Ultra-thin films of polymerised smectic liquid crystals. A study using AFM and ellipsometry

Abstract

Langmuir–Blodgett multilayers very often consist of 2D crystallites which scatter light. One way of avoiding this problem is to use preformed polymers but even here there is a difficulty as multilayers formed from them have a poor degree of order in the direction normal to the plane of the film. This defect is exemplified by the fact that a low-angle X-ray diffractogram obtained from such films consists, usually, of 3 or less Bragg peaks and, at the most, 5 observable peaks. In an effort to obtain better order in multilayers both within the plane and normal to it, we have examined the properties of multilayers of a compound which can exist in the smectic A phase and can be polymerised with the aid of UV light while they are in this phase. We have studied the thickness of a number of multilayers of this compound as they go through the phase change using ellipsometric techniques and indeed find a thickness change which corresponds to the change in monolayer thickness found by X-ray diffraction and reported in our earlier studies. The ellipsometer that we have used gives the average thickness of a circular region having a diameter of about 1 mm. In order to obtain a better understanding of the behaviour of films on a much smaller horizontal scale, we have used atomic force microscopy. We display a number of typical cross sections of AFM scans and show that, on a molecular scale, the films are smoother in the smectic phase than they are in the crystalline phase and that polymerisation tends to maintain a film in a smooth condition.