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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.

Controlling the reversibility, quantity, and extent of biomolecule interaction at interfaces has a significant relevance for biomedical and biotechnological applications, because protein adsorption is always the first step when a solid surface gets in contact with a biological fluid. Polymer brushes, composed of end-tethered linear polymers with sufficient grafting density, are very promising to control and alter interactions with biological systems because of their unique structure and distinct collaborative response to environmental changes. We studied protein adsorption and cell adhesion at polymer brush substrates which consisted of poly(N-isopropylacrylamide) (PNIPAAm), having a lower critical solution temperature (LCST), to control bioadsorptive processes by changing the environmental temperature. Preparing the PNIPAAm brushes by the "grafting-to"-method two differently synthesized PNIPAAm polymers were used, at which one possessed an additional hydrophobic terminal headgroup. It is known that hydrophobic moieties can influence protein adsorption significantly. The films were comprehensively analyzed by in situ spectroscopic ellipsometry, contact angle measurements, streaming potential, and atomic force microscopy. Our study was mainly focused on the investigation of the fibrinogen (FGN) adsorption responsiveness both on homo polymer PNIPAAm brushes with and without the hydrophobic terminal functionalization, and further on binary brushes made of the polyelectrolyte poly(acrylic acid) (PAA) and one of the prior described two PNIPAAm species. The results show that the terminal hydrophobic modification of PNIPAAm has a considerable impact on wettability, LCST, and morphology of the homo and the binary brush systems, which consequently led to an alteration of FGN adsorption. By using binary PNIPAAm-PAA brushes with different composition it was possible to induce stimuli dependent FGN adsorption with a considerable amplified switching effect by introducing a hydrophobic terminal residue to PNIPAAm. Cell adhesion studies with human mesenchymal stem cells reflected the results of the FGN adsorption.

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