Blue-phase liquid-crystal mixtures and their induced stabilization by photopolymerization Fernando Ely (SID Member) Luciana Quinalia dos Santos Ivan H. Bechtold Juliana Eccher Hugo Gallardo Luiz Fernando Zagonel Abstract — Experiments on mixtures of different nematic-liquid-crystal hosts with a chiral dopant to induce the appearance of blue phases (BPs) is reported. The phase behavior and transition tempera- tures of the mixtures were studied by temperature-dependent polarized optical microscopy, variable temperature X-ray diffraction, and reflectance measurements. After completely characterized, a selected LC mixture presenting BPs was submitted to polymer-stabilization using UV-curable mono- mers. By using a specific combination of a pro-mesogenic (B11) and a branched-chain acrylate (EHA) monomer, a uniform porous microstructure is formed providing a stable BP. Keywords — Liquid crystals, blue phase, photomonomer, doping. DOI # 10.1889/JSID19.11.781 1 Introduction The blue phases (BPs) are frustrated liquid-crystalline phases, originally discovered in 1888 1 and characterized by a bright platelet texture appearing between the isotropic liq- uid and chiral nematic (cholesteric) phase (N*). BPs have a unique three-dimensional structure stabilized by the coexis- tence of double-twisted cylinders and disclinations. BPs are subdivided into three subphases: the BPI, BPII, and BPIII, according to chirality and transition tem- perature. BPI and BPII have body-centered cubic and sim- ple-cubic structures, respectively. On the other hand, BPIII is thought to have an amorphous state and its symmetry is the same as that of an isotropic phase. BPs are often found in a narrow temperature range of about 1K, and until the year 2000 they were mostly a subject of fundamental research for scientists. However, that scenario changed in 2002 when Kikuchi et al. 2 were able to obtain BPs in a temperature range larger than 60°C by forming polymer networks quite similar to those used in PSCT (polymer-stabilized cholesteric textures). 3,4 This result enabled applications for BP in displays using the so-called blue-phase LCD mode (BP-LCD). The switching mechanism in a BP-LCD is elec- tric field-induced birefringence attributed to the local reori- entation of molecules by electro-optic Kerr effect. 5–7 In the voltage-off state, the BP Liquid-Crystal (BPLC) mixture is optically isotropic (∆n induced = 0) so that it appears black under crossed polarizers. In the presence of an electric field, ∆n induced gives rise to transmittance until a saturation value. BP-LCD prototypes with a superior 240-Hz refresh rate have recently been demonstrated. 8 The other main characteristics of this BP-LCD prototype is the absence of alignment layers, high contrast ratio (CR), wide viewing angle, and a response time of about 1 msec. However, some impor- tant practical issues, such as operating voltage, hysteresis, residual birefringence, and long-term stability, remain to be addressed before widespread applications can take off. In addition to BP-LCD operating by Kerr effect, the possibility of electrically switching the Bragg-reflected wavelength has been demonstrated in BP materials. 9 The mechanism of this switching is a field-induced distortion (electrostriction) of the BP cubic lattice that results in a shift of the spectral reflection peak. In this paper, we report experiments on mixtures of different nematic-liquid-crystal host with a chiral dopant to induce the formation of BPs. The mixtures, which showed BP, were subjected to polymer stabilization using UV-cur- able monomers after extensive characterization. 2 Experimental The liquid crystals and the dopant used in this study were purchased from Merck. Monomers used to form polymer networks in BP liquid crystals were EHA (2-ethylhexyl acry- late, Aldrich), BA (butyl acrylate, Aldrich), TMPTA (trimethylolpropane triacrylate, Aldrich), TMSPM (3-(Tri- methoxysilyl)propyl methacrylate, Aldrich), PUA (polyure- thane acrylate monomer, Cytec Industries), and NOA65 ® (Norland Optical Adhesives). The diacrylate mesogen B11 (Phenyl 4,4′-bis(4-(acryloyloxy)undecyloxy)benzoate) was synthesized at the Universidade Federal de Santa Catarina by H. Gallardo’s team. BME (benzoin methyl ether, Aldrich) and IC-651 (Irgacure 651, Cyba) refer to the photoinitiators (PI) used in the polymerization process. Polarized optical microscopy (POM) observations were per- formed by a Olympus B51x microscope coupled to a PC- controlled heating stage (HSC302, Instec). Reflectance measurements were performed in a UV-Vis-NIR spec- Based on a paper presented at the 30th International Display Research Conference/Latin Display held November 16–19, 2010, in São Paulo, Brazil. F. Ely and L. Quinalia dos Santos are with the Displays and Surfaces of Interaction Laboratory, CIT – Center for Information Technology Renato Archer, Rod. Dom Pedro 1, km 143.6, 13069-901 Campinas, SP, Brazil; telephone +55-19-3746-6045, e-mail: Fernando.ely@cti.gov.br. I. H. Bechtold and J. Eccher are with the Departamemto de Física, Universidade Federal de Santa Catarina, Florianópolis, SC, Brazil. H. Gallardo is with the Departamento de Química, Universidade Federal de Santa Catarina, CEP, Florianópolis, SC, Brazil. L. F. Zagonel is with the Laboratório Nacional de Luz Sincroton, Campinas, SP, Brazil. © Copyright 2011 Society for Information Display 1071-0922/11/1911-0781$1.00. Journal of the SID 19/11, 2011 781