Cross-polarization aligned samples

Figure 1.3. A thin section of bulk crystallized nylon, in polarized light, reveals a bright, birefringent and spheru-litic texture. (A) At high magnification, a classic Maltese cross pattern is seen, with black crossed arms aligned in the position of the crossed polarizers the sample was isothermally crystallized, and exhibits large spherulites. (B) The sample quenched during crystallization yielding large spherulites surrounded by smaller ones.

A typical experimental apparatus for studying photorefractivity in liquid crystals is illustrated in Fig. 2. Two coherent laser beams from an Ar+ laser are crossed in the sample, with a total of 5 mW of p-polarized output at 514 nm. The beams are unfocused and have a 1/e diameter of 2.5 mm. The liquid crystal composite is sandwiched between two ITO coated glass slides that are coated with octadecyl-silyl groups to induce the liquid crystal director to align perpendicular to the face of the glass slides, that is, homeotropic alignment [43], The cell thickness is determined by a Teflon spacer that is 12 to 100 p,m thick. A small electric field... 

Figure 4.21 shows a focal-conic fan texture of a smectic A phase in which is observable some defects represented by Figure 4.20(b). While Sa forms the fan-like focal-conic textures, the focal-conics formed by the Sc phase are often broken and less distinct. In addition, as stated above, Sa is uniaxial but Sc is biaxial. Sa may take a homeotropic alignment with the axis normal to the sample plane and extinct on POM with crossed polars. The two smectic phases are therefore distinguishable with POM. Nevertheless, it is desirable to include WAXS studies in order to identify the two with assurance (Section 4.3). 

A simple measurement of the total photoionization cross section (isotropic sample, cross section measured at a specified photon energy, integrated over all photoejection angles, without specification of the internal state of the photoion, without determination of ms of the ejected electron) contains no information about the distribution of Z, m -values of the ejected electron. However, measurable properties of the photoionization event can provide information about the mechanism of the photoionization process. The frequently measured quantities included f3, Aq, and A. The ft quantity describes the angular distribution of the photoelectrons and is defined analogously to the (3 for photodissociation (see Section 7.2.4), Ajp is the alignment ( Mm distribution) of the photoion. A (not to be confused with the spin-orbit constant) or alternatively P, is the spin-polarization of the ejected electron, which is relevant when the photoion has nonzero spin. 

Classification of LC alignment into planar or homeotropic is an oversimplification. Under suitable conditions, the nematic director may make an angle with the substrate surface (Fig. 24). The observations between crossed polarizers of such samples are the... 

Knowledge of the electrooptic behavior of the FLCPs is of the utmost importance for display device applications. One relevant parameter in this respect is the response time. As for the spontaneous polarization, the determination of the response time requires a uniformly aligned sample. The test cell is placed between crossed polarizers so that one tilt direction is parallel to the direction of one polarizer. The electrooptic effect is achieved by applying an external electric field across the cell, which switches the side chains from one tilt direction to the other as the field is reversed. A photodiode measures the attenuation of a laser beam when the cell is switched between the two states. Generally, the electrooptical response time is defined as the time corresponding to a change in the light intensity from 10 to 90% when the polarity of the applied field is reversed ( 10-9o)-... 

The stmcture of the SmA phase does not change if the mesogens are chiral, but some of its properties do, e.g. the response to an applied electric field [21], Therefore, the chiral SmA phase is denoted as SmA. Between crossed polarizers, the SmA, the SmA as well as the lamellar L phase initially form so-called batonnets under planar anchoring conditions if emerging directly from the isotropic phase. These batonnets then condense into a focal conic fan-shaped textiue. If the director is oriented parallel to the viewing direction, the texture appears black between crossed polarizers, which is referred to as homeotropic . It is also possible to find oily streaks within the homeotropic texture, if the alignment of the sample is imperfect. 

Fig. 5.21 Texture images C50/NMF mixtures as seen through crossed polarizers of a the N phase with 2 wt% of NMF at 66 °C, b the phase transition from the homeotropically aligned L phase to the filament texture of the TGBL phase in a sample with 2 wt% of NMF at 65.5 °C, c the Loj phase with 13 wt% of NMF at 50 °C and d the phase with 2 wt% of NMF at 55 °C...

Fig. 9. An initially aligned sample of rigid rod microtubule biopolymers in solution after it has been hit by radiation. The sample is viewed between crossed polarizers. The burn mark is centrally placed. The alternating dark and light bands are domains that have a different orientation. The transition from uniform alignment to domains with different orientation takes place in seconds after the sample is irradiated. The burn mark has dimensions of roughly 0.5 x 3 mm. The shift of the domains with respect to each other is approximately 0.5 mm as witnessed by the distortions of the bum mark.

Retardation is measured more accurately with compensators, with sample and compensator set at 45° between crossed polars. This alignment is critical, as the superposition of birefringent objects at arbitrary orientations produces extremely complicated effects [70, 71]. The compensator is adjusted until the specimen appears dark. Then the retardation of sample and compensator are equal and opposite, R(net) = 0. If the polarization color change on inserting a compensator shows an increase in retardation, then R(net) > R(specimen), and the compensator is adding its effect to that of the specimen. The... 

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