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 consists in associating different cholesteric pitches in the same film. In two previous papers, we proposed an efficient method to produce variable pitch films, 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 description of the evolution of the structure with respect to the processing time is given, and classified in three stages, corresponding to the three stages already stated from the optical properties. The relationship between the evolution of the structure and the optical properties is discussed.