Cholesteric liquid crystals with a helical pitch gradient: Spatial distribution of the concentration of chiral groups by Raman mapping in relation with the optical response and the microstructure

M. Belalia , Michel Mitov (1, 2) , C. Bourgerette , A. Krallafa (3) , M. Belhakem (4) , D. Bormann (5)

1 CEMES - Centre d'élaboration de matériaux et d'études structurales
2 CEMES-NeO - Nano-Optique et Nanomatériaux pour l'optique
3 USTO MB - Université des sciences et de la Technologie d'Oran Mohamed Boudiaf [Oran]
4 SEA2M - Laboratoire de Structure et Application des Matériaux Moléculaires
5 LPCIA - Laboratoire de physico-chimie des interfaces et applications

Cholesteric liquid crystals (CLC) selectively reflect light when the helical pitch is of the order of the wavelength of the incident beam propagating along the helix axis. The wavelength bandwidth, related to the optical anisotropy, is typically limited to a few tens of nanometers in the visible part of the spectrum, which is insufficient for applications such as white-or-black polarizer-free reflective displays and smart windows for the control of the solar light and heat. A way to make cholesteric films reflecting in a broad wavelength band consists in associating various cholesteric pitches in the same film. In this work, it is shown how a study by confocal micro Raman spectrometry mapping makes it possible to have access to information accounting for the local organization of CLCs in the case of graded pitch materials. These investigations will be correlated to the optical response and the transverse microstructure of the CLC material as investigated by transmission electron microscopy. An accurate analysis of the vibrational behavior evolution of the CO can be correlated to the evolution of the populations of the chiral and achiral groups in the case of the interdiffusion of two CLC substances with various stoechiometries. Besides an easy measurement of the Raman spectrum gives the opportunity to quantify the relative ratio of the mesogenic species and thus to go up by a simple way to the pitch of the helical structure.

Domaines: Optique [physics.optics]; Matière Molle [cond-mat.soft]

M. Belalia, Michel Mitov, C. Bourgerette, A. Krallafa, M. Belhakem, et al.. Cholesteric liquid crystals with a helical pitch gradient: Spatial distribution of the concentration of chiral groups by Raman mapping in relation with the optical response and the microstructure. Physical Review E : Statistical, Nonlinear, and Soft Matter Physics, 2006, 37 (5), ⟨10.1103/PhysRevE.74.051704⟩. ⟨hal-02887360⟩