Anchoring Effect on the Stability of a Cholesteric Liquid Crystal s Focal Conic Texture

Transcription

1 CHINESE JOURNAL OF PHYSICS VOL. 50, NO. 5 October 2012 Anchoring Effect on the Stability of a Cholesteric Liquid Crystal s Focal Conic Texture Tao Yu, 1, Lei Luo, 1, 2 Bo Feng, 1 and Xiaobing Shang 3 1 Department of Physics, Dalian Maritime University, Dalian, P. R. China 2 Department of Embedded System Engineering, Dalian Neusoft Institute of Information, Dalian, P. R. China 3 Tianma Micro-Electronics Co., Ltd., Shenzhen, P. R. China (Received February 25, 2011; Revised January 1, 2012) The relationship between the anchoring energy and stability of a cholesteric liquid crystal s focal conic texture was investigated in this paper. Both the polar anchoring energy and the transmittance of the focal conic textures were measured under various conditions. The experimental results indicate that the focal conic textures with large magnitudes of polar anchoring strength tend to be more stable in a wide temperature range. This means that the polar anchoring energy plays an important role in the thermodynamic stability of the focal conic textures. PACS numbers: Hn, e I. INTRODUCTION The cholesteric liquid crystal displays (Ch-LCDs) exhibit two stable states at zero electric field, i.e., the reflective planar (P) state with bright colors and the scattering focal conic (FC) state with dark backgrounds. The planar states can nearly reflect the entire visible wavelengths. Due to these unique characteristics, the Ch-LCDs demonstrate many excellent features, such as low power consumption, high contrast ratio and reflectivity, good color saturation, etc. [1, 2]. Hence, the reflective Ch-LC devices have great potential among the emerging e-papers, e-labels, signboards, and so on. When Hiji et al. studied the LC cell filled with the Ch-LC microcapsule droplets, they observed a phenomenon such that the contrast ratio was decreased a lot near the clear point, and the reason was that the FC texture tended to be transformed into the P texture [3]. This issue would severely hinder the large-scale applications of Ch-LCDs. In the past, many research works were focused on the effects of boundary conditions and temperatures on the threshold field and the opto-electrical properties of the cholesteric-nematic phases near the transition point [4, 5], however, studies of the anchoring effect on the stability of the FC texture were scarcely reported. In this paper, we first propose a new scheme to investigate the relationship between the anchoring energy and the stability of the FC textures. The scheme is illustrated as follows: the vertically aligned (VA) LC cell with two equally separate parts is fabricated, Electronic address: yutaorenhe@vip.sina.com c 2012 THE PHYSICAL SOCIETY OF THE REPUBLIC OF CHINA

2 VOL. 50 TAO YU, LEI LUO, BO FENG, ET AL. 805 then one of the two parts is filled with nematic liquid crystals (NLCs) and the other is filled with Ch-LCs. The polar anchoring strength and the stability of the FC textures are respectively evaluated on the two different parts of the same cell, and this procedure can ensure that the NLCs and Ch-LCs are under the same physical conditions for the device, such as the same surface boundary, cell thickness, etc. Based on the remarkable difference of the transmissions between the FC textures and P textures, the measurements of transmissions are adopted to characterize the stability of FC textures in a wide temperature range. Because the anchoring energy decreases with the increment of temperatures [6, 7], we investigated the stability of FC textures under different anchoring conditions created by the heating process, as well as the method of the rubbing/non-rubbing process. II. EXPERIMENT II-1. Material and Equipment The relevant materials used in the experiments are listed as follows: the cholesteric liquid crystal is formed by mixing the negative nematic LC (HCCH , Jiangsu Hecheng Chemical Materials Co., Ltd.) with the chiral dopant R-1011 and TBAB (tetra-n-butylammonium bromide), and the proportion by weight is HCCH /R- 1011/TBAB = 97.5%/2.47%/0.03%. The pitch of the Ch-LCs is about 1.25 µm at 25. Other materials include the glass substrates with indium tin oxide (ITO) layers (Anhui Bengbu Huayi Conductive Film Glass Co., Ltd.), homotropic polyimide 8088A (Daily Polymer Co.), UV curable adhesive NOA-65(Norland), and the spacers with the diameter of 10.5 µm (Nippon Electric Co.). The main experimental equipments comprise the rubbing machine (The Second Research Institute of China Electronics Technology Group Co.) and the LC parameters testing instrument (Instec Inc.). II-2. Device Fabrication The standard abluent, propanol, ethanol, and deionized water are used to wash the glass substrates followed by the brush and ultrasonic cleaning. After these procedures, the ITO patterns are fabricated by the photolithography method, then the polyamide acid of 8088A is spincoated on the glass substrate, and cured at the high temperature above 200 to form the homotropic polyimide (PI) layer. The PI layer is weakly rubbed by the rubbing machine. Two rectangular glue borders are formed by the screen printing process, and divide the whole cell into two equal parts with the same size 17 mm 11.5 mm, and spacer beads with the diameter of 10.5 µm are uniformly sprayed on the PI layer. Another glass substrate is assembled onto the one above with the press machine, and the glues were irradiated with an a1000 watt UV lamp, then an empty LC cell is finally completed (see Fig. 1). The NLCs and Ch-LCs are filled into the cell under capillary action.

3 806 ANCHORING EFFECT ON THE... VOL. 50 FIG. 1: The LC cell consists of two ITO glass plates coated with PI layers. Two rectangular glue borders are formed to divide the whole cell into two equal parts, which are filled with NLCs and Ch-LCs, respectively. II-3. Measurement of the Polar Anchoring Energy In our work, it is very difficult to measure the polar anchoring energy within the Ch- LC device, because of the complicated and extremely hard-to-get theoretical derivations. We propose the measurement of the polar anchoring energy of the NLC devices instead. This requires that both the NLCs and Ch-LCs are under the same device conditions, especially the same boundary conditions. The above II-2 mentioned scheme of the LC cell with two equal parts can meet the requirement. The NLCs and Ch-LCs are filled into the different parts of the same cell, respectively. Then the polar anchoring strength of the boundary can be measured by the NLCs part, and the stability of the FC textures can be evaluated through the Ch-LCs part. There are many methods to determine the anchoring energy, such as the field-off method, which contains the wedge-cells technique [8], and the light-scattering technique [9], and so on. The other type is the field-on method like the electric field technique and magnetic field technique. Both the electric-field and magnetic-field technique are established in the measurements of the LC Freedericks transition effects. The more widely used electric

4 VOL. 50 TAO YU, LEI LUO, BO FENG, ET AL. 807 field method to determine the polar anchoring strength was firstly proposed by Yokoyama and van Sprang [10], and Nastishin et al. developed this high electric-field technique without measurements of the LC capacitance [11]. Nie et al. extended this method to the measurement of the polar anchoring strength of VA cells [12]: ( ) R 1 (V V ) = 2K 33 R 0 W p d (V V ξ ) I(γ, k, v, π nc 2 ), V = α(1 ε )V th, ε α = 1/π 1 0 [(1 + r)(1 + k)]/x(1 + rx)dx, v = (n 2 e n 2 o)/n 2 e, where γ = (ε ε )/ε, ε and ε are respectively the dielectric permittivities that are parallel and perpendicular to the director; k = (K 11 K 33 )/K 33, K 11 and K 33 are the splay and bend elastic constants, respectively; R 0 = 2πd(n e n o )/λ is the maximum optical phase retardation of the VA cell; n o and n e are respectively the indexes for the ordinary and extraordinary rays; ξ = (ε 0 ε S/d)π(K 33 /ε) 1/2 ; S is the electrode area; C is the LC capacitance when V ; and W p is the polar anchoring energy. Through the linear fitting of Equation (1), the polar anchoring energy W p can be obtained without a measurement of the capacitance C LC. (1) TABLE I: The parameters of nematic liquid crystal HCCH at different temperature. Temperature ε ε ε K 11 (PN) K 33 (PN) 20 C C C C In the experimental procedure, we adopt the setup depicted in Fig. 2 and Fig. 3. The light source is a normally incident semiconductor laser with λ = 532 nm. The LC cell is placed in a temperature control chamber between the crossed polarizer P1 and analyzer P2. And the optical axes of P1 and P2 are respectively oriented at 45 with respect to the optical axis of the VA cells. The wave generator provides the LC cell with the signal voltages. Thus the intensity of the exit beams I can be written as I = I 0 sin 2 (R/2). Here I 0 is the light intensity through the polarizer P1; the intensity I is received by the photo detector. The optical phase retardation R can be derived from Equation (2) by the data analysis procedures, and R 0 is accessible through the extrapolation method [13]. By substituting the data of the measured R-V curve and the parameters of the LC cells into Equation (1), the polar anchoring energy W p of the NLC part can be derived at different temperatures. (2)

5 808 ANCHORING EFFECT ON THE... VOL. 50 FIG. 2: The automatic measuring setup for measurements of the RV curve of the VA cells, as well as the transmission of FC textures. FIG. 3: To measure the polar anchoring energy, the NLC cells are placed between the crossed polarizer and analyzer, and the optical axis of the LC cell is oriented at 45 with respect to the polarizer. II-4. Evaluation of the Stability of the FC Textures Due to the homeotropic alignment and the large ratio of d/p (cell thickness divided by the pitch P ), the Ch-LC initiately exhibits the planar texture rather than the focal conic texture after the Ch-LC is injected into the cell [14]. In our study, the central wavelength of the light reflected by the P texture is set near the infared region, so the P texture looks very transparent, and its transmittance is the maximum among all the states of the Ch-LCs [15]. With the applied voltage far exceeding V th,p FC (the threshold of P FC), the helical axes of the Ch-LCs are aligned almost parallel to the substrates; it is called the FC state which strongly scatters the light, and its transmittance is very low. Even when the electric field is removed, the FC state can still remain the same. With the temperature gradually rising, the FC state tends to become the P state, and the intermediate state consists of both the

6 VOL. 50 TAO YU, LEI LUO, BO FENG, ET AL. 809 P textures and the FC textures with its transmittance in between the two bi-stable states. These are verified by the images observed by the polarization microscope shown in Fig. 4. The FC state with low transmission is stable at room temperature (Fig. 4(b)), and it is seen that the FC textures in some regions are transformed into the P textures at 30 (Fig. 4(c)). At the temperature of 40, the P/FC ratio in the FC textures is further increased, resulting in a significant increment of the transmission (Fig. 4(d)). Since the direct measurements of the P/FC ratio in the FC textures are rather difficult and inaccessible, it is reasonable to measure the transmittance of the FC textures for the assessment of the stability of the FC textures. Based on the Ch-LC part of the above LC cell, the transmissions of FC textures at 30, 40, and 50 are sequentially measured by use of the experimental setup shown in Fig. 2, and the polarizer P1 and analyzer P2 are unnecessary in the measurements of the Ch-LC cells. We also investigated the stability of the FC textures of the Ch-LC cell with rubbed and non-rubbed conditions. III. RESULT AND DISCUSSION To verify that two equal parts of the above cell have the same boundary conditions, both of the two parts of the non-rubbed cell are filled with the nematic liquid crystals. And the polar anchoring energy W p can be measured by the above RV technique. Fig. 5 shows the practical curve of retardation vs. applied voltage, it is obvious that there is a linear relationship between (R/R 0 1)(V V ) and (V V ) within 9 V rms 18 V rms, so the polar anchoring energy W p can be derived from the slope of the linear region. The polar anchoring energy W p of the two parts is respectively calculated and listed in Table II. The results indicate that the two parts of the same non-rubbed LC cell have the same polar anchoring energy W p (both are J/m 2 ). The above procedures are also applied to the rubbed LC cell, and the result is that both of the two parts almost have the same W p ( J/m 2 and J/m 2, respectively) (see Fig. 6 and Table 2). Hence, it is certain that the boundaries of the two parts of the same LC cell play the identical role in the alignment of the same liquid crystals. This is crucial to the subsequent evaluations of the stability of FC textures by means of polar anchoring strength obtained through the NLCs. TABLE II: Cell Type W p of Part 1 (10 4 J/m 2 ) W p of Part 2 (10 4 J/m 2 ) Rubbed Non-Rubbed On the basis of the above results, one part of the non-rubbed LC cell is filled with NLCs, and the other part is filled with Ch-LCs, then the polar anchoring energy is measured from the NLCs part. Fig. 7 shows the polar anchoring energy of both rubbed and

7 810 ANCHORING EFFECT ON THE... VOL. 50 FIG. 4: The textures of Ch-LCs are observed by the polarization microscope: a) P texture, a transparent state; b) FC texture, a strong scattering state; c) Without the electric field, and placed in a heating chamber at 30, the FC texture is slightly changed into the P texture, resulting in a larger transmittance; d) More FC textures get back to P textures at 40, and the transmittance is further increased.

8 VOL. 50 TAO YU, LEI LUO, BO FENG, ET AL. 811 FIG. 5: There is a linear relationship between (R/R 0 1)(V V ) and (V V ) within 9 V rms 18 V rms, the polar anchoring energy is derived from the slope of the linear region. The polar anchoring energy of one part of the non-rubbed LC cell is J/m 2, and for the other part it is J/m 2. TABLE III: Temperature Cell Type W p (10 4 J/m 2 ) 20 C Rubbed C Non-rubbed C Rubbed C Non-rubbed C Rubbed C Non-rubbed C Rubbed C Non-rubbed 0.44 non-rubbed LC cells at different temperatures, the anchoring energy is decreased with an increment of the temperature, and the polar anchoring energy of the rubbed LC cells is larger than that of the non-rubbed LC cells at different temperatures. These phenomena are quite consistent with the previous studies [6, 7], and can be well explained with the theory discussed in Ref. [16]. The LC molecules of the non-rubbed LC cell orient randomly near the boundary, while the rubbing process enhances the LC molecules alignment on the surface, which slightly boosts the surface scalar order parameter S of the rubbed LC cell. The polar anchoring energy is dependent on the scalar order parameter S, so the polar anchoring energy of the rubbed LC cells is larger than that of the non-rubbed LC cells. The transmittances of the FC textures versus time are measured from the Ch-LCs part at 30, 40, and 50 (shown in Fig. 8), respectively. As the temperatures raises, the

9 812 ANCHORING EFFECT ON THE... VOL. 50 FIG. 6: There is a linear relationship between (R/R 0 1)(V V ) and (V V ) within 9 V rms 18 V rms, the polar anchoring energy is derived from the slope of the linear region. The polar anchoring energy of one part of the rubbed LC cell is J/m 2, and for the other part it is J/m 2. FIG. 7: The figure depicts the temperature dependence of the polar anchoring energy of the rubbed and non-rubbed LC cells. transmittances of the FC textures in both the rubbed and non-rubbed LC cells are correspondingly increased. This means that the FC textures are not stable and tend to be converted into the P textures, so the transmittances of the FC textures go up at high temperatures. Fig. 9 depicts the transmittances of the FC textures versus the relative polar anchoring energy from 20 to 50. It is seen that the stability of the FC textures of both the rubbed and non-rubbed LC cells deteriorates as the anchoring strength is decreased. The above phenomena may be explained as follows: the FC state is a semi-stable state

10 VOL. 50 TAO YU, LEI LUO, BO FENG, ET AL. 813 in thermodynamics, and its free energy is larger than that of the P textures. At room temperature, there is an energy barrier between the FC state and the P state, which can effectively hinder the transitions of FC textures to P textures, so the FC state has a durable retention. The pitch P of the dopant R-1011 is almost stable in the heating-up process [17], that contributes zero to the energy of the defects existing in the FC textures. The strong polar anchoring energy can keep the director orientation of the initial FC textures, the increment of the total free energy of the FC textures is prevented, so the energy barrier still stays between the FC state and P state, and the FC state can be well preserved. When the temperature is increased further, the extra energy contribution of the pitch P is negligible due to its temperature independence. However, the scalar order parameter S is decreased, and the polar anchoring energy accordingly diminishes according to Ref. [16]. The significant drop of the polar anchoring strength means that the surface cannot effectively anchor the LC molecules initial orientations, or prevent the total free energy of the FC textures from arising any more. The energy barrier between the FC state and the P state can be overcome by thermal fluctuations, leading to a larger proportion of the FC textures being changed into P textures. Hence, the polar anchoring energy may play a vital part in maintaining the FC textures, and the strong polar anchoring energy with larger magnitudes at a room temperature of 20 can effectively keep the FC state from being changed into the P state at higher temperatures. FIG. 8: Plots of the transmission versus time of the rubbed and non-rubbed Ch-LC cells at different temperatures.

11 814 ANCHORING EFFECT ON THE... VOL. 50 FIG. 9: From the left to the right: plots of the transmission versus anchoring energy of the rubbed and non-rubbed Ch-LC cells. IV. CONCLUSION By means of the novel scheme of one LC cell with two equal parts respectively filled with NLCs and Ch-LCs, the effect of the polar anchoring energy on the stability of the Ch-LC s focal conic textures was studied in detail. With the heating process and rubbing process, the polar anchoring energy was adjusted for the experimental requirements. The results indicate that the FC state with larger magnitudes of polar anchoring energy is more stable than that with weak polar anchoring strength; the polar anchoring energy plays a positive role in keeping the stability of the FC textures. This will be of benefit to the applications of the Ch-LCs, as well as other potential prospects, such as the study of chemical and biological sensors etc. Acknowledgements The authors are very grateful to Jiangsu Hecheng Chemical Material Co. Ltd. for providing us the nematic liquid crystals and technical supports, and special thanks to Dr. Jinggang Zhao (Chief Engineer of Dalian DKE LCD Co., Ltd.) for helping us with the fabrications of the LC devices. References [1] D. K. Yang and J. W. Doane, SID Symposium Digest 23, 759 (1992). [2] A. Kahn, E. Schneider, and E. Montbach et al., SID Symposium Digest 38, 54 (2007). [3] N. Hiji, C. Manabe, T. Kakinuma, and S. Yamamoto, SID 08 Digest 40, 600 (2008). [4] L. J. M. Schlangen, A. Pashai, and H. J. Cornelissen, J. Appl. Phys. 87, 3723 (2000).

12 VOL. 50 TAO YU, LEI LUO, BO FENG, ET AL. 815 [5] H. A. van Sprang and J. L. M. Van de Venne, J. Appl. Phys. 57, 175 (1984). [6] D. S. Seo, Y. Iimura, and S. Kobayashi, Appl. Phys. Lett. 61, 234 (1984). [7] H. Akiyama and Y. Iimaura, Mol. Cryst. Liq. Cryst. 350, 67 (2000). [8] S. Faetti and P. Marianelli, Phys. Rev. E 72, (2005). [9] M. Vilfan and A. Mertelj, Phys. Rev. E 65, (2002). [10] H. Yokoyama and H. A. van Sprang, J. Appl. Phys. 57(10), 4520 (1985). [11] Yu. A. Nastishin, R. D. Polak, and S. V. Shiyanovskii, Appl. Phys. Lett. 75(2), 202 (1999). [12] X. Nie, Y.-H. Lin, and S.-T. Wu, J. Appl. Phys. 98, (2005). [13] I. C. Khoo, S. T. Wu, Optics and Nonlinear Optics of Liquid Crystals, (World Scientific, Singapore, 1993). [14] C. G. Lin-Hendel, J. Appl. Phys. 53, 916 (1982). [15] P. G. de Gennes and J. Prost, The Physics of Liquid Crystals, 2nd ed. (Clarendon, Oxford, 1993), Chap. 6. [16] G. Barbero and A. K. Zvezdin, Phys. Lett. A, 270, 331 (2000). [17] Jun Geng et al., Appl. Phys. Lett. 89, (2006).