Photorefractive Effect of Polymer-Stabilized Ferroelectric Liquid

5792

J. Phys. Chem. C 2009, 113, 5792–5798

Photorefractive Effect of Polymer-Stabilized Ferroelectric Liquid Crystals Takeo Sasaki* and Yukihito Nakazawa Department of Chemistry, Faculty of Science, Tokyo UniVersity of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan ReceiVed: October 08, 2008; ReVised Manuscript ReceiVed: February 24, 2009

The photorefractive effect of polymer-stabilized ferroelectric liquid crystals (FLC) was investigated. Twotypes of monomers, a liquid crystalline monomer and a photoconductive monomer, were mixed with an FLC and the mixtures were photopolymerized in the ferroelectric phase. The polymer-network was formed in the FLC medium and a polymer-stabilized FLC was obtained. The polymer-stabilized FLC exhibited monostable switching properties. The photorefractive effect was evaluated by a two-beam coupling experiment. It was found that the photorefractive effect was strongly dominated by the preparation condition of the polymerstabilized FLCs. Introduction The photorefractive effect is a phenomenon wherein the refractive index of a material is modulated by the illumination of light.1-4 When laser beams interfere in a photorefractive material, charge separation occurs between the light and the dark positions of the interference fringe. A space-charge field (internal electric field) is built at the area between the light and the dark positions. The refractive index of the corresponding area is changed through electrooptic effects. Thus, a refractive index grating is formed at the interference fringe. This phenomenon enables us to create various types of photonic applications. The photorefractive effect in ferroelectric liquid crystals (FLCs) has been previously investigated.5-10 In FLCs, the spontaneous polarization responds to the internal electric field and a refractive index grating is formed. The response of the FLC to the applied electric field is fast, and a refractive index grating formation time of 10-20 ms was obtained in FLCs.7,10 FLCs are liquid crystals that form the chiral smectic C phase (SmC*).11 The SmC* possesses a helical structure so that the SmC* phase itself does not exhibit ferroelectricity. However, when the FLC is sandwiched between glass plates to form a film thickness of 2-10 µm, the helical structure of the SmC* phase uncoils and forms a surface-stabilized state. It is to be noted that the ferroelectricity of FLCs only appears in the surface-stabilized state. The preparation of a defect-free surface-stabilized state is difficult and requires sophisticated techniques. The presence of a defect leads to a smaller photorefractive effect.7 It has been reported that a defect-free FLC material was obtained in a polymer-stabilized FLC (PS-FLC).12-15 The PS-FLC materials were prepared by photopolymerization of a mixture of FLC, LC monomer, and photoinitiator at a temperature where the FLC material is in a SmC* phase, with the application of a monopolar electric field.12-15 The polymer-stabilized FLCs exhibit unique electro-optical properties. The direction of the spontaneous polarization is stabilized to a specific direction through interaction between FLC molecules and the polymer network. In this study, the photorefractive effect of the polymerstabilized FLC was investigated. The LC monomer and pho* To whom correspondence should be addressed. E-mail: sasaki@ rs.kagu.tus.ac.jp; tel.: +81-3-5228-8277; fax: +81-3-3225-2214.

Figure 1. Structures of the low-molecular-weight photoconductive compound (CDH), LC monomer mixture (UCL-001), photoconductive monomer (ECMHBP), and photosensitizer (TNF) used in the study.

toconductive monomer were mixed with FLC and photopolymerized. The effect of the polymer stabilization on the photorefractive effect was investigated. Experimental Section Materials. A commercially available FLC (FELIX-M4851/ 050, Clariant Co., Ps ) -14 nC/cm2, SmC* 65 SmA 70, N* 73 I (°C)) was used in this study. The structures of a low-molecular-weight photoconductive compound (CDH), a photoconductive monomer (ECMHBP), and a photosensitizer (TNF) are shown in Figure 1. A monoacrylate LC monomer UCL-001 (N 41 I (°C)) was obtained from DIC. IRGACURE369 and IRGACURE-651 (Chiba Specialty Chem.) were used

10.1021/jp808925g CCC: $40.75  2009 American Chemical Society Published on Web 03/19/2009

Polymer-Stabilized Ferroelectric Liquid Crystals

Figure 2. Schematic illustration of the beam incidence condition and the definition of the direction of the externally applied electric field.

as photoinitiators. The PS-FLC materials doped with a lowmolecular-weight photoconductive compound were prepared as follows. A mixture of FLC, LC monomer (2-10 wt%), photoinitiator (1 wt%), CDH (1 wt%), and TNF (0.1 wt%) was injected into a 5 µm gap glass cell equipped with an ITO electrode and a polyimide alignment layer. In order to form a highly homogeneous surface-stabilized state, the samples were heated to the isotropic phase and deliberately cooled to the SmC* phase at a rate of 0.1 °C/min using a hot plate (Mettler FP-80 and FP-82). The monomer/FLC mixture was then photopolymerized. Light (365 nm) from an ultrahigh pressure mercury lamp (10 mW/cm2) was irradiated onto the sample for 5 min. An electric field of -2.0 V/µm was applied to the mixture during the photopolymerization. The polarity of the electric field is defined as shown in Figure 2. The FLC materials polymerstabilized by the photoconductive monomer were also prepared using the same conditions as described above. Measurements. The photopolymerization of the monomer in the FLC medium was confirmed by gel-permeation chromatography (GPC, Tosoh Co., HLC-8020, SD-8000, AS-8010, and UV-8010). The textures of the FLC mixtures were observed by a polarizing optical microscope (POM; Olympus, BX-50, with Mettler, FP-80 and FP-82 hot stage). Spontaneous polarization (Ps) was measured by a triangular waveform voltage method (10 Vp-p, 100 Hz). The electro-optical hysteresis of the FLC sample was measured by a polarizing optical microscope with the application of 20 Hz, 20 Vp-p triangular waveform voltage. The photorefractivity was measured using a two-beam coupling experiment. The two-beam coupling experiment was performed by a p-polarized Ar+ laser (Laser Graphics, 165LGS-S, 488 nm, continuous wave, 2.5 mW, 1 mm diameter, probe pump beam intensity was 1:1). The orientation of the rubbing direction and the beam incidence plane are shown in Figure 2. The incident beam angles to the glass plane were 40° and 60°. An interval of the interference fringe was 1.87 µm. The photorefractive effect was measured with dc voltage applied to the sample in order to increase the charge separation efficiency. Results and Discussion PS-FLC Material with LC Monomer. The FLC mixtures containing the LC monomer UCL-001 (2-10 wt%), the IRGACURE-369 photoinitiator (1 wt%), the CDH photoconductive compound (1 wt%), and TNF (0.1 wt%) were photopolymerized in a 5 µm gap cell. The conversion of the monomer into a polymer as a function of the monomer concentration as measured by GPC is shown in Figure 3. An LC monomer concentration higher than 2 wt% was necessary for photopolymerization in the FLC (SmC*phase) under the irradiation conditions of this study. The low conversion of the monomer

J. Phys. Chem. C, Vol. 113, No. 14, 2009 5793

Figure 3. Conversion of the LC monomer UCL-001 mixed with FLC (FELIX-M4851/050), CDH (1 wt%), TNF (0.1 wt%), and a photoinitiator (1 wt%). IRGACURE-369 was used as an initiator. The conversions were measured after 366 nm irradiation (10 mW/cm2) for 5 min.

Figure 4. Polarizing microscope photographs of the FLC/CDH/TNF mixture (a), FLC/CDH/TNF/LC monomer mixture before (b and c) and after (d and e) photopolymerization: (b, d): 5 wt%, and (c, e): 10 wt% of LC monomer mixed with FLC.

was considered to be caused by the high viscosity of the SmC* phase and light absorption of CDH. A few defects were observed in the FLC doped with 10 wt% LC monomer before photopolymerization (Figure 4). After the UV irradiation, a striped texture appeared along the rubbing direction. This striped texture was not observed in the FLC mixture without the LC monomer after UV light irradiation. Thus, the striped texture indicates the formation of the polymer. After photopolymerization, a part of the LC polymer was excluded from the FLC. The FLC doped with 10 wt% of the LC monomer after irradiation (10 wt% PSFLC) strongly scattered light, so that it was impossible to measure the photorefractive effect in this case. The temperature dependence of the spontaneous polarization (Ps) and that of the risetime in the electro-optical switching of the FLC/CDH/TNF/LC-monomer mixtures before and after photopolymerization as well as those of the FLC/CDH/TNF mixture are shown in Figure 5. The magnitude of Ps of the

5794 J. Phys. Chem. C, Vol. 113, No. 14, 2009

Figure 5. Temperature dependence of spontaneous polarization (a) and risetime in electro-optical switching (b), of the FLC/CDH/TNF mixture without the LC monomer (0), FLC/CDH/TNF/LC-monomer mixture before (4) and after (2) photopolymerization. The concentration of the LC monomer was 5 wt%. Ps was measured with 100 Hz, 5 Vp-p/µm triangular waveform field. Electro-optical switching was measured with 100 Hz, 5 Vp-p/µm square waveform field. The risetime is defined as the time required to change the transmittance from 10% to 90% of the maximum value.

FLC/CDH/TNF/LC-monomer mixture increased after the photopolymerization. The risetime of the FLC/CDH/TNF/LCmonomer (5 wt%) was longer than that of the FLC/CDH/TNF mixture and was almost independent of temperature. However, the temperature dependence of the risetime of the identical mixture after photopolymerization was almost the same as that of the FLC/CDH/TNF mixture. The SmC* to SmA phase transition temperature increased after photopolymerization. These results indicate that the LC polymer formed in the LC phase was excluded from FLC mixture. The electro-optical hysteresis of the FLC/CDH/TNF/LCmonomer mixture before and after photopolymerization was measured. Since the FLC exhibits a bistable state, the electrooptical response of the FLC shows a hysteresis curve (optical hysteresis curve). The electro-optical hysteresis of the FLC/LC monomer mixture before and after UV irradiation was measured using a polarizing microscope (Figure 6). When the FLC is completely bistable, the center of the optical hysteresis curve is 0 V. The deviation of the center of the hysteresis curve from 0 V indicates that the FLC exhibits a monostable state.12-15 In the monostable state, the direction of the FLC molecules is stabilized in one direction. The FLC/CDH/TNF/LC-monomer mixtures were photopolymerized with application of a -2.0 V/µm DC electric field. After photopolymerization, the center of the hysteresis curve was shifted from 0 V to a positive voltage. This indicates that the FLC/CDH/TNF/LC-monomer mixture exhibits a monostable state. The direction of the Ps (and the alignment of the FLC molecules) was stabilized to the direction of the electric field applied during the photopolymerization. The monostability of FLC molecules is considered

Sasaki and Nakazawa

Figure 6. Optical hysteresis of FLC/CDH/TNF/LC monomer mixture before (a) and after (b) photopolymerization. The concentration of the LC monomer was 5 wt%. Light transmitted through the parallel polarizers without the ITO glass cell was defined as 100% transmittance.

Figure 7. Example of two-beam coupling signal in PS-FLC with LC monomer. The concentration of the LC monomer was 5 wt%. The sample temperature was 30 °C, and an electric field of -3.6 V/µm was applied.

to be produced by the strong interaction between the FLC molecules and the mesogenic part of the polymer.12 The photorefractivity of the polymer-stabilized FLC was investigated. An example of a two-beam coupling signal observed in the 5 wt% PS-FLC sample is shown in Figure 7. The transmitted intensity of one of the interfering beams in the sample increased and that of the other decreased. An asymmetric energy exchange was observed only when an external electric field was applied, indicating that beam coupling was not caused by a thermal grating. In order to calculate the two-beam coupling gain coefficient, it must be determined whether the diffraction condition is in the Bragg regime or in Raman-Nath regime. These diffraction conditions are distinguished by a dimensionless parameter Q.1

Q)

2πλL nΛ2

(1)



Figure 9. Polarizing microscope photographs of FLC/ECMHBP/TNF after photopolymerization in a 5 µm gap cell: (a) with application of negative electric field, and (b) with application of positive electric field. The concentration of ECMHBP was 5 wt%.

Figure 8. Electric field dependence of the gain coefficient and formation time of the FLC/CDH/TNF mixture and PS-FLC with the LC monomer. The concentration of the LC monomer was 5 wt%.

When Q > 1, it is defined as the Bragg regime of optical diffraction. In this regime, multiple scattering is not permitted and only one order of diffraction of light is produced. Conversely, when Q < 1, it is defined as the Raman-Nath regime of optical diffraction. In this regime, many orders of diffraction can be observed. Usually Q > 10 is required to guarantee that the diffraction is entirely in the Bragg regime. In the present experimental condition, Q is calculated to be 2.9-5.8 (L: interaction length ) 5.5-10.9 µm), so that the diffraction observed in this experiment is predominantly but not entirely in the Bragg regime, and there is partial contribution from Raman-Nath type diffraction. However, since we could not observe higher order diffraction, the two-beam coupling gain coefficient Γ was calculated assuming Bragg diffraction, as:2-4

Γ)

gm 1 ln D 1+m-g

(

)

(2)

where D ) L/cos(θ) is the interaction path for the signal beam (L ) sample thickness, θ ) propagation angle of the signal beam in the sample), g is the ratio of the intensities of the signal beam behind the sample with and without a pump beam, and m is the ratio of the beam intensities (pump/signal) in front of the sample. The formation time of the photorefractive effect is calculated as:

γ(t) - 1 ) (γ - 1)[1 - exp(-t/τ)]2

Figure 10. Temperature dependence of spontaneous polarization (a), risetime in electro-optical switching (b), and voltage dependence of the switching angle (c) of the FLC/ECMHBP/TNF mixture in the 5 µm gap cell before and after photopolymerization. The concentration of ECMHBP was 5 wt%. The risetime is defined as the time required to change the transmittance from 10% to 90% of the maximum value.

(3)

where γ(t) represents the transmitted beam intensity at time t divided by the initial intensity (γ(t) ) I(t)/I0), and τ is the formation time. The gain coefficient and the formation time of the 5 wt% PS-FLC sample were compared with those of the FLC/CDH/ TNF mixture (Figure 8). The gain coefficient of the photopolymerized sample was smaller than that of the nonpolymerized sample. The formation time of the PS-FLC was much longer than that of FLC/CDH/TNF. Considering that the electro-optical

response of PS-FLC was almost the same as that of the FLC/ CDH/TNF mixture, it is likely that the polymer network reduced the charge separation efficiency and the internal electric field. PS-FLC Material with Photoconductive Monomer. When the LC monomer in a FLC mixture is photopolymerized in the SmC* phase with the application of an electric field, the monostable state is obtained. The FLC mixed with a photoconductive monomer ECMHBP (5 wt%), a photoinitiator IRGACURE-651 (1 wt%), and TNF (0.1 wt%) was injected into an ITO glass cell and photopolymerized under application of a -2.0


Sasaki and Nakazawa

Figure 11. Optical hysteresis of the FLC/ECMHBP/TNF mixture before and after photopolymerization: (a, b) measured in the 5 µm-gap cell, and (c, d) measured in the 10 µm gap cell. The concentration of ECMHBP was 5 wt%.

Figure 12. Examples of two-beam coupling of the FLC/ECMHBP/TNF mixture before photopolymerization with application of -1.0 V/µm and +1.0 V/µm electric fields: (a, b) measured in the 5 µm-gap cell, and (c, d) measured in the 10 µm gap cell. The concentration of ECMHBP was 5 wt%.

V/µm electric field. Polarizing microscope photographs of the FLC mixture doped with 5 wt% ECMHBP after photopolymerization is shown in Figure 9a. No defects were observed even after photopolymerization. This indicates that the refractive index of the formed photoconductive polymer was the same as that of the FLC. However, many stripes along the rubbing direction appeared when the direction of the electric field was reversed to the positive direction (Figure 9b). The temperature dependence of Ps, the risetime in electro-optical switching, and the voltage dependence of the switching angle are shown in Figure 10. The Ps and the risetime of the FLC/ECMHBP/TNF

mixture did not change after photopolymerization. That was different from the results for the LC-polymer (Figure 5). It is considered that the interaction between the photoconductive polymer and FLC is weaker than that between the LC polymer and FLC. The switching angle of the photopolymerized FLC/ ECMHBP/TNF mixture was smaller than that of the identical mixture before photopolymerization. The FLC molecules did not respond to a low electric field. The formed polymer does not respond to the electric field. Thus, the response of the FLC molecules to an electric field was prevented by the polymer network formed in the FLC.



Figure 13. Examples of two-beam coupling of the FLC/ECMHBP/TNF mixture after photopolymerization with application of -1.0 V/µm and +1.0 V/µm electric fields: (a, b) measured in the 5 µm-gap cell, and (c, d) measured in the 10 µm gap cell. The concentration of ECMHBP was 5 wt%.

Figure 14. Electric field dependence of the gain coefficients and the formation times of the FLC/ECMHBP/TNF mixture before and after photopolymerization: (a, b) measured in the 5 µm-gap cell, and (c, d) measured in the 10 µm gap cell. The concentration of ECMHBP was 5 wt%.

The electro-optical hysteresis behavior in the FLC/ECMHBP/ TNF samples are shown in Figure 11. An electric field of -2.0 V/µm was applied to the mixture, which was then photopolymerized. The center of the hysteresis curve of the FLC/ ECMHBP/TNF mixture was shifted from 0 V to a positive voltage after photopolymerization. This indicates that the direction of the Ps was stabilized to the polarity of the electric field applied during the photopolymerization. The positive

polarity of the Ps is unstable in this case. Without an external electric field, Ps shows a negative polarity. The photorefractivity of the FLC/ECMHBP/TNF mixture before and after photopolymerization was investigated. Examples of two-beam coupling signals of the FLC/ECMHBP/ TNF mixture before and after photopolymerization are shown in Figures 12 and 13, respectively. Regardless of the polarity of the applied electric field, the photorefractive effect appeared


Figure 15. Electric field dependence of the gain coefficient and formation time of the FLC/CDH/TNF mixture and the photoconductivepolymer-stabilized FLC (5 wt% ECMHBP). The concentration of the CDH was 2 wt%, and TNF was 0.5 wt%. The sample temperature was 30 °C.

in the thin 5 µm gap cell. However, in the 10 µm gap cell, asymmetric energy exchange was not observed before photopolymerization. Asymmetric energy exchange was observed in the 10 µm photopolymerized samples under application of a negative voltage, which had the same polarity as the electric field applied during the photopolymerization. In the 10 µm gap cell, the applied voltage produces defects that cause light scattering. The light scattering precludes the formation of a refractive index grating. The change in transmitted intensity of the laser beams in Figure 13d (and Figure 12c,d) is caused by the thermal effect. It is considered that in the photopolymerized sample, the generation of defects was prevented by the polymer network, which led to an appearance of the photorefractive effect. A monodomain texture was observed under a polarizing microscope with the application of a negative electric field in the 10 µm samples photopolymerized with application of a negative electric field. When a positive electric field was applied to the 10 µm sample which was photopolymerized with a negative electric field, many stripes appeared in the texture. The stripes scatter the laser beams and hinder the formation of the refractive index grating. The polymer network reflecting the structure of the LC phase under negative field was formed under the experimental conditions of this study, so that the polymer network optically appeared with application of a positive electric field. Thus, the two-beam coupling was only observed with the application of an electric field with the same polarity as the electric field applied during the photopolymerization. The electric field dependence of the gain coefficients, and the refractive index grating formation times of the FLC/ ECMHBP/TNF mixture before and after photopolymerization, were investigated (Figure 14). The gain coefficient of the 5 µm sample before photopolymerization decreased with increasing electric field as a result of the increased defects in the SS state. However, the gain coefficient did not decrease in the sample after photopolymerization. This indicates that the photoconductive polymer formed in the FLC reduced the generation of the defects. The gain coefficient of the photopolymerized sample increased with increasing electric field and reached a constant value with an electric field higher than 2.0 V/µm. The electric field dependence of the gain coefficients of the FLC/CDH/THF mixture and the photoconductive-polymer-stabilized FLC doped with 0.5 wt% TNF are shown in Figure 15. The gain coefficient of the FLC/CDH/TNF mixture decreased with the applied

Sasaki and Nakazawa electric field higher than 1 V/µm because of the increased defects formed in the surface-stabilized state. However, the gain coefficient of the photoconductive-polymer-stabilized FLC did not decrease with the applied field. A large gain coefficient of 50 cm-1 was obtained in the photoconductive-polymer-stabilized FLC. It has been reported that ionic conduction plays a major role in the formation of the space-charge field in the photorefractive effect of FLCs.10 When laser beams are illuminated onto the photoconductive-polymerstabilized sample, photoinduced charge transfer occurs between the ECMHBP chromophore and TNF. The mobility of the cation of the ECMHBP polymer is much smaller than that of the TNF anion, and this difference in mobility is thought to be the origin of the charge separation. In this case, a large internal electric field is induced and result in a large gain coefficient. The refractive index grating formation time of the FLC mixture after photopolymerization was longer than that of the mixture before photopolymerization. It was considered that the mobility of the FLC molecules was reduced by the polymer network and led to a slower response. Conclusions The photorefractive effect of polymer-stabilized FLCs were investigated. An LC monomer and a photoconductive monomer were mixed with the FLC and photopolymerized. The polymerstabilized FLC exhibited a monostable switching property. Asymmetric energy exchange was observed in the polymerstabilized FLC. The photorefractive effect was induced in the PS-FLC material only with the application of a DC electric field with the same polarity as the applied voltage during the photopolymerization. Acknowledgment. This work was supported by a Grant-inAid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, and Tokuyama Science Foundation. References and Notes (1) Yeh, P. Introduction to PhotorefractiVe Nonlinear Optics; John Wiley: New York, 1993. (2) Moerner, W. E.; Silence, S. M. Chem. ReV. 1994, 94, 127. (3) Solymar, L.; Webb, J. D.; Grunnet-Jepsen, A. The Physics and Applications of PhotorefractiVe; Oxford: New York, 1996. (4) Ostroverkhova, O.; Moerner, W. E. Chem. ReV. 2004, 104, 3267. (5) Wiederrecht, G. P.; Yoon, B. A.; Wasielewski, M. R. AdV. Mater. 2000, 12, 1533. (6) Sasaki, T.; Kino, Y.; Shibata, M.; Mizusaki, N.; Katsuragi, A.; Ishikawa, Y.; Yoshimi, T. Appl. Phys. Lett. 2001, 78, 4112. (7) Sasaki, T.; Katsuragi, A.; Mochizuki, O.; Nakazawa, Y. J. Phys. Chem. B 2003, 107, 7659. (8) Talarico, M.; Termine, R.; Prus, P.; Barberio, G.; Pucci, D.; Ghedini, M.; Goelemme, A. Mol. Cryst. Liq. Cryst. 2005, 429, 65. (9) Talarico, M.; Goelemme, A. Nat. Mater. 2006, 5, 185. (10) Sasaki, T.; Moriya, N.; Iwasaki, Y. J. Phys. Chem. C 2007, 111, 17646. (11) Skarp, K.; Handschy, M. A. Mol. Cryst. Liq. Cryst. 1988, 165, 439. (12) Furue, H.; Miyama, T.; Iimura, Y.; Hasebe, H.; Takatsu, H.; Kobayashi, S. Jpn. J. Appl. Phys. 1997, 36, L1517. (13) Shikada, M.; Tanaka, Y.; Xu, J.; Furuichi, K.; Hasebe, H.; Takatsu, H.; Kobayashi, S. Jpn. J. Appl. Phys. 2001, 40, 5008. (14) Fujikake, H.; Takizawa, K.; Kikuchi, H.; Fujii, T.; Kawakita, M.; Aida, T. Jpn. J. Appl. Phys. 1997, 36, 6449. (15) Fujikake, H.; Murashige, T.; Sato, H.; Fujisaki, Y.; Kawakita, M.; Kikuchi, H.; Kurita, T. Jpn. J. Appl. Phys. 2003, 42, L186. (16) Sasaki, T.; Katsuragi, A.; Ohno, K. J. Phys. Chem. B 2002, 106, 2520. (17) Nakazawa, Y.; Sasaki, T. Liq. Cryst. 2006, 33, 159.

JP808925G

Photorefractive Effect of Polymer-Stabilized Ferroelectric Liquid

Recommend Documents