Mathias Ulbricht

Surface modification of ultrafiltration membranes by low temperature plasma II. Graft polymerization onto polyacrylonitrile and polysulfone

Low temperature plasma-induced surface modifications of polyacrylonitrile (PAN) and polysulfone (PSf) ultrafiltration (UF) membranes were studied. Treatment with water plasma and with He plasma drastically and almost permanently increased the surface hydrophilicity of PSf UF membranes. However, in contrast to the behavior of PAN UF membranes [23], the PSf surface pore structure was also changed as indicated by altered water permeabilities and reduced protein retentions. The lower permeability PSf membranes (nominal Mw cut-off 10 kD) showed slower but more extended conversion due to plasma excitation and stronger indications of pore etching effects in comparison with 30 kD cut-off membranes. Polymer peroxides on PAN and PSf membranes created by plasma excitation were monitored by the 2,2-diphenyl 1-picryl hydrazyl (DPPH) assay. Graft polymerization of hydrophilic monomers such as 2-hydroxy-ethyl methacrylate (HEMA) and acrylic or methacrylic acid onto PAN and PSf UF membrane surfaces was initiated via thermal decomposition of peroxides. The degree of modification could be adjusted by polymerization conditions. Graft polymer modified surfaces were characterized with the help of Fourier transform infrared attenuated total reflection (FTIR-ATR) and electron spectroscopy for chemical analysis (ESCA) spectra. The hydrophilic character of the modified surfaces was increased as compared to that of the parent membranes. With about 1–1.4 mmol/cm2 grafted HEMA, the contact angles (captive bubble technique; Θoctane/water) for PAN and PSf were reduced from 48 to 34° and from 92 to 43°, respectively. A clear dependency of PAN UF membrane water permeability on the amount of grafted monomer was observed. The monomer type influenced the water permeation flux per mole of grafted acrylate via specific swelling of the graft polymer layer in water. Hydrophilic PAN membranes, modified either by plasma treatment [23] or HEMA graft polymerization, showed significantly reduced fouling due to static protein adsorption, and improved protein UF performance. In particular, for water plasma treated PAN membranes with high initial retention, higher fluxes (up to 150%) with the same or even improved retentions were obtained. Hydrophilized PSf-g-HEMA membranes can provide improved performance in protein ultrafiltration over unmodified PSf UF membranes because pore etching effects are compensated for by the grafted layer yielding both improved filtrate flux (>30%) and protein retention of bovine serum albumin. Hence, plasma induced graft polymer modification of UF membranes can be used to adjust membrane performance by simultaneously controlling the surface hydrophilicity and permeability.

Photo-induced graft polymerization surface modifications for the preparation of hydrophilic and low-proten-adsorbing ultrafiltration membranes☆

Abstract Heterogeneous surface modifications of PAN UF membranes with either simultaneous or sequential UV irradiation-initiated graft polymerizations of monomers from water solutions were studied. Previous coating with a photo-initiator (benzophenone, BP) and saturation of the monomer solutions with BP mainly promoted simultaneous graft polymerization onto the membrane surface. Photo-induced formation and thermal decomposition of peroxides were assayed with the DPPH assay and then used for sequential grafting. For both approaches, and with acrylic acid, 2-hydroxyethyl methacrylate and various poly(ethylene glycol) methacrylates (PEG-MAs and MePEG-MAs) the impact of photo-initiation as well as monomer type and concentration was described. The polymerization conditions could be used as means to adjust the total degree of graft polymerization (DG) and the graft polymer chain length. Modified membranes were characterized with FTIR-ATR spectroscopy and contact angle measurements; the main conclusion was that even at high DG (1–2 mg/cm2) the modified layer on the outer surface was thin ( 400 μg/cm2) reduced contact angles, very little protein adsorption and almost no fouling due to BSA adsorption were observed. Results of UF experiments with γ-globulins. BSA and cytochrome C, applied as single and mixed protein solutions, suggested that with PAN-g-MePEG200MA, in contrast to the unmodified PAN UF membranes, protein/protein UF separations may become feasible because protein/polymer surface interactions are diminished.

Photograft-polymer-modified microporous membranes with environment-sensitive permeabilities

Abstract Intraporous heterogeneous modification of commercial nylon (Ny) and polypropylene (PP) microfiltration membranes, both of 0.2 μm pore size, with grafted poly(acrylic acid) (g-PAA) was accomplished by first coating the membranes with a photoinitiator, benzophenone (BP), and then UV irradiation in acrylic acid (AA) solutions in water. The degree of modification (DG) depended strongly on AA concentration (cAA) and UV time. As estimated from PAA homopolymer GPC analyses, average degrees of graft polymerization between 680 and 2200 were achieved by varying cAA between 10 and 50 g/l. PP-g-PAA and Ny-g-PAA membranes were characterized with scanning electron microscopy, revealing outer and intraporous surface coverage with g-PAA. FTIR-ATR and energy-dispersive X-ray spectroscopy data verified the chemical composition and a gradient of DG over membrane thickness, but modification also of the bottom layer. Membrane swelling and permeabilities depending on DG and pH — above and below pKgPAA — were studied. PP-g-PAA membranes were almost dimensionally stable, and with intermediate DG (around 1 mg/cm2) the ‘switch height’ of transmembrane permeability as function of pH was very high (by a factor of 100, even in 100 mM buffer). Ny-g-PAA membranes markedly changed the shape due to modification, swelling and additional pH changes; the mechanical stability — especially at DG > 1 mg/cm2 — was poor, and the permeability response to pH was less pronounced. Thus, the differences in membrane material hydrophilicity caused different modification surface selectivity, which for Ny was reduced due to sorption of BP and AA during coating and polymerization, respectively. Two different types of graftpolymer-modified microporous membranes resulted: almost ‘perfect’ pore filling for PP-g-PAA, but simultaneous matrix and pore modification for Ny-g-PAA.

Novel photochemical surface functionalization of polysulfone ultrafiltration membranes for covalent immobilization of biomolecules

The major objective of the work was to develop a heterogeneous modification method for attachment of reactive groups, suitable for covalent immobilization of active biomolecules on the surface of polysulfone ultrafilters without loss of membrane selectivity. For applying a polymer specific activation chemistry, the materials of commercial “polysulfone” UF membranes were identified using elemental analysis along with 1H NMR, FTIR-ATR and UV spectroscopy. Heterogeneous photoinitiated graft polymerization was realized using acrylic acid (AA) as model monomer and as carrier of reactive groups. Polymer structure (polysulfone, PSf, or polyethersulfone, PESf), coating with photoinitiator (benzophenone, BP, or benzoylbenzoic acid, BPC) and UV excitation energy (λexc220 > 300 or 350 nm) were the major parameters. Grafted polyAA (g-PAA) could be obtained under almost all conditions but with largely varying yields (DG). However, only with λexc350 nm, polymer and pore degradation could be excluded. A new selective initiation of graft polymerization onto PSf, H-abstraction by photoexcited BP derivatives from the methyl side groups, thus avoiding polymer chain scission, was proved indirectly. Modified structures were characterized spectroscopically, including visualization with SFM of laterally patterned surfaces generated by UV irradiation through a mask. UF tests of PSf-g-PAA and PESf-g-PAA UF membranes (DG ∼ 100…150 μg/cm2), prepared under “mildly degrading” conditions (λexc300 nm), indicated only slight permeability and selectivity changes compared with unmodified samples. Selective PSf functionalization (BPC coating, λexc350 nm; DG 5 μg/cm2) caused flux reductions and dextran selectivity increases by factors of ∼ 1.3. Covalent immobilization onto g-PAA-functionalized and carbodiimide-activated PSf or PESf membrane surfaces was studied with a protein (BSA), an enzyme (invertase, INV), an antibody-enzyme (IgG-POD) conjugate, and a peptide (“PC1”) as specific antigen of a monoclonal antibody. High binding capacities, up to 40 fold compared with a flat unmodified surface, were detected either directly (BSA) or indirectly via specific activity/binding assays (INV, IgG-POD, “PC1”). This indicated an increased outer membrane surface area due to multifunctional reactive and hydrophilic grafted polymer chains.

Impedometric herbicide chemosensors based on molecularly imprinted polymers

The technique of grafting polymerization has been used for preparation of thin films of molecularly imprinting polymers on the surface of polypropylene membranes and on hydrophobized gold electrodes. The herbicide desmetryn was used as a template. The solid supports used were hydrophobic, while the polymer was hydrophilic. The adsorbed layer of benzophenone, irradiated by UV-light, initiated a radical polymerization near the surface. Polymer films were characterized by weighing, contact angle measurements and impedance spectroscopy. The electrodes coated with the molecularly imprinted polymers displayed fairly specific binding of desmetryn, as detected by the decrease in the capacitance of the electrode. Only small capacitive effects were observed on addition of terbumeton or atrazine, while metribuzine displayed capacitance decrease similar to desmetryn.

Chemically and morphologically defined ultrafiltration membrane surfaces prepared by heterogeneous photo-initiated graft polymerization

Polyacrylonitrile (PAN) ultrafiltration (UF) membranes with varied surface hydrophilicity and charge have been prepd. by heterogeneous photo-initiated graft polymn. of various acrylates or methacrylates having polyethyleneglycol (PEG), carboxyl, sulfopropyl, dimethylaminoethyl or trimethylammoniumethyl side groups. A series of photo-modified membranes with about the same water permeabilities were obtained by selection of initial membrane pore size and appropriate modification conditions (monomer concn. and UV irradn. time, lexc>300 nm). A comprehensive characterization of the new membranes was achieved using a multi-method approach. Gravimetry, FTIR-ATR spectroscopy, SEM and UF expts. provided basic information about the degree of graft polymer modification (DG, up to 1 mg cm-2) and integrity of the macroporous membrane structure after modification. No extra layer (with a thickness >1 mm) was deposited on top of the UF membranes but, depending on DG, active layer pores were more or less covered, causing reduced water permeabilities and enhanced solute selectivities. The modified membranes were monitored with scanning force microscopy (SFM), contact angle (CA) and streaming potential (ZP) measurements. The water-swollen graft polymer layers had a monomer-specific texture with larger grain size, but in the sub-micrometer range, compared with the unmodified membranes (SFM). However, interface interactions in terms of zeta potential (ZP) and hydrophilicity (CA) were unambiguously governed by the chem. of the graft polymer side groups. Solid surface tensions (wetting/dewetting) from advancing and receding CAs, gsv (adv.)/gsv (rec.), ranged from 35 mN m-1/60 mN m-1 for PAN-grafted-poly(dimethyl-aminoethyl methacrylate) to 71.5 mN m-1/71.5 mN m-1 for PAN-grafted-poly(sulfopropyl acrylate) and PAN-grafted-poly(tri-methylammoniumethyl methacrylate), whereas unmodified PAN membranes had 49 mN m-1/67 mN m-1. Concn. potential measurements revealed that ion-exchange graft-polymer-modified PAN membranes behave as barrier for co-ions via a Donnan exclusion mechanism. All anal. data enabled a classification of the photo-modified UF membranes into permanently anionic (sulfonic acid) or cationic (trimethylammonium) as well as ionizable (carboxyl, dimethylamino) or non-ionic hydrophilic, flexible (PEG) types. The results provide guidelines for the development of new low-fouling UF and nanofiltration membranes.

Molecular imprinting of cellulose acetate-sulfonated polysulfone blend membranes for Rhodamine B by phase inversion technique.

A functional polymer, sulfonated polysulfone (SPS) with a degree of substitution of 0.10, was prepared and then blended with cellulose diacetate (CA) as the matrix polymer for the preparation of molecularly imprinted polymer (MIP) membranes via phase inversion from a casting solution containing a template. Polyethylene glycol was found to be compatible with these polymers and was subsequently used as pore forming agent. Optimization studies with the aim to enhance the membrane permeability, were carried out with respect to solvent, polymer blend composition and total polymer concentration. The effects onto pore structure were also studied by membrane water uptake and specific surface area measurements. In conclusion from these investigations, dimethyl sulfoxide was chosen as the solvent, the CA/SPS composition was maintained at 90/10 (w/w) to minimize non-specific template binding and the total polymer concentration was set at 13 wt.%. MIP membranes based on CA/SPS blends with Rhodamine B (RhB) as template were prepared using the optimized conditions. Results for re-binding of RhB during filtration through MIP as well as Blank membranes, prepared without RhB, provide evidence for surface imprinting of the porous membranes.

Novel high performance photo-graft composite membranes for separation of organic liquids by pervaporation

A new application of the ‘pore filling’ concept yielded high-performance composite membranes for the selective pervaporation (PV) separation of organic mixtures. Asymmetric polyacrylonitrile (PAN) membranes (average pore sizes of 7 or 12 nm) were used as matrix for polymeric PV separation phases which were in situ prepared by heterogeneous photoinitiated graft copolymerization. By this means, defect-free and stable layers were synthesized from various moderately hydrophilic (meth)acrylates, e.g. poly(ethylene glycol) (meth)acrylates. The impact of (meth)acrylate side-group functionality (hydrophilicity, size) and preparation parameters (monomer concentration, UV irradiation time) was analyzed using PV methanol removal from less polar hydrocarbons (cyclohexane or MTBE) as example. High selectivities (αmethanol/cyclohexane ⩽ 2000) and extraordinarily high permeate fluxes (J ⩽ 8 kg/m2 h) were achieved. Major reasons for the excellent performance were the small effective PV barrier thickness (< 1 μm; determined by the thickness of the selective layer of the matrix membrane) and the fixation of the graft polymer (covalent anchoring, suppressed swelling) in a matrix (PAN) which is not swollen under PV conditions. Due to the functionalization principle, a great variety of thin-layer composite membranes can be prepared, ‘tuning’ the PV selectivity by functionality of the graft polymer, thus providing possibilities to efficiently solve various PV separation problems.

Influences of solution chemistry and polymeric natural organic matter on the removal of aquatic pharmaceutical residuals by nanofiltration.

This study demonstrates the removal efficiency and the permeate flux behavior during cross-flow nanofiltration (NF) of aqueous solutions of five pharmaceutically active compounds (PhACs). Cephalexin, tetracycline, acetaminophen, indomethacin and amoxicillin were used as models of PhACs, and alginate was selected as model of natural organic matter (NOM). Two commercial composite NF membranes (SR2 and SR3) with different characteristics were used. The highest rejection was observed for tetracycline, i.e., 75-95% for membrane SR 2 and 95-100% for membrane SR 3, while the rejection was least for acetaminophen (32-36% for SR2 and 52-59% for SR3). As the pH of acetaminophen solution was increased (from 6 to 9) the rejection would increase. Changes of ionic content (from 10 to 20mM) lead to increase (from 89 to 93% for SR 3) or decrease (from 100 to 91% for SR2) of cephalexin rejection depending on the membrane used. The permeate flux would decrease with decreasing the pH solution and increasing ionic strength. The addition of alginate in the feed stream decreased the permeate flux, with lower reduction for SR3, and increased the PhAC rejection except for acetaminophen and amoxicillin. Both size and Donnan exclusions seemed to occur, and the effect of Donnan exclusion was more pronounced for the NF membrane having larger effective pore size (SR2).

Immobilization of enzymes in photochemically cross-linked polyvinyl alcohol.

Invertase and amyloglucosidase were entrapped in polyvinyl alc. membranes through UV irradn. of pendent styrylpyridinium groups. The influence of crosslinking on immobilization efficiency was studied using prepolymers with varied cross-linker d., the above mentioned enzymes of different mol. wt., and various substrates. It was found that the larger enzyme invertase is effectively immobilized even in polymers with very low contents of the crosslinking component. In contrast for an effective immobilization of amyloglucosidase, a higher degree of crosslinking is necessary. Although there is only a slight loss in amyloglucosidase activity, the apparent activity, esp. for the macromol. substrate starch, is low. This is contributed to a hindered diffusion of the substrates in the swollen hydrogel matrix. The influence of the diffusion is also reflected in the kinetic parameters Km and Vmax. The pH and temp. optima of entrapped amyloglucosidase are similar to those of the native enzyme in soln.