Optical properties of cholesteric (2‐hydroxypropyl) cellulose (HPC) esters

Acetoxypropylcellulose (APC) and propionic, n‐butyric, isobutyric, valeric, isovaleric, hexanionic, and heptanionic acid esters of hydroxypropylcellulose (HPC) (respectively PPC, BPC, iBPC, VPC, iVPC, HexPc, and HepPc) were prepared and characterized by differential scanning calorimetry, chromatography, polarizing microscopy, chemical methods, and spectroscopy. All these esters form thermotropic cholesteric liquid crystalline phases. The glass (T_g) and clearing (T_c) transition temperatures were determined. The stability interval of the mesophases appears to be greater in the case of longer‐linear side chains. The mesophases of APC, PPC, iBPC, VPC, and iVPC exhibit reflection bands in the visible region, at wavelengths that depend on temperature, moisture content, size and number of substituents, and degree of polymerization (\[ \overline {{\rm DP}} \]). The pitch of the cholesteric helical structure increases with side chain length and with increasing temperature. At room temperature, a fingerprint‐like pattern can be observed for HepPc. No reversal in the sense of the pitch with temperature variations was observed. The pitch of BPC increases with moisture content and with decreasing values of the degree of esterification (\[ \overline {{\rm DE}} \]) and \[ \overline {{\rm DP}} \]. A theory for cholesteric mesophases composed of helical rod‐like species and a model of elastic bend chain have been compared to the experimentally observed changes in the pitch with temperature and with the length of the side chain substituents (for iBPC, iVPC, BPC, PPC, and APC) and \[ \overline {{\rm DE}} \] and \[ \overline {{\rm DP}} \] (for BPC). © 1994 John Wiley & Sons, Inc.