Cholesteric phases and films from cellulose derivatives

Abstract

Opalescent films, which own their color to Bragg reflection of light, can be prepared from cholesteric cellulose derivatives. Both lyotropic and thermotropic systems can be realised. Here we describe the manipulation of the wavelength of selective reflection by (a) mechanical compression, (b) photoisomerization of pendant groups and (c) the water uptake of a cholesteric system. The lyotropic cholesteric phases of cellulose 3-chlorphenyl urethane and a mixture of acrylate-monomers can be transformed into a semi-interpenetrating network by photo curing. The composite is elastic and shows selective reflection. By compression of a thick film (1 mm), the maximum of the selective reflection shifts reversible to shorter wavelength. Cellulose urethanes with azo dyes as side groups were synthesized and used as mesogenes in lyotropic cholesteric phases. The wavelength of the selective reflection can be changed reversible by photoisomerizations of the side-groups. Hydroxypropylcellulose has been functionalized with acryloyl chloride. The degree of substitution of the resulting hydroxypropylcellulose acrylate is low enough to realize lyotropic cholesteric phases in water. It is possible to obtain colored gels by photocrosslinking. The maximum of the selective reflection shifts to shorter wavelength by drying. Reswelling of the gel causes a backshift of the selective reflection. The gel acts thus as humidity sensor. © Wiley-VCH Verlag GmbH, 2000.