Hydrophobic silica aerogels by silylation

Abstract

In this work, silica gel networks were modified with a silylating agent dimethoxy-methyl (3,3,3-trifluoropropyl) silane (SiF3) to obtain hydrophobic aerogels of various surface energy values. The baseline aerogels were synthesized from tetraethoxy silane (TEOS) using a twostep sol-gel process followed by supercritical drying in liquid carbon dioxide. The resultant aerogels were characterized using scanning electron microscopy, Instron tensile tester, contact angle goniometry, and nitrogen adsorption-desorption isotherms. Three modification methods were studied. In method 1, TEOS and SiF3 were combined before gelation; in method 2, SiF3 was added after TEOS was hydrolysed and before its condensation, and in method 3, SiF3 was added after the gels were produced from TEOS. It was found that method 3 produced the best results in terms of achieving high values of hydrophobicity and compressive properties. The data on solid state 13 C and 29 Si NMR spectra revealed chemical reactions between the silylating agents and the silanol groups on silica surface. The bulk density and the fractal dimensions of silica networks gleaned from small angle X-ray scattering (SAXS) data showed weak dependence on the degree of silylation. The silylation process rendered the aerogels strongly hydrophobic and also doubled its compressive modulus.

Synthesis and characterization of partially fluorinated aerogels and xerogels from environmentally-compatible precursors

“…The oven temperature was ramped from 70 to 250 C at a rate of 20 C min À1 . 1 H, 13 Supercritical uid extraction was carried out with an EM Science E3100 large chamber supercritical dryer (SCD). The dryer was equipped with a PolyScience refrigeration/heating circulator, series 9102, digital temperature controller.…”

“…More recently, Jana et al synthesized hydrophobic aerogels by adding the commercially available uo-rinated reactive silane, dimethoxy-methyl (3,3,3-triuoropropyl) silane, at different stages of the sol-gel synthesis. 13 The authors determined that the point at which the uorinated monomer was added had a signicant effect on the hydrophobicity of the resulting aerogels.…”

Flexible organofunctional aerogels

“…[2][3][4][5][6] Another promising way to improve the mechanical properties is to make aerogels hydrophobic by post-treatment with surface modification agents or by the hybridization of silica aerogels via a cocondensation approach of silicon alkoxides with organosilane precursors that contain hydrophobic groups. [7][8][9][10][11] Recently, different flexible organic-inorganic hybrid aerogels were made solely from a methyltrimethoxysilane (MTMS) or vinyltrimethoxysilane (VTMS) precursor. Rao et al produced flexible as well as bendable, superhydrophobic and low-density gels, that were dried supercritically and had a reversible elastic compression of up to 60%.…”

Nanomaterials in Advanced, High-Performance Aerogel Composites: A Review

“…The highest contact angle was obtained when the ratio of TMES: hexane was 8:1. Another study proposed an interesting method to obtain hydrophobic aerogels through modifying silica gel networks with silylating agent dimethoxy-methyl (3,3,3-trifluoropropyl) silane [143]. The aerogels were synthesized from tetraethoxy silane (TEOS) using a two-step sol-gel process followed by a supercritical drying with CO 2 .…”