Elsevier

Microporous and Mesoporous Materials

Volume 267, 1 September 2018, Pages 242-248
Microporous and Mesoporous Materials

Structure and thermal stability in hydrophobic Pluronic F127-modified silica aerogels

https://doi.org/10.1016/j.micromeso.2018.03.039Get rights and content

Highlights

  • Hydrophobic Pluronic F127-modified silica aerogels were prepared.

  • Hydrophobicity was promoted with hexamethyldisilazane as a silylating agent.

  • The porous structure was found fairly thermally stable up to about 500 oC.

  • The degradation of the silylation layer occurs sharply at about 350 oC.

Abstract

Hydrophobic ambient pressure drying (APD) aerogels were prepared from hydrolysis of tetraethylorthosilicate (TEOS) in solutions with different concentrations of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (F127). APD was carried out after silylation of wet gels with 20% by volume of hexamethyldisilazane (HMDZ) in n-hexane. The samples were analyzed by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The APD aerogels obtained in this process were submitted to heat treatment at 300, 500, 700 and 900 °C to study the pores stability. The samples were characterized by nitrogen adsorption. Wet gels are formed by mass-fractal domains, with fractal dimension close to 2.1 and characteristic size (ξ) spanning from about 9 nm (for the gel prepared without the addition of F127) up to values that exceed the maximum limit of the SAXS experimental setup, with increasing the concentration of F127. Nitrogen adsorption data showed that the pore volume (Vp) and the mean pore size (lp) of the aerogels increased with increasing the concentration of F127. The drying process diminished the characteristic size ξ and increased the dimension D of the mass-fractal domains and the size (r0) of the primary particles of the aerogels with respect to the wet gels. The characteristic size ξ of the mass-fractal of the aerogels was found significantly larger with increasing the concentrations of F127. Thermally treated aerogels exhibited a similar general behavior with temperature independent of the concentration of F127. The porosity was found fairly stable up to about 500 °C. The porosity started to be eliminated at 700 °C and it was found practically collapsed at 900 °C. The silylation layer on the silica surface of the present APD aerogels was promptly eliminated at about 350 °C yielding complete loss of hydrophobicity.

Introduction

Since the discovery of the amphiphilic properties and of the micelles formation in aqueous solution of the system copolymer tri-block poly(ethylene oxide) – poly(propylene oxide) – poly(ethylene oxide) [1,2], many scientific and technological advances have been achieved, using this and others similar surfactants in the preparation of the mesoporous silica, often resulting in interesting ordered pore structures [3,4]. In the preparation of composites, the copolymers block are an interesting class of structure modifier agents because the micelle structuration can determine the formation of the organized molds during the synthesis of the composites. These copolymers have advantage as their ordering way can be tailored by factors as solvent composition, molecular weight or copolymer architecture [3,4]. The use of the copolymer tri-block as impregnate structure agent in the silica polymerization process has allowed the preparation of ordered mesoporous silica with cubic arrangement and others forms [[4], [5], [6]].

Mesoporous silica shows interesting structural characteristics, including high specific surface area and pore volume, making this material of great scientific and technological interest for applications in many knowledge areas. Applications in catalysis, separation, adsorption, immobilization of enzymes, transport and controlled release of drugs, and nanotechnology, as suitable matrices for preparation of nanoparticles of advanced materials, are naturally areas of interest [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16]].

The copolymer F127 when used as a structure modifier serves to produce pores in the silica gels, not only with an ordered porous structure, but with possibility of formation of various pore classes in the gel. This copolymer also allows one to obtain pores with characteristic size greater than those generated by surfactants of smaller molecular weight.

The sol-gel process allows one to obtain silica particles of different sizes through the variation of reaction parameters as temperature, pH, reactant concentration, type and concentration of catalyst, etc. The chemical nature of the particle surface can also be tailored in the sol-gel process. The sol-gel process of silicon alkoxides and the chemistry involved in the process lead to a more control of the purity of the final products.

The system silica/surfactant has attracted the attention of researchers in the material area due to the facility in obtaining mesoporous silica gels. This is due to the fact that the surfactant molecules form micelles in acids solutions, which can be cylindrical, spherical or lamellar, depending on the synthesis conditions [17], constituting a wide range of structural possibilities in the gel formation. In this work, copolymer F127 was used as an impregnate structure agent and HMDZ as a silylation agent in the obtaining of hydrophobic ambient pressure drying aerogels. The results were analyzed by small-angle X-ray scattering (SAXS), nitrogen adsorption and thermogravimetric analysis.

Section snippets

Material and methods

A starting F127 solution was prepared by dissolving 4.0 g of copolymer Pluronic F127 in 30 ml of water and 120 ml of HCl 2 M, under mechanical stirring for 20 h at ambient temperature. Sols of silica were prepared from hydrolysis of a fixed amount of 25 ml of TEOS and 8 ml of water mixed to different quantities of the F127 solution (0, 10, 20 and 30 ml) to yield silica sols with different concentrations of F127. The hydrolysis was carried out under mechanical stirring at 35 °C for 60 min in

Wet gels

Fig. 1 (on the left) shows the SAXS intensity I(q) as a function of the modulus q of the scattering vector for the wet gels T0, T10, T20 and T30. The curve of the wet gel T0 can be associated with that produced by a system of mass-fractal clusters. The mass of a mass-fractal cluster scales with the length scale r as m(r) ∝ rD in an interval ξ ≫ r ≫ r0, where ξ is the characteristic length of the mass-fractal domain formed by primary particles of characteristic size r0, and D is the dimension of

Conclusions

Ambient pressure drying (APD) aerogels were prepared from the hydrolysis of tetraethylorthosilicate in water solutions of poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (F127), followed by silylation of the silica surface using a solution of 20% volume of hexamethyldisilazane (HMDZ) in n-hexane.

Wet gels can be described as mass-fractal structure, with mass-fractal dimension close to 2.1 and characteristic size ξ spanning from about 9 nm, for the gel prepared without F127, up to

Acknowledgment

This work was supported by the Brazilian Synchrotron Light Laboratory (LNLS) and by the Brazilian funding agencies: FAPESP, CAPES and CNPq.

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