Last updated A typically experimental setup for interfacial polymerization. One phase is above the interface, and the other phase is below. Polymerization occurs where the two phases meet, at the interface.
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.[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] As opposed to melt polymerization, interfacial polymerization reactions can be accomplished using standard laboratory equipment and under atmospheric conditions.[3]
This first interfacial polymerization was accomplished using the Schotten–Baumann reaction,[3] 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.[1][3] 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.[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.[1][2]
Mechanism
Five common types of interfacial polymerization interfaces (from left to right): liquid-solid, liquid-liquid, and liquid-in-liquid emulsion. There are two examples each for liquid-liquid and liquid-in-liquid emulsion, either using one monomer or two.
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.[1] In the liquid-liquid and liquid-in-liquid emulsion interfaces, either one or both liquid phases may contain monomers.[1][3] There are also other interface categories, rarely used, including liquid-gas, solid-gas, and solid-solid.[1]
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.[2] 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.[2][3] In the case of capsule synthesis, the size of the capsule is directly determined by the stirring rate of the emulsion.[2]
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.[2][3] 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.[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] 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.[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] In this case, a good support would consist of pores ranging from 1 to 100nm.[2] Free-standing films, by contrast, do not use a support, and are often used to synthesize unique topologies such as micro- or nanocapsules.[2] 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.[3][8] 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.[9] 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.[10]
Mathematical Models
Interfacial polymerization has proven difficult to model accurately due to its nature as a nonequilibrium process.[7][9][11] These models provide either analytical or numerical solutions.[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] In order to take into account different variables, this interfacial polymerization model is divided into three scales, yielding three different models: the kinetic model, the local model, and the macrokinetic model.[9]
The kinetic model is based on the principles of kinetics, assumes uniform chemical distribution, and describes the system at a molecular level.[9] This model takes into account thermodynamic qualities such as mechanisms, activation energies, rate constants, and equilibrium constants.[9] The kinetic model is typically incorporated into either the local or the macrokinetic model in order to provide greater accuracy.[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.[9] Parameters determined using the local model include the mass transfer weight, the degree of polymerization, topology near the interface, and the molecular weight distribution of the polymer.[9] Using local modeling, the dependence of monomer mass transfer characteristics and polymer characteristics as a function of kinetic, diffusion, and concentration factors can be analyzed.[9] One approach to calculating a local model can be represented by the following differential equation:
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] Although precise, no analytical solution exists for this differential equation, and as such solutions must be found using approximate or numerical techniques.[9]
In the macrokinetic model, the progression of an entire system is predicted. One important assumption of the macrokinetic model is that each mass transfer process is independent, and can therefore be described by a local model.[9] The macrokinetic model may be the most important, as it can provide feedback on the efficiency of the reaction process, important in both laboratory and industrial applications.[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.[7][11] One model is for thin film composite (TFC) membranes, and describes the thickness of the composite film as a function of time:
Where A0, B0, C0, D0, and E0 are constants determined by the system, X is the film thickness, and Xmax is the maximum value of film thickness, which can be determined experimentally.[11]
Another model for interfacial polymerization of capsules, or encapsulation, is also described:
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]
Applications
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]
Sensors
PANI nanofibers are the most commonly used for sensing applications.[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]
Fuel Cells
PPy-coated ordered mesoporous carbon (OMC) composites can be used in direct methanol fuel cell applications.[1][14] The polymerization of PPy onto the OMC reduces interfacial electrical resistances without altering the open mesopore structure, making PPy-coated OMC composites a more ideal material for fuel cells than plain OMCs.[14]
Separation/Purification Membranes
Composite polymer films synthesized via a liquid-solid interface are the most commonly used to synthesize membranes for reverse osmosis and other 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][4][5] For example, synthesizing a polymer with a pore size somewhere between the molecular size of hydrogen gas (H 2) and carbon dioxide (CO2) results in a membrane selectively-permeable to H 2, but not to CO2, effectively separating the compounds.[1][5]
Cargo-loading Micro- and Nanocapsules
Compared to previous methods of capsule synthesis, interfacial polymerization is an easily modified synthesis that results in capsules with a wide range of properties and functionalities.[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.[1][6]
In polymer chemistry, living polymerization is a form of chain growth polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is also much larger than the rate of chain propagation. The result is that the polymer chains grow at a more constant rate than seen in traditional chain polymerization and their lengths remain very similar. Living polymerization is a popular method for synthesizing block copolymers since the polymer can be synthesized in stages, each stage containing a different monomer. Additional advantages are predetermined molar mass and control over end-groups.
In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening ("curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and may be promoted by high pressure or mixing with a catalyst. Heat is not necessarily applied externally, and is often generated by the reaction of the resin with a curing agent. Curing results in chemical reactions that create extensive cross-linking between polymer chains to produce an infusible and insoluble polymer network.
Polyvinylidene fluoride or polyvinylidene difluoride (PVDF) is a highly non-reactive thermoplastic fluoropolymer produced by the polymerization of vinylidene difluoride. Its chemical formula is (C2H2F2)n.
Conductive polymers or, more precisely, intrinsically conducting polymers (ICPs) are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The main advantage of conductive polymers is that they are easy to process, mainly by dispersion. Conductive polymers are generally not thermoplastics, i.e., they are not thermoformable. But, like insulating polymers, they are organic materials. They can offer high electrical conductivity but do not show similar mechanical properties to other commercially available polymers. The electrical properties can be fine-tuned using the methods of organic synthesis and by advanced dispersion techniques.
Polypyrrole (PPy) is an organic polymer obtained by oxidative polymerization of pyrrole. It is a solid with the formula H(C4H2NH)nH. It is an intrinsically conducting polymer, used in electronics, optical, biological and medical fields.
Microencapsulation is a process in which tiny particles or droplets are surrounded by a coating to give small capsules, with useful properties. In general, it is used to incorporate food ingredients, enzymes, cells or other materials on a micro metric scale. Microencapsulation can also be used to enclose solids, liquids, or gases inside a micrometric wall made of hard or soft soluble film, in order to reduce dosing frequency and prevent the degradation of pharmaceuticals.
Electrospinning is a fiber production method that uses electrical force to draw charged threads of polymer solutions for producing nanofibers with diameters ranging from nanometers to micrometers. Electrospinning shares characteristics of both electrospraying and conventional solution dry spinning of fibers. The process does not require the use of coagulation chemistry or high temperatures to produce solid threads from solution. This makes the process particularly suited to the production of fibers using large and complex molecules. Electrospinning from molten precursors is also practiced; this method ensures that no solvent can be carried over into the final product.
An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of the twentieth century. A wide variety of synthetic membranes is known. They can be produced from organic materials such as polymers and liquids, as well as inorganic materials. Most commercially utilized synthetic membranes in industry are made of polymeric structures. They can be classified based on their surface chemistry, bulk structure, morphology, and production method. The chemical and physical properties of synthetic membranes and separated particles as well as separation driving force define a particular membrane separation process. The most commonly used driving forces of a membrane process in industry are pressure and concentration gradient. The respective membrane process is therefore known as filtration. Synthetic membranes utilized in a separation process can be of different geometry and flow configurations. They can also be categorized based on their application and separation regime. The best known synthetic membrane separation processes include water purification, reverse osmosis, dehydrogenation of natural gas, removal of cell particles by microfiltration and ultrafiltration, removal of microorganisms from dairy products, and dialysis.
In materials science, the sol–gel process is a method for producing solid materials from small molecules. The method is used for the fabrication of metal oxides, especially the oxides of silicon (Si) and titanium (Ti). The process involves conversion of monomers in solution into a colloidal solution (sol) that acts as the precursor for an integrated network of either discrete particles or network polymers. Typical precursors are metal alkoxides. Sol–gel process is used to produce ceramic nanoparticles.
Sodium polyacrylate (ACR, ASAP, or PAAS), also known as waterlock, is a sodium salt of polyacrylic acid with the chemical formula [−CH2−CH(CO2Na)−]n and has broad applications in consumer products. This super-absorbent polymer (SAP) has the ability to absorb 100 to 1000 times its mass in water. Sodium polyacrylate is an anionic polyelectrolyte with negatively charged carboxylic groups in the main chain. It is a polymer made up of chains of acrylate compounds. It contains sodium, which gives it the ability to absorb large amounts of water. When dissolved in water, it forms a thick and transparent solution due to the ionic interactions of the molecules. Sodium polyacrylate has many favorable mechanical properties. Some of these advantages include good mechanical stability, high heat resistance, and strong hydration.
Polyester is a category of polymers that contain one or two ester linkages in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include naturally occurring chemicals, such as in plants and insects, as well as synthetics such as polybutyrate. Natural polyesters and a few synthetic ones are biodegradable, but most synthetic polyesters are not. Synthetic polyesters are used extensively in clothing.
Nanofiber Seeding is a process to control the bulk morphology of chemically synthesized conducting polymers. Typically, catalytic amount of nanofiber seeds are added in prior to onset of nanofiber seeding polymerization (reaction), where seeds are served as the 'morphology directing agent' rather than conventional templates.
Nanofibers are fibers with diameters in the nanometer range. Nanofibers can be generated from different polymers and hence have different physical properties and application potentials. Examples of natural polymers include collagen, cellulose, silk fibroin, keratin, gelatin and polysaccharides such as chitosan and alginate. Examples of synthetic polymers include poly(lactic acid) (PLA), polycaprolactone (PCL), polyurethane (PU), poly(lactic-co-glycolic acid) (PLGA), poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV), and poly(ethylene-co-vinylacetate) (PEVA). Polymer chains are connected via covalent bonds. The diameters of nanofibers depend on the type of polymer used and the method of production. All polymer nanofibers are unique for their large surface area-to-volume ratio, high porosity, appreciable mechanical strength, and flexibility in functionalization compared to their microfiber counterparts.
Polymer engineering is generally an engineering field that designs, analyses, and modifies polymer materials. Polymer engineering covers aspects of the petrochemical industry, polymerization, structure and characterization of polymers, properties of polymers, compounding and processing of polymers and description of major polymers, structure property relations and applications.
In polymer chemistry, in situ polymerization is a preparation method that occurs "in the polymerization mixture" and is used to develop polymer nanocomposites from nanoparticles. There are numerous unstable oligomers (molecules) which must be synthesized in situ for use in various processes. The in situ polymerization process consists of an initiation step followed by a series of polymerization steps, which results in the formation of a hybrid between polymer molecules and nanoparticles. Nanoparticles are initially spread out in a liquid monomer or a precursor of relatively low molecular weight. Upon the formation of a homogeneous mixture, initiation of the polymerization reaction is carried out by addition of an adequate initiator, which is exposed to a source of heat, radiation, etc. After the polymerization mechanism is completed, a nanocomposite is produced, which consists of polymer molecules bound to nanoparticles.
Polyaniline nanofibers are a high aspect form of polyaniline, a polymer consisting of aniline monomers, which appears as discrete long threads with an average diameter between 30 nm and 100 nm. Polyaniline is one of the oldest known conducting polymers, being known for over 150 years. Polyaniline nanofibers are often studied for their potential to enhance the properties of polyaniline or have additional beneficial properties due to the addition of a nanostructure to the polymer. Properties that make polyaniline useful can be seen in the nanofiber form as well, such as facile synthesis, environmental stability, and simple acid/base doping/dedoping chemistry. These and other properties have led to the formation of various applications for polyaniline nanofibers as actuators, memory devices, and sensors.
A two-dimensional polymer (2DP) is a sheet-like monomolecular macromolecule consisting of laterally connected repeat units with end groups along all edges. This recent definition of 2DP is based on Hermann Staudinger's polymer concept from the 1920s. According to this, covalent long chain molecules ("Makromoleküle") do exist and are composed of a sequence of linearly connected repeat units and end groups at both termini.
In polymer chemistry, graft polymers are segmented copolymers with a linear backbone of one composite and randomly distributed branches of another composite. The picture labeled "graft polymer" shows how grafted chains of species B are covalently bonded to polymer species A. Although the side chains are structurally distinct from the main chain, the individual grafted chains may be homopolymers or copolymers. Graft polymers have been synthesized for many decades and are especially used as impact resistant materials, thermoplastic elastomers, compatibilizers, or emulsifiers for the preparation of stable blends or alloys. One of the better-known examples of a graft polymer is a component used in high impact polystyrene, consisting of a polystyrene backbone with polybutadiene grafted chains.
Cardo polymers are a sub group of polymers where ring structures are pendent to the polymer backbone. The backbone carbons bonded to the pendent ring structures are quaternary centers. As such, the cyclic side group lies perpendicular to the plane of the extended polymer chain. These side rings are bulky structures which sterically hinder rotation of the backbone bonds; they also disrupt chain packing and thus create greater free volume than found in conventional polymer structures. The rotational restrictions placed on the polymer backbone increase the value of the characteristic ratio, leading to what is referred to as a "rigid" or "stiff" backbone. The sterically driven large rotational potential produces a high glass transition temperature and the disrupted packing yields comparatively high values for gas and other small molecule solubilities in these polymers. Because of these physical effects, recent advances in membranes used for gas separation have used cardo polymers.
Catalyst transfer polymerization (CTP), or catalyst transfer polycondensation, is a type of living chain-growth polymerization that is used for synthesizing conjugated polymers. Benefits to using CTP over other methods are low polydispersity and control over number average molecular weight in the resulting polymers. Very few monomers have been demonstrated to undergo CTP.
1 2 3 De Cock, Liesbeth J.; De Koker, Stefaan; De Geest, Bruno G.; Grooten, Johan; Vervaet, Chris; Remon, Jean Paul; Sukhorukov, Gleb B.; Antipina, Maria N. (2010-09-17). "Polymeric Multilayer Capsules in Drug Delivery". Angewandte Chemie International Edition. 49 (39): 6954–6973. doi:10.1002/anie.200906266. PMID20645362.
1 2 3 4 5 6 Ji, J (2001-10-15). "Mathematical model for the formation of thin-film composite hollow fiber and tubular membranes by interfacial polymerization". Journal of Membrane Science. 192 (1–2): 41–54. doi:10.1016/S0376-7388(01)00496-3.
1 2 Morgan, Paul W.; Kwolek, Stephanie L. (November 1959). "Interfacial polycondensation. II. Fundamentals of polymer formation at liquid interfaces". Journal of Polymer Science. 40 (137): 299–327. Bibcode:1959JPoSc..40..299M. doi:10.1002/pol.1959.1204013702.
↑ MacRitchie, F. (1969). "Mechanism of interfacial polymerization". Transactions of the Faraday Society. 65: 2503. doi:10.1039/TF9696502503.
1 2 3 4 5 6 7 Ji, J.; Dickson, J. M.; Childs, R. F.; McCarry, B. E. (December 1999). "Mathematical Model for the Formation of Thin-Film Composite Membranes by Interfacial Polymerization: Porous and Dense Films". Macromolecules. 33 (2): 624–633. doi:10.1021/ma991377w. ISSN0024-9297.
1 2 Ji, J (2001-10-15). "Mathematical model for encapsulation by interfacial polymerization". Journal of Membrane Science. 192 (1–2): 55–70. doi:10.1016/S0376-7388(01)00495-1.
1 2 3 4 Huang, Jiaxing; Virji, Shabnam; Weiller, Bruce H.; Kaner, Richard B. (2004-03-19). "Nanostructured Polyaniline Sensors". Chemistry - A European Journal. 10 (6): 1314–1319. doi:10.1002/chem.200305211. ISSN0947-6539. PMID15034876.
1 2 3 Choi, Yeong Suk; Joo, Sang Hoon; Lee, Seol-Ah; You, Dae Jong; Kim, Hansu; Pak, Chanho; Chang, Hyuk; Seung, Doyoung (April 2006). "Surface Selective Polymerization of Polypyrrole on Ordered Mesoporous Carbon: Enhancing Interfacial Conductivity for Direct Methanol Fuel Cell Application". Macromolecules. 39 (9): 3275–3282. Bibcode:2006MaMol..39.3275C. doi:10.1021/ma052363v. ISSN0024-9297.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
