Polymer-stabilized blue-phase liquid crystal (BPLC) comprising fluorinated compounds with high resistivity and photochemical stability is demonstrated. The Kerr constant, driving voltage, and response time of this BPLC are measured using an in-plane switching liquid crystal cell. At 20 degrees C, the measured total response time is faster than 0.7 ms and Kerr constant is 2 nm/V-2. This fluorinated BPLC material is a promising candidate for next-generation photonic and display devices, because it can be used in active matrix addressed devices

Chen, Y.

Chojnowska, O.

Dabrowski, R.

Wu, S. T.

Yan, J.

University of Central Florida (UCF): STARS (Showcase of Text, Archives, Research & Scholarship)

University of Central Florida STARS Faculty Bibliography 2010s Faculty Bibliography 1-1-2014 Electro-optical properties of photochemically stable polymer-stabilized blue-phase material O. Chojnowska R. Dabrowski J. Yan University of Central Florida Y. Chen University of Central Florida S. T. Wu University of Central Florida Find similar works at: https://stars.library.ucf.edu/facultybib2010 University of Central Florida Libraries http://library.ucf.edu This Article is brought to you for free and open access by the Faculty Bibliography at STARS. It has been accepted for inclusion in Faculty Bibliography 2010s by an authorized administrator of STARS. For more information, please contact STARS@ucf.edu. Recommended Citation Chojnowska, O.; Dabrowski, R.; Yan, J.; Chen, Y.; and Wu, S. T., "Electro-optical properties of photochemically stable polymer-stabilized blue-phase material" (2014). Faculty Bibliography 2010s. 5187. https://stars.library.ucf.edu/facultybib2010/5187 J. Appl. Phys. 116, 213505 (2014); https://doi.org/10.1063/1.4903551 116, 213505© 2014 AIP Publishing LLC.Electro-optical properties of photochemicallystable polymer-stabilized blue-phasematerialCite as: J. Appl. Phys. 116, 213505 (2014); https://doi.org/10.1063/1.4903551Submitted: 05 August 2014 . Accepted: 24 November 2014 . Published Online: 04 December 2014O. Chojnowska, R. Dąbrowski, J. Yan, Y. Chen, and S. T. WuARTICLES YOU MAY BE INTERESTED INNear infrared Kerr effect and description of field-induced phase transitions in polymer-stabilized blue phase liquid crystalsApplied Physics Letters 108, 081107 (2016); https://doi.org/10.1063/1.4942604Fringing field-induced monodomain of a polymer-stabilized blue phase liquid crystalApplied Physics Letters 107, 241105 (2015); https://doi.org/10.1063/1.4938072Polymer-stabilized supercooled blue phaseApplied Physics Letters 101, 131904 (2012); https://doi.org/10.1063/1.4752461Electro-optical properties of photochemically stable polymer-stabilizedblue-phase materialO. Chojnowska,1,a) R. Dąbrowski,1 J. Yan,2 Y. Chen,2 and S. T. Wu21Institute of Chemistry, Military University of Technology, Warsaw 00-908, Poland2College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USA(Received 5 August 2014; accepted 24 November 2014; published online 4 December 2014)Polymer-stabilized blue-phase liquid crystal (BPLC) comprising fluorinated compounds with highresistivity and photochemical stability is demonstrated. The Kerr constant, driving voltage, andresponse time of this BPLC are measured using an in-plane switching liquid crystal cell. At 20 C,the measured total response time is faster than 0.7 ms and Kerr constant is 2 nm/V2. This fluorinatedBPLC material is a promising candidate for next-generation photonic and display devices, becauseit can be used in active matrix addressed devices. VC 2014 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4903551]I. INTRODUCTIONLiquid crystalline blue phase is a promising material forfast-response displays,1–3 three-dimensional lasers,4,5 and newphotonic devices.6 It exhibits several revolutionary features incomparison to conventional nematic materials, such as sub-millisecond response time,7,8 optically isotropic dark state,and no need for surface alignment layer. There are, however,a few remaining challenges that need to be overcome: hystere-sis, high operating voltage, and thermal instability.Three blue phases BPI, BPII, or BPIII exist between cho-lesteric and isotropic phase in a very narrow temperature range(from 0.1 K to 5 K). It is proposed9 that blue phase has periodiccubic lattice of double twist cylinders in which the LC directorrotates spatially about any radius of the cylinder and is parallelto the axis at the centre of the cylinder. The twist in each cylin-der can be right- or left-handed, twisting to 45 at the bound-ary. Matching these tubes in three-dimensional space isimpossible, which induces disclinations or defects in moleculararrangement. The blue phase occurs due to the balancebetween defects and local energy minimum of LC director andis a frustrated system. The structures of BPI and BPII havebeen identified in many experiments and theories.10 BPI hasbody-centred cubic symmetry, BPII has simple cubic structure,and BPIII is amorphous with the short distance order of doubletwist alone. In frustrated systems, different terms in the freeenergy dictate different molecular arrangements, and the com-petition between them results in a more complex structure.11Defects in blue phase make the structure less stable, thus, bluephase exists in a narrow temperature range.To widen the blue phase range, several methods havebeen explored, such as adding nanoparticles,12 bent-shapedmolecules,13 T-shaped molecules,14 polymer network,15 orcombining several methods together.16 The great majority ofreported and measured blue phase systems are stabilizedby a polymer network with a LC host having cyano com-pound to increase dielectric anisotropy. Usually, 4-cyano-40-pentylbiphenyl (5CB),17,18 4-cyano-40-(2-methyl)butylphenyl(CB15),19,20 and cyanoesters21 are mentioned. In our recentwork,22,23 we demonstrated that the presence of 5CB hasentailed reduction of BP temperature range and clearing point.Additionally, it complicates the temperature control of BP ma-terial doped with polymer precursor during UV exposure.In this paper, we report a polymer-stabilized blue phase(PSBP) system comprising fluorinated compounds with highresistivity and high photochemical stability. They can beapplied to devices addressed by an active matrix due to theirhigh voltage holding ratio (VHR).24–26 The reported systemshould be useful for display and non-display applications,such as spatial light modulators, photonic crystal fibers, andlasers.II. EXPERIMENTSA. Preparation of PSBPThe nematic LC host we employed is mixture 1911whose major compositions are fluorinated terphenyls,biphenyl, cyclohexylbiphenyls, and bicyclohexylbiphenyl.Table I lists the compound structures, phase transition temper-atures, and concentrations. Compounds 1–3 were provided byValiant Fine Chemicals and compounds 4–10 were synthe-sized in our laboratory. Some properties of mixture 1911 areas follows: melting point below 20 C, clearing point 82 C,birefringence Dn¼ 0.174 at 589 nm, dielectric anisotropyDe¼ 8.5 at 1 kHz, and bulk viscosity g¼ 30 mPas, all meas-ured at 20 C.We prepared a blue phase precursor, we blendedtwo chiral dopants: biphenyl-4,40-dicarboxylic acid bis-(1-methylheptyl) ester and [1,10;40,100]terphenyl-4,400-dicarbox-ylic acid bis-(1-methylheptyl) ester (both synthesized inour laboratory27) as well as two reactive acrylates: 1,4-bis-[4-(3-acryloyloxypropoxy) benzoyloxy]-2-methylbenzene(from CHEMOS), 1,1,1-tri-(acryloyloxymethyl)-propane (fromSARTOMER), and photoinitiator 2-hydroxy-2-methyl-1-phenylpropan-1-one (from SIGMA-ALDRICH) at 6.3 wt. %,6.3 wt. %, 6.0 wt. %, 4.0 wt. %, and 0.5 wt. % concentrations,respectively, to mixture 1911. Out of all the components onlydifunctional acrylate has nematic phase, neither the mono-functional acrylate nor the chiral dopants exhibit mesophase.a)Author to whom correspondence should be addressed. Electronic mail:ochojnowska@wat.edu.pl.0021-8979/2014/116(21)/213505/4/$30.00 VC 2014 AIP Publishing LLC116, 213505-1JOURNAL OF APPLIED PHYSICS 116, 213505 (2014)The mixture of nematic host, chiral dopants, and polymer pre-cursor was heated to 70 C to dissolve all components andthen it was filled into an empty cell at the same temperature(the mixture was at an isotropic state). To measure theelectro-optical properties of the material, we used an IPS cellwith following geometry: cell gap of 7.5 lm, indium tin oxideelectrode width of 10 lm, and electrode gap of 10 lm.The phase transition temperatures were determined by apolarizing optical microscope OLYMPUS BX51 equippedwith a LINKAM THMS600 heating stage. The temperaturewas controlled with an accuracy of 60.1 K. The precursorexhibits blue phase between 47.8 C and 52.4 C. To stabilizethe precursor, we heated the cell to 70 C, then cooled downto 47.7 C (the cooling rate was 1 K/min), and exposed to UVlight. The temperature in which the cell was irradiated was0.1 C below the transition from cholesteric to blue phase toavoid the transition to isotropic phase, which is possible dueto the heating during UV curing. Photo-polymerization wasperformed using a metal halide lamp (Hamamatsu PhotonicsLC6; k 365 nm) with 10 mW/cm2 intensity for 30 min at47.7 C. After polymerization, the cell was cooled down with1 K/min cooling rate. As observed from an optical micro-scope, the polymer-stabilized composite shows a stable bluephase from less than 30 C to 54.3 C (isotropic phase).Figure 1 shows the observed microscopic texture at 23 C.The voltage-dependent transmittance (VT) of filled IPScell was measured with a He-Ne laser (k¼ 633 nm) byapplying 100 Hz signal with rectangular waveform. Theinduced birefringence Dnind is related to wavelength k, elec-tric field E, and Kerr constant K asDnind ¼ kKE2: (1)However, we fit the VT curves with the extended Kerreffect model28 because the induced birefringence should besaturated in the strong electric field region according toEq. (2), where Dnsat represents the saturated induced bire-fringence and Es is saturated electric fieldDnind ¼ Dnsatð1 exp ½ðE=EsÞ2Þ: (2)In the weak field region, Eq. (2) can be expanded andreduced to Kerr effect (Eq. (1)) with the following relation:K ¼ Dnsat=ðkE2s Þ: (3)III. RESULTS AND DISCUSSIONSFigure 2 shows the normalized VT curves from 10 C to50 C. As the temperature increases, the on-state voltage(Von) increases as well. Since Kerr constant depends on theon-state voltage, the VT curves measured at each tempera-ture were fitted with extended Kerr effect model. The KerrTABLE I. Formula of components of the nematic host mixture 1911.No. Formula Phase sequence Concentration in mole fraction (%)1 Cr 65.1 N 108.2 Iso 20.92 Cr 67.6 N 98.6 Iso 7.83 Cr (60.9 N) 69.0 Iso 7.04 Cr 103.8 N 263.0 Iso 11.45 Cr 89.2 N 233.6 Iso 2.76 Cr 41.4 Iso 13.37 Cr 87.7 SmA 95.6 N 132.5 Iso 7.68 Cr 75.4 N 87.9 Iso 6.69 Cr 62.3 Iso 13.710 Cr 69.2 Iso 9.0FIG. 1. Polarized optical microscope image of PSBP at 23 C.213505-2 Chojnowska et al. J. Appl. Phys. 116, 213505 (2014)constants obtained at each temperature are shown in Fig. 3.The Kerr constant tends to saturate at low temperatures.29The highest Kerr constant was 2.1 nm/V2 at 10 C, while thelowest Kerr constant of 1.4 nm/V2 was observed at 50 C,which is close to the clearing point. A typical Kerr constantof a polymer-stabilized BP composite is about 1 nm/V2.30,31However, some BP materials with an extraordinarily largeKerr constant (K> 13.7 nm/V2) have been reported by usinga nematic host with a very large dielectric anisotropy(De> 100).32,33 The development goal for the PSBP materi-als is to increase Kerr constant to lower the driving voltage.To enhance Kerr constant, a liquid crystal with high birefrin-gence and large dielectric anisotropy is strongly desirable.Fast response time is one of the several outstanding fea-tures of BP in comparison to conventional nematic materials.It is less than 1 ms and is independent of the cell gap. Fastresponse time makes PSBP applicable to devices working inlonger ranges of electromagnetic radiation (infrared, giga-hertz, and terahertz frequency range), where the required LClayer is thicker. Response time was measured by applying avoltage corresponding to the maximum transmittance andthen removed instantaneously. The optical signals wererecorded by a photodiode detector and displayed in a digitaloscilloscope. Both rise time (son) and decay time (soff) weremeasured as the transmittance changes from 10% to 90%.Figures 4(a) and 4(b) show the measured temperature de-pendent rise time and decay time, respectively.The sum of rise time and decay time is in the range from1.266 ms (283 K) to 191 ls (323 K). At room temperature(293 K), rise time is 117 ls and decay time is 522 ls. Thesetimes are slightly shorter than those published in the literatureby other groups.34,35 The response time of a PSBP depends onthe type of polymer network, LC host, and chiral dopantsemployed. Generally, it decreases with increased degree ofcrosslinking, but the trade-off is increased operation voltage.High chemical and photochemical stability of fluori-nated liquid crystals opens many new applications where UVlight and visible light with high intensity are used. What ismore, compounds with fluorine substituents have high resis-tivity, which does not change after UV illumination, in con-trast to the compounds with cyano or isothiocyanato groups.The VHR of nematic base mixture 1911 is 99% and itslightly decreases to 98% after UV exposure. In contrast, theVHR is only 60% for cyano mixture like E7 and it decreasesto 51% after UV exposure.IV. CONCLUSIONSWe have demonstrated a polymer-stabilized blue phasecomposite based on a nematic mixture comprised fluorinatedFIG. 3. Temperature dependent Kerr constant of PSBP obtained by fittingwith extended Kerr model.FIG. 4. Measured temperature dependent (a) rise time and (b) decay time.FIG. 2. The VT curves of PSBP measured in an IPS cell at different tempera-tures, k¼ 633 nm. IPS cell: Electrode width¼ 10lm, electrode gap¼ 10lm,and cell gap¼ 7.5 lm.213505-3 Chojnowska et al. J. Appl. Phys. 116, 213505 (2014)compounds, which are chemically and photo-stable. Thevoltage holding ratio is very high VHR¼ 98%–99% anddoes not change after UV exposure. Proposed material hasKerr constant K¼ 2 nm/V2 at 20 C and k¼ 633 nm. Thecorresponding on-state voltage is 130Vrms in an IPS cell with10 lm electrode width and 10 lm electrode gap. The totalresponse time (sonþ soff) is only 640 ls measured at 20 Cby applying a voltage corresponding to the maximum trans-mittance. Since the presented fluorinated materials exhibit ahigh resistivity in contrast to other cyano materials used inBP systems, we believe our materials have potential forwidespread application in next-generation displays and pho-tonic devices.ACKNOWLEDGMENTSThis work was carried out under the Projects RMN 08-715 and PBS 847. The authors are indebted to SartomerCompany for providing trifunctional monomer.1H. Kikuchi and H. Higuchi, SID Int. Symp. Digest Tech. Papers 38, 1737(2007).2Z. Ge, L. Rao, S. Gauza, and S.-T. Wu, J. Disp. Technol. 5, 250 (2009).3Y. Li, M. Jiao, and S.-T. Wu, Opt. Express 18, 16486 (2010).4H. Coles and S. Morris, Nat. Photonics 4, 676 (2010).5S. Yokoyama, S. Mashiko, H. Kikuchi, K. Uchida, and T. Nagamura, Adv.Mater. 18, 48 (2006).6Y.-H. Lin, H.-S. Chen, H.-C. Lin, Y.-S. Tsou, H.-K. Hsu, and W.-Y. Li,Appl. Phys. Lett. 96, 113505 (2010).7K. M. Chen, S. Gauza, H. Xianyu, and S.-T. Wu, J. Disp. Technol. 6, 49(2010).8H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang, and T. Kajiyama, Nat.Mater. 1, 64 (2002).9P. P. Crooker, in Chirality in Liquid Crystals, edited by H. S. Kitzerowand C. Bahr (Springer, New York, 2001), pp. 186–122.10S. Meiboom, J. Sethna, P. Anderson, and W. Brinkham, Phys. Rev. Lett.46, 1216 (1981).11J. P. Sethna, in Theory and Applications of Liquid Crystals, edited by J. L.Ericksen and D. Kinderlehrer (Springer-Verlag, New York, 1987) p. 305.12H. Yoshida, Y. Tanaka, K. Kawamoto, H. Kubo, T. Tsuda, A. Fuji, S.Kuwabata, H. Kikuchi, and M. Ozaki, Appl. Phys. Express 2, 12501 (2009).13K. Z. Zheng, D. Shen, and P. Huang, New J. Phys. 12, 113018 (2010).14A. Yoshizawa, M. Sato, and J. Rokunohe, J. Mater. Chem. 15, 3285 (2005).15T. Iwata, K. Suzuki, N. Amaya, H. Higuchi, and H. Kikuchi,Macromolecules 42, 2002 (2009).16I. Dierking, W. Blenkhorn, E. Credland, W. Drake, R. Kocioruba, B.Kayser, and T. Michael, Soft Matter 8, 4355 (2012).17S.-K. Hong, H.-S. Choi, S. Shibayama, H. Higuchi, and H. Kikuchi, Mol.Cryst. Liq. Cryst. 528, 32 (2010).18H.-Y. Chen, H.-H. Liu, J.-L. Lai, C.-H. Chiu, and J.-Y. Chou, Appl. Phys.Lett. 97, 181919 (2010).19J. Yan, M. Jiao, L. Rao, and S.-T. Wu, Opt. Express 18, 11450 (2010).20K.-M. Chen, S. Gauza, H. Xianyu, and S.-T. Wu, J. Disp. Technol. 6, 318(2010).21C. Shott, J. Patrick, K. Adlem, L. D. Farrand, G. Berntz, A. Goetz, P. Best,and H. Plach, U.S. patent 7,794,621 (14 September 2010).22O. Chojnowska and R. Dąbrowski, Photonics Lett. Pol. 4, 81 (2012).23O. Chojnowska, R. Dąbrowski, P. Kula, Ł. Szczucinski, J. Dziaduszek, J.Yan, and S.-T. Wu, Liq. Cryst. 41, 15 (2014).24Y. Iwashita, Y. Umezo, K. Takeuchi, and H. Takatsu, DIC Tech. Rev. 8,39 (2002).25M. Hird, Chem. Soc. Rev. 36, 2070 (2007).26P. Kirch and M. Bremer, Angew. Chem., Int. Ed. 39, 4216 (2000).27P. Kula, J. Herman, and O. Chojnowska, Liq. Cryst. 40, 83 (2013).28J. Yan, H.-C. Cheng, S. Gauza, Y. Li, M. Jiao, L. Rao, and S.-T. Wu,Appl. Phys. Lett. 96, 071105 (2010).29F. Peng, Y. Chen, J. Yuan, H. Chen, S.-T. Wu, and Y. Haseba, J. Mater.Chem. C 2, 3597 (2014).30Y. Haseba, H. Kikuchi, T. Nagamura, and T. Kajiyama, Adv. Mater. 17,2311 (2005).31J. Yan, L. Rao, M. Jiao, Y. Li, H.-C. Cheng, and S.-T. Wu, J. Mater.Chem. 21, 7870 (2011).32L. Rao, J. Yan, S.-T. Wu, S.-I. Yamamoto, and Y. Haseba, Appl. Phys.Lett. 98, 081109 (2011).33Y. Chen, D. Xu, S.-T. Wu, S.-I. Yamamoto, and Y. Haseba, Appl. Phys.Lett. 102, 141116 (2013).34J. Yan and S.-T. Wu, J. Disp. Technol. 7, 490 (2011).35Z. Hussain, A. Masutani, D. Danner, F. Pleis, N. Hollfelder, G. Nelles, andP. Kilickiran, J. Appl. Phys. 109, 114513 (2011).213505-4 Chojnowska et al. J. Appl. Phys. 116, 213505 (2014)

Electro-optical properties of photochemically stable polymer-stabilized blue-phase material

O. Chojnowska

R. Dąbrowski

J. Yan

Y. Chen

S. T. Wu

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Journal of Applied Physics

https://stars.library.ucf.edu/cgi/viewcontent.cgi?article=6186&context=facultybib2010

Electro-optical properties of photochemically stable polymer-stabilized blue-phase material

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