Frequency Effects on Polymer-Stabilized Blue-Phase Liquid Crystals Yan Li*, Yuan Chen*, Jie Sun*, Shin-Tson Wu*, Shih-Hsien Liu**, Pao-Ju Hsieh**, Kung-Lung Cheng** , Jyh-Wen Shiu**, Shin-ichi Yamamoto*** and Yasuhiro Haseba*** *College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USA ** Industrial Technology Research Institute, Hsinchu, Taiwan *** JNC Petrochemical Corporation, Ichihara, Chiba 290-8551, Japan Abstract Frequency effects on polymer-stabilized blue-phase liquid crystals (BPLCs) are investigated. As the electric field frequency increases, Kerr constant decreases and dielectric heating increases. A physical model (extended Cole-Cole model) is proposed to fit the experimental results. These results are important on high frequency operation of BPLC displays. Author Keywords Frequency, Blue phase liquid crystal, Dielectric relaxation 1. Introduction Polymer-stabilized blue phase liquid crystal (BPLC) [1-7] exhibits several attractive features: optically isotropic dark state, no need for surface alignment layer, and submillisecond gray-to-gray response time [8,9]. Therefore, it is promising for next-generation display applications. Fast response time of BP LCDs not only reduces image blurs but also enables color sequential displays using red, green and blue LEDs. The elimination of spatial color filters triples the optical efficiency and spatial resolution density. However, color sequential displays require a much higher operation frequency (up to 1 kHz) than conventional displays (240 Hz) in order to suppress color breakup [10,11]. Therefore, it is important to investigate the frequency effects [12] on the Kerr constant of BPLC devices. In this paper, we report the electric field frequency effects on the Kerr constant of two polymer-stabilized BPLCs. A model called extended Cole-Cole model is proposed to fit the experimental results, and good agreement is obtained. As the electric field frequency increases, Kerr constant decreases and the associated dielectric heating gradually increases. 2. Experiments In our experiment, we prepared two IPS (in-plane-switching) cells whose electrode width and electrode gap are both 10 m, and cell gap is 7.5 m. Cell 1 was filled with a BPLC mixture consisting of 76.2 wt% JM2069-043 nematic LC host (from ITRI), 13 wt% chiral dopants [8% ISO-(6OBA) 2 and 5% CB15 (Merck)], 10 wt% monomers [6% RM257 (Merck) and 4% TMPTA (1,1,1- Trimethylolpropane Triacrylate, Sigma Aldrich)], and ~0.8 wt% photoinitiator. The cell was cooled to BPI phase and the precursor was cured by a UV light with ~365nm and intensity ~2 mW/cm 2 for 30 minutes. After UV curing, the polymer-stabilized BPLC composite was self-assembled, and the clearing temperature was measured to be T c ~44 °C. The relatively low T c is attributed to the low melting point (~4 o C) of the chiral dopant CB-15. Cell 2 was filled with Chisso JC-BP01M precursor, in which chiral dopant and monomers were premixed. The UV curing conditions were the same as reported in Ref. 8, and the T c of sample 2 is ~70 o C. Next, we measured the voltage-dependent transmittance (VT) curves of these two cells using a He-Ne laser (=633nm). The cells were placed in a temperature controller to maintain a constant temperature~22 o C. 3. Results Figure 1 shows the normalized VT curves of cell 1 at six frequencies: 100, 1k, 10k, 20k, 50k and 100k Hz. As frequency increases, the VT curve shifts rightward, indicating that the on- state voltage (V on ) increases (i.e., Kerr constant decreases) with frequency. 0 20 40 60 80 100 120 140 0.0 0.2 0.4 0.6 0.8 1.0 100Hz 1kHz 10kHz 20kHz 50kHz 100kHz Normalized Transmittance Voltage (V rms ) Figure 1. VT curves of sample 1 at different frequencies Fig. 2 shows the normalized VT curves of cell 2 at different frequencies. Similarly, the VT curves shift to the right side and Kerr constant decreases as frequency increases. 0 10 20 30 40 50 60 0.0 0.2 0.4 0.6 0.8 1.0 60Hz 120Hz 240Hz 480Hz 960Hz 2kHz 5kHz Normalized Transmittance Voltage (V rms ) Figure 2. VT curves of sample 2 at different frequencies But for this material, Kerr constant starts to decrease at a much 4.3 / Y. Li 22 • SID 2012 DIGEST ISSN 0097-966X/12/4301-0022-$1.00 © 2012 SID