Klaus-Jochen Eichhorn

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.

Protein adsorption on and swelling of polyelectrolyte brushes: A simultaneous ellipsometry-quartz crystal microbalance study

With a coupled spectroscopic ellipsometry-quartz crystal microbalance with dissipation (QCM-D) experimental setup, quantitative information can be obtained about the amount of buffer components (water molecules and ions) coupled to a poly(acrylic acid) (PAA) brush surface in swelling and protein adsorption processes. PAA Guiselin brushes with more than one anchoring point per single polymer chain were prepared. For the swollen brushes a high amount of buffer was found to be coupled to the brush-solution interface in addition to the content of buffer inside the brush layer. Upon adsorption of bovine serum albumin the further incorporation of buffer molecules into the protein-brush layer was monitored at overall electrostatic attractive conditions [below the protein isolectric poimt (IEP)] and electrostatic repulsive conditions (above the protein IEP), and the shear viscosity of the combined polymer-protein layer was evaluated from QCM-D data. For adsorption at the “wrong side” of the IEP an incorporation of excess buffer molecules was observed, indicating an adjustment of charges in the combined polymer-protein layer. Desorption of protein at pH 7.6 led to a very high stretching of the polymer-protein layer with additional incorporation of high amounts of buffer, reflecting the increase of negative charges on the protein molecules at this elevated pH.

Ordered structures and progressive transesterification in PC/PBT melt blends studied by FT i.r. spectroscopy combined with d.s.c. and n.m.r.

Abstract Melt blends of polycarbonate (PC) and poly(butylene terephthalate) (PBT) were characterized by their transesterification and crystallization behaviour using Fourier transform infrared spectroscopy (FT i.r.) as well as nuclear magnetic resonance (n.m.r.) spectroscopy and differential scanning calorimetry (d.s.c.). The transesterification can be analysed by FT i.r. spectroscopy using two different spectral features. The appearance of new bands is used for the direct proof, and changes of i.r. bands correlated with structural order for the indirect proof. The results from the PC-rich blend show that the indirect proof is very sensitive, but a detailed knowledge of the crystallinity is necessary for a correct analysis. The improvement of the compatibility of both components due to the formed copolymers has been observed in the blend PC/PBT 50/50 by i.r. microscopy. Furthermore, a splitting of the PBT carbonyl band was found and discussed. The changes of the observed two peaks correlate directly with the changes in the conformation of the tetramethylene sequences of PBT.

Biocompatible polymeric materials with switchable adhesion properties

We report on the design, investigation and comparison of the adhesive properties of poly-(N-isopropylacrylamide) and biocompatible polyethylene glycol-based thermoresponsive brushes. Specifically, a poly-(N-isopropylacrylamide), PNIPAM, poly-(oligo(ethylene glycol) methyl ether methacrylate-co-2-(2-methoxyethoxy)ethyl methacrylate), P(OEGMA-MEO2MA), and poly-(oligo(ethylene glycol) methyl ether methacrylate-co-oligo(propylene glycol) methacrylate), P(OEGMA-OPGMA) brushes were synthesised on planar silicon wafers and silica particles via the surface-initiated atom transfer radical polymerisation. Switching of swelling and surface charge were investigated using spectroscopic ellipsometric and zeta-potential measurements, respectively. Adhesion properties were studied in situ in aqueous solutions at different temperatures using three kinds of AFM probes with well defined tips geometries and radii. We found that: (1) adhesion properties of all polymer brushes reversibly switch with temperature—polymer brushes are completely non-adhesive below the Low Critical Solution Temperature (LCST) and become sticky above the LCST; (2) adhesive force scales nonlinearly with the probe radius; (3) adhesion energy, obtained using Derjaguin/Muller/Toporov approaches, decreases with the increase of the tip radius that is attributed to different depth of penetration of the probes in the polymer layer; (4) adhesion decreases in the sequence PNIPAM – P(OEGMA-MEO2MA) – P(OEGMA-OPGMA). Based on dynamic light scattering and cryo-TEM experiments, the latter effect is attributed to the formation of a thin oligoethylene glycol shell around the hydrophobic polymer core in the case of P(OEGMA-OPGMA). We expect that the P(OEGMA-OPGMA) system, which demonstrates less sticky adhesion properties, could be particularly promising for cell adhesion experiments and tissue engineering.

Ion-beam induced chemical and structural modification in polymers

In order to increase the sensitivity to moisture uptake of polyimide (PI) and polyethersulfone films applied in bimorphic humidity sensors 50, 130 and 180 keV boron ions with irradiation doses between 1013 and 1016 B+/cm2 were implanted. A complex investigation of the following features has been carried out: chemical changes in the surface regions by attenuated total reflection–FTIR spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS); optical properties by spectroscopic ellipsometry; hardness and elastic modulus by depth-sensing low-load indentation technique; conductivity of modified polymer films. It could be shown, that the partial destruction of chemical bonding under ion bombardment leads to the creation of new amorphous and graphite-like structures, which increase the surface film conductivity by several orders of magnitude, and enhances the sensitivity of these polymer films to moisture uptake. The ion-beam irradiation destroys the anisotropic features of the refractive index of PI layers leading to its isotropization. Radiation-induced changes in the layer structure result in an increase of the hardness and elastic modulus of the modified layers up to ten and six times, respectively. The hardness and refractive index depth profiles were determined. The detectable effective modification depth estimated from the depth profiles is 250–300 nm at an ion energy of 50 keV and 400–450 nm at an ion energy of 180 keV.

FTi.r. spectroscopy on electron irradiated polytetrafluoroethylene

Abstract The partial degradation of polytetrafluoroethylene caused by electron irradiation was investigated by infrared (i.r.) transmission, diffuse reflectance (DR) and photoacoustic (PAS) spectroscopy. We studied the influence of the radiation dose and the hydrolysis conditions on the structure. Irradiation produces COF groups with a band at 1884 cm −1 in the transmission spectra. Under ambient conditions these COF groups hydrolyse with the atmospheric humidity to free (1810 cm −1 ) and associated (1776–1792 cm −1 ) COOH groups. We could establish the presence and the nature of these groups in near surface regions and in the bulk phase. The formation of carboxylic groups in different states of association also depends on the hydrolysis conditions. Various forms of hydrogen bonding can be found for different associated COOH groups (1776 and 1792 cm −1 ). Hence we could assign the band at 1776 cm −1 to the vibration of a dimer consisting of two hydrogen bonded carboxyl groups. The band at 1792 cm −1 comes from ‘monomeric’ acid groups associated with water via hydrogen bonds. After hydrolysis under extreme conditions using alcoholic sodium hydroxide changes in band intensities and a strong new band at 1784 cm −1 were observed. This fact indicates a change of association states on the surface. Subsequently ester groups were formed in a surface acylation reaction of COF groups with ethanol which give this new band. From the DR and PAS spectra it can be concluded that most COOH groups are localized in the near surface region. In every case a small part of the COF groups remains after hydrolysis. The COF band in the spectra of hydrolysed PTFE only disappears when the permeation conditions for water vapour at the PTFE surface were improved. This fact indicates that COF groups in irradiated PTFE are not only localized on the surface of the PTFE particles. The results establish the hypothesis that a part of the COF groups is inaccessibly arranged in the inner part of the irradiated PTFE particle, and spontaneous hydrolysis is preferred in near-surface regions.

Characterization of ion-beam modified polyimide layers

Thin chemically modified polyimide films are widely used as functional layers for new microelectronic sensors. Modification of the chemistry of these polymers can lead to different mechanical, optical and electrical properties. Ion implantation is a preferred method to modify polyimide structures. In this work the ion-induced changes of chemical structures of three polyimides were analyzed by attenuated total reflection. Fourier transform infrared spectroscopy (ATR-FTIR); X-ray photoelectron spectroscopy (XPS); Raman spectroscopy; and spectroscopic ellipsometry and atomic force microscopy (AFM). The results indicate that during the implantation process the imide structures were partly destroyed. Carbon-rich, graphite-similar and amorphous structures were formed in the surface-near area of the polyimide layers. The changes in molecular structures especially depend on the dose of implanted boron ions.

Functionalization of solid surfaces with hyperbranched polyesters to control protein adsorption.

Thin films of hyperbranched polyesters were studied in dry state and in aqueous buffer solution regarding their swelling behaviour and protein adsorption potential. The influence of the degree of branching, the backbone structure, flexibility as well as the polarity was varied. By changing the backbone structure from aromatic, aromatic-aliphatic to aliphatic the surface properties can be controlled from protein active to protein repelling. The higher adsorption potential observed in comparison to linear polyesters is the result of the large amount of end groups allowing the formation of hydrogen bonds, and the larger swellability of the more flexible linear polymers. The protein adsorption process was studied intensively by in-situ spectroscopic ellipsometry. Different approaches towards a proper optical model for the vis-ellipsometry data evaluation for the determination of the correct layer thickness and refractive index are discussed. IR-ellipsometric measurements using a new in-situ cell gave the full chemical evidence for the formation of thin protein adsorption layer on the polymer films in the aqueous buffer environment.

FTIR spectroscopic studies of interfacial reactions between amino functionalized silicon surfaces and molten maleic anhydride copolymers

The interactions of two reactive maleic anhydride copolymers, a styrene maleic anhydride copolymer and an alternating copolymer of maleic anhydride and α-olefin side chains, with amino-functionalized surfaces were studied by means of FTIR-ATR spectroscopy. The interfacial processes were detected at a planar surface of a thermally oxidized silicon as internal reflecting element. This ATR element can be understood as a model for a glass fiber which is used as reinforcing material in polymer matrices. A typical glass fiber coupling agent, γ-aminopropyltriethoxysilane (γ-APS), was used to functionalize the silicon dioxide layer. It was found that the amino groups of the amino siloxane network layer, which were obtained by a coating procedure with γ-APS. react with the anhydride groups of the copolymer melt to form amic acid structures and imide structures depending on the chemical composition of the copolymers and the temperature.

Comparative Study of an Anisotropic Polymer Layer by Infrared Spectroscopic Techniques

Infrared spectroscopic ellipsometry (IRSE), reflection absorption IR spectroscopy (RAIRS), IR transmission spectroscopy, and best-fit calculations are applied in a cooperative study to determine the anisotropic optical properties of a thin polyimide layer in the spectral range 4000–500 cm−1. The employed anisotropic uniaxial optical layer model afforded very good agreement between the calculated and the experimental spectra obtained by the different complementary IR methods. The main advantage of IRSE is that it is possible to obtain data for the optical constants and the thickness (d = 1.81 μm) of the polyimide layer simultaneously within one experiment. From the ellipsometric spectra it was concluded that the layer structure can be regarded as possessing uniaxial symmetry where the layer is isotropic in directions (x, y) parallel to the sample surface. A qualitative determination of the anisotropic parameters of vibrational bands is possible by calculation of the ellipsometric spectra. The evaluation procedure can be improved by evaluation of polarized reflection spectra, provided the reference standard has been calibrated by ellipsometry. The oscillator parameters are then derived more accurately from the separate s- and p-polarized reflection spectra rather than from their ratio, which is measured in ellipsometry.