Optical Properties of Certain Cholesteric Liquid-Crystal Films

Abstract

Optical scattering data have been used to investigate molecular order in certain cholesteric mixtures. When films of these materials are deposited on a reflecting substrate, their colors can be shifted by several thousand angstroms for any condition of illumination and observation by a mechanical disturbance. The dispersive color reflections in both the undisturbed and disturbed films can be attributed to Bragg-type crystallite sites embedded in a material with refractive index n. When the films are mounted on a reflective substrate, there are effectively two angles of incidence. One angle corresponds to the actual angle of incidence and the other angle corresponds to the reflected beam. Accordingly, for some fixed combination of incidence and observation angles, two wavelength peaks should be observed for an isotropic distribution of scattering sites. Scattering data in both reflection and transmission indicate that the distribution is far from isotropic and that the effect of disturbance is to reorient the sites such as to produce a large increase in the intensity of light scattered from the direct beam and a corresponding decrease in the intensity of visible light scattered from the reflected beam. There is evidence that the basic helical pitch parameter is not changed significantly by the mechanical disturbance.

Chiral Nematic Liquid Crystals: Chiral Nematics: Physical Properties and Applications

“…Reflection spectra as a function of viewing angle (6-6,) for a chiral nematic reflecting green light at normal incidence. The data were taken [89] for ei = 6, using an isoenergetic white light source with a black, nonreflecting back substrate.…”

“…20 b) will selectively reflect light onto this substrate and the light will be back-reflected back towards the observer, who without polarizers would observe specular colors. A number of geometries that display this effect in chiral nematics have been discussed [89] and lead to the conclusion (see p. 447 of [ 3 3 ] ) that in such textures the helix axis is predominantly, or on average, in the plane of the sample, as would be the case if the helix axis is parallel to the local normal of the Dupin cyclides. This provides a further useful example of the 'Bragg' reflection model in chiral nematic liquid crystals and allows us to differentiate between the two types of focal-conic texture.…”

Characterization of Molecular Role in Pitch Determination in Liquid Crystal Mixtures

“…(12) It is possible to measure pitch to around 1% accuracy using this technique. Because of the fundamental absorption edge of ABUTA, it is difficult to do transmission measurements with films with reflectance maxima below 4200 A, and, since the oleyl cholesteryl carbonate rich films fell into this regime, these were measured following a dispersive-reflection technique outlined by F e r g a s~n .…”

Evidence for Polymorphism within the So-called "Blue Phase" of Cholesteric Esters. IV. Temperature and Angular Dependence of Selective Reflection

“…The results have to be discussed in terms of the de Vries theory of eholesterics [8] : For an ideally oriented cholesteric structure selective reflection occurs under the condi-= AJI COS <p . (1) If a uniformly oriented structure is not achieved totally, SR will be observed too under the condition (Pi^Ts [9,10]. Based on a model of local scattering regions acting as Bragg sites (cf.…”