LC dependent electro-optical properties of holographic polymer dispersed liquid crystals
Introduction
Holographic polymer dispersed liquid crystals (HPDLCs) have great potential applications viz. flat panel display, switchable lenses, information storage, guided-wave switches, graphic art which necessitate high diffraction efficiency, low driving voltage, and fast response time [1], [2], [3]. In principle, a great mismatch of refractive index between polymer and LC is a basic requirement for high diffraction efficiency of the film [4]. On the other hand, a high degree of phase separation between the polymer and LC is essential to approach the inherent index mismatch since LC dissolved in polymer and monomers in LC reduce the index mismatch between the two phases. Since phase separation increases with the molecular weight buildup of polymer, high conversion of monomers into polymer as well as a great solubility parameter gap between the polymer and LC is of basic concern to obtain good phase separation [5]. Low mixture viscosity and high polymer elasticity also contribute to the great phase separation from kinetic and viscoelastic points of view [6].
On the other hand, operating voltage of film depends on various facts such as droplet size and shape, film conductivity and dielectric constant, surface anchoring and so on [7], [8], [9]. Namely, the electro-optical properties of the films depend not only on the characteristics of resin and LC but also on the combinations of the two.
We have earlier concerned with the structure controls of polymer to augment the electro-optic performances of the films [10], [11], [12], [13]. In this work, we used four different types of LC to fabricate HPDLC films at a specific resin formulation based on the photopolymerization of urethane acrylate system, and report the effects of LC on the electro-optical properties of the film.
Section snippets
Materials and oligomer synthesis
Polyurethane acrylate oligomer was synthesized from polypropylene glycol (PPG, Mn = 400) and a molar excess of hexamethylene diisocyanate (HDI) to form isocyanate terminated prepolymers, followed by capping with hydroxyethyl methacrylate (HEMA). Detailed synthetic procedures are described in our early paper [12], [13].
The oligomer is highly viscous and immiscible with LC, which necessitates the use of reactive diluent such as ethyl hexyl acrylate (EHA).
Inherent properties of four different types
Morphology
Typical SEM and AFM morphologies of the holograms are shown in Fig. 1 where dark channels represent those previously occupied by LC and bright ones by polymer. Regardless of the type of LC, gratings are fairly well fabricated [16]. It is seen that SEM and AFM morphologies show the size of LC droplet, Bragg period as well as LC–polymer interfaces. Droplet size of LC affects diffraction efficiency, driving voltage, and response time. The droplet size of E7 is similar to that of ZLI-3417-100 and
Conclusions
Effects of type and content of LC on the electro-optical properties of holographic polymer dispersed liquid crystal based on polyurethane acrylate have been studied.
With the increase in refractive anisotropy, off-state diffraction efficiency increased due to the increased mismatch of refractive indices with polymer.
Driving voltage and response time could be optimized by changing the type of LC having appropriate dielectric anisotropy and viscosity. Driving voltage of 2.5 V/μm and response time
Acknowledgements
This work was supported by a grant-in-aid for the National Core Research Center Program of the Ministry of Science & Technology and the Korea Science & Engineering Foundation.
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Optical diffractometry of anisotropic holographic structure composed of liquid crystal and polymer phases with extended Bragg modes
2014, Thin Solid FilmsCitation Excerpt :The most simple periodic structure, or sinusoidal refractive-index modulation, in thick transparent grating produces single mode Bragg diffraction, which can be determined by Bragg formula approved at the sinusoidal periodicity. In practice, however, spatially periodic photopolymerization caused by interferometric exposure probably yields various profiles of LC/polymer phase-separated distribution, dependently on material system, exposure condition, and so on, as shown in several SEM micrographs in previous reports [15,23–27], and thus refractive-index modulation generated by such practical morphology is not necessarily sinusoidal but complex to be analyzed. In the present paper, the profiles of refractive-index modulations in HPDLCs were closely analyzed by the so-called θ–2θ optical diffractometry based on two-wave coupled wave theory employing not only the first mode Bragg diffraction but also the higher order ones, which has never been demonstrated.
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