Removal of pitch jumps from a chiral nematic liquid crystal Alison Ford, Stephen Morris and Harry Coles CMMPE, University of Cambridge, Department of Engineering, 9 JJ Thomson Avenue, Cambridge, CB3 0FA. Background.
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Removal of pitch jumps from a chiral nematic liquid crystalAlison Ford, Stephen Morris and Harry ColesCMMPE,University of Cambridge,Department of Engineering,9 JJ Thomson Avenue,Cambridge, CB3 0FA
In this poster we present a new method for eliminating pitch jumps from a chiral nematic liquid crystal when subjected to a variation in temperature. The transmission spectra are examined as a function of temperature for three different chiral nematic liquid crystals. E49 (commercially available from Merck) is used as the nematic host for two of these samples, each doped with a different chiral dopant. The temperature dependence of these two samples exhibit opposite trends and distinct discontinuous changes in the reflection wavelength. This discontinuous change is known as a pitch jumps. A combination of the two chiral dopants into the same host can eliminate the pitch jump effect for these samples.
N -1 turns
H.Zink, V.A.Belyakov, J. Exp. Theor. Phys, 85 (2), 285, 1997
V.A.Belyakov and E.I.Kats, J. Exp. Theor. Phys,91 (3), 488, 2000
V.A.Belyakov, P.Oswald and E.I.Kats, J. Exp. Theor. Phys, 96 (5), 915, 2003
P.Shibaev, V.Kopp, A.Genack and E.Hanelt, Liq. Cryst. 30, 1391, 2003
S. M. Morris, A. D. Ford, B. J. Broughton, M. N. Pivnenko and H. J. Coles, Proc. SPIE,5741, 118 (2005)
A single host was used, E49 + D1* which exhibited the following phase transition temperatures (measured on cooling):
I – (104 ºC) – N + I – (100 ºC) – N – (< 30 ºC) – Cr
The sample was filled into a planar aligned 7.5 mm thick cell and the transmission spectra were measured as a function of temperature. A typical transmission spectrum is shown in the figure on the right.
The central wavelength (l) is defined according to:
where n is the average refractive index and P is the pitch.
l = nP
A micrograph showing the discontinuous change in colour indicating a pitch jump.
Temperature dependence of the refractive indices
Temperature dependence of the pitch
The temperature dependence of the long and short wavelength band edges is shown in the figure on the right. The regions of continuous wavelength change are a result of the temperature dependence of the refractive indices whilst the discontinuous changes are due to the temperature dependence of the pitch.
On heating a N*LC, the pitch can either increase or decrease due to two competing mechanisms: thermal expansion causing an increase in the pitch and an increase in the average angular separation of molecules along the helix axis causing a decrease in the pitch. The dependence also depends on the other mesophases. For example on cooling a sample with an underlying smectic phase, the pitch will diverge on approaching the smectic phase.
E49 + D2*
E49 + D1*
Different Nematic Hosts with a Single Chiral Dopant
E49 + D2*
M21 + D2*
E49 + D2*
E49 + D1*
A chiral nematic sample was prepared by doping E49 with D1* and D2*. The temperature dependence of the transmission spectrum was measured and the results are shown in the right hand figure below. These results shown that a combination of the two different chiral dopants has eliminated the pitch jumps exhibited by the two individual samples (as shown in the left hand figure below).
E49 + D1* + D2*
In this poster we have shown that the temperature dependence of a chiral nematic liquid crystal is determined by the combination of the nematic host and the chiral dopant. Pitch jumps that arise due to a competition of forces can be eliminated by using a combination of chiral dopants that exhibit opposite temperature dependencies in the same nematic host. This is a simple process for eliminating pitch jumps, whilst maintaining the high quality alignment and strong anchoring energy associated with a thin cell and planar alignment.