Liquid-crystalline characteristics of cellulose derivatives: Binary and ternary mixtures of ethyl cellulose, hydroxypropyl cellulose, and acrylic acid

Abstract

Phase behavior in binary and ternary mixtures of ethyl cellulose (EC), hydroxypropyl cellulose (HPC), and acrylic acid (AA) was investigated by optical techniques including polarized-light microscopy (POM), refractometry, UV absorption, and circular dichroism (CD) spectrophotometries, and laser-light scattering. Both derivatives of cellulose formed a cholesteric liquid-crystal phase in concentrated solutions in AA. EC and HPC homopolymer solutions in AA showed iridescent colors in the concentration range of 42–50 wt% and 65–80 wt%, respectively. In the 30–50 wt% HPC/AA solutions, an optical image with fingerprint-like patterns was observed between crossed polars. CD revealed that the cholesteric helical sense was right-handed in the HPC/AA mesophase, while the EC/AA mesophase formed left-handed structures. Anisotropic samples of HPC/AA and EC/AA also showed a POM image with a grid-like texture, which was much coarser in the former system compared with that in the latter. A phase diagram constructed for the ternary system EC/HPC/AA was divided into five distinct regions: (1) isotropic (I) monophase, (2) I + HPC mesophase (HLC), (3) I + HLC + EC mesophase (ELC), (4) HLC + ELC, and (5) anisotropic monophase, with the optical appearance changeable depending on polymer composition and concentration. In region 5 the anisotropic system was apparently monophasic in POM observations; however, CD and UV results indicated that the biphasic separation into the two cholesteric structures still occurred. Optical anisotropy and phase behavior were also examined for the EC/ HPC binary blends prepared from solutions and melt.