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A blue phase is induced by bent-shaped monomer with allylic end bonds when doped into chiral nematic liquid crystal. The mechanism of blue phase induction and stabilization is investigated. Its temperature range is further widened to 10.2 K by UV irradiation with a slowly cooling process. The widening principle is distinguishable from traditional polymer stabilization mechanism. The study provides some useful insights into the molecular design of suitable bent-shaped dopants towards wide range blue phase liquid crystals.
©2011 Optical Society of America
1. Introduction
In recent years, blue phase liquid crystals (BPLCs) have attracted a lot of attentions. Blue phases (BPs) are always found in a highly twisted system and exist in a very narrow temperature range between a chiral nematic phase and an isotropic phase. They consist of double-twist self-assembled LC cylinders, stacked in three dimensions with cubic symmetry [1,2]. As a result of the structural symmetry, BPs are optically isotropic. Corresponding to the nano-scale cubic lattice, BPs provide selective Bragg reflections in the range of visible light and birefringence will be produced by Kerr effect when voltage applied. It endows BPLCs with very fast response time down to sub-microsecond. Due to the double-twisted arrangment, long-range order is impossible for BPs and defects are inevitable to occupy a three dimensional space in nano-scale. For general case, the temperature range of BPs is less than a few Kelvin (K). Therefore, although a lot of attentions have been paid on the blue phases in wide fields, such as fast light modulators and tunable photonic crystals, the real application of BPs has been drastically limited [3].
The coexistence of defects with BPLCs is associated with certain energy consumption. Normally, the blue phase is unstable unless certain energy is supplemented. To endow BPLCs with a wide temperature range, an artificial control of energy supplement has to be settled. So far much effort has been made on this subject. One notable attempt to widen the temperature range of BPs is by polymer stabilization [4]. Kikuchi et al. reported the stabilization of the three-dimensional cubic lattice in a defect confined polymer matrix and demonstrated an effective way to extend the temperature range of BPs [3]. Polymer-stabilized blue phase (PSBP) with temperature range over 60°C has been obtained [5]. As a result, the applications of BP materials in information display and non-display fields have been significantly increased [6–8]. However, some shortcomings such as high driving voltage, hysteresis and residual birefringence are still need to be settled [9]. For these reasons, new attempts towards BPLCs with wide temperature range are still in urgent demand. A strong twist force can improve the stability of BPs, so wide temperature range BPs could be generated through synthesis of liquid crystals with higher helical twisting power (HTP). Coles et al. reported a dimer liquid crystal with large flexoelectricity that has 44 K - BPs [10]. Yoshizawa et al. demonstrated some T-shaped molecules and binaphthyl derivatives with temperature ranges of 13 K and 29 K respectively [11,12]. In 2003, Nakata et al. noticed an exotic phenomenon that the blue phase can be induced by doping some achiral bent-shaped molecules into chiral nematic liquid crystals(N*LCs) [13]. The result is exciting, though the blue phase range is not exceeding 5 K and only a preliminary study on mechanism was presented. Recent studies demonstrated a wide temperature range BPs by doping with bent-shaped molecules into N*LCs host [14] and effects of terminal chain length on the bent-shaped molecule induced BPs were revealed [15].
In this work, we report a BPLC induced by doping achiral monomer with bent-shaped structure and allylic end bonds into a chiral nematic liquid crystal. By further UV curing in a precisely-temperature-controlled cooling process, a wide temperature range up to 10.2°C was achieved. The thermo-optical properties of the BPLC were investigated and the temperature widen mechanism of UV curing was analyzed through FTIR methodology.
2. Experiment
Two LC mixtures (I and II) were prepared. Mixture I was composed of eutectic nematic LCs (67.2%, Slichem, SLC-9023) and chiral dopant (32.8%, Merk, R811), while Mixture II was mixed by 7% of bent-shaped monomer ((para-(2-propenylbenzoate) diester of 2,5-bis(p-hydroxyphenyl)-1,3,4-oxadiazole (ODBP-Ph-pr), molecular structure is shown in Fig. 1(a) ) and 93% of Mixture I. The LC mixtures were capillary filled into 18 μm-thick cells with no alignment layers at isotropic phase (at 90°C in our experiments) and kept heating for 15 min to suppress the flow orientation. During the thermal-optical analysis, samples were heated to an isotropic phase first, and cooled down to a uniform chiral nematic phase at a rate of 0.5°C/min with temperature precisely controlled by a heating and cooling stage (Linkam SLT 120). The LC phases and textures were observed under a transmission polarizing microscope with orthogonal polarizers (Olympus BX-51).
Fig. 1 (a) Chemical structure and (b) the atomistic of ODBP-Ph-pr with a bent angle of 62° and dipole moment of 3.767D.
UV curing is executed under a monochromatic UV LED (365 ± 10 nm, 166 mW/cm2) for 10 min to combine the adjacent ODBP-Ph-pr molecules. Pure ODBP-Ph-pr was sandwiched in two CaF2 substrates and UV cured at a liquid state in a same process shown above. IR spectra of the sample fore-and-aft exposure were recorded on a IR spectrometer (PERKIN ELMER Spectrum GX).
3. Results and discussions
The phase transitions and temperature ranges of the mixtures are tested. During the cooling process, Mixture I would go from isotropic (ISO) phase directly into N*LC phase. The phase transition occurred at 76.0 °C, which is the clearing point of the material. Figures 2(a) –2(c) are the images recorded in the process. (a) shows a homogeneously dark state of ISO phase; (b) exhibits the appearance of N*LC phase at the clearing point, as focal conic texture is observed in this image; Keeping or decreasing the temperature would eventually lead to a uniform N*LC phase, as shown in (c). The inhomogeneous transmittance in the image results from the different orientations of LC in adjacent domains. However, during the same cooling process, Mixture II exhibited different phase transitions. Above the clearing point, it shows a same dark state (Fig. 2(d)) as Mixture I. At 78.7 °C, BP appeared. Colorful platelets are observed corresponding to the selective Bragg reflection of different lattices, as shown in (e). The transformation point shifting is mainly due to the composition difference between the two mixtures. (f) depicts the transition from BP to N* state, starting at 72.5 °C. Colorful platelets are vanishing and focal conic texture of N* appears. Further decreasing the temperature results in a uniform N*LC. The BP is an enantiotropic phase and the phase transition temperatures were summarized below:
Fig. 2 Micrographs of: (a) ISO state, (b) phase transition from ISO to N*, (c) N* of Mixuture I, and (d) ISO state, (e) BP, (f) phase transition from BP to N* of Mixture II. The in situ temperatures when taking each micrograph were labeled at the left bottom respectively. Each sale bar represents 10 μm.
In Mixture I, LC exhibits chiral nematic phase twisted-arranged with only one helical axis under the role of chiral R811. However, a blue phase range of about 6 K is induced by extra doping a small amount of ODBP-Ph-pr. It indicates the bent-shaped monomers play an important role in the induction of double-twisted arrangement, which is the feature arrangement of BPLCs. The role of molecular biaxiality in the blue phase has been studied both experimentally and theoretically [11,12]. As shown in Fig. 1(b), the geometry of ODBP-Ph-pr molecule is bent-shaped with two rigid arms and the axes are represented by dashed lines. The calculated bent angle of the molecules is 62° with a molecular dipole moment of 3.767D. As previous research demonstrated, large bent angle and long rigid body can generate a strong biaxiality [14]. The strong polarity confirms a large interaction of ODBP-Ph-pr with other molecules in the mixture. That means the ODBP-Ph-pr can molecularly interfuse the helical cylinders with one arm following the twisted arrangement normally, while the other arm will realign adjacent LC molecules and stretch to a new helical cylinder in corresponding direction. Therefore, double-twisted arranged BPLC is formed, as illustrated in Fig. 3 .
BP is a coexisting state of cubic lattices and defects as mentioned above, which makes the system unstable. Stability of BP mainly depends on the interfacial energy between double-twisted LCs and defects [3]. Normally, a strong twist will reduce this consumption and stabilize the system [16]. However, the bent-shaped monomer here is achiral, and it has been proved that the twist energy of the LC doped with this kind of molecules is almost invariant [14]. Therefore, a different mechanism should be introduced. Herein interfacial energy decreases as a result of the large molecular volume of the dopant, which extends the distance between molecules and decreases the van der Waals energy of the system [17]. Moreover, due to the inflexibility of ODBP-Ph-pr molecules, the rigid crosslinks further stabilize the double-twisted cylinder systems.
As mentioned previously, the BP of Mixture II appears at 78.7 °C and vanishes at 72.5 °C, which is presented in Fig. 4(a) . The BP range is 6.2 K. To widen the temperature range of BP, UV curing was carried out. We found that the BP temperature range could be optimized by keeping the system at BP state through the whole process. The exposing was carried out during cooling at a rate of −0.5 °C/min. The irradiation starts at 74.5 °C and ends at 69.5 °C with the system in uniform BP. After curing, the sample was examined again under a polarizing microscope. As shown in Fig. 4(b), the temperature range is widened to 10.2 °C, from 73.7 °C to 63.5 °C, which is 1.7 times as large as that before exposure.
Fig. 4 Phase sequence of the BP from the isotropic to nematic phases. (a) phase sequence before exposure; (b) phase sequence with a cooling rate of 0.5°C /min.
We consider that the photoreaction of ODBP-Ph-pr contributes to the widening of BP range. To prove it, IR spectra of pure ODBP-Ph-pr were measured before and after UV exposure, exhibited in Figs. 5(a) and 5(b) respectively. After curing, the characteristic peaks at 3075 cm- and 1640 cm- (corresponding to stretching vibrations of H-C = and C = C of allylic end bonds respectively) decrease considerably. Calculation reveals that over 10% of double bonds reacted. In Mixture II the concentration of ODBP-Ph-pr is only 7% (an optimized concentration range for the system is 7-9%, values apart from the range make against the formation of double twisted arrangement of LCs), indicates the chances of effective collision and thus the conversion efficiency would be dramatically reduced. We have measured the IR spectra of Mixture II fore-and-aft irradiation, and the corresponding peaks weakened slightly. That suggests not polymer but only some small molecules with longer rigid bonds were generated in curing. Thereby the mechanism of stabilization herein is quite different from traditional polymer stabilized BPs, which forms some polymer matrices in the defects [3–5].
As described in [14], longer rigid body of biaxial dopants would facilitate the BP stability, which is coincident with our experimental results. Long rigid body leads to decrease of interfacial tension, increase of elastic constant as well as shrinkage of the defect core radius, all of which facilitates the decrease of the interfacial energy between double-twisted LCs and defects, and makes the mixtures more stable. Besides, dipole moment would also increase as the extending of rigid body. A strong dipole moment leads to greater interaction with LCs and strengthens the biaxiality [18], which has a significant impact on the stabilization of BPs also.
4. Conclusion
In summary, the BP of N*LC is induced by doped with ODBP-Ph-pr with a temperature range of 6 K. Besides, ODBP-Ph-pr here also acts as photoreactive monomer. Through exposing the sample at BP state, a wider temperature range over 10 K is obtained. This study provides some useful insights into the molecular design of suitable bent-shaped dopants towards wide range blue phase liquid crystals.
Acknowledgments
This work is sponsored by the 973 Program with contract No. 2011CBA00200 and 2012CB921803, NSFC programs under contract No. 10874080, 60878047 and 61108065, the NSFJP program under contract No. BK2010360. The authors also thank the supports from PAPD and Fundamental Research Funds for the Central Universities.
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