Cavity Morphology of Polymer Dispersed System Utilizing Atomic Force Microscopy
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Cavity Morphology of Polymer Dispersed System Utilizing Atomic Force Microscopy Adam K. Fontecchio, Gregory P. Crawford1, and David Content2 Department of Physics and 1Division of Engineering, Brown University Providence, RI 02912, USA 2 NASA Goddard Space Flight Center Greenbelt, MD 20771, USA ABSTRACT Holographically-formed Polymer Dispersed Liquid Crystal have been imaged using atomic force microscopy (AFM) tapping techniques to study the droplet cavity morphology. Reflection-mode Bragg gratings were created using a 532 nm beam expanded laser to holographically form a grating structure within the liquid crystal / polymer film. The films were removed from their glass substrates prior to imaging by an atomic force microscope, and the digital files were analyzed. The surface structure is not smooth, as anticipated, but contains cavities with a dimpled morphology. We report our investigations and analysis including AFM images, image analysis, and liquid crystal / polymer alignment considerations. INTRODUCTION Holographically-formed Polymer Dispersed Liquid Crystal (H-PDLC) technology enables electrically controlled versions of applications such as beam steering, remote sensing wavelength filtration[1], diffraction[2], and reflective displays[3][4]. Performance improvements for the technology are actively sought, and the fundamental principles of operation are currently being researched. Currently, active areas of research include theoretical modeling[5], materials development[6], device development[7], and the morphological studies reported here. Electrically switchable Bragg grating materials are different from their polymer dispersed liquid crystal[8] ancestors: they are composed of periodic planes of liquid crystal rich and polymer rich regions, as shown in Figure 1, rather than a random droplet dispersion. When the ordinary refractive index of the liquid crystal, no, matches that of the polymer, np, the grating disappears into a transparent state upon the application of a voltage, as shown in Figure 1(a). A large difference in the indices of refraction between the liquid crystal droplet planes and the surrounding polymer results in high diffraction/reflection efficiency and low residual scattering in the zero field state, as shown in Figure 1(b). The performance characteristics of an H-PDLC, in both the electrical and optical regimes, depend on sample morphology, materials characteristics, sample size, and impurity levels. Phase separation and droplet formation are the key elements that determine the H-PDLC electro-optic performance. The coherent light scattering effect that is the basis of the Bragg grating structure depends on the controlled index mismatch of planes of liquid crystal and the polymer matrix. For this to occur, the liquid crystal and monomer components must be chosen with consideration given to viscosity, index of refraction, dielectric characteristics, and chemical purity. The H-PDLC is created when a mixture of liquid crystal and photopolymerizable monomer is exposed to an interference pattern
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