Characterization of Ordered Mesoporous Silica Films Using Small-Angle Neutron Scattering and X-ray Porosimetry

Abstract

Ordered mesoporous silica films were synthesized using pre-organized block copolymer templates in supercritical carbon dioxide. Poly(ethylene oxide-block-propylene oxide-block-ethylene oxide), PEO-b-PPO-b-PEO, films doped with p-toluenesulfonic acid (p-TSA) were infused with carbon dioxide solutions of silica precursors. Silica condensation proceeded selectively in the hydrophilic PEO domain due to selective partitioning of the acid catalyst in the template during its preparation. Removal of the organic template by calcination at 400 °C yielded robust mesoporous films. The films were characterized using X-ray diffraction (XRD), spectroscopic ellipsometry (SE), X-ray reflectivity (XR), Rutherford backscattering (RBS), forward recoil elastic spectroscopy (FRES), transmission electron microscopy (TEM), and small-angle neutron scattering (SANS). The pore structure and distribution was probed using contrast variation SANS porosimetry and X-ray porosimetry. Two distinct populations of pores are present in the films. Spherical mesopores were templated by the hydrophobic domain of the copolymer. Micropores in the pore walls are present due to removal of the hydrophilic PEO block, which forms an interpenetrating network (IPN)-like structure with the silicate network during infusion and condensation of the precursor. The wall density was determined to be (1.95 ± 0.05) g cm-3 and the overall porosity was found to be 39 vol %, with good agreement between the SANS and X-ray reflectivity results. A significant fraction of the total porosity, 44% to 53%, in these materials is attributed to the micropores in the wall structure.