Photochromic Organic-Inorganic Hybrid Nanocomposite Hard Coatings With Tailored Fast Switching Properties
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at. Res. Soc. Symp. Proc. Vol. 576 © 1999 Materials Research Society
observed [12]. In general, one may assume that a certain inner free volume of the host material would be required to avoid sterical hindering for the switching process. On the other hand, reactions of the photochromic dyes with moisture and oxygen from the environment, which lead to a decay of the photochromic activity [13] should be hindered by a minimum inner free volume of the host. In order to fullfill these demands, the use of an organic-inorganic nanocomposite coating matrix with "hard" and "soft" microstructural units allows the development of photochromic coatings with sufficient free volume for fast switching dyes in combination with macroscopic hard properties (rigid structures). A model system for photochromic coatings on glass from 3glycidyloxypropyl-trimethoxysilane (GPTS), bisphenole A (BPA) and methylimidazole (MI) [3,14] was used as a base line. Further developments in this field led to a new organicinorganic nanocomposite coating system, which is based on an epoxysilane as network former, different bisepoxides as spacers, and an organic amine as thermal cross-linker. This coating system is compatible with different photochromic dyes like oxazines, pyrans and
fulgides and also with surface modified ceramic nanoparticles as filler in order to increase the scratch resistance without changing the photochromic kinetics [15]. Coatings with switching times of only 2 s - 4 s were obtained [15], which are very short, compared to commercial photochromic eye glass lenses with switching times in the range of 1 - 3 minutes. The fast switching behaviour can be very useful for applications in the fields of automotive or sports. However, for other application fields (ophthalmic or architecture) slower switching could be of interest. Therefore, the general goal of this paper was to retard the photochromic behavior of spirooxazines by interactions with the nanocomposite system in order to adjust the kinetics to different applications without changing the direct photochromism into reverse photochromism [12]. To meet this goal, the polarity and amount of spacers was varied. As "probe" dyes, four spirooxazines with different polarity and molecular size were chosen. Ceramic nanoparticles, which do not influence the optical properties in the selected system [15], were not incorporated to keep the system simple at this stage. As an alternative, the decrease of the pore volume by different amounts of cross-linkers was investigated; these results will be published elsewhere [16]. EXPERIMENTAL
Sol preparation For the preparation of the photochromic coatings, GPTS (3-glycidoxypropyltrimethoxysilane)-pre-hydrolysate [3] and TEOS (tetraethoxysilane) in molar ratio of 5:1; spacer (CHMG: cyclohexanedimethanoldiglycidyl, PCF: poly(phenylglycidylether)-coformaeldehyd) with molar ratio relative to GPTS = 0:1, 1/5:1, 2/5:1, 3/5:1, 4/5:1 and 1:1; 10 mol% cross-linker IDA (isophorondiamine) relative to the epoxide groups; and 0.25 wt% photochromic dye (Blue A, C11,
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