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Electro-optical properties of the polymer-stabilized blue phase (PSBP) liquid crystals (LCs) with different monomer combination and concentration were studied. Kerr constant, voltage-dependent transmittance, response time, residual birefringence and hysteresis were measured for PSBP LCs prepared using acrylate and methacrylate monomer composites. Operating voltages of PSBP LCs prepared by acrylate composites were lower than those of PSBP LCs prepared by methacrylate composites. By contrast, residual birefringence and hysteresis of PSBP LCs prepared by acrylate composites were higher than those of PSBP LCs prepared by methacrylate composites. Faster response and lower hysteresis were observed in PSBP LCs with higher monomer concentration. It was found that monomer combination and concentration strongly affected the electro-optical properties of PSBP LCs.
©2011 Optical Society of America
1. Introduction
Electro-optical (EO) switching based on Kerr effect is an attractive phenomenon for the application to optical devices like a flat panel display. A blue phase (BP) LC is one of the promising Kerr materials because BP LC is optically isotropic without electric field and has a quite large Kerr constant [1,2]. BP LC has a double-twist-cylinder packing and is known to show three types of phases; BPI, BPII and BPIII. BPI and BPII have been assigned as a body center cubic and a simple cubic, respectively, while BPIII is nearly amorphous [3].
Typically, the temperature range of stable BP LC is very narrow, because their exotic structure is on a delicate balance between the intermolecular interaction and topological requirement [4]. This is an obstacle to the practical application and enormous efforts have been expended to widen these temperature ranges. In 2002, Kikuchi et al. reported that an in situ photopolymerization of monomers in a BP LC extends its temperature range over 60 K [5]. The stabilized phase is referred as ‘polymer-stabilized blue phase (PSBP) LC’. PSBP LC has many intriguing properties such as an optically isotropic character which eliminates the use of a surface alignment layer, faster EO response time of ca. submillisecond [5–7], and a widely available temperature range. However, some problems of the high operating voltage and the low contrast ratio due to the residual birefringence still remained to be overcome [8,9]. Regarding these problems, the operating voltage can be reduced sufficiently by using large Kerr constant liquid crystal materials which are still being developed [10]. Generally, a mixture of a mono-functional monomer and a di-functional monomer is used for in situ polymerization to stabilize a BP LC. Recently, a correlation between monomer ratio and residual birefringence was reported by Yan and Wu [8,11]. In their studies, it was found that residual birefringence was suppressed by the optimization of the monomer ratio. From a viewpoint of applications, a reduction of total monomer weight in PSBP LC is a very important factor, since it may improve the voltage holding ratio (VHR) to be higher and the driving voltage to be lower by decreasing residual monomer after ultraviolet (UV) curing. Therefore, reduction of the total monomer concentration with optimizing monomer ratios in PSBP LC is desirable to promising of the application of PSBP LC. Moreover, L. Rao et al. reported that there is also a correlation between the peak electric field and hysteresis of PSBP LCs using novel design of electrode shapes and dimension [12]. In this paper, we present the EO properties of PSBP LC with two sets of monomer combination, in which both mono- and di-functional monomers are acrylates or methacrylates. The total concentration of monomers was adjusted to be 11 wt% or 8 wt% in order to clarify the effect of the amount of monomer. Influences of monomer combination and total monomer concentration on the EO properties of PSBP LC are discussed. Moreover, the optimum total monomer concentration and the effect of electric field on residual birefringence were investigated.
2. Experimental
The host LC material with chiral dopant used in this study was JC-BP03C provided by Chisso Petrochemical Co. Ltd. Mono-functional monomer of dodecyl acrylate (C12A, Wako) or dodecyl methacrylate (C12M, Wako) and di-functional monomer of PLC or PLCM (Chisso Petrochemical Co. Ltd.) were mixed with JC-BP03C. The total monomer concentration and ratios in precursors are listed in Table 1 . Each precursor was injected into an in-plane switching (IPS) cell. The cell thickness was about 10 μm. A width and a distance of the IPS interdigitated electrode were 5 μm and 10 μm, respectively. The cell was heated at a slightly higher temperature than the N*-BPI transition point and irradiated with UV light of 10 mW/cm2 for 3 minutes using a mercury lamp (USHIO Inc., Deep UV500) where N* represents cholesteric phase. After UV curing, the formation of PSBP LC was checked using a polarizing optical microscope (Nikon Corp., LV100POL). The EO effects were observed using the experimental setup described by Choi et al. [1]. The EO signal was accumulated for 1 second by applying a square ac electric voltage, whose frequency was 100 Hz. In this study, voltage step was 0.9 V for all voltage-transmittance, hysteresis and residual birefringence measurements. Experiment on EO properties using smaller voltage steps and slower rates of steps are underway. All measurements were performed at 25°C controlled by a hot stage.
Table 1. Compositions of monomers in precursors to prepare PSBP LC
3. Results and discussion
3.1 Operating voltage and Kerr constant of PSBP LC
Figure 1 shows the measured voltage-dependent transmittance (VT) curves of PSBP LC samples at 25°C. Each VT curve was obtained by normalizing with the maximum transmittance. Operating voltage, Vp which is defined as a voltage at the maximum transmittance for each sample is shown in Table 2 . Comparing MLC1 with MLC3 prepared by precursors having 11 wt% of the total monomer concentration, Vp of MLC1 with acrylate composites was 49.7 V while that of MLC3 with methacrylate composites was over 70 V. Similarly, in the case of the total monomer concentration of 8 wt%, Vp of MLC2 was 38.0 V, while that of MLC4 was 51.4 V. According to these results, Vp of methacrylate composites were higher than that of acrylate composites even if the total monomer weight and the UV curing conditions were the same. So, it is considered that appropriate polymer networks which strongly anchored the host LC molecules were formed in the methacrylate composites compared to the acrylate composites, since the strong anchoring effect by polymer network in BP LC resulted in higher operating voltage.
Fig. 1 Measured VT curves of PSBP LC at 25°C. Solid markers with line represent transmittances on forward sweeping voltage and open markers represent backward sweeping voltage.
Table 2. Vp, Kerr constant K, residual birefringence and hysteresis of PSBP LCs
We calculated Δninduced as a function of the square of applied electric field from measured VT curves and depicted it in Fig. 2 . The Kerr constant was obtained by the slope of the Δninduced versus E2 curve due to Eq. (1). Values of the Kerr constant of our samples are listed in Table 2.
where Δninduced is induced birefringence by applied electric field, λ is the wavelength of the probe light (633 nm), K is Kerr constant and E is an applied electric filed. MLC2 showed the largest Kerr constant (8.16 nm/V2), followed by MLC1 (5.81 nm/V2), MLC4 (4.31 nm/V2) and MLC3 (3.33 nm/V2). From these results, PSBP LC prepared with lower monomer concentration led to the large Kerr constant in both acrylate and methacrylate composites. According to the literature [10], operating voltage is proportional to the reciprocal Kerr constant. Figure 3 shows the correlation between the reciprocal Kerr constant and operating voltage of our samples. In our case having different LC/monomer compositions such as monomer concentration and monomer types, a linear relationship was observed. This relationship will guide us in order to search for the optimized LC/monomer composition.Fig. 3 Vp versus K-1/2 of MLC1, 2, 3 and 4. Operating voltage, Vp is proportional to the reciprocal Kerr constant, K.
3.2 Residual birefringence
The residual birefringence of PSBP LC after application of an applied electric field occurs when the polymer-stabilization effect is not enough. Recently, Yan et al. reported a correlation between residual birefringence and total monomer concentration in PSBP LC [8,11]. They reported that the higher monomer concentration yielded lower residual birefringence, in other words PSBP LC were more stable when the monomer concentration was higher. The residual birefringence of each sample measured in this study is listed in Table 2. The residual birefringence is here defined as the difference in the transmittances at the zero voltage before and after application of the voltage. The residual birefringence showed the highest in MLC2 (0.5508%). On the other hand, the residual birefringence of MLC1 and MLC4 were 0.0164% and 0.0160%, respectively. The residual birefringence of MLC3 was nearly 0% (under detection limit of photodiode). These results indicate that there is a trade-off relation between the operating voltage (Vp) and the residual birefringence. However, it is remarkable that Vp and the residual birefringence of MLC4 yielded almost the same as those of MLC1 in spite of the small amount of monomers in MLC4. The methacrylate composites tend to provide favorable EO characteristics. The residual birefringence had a noticeable decreasing trend towards increasing temperature. Note that the residual birefringence depends on the magnitude of the applied maximum voltage. Here, we measured the residual birefringence of PSBP LCs with applied voltage of V = Vp and V = 70 V as shown in Table 3 in order to investigate the effect of electric field on residual birefringence. It was found from Table 3 that the residual birefringence can be optimized by controlling an applied electric field.
Table 3. Comparison of residual birefringence of PSBP LCs with applied voltage V ≤ Vp and V = 70V
3.3 Hysteresis effect
The hysteresis behavior of PSBP LC upon forward and backward sweeping voltages is a critical problem because it affects the accuracy of the gray scale control of the liquid crystal display (LCD). This value should be less than 5% for a practical application [13]. The hysteresis of PSBP LCs obtained in this study is listed in Table 2. The hysteresis showed the highest in MLC2 (7.29%) and it decreased as the following order MLC1 (1.71%), MLC4 (1.44%) and MLC3 (0.075%). These trends were the same as those of the residual birefringence, which would reflect the aggregated state of the polymer networks.
3.4 Response time
The response time is one of the indispensable factors in display devices. Especially, the gray scale response time is important, because most display images are in gray levels. We have measured the dark-to-gray (DTG) and the gray-to-dark (GTD) response time, which correspond to rise and decay time, respectively, at each of the applied voltage. Figure 4(a) depicts the voltage dependence of measured DTG response time. MLC1 was found to be slightly faster than MLC2 in the region of low applied voltage. However, in the region of higher applied voltage, DTG response time of MLC1 is almost the same as that of MLC2. Similar trends of MLC3 and MLC4 were observed as shown in Fig. 4(a). Accordingly PSBP LC samples with higher monomer concentration are considered to cause faster response than those with a lower monomer concentration. DTG response times were nearly constant at different applied voltages for all samples as shown in Fig. 4(a). Figure 4(b) depicts the GTD response times of all samples. GTD response time gradually increased with increasing the applied voltages as shown in Fig. 4(b). This trend was very prominent for PSBP LCs prepared with the methacrylate composites of MLC3 and MLC4.
3.5 Discussions
It was found that Vp of PSBP LC depended on the combination and the concentration of mono- and di- functional monomers. Let us discuss these results from the viewpoint of the polymerization rate of monomers. Generally, the polymerization rate of acrylate monomers is much faster than that of methacrylate monomers. Previously, Iwata et al. reported the polymer structures in PSBP LCs evaluated by small angle X-ray scattering (SAXS) measurements for PSBP LCs prepared with acrylate monomers and methacrylate monomers [14]. In their report, it was found that there were two types of polymer aggregation in PSBP LC, one had an identical periodic structure with the disclination network of BP and the other had non-periodic structure. The polymer with periodic structure could be concentrated in the disclination cores in BP and one with non-periodic structure might be formed at the grain boundary and/or the interface with the substrate. They found that the fraction of polymer with non-periodic structure increased as the polymerization-induced gelation time was shorter. In other words, if the polymer-crosslinking rate is too fast, the polymer could not be concentrated in the disclination core in a suitable manner and form an aggregation in BP, resulting in weak polymer networks along the disclination lines. It is assumed that the electric field-induced birefringence is mainly due to the local reorientation of the LC molecules. Strong polymer networks in the disclination lines may suppress these molecular reorientations, causing high operating voltage, and vice versa. This interpretation is consistent well with our experimental results that PSBP LCs prepared with acrylate composites showed lower operating voltage than those prepared with methacrylate ones.
An origin of the residual birefringence is due to insufficient relaxation of reoriented molecule and/or partial field-induced transition to N* phase [8,15]. Total monomer concentration and monomer ratios affect the residual birefringence. In our results, PSBP LCs prepared with total monomer weight of 8 wt% showed a large residual birefringence in comparison with those prepared with total monomer weight of 11 wt%. Especially, MLC2 showed fairly large residual birefringence. On the one hand, if the total monomer concentration was less than or equal to 6 wt%, PSBP LCs cannot be obtained, but on the other hand, if the total monomer concentration was higher than 14 wt%, the BPs showed partial phase separation and crystallization of monomers. Table 4 summarized Vp for various total monomer concentrations of PSBP LCs. Here, “A” represents acrylate composites, “B” denotes methacryalte composites, and “X” stands for an insufficient condition for PSBP LCs. It is therefore considered that there is an optimum total monomer concentration to minimize residual birefringence. Total monomer concentration of 11 wt% for PSBP LC prepared with acrylate composites and of 8 wt% for ones prepared with methacrylate composites are optimum monomer concentrations to achieve minimum residual birefringence in our materials. Further, as mentioned above, since stronger polymer networks could be constructed in methacrylate composites compared with acrylate ones, the polymer networks in disclination lines in PSBP LC plays an important role for the reduction of the residual birefringence. In a case that weak polymer networks are formed in disclination lines, some polymer aggregates reoriented with host LC molecule and/or partially induced N* phase when a voltage is applied may cause a residual birefringence.
Table 4. Vp for various total monomer concentrations of PSBP LCs
Our observation indicates that the PSBP LCs with a larger hysteresis showed noticeable tendency towards giving a larger residual birefringence. Because it has been reported that hysteresis was reduced after polymerization in BPI [13], the origin of the hysteresis of PSBP LC should not be related to that of polymer dispersed LC (PDLC). Although the appearance of hysteresis should closely connected to the residual birefringence, further investigations are necessary to clarify the mechanism.
DTG response times depicted in Fig. 4(a) showed little voltage dependence. In contrast, voltage-dependent GTD response times were observed in Fig. 4(b). Driving mechanism of PSBP LC on gray scale response has not yet been clearly understood. It is important that no voltage dependency is favorable for the practical application. In our results, both acrylate and methacrylate composites showed faster response time in higher monomer concentration. Previously, Hisakado et al. reports the dependency of the response time on the different total monomer concentration [6]. In their report, higher total monomer concentration results in faster response time probably due to a strong polymer-network. It is reasonable to say that the strong polymer-network would be constructed in the condition of higher monomer concentration, which would provide strong anchoring force preventing the local molecular reorientation. We are trying to improve it by optimizing PSBP process, the choice of monomers, and the development of host LCs.
4. Conclusions
The electro-optical properties of polymer-stabilized blue phases with different monomer combination and concentration were studied. Operating voltage (Vp) of PSBP LCs prepared by methacrylate composites was higher than that of acrylate composites on the same total monomer concentration. It is remarkable that the same value of residual birefringence was observed in PSBP LC prepared by acrylate composite of MLC1 (11 wt% of total monomer concentration) and methacrylate composite of MLC4 (8 wt % of total monomer concentration). PSBP LC prepared with acrylate composite of MLC2 (8 wt% of total monomer concentration) showed the largest residual birefringence and the highest hysteresis which indicate that polymer-stabilization effect was not enough. The gray scale response time was faster in PSBPs with high monomer concentration in both acrylate and methacrylate composites. These results indicate that the optimization of monomer combination and the total concentration improve the electro-optical properties of PSBP LCDs. Moreover, the optimum total monomer concentration and the effect of electric field on residual birefringence were experimentally confirmed.
Acknowledgments
The authors would like to thank Ms. Chihiro Tamura for in-plane-switching cell preparation and technical support. This work has been supported by Regional Research and Development Resources Utilization Type Program, A-STEP of Japan Science and Technology Agency.
References and links
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