Thermomechanical Analysis of Cholesteric Liquid Crystal Mesophases from Chitosan Upon Solvent Evaporation

Chitin, derived predominantly from crustacean shells and fungi, is the second most abundant natural polysaccharide worldwide. Chitosan, the deacetylated product of chitin, has potential in many environmentally-friendly applications due to its biocompatibility and antimicrobial properties. This polysaccharide also displays liquid crystalline behavior in solution, where strong hydrogen bonding can induce polymer chain self-assembly into cholesteric mesophases, in turn resulting in unique optical properties such as circular dichroism and selective reflectivity. The cholesteric pitch is directly related to the wavelength of maximum reflectivity of the ordered structure, which, for chitosan-based mesophases, corresponds to the infrared portion of the electromagnetic spectrum. Tailoring optical properties through control of the cholesteric phase behavior offers great potential for functional chitosan-based materials in a broad range of applications, from optical sensing to wearable devices. Here, we investigate how chitosan chain structure, such as degree of deacetylation (DDA) and molecular weight, and polysaccharide-solvent interaction impact cholesteric order formation and retention upon solvent evaporation. Our past work has studied gel-like cholesteric mesophases of chitosan and established the dependence of the critical concentration (C*, i.e. the onset of LC self-assembly) and the pitch-concentration relationship on polysaccharide chain structure. To further our understanding of this liquid crystalline system, we discuss the impact of solvent evaporation on the cholesteric LC order and its pitch. We examine the effects of chain structure and concentration on cholesteric order parameters and thermomechanical properties. We characterize how solvent evaporation conditions and kinetics affect the cholesteric pitch as measured by polarized light microscopy and qualitative observation by scanning electron microscopy. Subsequently we investigate the thermomechanical behavior of the dried films through differential scanning calorimetry and dynamic mechanical analysis to understand how decreased chain mobility in the solid state affects elastic and viscous moduli, along with thermal transitions such as glass transition temperature. This work provides foundational knowledge for the development of solid-state optical materials based on chitosan, paving the way for their integration in sensors and stimuli-responsive film applications.