Inorganic-Organic Composites by Sol-Gel Techniques
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INORGANIC-ORGANIC COMPOSITES BY SOL-GEL TECHNIQUES Helmut SCHMIDT Fraunhofer-Institut fUr Silicatforschung, WUrzburg, Federal Republic of Germany
Neunerplatz 2,
D-8700
ABSTRACT The sol-gel process opens the possibility of combining inorganic and organic units to new hybrid polymers. Organic units can be used for structural modification of the inorganic backbone, for creating new functions within an inorganic network and for building up organic polymeric chains. The materials show interesting perspectives with respect to structural (surface hardness, strength) and functional properties (e. g. diffusion, photocuring, incorporation of dyes, optical properties). A review over structural and functional properties of sol-gel derived inorganic-organic polymers (ORMOCERs = organically modified ceramics) is given. INTRODUCTION The sol-gel process is a synthesis route to inorganic nonmetallic materials. Reactive monomers, oligomers or colloids can be used as starting materials. They have to be "activated" in order to undergo a polycondensation step and to form polymeric networks. This can be achieved by various means but, in general, by creating reactive aMeOH groups, which are able to form sMe-O-Me= bonds during the condensation step. A convenient route is the use of alkoxides as precursors which react with water to hydroxides, condensing spontaneously to polymeric species (1). Similar reactions are well-known from inorganic salts, forming hydroxides and precipitates (2) by pH change. wMeOR =MeOH
H2 0 H20.. =MeOH (irreversible step) + ROMe= - - MeOMe= + HOR + HOME= - - MeOMe= + H2 0 R = alkyl
H2 0 =MeX .... =MeOH (reversible step, pH!) =MeOH + XMe= = MeOMe= + HX + HOMe= - MeOMe= + H20 X = anion (CI-,
(1)
(2) N03-,..)
Another route is the destabilization of colloidal sols by pH change either in organic solvents or in water. All these reactions lead to gels, containing solvent or air after drying. Due to the (in general) high specific surface areas of these gels, they contain adsorbed molecules from the processing steps (water or organics) or, in the case of the alkoxide route, unhydrolysed alkoxy groups. Organics are oxidized or pyrolyzed during heat treatments of gels, converting them into glasses or ceramics. Heat treatments are necessary to enhance diffusion
Mat. Res. Soc. Symp. Proc. Vol. 171. ©1990 Materials Research Society
processes (for crystallization) or to decrease viscosity (for glass formation) in gels. The viscosity of inorganic gels is high due to the three dimegsional crosslinking of the inorganic tetrahedron) leading to the typical building units (e. g. Si04 brittleness of ceramics. Decreasing network connectivity by introducing organic groupings can cause a remarkable decrease of viscosity of systems with pure inorganic backbones and leads to dense materials at temperatures between 50 and 150 °C, as shown in [1-2]. An analogue effect can be observed in inorganic glasses where the introduction of inorganic network modifiers leads to lower viscosities compared to fused silica. The combination of or
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