In situ SAXS investigation of structural changes in soft resorcinol–formaldehyde polymer gels during CO2-drying

Drying is one of the key steps in the preparation of resorcinol–formaldehyde (RF) gels. Supercritical drying has long been known to be a method by which the structure of RF aerogels has the greatest chance of being perfectly preserved. Although several investigations have been conducted into the effects of the applied drying parameters on the final gels, the structural evolution of the gels during the drying process is nevertheless poorly known. In this work we present an in situ small angle X-ray scattering study of (1) the changes that occur in the structure of RF networks during supercritical drying under various conditions, and (2) the kinetics of the drying process.During the pressurization stage the overall structure of the network remains unaffected by the rate of increase of the CO2 pressure. By contrast, depressurization with supercritical CO2 at rates in excess of 4 bar/min, or with liquid CO2 alone, causes bubbles to form and yields a shrunken state of the final network. This densification preferentially shrinks the larger pores, the smaller pores being much less affected. We find that liquid CO2 is also an efficient drying medium that can preserve the structure, but the depressurization stage must always start from the supercritical state. The incomparable advantage of supercritical CO2, however, is that its higher rate of molecular diffusion makes solvent exchange substantially faster than with liquid CO2.

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► In situ study of the drying process of RF gels with sc and liqCO2 is reported.
► The rate of depressurization is of major importance.
► Fast depressurization provokes bubble formation and isotropic shrinkage of the gels.
► LiqCO2 can be used, but the depressurization step must start from the supercritical state.
► Owing to its low diffusion rate, liqCO2 is not suitable in batch-type processes.