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Hierarchically porous monoliths: Organic polymers

Shaping chemical interfaces of hard and soft matter materials into physical morphologies that guarantee excellent transport properties is of central importance for technologies relying on adsorption, separation, and reaction at the interface. Polymer monoliths with a hierarchically structured pore space are widely used in flow-driven processes, whose efficiency depends on the morphology of the support material over several length scales (Figure 1). The strength of organic polymer monoliths is their versatility, i.e., the ease and flexibility of preparation as well as the wide variety of available surface chemistries and functionalization.

Compared with alternative support structures, particularly silica monoliths, polymer monoliths yield lower efficiency, which suggests a sub-optimal morphology. Whereas silica monoliths are highly efficient support structures due to their homogeneous macropore space and a thin mesoporous skeleton, the efficiency of organic-polymer monoliths appears to be limited by slow diffusion in the small pores of the polymer backbone and by a high degree of backmixing in the heterogeneous, constricted macropore space. It is our goal to identify the structural limitations of organic polymer monoliths regarding their efficiency in flow-driven applications (mass separations, heterogeneous catalysis) to provide a morphology-based foundation for directed synthetic efforts.

Based on their physical reconstruction, e.g., by serial block-face scanning electron microscopy, we systematically evaluate the structural features of polymer monoliths from the pore scale to the column scale. For that purpose, the reconstructions need to recover the entire column cross-section over a sufficient length (Figure 2, right panel) to enable the quantification of macroscopic inhomogeneities. Structural features of the void space and skeleton are subsequently evaluated using proven statistical methods for the morphological analysis of porous materials (Figure 2, left panel; Figure 3) and compared with data collected for alternative polymer and silica-based monolithic supports.

Figure: Ulrich Tallarek

Figure 1. Characteristic length scales of morphological heterogeneities, which have an impact on mass transport properties. Polymer monoliths with a hierarchically structured pore space are widely used in flow-driven processes, whose efficiency depends on the morphology of the support material over several length scales. Based on physical reconstruction we evaluate the structural features of polymer monoliths from the pore (transchannel) scale to the bed (transcolumn) scale.

Figure: Ulrich Tallarek

Figure 2. Chord length distribution (CLD) analysis for the void space and skeleton characteristic sizes as well as their heterogeneities. Chords are generated by randomly choosing points and projecting pairs of opposing vectors from these points equiangularly in several directions until they hit the solid-void interface. The distance spanned by a vector pair is a chord length. Chords for void space and skeleton can be stacked to give the respective CLDs, which are then fitted to a k-Gamma function, returning a descriptor for mean size (µ) and heterogeneity (k).

Figure: Ulrich Tallarek

Figure 3. Morphological parameters extracted from spatially resolved CLD analysis. Average size µ and size homogeneity k of void space (top half of the figure) and polymer skeleton (bottom half) are displayed as 1D axial or radial profiles in the eight outer frames and as full 2D plots by the inner color-coded maps. In contrast to the global CLD analysis (Figure 2) the spatially resolved maps detect systematic and interdependent morphological variations, which provide insigths into subtle details behind monolith preparation.

Highlighted publications:

  • T. Müllner, A. Zankel, A. Höltzel, F. Svec, U. Tallarek
    Morphological properties of methacrylate-based polymer monoliths: From gel porosity to macroscopic inhomogeneities.
    Langmuir     2017, 33, 2205–2214. DOI: 10.1021/acs.langmuir.7b00337
  • T. Müllner, A. Zankel, Y. Lv, F. Svec, A. Höltzel, U. Tallarek
    Assessing structural correlations and heterogeneity length scales in functional porous polymers from physical reconstructions.
    Advanced Materials     2015, 27, 6009–6013. DOI: 10.1002/adma.201502332
  • T. Müllner, A. Zankel, F. Svec, U. Tallarek
    Finite-size effects in the 3D reconstruction and morphological analysis of porous polymers.
    Materials Today     2014, 17, 404–411. DOI: 10.1016/j.mattod.2014.07.003