Interfacial polymerization
[1][2][3] There are several variations of interfacial polymerization, which result in several types of polymer topologies, such as ultra-thin films,[4][5] nanocapsules,[6] and nanofibers,[7] to name just a few.
[1][2] 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.
[3] This first interfacial polymerization was accomplished using the Schotten–Baumann reaction,[3] a method to synthesize amides from amines and acid chlorides.
[3] 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.
[2][8] 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.
[2][3] 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.
[3] 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.
[2] Free-standing films, by contrast, do not use a support, and are often used to synthesize unique topologies such as micro- or nanocapsules.
[10] Interfacial polymerization has proven difficult to model accurately due to its nature as a nonequilibrium process.
[9][11] The wide range of variables involved in interfacial polymerization has led to several different approaches and several different models.
[1][7][9][11] One of the more general models of interfacial polymerization, summarized by Berezkin and co-workers, involves treating interfacial polymerization as a heterogenous mass transfer combined with a second-order chemical reaction.
[9] The local model is used to determine the characteristics of polymerization at a section around the interface, termed the diffusion boundary layer.
[9] This model can be used to describe a system in which the monomer distribution and concentration are inhomogeneous, and is restricted to a small volume.
in which ci is the molar concentration of functional groups in the ith component of a monomer or polymer, t is the elapsed time, y is a coordinate normal to the surface/interface, Di is the molecular diffusion coefficient of the functional groups of interest, and Ji is the thermodynamic rate of reaction.
[9] More specific approaches to modeling interfacial polymerization are described by Ji and co-workers, and include modeling of thin-film composite (TFC) membranes,[11] tubular fibers, hollow membranes,[7] and capsules.
[1][12] These models take into account both reaction- and diffusion-controlled interfacial polymerization under non-steady-state conditions.
Where A0, B0, C0, D0, E0, I1, I2, I3, and I4 are constants determined by the system and Rmin is the minimum value of the inside diameter of the polymeric capsule wall.
[12] There are several assumptions made by these and similar models, including but not limited to uniformity of monomer concentration, temperature, and film density, and second-order reaction kinetics.
[7][11] Interfacial polymerization has found much use in industrial applications, especially as a route to synthesize conducting polymers for electronics.
[1][2] Conductive polymers synthesized by interfacial polymerization such as polyaniline (PANI), Polypyrrole (PPy), poly(3,4-ethylenedioxythiophene), and polythiophene (PTh) have found applications as chemical sensors,[13] fuel cells,[14] supercapacitors, and nanoswitches.
[1][2] These nanofibers have been shown to detect various gaseous chemicals, such as hydrogen chloride (HCl), ammonia (NH3), Hydrazine (N2H4), chloroform (CHCl3), and methanol (CH3OH).
[1] PANI nanofibers can be further fined-tuned by doping and modifying the polymer chain conformation, among other methods, to increase selectivity to certain gases.
[1][2][13] A typical PANI chemical sensor consists of a substrate, an electrode, and a selective polymer layer.
[13] PANI nanofibers, like other chemiresistors, detect by a change in electrical resistance/conductivity in response to the chemical environment.
[13] PPy-coated ordered mesoporouscarbon (OMC) composites can be used in direct methanol fuel cell applications.
[1][2][4] One added benefits of using polymers prepared by interfacial polymerization is that several properties, such as pore size and interconnectivity, can be fined-tuned to create a more ideal product for specific applications.
[1][2] Once synthesized, the capsules can enclose drugs,[6] quantum dots,[1] and other nanoparticles, to list a few examples.
Further fine-tuning of the chemical and topological properties of these polymer capsules could prove an effective route to create drug-delivery systems.
