Liquid crystal cells with five different polyimide alignment layers have been made for the study of liquid crystals alignment mechanisms. Two of the alignment layers are crystalline and three are amorphous. These cells have been filled with twenty different nematic liquid crystalline materials, and the relationship between the bulk properties and the pretilt angles has been studied. We find that the polarity of the liquid crystal is the best predictor for the pretilt angle. This effect can be explained by assuming that dimeric pairs of liquid crystal molecules are formed in the bulk, but that they are broken up on the crystalline surfaces.