Polyurethanes via Microemulsion Polymerization

Microemulsions are three or more component solutions comprising two immiscible liquids, typically water and oil. Miscibility is induced by a third amphiphilic component or surfactant. Since their discovery by Hoar and Schulman over 60 years ago,1 much theoretical and experimental development has transformed microemulsion science and technology into a mature field.2-4 Microemulsions are usually expressed as oil in water solutions wherein nanoscopic oil droplets (2-6 nm in diameter) are dispersed in water with the aid of a low radius of curvature surfactant. Reverse or inverse microemulsions have nanoscopic water droplets dispersed in a pseudo-continuous phase of oil, and the low radius of curvature in this case has opposite sign to the radius of curvature in the oil in water example. Since the droplets or domain lengths in such systems are nanoscopic, light is only very weakly scattered by the droplets, and such microemulsions appear to be transparent. A particularly interesting property of microemulsions is that when the oil and water and surfactant are suitably balanced, droplet “microstructure” can be replaced in part by a random bicontinuous structure comprising interdigitated water and oil domains, so that such a solution is effectively continuous in both water and oil.5-7 The average curvature in such systems is typically close to zero and comprises essentially equal amounts of positive and negative components. Such bicontinuous structures are of keen interest because they afford pathways of diffusive transport. Furthermore, on occasion the water component is replaced with some other immiscible liquid, typically a polar solvent of some kind, and in this report we utilize propylene glycol as such a “water replacement”. Microemulsion polymerization has heretofore been confined to examples wherein the oil or water domains, or both domains, comprise radical chain polymerization of monomers such as acrylates. A rich literature8-13 has been developed around such radical chain polymerization in microemulsions, and microlatexes of various monomers have been produced by incorporating oil monomers in oil in water microemulsions or by incorporating hydrogel monomers in water in oil microemulsions. In addition, nanoporous membranes have been produced by polymerizing bicontinuous microemulsions of radical chain polymerizing monomers.14,15 This communication reports the first application of microemulsion polymerization to produce polyurethanes. Polyurethanes form by addition step polymerization of diols with diisocyanates (radical chain polymerization is not involved). When the mole ratio of monomers is 1.00 ( 0.01, urethane polymers having more than 100 repeating pairs of monomers may be obtained.16 High molecular weights are usually obtained by incorporating cross-linking agents. We apply microemulsion polymerization here to immiscible monomers: propylene glycol (PG) and isophorone diisocyanate (IPDI). We use bis(2-ethylhexyl) sulfosuccinate, sodium salt (AOT), as the surfactant. AOT is widely appreciated as an excellent surfactant for producing reverse microemulsions of water in oils and monomers and for producing nonaqueous microemulsions with such polar liquids as PG, ethylene glycol, etc.17-19 The production of polyurethanes from immiscible monomers has limited applications, since stirring and emulsification are physically required until the reaction generates sufficient heat to carry itself forward. Nonstirred applications (such as injection molding) are unsuitable if the polymerization half-life is longer than the lifetime of the emulsion. Since microemulsions are thermodynamically stable solutions of the constituent components, mixing is driven by chemical free energy, and shear or stirring is not required to maintain a single-phase solution of the otherwise immiscible components. A starting point for appreciating microemulsions is to construct a phase diagram to distinguish single-phase microemulsion domains from liquid crystalline domains and multiphase domains. In Figure 1, we present a ternary phase diagram for the PG/IPDI/AOT system at room temperature (22 ( 1 °C). Three domains are identified. Domain 3, above 50% (w/w) in AOT, was not investigated, as we have not explored the structure or properties of this domain. The cap of this domain, AOT, is a waxy, amorphous solid (this is the natural physical state; AOT cannot be crystallized since it contains three chiral centers). Another prominent domain is the emulsion domain 2, which is a multiphase domain that comprises two microemulsion phases: an IPDI in PG * Corresponding author. E-mail jtexter@emich.edu. Figure 1. Room temperature ternary phase diagram illustrating large single-phase microemulsion region 1. The region 3 above 50% (w/w) AOT was not explored; the multiphase domain 2 comprises PG in IPDI and IPDI in PG microemulsions emulsified in each other. The “X” and the “+” represent loci of polymerization. 5841 Macromolecules 2004, 37, 5841-5843