Langmuir 1988, 4, 656-662
656
Spatial Distribution of the Phases in Water-in-Oil Emulsions. Open and Closed Microcellular Foams from Cross-Linked Polystyrene? Joel M. Williams* and Debra A. Wrobleski Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 Received August 18, 1987. In Final Form: December 15, 1987 The composition of styrene/divinylbenzene/sorbitan monooleate/water emulsions has a dramatic effect on the openness of the cells of the foam prepared by polymerizing the monomers and subsequently removing the water. Monomer concentrations were varied from 0.025 to 0.20 g/cm3, while the surfactant level was varied from 1.2% to 320% of the monomer. Most remarkably, the variation in cell openness, and hence the continuity of the oil phase, was insensitive to the amount of oil. The variation was, however, strongly related to the surfactant-to-oil ratio. The structures of the resulting foams provide further evidence that emulsions possess considerable spatial ordering of the phases and, hence, the molecular species associated with them at the time of polymerization.
Introduction Unileverl has described t h e production of porous, homogeneous, cross-linked polymeric block material having a dry density of less than 0.1 g/cm3 by a process in which monomers (styrene or substituted styrene plus comonomers) are polymerized after they have formed an emulsion having a high fraction (at least 90%) of internal phase comprised of water. T h e rigid open microcellular foams made by this process have potential use in high-energy physics experimenk2 Because of this interest, Los Alamos National Laboratory and workers at Lawrence Livermore National Laboratory? the Atomic Weapons Establishment (England),4 a n d Case Western Reserve University5 are evaluating the influence of changes in emulsion parameters on the microstructure of the final foam. During our initial investigations of making foams by polymerizing water-inoil (w/o) emulsions containing styrene/divinylbenzene as the oil phase, i t appeared t h a t t h e structure of t h e foam obtained could give significant information about t h e structure of the emulsion that led t o it. I n a previous paper: this fact was vividly illustrated; the foams obtained demonstrated that considerable spatial ordering occurred in the oil phase at t h e micrometer level down to less than 1/100th of a micrometer (
