Mesoporous Silica Formed with Triblock Copolymers

Abstract: The formation of highly ordered mesoporous silica materials, using PEO-PPO-PEO amphiphilic triblock copolymers (so-called Pluronics) as structure directing agents, has been studied. The materials are formed through the addition of a silica source to a polymer solution of low concentration (2.5 wt ) under acidic conditions. The products have BET surface areas of approximately 800 m2/g. The unit cell parameter is around 100  for hexagonally structured materials, and in the range of 150-250  for cubic materials. Small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) have been the main characterisation techniques. Pluronics with different lengths of the PEO- and PPO-blocks were used in the syntheses. It was shown that different block lengths influence the structure formed, as well as the pore size and wall thickness of the materials. Materials with hexagonal (p6mm), micellar cubic (Im3m) and lamellar structures were obtained using Pluronics with different PEO/PPO compositions. Temperature has also shown to be an important synthesis parameter, as the conformation of the polymers, as well as the silica polymerisation, is highly temperature dependent. Further, the effect of the addition of salt to the synthesis mixture has been studied. Through the addition of sodium iodide, a new large-pore bicontinuous cubic silica (Ia3d) was synthesised. The mechanism of formation for the materials has been studied in detail. In situ synchrotron SAXS, in situ time-resolved 1H NMR and time-resolved TEM have been used to follow the time evolution of the syntheses. The different structures are formed via different mechanisms, illustrating the importance of understanding the mechanism in order to influence and control the synthesis to obtain the desired product.