A population distribution model for the alignment of nematic liquid crystals

In polymer aligned liquid crystal cells, a surface layer exists. In this surface layer the in-plane distribution of the mesogens, the shape of the mesogens, and their average angle with the surface determine the pretilt angle in the bulk of the sample. A model to incorporate those effects has been proposed. This model gives quantitative agreement with a number of experiments for thick cells, where the optical effects of the surface layer can be neglected. By introducing a linear variation of the optical axis through the surface layer we get qualitative agreement between the model and the pretilt found in thin cells. The thickness of the surface layer is of the order 0·1-1·0 μm.