Temperature-independent pitch invariance in cholesteric liquid crystal Kyoo Sung Shim, Jeong Uk Heo, Soo In Jo, You-Jin Lee, Hak-Rin Kim, Jae-Hoon Kim, and Chang-Jae Yu Optics Express, Vol. 22, Issue 13, pp. 15467-15472 (2014) http://dx.doi.org/10.1364/OE.22.015467

Introduction Pitch variation by temperature gives rise to color shift in device applications. Pitch stabilization of CLC against temperature has been explored by introducing photopolymer to CLCs. In such polymer/CLC composite systems, the polymer structure suppresses thermal variations of CLC pitches and thus various CLC pitches formed at different temperature are stabilized even in a single-layered configuration. However, these methods are limited in switching properties such as high operating voltage of CLC devices.

In this work They demonstrated the temperature-independent pitch invariance of the CLCs by blending two chiral dopants with the opposite temperature dependencies of the cholesteric pitch. They mixed two chiral dopants with a certain mixing ratio, which was determined by the algebraic balance of slopes under an assumption of the linearly varied pitch by temperature below the cholesteric-isotropic phase transition. They demonstrated the various CLCs with different pitches exhibiting the pitch invariance to temperature, and they determined the HTP of the blended chiral dopant proposed here. The technique proposed here is expected to be useful to apply the reflective display applications with color invariance to temperature. To obtain the pitch invariance of the CLC to temperature

Fig. 1 Concept of a pitch invariance of the CLC independent of temperature. A dopant of S811 tends to shorten a helical pitch but the other S5011 tends to make the pitch longer with increasing temperature. In the mixed dopant with a suitable mixing ratio, the pitch invariance to temperature is obtained.

Fig. 2 The reflection spectra of the CLCs doped with (a) S811 (29 Fig. 2 The reflection spectra of the CLCs doped with (a) S811 (29.97 wt%) and (b) S5011 (2.15 wt%) with increasing temperature, and (c) the corresponding central wavelengths of reflection spectra. The solid lines in (C) depict the least-squares fits to a straight line. The slopes of the red and blue straight lines are fitted to be −4.33 and 2.17, respectively.

Table 1. Summary of the calculating procedure to determine the dopant mixing ratio.      sample      nematic LC      dopant      slope      mixing ratio      mixing amount      S811-doped CLC 　 100 mg      42.8 mg      −4.33      2.17/(4.33 + 2.17) = 0.33     42.8 × 0.33      = 14.29      S5011-doped CLC     100 mg      2.2 mg      2.17      4.33/(4.33 + 2.17) = 0.67     2.2 × 0.67      = 1.47 final mixing ratio of the S811 and S5011 dopants used here is 14.29:1.47 ≈16.2:1.7

They blended two chiral dopants of S811 (16. 2 mg) and S5011 (1 They blended two chiral dopants of S811 (16.2 mg) and S5011 (1.7 mg) and mixed them to the nematic LCs of 120, 100, and 80 mg for the CLCs with different pitches. 12.98wt% nematic LC:S811:S5011 = 120:16.2:1.7 15.18wt% nematic LC:S811:S5011 = 100:16.2:1.7 18.28wt% nematic LC:S811:S5011 = 80:16.2:1.7 Fig. 3 The reflection spectra of the CLCs with different concentrations of the mixed dopant (a) 12.98, (b) 15.18, and (c) 18.28 wt% with increasing temperature, and (d) the corresponding central wavelengths of reflection spectra.

It should be noted that central wavelength of reflection spectra is directly related to cholesteric pitch of CLC such that λ = (ne + no)P/2. ordinary (no) extraordinary (ne) indices of refraction of ML-0643 12.98wt% 15.18wt% 18.28wt% Fig. 3(d) the corresponding central wavelengths of reflection spectra.

In general, HTP of the chiral dopant is defined by the following relation that where P and c represent helical pitch in μm and concentration of the dopant, respectively. P = 1 C × HTP Fig. 4 The central wavelength as a function of the concentration of the mixed dopant. The solid line depicts the least-squares fit of the measured pitches to Eq. (1) to estimate a HTP of the mixed dopant. The HTP is calculated to be 17.32 ± 1.78 μm−1

Conclusions They demonstrated pitch invariance of CLCs to temperature by mixing two chiral dopants with opposite temperature dependencies of cholesteric pitch. They demonstrated temperature-independent pitch invariance for various CLCs with different pitches and HTP of blended chiral dopant proposed here was estimated to be 17.32 ± 1.78 μm−1.