【作者】 李鑫；

【导师】 路庆华；

【作者基本信息】 上海交通大学 ， 材料学， 2007， 博士

【摘要】 迅速发展的液晶取向技术,要求有工艺简单,易于实施的大面积图形化液晶取向方法来提高液晶显示器件的视角特性和灰度。激光诱导周期性表面微结构(LIPSS)对液晶分子有很好的取向性能,是理想的非接触液晶取向技术。本文以Nd: YAG激光器为光源,分别采用波长为266 nm、355 nm和532 nm纳秒脉冲激光,在聚酰亚胺(PI),聚对苯二甲酸乙二醇酯(PET)和聚(氨酯-酰亚胺)(PUI)等聚合物表面制备了LIPSS,研究了激光预辐射对LIPSS形成的影响,在此基础上探讨了LIPSS的可擦写特性,并将该性能应用于图形化液晶取向研究。本文还尝试了将一种新型的侧链型偶氮液晶树枝状高分子材料用于液晶取向研究。全文的主要内容和创新点如下:1.激光预辐射对LIPSS形成的影响。在LIPSS制备过程中,激光辐射容易导致聚合物表面消融,阻碍了LIPSS在众多领域的研究及应用。本文以355 nm的激光作为辐射光,首次研究了激光预辐射对LIPSS形成的影响。先用低于LIPSS形成阈值的激光对PI薄膜表面进行预辐射处理,然后进行第二次激光辐射制备LIPSS。结果表明预辐射后PI分子链段发生了各向异性取向。预辐射对LIPSS形成有很大的影响。在第二次激光辐射后,PI表面LIPSS形成的阈值与未预辐射的PI相比显著降低。在相同的能量下,预辐射PI表面形成LIPSS的深度大于未预辐射PI。激光的偏振方向对LIPSS的深度也有影响。预辐射和第二次激光辐射的偏振方向相互平行时或垂直时LIPSS的深度均比无预处理PI表面所制备LIPSS的深度大,但在预辐射和第二次辐射的激光偏振方向平行情况下LIPSS的深度大于两者垂直时LIPSS的深度。2. LIPSS的可擦写特性。用波长为355 nm的激光辐射表面已制备有LIPSS的PI基板,激光偏振方向垂直于初始LIPSS。用原子力显微镜(AFM)考察了不同激光能量下初始LIPSS的形貌变化,找到了LIPSS擦除所需的激光能量条件。当激光能量为7.0~9.6 mJ/cm2时,PI表面形成一种二维点阵结构;如果激光能量大于10.6 mJ/cm2,初始LIPSS即被完全擦除并被新形成的LIPSS取代,且新形成LIPSS的深度与初始LIPSS的深度相同。3. LIPSS诱导图形化液晶取向。采用两种方案实现大面积图形化液晶取向:(1)摩擦和LIPSS组合。首先对PI基板进行摩擦处理,然后加掩模板用激光扫描。在光辐射区域,液晶分子的取向方向由初始的摩擦方向转变为新形成的LIPSS方向;(2)LIPSS的可擦写特性。在PI表面上制备LIPSS,然后加掩模板用激光扫描。结果表明,液晶取向取决于新形成的LIPSS,同时也受聚合物分子链段取向的影响,因此液晶分子的取向方向不是完全沿LIPSS方向,而是与LIPSS的方向存在3o~4o的偏差。LIPSS可以重复擦写,具有很好的重复再现性,利用该特性在PI表面实现了多畴液晶取向。利用擦除LIPSS基板组装的液晶盒具有良好的光电响应特性。除PI外,利用波长为266 nm的激光在PET上也实现了图形化液晶取向。LIPSS的可擦写性除了实现多畴液晶取向,还可用于制备液晶衍射光栅。采用波长为532 nm的激光在PUI上得到了可电场调控的液晶衍射光栅。采用两种方案制备液晶衍射光栅:(1)摩擦和LIPSS组合;(2)LIPSS本身的可擦写特性。考察了光栅周期对光栅响应电压、衍射强度、偏振依赖性的影响。结果表明,由摩擦PUI基板制备的液晶光栅响应电压低于基于LIPSS基板的液晶光栅。由于光栅周期越小,衍射级次分散的衍射能量越多,使得周期较小的光栅1级衍射强度低于周期较大的光栅。低电压时液晶取向受取向层的影响较大,此时由于光栅内相邻栅区的液晶取向方向不同,导致光栅的偏振依赖性较强;随着电压增大,液晶分子逐渐沿电场排列,取向层对液晶分子的影响减小,因此光栅的偏振依赖性减弱。4.侧链型偶氮液晶树枝状高分子的光取向及其诱导液晶分子取向的研究。偶氮液晶树枝状高分子兼有液晶聚合物和树枝状高分子的特性,如高光敏性、高流动性和热致增强效应,这使得液晶树枝状高分子成为一种极具潜力的液晶取向材料。本文首次研究了侧链型偶氮液晶树枝状高分子在波长为355 nm的脉冲激光辐射下的光取向行为和LIPSS制备,并将其应用于液晶取向。AFM和偏振紫外光谱的结果表明,激光辐射可同时诱导LIPSS的形成和偶氮基团的各向异性取向,光取向的方向垂直于激光偏振方向,即垂直于LIPSS。考察了激光能量对面内和面外取向度的影响。面内取向度随激光能量的增大而增大,在4.0 mJ/cm2时达到一个最大值后开始随激光能量的增大而减小。对激光辐射的树枝状高分子进行热处理后,面内取向度显著增大。激光能量对面外取向度也有很大的影响。当激光能量小于1.8 mJ/cm2时,面外取向度且随着能量增大而增大;当激光能量大于1.8 mJ/cm2时,面外取向度随能量增大而减小。光取向的树枝状高分子薄膜对液晶分子有均一取向性能。液晶取向既可由分子链取向控制,也可由LIPSS控制。液晶分子在树枝状高分子表面的取向方向与激光能量有关。能量为0.6 mJ/cm2时,树枝状高分子表面没有形成LIPSS,此时液晶取向完全取决于偶氮基团的各向异性取向;能量为2.2 mJ/cm2时,液晶取向同时受LIPSS和分子链控制,液晶分子的取向方向介于LIPSS和偶氮基团的取向方向之间;当能量为3.0 mJ/cm2时,液晶分子完全由LIPSS决定,取向方向沿LIPSS。热处理后,薄膜表面的LIPSS被热擦除,同时偶氮基团的取向程度增大,此时液晶取向方向由原来的沿LIPSS方向转变为沿偶氮基团的分子链取向。更多还原

【Abstract】 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.更多还原