Birefringence dynamic scattering

Light valves were first produced on the basis of the classical semiconductors, ZnS, CdS, ZnSe, CdTe, and GaAs, in contact with nematic or chiral nematic liquid crystal [18]. The basic effects in liquid crystals included electrically controlled birefringence, dynamic scattering, and the cholesteric-nematic phase transition with the frequency response limited to a few Hertz. 

The rotational diffusion coefficient Dr of a rodlike polymer in isotropic solutions can be measured by electric, flow, and magnetic birefringence, dynamic light scattering, and dielectric dispersion. However, if the polymer has some flexibility, its internal motion makes it difficult to extract Dr for the end-over-end rotation of the chain from data of these measurements. In other words, Dr can be measured only for nearly rodlike polymers. 

Sensitized for blue-green or red light, photoconductive polyimides and liquid crystal mixtures of cyanobiphenyls and azoxybenzene have been used in spatial light modulators [255-261]. Modulation procedure was achieved by means of the electrically controlled birefringence, optical activity, cholesteric-nematic phase transition, dynamic scattering and light scattering in polymer-dispersed liquid crystals. 

The different methods - potentiometry conductometry turbidimetry vis-cometry " calorimetry , kinetira, sedimentation , dynamic flow birefringence, light scattering , high resolution H-NMR spectrometry , chromatography, spectroscopy , electron microscopy , and others have been used for the investigation of the formation and composition of polymer-polymer complex. ... 

This is too fast for nematics operated in dynamic scattering, but it is usable in electrically controlled birefringence (ECB). In ECB the nematic can respond to short duration pulses if the repetition frequency is high enough. The liquid crystal can summate a series of small excitations. If r is the pulse duration and T the repetition period, the optical retardation does not depend on r or T if these quantities are larger than the molecule relaxation time. The only important parameter is r/T. 

Liquid-crystal electro-optic phenomena can be divided into two categories—those caused only by dielectric forces and those induced by the combination of dielectric and conduction forces. The two conduction-induced phenomena discussed later are dynamic scattering and the storage effect. Four of the dielectric phenomena, or field effects as they are sometimes known, are discussed first (1) induced birefringence, (2) twisted nematic effect, (3) guest-host interaction, and (4) cholesteric-nematic transition. 

Experimental observations for induced birefringence, " 2 twisted nematic,and dynamic scattering 78,84 reasonable agreement... 

Although the effectiveness of the dynamic methods using a small-amplitude oscillatory strain to the rheo-optical studies of polymeric materials was demonstrated successfully from early days for birefringence, " light scattering, " and X-ray diffraction, relatively little had been reported on the use of IR probe, " " " until the introduction of DIRLD spectroscopy. " Because of the specificity of IR to certain submolecular constituents of the system, such a combination provides useful and direct information on the molecular origin of localized time-dependent deformation and relaxation behavior of materials. 

DIRLD spectroscopy belongs to the characterization techniques known as Vheo-optics. Rheo-optical analysis of materials utilizes electromagnetic probes in conjunction with application of macroscopic mechanical perturbations. It investigates primarily the microscopic or molecular responses of materials under deformation, flow, and relaxation. In dynamic rheo-optical methods, such as dynamic birefringence, light scattering, and X-ray diffraction, relationships... 

Figure 4.1. Time scales for rotational motions of long DNAs that contribute to the relaxation of the optical anisotropy r(t). Experimental methods used to study these motions in different time ranges are also indicated along with the authors and dates of some early work in each case. FPA, Fluorescence polarization anisotropy (Refs. 15, 18-20, and 87) TPD, transient photodichroism (Refs. 28 and 62) TEB, transient electric birefringence (Refs. 26 and 27) DDLS, depolarized dynamic light scattering (Ref. 116) TED, transient electric dichroism (Refs. 25, 115, and 130) Microscopy, time-resolved fluorescent microscopy (Ref. 176).

Dynamic Light Scattering and Transient Electric Birefringence Study of Poly(vinyI chloride) Micrc els... 

Analysis of polyelectrolytes in the semi-dilute regime is even more complicated as a result of inter-molecular interactions. It has been established, via dynamic light-scattering and time-dependent electric birefringence measurements, that the behavior of polyelectrolytes is qualitatively different in dilute and semi-dilute regimes. The qualitative behavior of osmotic pressure has been described by a power-law relationship, but no theory approaching quantitative description is available. 

Data of Dr were obtained for low molecular weight poly (y-benzyl-L-gluta-mate) (PBLG) samples in isotropic solutions by two methods. Mori et al. [129] used dynamic electric birefringence, while Zero and Pecora [130] and Kubota and Chu [131] applied depolarized dynamic light scattering. Figure 18 shows... 

Fig. 27a, b. Comparison of Eq. (50) with the Dr data for isotropic solutions of relatively short PBLG samples in the plot of Dr/Dr0 vs X a data from dynamic electric birefringence by Mori et al. [129] (the same data as shown in Fig. 18) b data from dynamic light scattering by Kubota and Chu [131] and Zero and Pecora [130], The solid curve, theoretical values predicted by Eq. (50)... 

Three of the experiments are completely new, and all make use of optical measurements. One involves a temperature study of the birefringence in a liquid crystal to determine the evolution of nematic order as one approaches the transition to an isotropic phase. The second uses dynamic laser light scattering from an aqueous dispersion of polystyrene spheres to determine the autocorrelation function that characterizes the size of these particles. The third is a study of the absorption and fluorescence spectra of CdSe nanocrystals (quantum dots) and involves modeling of these in terms of simple quantum mechanical concepts. 

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