Hydrophobicity of amorphous silica-based inorganic-organic hybrid materials derived from perhydropolysilazane chemically modified with alcohols

Abstract

Perhydropolysilazane (PHPS) was chemically modified with various alcohols (ROH, RCH3, n-C3H7, n-C5H11, n-C10H21, CH3OC2H4 or C2H5OC2H4) at a PHPS (Si basis) to ROH molar ratio of 3:1. The resulting alkoxy group-functionalized PHPS materials were successfully converted to amorphous silica-based inorganic-organic hybrids by exposure to vapour from aqueous ammonia at room temperature. Nitrogen sorption analyses demonstrated that these hybrids contained a small quantity of micropores less than 0.9 nm in size along with mesopores having a relatively wide pore size distribution (PSD). The PSD plots of the CH3O, CH3OC2H4O and C2H5OC2H4O functionalized samples had especially wide distributions extending to more than 50 nm. Water sorption tests showed that the hybrid synthesized from PHPS modified with n-C10H21OH was stable and did not exhibit significant capillary condensation even at higher levels of humidity above P/Po = 0.6. As a result, the number of water molecules adsorbed per square nanometre of the sample surface area at P/Po = 0.95 was as low as 4 mol nm-2. The relatively low density of hydrophilic silanol groups (0.21), smaller average mesopore size (3.3 nm) and longer hydrophobic alkyl chain of the n-C10H21O moiety were all thought to contribute to improving the hydrophobicity of this hybrid. These results indicate that n-C10H21OH is a useful alcohol modifier for synthesizing amorphous silica-based inorganic-organic hybrid materials with improved hydrophobic properties through a polymer precursor route