Second Order Nonlinear Optical Films by Alternating Polyelectrolyte Deposition on Hydrophilic and Hydrophobic Substrates
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Naval Air Warfare Center Weapons Division, Polymer Science and Engineering Branch China Lake, CA 93555-6100 b Naval Air Warfare Center Aircraft Division, Electro-Optic Sensors Branch Patuxent River, M D ABSTRACT Alternating polyelectrolyte deposition (APD) is a room temperature process that can be used to produce noncentrosymmetric ordered films of nonlinear optical polymers (NLOP). In this study, second-order NLOP APD films of a stilbazolium-substituted polyepichlorohydrin , poly (diallyldimethylammonium chloride), and polyacrylic acid were prepared by alternately dipping a substrate into aqueous solutions of each polymer. Evidence for uniform layer to layer deposition includes a linear increase of UV-Visible absorbance, thickness measurements , and quadratic increase of second harmonic generated light intensity as a function of film thickness. Films have been uniformly deposited up to 0.1 micrometer thick. The surface morphology, roughness, and film thickness have been characterized by Tapping Mode Atomic Force Microscopy (TM AFM ). Work is in progress to deposit thicker films of the same qualit y and to improve NLO figures of merit. INTRODUCTION APD is a technique used to form polyelectrolyte multilayers without the need for expensive equipment.1 A multilayer film is formed by alternately immersing the substrate in aqueous solutions of a polyanion and a polycation. Since the thickness of each layer is on the order of nanometers , APD allows precise control of the overall film thickness. APD films of thousands of layers have been reported. Coulombic forces are responsible for the formation of the APD films so the resulting structure is very robust. The technique has been used to produce films incorporating a range of materials including electrically conducting polymers ,2 conjugated polymers for light emitting diodes (LED),3 proteins,4 nanoparticles ,5 and noncentrosymmetric -ordered second order nonlinear optical polymers (NLOPs ).6 Noncentrosymmetric chromophore orientation has been observed in APD films by several research groups.7-9 Uniform NLO APD films have been
deposited with SHG increasin g quadratically with film thickness over one hundred bilayers , although χ(2) values were modest and comparable to quartz.10-11 More recently , our group has made NLO APD films in which both the polyanion and the polycation contain polarizable chromophores and the results suggest that both polyelectrolytes contribute to the observed SHG signal.12 Also, our group has demonstrated that the chromophore orientation in NLO APD films is stable at temperatures up to 150 °C.13 O
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5 PDADMAC/PAA 10 SPECH/PAA 5 PDADMAC/PAA 10 SPECH/PAA 5 PDADMAC/PAA
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Figure 1. NLO-active water-soluble polycation, SPECH, (left), and inactive water-soluble polycation, PDADMAC (middle), and polyanion PAA (right) used to make polar films by APD. For SPECH, n=4-5; for PDADMAC, n=1000-2000, and for PAA, n=750-800. Inset: Schematic view of layers resulting from the depositio
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