Structural design of mesoporous silica by micelle-packing control using blends of amphiphilic block copolymers

The formation of mesoporous silica materials has been studied using blends of diblock (C_nH_2n+1(OCH₂CH₂)_xOH, C_nEO_x, n = 12 - 18 and x = 2-100) and Pluronic triblock (EO_xPO₇₀EO_x, x = 5-100) amphiphilic block copolymers as the structure-directing agents and sodium silicate as the silica source. The mesostructure of the silica materials thus obtained, as determined by X-ray diffraction and transmission electron microscopy, changes as the volume of the hydrophilic EO group of the surfactant increases, from lamellar to two-dimensional hexagonal (p6mm), three-dimensional hexagonal, a cubic phase, and another cubic phase with Im3̄m symmetry. Particle morphologies of the mesoporous silica materials are correspondingly changed from sheetlike to irregular, facetted cubic shapes depending on the periodic mesoscale symmetry of the structure. It is reasonable that the structural transformations are due to the changes in hydrophobic surface curvatures, which can be controlled readily and precisely by using polymer blends with different hydrophilic headgroup sizes of the individual components. The hydrophilic-hydrophobic ratios between the EO headgroups and carbon tail groups were optimized in order to obtain highly ordered mesoporous silica materials. Mesoporous silica materials with the same structures but different unit cell sizes, which strongly suggests different pore sizes, can be readily obtained through the present synthetic approach.