Facile, tunable, and environmental friendly synthesis of silica/resorcinol–formaldehyde hybrid xerogels with ultra-low shrinkage using a cationic polyelectrolyte as a soft template

Silica/resorcinol–formaldehyde (RF) hybrid xerogels with low shrinkage were prepared efficiently in an environmental-friendly way through a two-step method and ambient drying process. Poly(diallyldimethylammonium chloride) (PDADMAC) was used as a soft template, aqueous solution of resorcinol and formaldehyde was used as organic source, and hydrosol of tetraethoxysilane and methyltriethoxysilane was used as inorganic source. Then, the hybrid xerogel sample was grafted with trimethylchlorosilane to improve its hydrophobicity. By adjusting the amount of PDADMAC, the minimum shrinkage can be controlled at about 2%. With the slight increase of pH from 2.13 to 2.98, the average pore size decreased sharply from 24.82 to 0.47 μm, and the specific surface area increased significantly from 0.63 to 26.51 m 2 /g, but it still maintained the characteristics of low shrinkage (1.16–2.18%), low density (0.20–0.25 cm 3 /g) and high porosity (~ 80%). Scanning electron microscopy (SEM) analysis showed that the polysiloxane formed a continuous coating on the surface of the RF resin skeleton, and the average particle size decreased from micron level to nanometer level. Mechanics testing indicated that its elastic modulus is as high as 37.26 MPa. Thermogravimetric analysis in air revealed that the weight loss rate of hybrid modified samples slowed down at the high-temperature range (200–600 °C), and the residual weight increased from 0.65 to 13.80% at 800 °C. The water contact angle test before and after hydrophobic modification demonstrated that the contact angle increased sharply from 0° (superhydrophobic) to 160° (superhydrophobic).