Electrically Switchable Reflectors of Chiral Gels - ACS Symposium

Chapter 21

Electrically Switchable Reflectors of Chiral Gels

Downloaded by PENNSYLVANIA STATE UNIV on September 7, 2012 | http://pubs.acs.org Publication Date: August 17, 2004 | doi: 10.1021/bk-2005-0888.ch021

Rifat A . M . Hikmet Philips Research, Polymers and Organic Chemistry, Prof. Holstlaan 4, 5656AA Eindhoven, The Netherlands (email: [email protected])

In-situ polymerisation of LC reactive molecules in the presence of non-reactive LC molecules leads to the formation of anisotropic networks containing free molecules (anisotropic gels). In this chapter gels obtained by polymerisation in the cholesteric phase will be described. In the cholesteric phase the optical rotary dispersion shows extremely high values and a band of light is split into two opposite circularly polarised components. These properties of cholesterics make them suitable to be used in passive optical components such as reflectors, polarizers, bandpass and notch filters. Cholesteric gels can be switched fast and in a controllable way. Using combination of heat and ultraviolet light in a pattern-wise manner, a single switchable gel layer reflecting various colours are produced. Homogeneity of the gels can be also be manipulated to increase the width reflection band to obtain electrically switchable silver coloured mirrors.

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© 2005 American Chemical Society In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

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Introduction Cholesteric liquid crystal phase is obtained when a nematic phase is doped with chiral molecules. Chiral molecules are optically active and are known to show optical rotary dispersion in the order of 1 /cm. However in the cholesteric phase they induce rotation of the long axes of the liquid crystal molecules (the director n) about a helix as shown in Figure 1.


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Figure 1 Schematic representation of a cholesteric Phase. Such a macromolecular arrangement leads to optical effects unique to this phase. For example, the optical rotary dispersion shows a dramatic increase and reaches values in the order of 100%m. Furthermore, a band of circularly polarized light having the same sense as the cholesteric helix is reflected while the band with the opposite sense is transmitted. The upper (k ) and lower (λ^η) boundaries of the reflected band are X =p*n and λ ^ ^ * ^ respectively where ρ is the cholesteric pitch corresponding to the length over which the director rotates 360°, n and ^ are the extraordinary and the ordinary refractive indices of a uniaxially oriented phase respectively. The reflected bandwidth (Δλ) is given by Δλ= ΚΪΠΚΒΚ =p*(n -n ). It has already been shown that such a cholesteic structure can be frozenin by cooling a liquid crystal polymer below its glass transition temperature (7) or polymerisation of liquid crystal molecules with reactive end groups (2) to be used in passive optical applications. There has also been much interest in the use of cholesteric materials in display applications where switching can be realized between various misaligned states. Switching characteristics of cholesterics, for example the presence of hysteresis has been used in addressing schemes to produce passive matrix displays (5). The scanning speed of the voltage has also shown to influence the switching characteristic of cholesterics. This effect has max

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In Chromogenic Phenomena in Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2004.


280 been used in gels where bistable switching took place between weakly scattering focal-conic texture and planar texture with misaligned helix. In our earlier attempts to produce cholesteric gels, which could be switched between defect free planar Grandjean texture and homeotropic texture we used liquid crystal diacrylates which were polymerized in the presence of non-reactive cholesteric liquid crystal (4). However fast switching to the defect free state could not be obtained. For this purpose new types of gels, which can be used in defect free switching of cholesteric liquid crystals were developed (5). In this review production, various properties, patterning of such gels in lateral directions (6) in order to obtain multi colour displays, and creating a pitch gradient along the cell thickness to broaden the reflection band to produce broad band switchable reflector will be described.

Materials

The reactive liquid crystals were synthesized at Philips research. Conventional liquid crystals were purchased from Merck. The polymerisation of the mixtures was initiated by means of UV radiation using photoinitiator Irgacure 651 purchased from Ciba Speciality Chemicals. The structures of the acrylates are shown in Figure 2.

C6M

Figure 2. Structures of the acrylates.


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Results and Discussion In conventional LC cells, long range orientation of LC molecules is induced by orientation layers at the cell. Switching is induced by applying an electric field across transparent electrodes present on the cell surfaces underneath the orientation layers. Upon removal of the field, the LC molecules revert to the initial orientation state under the influence of these orientation layers. As shown in Figure 1 cholecterics have a complicated helical structure and in order to obtain sufficient reflection the cell gap needs to be at least ten pitches thick. When such a structure is switched from a defect free planar orientation it is almost impossible for the system to reorient itself back to the initial state under the influence of the surface orientation layers. Instead, the cholesteric helix becomes oriented in various directions and the cell shows a scattering texture. In order to obtain fast switching a memory state needs to be built into such a cholesteric system. We tried to do this by creation of a lightly cross-linked network dispersed within the non-reactive LC molecules. This is done by in-situ polymerisation of a liquid crystal monoacrylate and diacrylate mixture in the presence of non-reactive LC molecules. The planar orientation of the cholesteric mixtures containing monomers with reactive groups is obtained in cells containing uniaxially rubbed polymer layers. The polymerisation of the reactive molecules is induced by UV radiationfreezing-inthe cholesteric configuration and orientation by creating a network containing non-reactive LC molecules (anisotropic gel). The gel structure is schematically represented in Figure 3.

Monomeric mixture

Anisotropic Gel

Figure 3. Schematic representation ofgelformation.



282 The network in these gels consists of monoacrylate molecules forming the side-chain polymers which are cross-linked by the diacrylates. The network is in strong interaction with non reactive LC which can be switched together with the side chain polymer upon application of an electric field. In these gels the functions of the diacrylate molecules which are present at fractions of a percent is two folds: i) They form the cross-links thus provide system memory function ii) They preserve the polymer structure and its distribution within the system preventing its diffusion. The second function is especially important in producing broadband reflectors and patterned gels described below. The switching behavior of the gels was studied using UV-Vis spectroscopy. In the gels, two different types of switching have been characterized. Figure 4 shows the first mode of switching in these gels. In this mode the reflection band shifts gradually to low wavelengths with increasing voltage before decreasing in magnitude and shifting back to higher wavelengths and disappearing at higher voltages.

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Figure 4. Transmission as a function of voltage. Shifting of the reflection band to lower wavelengths is a well understood effect associated with the cholesteric layers getting tilted with respect to the incident beam of light followed by helical unwinding. The threshold voltage associated with the helical deformation has been previously described as gridlike deformations or lattice dislocations in the literature. This transition, which is


283 also referred to as Helfrich's deformation, leads to the formation of the fingerprint texture which occurs above the critical voltage V ^ h given by (7)

v«*-Wa)"i-^-f

Electrically Switchable Reflectors of Chiral Gels - ACS Symposium

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