ALIGNMENT LAYER RELAXATION - A TECHNIQUE FOR ASSESSING THERMAL TRANSITIONS IN POLYMER-FILMS

We introduce a method for assessing the glass transition temperature ( T-g) of thin polymer films. The technique may be applied to any polyme r film that can effect liquid crystal alignment, and is demonstrated h ere for a commercial polyamide-imide. The method leverages the ability of the polymer film to align nematic liquid crystals on its surface, when that surface has been prepared by mechanical brushing. Relaxation of the alignment layer, brought about by thermal cycling through T-g, is seen to affect liquid crystal alignment, and thus serve as a T-g i ndicator. The technique reveals a three-order-of-magnitude change in t he measured property. The method allows the assessment of that portion of the film responsible for aligning liquid crystals, and provides an indication of the efficacy of alignment. Our results imply that the r elaxation of the surface in Probimide 32 occurs over a range of temper atures, and relaxation is not complete until the film is heated to a t emperature above the glass transition of the bulk polymer. (C) 1998 El sevier Science Ltd. All rights reserved.