Interplay of self-assembly and viscoelasticity in chiral liquid crystal gels

Abstract

Electro-optic and photonic devices that are robust against external stresses and deformation offer several technological advantages. A variety of device applications based on chiral liquid crystals (LCs) exhibiting blue phases (BPs), chiral nematic (N*), and ferroelectric Smectic (SmC*) phases have already been demonstrated. Chiral LC-gels are, therefore, among the best soft materials that can further extend device performance and functionality by combining electro-optic and photonic properties with mechanical stability. In view of this, a composite material composed of two highly chiral calamitic LCs and a low-molecular-weight organogelator was developed and investigated using a variety of experimental techniques. Optical microscopy, selective reflection, and x-ray diffraction studies show that the LC characteristics are very well retained in the chiral LC-gels, namely, BPIII-gel, BPII-gel, BPI-gel, N*-gel, twist grain boundary A phase (TGBA)-gel, and SmC*-gel, which form sequentially as the sample is gradually cooled from the isotropic phase. Rheological studies establish that the chiral LC-gels exhibit several features of a soft viscoelastic solid. This has been understood in terms of the mode of self-assembly of the gelator fibers and the ubiquitous defects pertinent to different LC phases. The major highlights of the study are the formation of a defect-mediated strong N*-gel with enhanced viscoelastic moduli and the induction of a TGBA-gel. The stretchable nature of a free-standing film of the BPI-gel that retains its shape over time is another interesting aspect of the present work.