The three-dimensional structure of flexible resorcinol-formaldehyde aerogels investigated by means of holotomography

Organic aerogels based on resorcinol-formaldehyde gels display remarkable properties due to their pronounced nanoporosity. Therefore, studies towards the understanding of their structure-property-relationship are of high value for the design of improved materials. X-ray tomography is a technique that has been used for the structural elucidation of porous materials, but so far no highly resolved three-dimensional structures of resorcinol-formaldehyde gels have been obtained under the classical absorption-based experimental X-ray setup. This paper reports on the successful analysis of a superflexible resorcinol-formaldehyde aerogel using zoom holotomography that yielded images with an unprecedented resolution in the sub-micrometer range. The preparation of suitable powder from monolithic superflexible resorcinol-formaldehyde, the experimental conditions for tomography, and data-processing to obtain a 3D-image of the dried gel sample are described. Macropores above ca. 75 nm could be identified and visualized. They were shown to adopt almost spherical shape and to display a low connectivity. A quantitative analysis of the pore space revealed that most of the identified pores are small macropores (diameter < 0.5 mu m), yet most pore volume is located in larger macropores of 1-4 mu m diameter.