Polymeric Membranes: Technical Challenges and Chemical Mitigation Strategies for Filtration and Separation
By Richard Maldanis, Ph.D., Nerac Analyst
Originally Published September 26, 2017
Whether performing chemical purification, blood detoxification, gas separation, desalination, wastewater treatment or recovery of effluents, separation technologies are essential to most sciences. Membranes, serving as a barrier or interphase between two adjacent phases, are constructed to separate, regulate and/or reject the transport of different types of media . These types of media can include a mixture of solids, liquids or gases. Polymeric membranes have many features that can be tailored and that are beneficial in membrane design including performance capabilities, cost and use versus ceramic and inorganic counterparts . However, there are also some limitations and areas that need to be optimized to allow for adequate filtration efficiencies.
In membrane separations, fouling/biofouling is a common problem occurrence . This occurs when particles or impurities from the solute adhere to the membrane surface. This leads to a reduced flux of the membrane, diminishing the amount of media that can be transported and separated across the membrane. Also, degradation of the membrane can occur if the fouling is significant. Hydrophobic polymers such as polyethersulfone or polyvinylidenefluoride often suffer from fouling problems during aqueous based filtrations . This is due to lower water permeation and less solute transport, thus allowing for particles to adhere to the membrane surface. The inclusion of hydrophilic additives or blending with water loving polymers are two methods used to decrease its fouling phenomena caused by hydrophobic polymers.
Copolymers and functionalized polymers having amphiphilic (hydrophilic and hydrophobic) properties also are utilized as blends in membrane fabrication to alter the flux properties of the polymer. Pluronic F127 , poly(tetrafluoroethylene-co-vinylpyrrolidone) , poly(methyl methacrylate-graft-poly(ethylene glycol) methacrylate) , and stimuli responsive polysulfone-graft-(poly(isopropylacrylamide-co-acrylic acid)-random-poly(methyl acrylate  have been demonstrated as copolymer additives to alter the hydrophilicity of the membrane leading to increased flux, and lower fouling. Areas of application of these membranes include ultrafiltration and oil-water separation.
Zwitterionic polymer additives, having both positive and negative functional groups, are able to enhance the electrostatic interactions with media and solutes . These additives, when blended with hydrophobic polymers, lead to membranes with better performance attributes. Types of zwitterionic compounds being explored for this include methacryloyloxyethylphosphorylcholine-co-poly(propylene glycol) methacrylate , sulfobetaine methacrylate , and poly(N,N-dimethylamino-2-ethylmethacrylate)-b-polydimethylsiloxane-b-poly(N,N-dimethylamino-2-ethylmethacrylate) . These additives have been reported to be useful in creation of antifouling membranes including polyvinylidene fluoride, polyvinylchloride and polyethersulfone.
Another hurdle with polymeric membrane systems is encountered with ultrafiltration and nanofiltration assemblies used in desalination. These types of membranes undergo more stresses and abrasions versus other types of membranes so it is important that they have adequate mechanical properties . Constructed of hollow fibers, these high performance membranes can undergo fiber deterioration by these stresses, thus diminishing the service life of the membrane. Nanocomposites can be used to improve the tensile and stiffness of the membrane surface.
Membranes that are used in high temperature or humid environments need to be able to withstand thermal stresses. Thermal stability is also another downfall of polymer membranes but can be modified through the inclusion of inorganic additives as well as high temperature polymers. Adequate dispersion and interbonding is needed to ensure these additives stay permanently in the polymer interface. The primary goal of these modifications is to raise the glass transition temperature (Tg) of the membrane, which correlates to its thermal stability. These types of high performance membranes are typically used for gas absorption including CO2 and H2S .
For membranes that require both thermal and mechanical properties, silica has been reported to increase these requirements in reverse osmosis and gas separation membranes. Polycarbonate/silica nanocomposite membranes with low silica loading provides thermal stability and mechanical strength enhancement, suitable for selective membranes for CO2, N2, and CH4 . Furthermore, the addition of fumed silica to cellulose acetate (CA)/polyethylene glycol composite membranes improved the glass transition temperature of the membrane to 92.4°C with a tensile strength of 8.2MPa and Young’s modulus of 854.0MPa . These membranes are suitable for reverse osmosis desalination.
Arabic gum was also found as a unique additive to polysulfone membranes in that it served as a pore forming agent during phase inversion synthesis . Not only did it increase the hydrophilicity, surface charge, flux, rejection and antibacterial properties, but also increased the mechanical properties of polysulfone membranes by 50%.
Blood proteins and cells also are known to cause clogging of pores in membranes when used for blood filtration for biomedical applications. For hemocompatible membranes, it is essential to lower the attraction of blood proteins to the membrane, but allowing for adequate diffusion of the blood through the membrane surface. Antifouling additives and polymers with hydrophilic counterparts have been shown to provide the membrane with lower protein adhesion .
Using the properties of two or more polymers allows one to tailor the performance capabilities and final properties of the final membrane. This includes optimizing the hydrophobic-hydrophilic balance which is required to avoid fouling issues. Scientists at Sichuan University are actively researching blood purification membrane blends. Their works include research on bio-inspired hemocompatibility and antifouling polyethersulfone membranes with heparin-mimicking polymers including copolymers of poly(acrylic acid-co-N-vinyl-2-pyrrolidone) and poly(2-acrylamido-2-methylpropanesulfonic acid-co-acrylamide)  and functionalized polyurethanes . The addition of the heparin-mimicking polymers was found to lower plasma protein adsorption and platelet adhesion to the membrane surface as well as prolong clotting times, resulting in better membrane flux.
Aramid nanofibers have emerged as a possible additive for use in polysulfone and polyethersulfone membranes. They have a water contact angle of 40 degrees but are non-dissolvable in water. Not only are they able to impart nonfouling, but also provide membranes with improved blood compatibility, limited protein adsorption, suppressed platelet adhesion and activation, inhibited coagulant factors and complementary factors activation. It is anticipated that this type of additive can be used to generate membranes for water purification and hemodialysis .
In polymer membrane fabrication, there are a number of factors and properties that can be manipulated to increase its performance capabilities.
How Nerac Can Help
Nerac can assist researchers in this area in optimizing the chemistry approaches as well as locating novel specialty additives and modifiers that can greatly increase the service life and meet the targeted specifications of the membrane system.
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