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Klemperer, Walter G. Ramamurthi, Sangeeta Dikshit 2011-05-07T12:01:08Z 2011-05-07T12:01:08Z 10000-01-01 1989 An analytical protocol involving gas chromatography (GC), gas chromatography-mass spectrometry (GCMS), and 29Si{1H} NMR spectroscopy, was developed to determine the structures of the polysilicate intermediates formed during silica sol-gel polymerization. This protocol allowed the structural identification of mono- through hexasilicate species (1<x<6 in [SixOy]9OCH3)a) formed during partial hydrolysis and condensation of tetramethylorthosilicate. The analytical protocol employing capillary gas chromatography was applied to monitor the stoichiometry, structures, and abundances of low molecular weight polysilcates (l<x<6 in [Si%Oy](OCHg)^) formed during partial hydrolysis of methanolic tetramethylorthosilicate under acidic (HC1) and basic (KOH) conditions. The same procedure was applied to study the redistributions of methanolic hexamethyldisilicate and octamethyltrisilicate under similar reaction conditions. The polysilicate molecular weight distributions (expressed in terms of mole percent of total silicon present) were determined as a function of the degree of polymerization. Under acidic conditions, the molecular size distributions showed maxima near the number average degree of polymerization, and diminished at higher and lower degrees of polymerization. Under basic conditions, on the other hand, the distributions had their maxima at the monomer percent and extended to very high molecular weights, yielding distributions far broader than the ones observed under acidic conditions. These features were found to be in agreement with the covalent network theories of silica gelation (Flory-Stockmayer theory). The weight fraction distributions of the type observed under acidic conditions were characteristic of linear polymers with low degrees of crosslinking. Weight fraction distributions observed under basic conditions were characteristic of branched polymers with high degrees of crosslinking. The low degree of crosslinking obtained under acidic conditions may arise from steric crowding that discourages branching in polysilicate structures. Finally, the physical properties of the resultant materials such as bulk and skeletal densities were explained in terms of the observed molecular weight distributions. A procedure was also developed to prepare monolithic silica glasses from mono-, tri-, and octasilicate precursors, under acidic (HCl) and basic (KOH) conditions. Bulk density measurements of these monoliths indicated that the octasilicate monoliths were relatively dense. Made available in DSpace on 2011-05-07T12:01:08Z (GMT). No. of bitstreams: 2 license.txt: 4922 bytes, checksum: 910b249b4beec47e7ab768910c8f966f (MD5) 8924925.pdf: 4097916 bytes, checksum: 12b7a59cbcdb261b71e556fc57fd263a (MD5) Previous issue date: 1989 Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Howard Ding (hding2@illinois.edu) on 2011-05-07T14:35:34Z Item is restricted indefinitely. Restriction data tranferred 2014-07-01T11:14:04-05:00 Original Data Group with Access UIUC Users [automated] Release Date: none Reason: ETDs are only available to UIUC Users without author permission ETDs are only available to UIUC Users without author permission U of I Only AAI8924925 (UMI)AAI8924925 http://hdl.handle.net/2142/19234 eng Copyright 1989 Ramamurthi, Sangeeta Dikshit Chemistry, Inorganic Molecular growth pathways in silica sol-gel polymerization text Chemistry Chemistry University of Illinois at Urbana-Champaign Dissertation Ph.D.