano group makes it possible for initially randomly oriented 5CB to align in a direction under a fixed electric field. Upon the removal of the electric field, the system eventually reverts to the randomly oriented state under the influence of thermal fluctuations of molecules. The process is schematically illustrated in Fig. 10.11, where 5CB molecules are depicted as ‘tadpoles’ with heads (cyanobiphenyls) and tails (pentyls) responding to the presence or absence of the external electric field. Similar 5CB liquid crystalline systems, typically under the influence of a sinusoidally alternating electric field, have been studied by several research groups using a step-scan FT IR spectrometer.13 39
4-pentyl-4'-cyanobiphenyl (5CB)
Figure 10.10 Molecular structure of 4-pentyl-4'-cyanobiphenyl (also known as 5CB)
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Generalized 2D Correlation Studies of Polymers and Liquid Crystals
Figure 10.11 Schematic representation of the electric field induced orientation and relaxation of nematic liquid crystalline 5CB. 5CB is depicted as a molecular tadpole with the cyanobiphenyl head group and a flexible pentyl tail
By using a polarized IR beam, it is possible to monitor the dynamic reorientation processes of 5CB under an electric field at the submolecular level. Because of the complex interactions between constituents, the field-induced molecular reorientation of 5CB does not happen instantaneously, and the observed reorientation responses can often be highly nonlinear with respect to time. Fortunately, generalized 2D correlation analysis can effectively sort out the complex evolutionary process of 5CB reorientation to extract the essential information about the field-induced reorganization of the liquid crystalline system. Figure 10.12 shows the synchronous 2D IR correlation spectrum of 5CB during the orientation process right after the application of an electric field. We observe the development of sharp synchronous cross peaks among bands associated with the cyano groups and biphenyls. This result indicates that they are reorienting together at the same rate under the application of the electric field.
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