Investigation Of The Silica Network During The Sol To Gel Transition And Final Xerogel Properties
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INVESTIGATION OF THE SILICA NETWORK DURING THE SOL TO GEL TRANSITION AND FINAL XEROGEL PROPERTIES PAUL R. SOSKEY, RANDALL E. NIKLES, GEORGE F. FATTMAN, ROBERT M. MININNI* AND DEBORAH A. GERENZA** *EniMont America Inc., 2000 Cornwall Rd., Monmouth Jct., NJ 08852 **Rheometrics Inc.,'One Possumtown Rd., Piscataway, NJ 08854 ABSTRACT Silica sols were produced with various catalysts (NaOH and HF). Dynamic small strain rheometry was used to probe the formation of the silica network from the sol state through the gelation point into the region of high modulus. These results were related to the dried xerogel properties of surface area and porosity. INTRODUCTION Various types of rheological measurements have been used to study the structure evolution of silica gel networks [l]. Many times large strain steady shearing flows are used to determine gel transitions, however, because the gel network is destroyed by the large strain, the gel point of the quiescent silica network actually occurs at an earlier time than measured. Others have used small strain, dynamic tests to follow gelation at a fixed frequency [2]. Winter and coworkers have shown that much information can be gained from the entire frequency domain as a function of the time of gelation in polymeric networks [3,4]. We will use these dynamic techniques to follow the gelation of silica gels from the initially mixed low viscosity fluid state, through the 3-D network growth (gelation), into the fully gelled (network syneresis) stage and relate these results to the porosity of the resultant dried xerogel. EXPERIMENTAL The tetramethoxysilane (TMOS) (Petrarch)/water silica sol system was studied using either hydrofluoric acid (HF) or sodium hydroxide (NaOH) (both from Fisher Scientific) as catalysts. The initial composition TMOS:H 2 0 was 1:16 with catalyst concentrations of 1.83x10- 5 M (low) and 1.85xl0- 3 M (high) in the water for both catalysts. A fifth system using no catalyst was prepared as a control. The initial reaction was carried out by vigorously stirring the components until a clear solution was obtained. The high NaOH catalyst sample did not form a clear sol prior to gelation and the sample was considered mixed when no phase separation was apparent. Mixing times for all samples ranged from 10-20 minutes and once mixed the samples were immediately analyzed. Due to the wide range in dynamic storage modulus (G') from the initial sol to the latter stages of gelation and beyond, two instruments were used in this study. A low viscosity fluids rheometer (Rheometrics RFS II) was used to investigate the solto-gel transition and a higher viscosity rheometer (Rheometrics RMS 800) was used to investigate further network formation beyond the gel point. Mat. Res. Soc. Symp. Proc. Vol. 180. @1990 Materials Research Society
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The RFS rheometer was fit with a couette fixture with dimensions: bob radius 16mm, bob length 33mm and cup radius 17mm. The sample was loaded into the fixture, the bob lowered and a layer of silicon oil (100 cp) was applied to the exposed sample a
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