Immobilization of the Cholesteric Structure Formed by Cellulose Phenylcarbamates and CaCO₃ Mineralization in the Liquid-Crystalline Composite Films

Abstract

Liquid-crystalline behavior of phenylcarbamate derivatives of cellulose in vinyl monomer solvents was investigated, and then calcium carbonate (CaCO₃) mineralization was conducted in colored films wherein a cholesteric structure of the cellulosic derivatives was immobilized by polymerization and cross-linking of the solvent monomer(s). Two sorts of derivatives, cellulose phenylcarbamate (CPC) and cellulose 3-chlorophenylcarbamate (3Cl-CPC), were used. Although both derivatives were hardly soluble in anionic monomers such as acrylic acid (AA) at high concentrations (>30 wt%), CPC formed a left-handed cholesteric mesophase in a mixture of N-vinyl pyrrolidone (VP)/ AA (10:0–9:1 in wt.) and 3Cl-CPC assumed a right-handed cholesteric arrangement in VP/AA/dimethyl sulfoxide (DMSO) (5:0:5, 3:2:5, or 2:3:5 in wt.); each mesomorphic solution imparted a vivid cholesteric color. By UV-induced polymerization and subsequent washing processes for the lyotropics, the original cholesteric architectures were successfully fixed into the resulting films of CPC/poly(N-vinyl pyrrolidone) (PVP), CPC/poly(N-vinyl pyrrolidone- co-acrylic acid) (P(VP-co-AA)), 3Cl-CPC/PVP, and 3Cl-CPC/P(VP-co-AA). After soaking the liquid-crystalline films in an aqueous salt solution containing Ca²⁺ and HCO₃⁻, we obtained calcic mineral-hybridized polymer composites. Wide-angle X-ray diffractometry (WAXD) and energy-dispersive X-ray (EDX) spectroscopy revealed that CaCO₃ crystals (in calcite form) were deposited inside the films when the anionic AA was present as a matrix component. In thermogravimetric analysis (TGA), it was observed that the flame resistance of the mineralized films was improved in the high-temperature range (>300°C), although the decomposition temperature tended to fall.