Control of pore size and condensation rate of cubic mesoporous silica thin films using a swelling agent

Large-pore mesoporous silica thin films have been synthesized by using poly(alkylene oxide) triblock copolymers (Pluronic F127) with poly(propylene oxide) (PPO) as a swelling agent. Various synthesis parameters such as the addition of PPO, the condensation catalyst (NH3) and the aging time have been systematically investigated in order to explore their influence on the formation of mesostructure as well as thickness and refractive indices of the mesoporous thin films. Mesoporous silica thin films with a uniform pore size as well as a large diameter of 12.5 nm were obtained by optimization of these parameters, resulting in a pseudocubic phase with a cage-like pore structure. However, the broad distribution of pore entrance size with a large diameter is observed at high concentrations of PPO. Porosity (vol.%) values calculated by the refractive indices of mesoporous silica films were well correlated with their pore volumes obtained by the BET analysis. Reflectometry using UV-visible light has been newly applied to investigate the cumulative effect of synthesis parameters on the porosity, thickness and refractive index of the formed mesoporous film. It was confirmed by reflectometry measurement that the resulting pseudocubic mesoporous silica film with a thickness of 392 nm exhibits a refractive index of 1.21 and a total porosity of 49 vol.%.