Studies of a Polymer Dispersed Ferroelectric Liquid Crystal
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the mechanical stability, easy processibility and possible enhancement of electro-optic properties in PDLCs, polymer dispersed ferroelectric liquid crystals (PDFLCs) have recently been developed [6,7]. Although PDFLCs have only recently been developed, results showing bistability [8], volume stabilization and effects of PDFLC preparation have been reported. Phase behavior and electro-optic properties of PDFLC systems have also been examined [9,10,111. The focus of this work is to examine some of the effects induced in an FLC system by introduction of a polymer network. Properties such as the ferroelectric polarization, rotational viscosity, polymerization kinetics and phase behavior will be discussed. EXPERIMENTAL The diacrylate used in this study was p-phenylene diacrylate (PPDA) from Polysciences, Inc. (Warrington PA). The ferroelectric liquid crystal was a 1:1 mixture of W82 and W7 (Displaytech, Boulder CO). Structures of the monomer and liquid crystals are shown elsewhere [9]. Photopolymerization was initiated using Irgacure 907 (Ciba-Geigy, Hawthorne NY). Enthalpies of transition were determined using a differential scanning calorimeter (PerkinElmer DSC 7) using a scan rate of 50 C per minute. Polymerizations were also performed in a differential scanning calorimeter modified with a photocalorimetric accessory (Perkin-Elmer DPA7). Ferroelectric polarization was obtained using an Automated Polarization Tester (Displaytech) by applying a 6 V/jim electrical field across a lOm rubbed polyimide indium tin oxide cell (Standish, Lake Mills WI) and integrating the induced current peak. Tilt angle and switching speed were examined utilizing polarizing microscopy. The tilt angle was determined by halving the angle of rotation between the two points of maximum and minimum transmission during switching. To determine optical response time, samples were placed in a 4pm cell (Standish) with surface treatments as mentioned above. A square wave electric field of 8 V/jim was then applied across the cell. HeNe laser light (10mW) at 630 nm was passed through the cell and its intensity was determined by an optical intensity sensor. The optical response time was then found using a digitizing oscilloscope. Rotational viscosity was obtained using the values of the ferroelectric polarization and optical response time as described previously [12]. All measurements were determined using surface stabilized samples [3]. RESULTS Phase Behavior The enthalpy of the isotropic to smectic A transition (AHLA) for polymer and monomer systems normalized by the enthalpy change of the pure FLC is presented in Figure 1. These values decrease monotonically and almost linearly with increasing monomer concentration. The percent of monomer is thus linearly related to the percentage of FLC molecules allowed to undergo typical liquid crystal transitions. In other words, the monomer is well dispersed within the FLC matrix, and each monomer molecule has an equal effect on the surrounding FLC molecules. This is not the case, however, after polymerization. A s
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