Structural changes in resorcinol formaldehyde aerogel seen by NMR.

Carbon aerogels prepared from resorcinol formaldehyde organic aerogels have a wide range of use due to their considerably large specific surface area. Since the applications mostly happen in wet form, e.g. in aqueous medium, NMR cryoporometry was employed to follow the porous behavior of an organic aerogel and its carbon derivative, as well as the textural changes after the pyrolysis. Water and cyclohexane were used as hydrophilic and hydrophobic probe molecules, respectively. In the polymer aerogel, saturated with water, by NMR we found spherical mesopores confined by the aerogel beads and wide channels in the macropore size-range separating the aggregated beads. After carbonization cylindrical pores were observed between the beads and the aggregates got closer to each other. On the other hand, the hydrophobic cyclohexane probed exclusively the macropores, which might be the result of local swelling. The micropore region both in the polymer and the carbonized form was explored only by the low temperature gas adsorption measurements. The comparison of the two methods confirmed that these techniques excellently complement each other in characterizing the micro-, meso- and macropores of solid porous materials: vapor adsorption is superior in characterizing the micro- and mesoporosity, while NMR cryoporometry provides information about the pore geometry and size distribution in the meso- and macropores. [Display omitted] • Characterization of pore structures under the conditions of applications. • Comparison of NMR and gas sorption methods on mesoporous materials. • Changes of the morphology by transition from polymer to carbon aerogel. • Wetting properties of polymer and carbon aerogels. [ABSTRACT FROM AUTHOR]