Synthesis of Porous Materials via Multiscale Templating Approaches: Emulsions, Nanoparticles, Supercritical Fluids, and
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Synthesis of Porous Materials via Multiscale Templating Approaches: Emulsions, Nanoparticles, Supercritical Fluids, and Directional Freezing Haifei Zhang1, Irshad Hussain1,2, James Long1, Bien Tan1, Mathias Brust1, Matthew Rosseinsky1, Steven Rannard1,3, Michael Butler4, and Andrew Cooper1 1 Department of Chemistry, University of Liverpool, Crown Street, Liverpool, L69 3BX, United Kingdom 2 NIBGE, Faisalabad, Pakistan 3 Unilever Research, Port Sunlight, L63 3JW, United Kingdom 4 Unilever R&D Colworth, Sharnbrook, MK44 1LQ, United Kingdom
ABSTRACT A “toolkit” of emulsion templating and directional freezing methods has been developed which allows the preparation of a wide variety of organic, inorganic, and metallic materials in a macroporous or hierarchically porous form.1,18,20 The various processes use water, organic solvents, or aqueous/organic emulsions as the template phase. We have shown that the organic solvent can be replaced by liquid CO2 in both the emulsion templating and directional freezing approaches, thus reducing organic waste and offering advantages in applications such as the preparation of biomaterials. INTRODUCTION There is significant technological and scientific interest in the preparation of materials with macroporous channels and with a hierarchy of pores. In most cases, “templating” procedures are used. In order to produce macropores, templates with dimensions of 50 nm or larger are generally required. A number of options exist (e.g., colloidal polymer particles, emulsion droplets, biological structures) each of which has its advantages and disadvantages in terms of cost, scaleability, and the degree of control and uniformity which can be achieved. Colloidal crystals, for example, offer exceptional regularity but there are issues connected with scale up. Emulsions by contrast are in general much less uniform but may be considerably more scaleable. We focus in this paper on the use of emulsions and aligned ice crystals as templates. Emulsions are heterogeneous mixtures of at least one miscible liquid dispersed in another in the form of droplets. In most cases, at least one of the liquids will be water or an aqueous solution. An emulsion is often described as either oil-in-water (O/W) or water-in-oil (W/O) where the first phase mentioned refers to the internal (or dispersed) phase. Generally, emulsions have average droplet sizes of at least several micrometers and the droplets have a rather broad size distribution unless special procedures are adopted (e.g., fractionation of the emulsion). Emulsions have been investigated extensively for decades and are used in a range of common practical applications. Concentrated emulsions have also been employed quite widely in the synthesis of porous materials.1,2 In the context of polymer synthesis, emulsions can be used in three ways (Fig. 1) as templates for the preparation of colloids, porous materials, and composite materials, respectively.1 In order to produce colloids, polymerization takes place in the dispersed
monomer phase, althoug
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