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A templating technique is proposed for improving the electro-optical properties of polymer stabilized blue phase (PSBP). After polymerizing a monomer-doped blue phase, the remaining chiral nematic liquid crystal was removed to create a porous free-standing cast that retained the 3D structure of the blue phase. The effects of the helical twisting power of the filling mesogen on the electro-optical properties of PSBP were studied. The driving voltage effectively declined as the proportion of chiral dopant in the filling material was decreased. Because of the achiral mesogen tended to be less twisted intrinsically, the elastic restoring force helped the electric field to unwind the helical structure, which was supported by the polymer anchoring force. Replacing the chiral agent with a reverse-handed chiral further reduced the driving voltage by 40% and doubled the value of the resulting figure of merit.
©2013 Optical Society of America
1. Introduction
Polymer-stabilized blue phase (PSBP) is an emerging candidate for use in photonic applications because of its ultrafast response, optically isotropic dark state, and ease of fabrication [1–7]. However, the high driving voltage that originates from the relatively small Kerr constant of the BP material limits its applicability. Numerous methods have been developed for reducing the driving voltage, including electrode design [8–10], operating mode design [11, 12], control on curing condition [13], alteration in polymer ingredients [14, 15] and the synthesis of materials with large dielectric anisotropy [16]. Recently, Choi et al. and Yan et al. proposed a simple and efficient method of enhancing the Kerr constant by lengthening the pitch of the blue phase liquid crystal [17, 18]. However, the lengthened pitch affects the Bragg reflection color and the visible range of the display. Besides, the formation conditions for the blue phase limit the increase in pitch. In the washout-refilled method, also known as the polymer templating method, the unpolymerized liquid crystal is first washed out with a solvent. The polymer network is then refilled to produce a new self-assembled material [19–22]. Accordingly, the washout-refill method provides the possibility of using a wide variety of liquid crystalline materials to form the “original” BP texture. Castles et al. were the first to demonstrate the use of templated BPs comprising 25–50 wt% reactive mesogen as casting material and the use of achiral mesogen as filling material [21]. However, high polymer concentrations produce electro-optical properties that are unfavorable for display applications.
This work fabricated templated BPs for a PSBP display by using common polymer concentrations approximating 12.5 wt%. The templated BPs varied in both the concentration and the handedness of the chiral dopant. The effects and mechanisms of their chirality on the driving voltage and the response time in templated BP systems were investigated.
2. Sample preparation
In the experiments, templated BPs were fabricated in the following steps. We firstly prepared a neat BPLC by mixing 11.5 wt% right-handed chiral dopant R1011 and nematic liquid crystals(LCs), 50 wt% JC-1041XX and 38.5 wt% 5CB. The precursor was subsequently formed by blending 87.1 wt% BPLC mixture with photo-curable prepolymers, 7.1 wt% reactive diacrylate mesogen RM-257, 5.4 wt% trimethylolpropane triacrylate (TMPTA), and 0.4 wt% photoinitiator 2,2-dimethoxy-2-phenyl acetophenone (DMPAP). After homogeneous mixing, the precursor was heated until it reached an isotropic state and dispensed into 7.5 μm-thick cells with indium-tin-oxide (ITO) layers [Fig. 1(a)]. The phase sequence of monomer-doped BP liquid crystals during cooling is isotropic-39.5°C-BPII-33.2°C -BPI-30.5°C-CLC. The samples were then cooled to 33.5 °C and irradiated with ultraviolet light (UV) at an intensity of 36 mW/cm2 for 6 mins to achieve phase separation [Fig. 1(b)]. After polymerization, the transition temperature from ISO to BP (Tc) is 53 °C, and the transition temperature from BP to CLC decreases to below 0 °C. When the polymer network had been generated, the samples were further cooled to room temperature. To complete the fabrication of the polymer templates, residual fluid molecules were washed out of all samples by immersion in hexane [Fig. 1(c)]. When the nonreactive mesogen was fully removed, the remaining hexane in the cells was evaporated by heating to 80 °C. Finally, the polymer templates were refilled with pure achiral or chiral nematics to form templated BPs [Fig. 1(d)]. The achiral nematic is the same as the nonreactive nematic LCs of the precursor. In contrast, the chiral nematics comprise the former achiral nematic and either a right-handed chiral dopant R1011 (concentration, 22 wt%, 18 wt%, 11.5 wt% or 5 wt%) or a left-handed chiral agent S1011 (concentration, 2 wt% or 4 wt%). The chiral dopant R1011 and chiral dopant S1011 had an identical helical twisting power (HTP) of 33 μm−1 but a different handedness. All of the refilled samples in step 4 in Fig. 1(d) were examined under R-POM to confirm that they were retained as BP platelet textures and to identify the phase sequences. Table 1 lists the transition temperatures from ISO to BP (Tc) observed with R-POM under different doping conditions. The same as in the original PSBP, the transition temperature from BP to CLC under varying doping conditions are all below 0 °C. When doping is performed using the same concentration of chiral dopant as in the original PSBP, the Tc is very much the same as that in the original PSBP. The experiments showed that Tc decreases as chiral concentration increases. If the chiral concentration doped is lower than that of the original PSBP, Tc would be higher than that of the original PSBP. Consequently, the BP range of the templated BP would become wider than that of the original PSBP.
Table 1. Electro-optical properties of templated BPs
3. Experimental setup
Vertical Field Switching (VFS) method was used for electro-optical measurements of the templated BPs owing to the uniform electric field [11, 12]. Figure 2 depicts the experimental setup used to implement VFS. A transparent container was filled with glycerol liquid and set between two crossed polarizers. The sample was then immersed in glycerol and oriented at an angle θ of 52°. Thus, the probe beam (He-Ne laser, λ = 632.8 nm) obliquely entered the sample at a high angle of incidence because of the index matching between glass and glycerol. On the opposite side of the analyzer, a detector was connected to a multimeter to measure the voltage-dependent transmittance (VT) curves, and an oscilloscope was used to measure the response time at room temperature (25°C). When measuring these electro-optical properties of the templated BP, 1 kHz square-wave AC signals were applied to the sample.
4. Results and discussion
Figure 3(a) shows an image of blue phase platelets of the PSBP fabricated by step 2 in Fig. 1(b) under the crossed R-POM. For further observation of the microstructure of the polymer, the sample was washed with hexane as shown in step 3 in Fig. 1(c) and one of the substrates was peeled off afterward. The polymer was then deposited with a thin layer of gold and examined with a scanning electron microscope (SEM) [Fig. 3 (b)]. The image shows that the domain size of the polymer network approximates 50nm, which is much smaller than the pitch of the original PSBPLC. It can be seen that the polymer was not only distributed in the defects but also in the double twist cylinders of the PSBP. Therefore, the filling achiral mesogen could be stabilized in the BP texture due to the anchoring force provided by polymer network with BP morphology. Figure 3(c) shows the templated BP doped with achiral mesogen under crossed R-POM. When the temperature rises above Tc, the blue phase platelets vanish [Fig. 3(d)]. The Bragg reflection property in liquid crystal phase confirms that the templated BPs possess periodic structures similar to that of the original PSBP.
Fig. 3 (a) R-POM image of the original PSBP, (b) SEM image of the polymer template and R-POM images of the templated BP in (c) BP and (d) ISO.
Figure 4 plots the measured VT curves for the templated BP. In all curves, the templated BP remained optically isotropic at null voltage, and birefringence could be induced by applying an electric field, which revealed the intrinsic characteristics of a typical BPLC phase. As the filling mesogen had the same R1011 concentration as the original PSBP (11.5 wt%), the on-state voltage was very similar (101 V). Since the favorable conditions of the polymer network was preserved, the templated BP worked as effectively as the original PSBP. When the fraction of R1011 increased, the on-state voltage of the templated BP increased. The voltage fell to 84 V when the polymer template was filled with only the achiral nematic. When chiral mesogen containing S1011 was used as a filling material, the on-state voltage declined further. Therefore, one can realize that the on-state voltage was abated not only by the increased intrinsic birefringence and dielectric anisotropy (which resulted from a decrease in the fraction of chiral dopant) but also by another effect in the templated BP system.
Fig. 4 The VT curves for templated BPs. Solid lines correspond to the curves with increasing voltage, and dash lines correspond to the curves with decreasing voltage. (Ch-D: chiral dopant).
Once a templated BP is formed from a stabilizing polymer template infused with mesogen, the molecules conform to the BP structure, even when most lack chirality. Meanwhile, an Elastic Restoring Force (ERF) is established among mesogenic molecules. The magnitude of the ERF is governed by both the twisting power and handedness of the filling mesogen relative to those of the original PSBP. When the mesogenic molecules are tilted by applying a voltage, the ERF may help or hinder the electric field to unwind the helical structure. The experimental results revealed the effect of ERF on electric field-induced birefringence under three different conditions. (i) If the filling mesogen has the same handedness as that of the original PSBP but has a shorter pitch, the ERF is exerted in the same direction as the polymer anchoring force. Hence, acquiring the same induced birefringence as the original PSBP requires a stronger electric field to counteract the ERF. (ii) If the filling mesogen has the same handedness but a longer pitch, or even if it has no chirality (nematic), then the ERF and the electric field synergistically unwind the helical structure because the mesogen tends to be less twisted intrinsically. (iii) If the filling mesogen exhibits reverse handedness, then the ERF is even higher than that in the second situation due to the inherently preferentially twist in the opposite direction. A decrease in the required driving voltage corresponds to an increased Kerr constant [23]. To demonstrate the underlying theory of this effect, the Rao’s model of the on-state voltage [8] is combined with the Gerber’s model of the Kerr constant (K) [24]. Therefore, the on-state voltage (Von) of a BP system can be estimated by applying the following equation:
where A is a device parameter that varies with the electrode configuration; λ is the probe beam wavelength ; Δn is the intrinsic birefringence; Δε is the dielectric anisotropy; ε0 is the permittivity of a vacuum; P is the pitch, and k is the effective elastic constant of this system. If the ERF is exerted against the polymer anchoring force [situations (ii) and (iii)], they cancel each other out, and only the residual anchoring force is resisted by the electric field. This phenomenon can cause a different drop in the effective elastic constant of the BP system in comparison with the original PSBP. Similarly, the systemic elastic constant is increased when both the ERF and the polymer anchoring force are exerted in the same direction [situation (i)]. The experimental data (especially the two S1011 curves) indicate that the change in on-state voltage resulted not only from the effects of intrinsic birefringence and dielectric anisotropy, but also from the effects of chirality-dependent ERF. The Kerr constants of all the templeted BPs were calculated and listed in Table 1. For the original PSBP, The Kerr constant was only ~0.37 nm/V2 owing to liquid crystal material and the polymerization conditions. But by means of the templating technique, the Kerr constant can be enhanced to 0.983 nm/V2. The VT curves for a cycle of ascending and descending voltage scans were measured to show the hysteresis property in Fig. 4. The hystereses under different doping conditions listed in Table 1 shows that hysteresis is small if the filling mesogen has the same handedness as that of the original PSBP. Additionally, hysteresis increases as chiral concentration decreases, which agree with the findings by Lan et al. [25] However, the addition of reverse-handed chiral agent substantially enlarged hysteresis. Therefore, the hysteresis was believed to be caused not only from the pitch-induced relaxation ability, but also due to the difference between the pitch of the original PSBP and that of the filling mesogen. When the pitch difference was elevated, the strong ERF prevented the polymer template from maintaining the BP texture and lowered the relaxation ability. Hysteresis might be improved by optimizing the polymerization condition or by varying the alignment condition [18].The ERF affected not only the driving voltage, but also the response time. Table 1 lists the examined electro-optical properties of each sample, including on-state voltage (Von), Kerr constant [23] (K), decay time (τ), hysteresis, and normalized figure of merit (N-FoM) [26]. All decay times measured for the templated BPs were under 1 ms; the fast response resulted from the dense polymer network. As the R1011 concentration decreased, the measured decay time almost remained unchanged. Based on the Gerber model, the decay time in a BP system can be expressed as [24],
where γ1 denotes the rotational viscosity. Although the systemic elastic constant declined as the R1011 fraction decreased, the speed of the response was apparently unchanged, probably because the drop in doping concentration of chiral also reduced the viscosity of the filling mesogen [27]. As the proportion of the chiral dopant decreased to zero, the decay time increased to 0.583 ms. When R1011 was replaced by S1011, the increase in decay time was highly dependent on the fraction of the reverse-handed chiral dopant, probably because the systemic elastic constant continued dropping whereas the rotational viscosity rose owing to the doping of the chiral agent. Figure of merit (FoM) is defined as the ratio of Kerr constant to response time. Reducing the R1011 concentration raised the FoM by decreasing the on-state voltage with a slightly increase in decay time. If the filling material was replaced by the one containing S1011, FoM further improved. However, the drawback of using a templated BP with reverse-handed chiral dopant is the relatively larger hysteresis. Moreover, in this case, 4 wt% was the S1011 fraction limit because the polymer template could not maintain the BP texture, due to the strong ERF induced by the higher doping concentration of the reverse-handed chiral agent.5. Conclusion
In summary, templated BPs were fabricated by washing out the nonreactive part of the original PSBP containing 12.5 wt% polymer and then refilling the template with mesogen containing varying concentrations and varying handedness of chiral dopant. The experimental results indicated that on-state voltage decreased as the R1011 ratio decreased. The on-state voltage further decreased when the chiral dopant was replaced with S1011 (with the opposite handedness to that of R1011). A model of the ERF effect on the effective elastic constant of a templated BP system was established to explain the variation of electro-optical properties. Refilling the polymer template with an achiral mesogen or a reverse-handed chiral mesogen efficiently reduces the driving voltage while only slightly increasing the response time over that of the original PSBP. In addition, the template method is compatible with other methods for improving properties of PSBPs. After applying those methods, the templating technique can still achieve an efficient increase in the Kerr constant and a corresponding increase in FoM.
Acknowledgment
The authors gratefully acknowledge InnoLux Corporation and the National Science Council of Taiwan under Contract No. NSC99-2119-M-110-006-MY3 for financially supporting this research. Ted Knoy is appreciated for his editorial assistance.
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