Electro&#x2013; and photosensitive azopolymer for alignment of liquid crystals

Denis Kasyanyuk; Nikolay Davidenko; Yuriy Kurioz; Irina Savchenko; Yuriy Reznikov

doi:10.1364/OE.23.026660

1. Introduction

Application of liquid crystals (LCs) usually requires specific orientation of LC molecules on the boundary surfaces. Depending on a LC device, homeotropic, tilted or planar alignment of the director with a different anchoring strength are necessary. Modern LCDs and LC optical elements often require complex spatial distributions of the easy orientational axis of LC on the aligning surface. Therefore, despite apparent achievements in the technology of LC alignment and control of a LC anchoring, improving of the alignment methods and introducing new aligning materials are still in the focus of the applied physics and engineering of LCs [1]. In this concern, high expectations are for alignment of LC by polymer films, in which an easy orientation axis is produced by irradiation with polarized light (photoalignment technique) [2]. Alignment of LCs by light provides effective control of basic anchoring parameters: easy orientation axis, pretilt angle and anchoring energy. The most attractive features of the photoalignment technique are their contactless and practically unlimited possibility producing complicated pattering of the LC director by irradiation of the aligning surface with different polarization in different spots.

In most cases the anchoring characteristics of photoaligning materials are controlled by changing the parameters of light irradiation (polarization, exposure doze and wavelength) and the irradiation geometry [2]. Additional control of the anchoring of a LC can be provided by different type of treatment of the photoaligning surface such as rubbing [3] or ion/plasma beam bombardment [4] as well as application of a magnetic field during irradiation [5,6]. Very attractive is the use both a light irradiation and electric field, but up to now electro-induced easy axis in the photoaligning material requires heating of the sample above the glass transition temperature [7] or adsorption of LC molecules on the aligning surface during filling of the cell [8]

In this work we introduce a new type of the aligning polymer materials, in which an easy orientation axis is induced both by a polarized light and by an electric dc-field. We obtained electro- and light-induced planar alignment of a nematic LC on a Co-containing azopolymer. Trans-cis isomerisation of the azobenzene fragments by light resulted in a photo-induced anisotropy, and orientation of permanent dipole moments of the polymer by dc-field caused electro-induced anisotropy. Both of these processes lead to formation of the LC easy orientation axis on the polymer surface.

2. Experiment and discussion

We used the azopolymer that contained coordinated Co-atoms disposed between two azobenzene fragments with electronodonor groups [9,10,12] (Fig. 1(a)). The metal- free azo-polymer (Fig. 1(b)) was also used in some experiments.

Fig. 1 Chemical structures of the metal-containing (a) azo-polymer 4-methacryloyloxy-2-(N,N-diethylamino)-(4’-carboxy-3′-oxy)azobenzene and metal-free azo-polymer (b) poly-4-methacryloyloxy-2-(N,N-diethylamino)-(4’-carboxy-3′-oxy)azobenzene.

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Both the polymers reveal a light-induced anisotropy due to trans-cis isomerisation of the azobenzene groups [13]. Co-free polymer, the azobenzene groups have some rotational degrees of freedom. In this case the linearly-polarized light irradiation results in an anisotropy axis due to consequent trans-cis-trans- isomerisation of the azobenzene groups that leads to their reorientation perpendicular to the polarization of light [14,15]. At the same time, the rotation of the azobenzene groups in the Co-containing polymer is difficult. In this case angular hole burning due to trans-cis isomerisation without rotation of azobenzene groups is the most probable mechanism of the light-induced anisotropy [14,15].

In addition to the light-induced anisotropy, both polymers reveal an electrically-induced anisotropy [9,10,12]. This anisotropy was found at the application of the electric field E≈1*10⁸ V/m perpendicular to the polymer film by a crown discharge. The authors of [10] explained this effect by the orientation of the permanent dipoles of the side-chains fragments in the electric field. The electrically-induced anisotropy of the Co-containing polymer was much larger than of the Co-free polymer. Analyses of the equilibrium permanent dipoles configuration of the side-chain fragments showed that the higher sensitivity of the Co-containing polymer was due to a break symmetry of the quasi-planar Co-free polymer structure by Co⁺² ion that results in significant increase of the permanent dipole moment.

In the experiments [9,10,12] the electrically-induced anisotropy axis was formed perpendicular to the plane of the polymer film but to get a planar or tilted LC alignment, anisotropy in the plane of the polymer is required. To know if it is possible to form electro-induced anisotropy in the plane of the polymer film, the droplet of the saturated solution of the polymer was deposited onto the glass substrate. The parallel strip-shaped copper electrodes were bonded to the surface of the substrate. The width of the strips and the gap between the strips was 2 mm. The solution was homogeneously distributed by a blade over the substrate and left covered for two days for evaporation of the solvent at room temperature. Observation in a Linnik interferometer showed that the resulting polymer films had a thickness, 1.8 ± 0.1 μm.

To induce the anisotropy by an electric field, ${\vec{E}}_{d c}$ , the dc-voltage in the range 0-500 V was applied to the electrodes. This voltage corresponded to the field, ${\vec{E}}_{d c}$ = 0,25 V/μm. A beam of the diode-pumped solid state laser (wavelength, λ = 532nm, intensity, I₀ ≈1.5 W/cm², beam waist, w ≈300µm) was used to induce the anisotropy by light.

The birefringence of the polymer films was measured by Senarmont interferometer technique [16,17]. In this technique, the elliptically polarized light behind a weakly anisotropic sample is converted to the linearly polarized light with a λ/4-plate. The polarization plane of output light was rotated with respect to the polarization plane of the original beam by the angle α that proportional to the phase retardation in the sample.

Δ φ = Δ n d = \frac{λ}{π} α

For the precise measurement of the angle α, the output beam from the λ/4-plate passed through high quality analyzer, which rotated by a step-motor (accuracy of the positioning 10⁻⁴ deg., rotation step 0.1°, rotation velocity 0.1 deg/s), and the dependence of the light intensity behind the analyzer on the angle θ between the transparency axis of the analyzer and the plane of the incident light, I_an,out(θ) was measured. The time needed obtaining the I_an,out(θ)-curve was about 100 s. The minimum of the dependence I_an,out(θ) determined the angle α. Numerical fitting of the I_an,out(θ)-curve allowed to measure α with the accuracy 0.1°. This corresponded to the measurement of the phase retardation with the accuracy 6*10⁻³ nm and the birefringence of the order of 10⁻⁵.

The test of the produced polymer films showed that both metal-containing and metal free polymer films were initially weakly anisotropic; the background birefringence of the films was of the order of Δn≈(5-10)*10⁻⁵ and together with the direction of the anisotropy axis depended on where on the film. Presumably, the background birefringence was due to orientation of the polymer chains during evaporation of the solvent at the film formation.

Electro–induced anisotropy

We studied the electrically-induced anisotropy in the polymer films applying a relatively weak field E_dc = 0.25 V/µm in the plane of the film at room temperature, at which the polymers were in the solid state. These experimental conditions are not-traditional. To induce anisotropy, usually a polymer film is heated above the glass transition temperature and poled by the corona discharge while cooling the polymer in the solid state. In this case the anisotropy appears in the direction perpendicular to the polymer plane due to the huge field in the corona discharge [10].

Application of the dc-voltage in the plane of the metal-containing film caused producing the anisotropy axes along the dc-field, ${\vec{E}}_{d c}$ , and the additional birefringence of the film. The dependence of the additional birefringence on the time of the application of the voltage U = 500 V, Δn_E (t), is presented in Fig. 2. After the beginning jump of Δn_E within first 100 s we observed a further small growth of birefringence in 10³ s time scale. The maximal achieved additional birefringence Δn_E ≈4*10⁻⁵. The electro-induced anisotropy was stable in time; it kept its value after the switching the electric field off.

Fig. 2 Dependence of birefringence of Co-containing film on time in presence of electric field. Upper plot: dependence of applied voltage on time. Bottom plot: dependence of electrically-induced birefringence on time.

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The same experiments with Co-free azopolymer have not revealed any electrically induced anisotropy. Therefore, it is the presence of cobalt causes sensitivity of the polymer to electric field. According to [10], orientation of the permanent dipoles of the Co-containing polymer along the electric field is responsible for the anisotropy in the dc-field.

Photo-induced anisotropy

To induce the anisotropy in Co-containing azopolymer films, we irradiated them with a linearly polarized Gaussian beam of diode-pumped solid state laser (I ≈1.52 W/cm²), which wavelength λ = 532 nm was around the maxima of the absorption bands of both polymers. Such irradiation resulted in the light-induced anisotropy axis perpendicular to the polarization of the light, ${\vec{E}}_{h ν}$ . The maximal value of the light-induced birefringence, Δn_hν ≈2.5*10⁻⁴ was in almost one order larger than the maximal value of the electrically-induced birefringence, Δn_E ≈8.4*10⁻⁵_. The dynamic Δn_hν(t) is presented in Fig. 3. One can see that the light-induced birefringence does not relax completely after switching the light-off and the permanent birefringence is formed. This effect was observed earlier by Puchkovs’ka et.al [11] and it was explained by orientational ordering of the main polymer chains during the reorientation of trans-isomers. At the randomization of trans-isomers after switching the light off, the heavy and long main chains keep their induced orientation that results in permanent anisotropy of the film.

Fig. 3 Dependence of birefringence of Co-containing film on time in presence of light. Upper plot: dependence of incident light intensity on time. Bottom plot: dependence of birefringence of the Co-containing film on the time.

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Combination of the electric field and the light field

We irradiated the Co-containing film with linearly polarized light, ${\vec{E}}_{h ν}$ , in the presence of the dc electric field, ${\vec{E}}_{d c}$ , directed perpendicular to the light polarization. The directions of light-induced anisotropy axis and electrically-induced anisotropy axis coincided in this case. It was found that the combination of the electric field, ${\vec{E}}_{d c}$ and the light field, ${\vec{E}}_{h ν}$ , led to amplification of the final birefringence in this geometry. The dependencies of the induced birefringence on time at the irradiation of the polymer film by light only and at the combined action of light and electric field is presented at Fig. 4. One can see that the addition of the electric field results in the increase of the final birefringence on ≈23%. It should be noted this value is larger than the increase on 12% expected at the independent action of the light field and electric field. We presume that influence of trans-cis isomerisation, which gains the rotation mobility of the side azobenzene fragments [14], on the value and the direction of the permanent electric dipoles, is responsible for this effect. Increase of the rotation mobility facilitates the electrically-induced rotation of permanent electric dipoles of the azo-fragments. Therefore the induced birefringence at the combined action of the electric and light fields is larger than the induced birefringence at the separation action of these fields.

Fig. 4 Dependence of birefringence of Co-containing film on time in presence of light and electric field. Upper two plots: dependencies of incident light intensity and electric field on time. Bottom plot: dependencies of birefringence of the Co-containing film on time; red circles; ${\vec{E}}_{d c}$ = 0.25 V/ μm, I = 1.4 W/cm², black squares; ${\vec{E}}_{d c}$ = 0, I = 1.4 W/cm², blue triangulars; ${\vec{E}}_{d c}$ = 0.25 V/ μm. The experimental errors coincide with the sizes of the experimental points on the plots.

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The effect of the electric field on the light-induced anisotropy of the Co-containing polymer is clearly demonstrated by the electrically-induced erase of the permanent component of the light-induced birefringence. After relaxation of the light-induced birefringence to the permanent state (see Fig. 3) we applied the field E_dc = 0.25 V/µm at 45° to the polarization of light, ${\vec{E}}_{d c}$ . It resulted in decreasing of the permanent component of the light-induced birefringence (Fig. 5).

Fig. 5 Dependence of the stationary birefringence of the film after application of the electric field perpendicularly photo-induced anisotropy axis on the value of the electric field.

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Alignment of liquid crystals

The photoalignment of the LC in the presence of the electric field was studied in combined LC cells made from the tested substrate and the reference substrate. The tested substrate consisted of a pair of the parallel ITO electrodes with the distance 2 mm in between. The substrate was covered with the Co-containing polymer layers above the electrodes. The reference substrate was covered with unidirectionally rubbed polyimide Kapton. The thickness of the cell, L ≈20 µm, was given by polymer spacers. The tested polymer layer and the polyimide layer provided a planar alignment with a small pretilt (≤ 1°) for the LC 4-pentyl–4’cyano–biphenyl (5CB, Merck).

Two areas of the tested substrates were irradiated by the Gaussian laser beam (λ = 532 nm, I≈1.25 W/cm²); one area was located on the electrode strip and was not affected by the electric field, and the other one was disposed between the electrodes. The field E_dc = 0.25 V/μm was applied either without or after the light irradiation.

The geometry of the experiment is presented in Fig. 6. The direction of rubbing of the reference substrate was parallel to the electrodes. This direction determined the initial orientation of the director, $\vec{n}$ in the cell. The polarization of the laser beam, ${\vec{E}}_{h ν}$ , made an angle α = 45° + π/2 with respect to the director, $\vec{n}$ , on the reference surface. The light-induced anisotropy axis, ${\vec{e}}_{h ν}$ (ψ_hν) was perpendicular to ${\vec{E}}_{h ν}$ , i.e. the angle ψ_hν = 45°. The direction of the electrically-induced anisotropy axis, ${\vec{e}}_{E}$ (ψ_E) was parallel to ${\vec{E}}_{d c}$ , i.e. perpendicular to the electrodes, ψ_E = 90°.

Fig. 6 Geometry of the LC alignment experiment.

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After the exposition by light and electric field the cells were filled with LC 5CB in the isotropic phase. The cell were placed on a hot metal block with the tested substrates facing up, and then slowly cooled to the room temperature over approximately 30 min. For these conditions the transition to the nematic phase started from the test substrate improved the quality of the LC alignment [18].

The alignment of the LC was examined in a polarizing microscope. The director twist structures were found in the irradiated areas both in the electrode region and in the area between the electrodes. Analysis of the twist structures (both on and between the electrodes) showed that the director; was parallel to the rubbing direction on the reference surface and deviated from this dire;ction on the test surface on the twist angle. The twist angle was maximal at the center of the irradiated spot and decreased to the periphery according to the Gaussian intensity distribution of the beam. Depending on the specific experiment conditions the following results were obtained.

The electric field only was applied to the substrate – no light irradiation

Inhomogeneous twist structure was observed between the electrodes only. The electrically-induced twist from the initial direction of the director was small but could be reliably detected; at the center of the irradiated area α = 3.5° ± 0.5° The formation of the twist structure points at the formation of an easy orientation axis, ${\vec{e}}_{E}$ (ψ_E), on the polymer surface. This axis coincides with the electrically induced anisotropy axis, which is parallel to E_dc and perpendicular to the initial direction of the director. The fact that the electrically-induced twist α << 90°, points at a weak anchoring of the LC with the substrate. According to [19], the twist angle, α is connected with the anchoring energy, W of the director on the aligning surface by the equation:

W = \frac{K_{22}}{L} \frac{2 \sin α}{\sin 2 (ψ - α)},

where K₂₂ is the twist elastic constant, ψ is the easy orientation axis angle that counts from the director. For our experimental conditions (K₂₂ = 5∙10⁻¹² N, L = 20 μm, α = 3.5°_, ψ = ψ_E = 90°), th1e electrically-induced anchoring energy, W_E = (2.5 ± 2) ∙10⁻⁷ J/m².

The substrate was irradiated by the light – no electric field was applied (Fig. 2(b)).

The irradiation resulted in the reliable homogeneous structures in the irradiated areas. The value of the twist angle increased with the increase of the light intensity and saturated at the angle α ≈37° at I = 1.25 W/cm² and exposition t = 120 s. According to (2), where $ψ_{n}$ = $ψ_{h ν}$ = 45°; α = 37°, the light-induced anchoring energy, W_hν = (1.1 ± 0.2)∙10⁻⁶ J/m². This value is typical for azo-polymer photoaligning materials [20].

The electric field followed by the light irradiation

The photos of the irradiated spots are presented in Fig. 7. The top photo (a) was taken after the light irradiation and the bottom photo (b) – after the irradiation at I = 1.25 W/cm² during 120 s and the following application of the electric field (E = 0.25 V/µm). The photos made at the positions of polarizers corresponded to the maximum darkness of the irradiated area. One can see the evident difference between two spots that point at the change of the light-induced twist angle after the application of the electric field. The analysis of the twist structures in the polarizing microscope showed that the light-induced twist angle at the center of the spot, α = 37° decreased to α = (28 ± 1)^o after application of the electric field despite the electrically-induced easy axis “encourages” to increase the twist angle.

Fig. 7 a) Photo of the irradiated area before application of the electric field; the angle between the polarizer and analyzer corresponds minimum intensity of the irradiated area. b) The same irradiated area after application of the electric field. c) Graphic representation of the complex anchoring energy of the LC induced by the light irradiation and tuned by the electric field.

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To understand the decrease of the twist after the application of the electric field, we considered formation of the anchoring of LC while the light and electric treatment in terms of the complex anchoring energy [21]. According to the complex representation, the complex anchoring energy is defined as ; ${\tilde{W}}_{a} = W_{a} e^{i 2 ψ}$ where $W_{a}$ is the azimuthal anchoring energy, ψ is the azimuthal angle of the easy axis. If a surface is exposed by subsequent treatments the resulting anchoring is determined by the sum of the partial complex anchoring: $\tilde{W_{a}} = \sum_{i} W_{a}^{(i)} e^{i 2 ψ_{i}}$ where $ψ_{i}$ is the direction of the easy axis ψ_i of i-th treatment, and $W_{a}^{(i)}$ is the corresponding anchoring energy.

One can visualize different surface treatments graphically presenting $\tilde{W_{a}}$ by vectors in the complex plane. For our experiment geometry the summarized action of the light- and electric treatment is shown in Fig. 7(c). These treatments forms the resulting anchoring

{\tilde{W}}_{a} = W_{a, h ν} e^{i 2 ψ_{h ν}} + W_{a, E} e^{i 2 ψ_{E}} .

As was shown above, the anchoring energy $W {}_{a, h ν}$ = 1.1∙10⁻⁶ J/m² and $W_{a, E}$ = 2.5∙10⁻⁷ J/m² at the separation actions of the light and the electric field. As was shown above, the application of the electric field after the light irradiation leads to decrease of the light-induced birefringence of the film (Fig. 5). We found that the birefringence decreased in 0.55 times after the irradiation at I = 1.25 W/cm² during 120 s and the following application of the electric field E = 0.25 V/µm during 600 s. Suggesting that the anchoring energy is proportional to the birefringence, one can estimate $W {}_{a, h ν}$ = 7∙10⁻⁷ J/m² after the application of the electric field. In this case, taking into account that $ψ_{h ν}$ = 45° and $ψ_{E}$ = 90°, the summarized easy axis reads:

{\tilde{W}}_{a, h ν + E} = W_{a, h ν + E} e^{i 2 ψ_{h ν + E}},

where the easy orientation axes after the combination treatment,

ψ_{h ν + E}

= 50° with the anchoring energy

W_{a, h ν + E}

= 6∙10⁻⁷ J/m². According to (2), the twist of the director α = 30° at these values

ψ_{h ν + E}

and

W_{a, h ν + E}

, which is close to the experimental number α = (28 ± 1)^o . Thus, the twist angle α decreases after the electric treatment despite the increase of the easy axis angle

ψ_{h ν + E}

= 50° in compare with initial easy axis angle

ψ_{h ν}

= 45°. This is caused by the reduction of the light-induced anchoring energy after the application of the electric field.

Taking into account that initial direction of the light-induced easy axis $ψ_{h ν}$ = 45° and the initial anchoring energy W_hν = 1.1∙10⁻⁶ J/m², the efficient tuning of the light-induced anchoring energy and the easy axis by the electric field is obtained.

3. Conclusions

In summary, electrically controlled photoalignment of the liquid crystal on the metal-containing azo–polymer is obtained. It is found that in addition to anisotropy caused by irradiation with polarized light, application of dc–electric field to the polymer film leads to its electrically-induced anisotropy even at room temperature. Therefore, the metal-containing azo–polymer is a unique material, in which both photo- and electro- alignment of liquid crystals are possible. Moreover, application of the electric field that was previously irradiated with polarized light causes changes of the light-induced anisotropy of the material. As a result, combination of the light and electric treatment allows the effective tuning of the light-induced easy orientation axis and anchoring energy of a liquid crystal by electric field on the polymer surface.

References and links

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Electro– and photosensitive azopolymer for alignment of liquid crystals

Abstract

1. Introduction

2. Experiment and discussion

Electro–induced anisotropy

Photo-induced anisotropy

Combination of the electric field and the light field

Alignment of liquid crystals

The electric field only was applied to the substrate – no light irradiation

The electric field followed by the light irradiation

3. Conclusions

References and links

Cited By

Figures (7)

Equations (4)

Optics Express