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We characterized the photoalignment anchoring energy of photocrosslinkable liquid crystalline polymers (PLCPs) doped in a liquid crystal (LC). PLCP-doped LC cells with homogeneous alignment were fabricated using rubbed polyimide (PI) films. The PLCP-doped LC cells were exposed to a linearly polarized ultraviolet laser beam, and were then annealed. As a result, the PLCP-doped LCs were realigned owing to the axis-selective photocrosslink reaction. We investigated the relationship between the surface anchoring strength of the rubbed PI films and the realignment direction. The result suggested that the photoalignment anchoring strength of the PLCP in the LC is higher than 4 × 102 J/m3 at the maximum. The photoalignment realized substantial realignment in the LC cell with the surface anchoring strength of 10−4 J/m2.
© 2016 Optical Society of America
1. Introduction
Photoalignment of liquid crystal (LC) molecules have gained attention in various photonic applications including information displays, diffractive optical elements, optical communications, and optical data storage [1–9]. In these applications, alignment technologies of LCs are very important because the optical anisotropy of LCs arises from the molecular alignment. Mechanical rubbing methods are widely used for obtaining planar LC alignments over large areas [10,11]. However, the rubbing processes have some issues [7]. These are also unsuitable to fabricate complex alignment structures such as multidimensional LC diffraction gratings. Photoalignment of LCs is realized using photoalignment films coated on substrates [5]. Various molecular and polymer films have been presented for photoalignment of LCs since the study on azobenzene by Gibbons et al. and the study on poly(vinyl cinnamate) by Schadt et al [1,2]. Gibbons et al. reported that LC cells with fine periodic alignment distributions were obtained using photoalignment films consisting of an azobenzene-containing polymer (azopolymer) [1]. Azobenzene molecules, which exhibit photochromism, realign when exposed to polarized ultraviolet (UV) light, owing to a trans-cis-trans photoisomerization reaction [12]. The photoalignment using azobenzene molecules is switchable for UV and visible light illumination because the photoisomerization reaction of azobenzene is reversible [5]. Azobenzene realizes both planar and homeotropic alignment due to the E/Z photoisomerization [5]. Photoalignment can also be achieved using azo-dye-doped LCs [13–22]. Fuh et al. demonstrated the feasibility of binary LC alignment of nematic LCs doped with an azo dye (methyl red), in which two alignment forms are in a pixel using the adsorption of methyl red onto the substrate surfaces [18]. We also reported multidimensional photoalignment of azo-dye-doped LCs with an initial alignment [15,16,21,22]. These studies demonstrated that azobenzene-containing LCs realize complex and fine alignment structures easily. However, azobenzene absorbs in the visible region and is thermally unstable. These can be problematic for photonic applications [12].
Photocrosslinkable liquid crystalline polymers (PLCPs) containing cinnamate side groups have also demonstrated attractive photoalignment properties [23]. The cinnamate side groups are photocrosslinked by irradiating them with polarized UV light. The crosslinked PLCP molecules induce a self-organized molecular alignment in the exposed PLCP film through subsequent annealing. PLCP films are thermally stable and transparent in the visible region. PLCP films with uniaxial alignment are also applicable in the alignment layers of low molecular weight LCs. Azimuth and tilt angles of LC molecules on the PLCP films can be controlled by the polarization state, exposure energy, and incident angle of the UV light beam [24]. Accordingly, PLCPs are suitable media for photoalignment of LCs. We also studied the photoalignment properties of PLCP-doped LCs (i.e., composites of low molecular weight LCs and PLCPs) recently [25,26]. Our experimental results demonstrated that the PLCP-doped LCs with homogeneous alignment were realigned by the UV illumination owing to the axis-selective photocrosslink of the PLCP in the composites [26]. It was also showed that the realigned LC composites respond to an applied voltage without hysteresis, because of spatially stabilized PLCP molecules [25]. We think that PLCP-doped LCs are attractive for photoalignment. However, the anchoring energy of photocrosslinked PLCP molecules in LC composites (i.e., the photoalignment anchoring energy) is not clear at this moment. We should quantify the photoalignment anchoring energy for applying PLCP-doped LCs as photoalignment media.
The purpose of the present study is to propose an evaluation method for the photoalignment anchoring energy of PLCPs doped in low molecular weight LCs. We fabricated PLCP-doped LC cells with rubbed polyimide (PI) alignment layers. The surface anchoring energy of the rubbed PI layers was controlled by varying the rubbing strength. The PLCP-doped LC cells were exposed to a linearly polarized UV light beam, and were then annealed. The realignment direction and birefringence of the PLCP-doped LCs were observed using a polarized visible laser beam. We investigated the relationship between the surface anchoring energy of the rubbed PI layers and the photoinduced realignment, and then discussed the photoalignment anchoring energy of the PLCPs.
2. Experimental method
Two PLCPs (P1 and P2) were employed as dopants for a low molecular weight nematic LC (E7, Merck). Figure 1 shows the chemical structures of the PLCPs. P1 has been used in our previous study [26]. In the present study, P2 was also used. We expected that the solubility of P2 is higher than that of P1 due to the long alkyl spacer. The in-plane order parameter of P2 films is nearly same as that of P1 films (0.7 at the maximum) [27,28]. The syntheses and photoreactions of P1 and P2 are reported in detailed elsewhere [27,28]. The mixtures of the PLCP (P1 or P2) and E7 were stirred at 180°C using a magnetic stirrer. The PLCP concentration C was varied in the range of C ≤ 5wt%. Homogeneous solutions could be obtained at 180°C when C ≤ 5wt% for both the PLCPs. The PLCP-doped LCs were injected into planar glass cells by the capillary method on a hotplate of 180°C, and then were cooled slowly to room temperature. The planar cells were prepared using glass substrates. The substrate insides were coated with PI alignment layer that gave planar alignment. The structure of the PLCP-doped LC cells is schematically illustrated in Fig. 2. The cell thickness was adjusted to 2.0 μm. The PI layers with a thickness of 0.1 μm were rubbed unidirectionally using a rubbing machine (MRJ-100, EHC, Japan). The rubbing strength was varied for controlling the azimuthal anchoring energy of the PI alignment layers [26,29]. Here, we determined the surface anchoring energy of the rubbed-PI films by the Neel wall method [30]. We separately fabricated planar LC cells using E7, PI-coated glass substrates, and 0.10 mm-spacers for measuring the Neel wall width w. The azimuthal anchoring strength of an alignment film is given by A = 2dK11/w2, where d is the cell thickness and K11 is the splay elastic constant of the nematic LC in the cell [30]. We calculated A based on w that was measured using a polarizing optical microscope (ECRIPS E200, Nikon, Japan). This calculation assumed that d = 0.10 mm, which is the experimental condition, and K11 = 10 pN, which is a literature value [31].
Fig. 2 Schematic illustration of the structure of the PLCP-doped LC cell. The rubbing direction of the PI films are parallel to the x-direction.
The obtained PLCP-doped LC cells were observed using the polarizing optical microscope. The observed images of the PLCP-doped LC cells with C = 5wt% and A ≈10−1 mJ/m2 are shown in Fig. 3. These were taken at room temperature. These observations demonstrated that the PLCP-doped LCs in the cells were uniaxially aligned along the rubbing direction. Light leaks in the dark conditions is presumably due to the aggregation of the PLCPs that takes place in the cooling processes. For applications of PLCP-doped LCs, the solubility of PLCPs will be important. The difference of the color in Figs. 3(b) and 3(d) are primarily associated with the difference of the retardation of the PLCP-doped LCs. We exposed the PLCP-doped LC cells to a linearly polarized UV laser (IK3501R-G, Kinmon, Japan) beam with a wavelength of 325 nm. The incident angle was set to be 0°. The angle between the initial director n, which is parallel to the rubbing direction, and the polarization direction of the UV laser beam EUV was set to be 45°. The exposure energy was adjusted to 2.0 J/cm2 for the P1-doped LC cells and 6.0 × 101 J/cm2 for the P2-doped LC cells, respectively. These were determined based on our previous studies [26–28]. After the UV illumination, the PLCP-doped LC cells were annealed at 100°C for 30 min using an oven, and were then cooled slowly to room temperature.
Fig. 3 Polarizing optical microscope images of the unexposed cells with A ≈10−1 mJ/m2 and C = 5wt%; (a), (b) the P1-doped LC cell and (c), (d) the P2-doped LC cell. P is the transmission axis of the polarizer. A is the transmission axis of the analyzer. r is the rubbing direction.
The alignment direction and birefringence of the PLCP-doped LCs were investigated for the unexposed cells, unannealed exposed cells, and annealed exposed cells based on the crossed nicols method [26]. A linearly polarized He-Ne laser with an operating wavelength of 633 nm was used for the probe beam. The observations were conducted at room temperature. Here, as seen in Fig. 2, we define as follows: the director in the unannealed or annealed exposed cell is nre, the angle between n and nre is the realignment angle θre, the angle between the n and EUV is the polarization azimuth angle θUV.
3. Results and discussion
By using measured θre, we characterized quantitatively the photoinduced realignment of the PLCP-doped LCs. The degree of realignment was defined as ζ = θre / θUV. Figure 4 shows ζ for the annealed exposed cells with C = 5wt%. For the unannealed exposed cells, we confirmed that ζ ≈0 (i.e., θre ≈0). However, apparent realignments were observed for the annealed exposed cells. This demonstrates that the realignments of the PLCP-doped LCs are induced by the photocrosslink of the PLCP molecules and the subsequent annealing [27,28]. The realignment angle was strongly depended on the surface anchoring energy of the rubbed PI films. As seen in Fig. 4, ζ was increased with decreasing A. We could realize ζ ≈1 (i.e., θre ≈θUV), which means that the LC molecules were realigned parallel to the polarization direction of the linearly polarized UV laser beam, for the P1-doped LC cells with weak azimuthal anchoring strength (10−2 ~10−1 mJ/m2). This result suggests that the anchoring energy of the photocrosslinked PLCP molecules in the mixtures (i.e., the photoalignment anchoring energy) is comparable to or higher than the azimuthal anchoring energy of the rubbed PI films. It was also suggested that the photoalignment anchoring energy of P2 was smaller than that of P1.
Fig. 4 Dependence of the degree of photoinduced realignment on the azimuth anchoring energy of the rubbed PI films (C = 5wt%). Filled and open circles represent the data for the P1- and P2-doped LC cells with annealing. The error bars are associated with the deviation of the Neel wall width.
Figure 5(a) shows the dependence of ζ on the PLCP concentration for the annealed exposed cells with A ≈10−1 mJ/m2. ζ was increased with increasing PLCP concentration. This result also suggests that the photoalignment anchoring energy of P1 is higher than that of P2. The surface azimuthal anchoring energy of the two rubbed PI films is written as
where S is the area, ϕe is the angle between the x-axis and the easy axis, ϕ is the angle between the x-axis and the director [32]. By using Eq. (1), we define formally the photoalignment anchoring energy of the PLCP in the bulk aswhere V is the volume, Ψ is the photoalignment anchoring strength, and ϕ′e is the angle between the x-axis and the photoalignment easy axis. When the elastic energy of the PLCP-doped LC is given as Felas, the total free energy of the PLCP-doped LC cell can be written asThe director of the PLCP-doped LC must satisfy ∂F/∂ϕ = 0. By assuming the PLCP-doped LC in the cell is homogeneously alignend (i.e., ∂Felas /∂ϕ = 0), we obtainwhere d is the cell thickness (d = V/S). Therefore, we can estimate the photoalignment anchoring strength based on the azimuthal anchoring strength of the rubbed PI film. Figure 5(b) shows Ψ calculated for the data of Fig. 5(a). The calculation assumed that ϕ = θre, ϕe = 0, ϕ′e = θUV = 45°, and d = 2.0 μm. The photoalignment anchoring strength was increased with increasing PLCP concentration. In the same PLCP concentration, the photoalignment anchoring strength of P1 was one order of magnitude higher than that of P2. More detailed work is necessary to clarify the difference of the photoalignment anchoring strength. As seen in Figs. 5(a) and 5(b), when the degree of realignment was close to 1, the photoalignment anchoring strength was in 4 × 102 to 3 × 103 J/m3.Fig. 5 Dependence on the polymer concentration (A ≈10−1 mJ/m2); (a) the photoalignment degree and (b) the photoalignment anchoring strength. Filled and open circles represent the data for the P1- and P2-doped LC cells with annealing. The error bars are associated with the deviation of the surface anchoring energy.
Figure 6 shows birefringence of the PLCP-doped LCs in the unexposed, unannealed exposed, and annealed exposed cells with A ≈10−1 mJ/m2. The birefringence Δn was slightly decreased by the UV illumination while it remained almost unchanged for the subsequent annealing. The decrease of Δn may imply that the photocrosslinked PLCP molecules degrade the order parameter of the mixture. Additionally, Δn of the P2-doped LC was smaller than that of the P1-doped LC. This fact may be associated with the difference of the solubility of the PLCPs in E7. Figure 7 shows the contrast ratio at the wavelength of 633 nm. For the homogeneous cell fabricated without using PLCPs, the contrast ratio was 470. The contrast ratio was clearly decreased by the PLCP doping. However, there were no apparent connections between the contrast ratio and the PLCP consentration, the UV irradiation, or the annealing. For applications of PLCP-doped LCs, it is necessary to evaluate the effects of the PLCP-doping on the alignment state from various aspects.
Fig. 6 Dependence of the birefringence of the PLCP-doped LCs on the polymer concentration; (a) the P1-doped LC cell and (b) the P2-doped LC cell (A ≈10−1 mJ/m2 and C = 5wt%). Squares, triangles, and circles represent the data for the unexposed, unannealed exposed, and annealed exposed cell.
Fig. 7 Dependence of the contrast ratio of the PLCP-doped LCs on the polymer concentration; (a) the P1-doped LC cell and (b) the P2-doped LC cell (A ≈10−1 mJ/m2 and C = 5wt%). Squares, triangles, and circles represent the data for the unexposed, unannealed exposed, and annealed exposed cell.
4. Conclusions
We evaluated quantitatively the photoinduced realignment of PLCP-doped LCs with homogeneous alignment. The PLCP-doped LCs in glass cells with rubbed PI layers were exposed to a linearly polarized UV laser beam. As a result, the PLCP-doped LCs were realigned in accordance with the polarization direction by the UV illumination and the subsequent annealing. The degree of realignment depended on the surface anchoring energy of the rubbed alignment films. The PLCP-doped LCs were realigned parallel to the polarization direction of the linearly polarized UV laser beam when the azimuthal anchoring strength of the rubbed PI layers was substantially low (< 10−4 J/m2). This result indicates that the anchoring strength of the photocrosslinked PLCP molecules in the mixtures (i.e., the photoalignment anchoring energy) is higher than that of the rubbed PI films. We think that this evaluation method serves for determining photoalignment anchoring strength of various photoalignable LCs.
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