Karina Grundke

Microwave CO2 plasma-initiated vapour phase graft polymerization of acrylic acid onto polytetrafluoroethylene for immobilization of human thrombomodulin.

The functionalization of polytetrafluoroethylene (PTFE) for human thrombomodulin (hTM) binding has been achieved by CO2 plasma activation and subsequent vapour phase graft polymerization of acrylic acid (AA). The PTFE surfaces after CO2 plasma treatment, AA grafting and hTM immobilization were characterized by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) spectroscopy, as well as by zeta potential and wetting measurements to quantitatively control each step of modification. The activity of immobilized hTM was estimated by the protein C activation test.

Insights on structural variations of protein adsorption layers on hydrophobic fluorohydrocarbon polymers gained by spectroscopic ellipsometry (part I)

Abstract Adsorption layers of two human plasma proteins (albumin-HSA, fibrinogen-FGN) on hydrophobic fluorohydrocarbon polymer (CHF) films were characterized in situ and ex situ by spectroscopic ellipsometry. The adsorbed layers were formed in phosphate buffered saline solutions of varied protein concentrations. Different optical five layer models were compared with respect to the evaluation of protein layers based on ellipsometric data. The Maxwell–Garnett effective medium approximation was concluded to be advantageous in providing a more realistic description of the layer structure as compared to the assumption of optical homogeneous layers. The applied models coincided with respect to the determination of the adsorbed amount of protein. Both the equilibrium surface concentration and the adsorption dynamics of HSA and FGN were found to depend on the solution protein concentration. The maximum adsorbed protein concentrations of the two proteins differed by ratios (HSA/FGN) of 1/4.5 in mass units and 1/0.83 in molar units (HSA: 1.0 mg m−2=15 nmol m−2; FGN: 4.5 mg m−2=12.5 nmol m−2). No reversibility of the adsorption of the two globulins on the polymer surface was observed upon dilution of the protein solutions with pure buffer. Coverage of the polymer surface with respect to the adsorbed molecules was achieved by different amounts of HSA or FGN depending on the transport conditions in the adsorption process. The observed variations of the surface area occupied by a given protein were apparently related to re-orientations and/or intramolecular changes of the adsorbed molecules. Structural parameters of the protein layers gained by the evaluation of the ellipsometric data support this conclusion.

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(12)E(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.

Liquid-fluid contact angle measurements on hydrophilic cellulosic materials

Abstract Contact angle measurements with captive bubbles have been used to quantify the wettability of different cellulosic materials in contact with pure water or aqueous protein solutions. The application of a new contact angle technique (axisymmetric drop shape analysis) in combination with atomic force microscopy permits new insights into the wetting behaviour of these hydrophilic surfaces. It was found that the unmodified cellulose membrane has a very smooth and homogeneous surface in the water-swollen state. There is almost no hysteresis between the advancing and receding water contact angles. The chemical modification of the cellulose with diethylaminoethyl groups results in a smooth but heterogeneous water-swollen membrane surface with fewer hydrophilic properties. A water-swollen cellulose acetate film was used as a model surface for wetting measurements with buffered protein solutions. It was found that different proteins (human serum albumin, fibrinogen) yielded different contact angles in their adsorbed state; after the adsorption of fibrinogen the contact angle of the pure aqueous buffer solution on the cellulose acetate was lower than the contact angle measured after the adsorption of human serum albumin.

Studies on the wetting behaviour of polymer melts on solid surfaces using the Wilhelmy balance method

Abstract A Wilhelmy balance method has been used to measure the wetting tension of polymer melts on solid surfaces at high temperatures. The application of thin quartz fibres or platinum wires allowed fast and reliable surface tension values for polypropylene and epoxy resin melts to be obtained without knowledge of the density of these melts. The measuring technique is also suitable for investigating the surface activity of additives or polar components in these polymer melts. The plot of γ cos Θ versus concentration for two commercially applied additives (benzoin, butyl acrylate) yielded different surface activities of these additives in an epoxy resin melt. The plot of γ cos Θ versus temperature provides different temperature coefficients of the wetting tension for each additive concentration. It was found that − d(γ cos Θ) d T was increased in the presence of the additives. The γ cos Θ versus temperature curves permit conclusions concerning the change in flow properties of polymer coating systems due to additives. Depending on the type of interaction at the solid/liquid interface — physical or chemical interactions — different wetting kinetics have been found for unmodified and chemiscally modified polypropylene melts on untreated and aminosilane-treated glass fibres. Thus, it is assumed that these measurements permit a more fundamental and quanttative understanding of the mechanism of interface formation in fibre-reinforced polymer composites.

Physico-chemical properties of surface-modified polymers

Contact angle, electrokinetic, and X-ray photoelectron spectroscopic (XPS) measurements have been used to study the surface properties of flame- and oxygen plasma-pretreated polypropylene/ ethylenepropylene-diene monomer rubber (PP-EPDM) blends and of ethylene vinyl acetate (EVA) copolymers grafted with carboxyl group-containing monomers. The contact angles of pure test liquids (water, methylene iodide, and ethylene glycol) were used to calculate the dispersive and polar components of the surface free energy according to Owens and Wendt, and the acid-base parameters according to Van Oss and co-workers. In addition, the acid-base properties of the differently pretreated polymers could be evaluated quantitatively by measuring the zeta potential vs. the pH in a 10-3 mol/l KCI solution. The zeta potential measurements show that oxygen plasma-treated PP-EPDM and grafted EVA indicate an acidic surface character, whereas the flame-treated PP-EPDM blends possess both acidic and basic surface groups. The basic sur...

Experimental studies of contact angle hysteresis phenomena on polymer surfaces – Toward the understanding and control of wettability for different applications.

Contact angle hysteresis phenomena on polymer surfaces have been studied by contact angle measurements using sessile liquid droplets and captive air bubbles in conjunction with a drop shape method known as Axisymmetric Drop Shape Analysis - Profile (ADSA-P). In addition, commercially available sessile drop goniometer techniques were used. The polymer surfaces were characterized with respect to their surface structure (morphology, roughness, swelling) and surface chemistry (elemental surface composition, acid-base characteristics) by scanning electron microscopy (SEM), scanning force microscopy (SFM), ellipsometry, X-ray photoelectron spectroscopy (XPS) and streaming potential measurements. Heterogeneous polymer surfaces with controlled roughness and chemical composition were prepared by different routes using plasma etching and subsequent dip coating or grafting of polymer brushes, anodic oxidation of aluminium substrates coated with thin polymer films, deposition techniques to create regular patterned and rough fractal surfaces from core-shell particles, and block copolymers. To reveal the effects of swelling and reorientation at the solid/liquid interface contact angle hysteresis phenomena on polyimide surfaces, cellulose membranes, and thermo-responsive hydrogels have been studied. The effect of different solutes in the liquid (electrolytes, surfactants) and their impact on contact angle hysteresis were characterized for solid polymers without and with ionizable functional surface groups in aqueous electrolyte solutions of different ion concentrations and pH and for photoresist surfaces in cationic aqueous surfactant solutions. The work is an attempt toward the understanding of contact angle hysteresis phenomena on polymer surfaces aimed at the control of wettability for different applications.

Determination of the surface tension of microporous membranes using wetting kinetics measurements

Abstract In this paper, a new method is introduced to determine the surface tension of the solid material from which a porous membrane is made. The method is based on the well known determination of the penetration velocity of the liquid that penetrates into a porous structure. A modified Washburn equation is applied to interpret the experimental data. Whereas the use of this equation has been well investigated on powder packages, it has not been used on microporous membranes until now. To test the applicability of the Washburn equation for microporous membranes, samples made from poly(tetrafluoroethylene), poly(ethylene), and poly(propylene), with different porosities, were investigated. All membranes were tested using a series of alkanes with different liquid surface tensions. A plot of the normalized penetration velocity as a function of the liquid surface tension shows a maximum if both liquid and solid surface tensions are equal. It was found that the use of the modified Washburn equation gives relevant solid surface tension values for different types of membranes and offers a way to determine the solid surface tension of the material from which a microporous membrane is made, independent of its pore geometry.

Surface Characterization of Polymers by Physico-Chemical Measurements

Abstract The possibility of characterizing dispersion forces and acid-base interactions by means of physico-chemical measurements is demonstrated by the examples of contact angle and zeta potential measurements, with special attention being given to the latter. This method has been applied, to characterize the effect of plasma and flame treatment on the adhesion behaviour of injection moulded poly(propylene) specimens. The results with respect to acidic or basic functional surface sites, as obtained by zeta potential measurements, correlate with the elemental surface compositions determined by XPS. There is no general interrelation between acidic and basic parameters determined by contact angle measurements and the results of zeta potential and XPS measurements.

Preparation and properties of thin films of Hyperbranched polyesters with different end groups

Hyperbranched polyesters (HBP) with different end groups were prepared as thin films. They were characterized with regard to their chemical composition, thickness, optical constants and morphology using infrared spectroscopy, spectroscopic ellipsometry, and atomic force microscopy. The surface properties of the films were determined by zeta-potential and contact angles measurements. The differences in the molecular structure and surface energetic and acid-base properties between HBP materials with carboxylic, hydroxy and acetoxy end groups result in differences in their swelling behavior in atmospheric humidity. The swelling behavior at different atmospheric humidity was observed in situ using spectroscopic ellipsometry and reflectometric interference spectroscopy. From the results it can be concluded that HBP films can be used potentially as sensoric materials.