Incorporation of Polyaniline Into a Silica Gel Via the Sol-Gel Technique
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INCORPORATION OF POLYANILINE INTO A SILICA GEL VIA THE SOL-GEL TECHNIQUE
F. NISHIDA*, B. DUNN*, E.T. KNOBBE*, P.D. FUQUA*, R.B. KANERt, and B.R. MATTESt
*Department of Materials Science and Engineering, University of California, Los Angeles. iDepartment of Chemistry and Biochemistry, University of California, Los Angeles.
ABSTRACT The sol-gel process has been used to incorporate a conjugated polymer, polyaniline, in silica gel. The emeraldine base form of the polymer is partially solubility in aprotic polar solvents which are compatible with the sol. The effect of sol environment on the emeraldine base form of polyaniline was investigated. The soluble emeraldine base form is maintained at pH ? 2.4 enabling good optical quality gels to be synthesized. The type of alkoxysilane and the water ratio used in preparing the sol influence the solubility of the emeraldine base.
INTRODUCTION The use of the sol-gel process to incorporate optically active organic molecules is well established. A number of optical properties have been demonstrated including luminescence, photochromism, laser action and optical gain [1-3]. From these studies it is evident that the gel serves as a chemically and thermally stable host material with good optical properties. In the work to date, the sizes of the incorporated guest molecules have been in the range of the gel pore diameter (or smaller). The molecules are likely to be encapsulated by the gel matrix and there is evidence that the optical properties are influenced by interaction with the matrix [4]. The present investigation extends this work on the synthesis of organic-doped gels by considering the incorporation of a polymeric species which is substantially larger than the pore diameter. In particular, polyaniline-doped gels have been synthesized to take advantage of the nonlinear optical properties of conjugated polymers. Conjugated polymers are promising candidates for nonlinear optical materials due to their extended network of delocalized pi-electrons. Recent work on conjugated polymers such as polydiacetylene [5] and polythiophene [6] have demonstrated large third order nonlinear susceptibilities, X(3). These conjugated polymers, however, generally exhibit poor processing characteristics. They are environmentally unstable and somewhat intractable. As a result, attempts to fabricate optical devices using conjugated polymers have been impeded. The conducting polymer, polyaniline, is an exception to this behavior as it is both environmentally stable and processible. The emeraldine base form of polyaniline exhibits partial solubility in aprotic polar solvents such as dimethyl formamide (DMF) and N-methyl pyrrolidinone (NMP). We have used this characteristic to synthesize silica gel monoliths containing 2-ethyl 3
polyaniline [7]. Degenerate four-wave mixing measurements of these materials indicate that xO) for the doped Rel3 was approximately 30% of the value obtained for CS2 at 1.06 ýim (i.e., = 5 x 10-- esu). In the present study, details of the synthesis of polyaniline-doped sili
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