Positron annihilation and N2 adsorption for nanopore determination in silica-polymer composites

Abstract

Silica has two different, fragmented and smooth, visual appearances when deposited onto porous Amberlite polymer substrate from TEOS at acidic and basic pH, respectively. The low temperature N₂ adsorption isotherms on both materials seem to be a combination of Type 1 and Type 4 isotherms with H2 hysteresis, indicating the presence of both micro (D < 2 nm diameter) and meso (2 nm < D < 50 nm) pores. Their mesopore distributions, computed by both BJH and DFT methods, show maxima near D∼5 nm with a narrower pore size distribution range than the organic support alone. For comparison, pores were also tested by positron annihilation lifetime spectroscopy (PALS) which can scan the full micro to mezo size range in one measurement without cooling and using any adsorbate molecules. These PALS results indicated the presence of D∼0.5, 0.8 and 1.5 nm diameter micropores and an average D∼5 nm diameter mesopores in both the base and the acid set materials. When the organic substrate is burned out from the two differently made composites, the new adsorption isotherms, BET surface areas, and differently measured pore sizes became distinctly different both from each other and from those of the parent materials.