Hierarchical Organization of Single Crystal Polymorphs of Azobenzene Chromophore in Anisotropic Media Subjected to Local Thermal Gradients upon Cold Spraying

Abstract

The present article entails the emergence of diverse crystal polymorphs following thermal quenching into various coexistence regions of binary azobenzene chromophore (ACh)/diacrylate (DA) solution and of azobenzene/nematic liquid crystal (E7) mixture. Development of various crystal topologies encompassing rhomboidal and hexagonal shapes can be witnessed in a manner dependent on thermal quenched depths into the crystal + liquid coexistence region of ACh/DA system. Upon spraying with compressed carbon dioxide (CO₂ ) fluid, the local temperature gradient is generated resulting in spherulitic morphology showing discrete lamellae undergoing twisting locally in some regions and branched dendrites or seaweeds in another. When ACh/E7 blend is sprayed using compressed CO₂ fluid, hierarchical organization of various discrete faceted single crystals including needle, rectangular, rhombus, and truncated hexagonal crystals radiating from the spherulite core can be discerned in a brighter region (off cross-polarization) polarized optical microscopy (POM) and nematic disclination in a darker cross-polarized region. Of particular interest is that the observed faceted single-crystal polymorphs in ACh/E7 may be contrasted to the lamellar twisting and branching observed in the ACh/DA system and plausible mechanisms of polymer spherulitic growth are discussed.

Keywords: azobenzene chromophores; crystal polymorphs; local thermal gradients; nematic liquid crystals; spraying compressed CO2 fluids.