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Morphological Properties of Methacrylate-Based Polymer Monoliths: From Gel Porosity to Macroscopic Inhomogeneities Tibor Müllner, Armin Zankel, Alexandra Hoeltzel, Frantisek Svec, and Ulrich Tallarek Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00337 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 19, 2017
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Morphological Properties of Methacrylate-Based Polymer Monoliths: From Gel Porosity to Macroscopic Inhomogeneities Tibor Müllner,† Armin Zankel,‡ Alexandra Höltzel,† Frantisek Svec,§ and Ulrich Tallarek†,* †
Department of Chemistry, Philipps-Universität Marburg, Hans-Meerwein-Strasse 4, 35032 Marburg, Germany
‡
Institute of Electron Microscopy and Nanoanalysis, Graz University of Technology, and Graz Centre for Electron Microscopy, Steyrergasse 17, 8010 Graz, Austria §
Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, 100029 Beijing, China
* Corresponding author. E-mail:
[email protected] KEYWORDS: Macropore space; Heterogeneity length scales; Three-dimensional reconstruction; Chord length distribution; Polymerization; Silica monolith template.
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ABSTRACT
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, for example, are widely used in flow-driven processes, whose efficiency depends on the morphology of the support material over several length scales. Compared with alternative support structures, particularly silica monoliths, polymer monoliths yield lower efficiency, which suggests a sub-optimal morphology. Based on physical reconstruction by serial block-face scanning electron microscopy we evaluate the structural features of a methacrylatebased polymer monolith from the pore scale to the column scale. The morphological data reveal a homogenous polymer skeleton with a solute-impenetrable core‒porous shell architecture and a heterogeneous macropore space that suffers from inhomogeneities at the short-range and the transcolumn scale. Although the morphology of the polymer phase is favorable to efficient mass transport, the performance of the polymer monolith is limited by severe transcolumn gradients in macroporosity and macropore size. We propose to overcome these morphological limitations by pursuing a preparation strategy that involves active rather than passive shaping of the macropore space, for example, by using silica monoliths as templating structures for polymer monolith preparation.
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INTRODUCTION Materials with a hierarchically structured pore space are employed as support structures in processes that require a large surface area for physical or chemical interactions combined with efficient molecular transport to and from the active sites,1 which applies to mass storage,2–4 energy applications,5–7 catalysis,8–10 and chemical separations.11,12 A network of larger macropores (with µm to mm pore size) allows percolation of a mobile phase (gas, liquid, or supercritical fluid) under an external force (gradients in electrical potential or pressure) applied to the material, enabling advective transport of the bulk fluid and solute molecules. A second network of smaller pores (meso- and/or micropores), accessible to solvent and solute molecules only by diffusion, provides a large surface area covered with functionalities tailored to a certain application. Thus, the design of improved morphologies exposing functional interfaces is a major challenge in the advanced engineering of processes relying on selective adsorption or reaction, and the preparation of materials with a hierarchically structured pore space in particular is fundamental to the operation of flow-driven processes for separation, storage, and catalysis.1,13 Macroporous‒mesoporous materials inside a containing geometry come in two different architectures: packed beds of discrete mesoporous particles, where the larger, flow-through pores are formed by the space between the macrosized particles, or continuous beds (monoliths) with interstitial macropores and a mesoporous skeleton. The most important flow characteristics of fixed beds, hydraulic permeability and the residence time distribution of solute molecules, are determined by the bed morphology, specifically the bed porosity (void volume fraction) and size of the flow-through pores. From the local pore scale up to the bulk material scale, monoliths offer more design variability than conventional particulate beds, allowing a unique combination of selectivity, mobile phase velocity, heat and mass transfer, and specific surface area.10,11,14–16
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For example, mechanically stable particulate packings can only be obtained at bed porosities between (roughly) 0.36 and 0.46,17 so that the size of their interparticle flow-through pores is inherently linked to the particle size. The interstitial porosity of monoliths (their macroporosity) typically ranges from 0.5 to 0.75 (with smaller and larger values possible),18 and the macropore size can be varied independently from the skeleton thickness.10,11 This allows a fine-tuning of the external surface area, which is especially important for applications involving large biomolecules, where the external surface area determines the adsorbent capacity.18 The solid phase of particulate or monolithic fixed beds can be a hard or soft matter material. The first category comprises mainly metal and nonmetal oxides, most prominently silica,11,19 the second category organic polymers.20 Porous silica is mechanically stable at high pressures and can be synthesized into particles with diameters ranging from submicron to sub-mm size as well as into monolithic structures. On the downside, silica-based materials tolerate a small pH-range, are incompatible with a number of solvents, and their surface modification is limited, difficult, and always leaves residual surface silanol groups that can interact with solute molecules in an undesirable fashion. Polymer-based materials have reduced mechanical stability, but are more resilient against aggressive chemicals and leave a chemically inert backbone after modification. Their most important asset, however, is the wide range of possible and straightforward surface modifications.20–22 The excellent hydraulic permeability of silica monoliths compared with packed beds of silica particles enables short analysis times, serving the demands of high-throughput screening applications.11,16 Furthermore, a thin, mesoporous skeleton (
