Conformal Internal Coating of Macroporous Co-Continuous MCF-silicas with Isocyanate Derived Polymers
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Since their discovery in 1992,1,2 templating sol-gel silica with structuredirecting agents has resulted in new materials with ordered mesoporous (2-50 nm) structure that is investigated for application in areas as diverse as high surface area supports for caralysts or as nanostructured drug storage and release platforms.3 Structure-directing agents are typically surfactants ranging from cationic detergents such as alkyl-trimethylammonium salts with C8-C18 hydrocarbon chains to non-ionic tertiary amines, polyethylene oxide or amphiphilic triblock copolymers such as poly(ethyleneoxide)-blockpoly( propyleneoxide)-block-poly(ethyleneoxide) (e.g., Pluronic P123). Depending on the surfactant and the conditions (e.g., kind and concentration of catalyst) the structure of the mesopores may vary (e.g., from cubic to hexagonal etc.) the surfactant forms templating micells, which can be enlarged by adding organic swelling agents as for example mesitylene to P123. The resulting materials are referred to as Mesoporous Cellular Foams (MCFs) and they contain a co-continuous three-dimensional macroporous (>50 nm) pore system consisting of interconnected spherical compartments with mesoporous walls.4 Owing to a combination of a low resistance to hydraulic flow with short diffusion path lengths within the mesoporous skeletal walls surrounding the macropores, such systems attract attention as chromatographic stationary phases,5 and in fact certain versions of this technology are already incorporated in monolithic separation columns marketed by Merck Co. under the trade name ChromolithTM.6 Typical preparation conditions for those materials involve phase separation induced by the templating agent, followed by gelation of an alkoxysilane. Ambient pressure drying removes the gelation solvent and a final high-temperature calcination removes the templating agent. These conditions induce shrinkage that not only makes reproducible in-place-of-use preparation of the monolith problematic but also causes chemical alternation of the silica surface, densification of the skeletal framework and partial collapse of the macropores. Here we explore a different approach for minimizing shrinkage, reduce cracking, increase mechanical strength and reproducibility of templated silica type of materials. For this we employ a method we developed recently by which we use the native -OH group surface functionality of silica in order to direct the polymerization of a di-isocyanate.7 Bi-continuous macro- /mesoporous monolithic silicas are prepared by Nakanishi's modification of Stucky's method,8 in which P123 (molecular weight~5,800) is used as templating agent while 1,3,5-trimethylbenzene (TMB) is employed as an expanding agent.3 the templating agent was washed off by Soxhlet extraction and the resulting wet gels were exposed to a solution of a di-isocyanate in acetone; unreacted di-isocyanate was washed off and the samples were dried with CO2 taken out supercritically. Comparative characterization was conducted for isocyanate-treated and non-treated samples. Isocyanate-treated monoliths undergo minimal shrinkage, they are much more robust than native samples while they maintain the surface area of the untreated monoliths.