Molecular Orientation of Liquid Crystal Monolayers on Polyimide Films Exposed to Linearly Polarized UV Light

Abstract

By using optical second harmonic generation (SHG), we have investigated the molecular orientation of liquid crystal (LC) monolayers in contact with polyimide films exposed to linearly polarized ultraviolet light (LPUVL) at the wavelength of 266 nm. For different exposures at 3, 8, 20, and 80 J/cm2, the second harmonic (SH) signal from the LC monolayer was measured as a function of the rotation angle of the sample around the surface normal. The SH signal has no rotation angle dependence, independent of the energy density of exposure. On the other hand, uniform parallel (homogeneous) alignment of bulk LC was observed for a LC cell made with two polyimide-coated substrates exposed to LPUVL at 8 J/cm2. From these results we found that the in-plane anisotropy of the LC monolayer in contact with the polyimide film is very small, if any, even though the polyimide film can induce the homogeneous alignment of bulk LC. The average tilt angle of LC molecules in the monolayers in contact with the polyimide films was also determined. We found that the average tilt angle of LC molecules measured from the surface normal decreases with the increase of UV exposure.

Surface anisotropy from photo-induced bond breaking at polymer surfaces: A sum-frequency vibrational spectroscopic study of polyimide

“…Indeed it has been reported that no anisotropy could be observed in second harmonic generation from an LC monolayer adsorbed on an LPL irradiated PI film. 19,28 To study photo-induced anisotropy in bond breaking by LPL irradiation, it is more appropriate to use a PI surface that already has the main chains aligned. Such an alignment can be achieved by mechanical rubbing of the surface.…”

45.3: Azo Dye Materials for the Alignment of Liquid Crystal

“…The thermal stability has been improved and become comparable to the polyimide matrix. Recently, the polyimide derivatives, which have crosslinked with the dyes, have shown very impressive alignment characteristics [5][6][7]. Since these materials have the similar stable characteristics as the polyimide, much research efforts have been undertaken in this direction.…”

Optical Second Harmonic Generation Study of a Rubbed Polyimide Film Containing Steroidal Structure Side Chains

“…As excitation we used the SHG output with photon energy of 2.33 eV from a harmonic unit driven by a Nd:YAG laser with a pulse duration of 30 ps and a repetition rate of 10 Hz. The photon energy of 2.33 eV was chosen according to literature [12][13][14]17]. The incident laser beam was directed onto the rubbed and unrubbed polyimide samples at 45…”

“…On the other hand, second harmonic generation (SHG) is becoming a powerful method and recently is used for studying in the field of surface and interfaces science [15,16]. There have been a lot of SHG measurements for determining the orientations of LC molecules or polymer chains [12][13][14][17][18][19][20] so we use SHG in this study.…”