Mesophases density measurements

Density and specific volume of mesophases are measured under atmospheric pressure with pycnometers [17-22], dilatometers [16, 23-25] (e.g., differential scanning dilatometers, DSD [26-31]), with a buoyancy method [32], and with a vibrating device, known as a digital precision density meter system, or densitometer (Anton Paar K.G.) [33-38], Devices for high-pressure measurements of specific volumes have been described by Kuss [39,40] and Ddrrer et al. [41], but were also mentioned earlier [42], More recently, a device for simultaneous... 

A giant optical nonlinearity of self-focusing type in the oriented mesophase of nematic liquid crystals (NLC) due to the director reorientation under the action of a light wave field is predicted. Self-focusing of He-Ne laser radiation with power lO" W and power density - 50 W/cm in a planar oriented 60 jum thick NLC layer has been carried out experimentally. The measured value of the nonlinearity effective constant 2 0.07 cm /erg corresponds to theoretical predictions, and turns out to be larger than the CS2 nonlinearity by sl0 times. 

NMR can, in principle, provide complementary information on motional processes in liquid crystals. The dipole-dipole interaction between a C-H pair and the quadrupolar interaction when the proton is replaced with a deuteron share the same principal interaction axis. In the case where the carbon is not directly bonded to a proton, there is still dipole-dipole relaxation by nearby protons, but it is also necessary to include an additional relaxation mechanism, the modulation of the chemical shift anisotropy. Proton spin decoupling is necessary to give well-resolved chemically shifted lines in the mesophase of liquid crystals. Furthermore, it is not practical to determine individual spectral density parameters from measured relaxation rates. Proton-proton dipolar interactions may not be ignored even when observation is exclusively confined to the resonant spin [5.31]. This is because proton relaxation causes population flow among the proton spin levels through dipolar (or scalar) coupling. As a consequence, cross-... 

The publication of density and specific volume studies on mesogens started soon after the discovery of liquid crystalline phases. In 1898/99 and 1905 Schenck [1, 2] reported specific volume data for PAA and p-meth-oxycinnamic acid. Until 1958/1960, with the publication of the Maier-Saupe theory [3], specific volume and density data on mesophases were the topics of only a few publications, and were always combined with results of other measurements [4—15]. A brief historical review of this aspect of liquid crystal research has been given by Bahadur [16],... 

Neutrons, like X-rays, are radiation that can be used to determine the structure of condensed matter such as liquid crystals. In principle they could be used to determine the period and range of any density waves, and hence perform the same type of structural determination as is usually made using X-rays as a probe. X-ray diffraction measurements have been used extensively to identify mesophases and investigate their translational order, and they were reviewed in the previous section. This type of investigation is rarely undertaken with neutrons because X-rays are much more appropriate for many reasons ... 

Overall molecular reorientations and internal motions take place in the 10 10 s time window and information about them can be accessed using relaxation rate measurements. By far the best approach is to use NMR experiments where deuterium Zeeman and quadrupolar spin-lattice relaxation times are measured on selectively deuteriated mesogens or on deuteriated probes dissolved in the mesophase. Frequency, orientation and temperature dependence of the spectral densities obtained in these relaxation studies give indications about the various motional modes and are extremely useful for testing orientational diffusion and chain dynamics models. Different deuterium relaxation experiments can be employed to extend the observable dynamic range the quadrupolar echo sequence gives spin-spin relaxation times sensitive to motions in the range s, while extremely slow motions... 

The expoimental methods of measuring transition temperatures, wh her for crystal to mesophase, mesophase to mesophase, or mesophase to isotropic liquid, fall into four categories. A high pressure cell with optical detection is a common method [1,2]. High pressure diff ential thermal analysis (DTA) systems have been built [3]. In some cases DTA has been combined with optical cells for visual as well as thermal detection [4]. A variation of thermal detection of phase transitions is the high pressure differential scanning calorimetry system [5]. The technique that provides pVT data is based on measuring volume or density as a function of pressure for various isotherms [6-10]. A unique method is based on the thermobarometer developed by Busine [11]. 

The experimental-theoretical study of mesophase formation in amphiphilic systems emphasizes the basic chemical, physical, and materials science aspects of the systems. The most commonly discussed mesophases, beyond the simple micelles discussed in Chapter 4, are lamellar aggregated micellar (packed in various cubic and hexagonal close-packed arrays), columnar or ribbon phases (rod-shaped micelles stacked in a two-dimensional hexagonal or rectangular array) microemulsions, and the cubic bicontinuous mesophases. The experimental techniques normally used to identify these mesophases are NMR Uneshape analysis, diffusion measurements, smaU-angle neutron and X-ray scattering, and optical texture analyses. In addition, reconstraction of electron density profiles and very low temperature transmission electron microscopy (TEM) have been used to elucidate the details of these mesostractures. 

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