Alignment, Optical Properties and Defect Formation of DSCG and SSY

Abstract

This thesis demonstrates the ability to achieve stable and uniform alignment in lyotropic chromonic liquid crystals (LCLCs) using surface-relief grating alignment, enabling detailed investigation of their optical properties and addressing a challenge that has limited their practical application compared to traditional thermotropic liquid crystals (TLCs). Achieving this excellent alignment allowed systematic investigation of the phase behaviour, optical properties, alignment mechanisms, and defect dynamics of two chromonic materials: Disodium Cromoglycate (DSCG) and Sunset Yellow (SSY). The materials were systematically characterized across different concentrations to produce phase diagrams, with further optical and physical property studies conducted on DSCG at 12 wt% and SSY at 32 wt% in the nematic phase using cell thicknesses of 12 μm. Small-angle X ray scattering revealed different molecular organizations: DSCG exhibited inter-columnar spacing of 43 Å with structural coherence (correlation length 64-79 Å), while SSY showed smaller inter-columnar spacing (25 Å) with reduced structural order (correlation length 33-43 Å). Both materials maintained consistent π-π stacking distances of 3.3 Å. Three surface treatments were systematically compared: untreated substrates, scratch alignment, and surface-relief grating alignment. Optical characterization demonstrated that surface-relief grating alignment enhances birefringence uniformity and provides stronger anchoring energies (7.38×10⁻⁵ J/m² for DSCG vs 2.10×10⁻⁵ J/m² for SSY) compared to traditional alignment methods. Surprisingly, despite DSCG's stronger surface anchoring energy (7.38×10⁻⁵ J/m²) and longer structural correlation length (64-79 Å), SSY achieved faster alignment times (15 minutes compared to 60 minutes for DSCG). 8 Tactoid formation during isotropic-nematic phase transitions revealed fundamental differences between the materials. DSCG exhibited unique "negative tactoids" (isotropic droplets embedded in nematic matrix) creating twist-induced birefringence, while SSY demonstrated conventional tactoid behaviour with morphological diversity (aspect ratios 1.05-1.65).