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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 1, Iss. 8 — Dec. 1, 2011
  • pp: 1478–1483
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Liquid crystal blue phase induced by bent-shaped molecules with allylic end groups

Ge Zhu, Xiao-Wen Lin, Wei Hu, Zhi-Gang Zheng, Hai-Feng Wang, Hong-Qing Cui, Dong Shen, and Yan-Qing Lu  »View Author Affiliations


Optical Materials Express, Vol. 1, Issue 8, pp. 1478-1483 (2011)
http://dx.doi.org/10.1364/OME.1.001478


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Abstract

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 OSA

1. Introduction

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

4. H. S. Kitzerow, H. Schmid, A. Ranft, G. Heppke, R. A. M. Hikmet, and J. Lub, “Observation of blue phases in chiral networks,” Liq. Cryst. 14(3), 911–916 (1993). [CrossRef]

]. 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

3. H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, and T. Kajiyama, “Polymer-stabilized liquid crystal blue phases,” Nat. Mater. 1(1), 64–68 (2002). [CrossRef] [PubMed]

]. Polymer-stabilized blue phase (PSBP) with temperature range over 60°C has been obtained [5

5. Y. Hisakado, H. Kikuchi, T. Nagamura, and T. Kajiyama, “Large electro-optic Kerr effect in polymer-stabilized liquid-crystalline blue phases,” Adv. Mater. (Deerfield Beach Fla.) 17(1), 96–98 (2005). [CrossRef]

]. As a result, the applications of BP materials in information display and non-display fields have been significantly increased [6

6. Y. H. Lin, H. S. Chen, H. C. Lin, Y. S. Tsou, H. K. Hsu, and W. Y. Li, “Polarizer-free and fast response microlens arrays using polymer-stabilized blue phase liquid crystals,” Appl. Phys. Lett. 96(11), 113505 (2010). [CrossRef]

8

8. Y. Li, M. Z. Jiao, and S. T. Wu, “Transflective display using a polymer-stabilized blue-phase liquid crystal,” Opt. Express 18(16), 16486–16491 (2010). [CrossRef] [PubMed]

]. However, some shortcomings such as high driving voltage, hysteresis and residual birefringence are still need to be settled [9

9. K. M. Chen, S. Gauza, H. Q. Xianyu, and S. T. Wu, “Hysteresis effects in blue-phase liquid crystals,” J. Disp. Technol. 6(8), 318–322 (2010). [CrossRef]

]. 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

10. H. J. Coles and M. N. Pivnenko, “Liquid crystal ‘blue phases’ with a wide temperature range,” Nature 436(7053), 997–1000 (2005). [CrossRef] [PubMed]

]. Yoshizawa et al. demonstrated some T-shaped molecules and binaphthyl derivatives with temperature ranges of 13 K and 29 K respectively [11

11. A. Yoshizawa, Y. Kogawa, K. Kobayashi, Y. Takanishi, and J. Yamamoto, “A binaphthyl derivative with a wide temperature range of a blue phase,” J. Mater. Chem. 19(32), 5759–5764 (2009). [CrossRef]

,12

12. A. Yoshizawa, M. Sato, and J. Rokunohe, “A blue phase observed for a novel chiral compound possessing molecular biaxiality,” J. Mater. Chem. 15(32), 3285–3290 (2005). [CrossRef]

]. 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

13. M. Nakata, Y. Takanishi, J. Watanabe, and H. Takezoe, “Blue phases induced by doping chiral nematic liquid crystals with nonchiral molecules,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 68(4), 041710 (2003). [CrossRef] [PubMed]

]. 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

14. Z. G. Zheng, D. Shen, and P. Huang, “Wide blue phase range of chiral nematic liquid crystal doped with bent-shaped molecules,” New J. Phys. 12(11), 113018 (2010). [CrossRef]

] and effects of terminal chain length on the bent-shaped molecule induced BPs were revealed [15

15. Z. G. Zheng, D. Shen, and P. Huang, “The liquid crystal blue phase induced by bent-shaped molecules with different terminal chain lengths,” New J. Phys. 13(6), 063037 (2011). [CrossRef]

].

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)
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.
) 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).

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)
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.
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:

N*72.566.7BP78.770.6ISO

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)
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.
. 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.

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)
Fig. 5 IR spectra of ODBP-Ph-pr before (a) and after exposure (b).
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

3. H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, and T. Kajiyama, “Polymer-stabilized liquid crystal blue phases,” Nat. Mater. 1(1), 64–68 (2002). [CrossRef] [PubMed]

5

5. Y. Hisakado, H. Kikuchi, T. Nagamura, and T. Kajiyama, “Large electro-optic Kerr effect in polymer-stabilized liquid-crystalline blue phases,” Adv. Mater. (Deerfield Beach Fla.) 17(1), 96–98 (2005). [CrossRef]

].

As described in [14

14. Z. G. Zheng, D. Shen, and P. Huang, “Wide blue phase range of chiral nematic liquid crystal doped with bent-shaped molecules,” New J. Phys. 12(11), 113018 (2010). [CrossRef]

], 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

18. M. A. Bates, “Bent core molecules and the biaxial nematic phase. A transverse dipole widens the optimal angle,” Chem. Phys. Lett. 437(4-6), 189–192 (2007). [CrossRef]

], 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

References and links

1.

H. Stegemeyer, T. Blumel, K. Hiltrop, H. Onusseit, and F. Porsch, “Thermodynamic, structural and morphological studies on liquid-crystalline blue phases,” Liq. Cryst. 1(1), 3–28 (1986). [CrossRef]

2.

W. Y. Cao, A. Muñoz, P. Palffy-Muhoray, and B. Taheri, “Lasing in a three-dimensional photonic crystal of the liquid crystal blue phase II,” Nat. Mater. 1(2), 111–113 (2002). [CrossRef] [PubMed]

3.

H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, and T. Kajiyama, “Polymer-stabilized liquid crystal blue phases,” Nat. Mater. 1(1), 64–68 (2002). [CrossRef] [PubMed]

4.

H. S. Kitzerow, H. Schmid, A. Ranft, G. Heppke, R. A. M. Hikmet, and J. Lub, “Observation of blue phases in chiral networks,” Liq. Cryst. 14(3), 911–916 (1993). [CrossRef]

5.

Y. Hisakado, H. Kikuchi, T. Nagamura, and T. Kajiyama, “Large electro-optic Kerr effect in polymer-stabilized liquid-crystalline blue phases,” Adv. Mater. (Deerfield Beach Fla.) 17(1), 96–98 (2005). [CrossRef]

6.

Y. H. Lin, H. S. Chen, H. C. Lin, Y. S. Tsou, H. K. Hsu, and W. Y. Li, “Polarizer-free and fast response microlens arrays using polymer-stabilized blue phase liquid crystals,” Appl. Phys. Lett. 96(11), 113505 (2010). [CrossRef]

7.

H. Y. Liu, C. T. Wang, C. Y. Hsu, T. H. Lin, and J. H. Liu, “Optically tuneable blue phase photonic band gaps,” Appl. Phys. Lett. 96(12), 121103 (2010). [CrossRef]

8.

Y. Li, M. Z. Jiao, and S. T. Wu, “Transflective display using a polymer-stabilized blue-phase liquid crystal,” Opt. Express 18(16), 16486–16491 (2010). [CrossRef] [PubMed]

9.

K. M. Chen, S. Gauza, H. Q. Xianyu, and S. T. Wu, “Hysteresis effects in blue-phase liquid crystals,” J. Disp. Technol. 6(8), 318–322 (2010). [CrossRef]

10.

H. J. Coles and M. N. Pivnenko, “Liquid crystal ‘blue phases’ with a wide temperature range,” Nature 436(7053), 997–1000 (2005). [CrossRef] [PubMed]

11.

A. Yoshizawa, Y. Kogawa, K. Kobayashi, Y. Takanishi, and J. Yamamoto, “A binaphthyl derivative with a wide temperature range of a blue phase,” J. Mater. Chem. 19(32), 5759–5764 (2009). [CrossRef]

12.

A. Yoshizawa, M. Sato, and J. Rokunohe, “A blue phase observed for a novel chiral compound possessing molecular biaxiality,” J. Mater. Chem. 15(32), 3285–3290 (2005). [CrossRef]

13.

M. Nakata, Y. Takanishi, J. Watanabe, and H. Takezoe, “Blue phases induced by doping chiral nematic liquid crystals with nonchiral molecules,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys. 68(4), 041710 (2003). [CrossRef] [PubMed]

14.

Z. G. Zheng, D. Shen, and P. Huang, “Wide blue phase range of chiral nematic liquid crystal doped with bent-shaped molecules,” New J. Phys. 12(11), 113018 (2010). [CrossRef]

15.

Z. G. Zheng, D. Shen, and P. Huang, “The liquid crystal blue phase induced by bent-shaped molecules with different terminal chain lengths,” New J. Phys. 13(6), 063037 (2011). [CrossRef]

16.

H. Kikuchi, “Liquid crystalline blue phases,” Struct. Bonding 128, 99–117 (2008). [CrossRef]

17.

J. Klosterman, L. V. Natarajan, V. P. Tondiglia, R. L. Sutherland, T. J. White, C. A. Guymon, and T. J. Bunning, “The influence of surfactant in reflective HPDLC gratings,” Polymer (Guildf.) 45(21), 7213–7218 (2004). [CrossRef]

18.

M. A. Bates, “Bent core molecules and the biaxial nematic phase. A transverse dipole widens the optimal angle,” Chem. Phys. Lett. 437(4-6), 189–192 (2007). [CrossRef]

OCIS Codes
(160.3710) Materials : Liquid crystals
(160.4670) Materials : Optical materials
(160.4890) Materials : Organic materials
(260.1440) Physical optics : Birefringence

ToC Category:
Liquid Crystals

History
Original Manuscript: September 15, 2011
Revised Manuscript: October 22, 2011
Manuscript Accepted: October 23, 2011
Published: November 4, 2011

Virtual Issues
Liquid Crystal Materials for Photonic Applications (2011) Optical Materials Express

Citation
Ge Zhu, Xiao-Wen Lin, Wei Hu, Zhi-Gang Zheng, Hai-Feng Wang, Hong-Qing Cui, Dong Shen, and Yan-Qing Lu, "Liquid crystal blue phase induced by bent-shaped molecules with allylic end groups," Opt. Mater. Express 1, 1478-1483 (2011)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-1-8-1478


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References

  1. H. Stegemeyer, T. Blumel, K. Hiltrop, H. Onusseit, and F. Porsch, “Thermodynamic, structural and morphological studies on liquid-crystalline blue phases,” Liq. Cryst.1(1), 3–28 (1986). [CrossRef]
  2. W. Y. Cao, A. Muñoz, P. Palffy-Muhoray, and B. Taheri, “Lasing in a three-dimensional photonic crystal of the liquid crystal blue phase II,” Nat. Mater.1(2), 111–113 (2002). [CrossRef] [PubMed]
  3. H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, and T. Kajiyama, “Polymer-stabilized liquid crystal blue phases,” Nat. Mater.1(1), 64–68 (2002). [CrossRef] [PubMed]
  4. H. S. Kitzerow, H. Schmid, A. Ranft, G. Heppke, R. A. M. Hikmet, and J. Lub, “Observation of blue phases in chiral networks,” Liq. Cryst.14(3), 911–916 (1993). [CrossRef]
  5. Y. Hisakado, H. Kikuchi, T. Nagamura, and T. Kajiyama, “Large electro-optic Kerr effect in polymer-stabilized liquid-crystalline blue phases,” Adv. Mater. (Deerfield Beach Fla.)17(1), 96–98 (2005). [CrossRef]
  6. Y. H. Lin, H. S. Chen, H. C. Lin, Y. S. Tsou, H. K. Hsu, and W. Y. Li, “Polarizer-free and fast response microlens arrays using polymer-stabilized blue phase liquid crystals,” Appl. Phys. Lett.96(11), 113505 (2010). [CrossRef]
  7. H. Y. Liu, C. T. Wang, C. Y. Hsu, T. H. Lin, and J. H. Liu, “Optically tuneable blue phase photonic band gaps,” Appl. Phys. Lett.96(12), 121103 (2010). [CrossRef]
  8. Y. Li, M. Z. Jiao, and S. T. Wu, “Transflective display using a polymer-stabilized blue-phase liquid crystal,” Opt. Express18(16), 16486–16491 (2010). [CrossRef] [PubMed]
  9. K. M. Chen, S. Gauza, H. Q. Xianyu, and S. T. Wu, “Hysteresis effects in blue-phase liquid crystals,” J. Disp. Technol.6(8), 318–322 (2010). [CrossRef]
  10. H. J. Coles and M. N. Pivnenko, “Liquid crystal ‘blue phases’ with a wide temperature range,” Nature436(7053), 997–1000 (2005). [CrossRef] [PubMed]
  11. A. Yoshizawa, Y. Kogawa, K. Kobayashi, Y. Takanishi, and J. Yamamoto, “A binaphthyl derivative with a wide temperature range of a blue phase,” J. Mater. Chem.19(32), 5759–5764 (2009). [CrossRef]
  12. A. Yoshizawa, M. Sato, and J. Rokunohe, “A blue phase observed for a novel chiral compound possessing molecular biaxiality,” J. Mater. Chem.15(32), 3285–3290 (2005). [CrossRef]
  13. M. Nakata, Y. Takanishi, J. Watanabe, and H. Takezoe, “Blue phases induced by doping chiral nematic liquid crystals with nonchiral molecules,” Phys. Rev. E Stat. Nonlin. Soft Matter Phys.68(4), 041710 (2003). [CrossRef] [PubMed]
  14. Z. G. Zheng, D. Shen, and P. Huang, “Wide blue phase range of chiral nematic liquid crystal doped with bent-shaped molecules,” New J. Phys.12(11), 113018 (2010). [CrossRef]
  15. Z. G. Zheng, D. Shen, and P. Huang, “The liquid crystal blue phase induced by bent-shaped molecules with different terminal chain lengths,” New J. Phys.13(6), 063037 (2011). [CrossRef]
  16. H. Kikuchi, “Liquid crystalline blue phases,” Struct. Bonding128, 99–117 (2008). [CrossRef]
  17. J. Klosterman, L. V. Natarajan, V. P. Tondiglia, R. L. Sutherland, T. J. White, C. A. Guymon, and T. J. Bunning, “The influence of surfactant in reflective HPDLC gratings,” Polymer (Guildf.)45(21), 7213–7218 (2004). [CrossRef]
  18. M. A. Bates, “Bent core molecules and the biaxial nematic phase. A transverse dipole widens the optimal angle,” Chem. Phys. Lett.437(4-6), 189–192 (2007). [CrossRef]

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