Cholesteric phases and films from cellulose derivatives

Opalescent films, which own their color to Bragg reflection of light, can b e prepared from cholesteric cellulose derivatives. Both lyotropic and therm otropic systems realised. Here we describe the manipulation of the waveleng th of selective reflection by (a) mechanical compression, (b) photoisomeriz ation of pendant groups and (c) the water uptake of a cholesteric system. T he lyotropic cholesteric phases of cellulose 3-chlorphenyl urethane and a m ixture of acrylate-monomers can be transformed into semi-interpenetrating n etwork by photo curing. The composite is elastic and shows selective reflec tion. By compression of a thick film (1 mm), the maximum of the selective r eflection shifts reversible to shorter wavelength. Cellulose urethanes with azo dyes as azo es as side groups were synthesized and used as mesogenes i n lyotropic cholesteric phases. The wavelength of the selective reflection can be changed reversible by photoisomerizatios of the side-groups. Hydroxy propylcellulose has been functionalized with acryloyl chloride. The degree of substitution of the resulting hydroxypropylcellulose acrylate is enough to realize lyotropic cholesteric phases in water. It is possible to obtain colored gels by photo-crosslinking. The maximum of the selective reflection shifts to shorter wavelength by drying. Reswelling of the causes a backshi ft of the selective reflection. The gel thus as humidity sensor.