A fractal description of pore structure in block-copolymer templated mesoporous silicates

Traditional nitrogen physisorption isotherm analysis of triblock copolymer mesoporous silicates interprets the pore structure as a bimodal distribution of mesopores and smaller micropores. Samples of SBA-15, SBA-16 and SBA-11 were prepared and calcined between 300 and 900 °C. Adsorption analysis using a fractal Frenkel–Halsey–Hill approach reveals an alternative interpretation, that the micropores are a manifestation of the fractal silica – polyethylene oxide polymer interface. We show that calcination at higher temperatures is effectively an annealing process that smoothes the mesopore surface, such that by 900 °C the surface dimension is ∼2. We also use the well-characterized pore structure of SBA-15 and related silicates to examine and discuss several different methods for extracting the fractal dimension from isotherm data.