Evidence for retaining crosslinked liquid-crystalline order in hydroxypropyl cellulose solid films by polarized microscopy

Abstract

Liquid-crystalline solutions of hydroxypropyl cellulose (HPC) in water or alcohols were mixed with p-formaldehyde (crosslinking agent) and hydrochloric acid (HCl, catalyst). HPC solid films were cast from those solutions, which were sheared, and fixing of the liquid-crystalline order was tried by chemical crosslinking. The texture of those solutions in the course of casting was observed by polarized microscopy and a solubility test of the resultant HPC cast films in water and alcohol was performed. Those liquid-crystalline solutions with or without p-formaldehyde and HCl formed a band texture after cessation of shear. This formation of the band texture is a characteristic of liquid-crystalline solutions. The band texture of aqueous solution decayed rapidly with time and finally changed to a globular texture, whereas the band texture of alcoholic solutions did not decay. The band or globular texture remained in the solid cast films. The HPC solid films cast from liquid-crystalline solutions with p-formaldehyde and HCl, which exhibited the band or globular texture, were insoluble in both water and alcohol. Our findings reveal that the liquid-crystalline order is perpetuated in the HPC solid cast films by crosslinking.