Structures of inorganic polymers in sol-gel processes based on titanium oxide

Abstract

Inorganic gels and precipitates have been made through hydrolysis and condensation of the precursor Ti(${\mathrm{OBu}}^{\mathit{n}}$ ${)}_4$ dissolved in n-butanol. The condensation of uninhibited precursors leads to precipitation; the selective inhibition of condensation through ${\mathrm{H}}^{+}$ ions prevents precipitation and leads to gelation. The structures of the polymers which result from condensation in either case have been examined through small-angle x-ray scattering. It has been found that these structures vary continuously with the inhibition ratio. At high inhibition, the polymers are tenuous objects with a self-similarity exponent ${\mathit{d}}_{\mathit{f}}$≊2; they invade the whole sample volume to form transparent gels. At intermediate ratios, the polymers become bushy with a self-similarity exponent ${\mathit{d}}_{\mathit{f}}$>2 and they form turbid gels. Finally, precipitation occurs when ${\mathit{d}}_{\mathit{f}}$ reaches 3. These nonuniversal values of the exponents result from nonstationary growth modes, where a few large polymers grow first, and then densify through the capture of unused monomers.