Low Temperature and High Frequency Effects on Blue Phase Liquid Crystals Fenglin Peng, Yuan Chen, Jiamin Yuan, Haiwei Chen, Shin-Tson Wu*, Yasuhiro Haseba** * College of Optics and Photonics, University of Central Florida, Orlando, FL., USA ** JNC Petrochemical Corporation, Ichihara Research Center, Ichihara, Chiba, Japan. Abstract We report the low temperature and high frame rate operation limits of a polymer-stabilized blue phase liquid crystal (BPLC). Debye relaxation sets another practical application limit even the temperature is still above melting point. Doping a diluter compound to the BPLC host greatly extends this low temperature operation range in terms of dielectric relaxation frequency and response time. 1. Objectives and Background Polymer-stabilized blue phase liquid crystal (PS-BPLC) [1-4] holds great promises for color sequential displays because of its submillisecond gray-to-gray response time [5,6]. In order to reduce operation voltage, the host BPLC compounds usually possess a huge dielectric anisotropy (∆ε>100) [7]. As a result, its viscosity is ~10X higher than that of a commonly used nematic LC. Two problems associated with such a high viscosity BPLC: 1) a fairly low Debye dielectric relaxation frequency (fr ~1-2 kHz vs. ~100kHz for a low viscosity nematic), and 2) increased response time at low temperatures. For color sequential displays [8,9], the required frame rate is 3X higher than normal, i.e., at least 360Hz. As the operation frequency approaches fr, ∆ε would decrease dramatically, which translates into unmanageably high operation voltage. This effect becomes more serious in the low temperature region, where the viscosity increases exponentially and fr further decreases. Therefore, dielectric relaxation could set another practical operation limit even the temperature is still above the melting point of the BPLC material. To overcome these problems, in this paper we develop a model to correlate the temperature and frequency effects on the Kerr constant (K) of BPLC materials. Excellent agreement between model and experimental data is obtained. Based on our model, we find that for each BPLC there is an optimal operation temperature at which the Kerr constant has a maximum value, i.e., the device operation voltage is the lowest. Moreover, by doping a diluter compound to our BPLC we can greatly extend the low temperature operation limit, which is imposed by the low fr. 2. Experiment and results In our experiment, we prepared two samples employing JC- BP06N and JC-BP07N as LC hosts. While ∆ε at low frequency limit of JC-BP06N is 648, which is almost twice of JC-BP07N (∆ε~332). The BPLC precursors consist of 88.17 wt% LC host with 2.92 wt% of chiral dopant R5011, 9 wt% of monomers (5.24 wt. % RM257 & 3.46 wt. % TMPTA (1,1, 1-Trimethylolpropane Triacrylate)) and 0.21 wt% photoinitiator. Samples were filled into two in-plane-switching (IPS) cells whose electrode width is 8µm, electrode gap is 12µm, and cell gap is 7.34µm. Both cells were cooled to BP-I phase and cured by an UV light with λ~365nm and intensity ~2mw/cm 2 for 30 min. After UV curing, the PS-BPLC composite was self-assembled. For convenience, we call two cells as JC-BP06 and JC-BP07. Next, both cells were placed on a Linkam heating/freezing stage controlled by a temperature programme (Linkam TMS94). We measured the voltage-dependent transmittance (VT) by sandwiching the heating stage between two crossed polarizers at two frequencies: 240Hz and 480Hz (square-wave AC voltage). A He-Ne laser (λ=633nm) was used as probing beam and the transmitted light was focused by a lens, so that different diffraction orders can be collected by the detector. [10] Figure 1(a) shows the normalized VT curves of JC-BP06 measured from 40 o C to 0 o C at 480 Hz. As the temperature decreases, the VT curves shift left first and then rightward, indicating Von bounces back at low temperatures. The lowest Von occurs at 20 o C. As the temperature is below 3 o C, which is still above the melting point of the BPLC (Tmp= −2 o C), the transmittance is lower than 5% even the applied voltage (Vapp) has reached 65V. Therefore, this imposes a practical low temperature operation limit of PSBP. Similar phenomenon was also observed for JC-BP07. We will further discuss this temperature limit later. Figure 1. (a) VT curves of JC-BP06 at different temperatures with 480Hz AC voltage signal, and (b) Kerr constants for each temperature of JC-BP06 and JC-BP07 and fitting curves using Eq. (3). 14.1 / F. Peng 164 • SID 2014 DIGEST ISSN 0097-966X/14/4501-0164-$1.00 © 2014 SID