PEG-mediated silica pore formation monitored in situ by USAXS and SAXS:Systems with properties resembling diatomaceous silica

Abstract

Poly(ethylene glycol) (PEG) was employed as templating agent for the synthesis of porous silica. The effect of PEG chain length and of the PFG/silica ratio on textural properties (fractality, pore size, and pore distribution) were investigated by monitoring the development of silica-PEG intermediates using ultrasmall and small-angle X-ray scattering analysis with high-brilliance synchrotron radiation to obtain sufficient radiation intensity for dynamic results at the subminute scale. We show that even a simple structure-directing polymer, such as PEG, results in silicas with pores of diameters spanning a range of less than 2 nm up to 20 nm, depending on polymer molecule length and polymer/silica ratio. Flocculation may well be the most important distinction between silicas prepared with small and large PEG. In this view, small PEG600 gets encapsulated by silica and forms pools within the silica framework, whereas large PEG20,000 is entangled in a mass of silica spheres, making enclosure by silica or phase separation impossible. Both polymer chain length and the polymer/silica ratio govern the relative importance of flocculation, phase separation, and hydrophobic silica-PEG interactions steering the silica polymerization. Increase in hydrophobicity results in a larger surface area and a more uniform pore size distribution, art effect confirmed by scanning electron microscopy (SEM) and observations of physical adsorption of nitrogen gas (BET). Polymers, such as PEG, may well be an inexpensive and versatile substitute and model for polypeptides known as structure-directing agents in biomineralization if silicas resembling natural ones, notably the ones present in such a huge diversity in algae of the group of diatoms, are the focus of scientific attention, e.g., for biomimicking with a view on industrial applications