Rheology of titanium oxide based gels: determination of gelation time versus temperature

The gelation of hydrolysed alcoholic solutions of acetylacetone complexed titanium butoxide was studied through rheological measurements. The gelation time was determined through the evolution of the storage and the loss moduli versus time at several frequencies using the Winters criteria. Fractal dimensions determined from rheological measurements are very close to self-similarity exponents obtained from SAXS experiments (df=2.4±0.1) and correspond to bushy structures. The effects of temperature and hydrolysis ratio on gelation were also studied. Increasing hydrolysis ratio and/or temperature speed up the kinetics of hydrolysis–condensation reactions and thus decreases the gelation time tg however at the expense of an efficient crosslinking. Indeed at t=tg the titanium oxo-polymers obtained at shorter tg are less dense and less elastic than the networks formed at longer tg. An apparent activation energy (around 120 kJ mol−1) which was found to be independent of hydrolysis ratio, was calculated from the values of gelation times determined at various temperatures.