A membrane is an interface between two adjacent phases acting as a selective barrier, regulating the transport of substances between the two compartments. In essence, a membrane is nothing more than a discrete, thin interface that moderates the permeation of chemical species in contact with it. This interface may be molecularly homogeneous, that is, completely uniform in composition and structure, or it may be chemically or physically heterogeneous, for example, containing holes or pores of finite dimensions or consisting of some form of layered structure. A normal filter meets this definition of a membrane, but, by convention, the term filter is usually limited to structures that separate particulate suspensions larger than 1 to 10 μm. Membranes can separate particles much smaller than 1 μm (down to nanometer range).

The key property that is exploited is the ability of a membrane to control the permeation rate of a chemical species through the membrane. In separation applications, the goal is to allow one component of a mixture to permeate the membrane freely, while hindering permeation of other components. Separations with membranes do not require additives, and they can be performed isothermally at low temperatues and—compared to other thermal separation processes—at low energy consumption. Also, upscaling and downscaling of membrane processes as well as their integration into other separation or reaction processes are easy.

The vast majority of membranes used commercially are polymer-based. However, in recent years, interest in membranes formed from less conventional materials has increased. Ceramic membranes, a special class of microporous membranes, are being used in ultrafiltration and microfiltration applications for which solvent resistance and thermal stability are required. Dense metal membranes, particularly palladium membranes, are being considered for the separation of hydrogen from gas mixtures, and supported liquid films are being developed for carrier-facilitated transport processes.

Advanced membranes of the next generation will have functions beyond just being selective barriers with high performance (flux, stability, etc.). The advanced polymer membranes will often be based on tailored functional macromolecular architectures instead of just ‘bulk polymer’ properties. The combination of membranes with catalysis is intensively studied and, occasionally, already used on a technical scale. ‘Smart’ membranes with changing selectivities or adaptive surfaces can be created using approaches currently under investigation in research labs. Examples for such stimuli-responsive membranes show that a synergistic interplay of pore structure and tailored functional macromolecular systems can be used to create ‘biomimetic’ membranes.

Membrane for water purification systems
Some of the most successful commercially available RO membranes are composite structures with aromatic polyamide separation layers. Unfortunately, such surfaces attract organic and biological species which on contact easily form bio-film as the first step in the biofouling process that creates an additional filtration barrier and reduces the flux of permeating water. Because of this, actions must be taken in order to compensate for undesired flux decline, including increasing the operating pressure by as much as 50% in some cases, interrupting production for cleaning the membranes by frequent chlorination, and replacing the membrane elements approximately every 3 years. In fact, the costs of energy to run the high-pressure pumps and to remedy membrane fouling (where the latter often amounts to ca. 30% of the total operating expense) are two main factors that control the economics of present day water production by RO processes.

MMI has recently used its dendrimer nanotechnology to develop effective anti-biofouling coatings for reverse osmosis (RO) membranes. The coatings are prepared by crosslinking amine-functional polyamidoamine (PAMAM) dendrimers and PAMAM-polyethylene glycol (PAMAM-PEG) multi-arm stars with difunctional PEG crosslinkers. The resulting coatings significantly reduced biofouling of the membrane surfaces without affecting their % salt rejection and only moderately reducing their permeate fluxes.

Membranes for fuel-cell systems
Enormous effort has been devoted in the last two decades to the improvement of membranes for fuel-cell systems.  Fuel cells are electrochemical cells that covert hydrogen, methanol (or other fuels) into electrical energy.  The two most important categories of fuel cell are solid oxide (SOFC) and polymer electrolyte membrane (PEMFC).  The former tend to be used in stationary power and co-generation applications, and the latter tend to be used in automotive and portable electronic power applications.  The high operating temperatures of SOFCs (>600°C) rule out the use of polymers, but many polymer material challenges may be found in the PEMFC area. Fuel cell proton exchange membranes (PEMs) need to have high proton conductivity across a wide temperature and humidity range, low fuel permeability (to prevent crossover), good mechanical strength and dimensional stability, good resistance to acidic and oxidative conditions (i.e., protons, and peroxides at the cathode), good catalyst compatibility and the ability to operate above 80°C (to increase catalyst efficiency and to minimize the size of automotive radiators).  The current industry standard material for hydrogen or methanol fuel cell proton exchange membranes (PEMs) is Dupont’s Nafion® sulfonated fluoropolymer.  Sulfonated aromatic polymers of various compositions and architectures have also been extensively studied. There have been  many attempts to improve fuel cell performance by adding microscale or nanoscale particles to these PEM polymers (e.g., zirconium phosphate, calcium phosphate,  titanium dioxide nanoparticles, silica and nanosilica).  Hydrophilic particles are used to improve membrane water retention, leading to improved conductivity in low humidity conditions, but water-soluble hydrophilic particles can be at risk of being leached out of the membrane.  Particles also improve the physical properties of the membrane and reduce methanol permeability, but often decrease the conductivity because they interfere with the paths and mechanisms of proton conduction. One way to circumvent this trade-off is to use particles functionalized with proton conducting groups, e.g., silica carrying sulfonic acid groups or zeolites carrying sulfonic acid groups.  Such groups also improve compatibility between the reinforcing filler and the sulfonated matrix that carries the filler.  MMI has pioneered the use of sulfonated polyhedral oligosilsesquioxane (POSS) nanoadditives in fuel cell membranes to improve proton conductivity and dimensional stability.

Membranes in Li-air batteries
Lithium-metal batteries approach the energy density of fuel cells without the plumbing needed for these devices; in theory, the maximum energy density is more than 5,000 watt-hours per kilogram, or more than 10 times that of today's lithium-ion batteries. Lithium metal-air batteries are also very lightweight because it's not necessary to carry a second reactant.

One of the important components of Li-air battery is the membrane that separates the internal components of battery from the outside atmosphere (air). MMI has developed novel polymer membranes that are highly permeable to oxygen (>2000 Barrer), and at the same time impermeable to organic carbonates, ethers, and moisture (£ 10 Barrer).

Our approach is based on unique hyperbranched polymer technology with a chemical composition that will pair high oxygen permeability with strong chemical resistance, as well low moisture permeability. The hyperbranched polymer approach also provides the ability to tune in appropriate levels of crosslinking density to optimize the mechanical properties of the material. The composite membranes fabricated from siloxane and perfluoro based polymers are the ideal choice for this application because of their unrivaled chemical stability, high oxygen permeability, and excellent organic solvent barrier properties. The membranes will be used in the air cathode of lithium-air batteries, a unique battery system because the cathode material is not stored in the battery but simply extracted from ambient air and used as needed. The high oxygen permeability expected from this membrane will allow the Li-air battery to use ambient air as its oxygen source, and realize high specific energies. Lithium-air batteries show great promise as the ultimate battery in terms of energy and power density and their market potential is in billions of dollars. The membrane developed and being perfected by Oxazogen will help make the lithium-air battery a marketable success.

Membranes in sensor systems
A chemo- or biosensor is a system consisting of a receptor coupled with a transducer to a detector, thus enabling the conversion of a chemical signal—binding to the receptor—into a physical signal. Many technically established sensor systems or sensors in the research lab involve membranes, their structure may be rather diverse but they should fulfill at least one of the main functions: (1) barrier between the sensor system and its environment, allowing selective access (e.g. of the analyte only) to the receptor or/and protecting the receptor from disturbing influences of the environment; (2) matrix for the immobilization of the receptor or/and tool for bringing it into proximity to the detector—if the transducer is a separate chemical species, the membrane is also the means to integrate the entire sensing system.