2 The Art of Making Polymeric Membranes
2.4 Modification of Membranes
The use of synthetic materials in biomedical applications has increased dramatically during the past few decades. Surface properties of polymers are of fundamental impor-tance in many branches of industrial applications (e.g., separation of gasses, liquid mix-tures, bonding, coating, adhesion, etc.). Performances of membranes also depend on the properties of their surfaces, since membrane performance is strongly influenced by the surface phenomena. Hence, it is very natural that much attention has been paid to the membrane surface modification. Surface contamination which may lead to dete-rioration in membrane performance is also known to be governed by the membrane surface properties. Detailed methods for modification of synthetic membranes are discussed by Khulbe et al. [63]
2.4.1 Modification of Polymeric Membrane by Additives/Blending
The blending or simple mixing of polymers is an easy and inexpensive method of modi-fying various properties of a polymer such as flexibility, heat distortion and process-ability. Property of polymer blend may be directly related to compatibility or miscibility of polymers forming a blend [64]. It was reported by Clark et al. [64] that the surface composition of homopolymer blend was governed by a combination of the homopoly-mer molecular weight and degree of crystallinity.
The PES-based membranes show high permeability for low molecular weight pro-teins when used as hemodialysis membranes. Due to this characteristic, PES mem-branes are widely employed in biomedical fields such as artificial organs and medical devices used for blood purification, e.g., hemodialysis, hemodiafiltration, hemofiltra-tion, plasmapheresis and plasma collection [3]. When contacting with blood, proteins are rapidly adsorbed onto the surface of PES membrane and protein layer which may lead to undesirable results, such as platelet adhesion, aggregation and coagulation. This phenomenon is due to the relatively hydrophobic character of PES membrane. Thus, the blood compatibility of PES membrane is not adequate, and injections of anticoagulants are needed during its clinical applications. To make PES membrane more hydrophilic
50 Handbook of Polymers for Pharmaceutical Technologies
from hydrophobic, another hydrophilic polymer such as poly(vinylpyrrolidone) (PVP) is added (blended).
To make the modified surface properties (hydrophilic/hydrophobic) more perma-nent, surface-modifying macromolecules (SMMs) were developed. SMM has an amphi-phatic structure consisting theoretically of a main polyurethane chain terminated with two low polarity polymer chains (i.e., fluorine segments) [65].
It is known that, in a polymer blend, thermodynamic incompatibility between poly-mers usually causes demixing of polypoly-mers to occur. If the polymer is equilibrated in air, the polymer with the lowest surface energy (hydrophobic polymer) will concentrate at the air interface and reduce the system’s interfacial tension as a consequence. The pref-erential adsorption of a polymer of lower surface tension at the surface was confirmed by a number of researchers for miscible blend of two different polymers. Based on this concept, surface modifying macromolecules (SMMs) as surface-active additives were synthesized and blended into polymer solutions of PES. Depending on the hydropho-bic [66] or hydrophilic [67] nature of the SMM, the membrane surface becomes either more hydrophobic or hydrophilic than the base polymeric material.
2.4.2 Coating
The membrane surface can be modified by contacting the surface of one side of the polymeric (A) membrane with a solution of a different polymer (B). A thin layer of polymer (B) is left on top of the membrane of polymer (A) after solvent evaporation.
Some post-treatment can also be applied.
2.4.3 Surface Modification by Chemical Reaction
The membrane surface can also be modified by chemical reaction. It is reported that direct fluorination can be effectively used to enhance commercial properties of polymer articles, such as barrier properties of polymer vessels, bottles, and packaging films and envelopes;
gas-separation properties of polymer membranes; adhesion and printability properties of polymer articles; and mechanical properties of polymer-based composites [68].
Dai et al. [69] modified the surface of microporous polypropylene (PP) membranes with phospholipid polymer using a new economic and convenient method. The process included the photo-irradiated graft polymerization of N-N-dimethylaminoethyl meth-acrylate (DMAEMA) and the ring-opening reaction of the grafted polyDMAEMA with 2-alkyloxy-2-oxide-1,3,2-dioxophospholanes (AOP). The FTIR spectra confirmed the chemical changes of the membrane surface and supported that PP membrane with excellent blood compatible surface could be fabricated by their novel method.
2.4.4 Interfacial Polymerization (IP)/Copolymerization
As already explained in Section 3.15, this is a process of polymerization in which two reactive monomers, each dissolved in different solvents that are mutually immiscible, react at the interface between the two solutions. This process provides a method for depositing a thin layer upon a porous support. In this case, a polymerization reaction occurs between two very reactive monomers (or one prepolymer) at the interface of two
The Art of Making Polymeric Membranes 51
immiscible solvents. This technique is well established for the fabrication of RO and NF membrane.
Chu et al. [70] demonstrated a simple and effective route for the hydrophilic sur-face modification of ceramic-supported PES membranes by synthesizing a poly(vinyl alcohol) (PVA)/polyamide (PA) composite thin surface layer with an interfacial polym-erization method (IP) method. The reaction of the interfacial polympolym-erization is sche-matically shown in Figure 2.7. A prepared tubular ceramic-supported PES membrane (both ends sealed) was immersed in a terephthaloyl chloride solution in benzene and
Figure 2.7 Reaction of interfacial polymerization of fabricating PVA/PA composite surface layer [70].
Table 2.1 Experimental design for surface modification of PES membranes.
Sample No. Description of modification method
1 Virgin PES membrane
2 UV/ozone for 20 min and 20% PVA grafting 3 UV/ozone for 20 min and 20% PEG2000 grafting 4 UV/ozone for 20 min and 1% chitosan grafting 5 Interfacial polymerization using 1% PVA 6 Interfacial polymerization using 1% PEG2000 7 Interfacial polymerization using 1% chitosan
52 Handbook of Polymers for Pharmaceutical Technologies
dimethyl benzene. After removing it from the solution, it was immersed (after drying) in the aqueous solution containing PVA and PA to form a composite membrane surface layer via interfacial polymerization.
Liu and Kim [71] modified the PES membrane surfaces using grafting and IP via UV/ozone pretreatment to graft PVA, PEG and chitosan on three samples of PES, and coating PVA, PEG and chitosan layers through interfacial polymerization. The descrip-tion of the modificadescrip-tion experimental design is shown in Table 2.1.
2.4.5 Plasma Polymerization/Treatment
Plasma polymerization process is a technique that allows us to obtain highly crosslinked polymers from nonfunctional monomers that are not utilized in conventional polymer synthesis. Plasma surface modification can improve biocompatibility and biofunctionality.
In the plasma surface modification process, glow discharge plasma is created by evacuating a plasma reactor, usually made of quartz because of its inertness, and then refilling it with a low pressure gas. The gas is then energized using techniques such as radiofrequency energy, microwaves, alternating current, or direct current. The energetic species in gas plasma include ions, electrons, radicals, metastable species, and photons in the shortwave ultraviolet (UV) range. When membrane surfaces are brought into contact with gas plasmas the surfaces are bombarded by these energetic species, and their energy is transferred from the plasma to the solid. As a result, the surface of the membrane is etched, leaving many reactive sites (mostly radicals) on the surface. When an organic vapor or a monomer is introduced into the plasma reactor, polymerization takes place at the reactive sites. This is called plasma polymerization or plasma-chemi-cal modification. Plasma modification is generally used to introduce functional groups to the membrane surface.
Inorganic gas plasma is known to promote the implantation of atoms, radical generation, and etching reactions, and is called a nonpolymer-forming plasma. It is reported that highly reactive particles from gas plasma can etch a surface very gradu-ally. Nitrogen-based plasma inorganic systems such as N2, NH3, Ar/NH3, and O2/NH3 are used to produce hydrophilic, low-fouling membranes [72].
2.4.6 Surface Modification by Irradiation of High Energy Particles
Irradiation plays an important role in modifying the polymer materials. The interaction of the ions with polymer leads to bond breaking, formation of free radicals and various phenomena that are induced by the complex secondary chemical processes along the trajectory of the ions [73,74]. Electron beams have established themselves as potential tools both in basic as well as applied sciences. The properties of polymers/polymeric membranes can be tuned by various techniques like plasma etching, irradiation with photons, ions, and electrons [75]. Since high energy electromagnetic and particle radi-ation exhibit properties of controlled penetrradi-ation and intensities, they are especially suitable for synthesis and modification of polymeric biomaterials/membranes without the need of usually toxic additives. These methods are being used for the synthesis of functional polymers in the forms of macro- and microgels, micro- and nanospheres, and functionalization of surfaces.
The Art of Making Polymeric Membranes 53 2.4.7 UV Irradiation
Treatment with UV/ozone has been used as a means of removing organic contaminants from different polymer surfaces. However, UV/ozone treatment has also been used to increase the wettability of poly(ethylene terephthalate) (PET), polyethylene (PE), polypropylene, different rubbers (vulcanized styrene-butadiene-SBR, unvulcanized styrene-butadiene-SBS). This UV/ozone treatment results in an increase in the surface energy of the polymer through oxidation of the polymer.
2.4.8 Ion-Beam Irradiation
Ion-beam irradiation, even at a small dose, alters the microstructure of the surface layer of the polymer, and high fluence of irradiation results in a large number of small-size microvoids in the surface.
Xu and Coleman [76] modified the 6FDA-pMDA (polyimide) films by irradiat-ing ion beam and studied the structure and morphology by AFM. The AFM images data indicated that free-standing polyimide films had deep surface valleys which could extend to a depth of several micrometers.
2.4.9 Surface Modification by Heat Treatment
Membrane surfaces can also be modified by heat treatment. The PES HFMs were pre-pared by dry-wet spinning method and heated in an oven at 120, 150, and 180oC. The membrane shrank by heating. It was noticed that pore size decreased from 8.16 nm for untreated hollow fiber to 3.8 nm with 1 minute heating and then increased to about 6 nm with 5 min heating at 150oC. With an increase in heating temperature, the pore size of the membrane decreases [77,78]. Charkoudian et al. reported increased levels of pro-tein adsorbed in thermally treated poly-acrylamide-modified PVDF microporous mem-branes in comparison to thermally untreated polyacrylate-modified memmem-branes [79].
2.4.10 Graft Polymerization/Grafting
Grafting can also be applied for the surface modification of the membrane. Although the method should work for any polymeric materials, most of the recent works on membrane surface graft polymerization were on polyamide thin-film composite (TFC) membranes or porous polypropylene membranes.
2.4.11 Other Techniques
Several surface modification techniques that do not belong to any of the above methods are summarized below.
Polyimide membranes were modified by immersing the films in the diamine/metha-nol solution for a stipulated period of time (crosslinking) [80]. Maekawa et al. [81]
modified the internal surfaces of the pores of poly(ethylene terephthalate) membranes using the alkylation reaction of the carboxylic acids on surfaces. Reid et al. [82] modi-fied the surface of poly(3-(2-acetoxyethyl)thiophene) membranes through surface
54 Handbook of Polymers for Pharmaceutical Technologies
hydrolysis under both basic and acidic conditions. Yu et al. [83] modified the surface of poly(L-lactic acid) (PLLA) membrane in a layer-by-layer (LBL) self-assembly man-ner for the improvement of hydrophilicity, antibacterial activity, etc., via polyelectro-lyte multilayer (PEM) immobilization. This membrane can be used for application in hemodialysis devices.
Molecular imprinting technology (MIT) is also a method used for the modifica-tion of membrane surfaces. This technology allows preparing polymeric materials with specific binding sites toward target molecules through polymerization or phase inver-sion in the presence of template. This can be achieved if the target is present during the polymerization process, thus acting as a molecular template. Monomers carrying cer-tain functional groups are arranged around the template through either noncovalent or covalent interactions. Following polymerization with a high degree of crosslinking, the functional groups are held in position by the polymer network. Subsequent removal of the template by solvent extraction or chemical cleavage leaves cavities that are comple-mentary to the template in terms of size, shape and arrangement of functional groups.
These highly specific receptor sites are capable of rebinding the target molecule with a high specificity, sometimes comparable to that of antibodies. Molecularly imprinted polymers have therefore been dubbed “antibody mimics.” It has been shown that they can be substituted for biological receptors in certain formats of immunoassays and bio-sensors. They have also been used as stationary phases for affinity separations, for the screening of combinatorial libraries, and as enzyme mimics in catalytic applications.