Particular thermally induced phase separation of ... - OSA Publishing

Particular thermally induced phase separation of liquid crystal and poly(N-vinyl carbazole) films and its application Yuan-Di Chen,1 Andy Ying-Guey Fuh,1,2,3,4 and Ko-Ting Cheng2,3,* 1

3

Department of Photonics, National Cheng Kung University, Tainan 701, Taiwan 2 Department of Physics, National Cheng Kung University, Tainan 701, Taiwan Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan 4 [email protected] * [email protected]

Abstract: In the current study, a method of particular thermally induced phase separation (TIPS) of liquid crystals (LCs) and polymers is presented. The method involves a combination of dissolution process and TIPS. The LCs and poly(N-vinyl carbazole) (PVK) play the roles of solvent and solute, respectively, during the processes of particular TIPS. The nematic LC sample fabricated by two substrates coated with uniform PVK films is heated and then cooled, generating the rough PVK layers onto the surfaces of the substrates. The LC sample having rough PVK layers produces micron-sized, multiple domains of disordered LCs that can scatter incident light. Additionally, an application of a scattering mode light shutter, having the advantages of low driving voltage, polarization-independent scattering, fast response, high contrast ratio, and being polarizer free, is reported. ©2012 Optical Society of America OCIS codes: (160.3710) Liquid crystals; (160.5470) Polymers; (230.0230) Optical devices.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

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Received 14 May 2012; revised 27 Jun 2012; accepted 29 Jun 2012; published 10 Jul 2012

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1. Introduction In recent decades, liquid crystals (LCs) have been widely developed for use in many optical devices, such as light shutters [1, 2], switches [3], waveplates [4], lenses [5], and so on. Among them, scattering mode LC light shutters have been fabricated by phase transition/separation methods in polymer-dispersed LCs (PDLCs) [6–8], polymer-stabilized cholesteric textures [1–3]. The approaches to PDLC preparation include encapsulation (emulsification) and phase separation; the latter process is the primary method of manufacture. Phase separation can be achieved via three mechanisms [9], namely, thermally induced phase separation (TIPS), solvent-induced phase separation (SIPS), and polymerization-induced phase separation (PIPS). The used LC is mixed thoroughly with the polymer/monomer to form a homogeneous solution. The separation of the two phases results in the formation of micron-sized LC droplets/polymer balls in a continuous polymer/LC matrix [10]. The ordinary refractive index (no) of the used LCs should be close to the refractive index (np) of the used polymer for the application of light shutters. Regarding the boundary of two materials with discontinued refractive indexes, the refractive index mismatch between the LC droplets (polymer balls) and polymer (LC) matrix leads to strong scattering in the field-OFF scattering state. On the other hand, the directors of the LCs (∆ε > 0) are aligned along the applied electric field in the field-ON transparent state. The unidirectional director provides almost insignificant differences (no = np) in the refractive indexes of neighboring polymers such that the PDLC appears perfectly transparent. However, the polymer matrix walls produce a strong surface anchoring effect that increases driving voltages, indicating that the operated voltage of a traditional PDLC scattering mode light shutter is high, i.e. over 30 V [9, 10]. Accordingly, many scientists have recently paid much attention to such approaches as surface rubbing effect [11], dichoric dyes [12], and others, to reduce the operated voltage of PDLCs. Another key role in this study used to demonstrate the method of particular thermally induced phase separation (TIPS) is a photoconductive polymer material, poly(N-vinyl carbazole) (PVK). PVK has recently received considerable interest from researchers worldwide [13–17]. Golemme et al. reported high-resolution photorefractive PDLCs using PVK as a matrix to fabricate highly efficient gratings [13]. Huang et al. presented an electro- and photo-controllable spatial filter based on an LC film with a photoconductive layer of PVK [14, 15]. PVK materials have been demonstrated to improve the efficiency of light emitting diodes. In addition, the mechanical rubbing of a PVK layer can also induce the homogeneous alignment #168474 - $15.00 USD

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of LCs with their easy axes perpendicular to the rubbing direction [16]. In a previous study, the alignment direction of a rubbed PVK film has been found to successfully switch toward the rubbing direction through thermal treatment. The angle of re-orientation of the director axis increases with the temperature within a specific range. Thus, the current study concluded that the ability of LC alignment to be modified from twisted nematics to homogeneous alignment using thermal treatment [17]. In the present study, the reported phase separation approach can be used to produce a rough PVK film that “re-aligns” LCs into multiple and micron-sized LC domains. The fabrication involves using disordered LC alignment based on thermally treated double-sided PVK films (no rubbing treatment). Two main mechanisms of scattering for such a light shutter exist, namely, surface scattering and volume scattering [18, 19]. Surface scattering is produced by the interface roughness between LC and the rough PVK surfaces, whereas volume scattering is caused by an inhomogeneous change in refractive index induced by the disordered LC alignment (multi-domains). The domain size generated determines the scattering performance [20, 21]. Moreover, the demonstrated electrically controllable LC light shutter in the scattering mode has the advantages of low driving voltage, fast response, being polarizer free, and high contrast ratio, indicating its extremely promising potential applications. The electro-optical properties of the light shutter and the morphologies of the PVK layers are examined in detail. 2. Experiments The chemical structures of the materials used in this study are shown in Fig. 1. The nematic LC material [Fig. 1(a)] used in the current work was K15 (no = 1.5309, clearing temperature TC = ~35 °C), purchased from Merck. The photoconductive polymer [Fig. 1(b)] was PVK (nPVK = 1.68), purchased from Aldrich. The fabrication processes are different from those of conventional PDLCs sample as described above. A solution of chlorobenzene solvent with PVK at a weight ratio of 98.36:1.64 was prepared to coat the PVK (powder) film onto indium-tin-oxide (ITO)-coated glass slides. The solution was then spin-coated onto the ITO-coated glass slides. The substrates were pre-baked in an oven at 80 °C for 20 min, and post-baked at 120 °C for 120 min after coating. The thickness of the fabricated PVK film, no mechanical rubbing, was measured to be of sub-micrometer (~0.2 µm) order using the Alpha-Step IQ Surface Profiler (KLA-Tencor). Moreover, two non-rubbed PVK-coated glass substrates were combined to fabricate an empty cell, whose cell gap was 6 µm. Finally, the nematic LC (K15) was homogeneously filled into the empty cell, and the edges of the cell were sealed with epoxy to produce a sample. The fabricated LC sample was very stable at room temperature (~25 °C). The details of the proposed particular TIPS are described later.

Fig. 1. Chemical structures of compounds used in this paper. (a) nematic LCs (K15), and (b) photoconductive polymer (PVK).

3. Results and discussion Figure 2 shows the plot of the variation in transmittance with the temperature of an LC (K15) sample fabricated from two non-rubbed PVK-coated ITO glass substrates. Experimentally, a red probed laser beam derived from a He-Ne laser (λ = 632.8 nm) was normally incident onto the temperature-controlled LC sample. The transmitted light was collected by a photo-detector placed behind the LC sample. The high transmittance remained almost unchanged when the LC sample was gradually heated (black dots) from 25 °C to 60 °C at a heating rate of ~5 °C/min. #168474 - $15.00 USD

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The temperature of the LC sample was maintained at 60 °C for 8 min to dissolve PVK homogeneously, which was experimentally optimized. Subsequently, the LC sample was cooled (red squares) to 25 °C at a cooling rate of ~5 °C/min. The transmittance abruptly dropped at approximately 34 °C during the cooling process. This point is close to the clearing temperature (~35 °C) of the used LCs (K15), given that the LC phase is transformed from isotropic to nematic at this temperature. The transparent sample became a scattering (opaque) sample after thermal treatment via the particular TIPS processes. Notably, the small variations in transmittance are caused by the temperature-dependent refractive indexes of LCs and the thermal disturbance [9, 22]. The switching temperature (TS), which is defined as the temperature required to switch the LC sample from the transparent to the scattering mode, depends on the selected LCs. Experimentally, the TS of the three kinds of nematic LCs, K15 (Merck), E7 (Fusol material), and MDA-00-3461 (Merck), were ~35, ~61, and ~92 °C, respectively. TS was almost equal to the clearing temperatures, suggesting that the LC sample should be heated to a temperature higher than the clearing temperature and then cooled to generate disordered LC alignment. The insets (a) and (b) of Fig. 2 show the photographs of the LC sample at room temperature (~25 °C) before (transparent) and after (scattering) thermal treatment via the particular TIPS, respectively. The LC sample was heated at 60 °C and then cooled at 25 °C at a rate of 5 °C/min. The scattering state of the thermally treated LC sample was also stable at room temperature (~25 °C). This finding indicated that the disordered LC alignment layer was permanent at temperatures below the clearing temperature of the LCs. Restated, the thermal stability of the LC light shutter can be improved by selection of LCs with proper clearing temperature. Notably, the thermal treatment conditions, such as switching temperature, for different LC materials will be different.

Fig. 2. Variations in stable transmission in relation to the temperature during heating (black dots) and cooling (red squares) of the LC sample fabricated from two non-rubbed PVK-coated glass substrates. The LC sample at 25 °C; (a) before (transparent) and, (b) after (scattering) thermal treatment via the particular TIPS.

The insets (a) and (b) of Fig. 3 show the images of the thermally treated LC sample (scattering mode) at ~25 °C under parallel- and crossed-polarized optical microscopy (POM), respectively. The multi-domains of the LCs with disordered alignments presented as different colors due to their different birefringence properties (phase retardations). The sizes of the multi-domains are measured to be in the order of micrometers. Macroscopically, light incident onto the thermally-treated LC sample with multi-domains resulting from the rough PVK layer was scattered because of the following two reasons. One was the interface roughness between LCs and PVK (surface scattering), and the other was the refractive index mismatches between the boundaries of the LC domains (volume scattering). A plot of the transmittance against the polarization of the incident light (He-Ne laser) of the multi-domain scattering region [Fig. 3] revealed that the transmittance was produced by the slight light leakage penetrating the LC sample. Experimentally, the extremely low transmittance (high scattering) was independent of the polarizations of the incident light, which suggests that the multi-domains of LC were uniformly and randomly dispersed.

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Fig. 3. Transmittance as a function of the polarization state of the incident light. LC sample after thermal treatment via the particular TIPS at ~25 °C observed under (a) parallel- and (b) crossed-POM. P and A are the transmission axes of the polarizer and analyzer of the POM, respectively.

According to the experiment results, the original non-rubbed PVK film fabricated by pre-baking (at 80 °C for 20 min) and post-baking (at 120 °C for 120 min) after coating onto a substrate was uniform and cannot provide any alignment anchoring to disturb the alignment of the LCs. Accordingly, the original LC samples were transparent and stable at room temperature (~25 °C), as shown in the inset (a) of Fig. 2. The coated PVK dissolved in the LCs after heating to temperatures higher than its TS, which was close to the clearing temperature. This finding indicated that the isotropic LCs dissolved the coated PVK materials at temperatures higher than TS. The temperature of the LC sample was maintained at the setting temperature for several minutes to dissolve PVK homogeneously. Subsequently, the heated LC sample was cooled to room temperature. During this cooling process, TIPS occurred [9]. Notably, the dissolved polymers (PVK) in the LCs tended to diffuse toward the low-temperature sides and re-form on the substrate surface. Moreover, the effects of the setting temperature of the thermal treatment, cooling rate, and cell gap on the formation of the double-sided PVK films were also elucidated. Three LC samples with double-sided PVK films (cell gap ~6 µm) were heated to 40, 60, and 80 °C at a heating rate of 30 °C/min to investigate the effect of the setting temperature on the formation of PVK films. The setting temperature of the LC sample was maintained for 8 min to dissolve PVK homogeneously, followed by cooling to room temperature (25 °C) at a rate of 30 °C/min. The SEM morphologies of the LC samples after thermal treatment at the setting temperatures of 40, 60, and 80 °C are shown in Figs. 4(a)-4(c), respectively. The morphologies of the rough PVK film in Fig. 4(c) are denser than those in Figs. 4(a) and 4(b). This result indicated that the roughness and amount of rough PVK increased with the setting temperature. The branch-like structures of the rough PVK film also became more prominent with a higher setting temperature, probably because the PVK solubility in isotropic LCs increases with the LC temperature. A separate experiment (data not shown) revealed that the scattering performance of the LC sample shown in Fig. 4(a) was very poor. The scattering performance of the thermally treated LC sample continuously increased and became saturated at the setting temperature of ~60 °C with higher setting temperature. Optically, the measured transmittances of the LC samples in Figs. 4(b) and 4(c) were identical. These findings indicated that the setting temperature of thermal treatment is the key in the fabrication of the scattering mode LC light shutter.

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Fig. 4. SEM images of LC samples after thermal treatment with setting temperatures of (a) 40, (b) 60, and (c) 80 °C at a heating rate of 30 °C/min. The temperature of the LC sample was maintained at the setting temperature for 8 min, and then cooled to 25 °C at a cooling rate of 30 °C/min.

Four fabricated LC samples with double-sided PVK films (cell gap = ~6 µm) were heated to 60 °C at a heating rate of 30 °C/min to study the effect of the cooling rate on PVK film formation. The temperatures of the LC samples were maintained at 60 °C for 8 min to dissolve PVK homogeneously, and then cooled to room temperature (25 °C) at cooling the rates of 30, 10, 5, and 1 °C/min [Figs. 5(a)-5(d)]. The SEM images revealed that the dimension and roughness of the branch-like structures increased with increasing cooling rate. The scattering performance of the LC samples directly increased with the cooling rate. In a previous study [23], the surface morphology of polymers fabricated via TIPS at a slow cooling rate is more uniform and regular than those at a fast cooling rate. A rougher PVK structure is associated with higher scattering. The effect of the cooling rate in TIPS is similar to that of the light intensity in PIPS by illumination. Moreover, a separate experiment (data not shown) shows that the scattering capability decreases with the increase of the cell gap. Experimentally, the processes and conditions of the thermal treatment were identical to those used in Fig. 4, except that the cell gaps varied and the cooling rate was fixed at 30 °C/min. The distribution of the LCs in the bulk of the large-cell-gap LC sample after thermal treatment was more difficult to disturb and tended to preserve its original alignment, thus leading to large LC domains. The driving voltage also increased with increased cell gap, resulting in the reduced performance of the LC light shutter.

Fig. 5. SEM images of LC samples after being heated to 60 °C at a heating rate of 30 °C/min. The temperature of the LC sample was maintained at 60 °C for 8 min, and then cooled to 25 °C at the cooling rates of (a) 30, (b) 10, (c) 5, and (d) 1 °C/min.

In order to verify the generated rough PVK structures in the bulk of the scattering LC sample, the cross-section of the LC sample, fabricated under treatment conditions consistent with those in Figs. 4(b) and 5(a), was examined by a SEM. The cross-section SEM image, shown in Fig. 6, clearly indicates that the rough surfaces structures of these two substrates (side-view) and the bulk structures of the thermally treated LC sample (scattering mode). Moreover, obviously, several tiny branches, crossing through the bulk of the sample (from top to bottom substrates), with diameter of ~1 µm were formed.

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Fig. 6. Cross-section SEM image of the thermally treated LC sample

Fig. 7. Measured transmission of the fabricated scattering mode LC light shutter as a function of an applied AC (1 KHz) voltage. Insets show photographs of the LC light shutter at 25 °C with the applied AC voltages of (a) 0 and (b) 18 V.

Figure 7 shows the plots of the measured transmittance of the scattering mode LC light shutter [fabricated under treatment conditions consistent with those in the insets (a) and (b) of Fig. 2] as a function of an applied alternating current (AC; 1 KHz) voltage. Transmittance was defined as the ratio of the intensity of the transmitted beam through the thermally-treated LC sample to that through an empty cell, such that the transmission through an empty cell was equivalent to 100%. The required voltage to achieve maximum transmittance (~76.7%) was about 18 V. A low applied AC voltage of ~13 V was required to switch the LC light shutter from the scattering mode to 90% of its maximum transmittance. Notably, the small cell gap and the weak surface anchoring resulting from the rough PVK layers are the keys to reduce the driving voltage. Additionally, the operated voltage, which was inversely proportional to the square root of the dielectric anisotropy (∆ε), can be reduced to a greater extent using LCs with higher ∆ε values. The contrast ratio, defined as the maximum transmittance (V = ~18 V) over the minimum transmittance (V = 0 V), of the light shutter was calculated to be 300:1. Photographs of the LC light shutter at 25 °C with applied AC voltages of 0 and 18 V are shown in the insets (a) and (b) of Fig. 7, respectively. Moreover, the LC (∆ε > 0) domains became aligned along the electric field as an AC voltage was applied, leading to a reduction in the refractive index mismatch (volume scattering). Thus, the scattering mode LC light shutter can be electrically switched from the opaque to the transparent mode. Notably, the transmittance cannot reach 100% considering that the fog-like scattering (surface scattering) was caused by the refractive index mismatch between no of the selective LCs and nPVK. Accordingly, proper selection of the used LCs with no equivalent to nPVK can enhance transmittance. Theoretically, the surface scattering, resulting from the refractive index mismatch between neff (1.53~1.7) of the LCs and nPVK (1.68), should be increased with the applied voltage. According to the transmittance of the light shutter with applied AC voltages of 0 and 20V, shown in Fig. 7, the volume scattering provides the main contribution to demonstrate the electrically switchable LC light shutter. Additionally, the small hysteresis (∆V/Vpeak = ~3.5%) was still an issue for this scattering mode light shutter. This issue is attributed to the fact that a little bit of the PVK branches crossed from one substrate to the other one, according to the cross-section SEM image

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(Fig. 6). The structures result in the small hysteresis. Accordingly, the small hysteresis can be decreased by reducing the thickness of PVK films. Figure 8 shows the dynamic response of the fabricated scattering mode LC light shutter when an AC voltage pulse is applied. The amplitude and the frequency of the pulse are 20 V and 1 KHz, respectively. The measured rise and fall times refer to the period required to change the transmittance of the light shutter from 10% to 90% and from 90% to 10% of its maximum transmittance, respectively. The rise and fall times for the scattering mode LC light shutter were about 2.25 and 3.22 ms, respectively. Notably, the backflow effect, which increases the response time, is not observed in Fig. 8. Moreover, the inner PVK structures of the LC light shutter provide a weak surface anchoring to result in the fast orientation of LC molecules, i.e. fast response [9]. However, the electrically induced carrier injection from ITO/PVK into LC should be considered because PVK is well known photo-conducting polymers for UV spectral region. According to Refs. 24 and 25, DC field and applied voltage with low frequency but not the applied voltage with high frequency, may result in the accumulation of charge carriers inside the LC bulk, which will reduce the performance of the LC light shutter severely. In case of reduction of performance, anti-UV coatings onto the substrates can be adopted to eliminate the carrier injection, produced by PVK.

Fig. 8. Dynamic response (red line, plotted on the primary axis) of fabricated scattering mode LC light shutter when an AC voltage pulse (blue line, plotted on the secondary axis) of 20 V (1 KHz) is applied.

4. Conclusion In conclusion, a particular TIPS method of LCs and polymers was presented. The method was used in the successful fabrication of a scattering mode LC light shutter from LC samples consisting of double-side PVK films. Permanent scattering resulted from the formation of multiple and micron-sized domains of disordered LCs. The fabricated LC light shutter possessed the advantages of low driving voltage, fast response in the order of milliseconds, independent of polarization, high contrast ratio (~300:1), and being polarizer free. The effects of different switching temperatures, cooling rates, and cell gaps on the formation of LC light shutter were also investigated. Moreover, the electrically switchable LC light shutter in the scattering mode had extremely promising potential applications, such as in energy-efficient smart windows and scattering mode LC displays. Acknowledgment The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Grant No. NSC 98-2112-M-006-001-MY3 and NSC 99-2112-M-006-002-MY3. Additionally, this work is partially supported by Advanced Optoelectronic Technology Center as well.

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