Synthesis and Spectroscopie Analysis of Smart Photochromic Materials
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ABSTRACT Sol-gel processing was used to dope photochromic materials into metal alkoxide-polymer and pure polymer materials. The films on silicon and quartz substrates were examined, with and without UV irradiation, by UV/VIS spectroscopy, ellipsometry and FTIR. The UV/VIS spectroscopy showed that the doped matrices were photochromic in the visible and near infrared. The ellipsometric data were obtained with a variable angle of incidence spectroscopic ellipsometer (VASE). The Cauchy and a combined Cauchy-Lorentz model were used to fit the unirradiated and irradiated films, respectively. The optical constants of the films showed significant changes upon irradiation. This means that the absorption coefficient and hence the emissivity of the films is being modulated with UV irradiation. The VASE-fitted thicknesses of the films were in the range of 1 to 6 microns. In the FTIR spectra, the spiropyran doped samples have shown IR transmission changes in the two spectral regions (6-7 gtm and 7.5-8.5 gtm) where changes are expected due to band opening. The transmission ratio for UV irradiated to unirradiated samples decreased by as much as about 24% at a particular IR wavelength.
1. Introduction In the past few years, photochromic dyes which show reversible photoisomerization have attracted considerable interest from the viewpoint of applications such as novel photometry, optical switching and memory devices[I-3]. Various photochromic dyes have been synthesized and improved reversibility and stability have been reported[4-5]. In this study sol-gel processing was used to develop a smart photochromic material for space applications whose infrared emissivity changes in response to solar radiation. The sol-gel technique can be used to produce both thin and thick films without cracking. This can be attributed to a mechanical relaxation of these materials, caused by various mechanisms such as hydrolysis and polycondensation[6-7]. The mechanical properties and thermal resistance of a pure photochromic dye can be improved by putting it into a matrix. One such matrix is a metal alkoxide-polymer. The metal alkoxidepolymer matrix can be prepared by the incorporation of metal alkoxides into organic polymeric materials via the in-situ polymerization of inorganic alkoxides by the sol-gel processing technique. In several published reports[8-10], various methods have been utilized to promote compatibility between the inorganic network and either an inorganic or organic polymer. These methods include reacting a metal alkoxide directly with organic polymers or oligomers that are end-capped with or that contain functional groups capable of entering into a cross-reaction with the inorganic polymers[ 11-13]. The other matrix is the polymer poly methyl methacrylate (PMMA). Even though this polymer is not expected to survive well in space, we are using it as a convenient way to test the incorporation of photochromic dyes into polymers. Also, this matrix is potentially useful for applications such as optical recording and switching devices[ 14-16]
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