Effect of drying rate on mesoporous silica morphology templated from PEO-PPO-PEO block copolymer assemblies

Abstract

Since the first synthesis of mesoporous MCM-41 by Mobil Corporation in 1992 [1], the use of surfactant as template or structure directing agent has opened new opportunities in the synthesis of porous materials. Nonionic triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) copolymers have been successfully used as surfactant templates for the synthesis of a series of periodic mesoporous silicas including hexagonal, lamellar, and cubic structures [2, 3]. Considerable attention has been paid to controlling the pore size and enhancing the porosity by varying such synthetic parameters as the aging time and temperature. Obviously, these heat treatments during the preparation are important in determining the final mesoporous structure of silicas. The finally solidified silica structure is considered to be determined by a competitive process of two simultaneously occurring solidifications of the structure directing agent and silica, whose structural arrest is due to desiccation and condensation polymerization, respectively. This means that the finally replicated mesoporous structure of silica does not necessarily agree with the thermodynamically preferred phase structure of the structure directing agent. Therefore, exploring the possibility of giving a variety in the final silica structure by heat treatments can be an intriguing research subject from the viewpoint of materials nanostructure manipulation. Until now, most of the mesoporous silicas were prepared by templating of surfactant and block copolymer liquid crystalline mesophases which usually give rise to an open pore structure. However, little attention has been paid on the closed pore structure, although in some cases, closed pores are more desirable than open pores due to the mechanical strength or resistance to humidity. Therefore, clarifying important factors for the closed pores is an interesting research subject which is expected to be valuable for preparing low dielectric films with high closed porosity. The authors have shown that the rapid drying process of the nanocomplex of silicate and pluronics is favorable for the formation of closed nanopores, where the enclosure of the pores is promoted due to the solidification of silica prior to the formation of the liquid crystalline phase of pluronics molecular assemblies [4]. Here, we report the effect of drying rate on the morphology of mesoporous silicas templated from the pluronic triblock copolymer aggregates, and the formation of closed pores in the mesoscopic structure of the silica. Mesoporous silicas were synthesized using triblock copolymer Pluronic P123 (Mav = 5800), EO20PO70 EO20, as surfactant template. Agents with the molar ratio of 0.04 (TEOS), 1 (water), 0.022 (Ethanol), and 0.001 (HCl) were mixed in a glass beaker and stirred at room temperature until a transparent solution was obtained. Then P123 was added and dissolved in this homogenous solution. The weight fraction of P123 to water was 10%. Samples were prepared by three different drying methods to carry out the preparation of the samples at three rates of drying. Sample A was obtained by keeping the beaker in a drying oven at 90 ◦C for a day with a cap for slower drying. Sample B was obtained by transferring the precursor solution into a glass dish and keeping it in a drying oven at 90 ◦C for a day without a cap on it. For drastically more rapid drying, sample C was obtained by spraying the resulting solution onto a TEFON-coated plate, thermostatted at 90 ◦C followed by complete desiccation in a drying oven for several hours. Finally, all the dried samples were calcined at 600 ◦C for 5 h in air to remove the pluronic P123 template. The closed porosity (Pc) of the mesoporous silica was evaluated from the apparent mass density of the sample measured by helium pycnometer where the bulk density of silica matrix was assumed to be 2230 kg/m3. The open porosity (Po) was obtained from the adsorpted amount of nitrogen at the relative pressure of 0.9814 at 77 K. Fig. 1 shows the closed porosity and the ratio of the closed porosity to the open