Working principle in serial block face scanning electron microscopy.
Working principle in serial block face scanning electron microscopy.

The morphology of porous materials is important to understanding a wide range of phenomena from the catalytic properties of nanoparticles to the behavior of soil. Now, an international team has demonstrated how image analysis based on SBF-SEM (serial block face-scanning electron microscopy) can be used to ascertain micrometer and submicrometer morphological features from porous polymeric materials. The findings suggest that SBF-SEM represents a promising method for such applications and to reveal how finite-size effects influence the determination of key structural parameters and mass transport behavior in the material. [Tallarek et al., Materials Today (2014) DOI: 10.1016/j.mattod.2014.07.003]

Chemists Ulrich Tallarek and Tibor Müllner of the Philipps-Universität Marburg, Germany, and colleagues Armin Zankel of the Institute for Electron Microscopy at Graz University of Technology, Austria and Frantisek Svec of The Molecular Foundry at the E.O. Lawrence Berkeley National Laboratory, USA, explain how hierarchical, porous polymeric scaffolds can be prepared with micro-, meso- and macro-porous domains within. The morphology can in some instance be tailored to boost efficiency in catalysis, improve fuel cell design, enhance battery and electrode development and also be exploited in gas separation and storage. Thus tailoring morphology can be useful in a wide range of materials applications.

Of course, tailor-made materials require sophisticated characterization techniques and these are sadly lacking. SEM and transmission electron microscopy (TEM) are both reliable analytical methods but are limited to two dimensions, which does not at first glance bode well for characterizing 3D porous solids. Of course, FIB (focused ion beam) as an extension of SEM has proved useful, the team says, but this is limited to characterizing small sample volumes if time is also limited.

The team has now shown how a technique developed a decade ago, SBF-SEM, might be used in this context, and allow the study of much greater cross sections than FIB-SEM provided the sample can be sliced thinly with a diamond knife. As such, the team give a proof of principle with a porous polymeric sample of hypercrosslinked poly(styrene-divinylbenzene) confined to capillary of fused silica with an internal diameter of 100 micrometers. SBF-SEM can then be used to scan the whole area of the capillary's internal cross section after slicing and an image reconstructed.

The team concludes that their approach could enable materials scientists to identify and improve morphological features that underlie the performance of a given material and so optimize the properties required of that materials. They add that the same approach to reconstruction of the polymer morphology can also allow realistic 3D models to be built for the simulation of flow, mass transport, sorption, and reactions of such materials with a wide range of applications in research to establish quantitative morphology-transport relationships. "The derived morphology-transport relationships build the basis for systematic further optimization of the material in a particular application," Tallarek told Materials Today.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".

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