Hydrophobic Polymer Surfaces Research Articles

Size-Selective Protein Adsorption to Polystyrene Surfaces by Self-Assembled Grafted Poly(ethylene Glycols) with Varied Chain Lengths

We report about the surface modification of polystyrene (PSt) with photoreactive alpha-4-azidobenzoyl-omega-methoxy poly(ethylene glycol)s (ABMPEG) of three different molecular weights (MWs of approximately 2, approximately 5, and approximately 10 kg/mol) and with two poly(ethylene glycol)/poly(propylene glycol) triblock copolymers (PEG-PPG-PEG) of about identical PEG/PPG ratio (80/20, w/w) and MW(PEG) of approximately 3 and approximately 6 kg/mol, all via adsorption from aqueous solutions. For ABMPEGs, an additional UV irradiation was used for photografting to the PSt. Contact angle (CA) and atomic force microscopy data revealed pronounced differences of the hydrophilicity/hydrophobicity and topography of the surfaces as a function of PEG type and concentration used for the modification. In all cases, an incomplete coverage of the PSt was observed even after modification at the highest solution concentrations (10 g/L). However, clear differences were seen between PEG-PPG-PEGs and ABMPEGs; only for the latter was a nanoscale-ordered interphase structure with an influence of MW(PEG) on the PEG density observed; after modification at the same solution concentrations, the density was significantly higher for lower MW(PEG). The adsorption of three proteins, myoglobin (Mgb), bovine serum albumin (BSA), and fibrinogen to the various surfaces was analyzed by surface plasmon resonance. Pronounced differences between the two PEG types with respect to the reduction of protein adsorption were found. At high, but still incomplete, surface coverage and similar CA, the shielding of ABMPEG layers toward the adsorption of Mgb and BSA was much more efficient; e.g., the adsorbed Mgb mass relative to that of unmodified PSt was reduced to 10% for ABMPEG 2 kg/mol while for both PEG-PPG-PEGs the Mgb mass was still around 100%. In addition, for the ABMPEG layers an effect of MW(PEG) on adsorbed protein mass-decrease with decreasing MW-could be confirmed; and the highest Mgb/BSA selectivities were also observed. A "two-dimensional molecular sieving", based on PEG molecules having a nanoscale order at the hydrophobic substrate polymer surface has been proposed, and the main prerequisites were the use of PEG conjugates which are suitable for an "end-on" grafting (e.g., ABMPEGs), the use of suitable (not too high) concentrations for the surface modification via adsorption/self-assembly, optionally the photografting on the substrate (possible only for ABMPEG), and presumably, a washing step to remove the excess of unbound PEGs. The results of this study also strongly support the hypothesis that the biocompatibility of hydrophobic materials can be very much improved by PEG modifications at surface coverages that are incomplete but have an ordered layer structure controlled by the size and steric interactions of surface-bound PEGs.

Surface Grafting on Polymer Surface Using Physisorbed Free Radical Initiators

A new strategy for creation of radicals on an inert polymer surface has been developed. This new method combines free radical polymerization with a surface graft polymerization. The polymer film, poly(dimethylsiloxane) (PDMS), was selected as an inert model material and was immersed into acetone containing azo-bis-isobutyrylnitrile (AIBN). We speculate that the AIBN becomes physically adsorbed or partially penetrates the hydrophobic polymer surface. The polymerization of water-soluble monomers was initiated by the absorbed initiator and polymer surface became modified by the hydrophilic monomers. We speculate this process was facilitated by the poor diffusion of physisorbed radicals derived from AIBN into an aqueous medium. ATR−IR, XPS, and contact angle measurements were used to characterize the surface modification. This method provides a potential modification method for the coating of many polymer surfaces possessing complicated geometrical shapes. The principle relies on the insolubility of the physi...

Modes of conformational changes of proteins adsorbed on a planar hydrophobic polymer surface reflecting their adsorption behaviors

Infrared spectra of hen egg white lysozyme and bovine serum albumin (BSA) adsorbed on a solid poly tris(trimethylsiloxy)silylstyrene (pTSS) surface in D 2O solution were measured using attenuated total reflection (ATR) Fourier transform infrared spectroscopy. From the area and shape of the amide I′ band of each spectrum, the adsorption amount and the secondary structure were determined simultaneously, as a function of adsorption time. We could show that the average conformation for all the adsorbed lysozyme molecules was solely determined by the adsorption time, and independent of the bulk concentration, while the adsorption amount increased with the bulk concentration as well as the adsorption time. These results suggest that lysozyme molecules form discrete assemblies on the surface, and that the surface assemblies grow over several hours to have a definite architecture independent of the adsorption amount. As for BSA, the extent of the conformational change was solely determined by the adsorption amount, regardless of the bulk concentration and the adsorption time. These differences in the adsorption properties of lysozyme and BSA may reflect differences in their conformational stabilities.

Spontaneously Formed Hydrophilic Surfaces by Segregation of Block Copolymers with Water-Soluble Blocks

A block copolymer of deuterated polystyrene (dPS) and 2-[2-(2-methoxyethoxy)ethoxy]ethyl methacrylate (PME3MA) spontaneously exposes the PME3MA block, which is soluble in water, to the surface in a vacuum. dPS−PME3MA mixed with polystyrene (PS) segregates to the PS surface and changes the hydrophobic PS surface into hydrophilic surface. Neutron reflectivity, X-ray photoelectron spectroscopy, and dynamic secondary ion mass spectrometry probe the surface segregation of dPS−PME3MA to the surface of PS due to its PME3MA block. Sum-frequency generation vibrational spectroscopy detects exclusively the triethylene oxide (3EO) side chains of PME3MA regardless of a small fraction of dPS−PME3MA in the mixture, suggesting the conformation of the side chains influencing the segregation behavior. Because of the amphiphilic character of the methoxy-terminated 3EO side chains, the PME3MA block exposes the terminal methyl groups of the side chains to the surface to reduce the free energy of the system. However, such a hydrophobic methyl surface layer barely covers the EO part of the side chains; the surface quickly reconstructs and becomes hydrophilic upon contact with water. This is a technologically important demonstration of converting a hydrophobic polymer surface to a hydrophilic surface just by adding a small amount of block copolymer.

Wetting characteristics of aqueous rhamnolipids solutions

The wetting properties of surfactants on solid surfaces form the basis of many industrial and biological processes. The preferential adsorption of the surfactants from aqueous solutions onto solid surfaces alter the adhesion tension of the surface and this behavior may cause partial to complete wetting of the surfaces by the aqueous surfactant solutions. However, different types of surfactants show different wetting characteristics. To study the wetting properties of biologically produced rhamnolipids (RL), advancing contact angles of the aqueous solutions of the RL mixture of R 1 and R 2 in a ratio of R 2/ R 1 = 1.1 were measured as a function of surfactant concentration. For a comparison of the wetting performance, sodium dodecyl sulfate (SDS) was chosen as the reference surfactant. A hydrophilic glass surface, a hydrophobic polymer, polyethylene terephthalate (PET), and gold surface were used as the solid surfaces to determine the wetting characteristics of rhamnolipids. At low surfactant concentrations (RL concentration <3 × 10 −5 M, SDS concentration <3 × 10 −4 M) contact angle ( Θ) varied in a certain range depending on the character of the surfactant interactions with the surface. This was followed by a decrease in contact angle. Parallel to this behavior, at low surfactant concentrations the adhesion tension decreased, then remained constant and an increase at higher surfactant concentrations was obtained on hydrophobic surfaces. On hydrophilic surfaces a steady decrease in adhesion tension was observed with both surfactant solutions.

Force measurements between Teflon AF and colloidal silica particles in electrolyte solutions

The interaction force between a very hydrophobic polymer surface and colloidal silica particles with a roughness of 10–15 nm has been measured in aqueous solutions of KOH and KCl using an atomic force microscope. The interaction can be described according to the DLVO theory by an electrical double-layer force that is repulsive at long distances and attractive at short distances and an attractive van der Waals force. The electrical double-layer potentials are compared to the zeta potentials of Teflon AF and the silica spheres. The roughness of the silica particles leads to an underestimation of the short-range attraction and the surface potential. Both KCl and KOH solutions affect the potential of the interacting surfaces. OH − ions that adsorb preferentially to the Teflon AF surface create higher potentials than Cl − ions. Range and strength of the attractive interaction are not affected by KCl solutions but reduced by addition of KOH. This can be explained by decreasing potential differences between the silica sphere and Teflon AF with increasing KOH concentration. In addition, the preferential adsorption of OH − ions may lead to a reduction of the van der Waals interaction. The presence of nanobubbles, too, might play a role.

Protein-mediated boundary lubrication in arthroplasty

Wear of articulated surfaces can be a major lifetime-limiting factor in arthroplasty. In the natural joint, lubrication is effected by the body's natural synovial fluid. Following arthroplasty, and the subsequent reformation of the synovial membrane, a fluid of similar composition surrounds the artificial joint. Synovial fluid contains, among many other constituents, a substantial concentration of the readily adsorbing protein albumin. The ability of human serum albumin to act as a boundary lubricant in joint prostheses has been investigated using a pin-on-disc tribometer. Circular dichroism spectroscopy was employed to follow the temperature- and time-dependent conformational changes of human serum albumin in the model lubricant solution. Effects of protein conformation and polymer surface hydrophilicity on protein adsorption and the resulting friction in the boundary lubrication regime have been investigated. Unfolded proteins preferentially adsorb onto hydrophobic polymer surfaces, where they form a compact, passivating layer and increase sliding friction—an effect that can be largely suppressed by rendering the substrate more hydrophilic. A molecular model for protein-mediated boundary friction is proposed to consolidate the observations. The relevance of the results for in vivo performance and ex vivo hip-joint testing are discussed.

Nanobubbles give evidence of incomplete wetting at a hydrophobic interface

The appearance of a hydrophobic surface, namely a crystalline (111) Si wafer coated with a thick soft polystyrene film, and the morphological changes along this interface depending on the polarity of an adjoining liquid phase were studied with magnetic tapping mode atomic force microscopy. Interfacially associated nanobubbles of decreasing size and number are observed as the hydrophobicity of the subphase increases. The disturbance of the water structure in the contact region induces the formation of nanobubbles. The topology of the interface is visualized, starting with the dry polymer under normal atmosphere conditions and observing the changes as air is replaced by a series of liquids. With water, the surface coverage of the substrate with bubbles is almost a close-packed configuration. The bubble shape is well approximated by spherical caps of a rather low aspect ratio. The Gaussian size distributions of bubble shape parameters are discussed. The contact angle of the nanobubbles is substantially smaller than the corresponding number measured for a macroscopic droplet. This apparent discrepancy might be resolved if the nanobubbles were assumed to exist along the interface as a connecting sublayer between a depleted water film at the hydrophobic polymer surface and an adsorbed macrodroplet.

Dynamic Alterations of Fibronectin Layers on Copolymer Substrates with Graded Physicochemical Characteristics

Desorption and exchange of preadsorbed fibronectin layers in pure buffer solution and solutions of human serum albumin or fibronectin, respectively, were studied in dependence on the physicochemical characteristics of maleic acid copolymer films used as substrates. Although the preadsorbed amount of fibronectin differed only slightly, the protein was found to exhibit a significantly enhanced anchorage at the more hydrophobic polymer surface as compared to the more hydrophilic and more negatively charged polymer surface. The preadsorbed fibronectin layer was most efficiently exchanged by fibronectin (i.e., in the homodisplacement process) while pure buffer solution and human serum albumin solutions induced desorption or exchange of fibronectin to lower and similar degrees. An increase of the total adsorbed amount of protein due to additional adsorption of fibronectin or human serum albumin accompanied the partial exchange of the preadsorbed fibronectin in the displacement experiments. Evaluation of the kinetics of desorption and exchange of fibronectin at any of the substrates revealed two kinds of surface-attached protein populations--a fast desorbing species and a species with a slow desorption and exchange rate. By a multivariate regression analysis the surface characteristics of the polymer substrate were confirmed to determine the degree of protein desorption and exchange while the dynamics of the layer alteration was found to solely depend on the diffusion behavior of the proteins.

Unusual wetting dynamics of aqueous surfactant solutions on polymer surfaces

Static and dynamic contact angles of aqueous solutions of three surfactants—anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (DTAB), and nonionic pentaethylene glycol monododecyl ether (C 12E 5)—were measured in the pre- and micellar concentration ranges on polymer surfaces of different surface free energy. The influence of the degree of substrate hydrophobicity, concentration of the solution, and ionic/nonionic character of surfactant on the drop spreading was investigated. Evaporation losses due to relatively low humidity during measurements were taken into account as well. It was shown that, in contrast to the highly hydrophobic surfaces, contact angles for ionic surfactant solutions on the moderately hydrophobic surfaces strongly depend on time. As far as the nonionic surfactant is considered, it spreads well over all the hydrophobic polymer surfaces used. Moreover, the results obtained indicate that spreading (if it occurs) in the long-time regime is controlled not only by the diffusive transport of surfactant to the expanding liquid–vapor interface. Obviously, another process involving adsorption at the expanding solid–liquid interface (near the three-phase contact line), which goes more slowly than diffusion, has to be active.

Adhesion of spray primers to plasma polymer coatings

AbstractThe adhesion failure of a primer on a plasma‐polymer‐deposited substrate surface could occur at the interface of the substrate/plasma polymer or at the interface of the plasma polymer/primer. We examined the adhesion of spray (solventborne or waterborne) primers on plasma polymer coated substrate with the substrate/plasma polymer systems in which the adhesion of the plasma polymer of trimethylsilane (TMS) to the substrate was good. Waterborne primers adhered well to quite hydrophobic plasma polymer surfaces, except the untreated surface of TMS. The plasma polymer of TMS appeared to contain a significant amount of oligomers, which were not in the three‐dimensional network of the plasma polymer coating and caused poor adhesion of primers. The plasma treatment of the deposited plasma polymer of TMS increased the adhesion characteristics of the primers dramatically. The oxygen‐plasma treatment, however, made the interface water sensitive and made the wet adhesion poor. The argon‐plasma treatment of the plasma polymer of TMS was found to be a viable process that yielded excellent adhesion of spray primers. The plasma polymer of TMS deposited by a closed system, without further plasma treatment, yielded an excellent foundation for the application of spray primers. Waterborne, nonchromated primers adhered exceedingly well and survived 8 h of boiling in water (with a scribed surface) and became virtually nonstrippable by conventional stripping agents. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1443–1457, 2002

Development of ultrafiltration membranes from acrylonitrile co-polymers

Hydrophobic polymer surfaces show higher tendency to protein adsorption and bacteria attachment, thus hydrophobic polymeric membranes foul rapidly in water purification operations. A change in membrane surface properties can reduce fouling; this may be accomplished by increasing the hydrophilicity of the membrane surface, and by using a membrane with smaller pore size. The ultrafiltration membranes were prepared via phase inversion process in our laboratory. Negatively charged hydrophilic ultrafiltration membranes were prepared from acrylonitrile-vinyl acetate (CP16)/Acrylonitrile-vinyl acetate-sodium p-sulfophenyl methallyl ether (CP24). Scanning electron microscopy (SEM) revealed the asymmetric structure of these membranes. The roughness of the surface was measured by atomic force microscopy (AFM). The basic characteristics of these membranes like water permeability, water content and membrane selectivity were also measured.

Protein patterning via radiation-assisted surface functionalization of conventional microlithographic materials

Several technological options available for protein high resolution patterning on conventional microlithographic materials were investigated: (i) two mechanisms for radiation-assisted selective attachment on polymer surface, i.e. chemical linkage and hydrophobicity-controlled patterning; (ii) two classes of microlithographic materials, i.e. diazo-naphto-quinone (DNQ)/novolak and poly( tert-butyl-methacrylate-co-methyl-methacrylate) [P(tBuMA-co-MMA)] resists; (iii) two pattern exposure techniques, i.e. contact printing using near-UV light on DNQ/novolak resist, and projection printing using e-beam lithography on P(tBuMA-co-MMA) resist; and (iv) two patterning tones, i.e. positive and negative tone lithography. The mechanism based on crosslinking protein amino groups to carboxylic groups generated by photolysis can be used to print (i) high contrast, positive or negative tone, protein features on DNQ/novolak resists; and (ii) good contrast, positive tone protein features on P(tBuMA-co-MMA) resist. The hydrophobicity-controlled mechanism based on the hydrophobic interaction between the protein and hydrophobic unexposed polymer surface can be used to print high contrast, negative tone, protein features on P(tBuMA-co-MMA) resist with 125 nm resolution. It was found that optical and e-beam lithography materials and techniques can be easily transferred in bio-microlithography, with impact on biodevices fabrication and combinatorial chemistry.

Synthesis, characterization and surface analysis using dynamic contact angle measurements of graft copolymers: poly(methyl methacrylate)-g-poly(dimethylsiloxane) and poly(methyl methacrylate)-g-poly(trifluoropropylmethylsiloxane)

A series of poly(methyl methacrylate)-g-poly(dimethylsiloxane) and poly(methyl methacrylate)-g-poly(trifluoropropylmethylsiloxane) copolymers have been synthesized via the ‘macromonomer’ method by which monofunctional polysiloxane macromonomers are copolymerized with methyl methacrylate. The graft copolymers have been characterized by gel permeation chromatography, 1H n.m.r. spectroscopy and differential scanning calorimetry. Dynamic contact angle (DCA) analysis afforded advancing and receding water contact angles over 15 days immersion in water. The initially hydrophobic polymer surfaces rapidly rearrange in the presence of water to more hydrophilic surfaces. The effects of varying the amount and type of siloxane in the copolymer on DCA results are discussed. The fouling and fouling release behaviour of coatings formulated from the graft copolymers have been evaluated in Chesapeake Bay. Fouling and fouling release behaviour are discussed in the light of DCA results

A novel method to introduce durable hydrophilicity onto hydrophobic polymer surface by plasma treatment

We have examined a novel method to fabricate a durable hydrophilic surface of hydrophobic polymeric materials modified by plasma treatment. The method involves a trapping of maleic anhydride-containing polymer onto poly(ethylene naphthalate) (PEN) by a plasma-induced crosslinking reaction of PEN followed by hydrolysis of maleic anhydride moiety to generate hydrophilic carboxyl groups on the PEN surface. In fact, the PEN film surface thus treated has shown an effective stability of wettability based on the water contact angle measurement.

Thermosensitive hydrogel coatings: Synthesis and heparin release

AbstractThe synthesis and drug release properties of crosslinked N‐isopropylacrylamide (NiPAAm) copolymer coatings on the surface of a hydrophobic poly(ester‐urethane) tubing were examined. A method was designed to coat hydrophobic polymer surface with a thermosensitive gel layer. Crosslinked NiPAAm copolymer coatings were synthesized using UV‐initiated polymerization. The feasibility of using NiPAAm based gels as heparin releasing thermosensitive coatings was investigated. Heparin, a high molecular weight hydrophilic solute, was loaded into hydrogels using a simple solution sorption technique. The release of heparin from NiPAAm copolymer gel coatings was compared to that of crosslinked NiPAAm copolymers. The gel coatings demonstrated a more gradual and prolonged heparin release as compared to gel disks of the same composition.

The visualization of water transport through hydrophobic polymer coatings applied to building sandstones by broad line magnetic resonance imaging

It is shown that broad line gradient echo magnetic resonance imaging is an ideal tool with which to study the ingress of hydrophobic polymer surface treatments into porous building materials, specifically in this case sandstone. It is further shown that the method can be used to quantitatively visualize the movement of water into and through the treated material, both from the surface and bulk, as a function of time. The influence of treatment cure time, cure temperature and substrate hydration on the subsequent water transport have been investigated. For the first time direct evidence of water pumping through a treated surface is presented.

Bulk surface photografting process and its applications. I. Reactions and kinetics

A bulk surface photografting process which is conducted in the interface between two polymer films was designed and investigated. The bulk surface photografting is a highly efficient process. With this method, the surface of hydrophobic polymers can be made hydrophilic in less than 2 s and a grafted layer 5 μm thick can be obtained in 30 s. The kinetic investigation shows that the bulk surface photografting polymerization involves a four-step reaction process: induction period, surface initiation, successive polymerization, involves a four-step reaction process: induction period, surface initiation, successive polymerization, and solid-phase crosslinking. The photoreduction of benzophenone (BP) takes place in the interlayer between the LDPE films and is a three-stage process: surface photoreduction, secondary photoreduction, and solid-state photoreduction. With regard to the photoreduction of the initiator caused by hydrogen abstraction, the kinetic studies show that the photoreduction rate has a first-order dependence on the BP concentration. The activation energy of the hydrogen abstraction reaction is about 28.5 KJ (6.8 kcal)/mol. With regard to the photografting polymerization reaction, the reaction order of the rate Rp with respect to the monomer is unity and 0.89 with respect to BP. This means that the termination reaction takes place mainly by combination of polymer chain free radicals and semipinacol free radicals from BP. The activation energy of the overall polymerization reaction is around 8.8 kJ (2.1 kcal)/mol. © 1996 John Wiley & Sons, Inc.

Water-Induced Surface Rearrangements of Poly(dimethylsiloxane−urea−urethane) Segmented Block Copolymers

An investigation of the surfaces of linear, segmented block copolymers of poly(dimethylsiloxane−urea−urethanes) by dynamic contact angle analysis is reported. The polymer films are immersed in water, the time-dependent advancing and receding contact angles are observed, and the contact angle hysteresis is reported. The initially hydrophobic polymer surfaces are observed to become more hydrophilic with long-term exposure to water. The advancing contact angles are relatively constant with immersion time; the receding contact angles decrease to some equilibrium value after a few days' exposure to water. It is proposed that the surfaces reorganize by a mechanism in which the hard block urethane−urea domains migrate through the soft block silicone to the polymer−water interface. The surface reorganization kinetics are discussed in terms of the effects of annealing as well as the average molecular weight of the soft block.

Hydrocoating: a new method for coupling biomolecules to solid phases

Solid-phase immunoassays such as enzyme-linked immunosorbent assays require one of the assay components to be immobilized. Most frequently this is achieved by passive adsorption of the antigen or antibody to a hydrophobic polymer surface composed of, e.g., polystyrene. Alternatively the biomolecule can be bound indirectly via passively adsorbed carrier proteins or directly via functional groups on the solid phase using cross-linking agents. Here we describe a new technique — hydrocoating — for covalent immobilization of biomolecules, such as peptides, in highly hydrophilic surroundings. Peptides were immobilized on microtiter plates via covalent bonds to an activated hydrophilic polymer. Soluble dextran was activated using 2,2,2-triflouroethanesulphonyl chloride (tresyl chloride) leading to activation of hydroxyl groups on the dextran polymer. This activated dextran molecule was immobilized on a surface containing amino groups leaving a sufficient number of active groups for secondary binding of other biomolecules. Peptides, that were either undetectable or poorly recognized when adsorbed on polystyrene, were readily recognized when immobilized by the hydrocoating technique. Furthermore, peptides immobilized by this method were recognized 5–10-fold better compared to the same peptides immobilized covalently on a surface containing secondary amino groups. The technique appears to provide an alternative to passive adsorption of biomolecules on solid phases and may be useful in the future development of immunoassays.