Polymer Brushes for Membrane Separations: A Review - ACS Applied

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Review

Polymer Brushes for Membrane Separations: A Review John Joseph Keating, Joseph Imbrogno, and Georges Belfort ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09068 • Publication Date (Web): 06 Oct 2016 Downloaded from http://pubs.acs.org on October 6, 2016

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ACS Applied Materials & Interfaces

September 30, 2016 Polymer Brushes for Membrane Separations: A Review John J. Keating IV, Joseph Imbrogno, and Georges Belfort* Department of Chemical and Biological Engineering and Center for Biotechnology and Interdisciplinary Studies, Rensselaer Polytechnic Institute, Troy, NY 12180 *Corresponding author: Abstract. The fundamentals and applications of polymer brush modified membranes are reviewed. This new class of synthetic membranes is explored with an emphasis on tuning the membrane performance through polymer brush grafting. This work highlights the intriguing performance characteristics of polymer brush modified membranes in a variety of separations. Polymer brushes are a versatile and effective means in designing membranes for applications in protein adsorption and purification, colloid stabilization, sensors, water purification, pervaporation of organic compounds, gas separations, and as stimuli responsive materials.

Keywords: Surface modification; polymer brush; synthetic membrane; desalination; pervaporation; gas separation; protein adsorption; stimuli responsive

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Introduction: Several reviews have been written which focus upon the synthesis and characterization,1 the stimuli-responsiveness,2-3 or the broad applicability of polymer brushes to a myriad of applications.4 Furthermore, the use of polymer brushes has been previously reviewed only with respect to subsets of the membrane field, such as biomedical applications.5 A condensed review of the applications of polymer brushes throughout several key aspects specific to the membranes field, however, has not been written. This review fulfills that task and presents the exciting benefits of membrane surface modification via polymer brushes throughout several key membrane separation processes. The biological analog for synthetic polymer brushes is the endothelial glycocalyx layer (EGL). The EGL is a macromolecular carbohydrate extracellular matrix made up of proteoglycans and glycoproteins.6-7 The EGL acts as a modulator for permeability in the capillary exchange of water, as a mechanotransducer of fluid shear stress, and as a blood cell interactions regulator.6, 8 Another biological example is the nucleoporin protein brush inside the pores of the quintessential protein selector, the nuclear pore complex.9 Similarly, synthetic modification is an excellent approach to tailoring surface properties.4, 10 There are two main processes by which surfaces can be modified: physisorption and chemisorption.11 Physisorption refers to coatings, such as polymers, which are not covalently attached to a surface. Electrostatic and van der Waals interactions between a polymer and a surface, for example, can result in the polymer physically adhering to a surface, albeit with relatively low binding energy. The limitation of this method lies in the low interaction energy between the polymer and surface, which can cause desorption of the adsorbed film over time, resulting in delamination. In contrast, chemisorption involves forming a covalent bond between the surface and coating. This can be achieved by grafting a polymer to the surface12-15 or grafting from the surface.16-19 The grafting-to approach involves reacting an endfunctionalized pre-formed polymer with suitable moieties exposed on a surface. The main drawback of this method is the inherent diffusion limitation affecting the grafting reaction.20 The 2 ACS Paragon Plus Environment

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grafting-from method involves creating or attaching an initiating species on the surface, followed by growth of the polymer layer from the surface as monomer is continually incorporated into the growing chain. The initiating species can comprise specific molecules (e.g. controlled radical polymerization initiators) or free-radicals. This method is less hindered by diffusion as compared with the grafting-to method, since relatively small monomeric units, rather than an entire polymer, are needed to diffuse to the end of the growing chain. The grafting-from method will be the main surface modification technique utilized in this review to modify the surface chemistry of membranes. As polymer chains grow from the surface of a material, they eventually begin to interfere with each other. Therefore, at sufficiently high grafting density (i.e. chains/nm2), the polymer chains are forced to extend away from the surface to minimize their free energy.21-25 The free energy of the system is a function of both enthalpic (i.e. interaction) and entropic (i.e. elastic) energy contributions of the polymer chains.26 As the polymer chains are forced to stretch further away from the surface as the grafting density increases, the equilibrium distance of the chain ends from the surface (i.e. chain height, h) becomes greater than the characteristic dimension of the polymer in solution (i.e. radius of gyration, Rg).27 This signifies the transition from the “mushroom” regime to the “brush” regime,28 as shown in Figure 1.

Figure 1. Polymer chain (a) in solution, (b) grafted in mushroom regime, (c) grafted in brush regime. 3 ACS Paragon Plus Environment


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Each monomer unit in the brush contains a non-polymeric side group labeled ‘R’, which can simply be hydrogen or a larger chemical functionality depending on the choice of monomer. The fact that these ‘R’ groups are non-polymeric differentiates them from molecular brushes, also known as bottle brushes.29 Polymer chains in the mushroom regime retain similar dimensions as the free chain would exhibit in solution (i.e. Rg). The brush regime, however, results when the distance of the chain ends from the surface is much greater than the end-to-end distance of the chain in solution. Both the chemical and morphological structure of polymer brushes are hypothesized to contribute to the overall separation process. The selectivity of a membrane for species ‘i’ over species ‘j’ is defined as =

, where and are the permeabilities of species ‘i’ and ‘j’,

respectively.30 According to the solution-diffusion mechanism proposed for reverse osmosis, gas separations, and pervaporation, the permeability of a given species through a membrane is defined as the product between its diffusivity in the membrane material, D, times its solubility in the membrane material, known as its sorption coefficient, K. Therefore, the selectivity can be written

as the product of two ratios: = , where the ratio is known as the mobility ratio

and the ratio is known as the solubility ratio.30 Polymers below their glass transition

temperature at ambient conditions (e.g. poly(ether sulfone)), are rigid and generally selectivity in such membranes is dominated by the mobility ratio, which favors the permeation of the smaller molecule.30 Therefore, these membranes find wide use where smaller molecules are selectively permeated (e.g. nanofiltration). Polymers above their glass transition temperature, such as poly(dimethyl siloxane), generally have their selectivity governed by the solubility ratio, since the free rotation of the polymer chains has a smaller impact on the mobility ratio of permeants.30 The performance of these rubbery polymers is therefore determined to a greater extent by its chemistry and the relative ability of permeants to dissolve in the membrane material. This makes it possible 4 ACS Paragon Plus Environment

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to selectively permeate larger molecules (e.g. isobutanol) over smaller molecules (e.g. water) due to a higher solubility of the larger component, if it exists.31 Membranes of this type find commercial use as pervaporation membranes. Polymer brushes are not as simple to define as “glassy” or “rubbery.” The glasssy behavior of polymer chains increases as chains are able to pack more efficiently, and this behavior is different in a polymer melt than on a surface which is modified by a certain grafting density of chains. A glassy polymer in the melt may not be as “glassy” when one end is immobilized on a surface at a low density. High grafting densities and brush molecular weights would be rquired to achieve a true glassy brush. Therefore, we hypothesize that polymer brushes should enhance a separation more profoundly through the solubility ratio than the mobility ratio, and the number of contacts between the brush (i.e. grafting density) and the solute(s) is a governing factor. Polymer brushes, as a result of being tethered to a surface from one end, have different properties than polymers in solution, such as wetting behavior,32 responsiveness to external stimuli,32-37 and colloid stabilization.38 Specifically, articular cartilage proteoglycans are capable of reducing friction between joints in the body lessening wear.39-40 The morphology of the polymer brushes, in addition to their chemical structure, determine the responsiveness of the material to external stimuli such as temperature, pH, or ionic strength. Therefore, several characterization methods are used to determine morphological properties of the polymer brushes. Properties such as the molecular weight, polydispersity index, thickness, and surface grafting density of the polymer brushes can be characterized using suitable methods.1 In order to obtain better control over the polydispersity of grafted brushes, a recent method has been developed known as the “grafting-through” method (Figure 2).41 This method consists of supplying monomers through the surface of a cellulose dialysis membrane.41 Atom Transfer Radical Polymerization (ATRP) initiators, which are attached to the permeate-facing surface of the dialysis membrane, react with the monomers passing through to create brushes. Supplying monomer to initiating sites in this manner reverses the monomer concentration gradient as compared with the traditional grafting5 ACS Paragon Plus Environment


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from mechanism, where the monomer concentration is highest further from the membrane surface. The grafting-through method increases the local monomer concentration closer to the membrane, enabling shorter brushes to grow faster than longer brushes. The resulting brush morphology is a thicker layer with lower roughness than obtained from the traditional grafting-from method.

Figure 2. Schematic depicting the difference between (a) the grafting-from method and (b) the grafting-through method. Red dots are monomer, green triangles are surface attached initiators, and the blue bar is the surface. The arrows at the left show the direction of monomer mass transport. Reproduced with permission from reference 41. Copyright 2016 American Chemical Society.

The thicker brush layer obtained from the grafting-through method is clearly shown in Figure 3.

Figure 3. AFM images depicting (a) unmodified cellulose dialysis bag, (b) brush modified dialysis bag synthesized from grafting-from approach, (c) brush modified dialysis bag synthesized from grafting-through approach. Reproduced with permission from reference 41. Copyright 2016 American Chemical Society. 6 ACS Paragon Plus Environment

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Additional methods for surface preparation prior to polymer brush modification which are referred to in this review include atmospheric pressure plasma polymerization, interfacial polymerization, and layer-by-layer (LBL) thin films. The atmospheric pressure polymerization (APP) method consists of promoting a gas or combination of gases into the plasma state typically through radio frequency power applied between electrodes. The resulting plasma, which can contain electrons, ions, free radicals and ultraviolet light impinge on a polymeric surface to create new surface functionalities capable of polymerization. Interfacial polymerization consists of a polymerization reaction occurring at the interface between an organic phase and aqueous phase, typically resulting in a crosslinked aromatic polyamide structure for use as reverse osmosis membranes. The layer-by-layer (LBL) technique involves the adsorption of a polyelectrolyte onto a surface, followed by the adsorption of an oppositely charged polyelectrolyte. This process can be repeated until the desired film thickness is achieved. The outermost polyelectrolyte layer can subsequently be functionalized with an appropriate small molecule which can serve as an initiating site for polymerization. The morphological information, in conjunction with chemical information of the brushes, can be used to engineer materials for particular applications. Polymer brushes have found application in several types of membrane separations, including protein adsorption (resistance) and separation,42-46 solution-diffusion separations such as water purification,47-48 pervaporation of organic compounds,31, 49 and gas separations,50-51 as well as in stimuli responsive materials.52-55 After a discussion of fundamental characterization methods, synthetic membrane applications are investigated along with the morphological characteristics of the polymer brushes grafted to these membranes. 2.0 Fundamental Characterization of Brush Surfaces: A brief review of major techniques used in polymer brush characterization is necessary to lay the foundation for brush enhancement of separation processes. It should be noted that the characterization of polymer brushes grafted to polymeric membrane materials is extremely 7 ACS Paragon Plus Environment


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difficult, and therefore many researchers attempt to characterize brush layers on surrogate surfaces made of silica, gold, or other materials from which the polymer chains are easily cleaved or have different properties which make characterization simpler (e.g. refractive index).56 A myriad of different methods are used to characterize properties of polymer brushes such as: chemical composition and structure; thickness or height; molecular weight and molecular weight distribution (i.e. polydispersity); brush density; topography and surface structure; stiffness; conformation and swelling; and polymerization kinetics. Estimating the surface grafting density of brushes (chains/nm2) requires a combination of techniques and is difficult to obtain.1 Knowledge of the chemical properties and morphological structure of brushes is paramount to understanding differences in separation performance. The most common scanning probe microscopy technique used to characterize polymer brushes is Atomic Force Microscopy (AFM). This technique has been used to characterize surface topography, root mean square (RMS) surface roughness, and brush height of poly(NIsopropylacrylamide) grafted from a silicon surface.57 The thermoresponsive change in thickness of a poly(N-Isopropylacrylamide) grafted layer,33 the heights of polyelectrolyte brushes in various solvents,58 and the molecular weight and polydispersity of poly(N,Ndimethylacrylamide)59 have also been measured using this technique. In addition, AFM has been used to detect conformational changes of poly(4-vinylpyridine) grown from gold surfaces via ATRP at different values of pH.34 A drawback of AFM is that the brushes can be mechanically compressed by the probe tip. This may yield brush thickness results that are smaller than those measured by ellipsometry.1 Ellipsometry is a specular spectroscopy technique that measures the change in polarization of a light source upon reflection or transmission and the data is fit to a mathematical model. This method is particularly suited to measuring the thickness (i.e. height) of a brush above a surface. The height of a poly(N-Isopropylacrylamide) brush in water at different temperatures,57 poly(methyl methacrylate) brushes on a silicon surface,60 polyampholyte brushes,36 and 8 ACS Paragon Plus Environment

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polystyrene brushes grafted to a silicon wafer through a PGMA anchoring layer15 have all been measured using ellipsometry. Infrared Spectroscopy (IR) is frequently used for confirming the presence of functional groups on a surface.61-62 Small shifts in IR peak positions can be used to determine polymer crystallinity, such as the crystallinity of bolaamphiphiles on polymeric substrates.63 Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) is especially useful to confirm the presence of functionalities introduced onto a surface. This technique measures chemical information to a depth of approximately a few microns into the surface away from the solution. For brush grafting from vinyl monomers, chemical attachment can be verified by the introduction of moieties onto the surface that are present in the monomer, along with the lack of carbon-carbon double bond peak (which occurs at ~1615 cm-1). 52, 64 The conversion of the double bond to a single bond is the only difference between the vinyl monomer and the polymer when analyzed with ATR-FTIR. Therefore, the carbon-carbon double bond peak must not be present to confirm vinyl monomer grafting using ATR-FTIR and to rule out monomeric physisorption. In addition, if it can be confirmed that only chemically attached brushes are present on the surface without residual homopolymer and monomer, 1H and 13C Nuclear Magnetic Resonance (NMR) spectroscopy can be used to determine the moles of monomer grafted per unit area of membrane.64 The Surface Plasmon Resonance (SPR) technique has been used to determine conformational changes of brushes grafted onto a silicon surface65 as well as to determine conformational changes of poly(4-vinylpyridine) grown from gold surfaces via ATRP at different pH values.34 The Quartz Crystal Microbalance with Dissipation monitoring (QCM-D) technique66-68 measures mass per unit area deposited on a quartz crystal through measuring the change in frequency of the crystal resonator. By measuring the dissipation of the QCM frequency, one can determine viscoelastic properties such as the storage and loss moduli of the adsorbed material. QCM has been used to show a frequency shift as pH was varied in a system consisting of a 9 ACS Paragon Plus Environment


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poly(acrylic acid) brush grafted onto the crystal.61 The technique had also provided information on the collapse of polyelectrolyte brushes in an ionic environment.69 This method is best suited to studying conformational changes of brushes, rather than giving physical dimensions of brushes. X-Ray Photoelectron Spectroscopy (XPS) is used to determine the chemical composition of brushes grafted onto a surface. Chemical information is measured to a depth of ~10 nm below the surface, which makes XPS much more surface sensitive than ATR-FTIR. XPS has been employed to determine the chemical composition of a silicon surface grafted with poly(NIsopropylacrylamide)57 and to confirm a poly(allyl alcohol) grafter layer was brominated after a modification.61 Another use of X-rays in characterization, X-Ray Reflectivity (XRR), aided in the determination of the thickness of native oxide layers and polymer brush layers on a silicon surface57 and the dewetting of polystyrene.70 Size Exclusion Chromatography (SEC) is used to determine the molecular weight and molecular weight distribution of polymers. A sacrificial initiator was added to an ATRP reaction mixture that produced homopolymer in parallel with grafted polymer and SEC was used to determine the number average molecular weight (Mn) of the homopolymer chains.60 The homopolymer produced has been previously shown to be close in molecular weight to the grafted polymer.1 A drawback of this method is that it can be very difficult to detach polymers from the surface of a membrane without destroying the polymer brush since strong acids are usually used.1 Therefore, analogous studies may need to be completed on surrogate substrates so the brushes can be removed without resorting to the use of powerful acids. For example, brushes may be grown from gold surfaces with terminal thiol groups, which can be easily reduced with iodine and removed.71 The brushes can then be recovered and passed through a SEC system to determine molecular weight and polydispersity index. Dynamic Scanning Calorimetry (DSC) was employed to determine the percent crystallinity of grafted brushes along with mercury porisometry to determine the change in pore size upon grafting.64 Tensile strength measurements of the membranes before and after grafting 10 ACS Paragon Plus Environment

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also give insight into the mechanical properties of the grafted surfaces.72 TEM is also a popular method to investigate changes in membrane morphology upon grafting.73 The kinetics of the grafting process also must be understood. This is particularly true when grafting using surface plasma treatment due to the number of variables involved (i.e. plasma power, plasma head distance from surface, irradiation time, etc.).74-75 Polymerization kinetics can be studied using QCM-D and in-situ ellipsometry, for example. Although not an exhaustive list,1 these characterization methods have found frequent application in the analysis of polymer brushes. As mentioned previously, the characterization of brushes directly on the membrane surface can be extremely difficult. This results from attempting to resolve one polymeric layer from another, which in general have very similar physical properties. For example, measurement of brush height using ellipsometry takes advantage of differences in refractive index between the substrate and the grafted polymer layer. If the refractive indices are too similar, as in the case of polymeric brushes atop a polymeric membrane, an accurate measurement of brush height may not be possible. In order to measure the molecular weight of polymer chains using SEC, the chains need to be detached from the underlying substrate. This may be relatively simple if the chains are attached to a surface via a chemical bond that can be selectively cleaved with a reducing agent or an acid, but this is typically not the case when grafting brushes from polymeric membrane materials. Strong acids, for example, may non-selectively hydrolyze ester groups in methacrylate based brushes or degrade the polymeric support membrane as well. Cross-section SEM imaging of brushes may not be possible if the brush layer is too thin, also rendering this technique challenging. Therefore, characterization of polymer brush layers is necessary to understand the morphological impact of the brush structure on a membrane’s separation performance, but in general is a difficult task to undertake. 2.1 Protein Resistant Surfaces and Purification: Polymer brush modified membranes have been used in several scenarios involving the adsorption of proteins from solution as well as in the separation of proteins from mixtures. Poly(2-hydroxyethyl methacrylate) (PHEMA) brushes have 11 ACS Paragon Plus Environment


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been synthesized atop porous alumina membranes 0.2 µm in diameter using ATRP.50 The brushes were then derivatized with nitrilotriacetate (NTA) and Cu2+ ions. In addition to surface modification, the inside of the membrane pores were modified with the PHEMA-NTA-Cu2+ brushes using convective flow of reactants through the pores. The PHEMA-NTA-Cu2+ brushes were able to bind 130 mg cm-3 of bovine serum albumin (BSA), which is several times higher than other protein binding membranes. Figure 4 shows a breakthrough curve for the binding of BSA with PHEMA-NTA-Cu2+ brushes. The initial BSA loading concentration was C0 = 0.008 mM. The data show essentially all of the inflowing BSA is adsorbed by the brush layer until breakthrough occurs just above 2 mL of collected permeate.

Figure 4. Normalized breakthrough curve for the binding of BSA with PHEMA-NTA-Cu2+ brushes. Reproduced with permission from reference 50. Copyright 2008 American Chemical Society.

Taking the difference between the feed (C(t=0)/C0 = 1) and permeate (C(t)/C0) BSA concentrations and integrating above the curve with respect to volume yields the binding capacity of the membrane.76 In order to introduce increased specificity to the binding of proteins, PHEMANTA-Ni2+ brushes were grafted from a porous alumina support.50 The nickel complex selectively binds proteins with polyhistidine tags. A solution of 1 mg mL-1 of BSA and 0.05 mg mL-1 of 12 ACS Paragon Plus Environment

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hexa-histidine-tagged ubiquitin (His6-U) was loaded onto the PHEMA-NTA-Ni2+ brush membrane. The membrane was subsequently eluted with imidazole buffer to remove protein bound to the membrane. A gel electropherogram of the eluent is depicted in Figure 5.

Figure 5. Electropherogram of Lane 1: 10% BSA with 0.3 mg mL-1 His-U and Lane 2: PHEMANTA-Ni2+ eluent. Reproduced with permission from reference 50. Copyright 2016 American Chemical Society.

A strong band for His-U was observed in the eluent along with no observable BSA band. The eluent was greater than 99% His-U even though the feed had a 20-fold excess of BSA. In another example, porous alumina membranes were also modified with PHEMA-NTANi2+ polymer brushes using ATRP.77 The brushes were also used to bind polyhistidine tagged ubiquitin (His-U). Figure 6 is a schematic of the binding of His-U with PHEMA-NTA-Ni2+ brushes grown from an alumina support. It is clear that the nickel ion coordinates with the side chains of the histidine residues tagged to the targeted protein for adsorption. Six histidine residues are used to tag the desired protein for chelation with the nickel ion. Two histidine tags positioned three residues apart in an α-helix secondary structure have previously been shown to exhibit a strong bi-dentate chelation effect, whereas histidine tags positioned only two or four residues apart did not.78



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Figure 6. Schematic of the binding of His-U with PHEMA-NTA-Ni2+ brushes. Reproduced with permission from reference 77. Copyright 2007 American Chemical Society.

The geometric requirements of the secondary structure of the histidine tagged protein is therefore critical for obtaining a higher binding strength with the metal ion. Solutions of His-U in phosphate buffer were pumped through the membrane using a peristaltic pump. The binding capacity of HisU with the brush membrane was 120 mg cm-3. The purity of the eluent was greater than 99% HisU in a mixed feed of BSA, myoglobin, and His-U as determined by gel electrophoresis. Room temperature adsorption isotherms of His-U binding as a function of both His-U feed concentration and time (kinetics) are plotted in Figure 7. The adsorption isotherms appear to fit a Langmuir model for adsorption with a maximum adsorption of 4 µg cm-2. The binding of His-U to the brush becomes saturated at approximately 0.04 mg mL-1. We see that ~90% saturation of the brush layer occurs in 20 minutes for a 0.05 mg mL-1 His-U solution.


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Figure 7. Adsorption of His-U as a function of: (a) solution concentration and (b) time. Reproduced with permission from reference 77. Copyright 2007 American Chemical Society.

The breakthrough curves of BSA, myoglobin and His-U through the brush membrane are plotted in Figure 8. A sharper breakthrough curve is observed for His-U, which could be due to a greater diffusion rate through the brush due to its smaller size. Swelling membranes in a solution of photo-initiator prior to UV initiated grafting has also been employed as a viable technique to modify membranes for protein adsorption. Polypropylene membranes have been swelled with a solution of benzophenone (BP) initiator in heptane.79 Acrylic acid (AA), acrylic acid/acrylamide (AAAAm) copolymers, and acrylic acid crosslinked with high amounts of methylenbisacrylamide (AAHMBAA) and low amounts of methylenbisacrylamide (AALMBAA) were UV grafted to the polypropylene support and evaluated for adsorption of lysozyme.79



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Figure 8. Concentration ratio versus accumulated permeate volume. Normalized breakthrough curves for BSA, myoglobin, and His-U through the PHEMA-NTA-Ni2+ brush membrane. Reproduced with permission from reference 77. Copyright 2007 American Chemical Society.

The AAHMBAA brush membrane (which has the highest density of crosslinks) was found to be least affected by pH and ionic strength changes, maintained the highest permeability, and adsorbed the most lysozyme (proportional to the area above the curves). The breakthrough curves (no reproduced here) showed that the AAHMBAA brush membrane exhibited the longest time before lysozyme breakthrough. The slopes of all breakthrough curves for the brush-modified membranes (which are related to the dispersion of the system76) were less steep than the curves for the empty module and the unmodified PP membrane. The lower slope of the breakthrough curves for the brush modified membranes indicates that the binding kinetics of lysozyme adsorption significantly impacted the system dispersion. The Peclet number describes the relative importance of convection to diffusion. The Peclet number was ~ 20 for the system with the unmodified polypropylene membrane. When the Peclet number is below 40, the effect of axial dispersion is considered to be significant.80 The breakthrough curve for the AAHMBAA brush was the steepest of the modified membranes, which correlated with the highest Peclet number. This suggests the 16 ACS Paragon Plus Environment

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binding kinetics are fastest for this brush, which has the highest degree of crosslinking. Also, increased crosslinking reduces the extension of the brushes compared with non-crosslinked brushes, which is more favorable for protein binding.79 A highly extended brush may present an increased diffusion limitation of the system, effectively increasing the system Damkohler number (a measure of reaction to diffusion rates). Typically, breakthrough curves in non-brush modified membrane adsorbers are much steeper than those for the brush modified membranes discussed below. Breakthrough curves have been analytically calculated for human IgG on non-brush modified biomimetic affinity membranes.76 The breakthrough curves were fit with a bi-Langmuir isotherm, which was the sum of independent irreversible and reversible Langmuir adsorption isotherms. The irreversible adsorption was modeled with an infinitely high reaction rate constant, which was responsible for the initial steep increase in the adsorption isotherm as protein became instantaneously bound to the specific adsorption sites. The broadening of the isotherm near saturation was due to the much slower reversible adsorption. Comparatively, the brush modified membranes appear to have less steep slopes initially, and it is possible that the increased diffusion limitations due to the brush are the cause. As with the EGL, there is much to be learned from nature regarding the role of brushes in separations. Researchers have attempted to replicate or mimic nature’s use of brushes to enhance membrane performance in many areas. Bioinspired surface modification of cellulose triacetate membranes has been utilized in a submerged osmotic membrane reactor.81 Cellulose triacetate membranes were modified by polydopamine attachment followed by the grafting of poly(ethylene glycol).81 The polydopamine was used to mimic the behavior of adhesive proteins, which increases the hydrophilicity of surfaces. Throughout 61 days of operation, the modified membranes showed a lower flux decline than the unmodified cellulose triacetate membranes. Also, modifying polymers, blending polymers, and adding functionalized or nonfunctionalized nanoparticles to casting solutions prior to membrane formation has also been an active area of research recently.82-84 Amphiphilic poly(ether sulfone)-b-poly(2-hydroxyethyl 17 ACS Paragon Plus Environment


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methacrylate) was blended with poly(ether sulfone) homopolymer and cast into membranes using the phase inversion process.82 Poly(sulfobetaine methacrylate) was subsequently reacted with the exposed hydroxyl groups on the PHEMA blocks of the membrane surface. The resulting membrane was utilized to separate proteins from a protein solution and to separate oil from oil-inwater emulsions. In another case, the water permeability, BSA rejection, and anti-fouling properties of poly(ether sulfone) (PES) membranes were improved significantly through creating a membrane from a casting solution of PES blended with PHEMA-grafted SiO2 nanoparticles.85 2.2 Polymer Brushes for Hemocompatibility: A special application for polymer brush modifications concerns increasing the hemocompatibility of various materials, many of which are membrane based, that are inserted into the bloodstream. Materials such as drug delivery devices and sensors may be introduced into the bloodstream and need to be designed such that the patient is unharmed and that they function efficiently. Several useful materials that have excellent mechanical properties are comprised of hydrophobic surfaces that attract proteins and platelets in the blood. This is a major problem because the adsorption of these proteins onto the surface of the membrane causes thrombogenesis (i.e. blood clotting) and subsequent embolism formation, which can cause death. Therefore, effort has been made to increase the hydrophilicity of membrane surfaces introduced into the bloodstream to prevent thrombogenesis. In order to synthesize hemocompatible membranes, atmospheric pressure plasma (APP) polymerization and low pressure plasma (LPP) polymerization was used to graft poly(ethylene glycol methacrylate) (PEGMA) macromonomers (500 Da) to the surface of poly(vinylidene fluroide) PVDF membranes (Figure 9).86 It is not necessary for polymers to be light-sensitive for the plasma irradiation to activate a polymer surface.87 PVDF membranes used were 110 µm thick with an average pore diameter of 0.1 µm.86


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Figure 9. PVDF surface modification techniques to synthesize PEGMA brushes: (a) LPP, (b) APP. Reproduced with permission from reference 86. Copyright 2011 American Chemical Society.

These membranes have excellent mechanical properties but have a hydrophobic surface that is prone to adsorption of proteins from blood plasma leading to thrombogenesis.

Grafting of the

hydrophilic PEGMA brushes decreased protein adsorption on the membrane surface and promotes hemocompatibility.86 The grafting yield was defined as the increase in mass of the modified membrane as compared to the unmodified PVDF membrane per cm2. The grafting yield was slightly higher for LPP polymerization. In both cases, the degree of grafting increases with increased plasma irradiation time. As grafting yield is increased, the water contact angle decreased and approached a constant value of about 20º.86 The grafting decreases the water contact angle from ~115⁰ for the native PVDF surface to ~20⁰ for the PEGMA brush modified surface. In addition, a related measure to compare the hydrophilicity of surfaces is to measure the wettability, which is defined as the cosine of the measured contact angle.88 SEM micrographs of the APP modified membrane top surfaces and cross sections are depicted in Figure 10 as a function of plasma treatment time. 19 ACS Paragon Plus Environment


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Figure 10. SEM micrographs as a function of APP treatment time for PEGMA brush modified PVDF membranes. (a-c): top view, (d-f): cross-section. Reprinted in part with permission from reference 86. Copyright 2007 American Chemical Society.

The micrographs show the pores on the top surface of the membrane become covered with increasing plasma treatment time, and become completely covered by 60 seconds of plasma treatment. We see that a thin brush film can be observed on the membrane surface at 60 seconds plasma treatment time, as highlighted by the inset in micrograph ‘f’. X-ray Photoelectron Spectroscopy (XPS) analysis of both the LPP and APP grafted surfaces has also been compared. XPS analysis shows that there is significantly more crosslinking in the LPP process as compared to the APP process. The authors reason that the increased crosslinking may be related to the higher diffusivity of plasma species through the brush layer, wherein the diffusivity is inversely proportional to the pressure. Since the pressure is much lower in the LPP modification chamber, the plasma species can create scissions of the brush throughout a greater portion of the film, resulting in a higher propensity for crosslinking reactions. Another technique that could have been used here to indicate an increased amount of crosslinking is QCM-D. This technique has been used previously to study rigidity differences between adsorbed polyelectrolyte layers67, 89, adsorbed gels and brushes66, and linear and crosslinked proteins68. From QCM-D measurements,


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viscoelastic properties such as the shear viscosity and shear modulus can be calculated assuming the brush thickness. The |∆D/(∆f/n)| ratio, which is the change in dissipation divided by the change in frequency, is inversely proportional to the layer rigidity as previously reported (Figure 11).89

Figure 11. Formation of layer-by-layer assembly of adsorbed poly(L-lysine) and poly(styrene sulfonate). (a) The |∆D/(∆f/n)| ratio of the adsorbed layer obtained due to adsorption of PLL and PSS in a layer-by-layer assembly. The dashed line with prior to buffer wash, the solid line with after buffer wash. (b) A schematic of the layer-by-layer assembly. Reprinted in part with permission from reference 89. Copyright 2008 Elsevier.

Differences in the moduli, for example, can be used to differentiate between more rigid (e.g. crosslinked) and less rigid (i.e. linear) brushes. The brush membranes were also evaluated for their hydration capacity and ability to adsorb fibrinogen, which is a protein present in copious amounts in human blood. Hydration capacity is defined as the difference in wet weight between the modified and virgin PVDF membranes, normalized by the wet weight of the virgin PVDF membrane. The hydration capacity of both LPP and APP brush membranes increases and



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fibrinogen adsorption decreases with grafting yield. The lower crosslinking of the APP brushes is observed to afford less fibrinogen adsorption. It is reasonable to conclude that non-crosslinked brushes have increased mobility that leads to less fouling of the membrane surface. Surface motion for less fouling is new route to reduce fouling. The LPP brush decreases fibrinogen adsorption by ~ 60% and the APP brush decreases fibrinogen adsorption by ~ 85% as compared with the virgin PVDF membrane. The native PVDF and brush membranes were also evaluated for protein adsorption from human platelet poor plasma solution. The proteins assayed for adsorption were -globulin, fibrinogen, and Human Serum Albumin (HSA). The adsorption was determined by the Enzyme-Linked Immunosorbent Assay (ELISA) technique. All adsorption data was normalized by the adsorption of the virgin PVDF membrane. For both the LPP and APP brush membranes, protein adsorption generally decreased with plasma irradiation time. For all three proteins evaluated, the APP brush membrane at 60 s plasma treatment time had the greatest antifouling, and generally performed better than the LPP brush membranes. The APP brush membrane was also shown to have the least platelet adsorption and longest time before the onset of blood clotting. Poly(sulfonebetaine methacrylate) (PSBMA) zwitterionic brushes have also been utilized to increase the hemocompatibility of polypropylene (PP) fibrous membranes using the LPP and APP methods.90 The pore diameter of the PP fibers was 3 µm. The PP fibers were soaked in a SBMA monomer solution and dried for 24 h and then APP and LPP were used to modify the precoated SBMA fibers. The grafting yield is much greater for the APP brush membranes as compared with the LPP brush membranes over the plasma irradiation times investigated. The diiodomethane contact angle for both the LPP and APP brush membranes increased as plasma irradiation times is increased, and both methods asymptotically approached a contact angle of ~ 135⁰. Thus, as expected, the hydrophilicity of the membranes was increased through the grafting


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of PSBMA. SEM images of the PSBMA grafted PP fibrous membranes as a function of plasma treatment time for both the LPP and APP modifications are presented in Figure 12.

Figure 12. Top-view SEM micrographs of PSBMA brush membranes. (a,g): Virgin PP membrane, (b-f) LPP and (h-l) APP brush membranes each at plasma treatment times of 15, 30, 60, 90, 120 s, respectively. Reproduced with permission from reference 90. Copyright 2012 American Chemical Society.

It is apparent that there is increased coverage of PSBMA as a function of plasma treatment time for both LPP and APP brush membranes. The 120 s APP brush appears more fully covered than the corresponding LPP brush. XPS analysis shows the LPP brushes are crosslinked whereas the APP brushes have a much lower degree of crosslinking. The APP brush was shown to outperform the LPP brush membrane by exhibiting higher resistance to protein adsorption, platelet adsorption, and increased blood clotting time. Zwitterionic brushes have also been used previously to identify a multitude of resistant surfaces.91-94 Another example of a biologically inspired polymer brush modification is to increase the hemocompatibility of PES.83 This polymer is too hydrophobic to be used as a hemodialysis 23 ACS Paragon Plus Environment


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membrane without the use of anti-coagulants, which can have adverse effects on the patient. Heparin, a polysaccharide, has been shown to increase the anti-coagulation ability of dialysis membranes.95 Carbon nanotubes were grafted with heparin-mimicking copolymer brushes using ATRP.83 The monomers comprising the copolymer brush include sodium styrene sulfonate (SS) and ethylene glycol methyl ether methacrylate (EGMA). The functionalized carbon nanotubes were blended with PES and cast into membranes using phase inversion. The resulting membranes exhibited better antifouling and anti-coagulant performance as compared with the pristine PES membranes. Whether improving the anti-fouling properties of hemodialysis membranes or other membrane devices in contact with blood, polymer brush modification is a versatile technology in the biomedical field. 3.0 Polymer Brushes for Water Purification: Increasing Salt Rejection and Reducing Scaling Polymer brushes were used to increase the salt rejection of brackish water purification membranes while reducing the scaling propensity of these membranes. Hydrophilic poly(methacrylic acid) (PMAA) and poly(acrylamide) (PAAm) brushes were grafted from polyamide membranes on a polysulfone support using APP.96 Polyamide membranes were synthesized on polysulfone supports using interfacial polymerization. The pre-grafted polyamide membrane synthesized by interfacial polymerization had NaCl rejections of ~ 30% when filtering a 1,000 ppm NaCl solution. Therefore, this membrane acted as a nanofiltration membrane. Brackish water filtration, scaling with respect to the mineral gypsum, and antifouling behavior with respect to BSA and alginic acid were investigated. In addition, zeta potential measurements were performed on the commercial RO membrane and the brush modified polyamide membranes. The zeta potential measurements were calculated by measuring the streaming potential of 10 mM KCl solution at pH = 6.5. Since the pKa of the carboxylic acids of PMAA is ~ 4.5-5, at pH = 6.5 most of the carboxylic acid groups would be deprotonated, resulting in a greater negative surface zeta potential for the PMAA brush membrane as compared with the commercial polyamide


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membrane. The results of the streaming potential experiments used to calculate the zeta potential of the membranes are plotted in Figure 13.

Figure 13. Zeta potential of commercial polyamide (LFC1) and PMAA and PAAm brush membranes as a function of grafting solution concentration. Reproduced with permission from reference 96. Copyright 2010 Royal Society of Chemistry.

The commercial polyamide membrane (LFC1) (Hydranautics, Oceanside, CA) refers to the commercial polyamide RO membrane and the PMAA and PAAm membranes are evaluated based on their grafting solution concentrations.

The PAAm brush membranes have a very small,

negative zeta potential. This is expected since the PAAm is a neutral polymer, whereas the large negative zeta potential of PMAA is due to its charged, polyelectrolyte nature. Contact angle measurements of the brush membranes were performed using the captive air bubble method.97 This method is preferable when analyzing the contact angle of membranes, which can allow the solvent to diffuse into its structure through capillary forces thereby altering the measured contact angle with time.98 The contact angle was determined to be much higher for the PAAm brush membrane as compared with the PMAA brush membrane. This is expected due to the more hydrophilic nature expected of the charged PMAA brush membrane and is in agreement with the



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zeta potential measurements (Figure 13). Both the PMAA and PAAm brush membranes had lower contact angles than the commercial polyamide membrane, indicating the brush membrane modifications result in more hydrophilic surfaces. The results of the scaling experiments with gypsum for both the PMAA and PAAm membranes are plotted in Figure 14.

Figure 14. Flux decline as a function of scaling time for (a) PMAA and (b) PAAm brush membranes as a function of grafting solution concentration. Initial flux, J0 = 2.95 x10-4 cm s1 . Reproduced with permission from reference 96. Copyright 2010 Royal Society of Chemistry. The flux data (normalized by the initial flux, J0 = 2.95 x10-4 cm s-1) are plotted for the various concentrations, [M0], used to synthesize the brush membranes as a function of filtration time for the PMAA brush membranes (panel a) and the PAAm brush membranes (panel b). The scaling experiments show both the PMAA and the PAAm brush membranes have a much longer induction time before the onset of flux decline as compared with the commercial polyamide RO (LFC1) membrane. The commercial polyamide membrane has an induction time of ~ 2.2 h before scaling


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occurs (and normalized flux declines) as compared with an induction time of 6.2 h for the 2.35 M PMAA brush membrane. Interestingly, the PAAm brush membrane at [M0] = 0.2 and 0.3 M do not show any flux decline over the period of 8 h tested. Figure 15 is an optical image of the commercial polyamide RO membrane and PMAA brush membrane surfaces after 8 h of gypsum scaling experiments.

Figure 15. Optical images of: (a) commercial polyamide (LFC1), and PMAA brush membrane surfaces as a function of grafting solution concentration after 8h of gypsum scaling for (b) 0.57 M, (c) 1.17 M, and (d) 2.35 M. Reproduced with permission from reference 96. Copyright 2010 Royal Society of Chemistry.

It is evident that the presence of the PMAA brush significantly reduces the adsorption of gypsum to the membrane surface. When filtering the BSA protein solution, the PMAA and PAAm brush membranes had similar flux declines as the commercial polyamide RO membrane. When filtering alginic acid, the commercial and PMAA brush membranes showed similar flux declines, but the



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PAAm showed a much greater flux decline. Although the PMAA brush and commercial polyamide RO membranes had similar behavior with respect to BSA and alginic acid fouling, PMAA had much better antiscaling performance. Finally, the PMAA and PAAm brush membranes increased the salt rejection of the polyamide support membrane from ~30% to ~95% when filtering a 1,000 ppm NaCl solution in crossflow. The commercial polyamide membrane used in this work also had a salt rejection of ~ 95%. Both brushes also had higher permeabilities than the commercial polyamide RO membrane. PMAA brush membrane permeabilities (2.0 – 3.4 x 10-10 m s-1 Pa-1) and PAAm brush membrane permeabilities (1.8-2.8 x 10-10 m s-1 Pa-1) were greater than the commercial polyamide RO membrane permeability (1.5 x 10-10 m s-1 Pa-1). Therefore, not only were the brush membranes capable of reducing scaling propensity and maintaining biocompatibility (at least for the PMAA brush membrane), they were also capable of attaining the similar salt rejection values. This is significant because a polymer brush was capable of transforming a nanofiltration membrane with low salt rejection into an RO membrane with salt rejection competitive with commercial RO membranes. PMAA brushes have been grafted onto polyamide nanofiltration support membranes as well in attempt to reduce gypsum scaling.99 The PMAA brushes were grafted using the APP method. The PMAA layer thicknesses were measured using ellipsometry as a function of grafting time for various monomer concentrations and two different reaction temperatures. The brush thickness increased as monomer concentration increased and reaction time was increased. Brush thickness decreased, however, on increasing reaction temperature from 60-70⁰C. This is reasoned to be due to increased chain transfer and homopolymerization in solution as reaction temperature is increased, allowing for surface chains to be terminated at a higher rate. The membrane surface roughness was also shown to increase with [M0]. The PMAA brush membranes exhibited NaCl rejections of ~ 95% as compared with the polyamide support membrane rejection of ~30% using a 1,000 ppm NaCl feed solution. The permeability of the PMAA brush membrane was higher than


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that of a commercial polyamide RO membrane while maintaining a comparable salt rejection. The results of the membrane scaling experiments with gypsum are plotted in Figure 16.

Figure 16. Normalized flux as a function of time for the commercial polyamide RO membrane (full diamonds) and the PMMA brush membranes at 5 (open squares), 10 (open triangles), and 20 (full circles) vol% grafting solution monomer concentrations. Initial flux, F0 = 2.95 x10-4 cm s-1. Reproduced with permission from reference 99. Copyright 2010 Elsevier.

The membrane flux was normalized by the initial membrane fluxes (F0 = 2.95x10-5 cm s -1). The induction time before the onset of gypsum scaling for the commercial polyamide RO membrane was ~4.5 h while all PMMA brush membranes had longer induction times before the onset of scaling. The 20 vol % grafting solution concentration had the longest induction time of 10.2 h. Therefore, the PMAA brush membrane exhibited a higher antiscaling propensity while maintaining industry competitive salt rejections. The polyamide nanofiltration membrane was transformed into a RO membrane via the grafting of the brush. 3.1 Water Softening and Wastewater Purification: Layer-by-layer (LBL) adsorption of polyelectrolyte films on porous membranes have been used for water softening100 and removal of by-products from wastewater streams.101 This method consists of adsorbing alternating layers of 29 ACS Paragon Plus Environment


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polyelectrolytes onto a surface (Fig. 11b). This is similar to the grafting-to surface modification technique where pre-formed polymers are coated onto a surface. The difference here is there is mostly electrostatic attraction and no covalent bond between the surface and the polyelectrolytes.89 Poly(styrene sulfonate) (PSS) and poly(diallyldimethyl ammonium chloride) (PDADMAC) were adsorbed onto porous alumina substrates with 0.2 µm diameter pores using LBL deposition.100 These membranes exhibited Cl-/F- and Br-/F- selectivities greater than 3 and fluxes 3-fold higher than commercial membranes.100 Selectivity of A/B was defined as the permeability of ‘A’ divided by the permeability of ‘B’. Fluoride rejection was greater than 70% and was fairly constant over pressures of 3.6 to 6 bar. The author attributes the selectivity of Cl- and Br- over F- to the larger Stokes radius (i.e. hydrodynamic radius) of F- as compared with Cl- and Br-. Membranes for the separation of monovalent anions from drinking water is promising especially since removing F- with ion exchange resins can be difficult due to the tendency of these resins to bind Cl- more strongly. The selective removal of dyes, sucrose, and amino acids from NaCl solutions has been accomplished using nanofiltration membranes synthesized by LBL deposition of poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) onto porous alumina membranes.101 Separation of dyes from wastewater effluent streams is needed since these dyes cannot be discharged directly back into the environment. The recovery of the dyes is very beneficial for the textile industry since the efficiency of attaching the dye to fabrics can be quite low and recovering the dye can reduce raw material costs. These membranes should have a low salt rejection, since increased salt rejection increases the osmotic pressure that needs to be overcome to drive the separation. The PSS/PAH polyelectrolyte membranes exhibit NaCl/sucrose selectivity of 130 and NaCl/dye selectivity of 2200. Rejections are 99.4% and > 99% for sucrose and dye, respectively. NaCl rejections were < 20%. Glutamine rejections of 7580% were also achieved for the LBL membrane. Figure 17 depicts an SEM cross-section micrograph of the porous alumina membrane coated with PSS/PAH polyelectrolytes.


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Figure 17. SEM cross section of porous alumina coated with PSS/PAH polyelectrolyte film. Reproduced with permission from reference 101. Copyright 2006 American Chemical Society.

The film is present on top of the alumina as well as inside the pores. Hydroxyl functionalized silica fibrous membranes were modified with covalently bound perfluorinated poly(ethylene glycol) (PEG) surfactants for application in emulsion separations.102 These amphiphiles increased the hydrophilicity of the membrane surface and were able to reject oil from oil-in-water emulsion feed streams. 4.0 Polymer Brushes in Gas Separations: Polymer brushes have also been used in the creation of gas separations membranes. LBL adsorption of PSS/PAH polyelectrolytes onto porous alumina followed by reaction of the outer layers of the polymer coating with 2-bromopropionyl bromide has been used to create gas separation membranes.50 This reaction introduced a reactive group on the surface capable of initiating ATRP. Crosslinked poly(ethylene glycol dimethacrylate) (PEGDMA) brushes were subsequently grown from the surface via the ATRP method. Figure 18 shows the growth of PEGDMA EGDMA brushes from the multilayer polyelectrolyte adsorbed on the alumina surface.



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Figure 18. Growth of crosslinked PEGDMA brushes from a multilayer polyelectrolyte film (MPF) using ATRP from a porous alumina surface. Reproduced with permission from reference 50. Copyright 2008 American Chemical society.

The synthesis begins with the LBL deposition of the PSS/PAH multilayer polyelectrolyte film (MPF), followed by derivatization with an ATRP initiator (using trimethylamine, Et3N, as a catalyst), and subsequent growth and crosslinking of the PEGDMA brush. Films 50 nm thick gave selectivities of 18 and 26 for CO2/CH4 and CO2/N2, respectively. Similarly, room temperature ATRP grafting of crosslinked PEGDMA and linear poly(2hydroxyethylmethacrylate) (PHEMA) from polyelectrolyte layers adsorbed onto a porous alumina support has also been used to create gas separation membranes.51 PEGDMA exhibited CO2/CH4 and O2/N2 selectivities of 20 and 2, respectively. The linear PHEMA brush membranes showed very little selectivity. Derivatization of the hydroxyl groups of the PHEMA bushes with pentadecafluorooctanoyl increased the CO2/CH4 selectivity to ~ 8. The concept of derivatizing a


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brush post-synthesis to create changes in separation performance is very versatile and provides limitless opportunities to investigate other surface chemistries. The cross-linked brushes showed higher selectivity than the linear brushes. The fluxes of different gases through the crosslinked PEGDMA and linear PHEMA brushes as a function of the transmembrane pressure are plotted in Figure 19.

Figure 19. Flux of different gases through brush modified membranes as a function of transmembrane pressure for (a) PEGDMA and (b) PHEMA. Reproduced with permission from reference 51. Copyright 2008 Elsevier.

The graph in panel ‘a’ represents gas fluxes through PEGDMA while the graph in panel ‘b’ represents fluxes through PHEMA brush membranes. CO2 shows the highest flux through the polar PEGDMA brush membrane. It is reasonable that this increased flux could be due to the affinity of the individually polar carbon-oxygen bonds of CO2 with the ester and hydroxyl functionalities of the PEGDMA brushes. The gas fluxes through the PHEMA brush membrane are in order from lowest to highest molecular weight. This is characteristic of Knudsen diffusion, which is proportional to the inverse square root of the gas molecular weights.103 Thus, hydrogen would have the highest and CO2 the lowest fluxes since they have the lowest and highest molecular



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weights, respectively. The higher solubility of CO2 in the fluorinated PHEMA derivative shows non-Knudsen behavior and exhibits the higher CO2 selectivity. 5.0 Polymer Brushes in Pervaporation: Polymeric membranes have found use in pervaporation for separation of volatile organics from aqueous mixtures.104 Interesting polymeric membranes, including block copolymers, have exhibited selectivity of organics over water.105 Polymer brushes have also found use in pervaporation membranes. PHEMA polymer brushes were grown from the surface of porous alumina using ATRP to separate several organics from water.49 The PHEMA brushes were subsequently derivatized with octyl, hexadecyl, and pentadecafluorooctyl side chains to increase the selectivity of the brush. The performance of the derivatized brushes was greater than the commercial poly(dimethyl siloxane) (PDMS) membrane. Separation factors as high as 500 with fluxes an order of magnitude higher than commercial PDMS membranes were synthesized. Increasing the derivatized side chain length from 8 to 16 carbons long increased the separation factor of the membrane and reduced the flux. It is reasonable that the longer 16 carbon side chains pack in a more crystalline arrangement than the 8 carbon side chain, creating a denser structure which increases separation and decreases flux. The fluorinated PHEMA brush exhibited the highest flux. All brush membranes synthesized showed ability to separate ethanol, ethyl acetate, trichloroethylene, dichloromethane, and benzene from water. Organics have also been separated through growth of stearyl methacrylate (C18) brushes from a poly(ether sulfone) (PES) nanofiltration membranes using the APP method.31 The C18 brush membrane exhibited a separation factor of ~ 10 for isobutanol from water. This C18 brush membrane had a 40% higher separation factor at a similar flux as compared with the commercial PDMS membrane (Figure 20).


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Figure 20. Separation factor (α) versus permeation flux (J) for a range of different brush membranes tested with feed containing in vitro enzymatic reaction products. Commercial PDMS membranes (blue circles) (Sil5 and Sil20), pure monomers (black diamonds), C18:C6 monomer mixtures (green squares), and plasma only (red triangles); the red dotted line indicates no separation (α = 1), and the red solid line shows the lower limit of J versus α for performance of new membranes over existing membranes. Monomers: polyethylene glycol (PEG), styrene, hexyl methacrylate (C6), isobutyl methacrylate (C-B4), and stearyl methacrylate (C18). Insert: degree of grafting (DG) for vinyl monomers: hexyl methacrylate (C6), isobutyl methacrylate (C-B4), stearyl methacrylate (C18), and mixtures of C18/C6. Reproduced with permission from reference 31. Copyright 2015 American Chemical Society.

6.0 Stimuli Responsive Polymer Brush Membranes: An interesting property of polymer brush systems is the ability to change conformation in response to changes in external stimuli (e.g. solvent, temperature, and pH).106 This tunable behavior of polymer brushes can be combined with membranes and capitalized on to control flow based on external stimuli.107-108 Tunable membranes were created through modification of poly(ethylene terephthalate) (PET) track membranes through UV grafting of N-isopropyl acrylamide to the membrane surface using benzophenone (BP) as initiator.109 The Lower Critical Solution Temperature (LCST) phase



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behavior of the resulting poly(N-isopropyl acrylamide ) (PNIPAAm) brush at 33⁰C allowed transition from a collapsed to an extended conformation by cooling below the LCST. This allowed for the brushes to act as a thermal valve, allowing flow through the membrane at temperatures above the LCST (i.e. when the brush is in the collapsed state) and blocking flow when the temperature is decreased below the LCST (i.e. when the brush is in the extended state). A schematic of the pore covering ability of the thermosensitive PNIPAAm brush membrane is shown in Figure 21.

Figure 21. Schematic depicting the thermal valve effect of the PNIPAAm brushes as function of degree of grafting and at temperature below and above the LCST. Reproduced with permission from reference 109. Copyright 2003 Elsevier.

At a degree of grafting above 1%, the brush was able to exhibit the thermal valve effect. At low degree of grafting, the brushes are too short to completely cover the membrane pores regardless of the temperature. As the degree of grafting is increased, the brushes are long enough to cover the membrane pores when the temperature is below the LCST but are unable to cover the pores when the temperature is above the LCST. This is the desired thermal valve effect. The LPP method has been used to graft PNIPAAm brushes to the surface of porous polyethylene and the resulting


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brushes were shown to act as a thermal valve.110 pH responsive membranes have been created through grafting of poly(methacrylic acid) (PMAA) brushes from track etch polycarbonate membranes by glow discharge treatment.53 The pH was adjusted to extend/collapse the brushes in order to gate the flow through the membrane. Acid-sensing ion channels (ASICs), found in peripherical sensory neurons and in the neurons of the central nervous systems, allow the passage of cations only under specific values of pH.111 Hybrid separation systems that mimic ASICs have been designed to allow such transport of cations only under specific values of pH.112 Figure 22 is a schematic of this selective ionic transport mechanism.

Figure 22. Schematic depiction of ion transport through the zwitterionic brush grafted mesoporous silica surface at A) pH > 5 and B) pH < 5. Reproduced with permission from reference 112. Copyright 2009 American Chemical Society. The zwitterionic brush poly(methacryloyl-L-lysine) was used to modify the surface of mesoporous silica and the transport of cations and anions was measured.112 The isoelectric point of the brush is at pH = 5, therefore, at pH > 5 the brush is negatively charged and should be cation selective. At pH < 5, the brush is positively charged and should be anion selective. However, at pH < 5, both 37 ACS Paragon Plus Environment


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anions and cations are rejected. This is due to the native silanol groups present on the mesoporous silica scaffold containing negative charges at pH < 5, while the brush is positively charged in this pH range, leading to rejection of both cations and anions.112 At pH > 5, both the silanol groups and the brush are negative, yielding the cation selectivity. The composite hybrid material, therefore, behaves differently than the sum of its parts. A similar approach was taken to fractionate proteins of similar molecular weight but different isoelectric points using grafted charged ultrafiltration membranes.113 7.0 Conclusion: Surface modification with polymer brushes is increasingly attractive for modifying synthetic membranes and plays an important role in several areas of membrane separations. Brush modified membranes are capable of binding copious amounts of protein from solution, as well as selectively binding affinity tagged proteins. They are also able to repel proteins from hydrophobic membranes or non-porous films that may be used as sensors or drug delivery devices in the bloodstream. Increasing the hydrophilicity of devices implanted into the body can prevent thrombogenesis and increase the range of materials for biomedical devices. Brush membranes have been successfully implemented in several water purification applications. Polymer brushes were designed to increase the antifouling and anti-scaling properties of membranes. Brush membranes were used to increase rejection of dyes and small molecules from wastewater effluent streams. The recovery of dyes from textile plants is an environmental as well as economic benefit for the textile industry. The separation of monovalent ions, such as fluoride from chloride ions, shows the specificity which charged polymer brushes can afford to water softening membranes. Furthermore, polymeric nanofiltration membranes can be transformed into reverse osmosis membranes through the grafting of polymer brushes. These brush membranes exhibit similar fluxes and salt rejections as commercial polyamide reverse osmosis membranes, but with lower mineral scaling propensities. Brush membranes were shown to have selectivity in gas separations, most notably separating CO2/N2 with a selectivity of 26. The performance of pervaporation of several organic 38 ACS Paragon Plus Environment

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compounds from water was enhanced through the grafting of polymer brushes, with selectivities above commercial “gold standard” PDMS membranes. Responsive brushes change conformation based on external stimuli and can be used to control flow through a membrane. Conditions such as temperature, pH, and solvent can be changed to bring about extended or collapsed brush conformations which regulate flow through the membrane. Finally, the importance of proper characterization of polymer brush modified membranes was outlined as well as several characterization methods. The difficulty in characterizing brush layers directly atop of polymeric membranes was highlighted. Methods for proper morphological characterization of such brush layers need to be improved. Further research on correlating the chemical nature of the brush and its morphological properties with actual membrane filtration performance is a key step needed before commercialization and will allow confident engineering design of brush membranes for niche applications. Brush membranes are an emerging class of membranes with numerous possibilities for application in a broad range of separations.

8.0 Acknowledgements We acknowledge the Department of Energy, Basic Energy Sciences Division (DE-FG0209ER16005) for funding our research on fundamental aspects brush modified membranes and thank Howard P. Isermann for first-year graduate fellowships.



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Polymer Brushes for Membrane Separations: A Review - ACS Applied

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