Macroporous silica using a "sticky" Stober process
The generation of macroporous silica structures using a sol-gel process generally requires the presence of high molecular weight, water-soluble polymers. We demonstrate that significantly lower molecular weight poly(ethylene glycol) (PEG) polymers can drive a particle aggregation process to generate macroporous silica. Compared to unfunctionalized PEGs (HO-PEG-HO, molecular weight > 10 000 g mol-1), PEG polymers with allyl (A-PEG-A) or silyl end groups (CH2)3Si(OEt)3 (Si-PEG-Si), with molecular weights of 2000 g mol-1 or greater, lead to monolithic macroporous structures derived from aggregates of nearly monodisperse particles. Lower molecular weight (less than 1000 g mol-1) allyl or silyl PEG, or hydroxy-terminated PEG-OH, lead to mesoporous silicas resulting from the gelation of weakly, electrostatically stabilized, nearly monodisperse silica particles that can readily reorganize in the floc into a tightly packed, mesoporous structure. With longer chains or higher concentrations of A-PEG-A or Si-PEG-Si stabilizers, electrosterically stabilized silica particles grow to larger sizes prior to fusing, and on aggregation give open structures without structural rearrangement - structures do not change after initial 'sticky' contact between particles. Macroporosity is particularly obvious after thermolysis: structures with 80 m2 g-1 surface area increase to about 600 m2 g-1 after calcination removes the organic material. The influence of molecular weight, end groups, pH changes and catalyst addition on the macroporosity and structure of the materials is analyzed with SEM, TGA and porosimetry (nitrogen and mercury). PEG groups remain entrained within the silica, and exposed at interfaces prior to calcination. The use of reaction conditions that induce reactivity in the alkoxysilane-terminated PEGs (i.e., hydrolysis and condensation into the network with acid, base or KF catalysis) leads to an increase in the amount of PEG incorporated in the silica, but only under basic conditions. The use of A-PEG-A or Si-PEG-Si stabilizers permits the controlled formation of biocompatible macroporous monolithic silica structures at neutral pH, arising from the aggregation of electrosterically stabilized, monodisperse particles in a 'sticky' Stober process.