Laser-Induced Patterning Liquid Crystal Alignment

With the rapid development of liquid crystal (LC) aligning techniques especially those for larger-screen and high-resolution displaying, it's desired for easy, facile and large-area patterning LC alignment methods to improve view-angle and grey-scale characteristics of LC devices. Laser-induced periodic surface structure (LIPSS) possesses good ability to align LC molecules and is an ideal non-contact LC alignment method. In this work, LIPSS was prepared on several polymer materials such as polyimide (PI), poly(ethylene terephthalate) (PET) and poly(urethane-imide) (PUI) by nanosecond Nd: YAG laser at a wavelength of 266 nm, 355 nm and 532 nm, respectively. The effect of laser pre-irradiation was investigated and the erasable and re-writable nature of LIPSS was explored for the first time, which was then applied in patterning LC alignment. Furthermore, we probed into LC alignment on a side-chain azo liquid crystalline dendrimer, which acts as promising LC aligning materials. The main contents were as the following:1. The effect of pre-irradiation on LIPSS formation. Although LIPSS is proved to be an effective way to prepare microstructures on polymer surfaces, laser ablation accompanied by the formation of LIPSS limits its application in many fields. In this work, the effect of pre-irradiation on LIPSS formation was explored using 355 nm laser as the light source. LIPSS was prepared by the two-step laser irradiation: pre-irradiation at the laser fluence below the threshold of LIPSS formation and then exposed to laser again to fabricate LIPSS. It's found that anisotropic orientation of PI segments was induced by pre-irradiation. After the second irradiation, the threshold of LIPSS formation remarkably decreased compared to the initial PI. Upon pre-irradiation, fine LIPSS with larger amplitude can be prepared at rather mild laser fluence. The laser polarization direction of pre-irradiation imposed influence on LIPSS amplitude: LIPSS of pre-irradiated PI was deeper compared to that of the initial PI whether the laser polarization directions of the twice irradiations are parallel or perpendicular to each other. While the LIPSS amplitude was larger in the case of parallel irradiation than that in the case of perpendicular irradiation.2. The erasable and re-writable properties of LIPSS. PI with LIPSS was irradiated by laser with the polarization direction perpendicular to original LIPSS. The morphology of original LIPSS showed strong dependence on the laser fluence. Two-dimensional dot-like periodic structures were formed when the laser fluence ranging from 7.0 to 9.6 mJ/cm2. If the laser fluence exceeded 10.6 mJ/cm2, original LIPSS can be totally erased and new LIPSS was generated with similar amplitude.3. LIPSS-induced patterning LC alignment. Pre-buffed PI or PI with LIPSS was exposed to laser through photomasks. Based on the erasable and re-writable properties of LIPSS, large-area patterning LC alignment was achieved on the buffing or LIPSS substrates. The direction of LC alignment changed from original buffing or LIPSS to the new-formed LIPSS in the laser irradiated regions. LC alignment was completely controlled by the LIPSS formed subsequently and simultaneously affected by orientation of polymer segments. As a result, LCs were not aligned along LIPSS strictly but deviated from LIPSS by an angle of 3o~4o. Furthermore, LIPSS can be re-written repeatedly and exhibited good reproducibility. Based on this, multi-domain LC alignment was obtained on PI substrate. The opto-electric characteristics of LC cell fabricated by the re-written LIPSS were as good as the cell assembled by buffing technique. In addition, the method is available to a wide range of polymeric substrates, including flexible ones such as PET.Electrically controlled LC diffraction grating was fabricated on buffing PUI or LIPSS substrate based on the erasable and re-writable abilities of LIPSS. The 0st and 1st order diffraction intensity undergoes significant changes under the applied electric field. LC grating on buffing substrate has lower response voltage than the LC grating on LIPSS substrate. 1st order diffraction intensity increased with the increase of grating periodicity and showed strong polarization dependence at lower applied voltages.4. Photoorientation of side-chain azo liquid crystalline dendrimer and its effect on LC alignment. Azo liquid crystalline dendrimers possess both merits of liquid crystalline polymers and dendrimers, which endows themselves charming properties such as high photosensitivity, high fluidity and thermal enhancement of orientation order. LIPSS fabrication and photoorientational phenomenon were investigated for the first time on side-chain azo liquid crystalline dendrimer by irradiation with 355 nm pulsed laser. AFM and polarized UV spectroscopy indicated that LIPSS formation and anisotropic orientation of azo groups were induced simultaneously after pulsed laser irradiation. The direction of photoorientation was perpendicular to laser polarization direction and dependent on the laser fluence. The in-plane orientation of azo groups presented parabola-shaped behavior with the increase of laser fluence. The out-of-plane reorientation of azo groups was closely related to the laser fluence. Annealing of irradiated film at its smectic liquid crystalline temperature resulted in dramatic enhancement of in-plane orientation and out-of-plane redistribution of azo groups.LC alignment on the irradiated film was controlled by a cooperative effect of surface topography and anisotropic molecular orientation simultaneously. Both factors can be modulated by annealing treatment. After annealing, the alignment of LC molecules can be changed from the direction of LIPSS to that of the molecular orientation.