Microstructure of variable pitch cholesteric films and its relationship with the optical properties

A way to make cholesteric films reflecting in a broad wavelength band consi sts in associating different cholesteric pitches in the same film. In two p revious papers, we proposed an efficient method to produce variable pitch f ilms, based on a thermal processing, and we studied the optical properties of these films with respect to the time of processing. In the present paper , we study the microstructure of such films with respect to the processing time by means of transmission electron microscopy. The cholesteric phase is shown to be very well ordered. Within a wide range of annealing times, its periodicity is progressive from one face to the other of the film. A descr iption of the evolution of the structure with respect to the processing tim e is given, and classified in three stages, corresponding to the three stag es already stated from the optical properties. The relationship between the evolution of the structure and the optical properties is discussed.