New Transparent Methylsilsesquioxane Aerogels and Xerogels with Improved Mechanical Properties

Abstract

Since they were first prepared in 1931 via supercritical drying (SCD), [1] aerogels have been utilized for rather specific applications in spite of their wide range of extraordinary properties, such as high optical transparency (> 90 %), large surface areas (ca. 1000 m 2 g -1 ), low refractive indices (< 1.01), high thermal and acoustic insulation values, and low dielectric constants (ca. 1.1). An aerogel-based Cherenkov detector has been used to detect charged elementary particles, thus contributing to the development of high-energy physics. [2,3] In the field of space development, [4] the first aerogel cosmic dust collector was launched on February 7, 1999 and returned to Earth on January 15, 2006, successfully bringing back the dust of the comet "Vilt 2" after a seven-year journey.[5] Despite these applications, aerogels are not routinely found in our daily life because they collapse easily and are difficult to prepare in a large-scale industrial setting. The unique characteristics of aerogels stem from the fact that they are composed mostly of air. As the skeletons that comprise their porous morphology are too thin, aerogels are brittle, and they must be dried with the utmost care by using SCD, for example, to preserve their fragile pore structure. To overcome such challenges for silica-based aerogels, numerous attempts to strengthen the gel skeletons by aging, [6,7] crosslinking with organic polymers, [8][9][10] modifying surfaces hydrophobically, [11][12][13] and conventional drying [14,15] have been explored.Prakash et al. [13] have obtained aerogel films with porosity up to 98.5 % by adding hydrophobicity through the use of trimethylchrolosilane constituents on bare silica networks. This method enables the so-called spring-back phenomenon, in which temporarily shrunk gel networks recover their original forms like a sponge. Another prevalent approach is to use trifunctional silicon alkoxides together with conventional tetrafunctional silicon alkoxides such as tetramethylorthosilicate (TMOS) or tetraethylorthosilicate (TEOS) in a sol-gel route. [16,17] Incorporating trifunctional monomers with one hydrophobic group, such as alkyltrialkoxysilanes, also makes the resultant gel surface hydrophobic [18,19] but sacrifices the transparency aspect. [20] As silanol groups that are closely located to each other lead to persistent shrinkage by forming siloxane bonds during drying, [21] incorporating hydrophobic groups to reduce the chances of such condensations prevents persistent shrinkage. The repulsive interactions between hydrophobic moieties also promote the spring-back effect. An in situ treatment of alkyltrialkoxysilanes rather than a post treatment by a silane coupling agent on bare silica is an effective way to achieve hydrophobicity in a simple manner. However, trifunctional alkoxides usually form cyclic and cagelike closed species that hinder the homogeneous gelling of a starting solution, especially when the monomer concentration is low (unfortunately in aerogel preparation, monomer concentration must be ...

Modeling silica aerogel optical performance by determining its radiative properties

“…[1][2][3][4] In particular, the optical transparency of silica aerogel in the solar spectrum is crucial for many solar related applications [5][6][7][8] and has been studied extensively. [9][10][11][12][13][14][15][16][17][18] J. Fricke et al showed that the optical transparency of silica aerogels is tied closely to its microstructure which acts as Rayleigh scattering centers.…”

Sol-gel synthesis, porous structure, and mechanical property of polymethylsilsesquioxane aerogels

“…14), 15) A modified 2-step acid/base sol-gel process utilizing hydrolysis of urea enhances the homogeneous gelation of MTMS. Additionally, an adequate surfactant is employed to suppress macroscopic phase separation which otherwise would develop in the micrometer scale and eliminate transparency.…”

Advances in monolithic porous materials tailored in liquid media: around inorganic oxides and organic polymers

“…With appropriate surfactant, such as n-hexadecyltrimethylammonium salt (CTAB for bromide or CTAC for chloride) or poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymers, phase separation is effectively suppressed and transparent aerogels can be obtained. 20) The PMSQ aerogels obtained after supercritical drying exhibit similar properties to the carefully-prepared conventional silica aerogels such as low density (<0.15 g cm ¹3 ) and high porosity (³90%), and high visible-light transparency (³90% at 550 nm through a 10 mm-thick-equivalent sample). 20) 22) The typical PMSQ aerogels prepared with CTAB consist of a well-defined mesoscopic pore structure with 30 50 nm pores and solid frameworks formed by nano particles (³10 nm in diameter).…”

“…20) The PMSQ aerogels obtained after supercritical drying exhibit similar properties to the carefully-prepared conventional silica aerogels such as low density (<0.15 g cm ¹3 ) and high porosity (³90%), and high visible-light transparency (³90% at 550 nm through a 10 mm-thick-equivalent sample). 20) 22) The typical PMSQ aerogels prepared with CTAB consist of a well-defined mesoscopic pore structure with 30 50 nm pores and solid frameworks formed by nano particles (³10 nm in diameter). Owing to the methyl groups, the PMSQ aerogels naturally exhibit hydrophobicity without the surface modification.…”