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Potential Sources for Interfacial Polymerization:

Recent progress in interfacial polymerization (review)

As the most recent review articles, this source covers a large range of topics and, along with "current trends in interfacial polymerization chemistry" will probably be the most references. Important topics include types of interfacial polymerization systems, industry applications, and current challenges/opportunities in the field of interfacial polymerization.

Current trends in interfacial polymerization chemistry (review)

Similar to the previous "recent progress in interfacial polymerization", this source covers a broad range of topics and will be used in conjunction to provide an overview of interfacial polymerization. Important topics emphasized in this article include a summary of the chemistry behind interfacial polymerization, its shortcomings, and possible future directions.

Mathematical modeling of interfacial polycondensation

This source provides a detailed description of the modeling of interfacial polymerization. In this way, the article focuses on the physics of interfacial polymerization rather than the chemistry or applications. This source will be used to describe kinetics and other physical properties of interfacial polymerization.

Interfacial polycondensation. I.

As the first article to describe interfacial polymerization, this source is a good reference for the origins of interfacial polymerization, and the initial conditions used to synthesize polymers. This source also goes into the process and chemistry of creating an interfacial polymerization synthesis.

Interfacial polycondensation. II. Fundamentals of polymer formation at liquid interfaces

Following the previous article, this source focuses more on physical properties of interfacial polymerization or the polymer itself, such as polymer solvent sensitivity, reaction concentration, duration of polymerization, and the effect of additional impurities.

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Interfacial Polymerization (Outline)

Summary/Most Important Information

*Include image of Interfacial Polymerization setup created by me (imagine this image from Wikimedia Commons, but nicer)

History

• conception in 1959, original synthesis
• Current/latest research

Mechanisms

• Reaction conditions and reaction design/synthesis parameters

• 4 main types of interfaces (liquid-solid, liquid-gas, liquid-liquid, and liquid-in-liquid emulsion)
• 5 types of polymerization systems (liquid/monomer-solid, liquid/monomer-liquid, liquid-monomer/liquid-monomer, liquid/monomer-in-liquid, liquid/monomer-in-liquid/monomer)
• Other rare interfaces

Mathematical Models

1. Kinetic model
2. Local (microscopic) model
3. Macrokinetic (macroscopic) model

Applications

• In industry
  • sensors, supercapacitors, batteries, purification/seperation
• In research
  • molecular cargo loading, synthesis of nano 0D, 1D, 2D and 3D particles
  • *Incude image of polyaniline nanofibers from Wikimedia Commons

See Also

References

Each bullet point in the outline is supported by one or more sources. Each sources is followed by a citation for the source with the most useful information. Within the bullet points, further facts will also be cited. When facts are supported by multiple sources, all supporting sources will be cited.

Notes:

"Recent progress in interfacial polymerization"

IP used to synthesize polymer particles and membrane

IP takes place at the interface of two immiscible phases/layers

2 theory models of IP

5 Typical polymerization systems

• liquid/monomer-solid
• liquid/monomer-liquid
• liquid-monomer/liquid-monomer
• liquid/monomer-in-liquid
• liquid/monomer-in-liquid/monomer

Used in sensors, supercapacitors, batteries, purification(?), and molecular(?) cargo loading

4 types of interfaces, with monomer in liquid

• liquid-solid
• liquid-gas
• liquid-liquid
• liquid-in-liquid emulsion

When there are two liquid layers, one or both contain monomer

When interfacial pressure of monomer monolayer exceeds equilibrium spreading pressure of polymer, polymer is stuck at interface

Polymer continues to grow at interface

For liquid/monomer-solid system, polymerization begins at interface and film forms on solid

For liquid/monomer-liquid and liquid/monomer-in-liquid systems, polymerization begins on one side

For liquid/monomer-in-liquid and liquid/monomer-in-liquid/monomer systems, polymerization begins on both sides

Liquid/monomer-liquid and liquid/monomer-in-liquid systems used for a homogenous polymer composition

Liquid/monomer-liquid and liquid/monomer-in-liquid systems used for a composite polymer composition

Other, rare interface categories: liquid-gas, solid-gas, and solid-solid

2 Theory models of IP

1. L_m-S
   • Interface consists of porous solid and monomer solution
   • Diffusion and polymerization rate greatly affect film growth rate
   • Assumptions of mathematical model
   • 1. formed polymer cannot be dissolved in liquid phase
   • 2. density of film is uniform during polymerization
   • 3. polymerization occurs in the "polymerization zone", which has a uniform density
   • 4. polymerization follows second-rate kinetics with polymerization rate proportional to monomer concentration
   • General relationship between time and thickness: (insert equ 1 here)
   • Suggests that film thickness increases briefly and then levels off, this is consistent with experimental data
2. L_m-in-Lm

Theory models suggest that controlling polymer thickness is possible

L_m-S fabrication method

L_m-L fabrication method

L_m-L_m fabrication method

L_m-in-L fabrication method

L_m-in-L_m fabrication method

Applications:

• popular method for synthesizing conducting polymers such as PANI, PPy, poly(3,4-ethylenedioxythiophene) (PEDOT), and polythiophene (PTh)
• PANI nano fibers used for sensing of gases
• PEDOT nanoneedles used for switches
• graphene oxide/PANI nanocomposite used for supercapacitors
• PPy–ordered mesoporous carbon (OMC) composites for fuel cells
• As electronics shrink down, there will be more need for nano conducting polymers
• Membranes used for separation/purification
• Polymer capsules used for cargo-loading

Challenges to IP

"Interfacial polycondensation. I."

IP first used in 1959, called "Interfacial Polycondensation" by Emerson L. Wittbecker and Paul W. Morgan

First IP was Schotten-Baumann reaction of an acid chloride and activated hydrogen

Choice of organic solvent affects many things including diffusion, reaction rate, and solubility/permeability of growing polymer

Organic solvents immiscible with water and and inert to reactants/"reactive intermediate" are good choices

"Mathematical modeling of interfacial polycondensation"

Models for interfacial polymerization are typically split up into three stages: diffusion of the monomer in its phase, diffusion through the film, and polymerization.

"Current trends in interfacial polymerization chemistry"

IP allows large-scale production of ultra thin films, hollow nanospheres, and nanofibers

Because polymer is constrained to interface, reactants are more likely to encounter growing polymer than other monomers, as a result higher molecular weights can be obtained compared to bulk polymerization

Properties of formed polymer depend greatly on reactivity and and local concentration of monomer, stability of interface, and number of reactive groups on monomers

Monomers used for IP have at least 2 functional groups

Typically one phase contains a nucleophile and the other an electrophile, electrophiles are generally dissolved in the organic layer due to reactivity with water

Using a difunctional monomer results in a linear polymer chain

Synthesis parameters for IP

3 different modeling approaches

Most commonly aqueous and hexane phases are used

Additional surfactants can be used to improve properties

Most IP layers are synthesized on a porous support

Most polymers prepared by IP are polyamides

Polyurethanes and polyurea most commonly used for preparation of nano- and microcapsules

MOFs

Interfacial Polymerization (First Draft)

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.

In polymer chemistry, interfacial polymerization is a type of step-growth polymerization process taking place at the interface between two immiscible phases (generally two liquids), typically resulting in 2D or 3D polymer that is constrained to the interface. There are several variations of interfacial polymerization, which results in several types of polymer topologies, such as ultra-thin films, nanospheres, 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.

An example of a Schotten-Baumann reaction. Benzylamine reacts with acetyl chloride under Schotten-Baumann conditions to form N-benzylacetamide.

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, 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.

Mechanisms

5 different, commonly-used interfaces in interfacial polymerization. The liquid-liquid and liquid-in-liquid emulsion interfaces can have either one or two monomers present in the phases.

In summary, the most widely 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 interfaces.

In a liquid-solid interface, polymerization begins at the interface, and results in a polymer attached to the surface of the solid phase. In liquid-liquid interfaces 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, and 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.

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.

Mathematical Models

Interfacial polymerization has proven difficult to model accurately due to its nature as a nonequilibrium process. These models provide either analytical or numerical solutions. Additionally, the wide range of variables involved in interfacial polymerization has led to several different approaches and several different models. 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. 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.

The kinetic model is based on the principles of kinetics, assumes uniform chemical distribution, and describes the chemical interactions of the system at a molecular level. This model takes into account thermodynamic qualities such as mechanisms, activation energies, rate constants, and equilibrium constants. The kinetic model is typically incorporated into either the local or the macrokinetic model in order to provide greater accuracy.

The local model is used to determine the characteristics of polymerization at a section around the interface, termed the diffusion boundary layer. 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. Parameters determined using the local model include the mass transfer weight, the degree of polymerization, topology near the interface (reaction zone), and the molecular weight distribution of the polymer. 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. One approach to calculating a local model can be represented by the following differential equation:

${\partial c_{i} \over \partial t}={\partial \over \partial y}(D_{i}{\partial c_{i} \over \partial y})+J_{i}$

in which c_i 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, D_i is the molecular diffusion coefficient of the functional groups of interest, and J_i is the thermodynamic rate of reaction. Although precise, no analytical solution exists for this differential equation, and as such solutions must be found using approximate or numerical techniques.

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 particular local model. 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.

More specific approaches to modeling interfacial polymerization are described by Ji and co-workers, and include modeling of thin-film composite (TFC) membranes, tubular fibers, hollow membranes, and capsules. These models take into account both reaction- and diffusion-controlled interfacial polymerization under non-steady-state conditions. One model is for thin film composite (TFC) membranes, and describes the thickness of the composite film as a function of time:

$t=-({E_{0} \over B_{0}}+{A_{0}D_{0} \over B_{0}^{2}}+{C_{0}A_{0}^{2} \over B_{0}^{2}})\ln(1-{X \over X_{max}})-{C_{0} \over 2B_{0}}X^{2}-({D_{0} \over B_{0}}+{C_{0}A_{0} \over B_{0}^{2}})X$

Where A₀, B₀, C₀, D₀, and E₀ are constants determined by the system, X is the film thickness, and X_max is the maximum value of film thickness, which can be determined experimentally.

Another model for interfacial polymerization of capsules, or encapsulation, is also described:

$t=A_{0}{R_{min}}^{5}E_{0}I_{4}+B_{0}{R_{min}}^{4}E_{0}I_{3}+C_{0}{R_{min}}^{2}E_{0}I_{2}+D_{0}{R_{min}}E_{0}I_{1}$

Where A₀, B₀, C₀, D₀, E_0, I₁, I₂, I₃, and I₄ are constants determined by the system and R_min is the minimum value of the inside diameter of the polymeric capsule wall.

There are several assumptions made by these and similar models, including but not limited to monomer concentration uniformity, temperature uniformity, film density uniformity, and second-order reaction kinetics.

{closing remarks???}

Applications

Interfacial polymerization has found much use in industrial applications, especially as a route to synthesize conducting polymers for electronics. 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, fuel cells, supercapacitors, and nanoswitches.

Interfacial polymerization is also widely used to synthesize composite polymers. The materials used to form composite polymers via interfacial polymerization include silica, carbon nanotubes, gold nanoparticles, metal-organic frameworks (MOFs), and graphene. Since composite polymers can have a wide range of properties based on starting materials and reaction conditions, they have proven effective in several industrial applications, including but not limited to: electronics, separation/purification membranes, and cargo-loading micro- and nanocapsules.

Sensors

PANI nanofibers are the most commonly used for sensing applications. These nanofibers have been shown to detect various gaseous chemicals, such as hydrogen chloride (HCl), ammonia (NH₃), Hydrazine (N₂H₄), chloroform (CHCl₃), and methanol (CH₃OH). PANI nanofibers can be further fined-tuned by doping and modifying the polymer chain conformation, among other methods, to increase selectivity to certain gases. A typical PANI chemical sensor consists of a substrate, an electrode, and a selective polymer layer. PANI nanofibers, like other chemiresistors, detect by a change electrical resistance/conductivity in response to the chemical environment.

Fuel Cells

PPy-coated ordered mesoporous carbon (OMC) composites can be used in direct methanol fuel cell applications. 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 simple OMCs.

Seperation/Purification Membranes

Composite polymer films synthesized via a liquid-solid interface are the most commonly used to synthesize membranes used for reverse osmosis and other applications. 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. For example, synthesizing a polymer with a pore size somewhere between the molecular size of hydrogen gas (H2) and carbon dioxide (CO2) results in a membrane selectively-permeable to H2, but not to CO2, effectively separating the compounds.

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. Once synthesized, the capsules can enclose drugs, quantum dots, 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.

See Also

Polymerization