Study of the orientation of liquid crystal molecules on polyimide alignment films by FTIR with polarisation mode

The orientation of liquid crystal molecules on an alignment film was studied using Fourier transform infrared spectrometer (FTIR) with polarisation mode. The standard laboratory instrument was used to simultaneously determine the alignment direction and pretilt angle of the liquid crystal molecules. The dipole of the infrared active moiety of the rigid biphenyl of liquid crystal molecules can be used to represent their orientation, and the polarised light electrical vector coinciding with the dipole of the infrared active moiety increases in absorption intensity. Thereby, the infrared light absorbance of liquid crystal molecules in liquid crystal cells as a function of the polarised angle of incident light reveals the orientation of molecules, and the size of pretilt angle can be calculated. In particular, this method has no restriction on the range of pretilt angle.