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

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Abstract

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.

Introduction

In the two last decades, sol-gel techniques in relation with materials chemistry has been extensively investigated and employed to explore new approaches in obtaining oxide materials with improved properties [1]. The chemistry involved in sol-gel processes is based on hydrolysis and condensation reactions [2] (, ) of metallo-organic compounds such as metal alkoxides (M(OR)n).MOR+H2O→MOH+ROH HydrolysisMOH+MOX→MOM+XOH (X=H or X=R) OxolationMOH+MOH→M(OH)2M Olation

The structure and the morphology of the resulting macromolecular oxide based network strongly depend on the relative contribution of each of these reactions [3], [4], [5]. The recombination of these metal-oxo-polymers can produce bushy structures which invade the whole volume, forming gels when these oxo-polymers reach macroscopic sizes [3], [4], [5].

The knowledge of the evolution of the rheological properties of the sol-gel derived system is of great importance for the shaping of materials in technological processes. The small deformation applied in rheological dynamic tests can be considered to interfere only slightly with the gelation mechanism. Rheology is a powerful analytical tool to probe the change in viscosity and elasticity at the gelation threshold where a three dimensional network is built up and to follow structural and textural evolution of the gels. Moreover, this information is complementary to techniques such as NMR and SAXS measurements.

Most of rheological studies devoted to sol-gel derived materials concern silica based network built from silicon alkoxide [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. Only a few data are available for transition metal oxide gels obtained through the hydrolysis of transition metal alkoxides [16], [17], [18], [19], [20] because their sol-gel process is rather complex compared to silica chemistry. Among transition metal oxide based network, titanium oxo-polymers or titanium based gels are precursors for titania based materials whose technological importance has been well known for a long time. Applications of titanium dioxide colloids and thin films are numerous, including photovoltaic, electrochromic, photochromic, electroluminescence and catalytic devices and sensors [1], [21].

Titanium oxo-polymers can be obtained through the hydrolysis of Ti(OR)4 alkoxide. However, the major problem is to control hydrolysis–condensation rates which are generally too fast, resulting in the formation of precipitates with a high degree of microstructural disorder [22]. An attractive strategy is to use hydroxylated complexing ligands such as carboxylic acids, β-diketones and allied derivatives which modify the reactivity of the precursors by increasing the coordination of the metallic center and substituting easily hydrolyzable alkoxy groups by chelating acetylacetonato ligands [23]. A careful tuning of the experimental conditions (initial hydrolysis ratio H=[H2O]/[Ti], complexing ratio A=[AcacH]/[Ti], temperature, solvent) allows to obtain indeed transparent sols and gels made of titanium oxo-polymers [23], [24].

The present paper is devoted to the rheological study of polymers obtained from the hydrolysis of titanium butoxide in the presence of acetylacetone as an inhibitor of gelation. This study is mainly focused on the effects of temperature and hydrolysis ratio on the cross-linking process.

Section snippets

Sample preparation

The precursor of the sol-gel derived system under investigation was acetylacetone complexed titanium butoxide. Titanium butoxide Ti(OBun)4 was purchased from Fluka whereas butanol and acetylacetone (AcacH) were purchased from Prolabo and used without further purlfication. Titanium butoxide was checked by IR spectroscopy in order to detect possible hydrolysis. Acetylacetone diluted in butanol was added dropwise under magnetic stirring to a butanolic solution of Ti(OBun)4. Further hydrolysis was

Results and discussion

The definition of the gelation time is of decisive importance in material processing and has been the subject of several debates. However its determination is not a simple procedure.

Qualitative observations have been proposed to determine this critical time by the instant at which the material lost its fluidity [31]. This arbitrary method presents the disadvantages to be qualitative and often not reproducible. So different kinds of rheological measurements have been performed since a few years

Conclusion

In this paper we have studied the rheological properties of the cross-linking of alcohol solutions of titanium butoxide in the presence of complexing ligand.

Information on evolving microstructures have been obtained from the measurements of storage G′ and loss G′′ moduli versus time at different frequencies.

Gel points have been obtained according to Winters criterion which states that viscoelastic properties of chemical and physical gels follow a power law near the critical gel point.

A

Acknowledgements

The authors are grateful to Professor Daniel Quemada for useful discussions and to Professor Patrice Flaud to critical reading of the manuscript prior to its publication.

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