Emulsion-Derived Foams: Preparation, Properties, and Application
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additional internal phase. For a collection of monodispersed spheres this packing limit corresponds to 74% by volume and is the accepted definition of a high internal phase emulsion, i.e., an emulsion with an internal phase greater than 74%.4 The structure of the emulsion is now analogous to soap bubbles, with thin films surrounding and separating the drops. Note also that, compared to "standard" emulsion polymerization, this is the exact reverse or "inverse" of that process. In the standard route, the dispersed oil phase is polymerized to yield a collection of polymeric spheres; with inverse emulsion, the continuous phase is polymerized, yielding a rigid matrix. If a polymerizable monomer, for example styrene, is incorporated as the continuous oil phase in a water/oil (WO) high internal phase emulsion, then polymerization of the monomer yields a continuous polymeric matrix structure containing compartmentalized droplets of water. During the polymerization step, holes are formed in the thin films separating the droplets, and an open structure is formed. The water is easily removed to produce a foam of the corresponding structure. Con-
ventional emulsion technology allows the average droplet size and distribution to be controlled at the mixing stage of the process. Droplet or cell sizes in the range of 1-100 microns can be readily generated. Coherent, contiguous structures can be formed from emulsions containing as little as 3% by volume of the continuous phase. The technique is applicable to a range of materials, both organic and inorganic, giving foams that are finding use in diverse applications, as described later in this article. Variations of emulsion-foam formulation are summarized in Table I. The essential differences relate to (1) the choice of starting monomer and (2) whether a WO or O/W emulsion is employed. Foams can be crosslinked via various routes, as described in the Table. Many systems exhibit excellent emulsion stability; others, such as the methacrylonitrile (MAN) derivatives, are less stable. In these cases, electrolytes, such as CaCl2 or NaCl, may be added to confer sufficient stability to allow satisfactory polymerization. After the polymerization step, the voids in the polymer foams are filled with the dispersed phase, or temporary pore former. The cells may also contain residual surfactant and/or electrolyte. Cleaning protocols need to be designed for the specific system under investigation. For example, in the polystyrene case mentioned earlier, the "wet" foam will contain water, low hydrophobic-lyophobic balance (HLB), surfactant, electrolyte, and excess initiator. A successful cleaning scheme, which leaves the foam surface hydrophobic, involves a wash with water to remove inorganics, followed by isopropanol to remove residual surfactant, and finally drying at temperatures up to 80°C. Because of the cellular dimensions of the foam microstructure, it is necessary to use extended wash/soak times (as in a Soxhlet extractor), or if the foam is monolithic, the solvents may be pumped through the
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