SYNOPSISThe transmittance, flexure strength, Young's modulus, and Vickers hardness of poly (methyl methacrylate) ( PMMA ) , filled with fumed silica, was measured. Transmittance decreased with increasing content of filler. At 2 vol % filler content, composites had a higher transmittance with a lower surface area of fumed silica (larger primary particle size) because the lower surface area filler was better dispersed. At 4 vol % filler content, composites had a higher transmittance with a higher surface area fumed silica (smaller primary particle size). Flexure strength and Young's modulus of the composites was measured using three point bending. Addition of fumed silica led to a decrease of strength. Also, addition of fumed silica led to a n increase of Young's modulus and Vickers hardness.
I NTRODU CTl O NIn recent years, a variety of highly transparent oxide /polymer composite materials have been investigated. One class of these transparent composites has received names such as ormocers (organically modified ceramics ) and ormosils (organically modified silicates) .'p2 A second class of transparent oxide /polymer composite materials is poly ( methyl methacrylate) ( PMMA ) impregnated silica gel^.^,^ These composites often possess high transparency because the scale of the individual phases is on the molecular or nanometer level.Transparent oxide /polymer composite materials often combine useful attributes of both the oxide and polymer constituents. For example, ormosils are both transparent and hard. Ormosil coatings, applied on polycarbonate lenses, have been shown to provide abrasion resistance.' Another possible application of these composites is as a host for an organic dye molecule. Like polymers, these composites can be processed at or near room temperature, allowing incorporation of organic dye molecules. Yet these composites have improved thermal properties with * To whom correspondence should be addressed. respect to polymers due to the ceramic phase. When used as hosts for organic dyes, transparent oxide/ polymer composite materials often improve the photostability and thermal stability of the dye molecules, in comparison to polymer hosts. For example, organic laser dyes incorporated in ormosils showed improved ph~tostability.~ Also, an electro-optically active aniline dye showed improved thermal stability when incorporated in PMMA-silica gel composites? Over the years, a number of polymers have been filled with submicron fumed or precipitated silica to yield transparent composites. Filled polymers are typically not nearly as transparent as the new classes of transparent composites described above. Nevertheless, one outstanding advantage of filled polymers is that they can easily be prepared in very large size. Like ormosils and PMMA-silica gel composites, filled polymers show improvement in thermal properties over bulk polymers. For instance, acrylicfumed silica composites show an increase in the glass transition temperat~re.~ This effect has been attributed to a boundary layer surrounding the fille...
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