Polymer membranes

Polymer membranes – or simplfied, plastic membranes – are thin, selectively permeable layers made of organic plastics. They act as controlled barriers: certain substances, particles, ions or gases can pass through, whilst others are retained. As a result, they are now indispensable in many fields – from water treatment and medical technology to energy conversion.

 

What is a polymer membrane?

A membrane can, in simple terms, be thought of as a very thin filter. The comparison is useful, but not complete, because many membranes do not “filter” only according to particle size, they may also discriminate according to charge, solubility, diffusion rate or chemical interaction with the membrane material (See our Basic Information article “What are membranes?”

Polymers are particularly important membrane materials because their properties can be adjusted in a targeted way. Depending on their chemical structure and manufacturing process, they can be porous or dense, neutral or charged, hydrophilic or hydrophobic, flexible or mechanically robust. They can also be processed efficiently into flat-sheet membranes, hollow fibers, capillaries or tubular membranes.

 

Applications

Polymer membranes in separation technology

The largest industrial application area for polymer membranes is separation technology. Here they replace or supplement conventional processes such as distillation, precipitation or adsorption. Their advantage is that they often operate without a phase change and can therefore be energy-efficient.

In water treatment polymer membranes are widely used:

  • Microfiltration removes particles, bacteria and turbidity.
  • Ultrafiltration retains macromolecules, viruses and colloids.
  • Nanofiltration separates small organic molecules and multivalent ions.
  • Reverse osmosis removes salts and very small, dissolved substances, for example in seawater desalination or ultrapure-water production.

 

The precise choice of material depends on pressure, temperature, pH, cleaning chemicals and susceptibility to fouling.

 

In food- and bioprocessing polymer membranes are equally important, for instance in the following applications:

  • Concentration of proteins
  • Clarification of fruit juices
  • Microbial removal
  • Processing of fermentation broths
  • Production of pharmaceutical intermediates

 

As many products are temperature-sensitive, membrane separation is particularly attractive: unlike distillation, it can often be carried out at moderate temperatures. Dense polymer membranes are used for gas separation, for example oxygen/nitrogen separation, hydrogen recovery, carbon dioxide removal or natural-gas processing. Separation is usually based on one gas being more soluble in the polymer or diffusing through it faster than another.

Pervaporation is relevant for liquid organic mixtures. In this process, one component evaporates after passing through the membrane. Applications include the dehydration of alcohols and the removal of trace organic substances. A central development goal remains the optimum compromise between permeability and selectivity: a membrane should allow as much material as possible to pass per unit time while still separating it very precisely.

 

Medical applications

Polymer membranes are widely used in medicine. In hemodialysis, membranes partially take over the filtering function of the kidneys. They remove urea, excess salts and water from the blood while retaining blood cells and larger proteins. In heart–lung machines and extracorporeal membrane oxygenation (ECMO), gas-permeable polymer membranes enable oxygen and carbon dioxide exchange without directly mixing the blood and gas phases. Other membrane systems are used for plasma separation or therapeutic blood purification.

Polymer membranes can also control the release of active pharmaceutical ingredients, for example in medical patches, implants or depot formulations. The membrane determines how quickly a drug moves from a reservoir into the body.

 

Other application areas

Polymer membranes play a central role in energy technology — in polymer-electrolyte fuel cells, water electrolysis, as proton-exchange membranes and anion-exchange membranes and in batteries.

Breathable, water-repellent textiles often contain microporous or dense polymer membranes. They are intended to keep liquid water out while allowing water vapor to escape. Such materials are used in outdoor clothing, protective wear and medical protective textiles.

In food packaging, polymer membranes or membrane-like films can control the exchange of gases and water vapor, thereby extending the shelf life of sensitive products. For fruit and vegetables, for example, appropriately adjusted oxygen and carbon dioxide permeability is important.

Membranes are also used in sensors. They can selectively allow certain substances to pass through, protect electrodes or control the environment around a measurement system. Examples include gas sensors, biosensors and electrochemical measuring systems.

 

Polymer membrane materials

The following overview presents common polymer membrane materials, organized according to the application areas described above. Membranes differ primarily in their internal structure. Membranes with pores of different sizes are often used for microfiltration and ultrafiltration. Dense membranes, in which substances dissolve in the polymer and pass through it, are important for gas separation and pervaporation. Charged membranes contain fixed ionic groups, preferentially allowing counterions to pass and retain ions with the same charge. For particularly high-performance membranes, thin-film composite membranes are often produced, in which an ultrathin active layer are positioned on a porous support.

Application area Typical membrane materials Notes
Microfiltration (MF) Polyvinylidene fluoride (PVDF), polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), cellulose acetate (CA), polyethersulfone (PES) Usually porous membranes; PVDF and PES are common because of their good mechanical and chemical properties.
Ultrafiltration (UF) Polysulfone (PSU), PES, PVDF, polyacrylonitrile (PAN), cellulose acetate, regenerated cellulose Used, for example, in water treatment, biotechnology and food technology.
Nanofiltration (NF) Aromatic polyamides, poly(piperazine amide), sulfonated polymers, cellulose acetate, polybenzimidazole (PBI) Often used as thin-film composite membranes on porous supports.
Reverse osmosis (RO) Aromatic polyamides, cellulose acetate, polyamide thin-film composites Polyamide composites dominate modern seawater and brackish-water desalination.
Gas separation Polyimide (PI), polysulfone, cellulose acetate, polydimethylsiloxane (PDMS), polycarbonate, poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), polymers of intrinsic microporosity (PIMs) Material choice depends strongly on the gas mixture, e.g. CO2/CH4, O2/N2 or H2/CO2.
Pervaporation Polyvinyl alcohol (PVA), PDMS, polyimide, chitosan, cellulose acetate, polyether block amide (PEBA), crosslinked polyamides Hydrophilic membranes are used for dehydration; organophilic membranes for organic substances.
Electrodialysis Sulfonated polystyrene–divinylbenzene copolymers, quaternized polymers, perfluorinated ion-exchange polymers, PES-based ion exchangers Cation- and anion-exchange membranes with fixed ionic groups.
Fuel cells Perfluorosulfonic-acid polymers, e.g. Nafion and Aquivion; sulfonated poly(ether ether ketone) (sPEEK), sulfonated polyimides, PBI Proton-exchange membranes; PBI is particularly relevant for high-temperature PEM fuel cells.
Water electrolysis Nafion, Aquivion, sulfonated aromatic polymers, quaternized poly(arylene ethers), PBI, anion-exchange polymers Depending on the technology, used as a proton- or anion-exchange membrane.
Redox-flow batteries Nafion, sulfonated PEEK, sulfonated polyimides, PBI, polyolefin separators, quaternized polymers Low permeability to active species and high ionic conductivity are important.
Lithium-ion battery separators Polyethylene, polypropylene, PP/PE/PP multilayer separators, PVDF, PVDF-HFP Usually microporous separator films; thermal safety is critical.
Dialysis Polysulfone, PES, polyamide, PAN, cellulose triacetate, regenerated cellulose Modern dialysis membranes are often synthetic high-performance membranes.
Blood oxygenation / ECMO Polymethylpentene (PMP), polypropylene, silicone/PDMS PMP is important for long-term applications because of its low plasma leakage.
Plasma separation / blood purification PES, polysulfone, polypropylene, cellulose acetate, PVA-based materials Pore size and biocompatibility are decisive.
Controlled drug release Ethylene-vinyl acetate (EVA) copolymers, polyurethanes, silicones/PDMS, polylactide (PLA), polylactide-co-glycolide (PLGA), cellulose acetate Used in patches, implants, depot and osmotic systems.
Functional textiles Expanded PTFE (ePTFE), polyurethanes (PU), polyester-based membranes, polyether esters Water-repellent but water-vapor-permeable; microporous or dense hydrophilic structures.
Packaging Polyethylene, polypropylene, polyethylene terephthalate (PET), ethylene-vinyl alcohol copolymer (EVOH), polyamide, polylactide Gas and water-vapor barriers for food and technical packaging.
Sensors Nafion, PTFE, polyurethane, PVC, PDMS, cellulose acetate, hydrogels Membranes act as selective layers, protective layers or diffusion barriers.
Membrane contactors Polypropylene, PVDF, PTFE, polyethylene Often hydrophobic porous hollow-fiber membranes for gas–liquid contact.
Organic-solvent nanofiltration Polyimide, PBI, crosslinked polyamides, PDMS composites, PEEK-based polymers Requires membrane materials with particularly high solvent stability.

 

Ceramic supports with polymer coatings

To improve membrane stability, mechanically and thermally robust ceramic supports made of aluminum oxide (Al2O3), titanium dioxide (TiO₂), zirconium dioxide (ZrO₂) or silicon carbide (SiC) are coated with thin polymer functional layers to form so-called hybrid or composite membranes. A central challenge is the durable adhesion of the polymer layer to the ceramic surface. Different coefficients of thermal expansion, mechanical loading and swelling in solvents can cause delamination. Chemical functionalization of the ceramic surface or chemical bonding of the polymer layer can significantly improve adhesion strength.

 

Special case: polyelectrolytes on ceramic supports

A particularly flexible and precise method for functionalizing ceramic supports is the deposition of polyelectrolyte multilayers based on electrostatic self-assembly. This process enables ultrathin charged layers with adjustable thickness, charge density and composition. The basic idea is simple: a ceramic support is alternately immersed in solutions of a polyanionic and a polycationic polymer. During each immersion step, a polyelectrolyte layer adsorbs onto the previously applied, oppositely charged layer (layer-by-layer deposition), driven by electrostatic attraction. Washing between the individual steps removes excess polymer. The result is an ordered multilayer system whose total thickness is controlled by the number of layers. Typically, each bilayer has a thickness ranging from a few nanometers to several tens of nanometers.

The selectivity of such membranes depends on pH, ionic strength and temperature. This is both a challenge and an opportunity: under defined conditions, very high rejection rates for multivalent ions can be achieved while monovalent ions preferentially pass through.

Common polyelectrolyte pairs include:

  • Poly(allylamine hydrochloride) (PAH) / poly(styrenesulfonate) (PSS)
  • Poly(ethyleneimine) (PEI) / poly(acrylic acid) (PAA)
  • Chitosan / alginate
  • Sulfonated polymers / quaternary ammonium polymers

 

The layer-by-layer deposition principle with polyelectrolytes described here is used in the MaterialNeutral project KeraRes for membranes designed to produce drinking and process water in a resource-efficient manner.

 

Advantages of polymer membranes

Polymer membranes are used in many areas because they combine several decisive advantages. First, they offer a broad range of materials: chemical structure, porosity, charge and surface chemistry can be tailored to the intended application. This is complemented by flexible processing, since polymers can be produced and shaped into membranes from solution, from the melt or through in-situ polymerization. The resulting membrane types – whether flat-sheet membranes, hollow fibers or capillaries – can also be manufactured industrially in large quantities using scalable processes. In addition, many membrane-based separation processes are particularly energy-efficient. Finally, compact membrane modules can provide a very large active separation area in a comparatively small space – a major advantage over other processes.

 

Limitations and challenges

Despite their versatility, polymer membranes also have limitations. Compared with ceramic or metallic membranes, they are often less resistant to temperature, chemicals and mechanical stress. Extreme pH values, strong oxidizing agents, organic solvents, pressure loads or long-term swelling can be particularly critical.

Another problem is fouling: deposits, biofilms or adsorbed molecules reduce flux and alter separation performance. Surface modification, suitable pretreatment, regular cleaning and optimized process design are therefore essential.

Research focuses, among other things, on narrower pore-size distributions at high porosity, thinner and defect-free separation layers, greater chemical and thermal stability, improved fouling resistance, recyclable or bio-based membrane materials, and cost-effective membranes for energy and environmental technology.

 

Toxicity

The toxicological and ecological assessment of the polymers used is becoming increasingly important in material selection. Particular attention is paid to per- and polyfluoroalkyl substances, known as PFAS. Materials such as PTFE, PVDF or perfluorinated ionomers such as sulfonated tetrafluoroethylene polymers owe their outstanding chemical resistance and water-repellent properties (hydrophobicity) to the fluorine in their polymer structures.

As high-molecular-weight and crosslinked solids, these membrane polymers are largely inert themselves and, in their installed state, are generally regarded as not acutely toxic. More problematic, however, are short-chain PFAS, fluorinated auxiliaries and process chemicals released during manufacture, processing and disposal; these substances are classified as persistent, bioaccumulative and, in some cases, endocrine-active. The European Chemicals Agency ECHA and the EU Chemicals Strategy envisage wide-ranging restrictions on PFAS that may also affect membrane materials and their manufacturing processes. Research and industry are therefore increasingly interested in fluorine-free alternatives — for example, based on sulfonated poly(ether ether ketone) (sPEEK), polybenzimidazole or other high-performance aromatic polymers — intended to provide comparable functionality without the toxicological footprint of PFAS.

The other polymer membrane materials mentioned cannot be generally classified as either toxic or harmless. As finished, high-molecular-weight membranes, many polymers are relatively stable and inert; potential concerns may instead relate to manufacturing, additives, solvents, monomers and processing aids used in production, and possible degradation products during disposal.

The polyelectrolytes identified as a special case may have biological effects that require critical assessment. Strongly cationic polyelectrolytes are particularly relevant because they interact with negatively charged cell membranes, proteins and biological surfaces and can thereby cause membrane damage, cellular stress or aquatic toxicity. The risk is lower in firmly bound layer-by-layer coatings, but this must be verified through leaching and migration tests.

 

Outlook

Polymer membranes are key materials for many future challenges. They help purify water, convert energy more efficiently, make medical therapies safer and use resources more carefully. Their particular strength lies in the combination of tunable chemistry, technical processability and high performance. Future developments will likely focus on more robust materials, more sustainable manufacturing processes and more precisely controlled structures with lower toxicity and reduced environmental impact. This could enable polymer membranes not only to improve existing separation processes, but also to open up new applications in hydrogen technologies, the circular economy, CO2 separation, medical technology and intelligent protective systems.

 

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Created 2026

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