The molecular orientation of steroidal side chains at rubbed polyimide (PI) surfaces is studied by sum frequency generation (SFG) vibrational spectroscopy. The main objective is to find a correlation between the molecular structure of the PI film and the liquid crystal alignment on the polymer. Analysis of the SFG spectra shows that rubbing of the polymer film appears to cause conformational changes in the methyl group of the polymer side chain near the steroidal structure. However, rubbing does not significantly influence the orientation of the isopropyl group at the end of the polymer side chain. This shows that the liquid crystal alignment is not correlated with the orientation of the isopropyl group.

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We have studied the optical second harmonic generation (SHG) of a rubbed novel polyimide (PI) film having steroidal structure side chains prepared by spin coating on an indium tin oxide (ITO)-coated glass substrate. The second order nonlinear susceptibility χ (2) ijk elements were obtained from the fitting of the SHG data. The symmetry of the polymer chains roughly belonged to Cs symmetry with the rubbing direction parallel to the mirror plane. The average polar (tilt) angle of the rubbed PI chains was determined to be around 16• by using the measured χijk elements.

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