Interfacial polymerization is a type of step-growth polymerization in which polymerization occurs at the interface between two immiscible phases (generally two liquids), resulting in a polymer that is constrained to the interface. There are several variations of interfacial polymerization, which result in several types of polymer topologies, such as ultra-thin films, nanocapsules, and nanofibers, to name just a few.
History Interfacial polymerization (then termed "interfacial polycondensation") was first discovered by Emerson L. Wittbecker and Paul W. Morgan in 1959 as an alternative to the typically high-temperature and low-pressure melt polymerization technique. As opposed to melt polymerization, interfacial polymerization reactions can be accomplished using standard laboratory equipment and under atmospheric conditions. This first interfacial polymerization was accomplished using the Schotten–Baumann reaction, a method to synthesize amides from amines and acid chlorides. In this case, a polyamide, usually synthesized via melt polymerization, was synthesized from diamine and diacid chloride monomers. The diacid chloride monomers were placed in an organic solvent (benzene) and the diamene monomers in a water phase, such that when the monomers reached the interface they would polymerize. Since 1959, interfacial polymerization has been extensively researched and used to prepare not only polyamides but also polyanilines, polyimides, polyurethanes, polyureas, polypyrroles, polyesters, polysulfonamides, polyphenyl esters and polycarbonates. In recent years, polymers synthesized by interfacial polymerization have been used in applications where a particular topological or physical property is desired, such as conducting polymers for electronics, water purification membranes, and cargo-loading microcapsules.
Mechanism
The most commonly used interfacial polymerization methods fall into 3 broad types of interfaces: liquid-solid interfaces, liquid-liquid interfaces, and liquid-in-liquid emulsion interfaces. In the liquid-liquid and liquid-in-liquid emulsion interfaces, either one or both liquid phases may contain monomers. There are also other interface categories, rarely used, including liquid-gas, solid-gas, and solid-solid. In a liquid-solid interface, polymerization begins at the interface, and results in a polymer attached to the surface of the solid phase. In a liquid-liquid interface with monomer dissolved in one phase, polymerization occurs on only one side of the interface, whereas in liquid-liquid interfaces with monomer dissolved in both phases, polymerization occurs on both sides. An interfacial polymerization reaction may proceed either stirred or unstirred. In a stirred reaction, the two phases are combined using vigorous agitation, resulting in a higher interfacial surface area and a higher polymer yield. In the case of capsule synthesis, the size of the capsule is directly determined by the stirring rate of the emulsion. Although interfacial polymerization appears to be a relatively straightforward process, there are several experimental variables that can be modified in order to design specific polymers or modify polymer characteristics. Some of the more notable variables include the identity of the organic solvent, monomer concentration, reactivity, solubility, the stability of the interface, and the number of functional groups present on the monomers. The identity of the organic solvent is of utmost importance, as it affects several other factors such as monomer diffusion, reaction rate, and polymer solubility and permeability. The number of functional groups present on the monomer is also important, as it affects the polymer topology: a di-substituted monomer will form linear chains whereas a tri- or tetra-substituted monomer forms branched polymers. Most interfacial polymerizations are synthesized on a porous support in order to provide additional mechanical strength, allowing delicate nano films to be used in industrial applications. In this case, a good support would consist of pores ranging from 1 to 100 nm. Free-standing films, by contrast, do not use a support, and are often used to synthesize unique topologies such as micro- or nanocapsules. In the case of polyurethanes and polyamides especially, the film can be pulled continuously from the interface in an unstirred reaction, forming "ropes" of polymeric film. As the polymer precipitates, it can be withdrawn continuously. It is interesting to note that the molecular weight distribution of polymers synthesized by interfacial polymerization is broader than the Flory–Schulz distribution due to the high concentration of monomers near the interfacial site. Because the two solutions used in this reaction are immiscible and the rate of reaction is high, this reaction mechanism tends to produce a small number of long polymer chains of high molecular weight.
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