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As an alternative to the rubbing method, the photoalignment method has been extensively developed for liquid crystal displays owing to its various advantages including the absence of any contamination problems, feasibility of multi-domain structure formation, and low temperature processing capability. However, cinnamate-based photo-reactive materials that are widely used in photoalignment suffer from several limitations including low pretilt angle and limited control of tilted-up direction of the liquid crystals compared to that of the rubbing process. In this work, we demonstrate a simple and convenient method to generate the pretilt angle, compared to that achieved via the rubbing method, by means of a simple directional peel-off process using a photo-reactive cinnamate oligomer having flexible molecular chain.
©2009 Optical Society of America
1. Introduction
Researches on the liquid crystal alignment layer are essential for the uniaxial control of the direction of liquid crystals in liquid crystal displays (LCDs). Among various methods to align liquid crystals, the rubbing method has been widely used for the generation of optical anisotropies by the orientation of polymer molecules due to its simple and rapid process. However, there are still some obstacles impeding the complete success of this method, including static electricity, dust, defects in the alignment layer, and inability to align liquid crystals with multi-domains [1].
Recently, several liquid crystal alignment techniques have been developed as alternatives to the rubbing process typically including photoalignment [2,3], imprinting [4], oblique evaporation [5], and Langmuir-Blodgett film [6]. In particular, photoalignment based on the generation of an anisotropic distribution of the photo-reactive moieties such as azobenzene [7], cinnmate [8–12], coumarin [13], and polyimide [14–16] by irradiation of linearly polarized UV (LPUV) light has gained substantial attention, as it can be used to achieve a multi-domain structure and be carried out under low temperature. However, photoalignment suffers from several limitations such as limited creation of the pretilt angle and difficulties in aligning liquid crystals with a certain direction generally: cinnamate-based photo-reactive materials that are widely used in photoalignment method exhibited almost zero pretilt angle [17]. For the practical applications of liquid crystal display devices, proper pretilt angle is required to achieve high cell performance: the pretilt angle prevents disclination of liquid crystals which is overlaps or collisions between liquid crystals at low pretilt angles as the alignment direction of liquid crystals is changed by application of an electric field. In addition, the generation of a uniform tilting-up direction is also important, because liquid crystals with random alignment directions are not useful as a component to control the direction of light.
To address this problem of limited creation of pretilt angle, oblique angle, multi-step, and multi beam illumination of cinnamate-based photoalignment layer were proposed: these controlled irradiations of incident light lead to the breakage of degeneracy and result in the pretilt angle [17]. However, these methods for the generation of pretilt angle still reported a fairly low pretilt angle (below 0.5 degree). Additionally, Kim et al. controlled the liquid crystal tilt up direction and the pretilt angles by a directional peeling of a Rigiflex mold on a photocurable polymer surface: they suggested the unidirectional orientation of the free end-group of photocured polymer could be generated by adhesion-driven pulling force between the photocurable material and poly(dimethylsiloxane) (PDMS) film [18]. However, although the reported value was relatively higher (about 1 degree) than those of cinnamate-based photoalignment layer, it needs to be further enhanced for the high performance of liquid crystal-based devices.
In this study, we demonstrate that a directional peel-off process of a cinnamate-based oligomer as a photoalignment material can generate the pretilt angle of liquid crystals (3 degrees) that are much higher than previously reported values (below 1 degree). The success of the proposed approach stems from the high flexibility of molecular chain, the ability of chemical alignment by itself, and the larger number of molecules that can participate in the tilt-up process compared with the case of a photocurable polymer. To demonstrate the versatility of oligomers as an alignment layer for a directional peel-off process, we performed a comparative study of a photo-reactive oligomer (2Ci-BD) and polymer (poly(vinyl cinnamate), PVCi). The influence of the surface property of PDMS film, controlled by ultraviolet/ozone (UVO) treatment, on the pretilt angle was also investigated.
2. Synthesis and characterization of the 2Ci-BD oligomer
1,4-Butanediyl dicinnamoyl ether (2Ci-BD) oligomer used as an alignment material, was synthesized by reaction between the acroyl chloride group of cinnamoyl chloride (Aldrich Co.) and the hydroxyl group of 1,4-butanediol (Aldrich Co.) in tetrahydrofuran (THF) (Aldrich Co.). The solution was stirred for at least 24 h for sufficient reaction. The produced oligomer was separated by centrifugation, and washed over five times with methanol to remove both impurities and unreacted monomers. Finally, the 2Ci-BD oligomer was obtained by drying the solution under vacuum at room temperature for 24 h. The 1H NMR spectrum (500 MHz) was recorded on a Bruker AMX 500 with tetramethylsilane (TMS) as an internal standard.
Figure 1 indicates the molecular structure and the 1H NMR spectrum of the 2Ci-BD oligomer. The proton peaks of the 1,4-butanediol appear at 3.72 ppm and those of the 2Ci-BD oligomer which are denoted by b, appear at 4.26 ppm. The extent (%) of reaction between cinnamoyl chloride and 1,4-butanediol was calculated by comparing the integral values of the peaks at 4.26 ppm and at 3.72 ppm. The percentage of synthesized oligomer and unreacted 1,4-butanediol is determined on this basis to be 98.7 and 1.3, respectively. The amount of impurities was negligible.
Fig. 1 (a) Molecular structure of photoreactive 2Ci-BD oligomer. (b) 1H NMR spectrum of 2Ci-BD oligomer in CDCl3.
3. Fabrication of liquid crystal cell for enhanced pretilt angle by a directional peel-off
The experimental procedure to generate the pretilt angle of photoalignment by using a flexible oligomer and a directional peel-off process is schematically illustrated in Fig. 2 . A 2 wt% solution of 2Ci-BD oligomer in cyclohexanone was spin-coated onto a glass substrate treated by hexamethyl disilazane (HMDS) for 30 s at 500 rpm, and the cast thin film was baked at 60 °C for 1h. The photo-dimerization reaction of 2Ci-BD oligomer was carried out by irradiation of LPUV light [19]. LPUV light was obtained by passing light from a 500W mercury arc lamp (Oriel) through a UV linear dichroic polarizer (27320, Oriel) and a UV bandpass filter (59800, Oriel). The intensity of the irradiated LPUV light was 1.2 J∙min−1 (0.02 W∙m−2). After irradiation of LPUV onto the 2Ci-BD oligomer layer, we use the roller of rubbing machine for the unidirectional attachment to the 2Ci-BD oligomer layer and detachment of PDMS film from the 2Ci-BD oligomer layer. Figure 3 shows the digital photograph of a rubbing machine (Ulsan general trading company) used in this experiment. The roller covered with elastomeric PDMS film and at the same time the stage with the cinnamate-based oligomer layer will move along the orientation direction of photo-aligned dimers. During this period, the PDMS films attach to the cinnamate-based oligomer layers first and then the films will peel-off from the oligomer layer soon with the continued rolling. The PDMS films used in a directional peel-off process by a rubbing machine was flexible enough to wrap onto the surface of roller, and they were also durable under continuous rolling. The rotation speed of roller and the movement speed of sample stage were controlled simultaneously by a motorized system. Therefore, a directional peel-off process by a rolling of PDMS film was conducted reproducibly in this experiment.
Fig. 2 Schematic illustration of procedures for fabricating alignment layer to generate pretilt angle.
The PDMS surface was treated by UVO (Ozone cure 16, Minuta Technology Co.) with various irradiation times to control the adhesion force between the 2Ci-BD oligomer layer and the PDMS surface. The equipment generated UV emissions at 185 nm and 254 nm, and the distance between the UV lamp and PDMS films was 20 mm. Hydrophilicity of the PDMS surface as a function of time for UVO treatment was measured by the sessile drop technique using a contact angle analyzer (Phoenix 300 Plus, SEO Co.). A homogeneously aligned liquid crystal cell was made by assembling a pair of glass substrates coated with 2Ci-BD oligomer aligned layers with monodisperse polystyrene bead spacers (10 μm). Nematic liquid crystals (E7, Merck Co.) were injected into the cell by capillary motion at 65°C, upon which the liquid crystals become the isotropic phase, followed by cooling down of the cell to room temperature. Pretilt angles of liquid crystals were measured by means of the crystal rotation method [20].
4. Liquid crystal alignment properties of 2Ci-BD oligomer layer
4.1 Effect of UVO treatment on the PDMS surface
Figure 4(a) illustrates the change in contact angle of the PDMS surface with UVO treatment, which is measured with deionized water. The contact angles on the PDMS surface were reduced with the time of UVO treatment. These results indicate that the hydrophobic PDMS surface at the initial state is gradually modified to the hydrophilic PDMS surface with UVO treatment. The UVO treatment initially breaks the molecular chains, such as C-H and Si-C of the PDMS, leading to the formation of methylene and silicon radicals. The radicals then react with activated oxygen species, which is generated by the photolysis of molecular oxygen at 185 nm [21]. Eventually, the methyl groups of PDMS surface are substituted with hydroxyl groups by UVO treatment. After the UVO treatment on the PDMS surfaces, the degree of hydrogen bonding between the oxygen groups of cyclobutane derivatives formed from 2Ci-BD oligomer and hydroxyl groups generated on the hydrophilic PDMS surface increased.
Fig. 4 Contact angles of PDMS layer as a function of time of UVO treatment. The inset images represent the water droplet shapes on the PDMS surface.
4.2 Pretilt angles of cinnamate-based oligomer and cinnamate-based polymer
As mentioned in introduction, according to Kim et al. [18], the adhesion force between the photocurable alignment layer and PDMS film gives rise to a pulling force on the free end groups of chemical chains of the alignment layer. In particular, this pulling force is driven by adhesion force between alignment layer and PDMS film. More importantly, the tilt-up direction of free end group is in the direction commensurate with the peeling direction of PDMS film. As a result, uniaxially aligned- and tilted- end groups of the chemical chains of the alignment layer lead to the generation of pretilt angle of liquid crystals.
In line with this, to investigate the generation of pretilt angles of liquid crystals on cinnamate-based oligomer (2Ci-BD) layer by a directional peel-off process in our system, we measured the pretilt angles of liquid crystals on the 2Ci-BD oligomer layer with and without the directional peel-off process and calculated the ratio of work of adhesion between three different surfaces to explain the measured pretilt angle results. The three different surfaces are glass substrate, 2Ci-BD oligomer layer, and PDMS film with and without UVO treatment. Using 2Ci-BD oligomers, comparison between the results derived with and without the directional peel-off process was performed, and the measured pretilt angles are shown in Fig. 5 . Initial value of pretilt angle of liquid crystals on 2Ci-BD oligomer layer without a directional peel-off process was nearly zero. On the other hand, in the case of the 2Ci-BD oligomer layers with a directional peel-off process, the pretilt angles of liquid crystals were enhanced with UVO treatment time (up to 5 min) of PDMS film. Especially, the maximum pretilt angle of 3 degrees appeared at 5 min of UVO treatment: this value of 3 degrees is much higher than that from a directional peel-off of conventional polymer (UV curable resin) (less than 1 degree) [18].
Fig. 5 Pretilt angles of photo-reactive oligomer (2Ci-BD) layer and polymer (PVCi) layer with respect to the time of UVO treatment of PDMS film.
To rationalize these results, we employed the adhesion theory, by which the adhesion between two materials can be interpreted by the secondary forces such as hydrogen bond and van der Waals force [22]. This ratio of work of adhesion of the two different interfaces was evaluated to explain the directional peel-off effect by PDMS film on the 2Ci-BD oligomer layer by the Owens-Wendt geometric mean equation [23]. According to this equation, the work of adhesion is a function of surface tension, and thus can be calculated by measuring the contact angles of the dispersion component and polar component. The ratio of the product of work of adhesion and surface area of the two different interfaces is defined as R ( = W23∙A23/W12∙A12) in this work. W23 and A23 are the work of adhesion and surface area between alignment materials and PDMS film, respectively. W12 and A12 are the work of adhesion and surface area between the glass substrate and alignment materials, respectively. In our study, the work of adhesion mainly depends on the hydrogen bonding at interfaces, and this acts as a critical role in the generation of the pretilt angle by a directional peel-off process. The hydrogen bonding between cinnamate-based oligomer layer and PDMS film was controlled by adjusting the UVO treatment time of PDMS as described in Fig. 4.
Figure 6 shows the ratio of W23 and W12 as a function of time of UVO treatment on PDMS film. A23 and A12 could be ignored, since the two parameters have the same values. In the case of PDMS film treated with UVO up to 5 min, the R values were below unity and the pretilt angles were increased with UVO treatment time. The creation of pretilt angle is because the work of adhesion between the 2Ci-BD oligomer layer and the PDMS surface is increased with UVO treatment time. In this region, alignment layer remained undetached due to the relatively high work of adhesion between the alignment layer and the substrate, although the work of adhesion between the alignment layer and the PDMS surface is enhanced: as shown in Fig. 6, R value was close to unity but less than unity at the UVO treatment for 5 min. As aforementioned, we obtained a maximum pretilt angle of around 3 degrees that is much higher than previously reported ones (less than 1 degree) [18] for the case of PDMS film treated by UVO for 5 min.
Fig. 6 Ratio of work of adhesion between three different surfaces (Surface 1: HDMS treated glass substrate, surface 2: photo-aligned 2Ci-BD oligomer layer, and surface 3: PDMS film without and with UVO treatment) as a function of time of UVO treatment. The ratio of work of adhesion can be defined by R = W23∙A23/W12∙A12 [23]. Since A23 and A12 have the same values, R can be simply expressed as the ratio of W23 to W12..
As a tool to analyze the degree of detachment of 2Ci-BD oligomer layer after a directional peel-off process, we selected Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR, IFS66V/S & HYPERION 3000) spectroscopy. Figure 7 displays the ATR-FTIR spectra of PDMS film as a function of the time of UVO treatment of PDMS film after a directional peel-off process. In the wavelength range of 3000−930 cm−1, the peak at 1016 cm−1 corresponding to the Si-O-Si stretching vibration mode of PDMS film was selected for the normalization, because the amount of the Si-O-Si bond was same in all samples (Fig. 7(a)). 1703 cm−1 and 1725 cm−1 are attributed to the C = O stretching at conjugated position (before UV irradiation) and the C = O stretching at unconjugated position (after UV irradiation) of 2Ci-BD oligomer, respectively. As shown in Fig. 7(b), in the case of pristine PDMS and 5 min UVO treated PDMS film, there were no bands at 1703 cm−1and 1725 cm−1, indicating that the aligned dimers of 2Ci-BD oligomer were successfully tilted-up without any detachment of alignment layer. Therefore, this result can be direct evidence that PDMS film treated with UVO up to 5 min could not detach the 2Ci-BD oligomer.
Fig. 7 ATR-FTIR spectra of PDMS film with respect to the time of UVO treatment of PDMS after a directional peel-off process: (a) in the wavelength range of 3000–930 cm−1 and (b) in the wavelength range of 1800–1650 cm−1.
In contrast, pretilt angles of liquid crystals were reduced with UVO treatment time in the case of the PDMS film treated with UVO for more than 10 min as shown in Fig. 5. In these cases, detachment of the oligomer materials from the alignment layer is possible, because R values were larger than unity as indicated in Fig. 6. These results comprise strong evidence that the oligomer materials were detached from the glass substrate due to much higher degree of the work of adhesion between the 2Ci-BD oligomer layer and the PDMS surface compared to that between the glass substrate and the 2Ci-BD oligomer layer: the higher hydrophilicity of PDMS film leads to the high adhesion force between the 2Ci-BD oligomer layer and PDMS film. As a result, the alignment property is spontaneously deteriorated due to the decrease in the amount of residual alignment materials on the substrate. The ATR-FTIR spectra also support this result as shown in Fig. 7(b). The peaks of 2Ci-BD oligomer were clearly shown in the case of PDMS film treated with UVO more than 10 min, because oligomers were detached from the glass substrate by a directional peel-off process.
In addition, we carried out comparative studies for a flexible cinnamate-based oligomer (2Ci-BD) and a cinnamate-based polymer (PVCi) which has rigid main chains to demonstrate the versatility of the flexible oligomer layer for the generation of pretilt angle (Fig. 5). In the case of PVCi, the pretilt angles remained unchanged even under higher adhesion between the polymer layer and hydrophilic PDMS surface. This could be ascribed to the low flexibility of the crosslinked main chain of polymer and embedded cinnamate groups in the polymeric chain. Therefore, we established that a highly flexible molecular chain is advantageous for the tilting-up of molecules by a directional peel-off process.
From these results, we conclude that a controlled directional peel-off process by varying UVO treatment time of PDMS film provides the efficient way for the generation of pretilt angle of liquid crystals. Accordingly, the work of adhesion between the 2Ci-BD oligomer layer and PDMS film could be increased for tilting- up molecules on the oligomer layer as shown in Fig. 6.
4.3 Properties of 2Ci-BD oligomer layer
The liquid crystal cells made from 2Ci-BD oligomer films with and without a directional peel-off process by the 5 min UVO treated PDMS film show perpendicular liquid crystal alignment to the polarization direction of the incident UV light as described in Fig. 8 . These results indicate that the orientation direction of liquid crystals is maintained uniformly even under an additional peel-off process. However, the order parameter is lower than that of PVCi due to the crystallinity of 2Ci-BD oligomer.
Fig. 8 Polar plot of liquid crystal orientation on the alignment layer of 2Ci-BD: (a) before the directional peel-off process and (b) after the directional peel-off process.
A differential scanning calorimetry (DSC) analysis was performed to confirm the crystallinity of 2Ci-BD oligomer, as shown in Fig. 9 . A crystallization exothermic peak is observed at 88 °C in the DSC thermogram at a heating rate of 10 °C/min. This suggests that the highly ordered oligomers were bound tightly during the irradiation of LPUV light for unidirectional alignment of molecules. Therefore, the lower order parameters appear in the case of 2Ci-BD oligomer, since the number of oligomers that can participate in a dimerization reaction by LPUV light was reduced. We are now developing a method to avoid the crystalline effect on the alignment layer by using chiral molecules in our next paper and the results will be reported later.
After the directional peel-off process, homogeneous orientation of the liquid crystals is also confirmed by a polarized optical microscopic (POM) analysis. Figure 10 presents polarized optical micrographs from liquid crystal cell prepared with 2Ci-BD oligomer with a directional peel-off process by the 5 min UVO treated PDMS film. The assembled liquid crystal cell was rotated and observed under crossed polarizers. As the cell rotates, the maximum dark state (Fig. 10(a)) and bright state (Fig. 10(b)) of the liquid crystal cell appear in an alternating way. Hence, it is confirmed that an anisotropic photoalignment layer structure is constructed without defects after a directional peel-off process on the oligomer layer, because there is no detachment of alignment layer as already demonstrated.
Fig. 10 Polarized optical microscopic images of liquid crystal cell prepared with 2Ci-BD oligomer after a directional peel-off process. The directions of polarizers (A and P) and the photoalignment direction of liquid crystals are denoted as arrows in the images.
5. Conclusions
In summary, we demonstrate that a pretilt angle of liquid crystals can be simply generated by a directional peel-off process of the photo-reactive cinnamate oligomer materials. In particular, the largest contribution to our strategy was the use of cinnamate-based oligomer having molecular flexibility instead of polymer-based cinnamate materials that are typically used for the liquid crystal photoalignment layer. From the comparison of pretilt angles between photo-reactive oligomer and polymer materials, we indeed conclude that the molecular flexibility of photoalignment layer acts as a critical role in the creation of pretilt angle of liquid crystals by a directional peel-off process. More importantly, it should be noted that the pretilt angle (3 degrees) obtained from the optimization of the work of adhesion is much higher that of conventional polymer materials (generally below 1 degree). Therefore, we believe that the directional peel-off process of photo-reactive oligomer rather than polymer materials can offer a new avenue toward the generation of a pretilit angle of liquid crystals in experimentally convenient and efficient way.
Acknowledgments
This publication was based on work supported by the ‘Center for Nanostructured Materials Technology’ under ‘21st Century Frontier R&D Programs’ of the Ministry of Education, Science, and Technology, Korea, grant (code#: 09K1501-02510). We also appreciate partial support from the Brain Korea 21 Program.
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