6 Materialchemie
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Interactions between a polymer and a substrate interface play a vital role in understanding the improvement in thin film material properties as well as serving as a model for nanocomposites. For any non-repulsive polymer-substrate interactions, polymer segments form an irreversibly adsorbed layer and show a slowdown in the glassy dynamics and thus an increase in the thermal glass transition temperature compared to the bulk-like values. The growth kinetics of the adsorbed layer showed a deviation for both poly (bisphenol-A carbonate) (PBAC) and polysulfone (PSU), two bulky polymers containing a functional group (phenyl ring) in the backbone, compared to conventional polymers previously studied like poly-2-vinyl pyridine (P2VP). This deviation was attributed to the bulkiness of the phenyl rings. . Further investigations into the influence of the adsorbed layer on glassy dynamics were conducted. The molecular mobility and glass transition for thin films of PBAC and PSU were compared to bulk samples of each polymer. Broadband dielectric spectroscopy, atomic force microscopy, and ellipsometry were primarily used and additionally supported by sum frequency generation spectroscopy.
Interactions between a polymer and a substrate interface play a vital role in understanding the improvement in thin film material properties as well as serving as a model for nanocomposites. For any non-repulsive polymer-substrate interactions, polymer segments form an irreversibly adsorbed layer and show a slowdown in the glassy dynamics and thus an increase in the thermal glass transition temperature compared to the bulk-like values. The growth kinetics of the adsorbed layer obey a two-step mechanism: formation of immobilized layer with flat segmental conformations and a loosely bound layer with stretched chains pinned to the surface. Here the adsorbed layer was studied for: poly (bisphenol-A carbonate) (PBAC) and polysulfone (PSU), two bulky polymers containing a functional group (phenyl ring) in the backbone and compared to poly (2-vinyl pyridine) (P2VP), where the backbone is a vinyl-derivative and the functional group (pyridine) is in the side chain. The growth kinetics for PBAC and PSU were found to deviate from the well-known mechanism, observed for polymers such as P2VP. Atomic force microscopy and ellipsometry were used for this investigation and was additionally supported by broadband dielectric spectroscopy.
The investigations into the complicated effects of film thickness on bulk properties of thin polymer films has yielded conflicting results. The reduction in molecular mobility, and with it an increase in the glass transition temperature, for thin films of poly (bisphenol A carbonate) (PBAC) was assigned to the formation of an adsorbed layer. The adsorbed layer was obtained by washing away the loosely bounded chains using a good solvent. Next, using atomic force microscopy (AFM), the thickness of each sample was measured after annealing for various times at three different annealing temperatures. The growth of this adsorbed layer was shown to deviate from the previously reported 2-step mechanism seen for other polymers. For PBAC, after very long annealing times at high temperatures the thin films were dewetted, where segments of the adsorbed layer were removed from the substrate.
Thin polymer films are of vital importance due to their low production costs and wide range of applications in sensors, electronics, and coatings. Their geometry is ideal for the study of confinement effects, specifically one-dimensional confinement, on the thermodynamic properties and segmental dynamics of polymers. For this investigation, films of PC are prepared on both glass and silica substrates to measure the dielectric and calorimetric behavior as a function of film thickness. The methodology consists of broadband dielectric spectroscopy (BDS), ellipsometry, and atomic force microscopy (AFM). Using ellipsometry, the glass transition shows an increase with decreasing film thickness. Compared to a previous investigation of PC where dilatometry was used, an increase in PC was seen, but now only at thicknesses below 20 nm. Therefore, the critical thickness of PC, where the bulk-like layer no longer percolates through the film, is dependent on the method probing the glass transition. The dielectric behavior shows a deviation from bulk behavior with decreasing film thickness, although still showing a Vogel-Fulcher-Tammann (VFT) temperature dependence for all films. The influence of the adsorbed layer on the molecular mobility was also addressed.
The composition of the surface layer in dependence from the distance of the polymer/air interface in thin films with thicknesses below 100 nm of miscible polymer blends in a spatial region of a few nanometers is not investigated completely. Here, thin films of the blend poly vinyl methyl ether) (PVME)/polystyrene (PS) with a composition of 25/75 wt% are investigated by Energy Resolved X-ray Photoelectron Spectroscopy (ER-XPS) at a synchrotron storage ring using excitation energies lower than 1 keV. By changing the energy of the photons the information depth is varied in the range from ca. 1 nm to 10 nm. Therefore, the PVME concentration could be estimated in dependence from the distance of the polymer/air interface for film thicknesses below 100 nm. Firstly, as expected for increasing information depth the PVME concentration decreases. Secondly, it was found that the PVME concentration at the surface has a complicated dependence on the film thickness. It increases with decreasing film thickness until 30 nm where a maximum is reached. For smaller film thicknesses the PVME concentration decreases. A simplified layer model is used to calculate the effective PVME concentration in the different spatial regions of the surface layer.
Tetraethyl orthosilicate (TEOS) was used as a chemical precursor to deposit ultra-thin SiO x C y plasma polymer films onto mild steel surfaces for preventing the corrosion process. The structure–property relationships of the coatings were evaluated by X-ray Photo Spectroscopy (XPS), X-Ray Diffraction (XRD), Fourier Transform InfraRed spectroscopy (ATR-FTIR) and Energy Dispersive X-ray spectroscopy (EDX) completed with Scanning Electron Microscopy (SEM). The SEM micrographs confirmed a pinhole-free surface morphology of the low-pressure deposited plasma polymer films. The TEOS molecules become fragmented in the plasma by numerous collisions with energy-rich electrons and heavier particles. Recombination of fragments and condensation onto the steel substrate is responsible for the formation of organic SiO containing plasma polymer layers. Such thin layers consist of predominantly SiO x structures. Their properties are determined largely by the gap distance between the two samples used as electrodes in the plasma. The efficiency of the corrosion-protecting coating was compared with uncoated samples. The corrosion protection was determined by exposure of samples to 3.5% NaCl aqueous solutions. For this purpose, polarization and Electrochemical Impedance Spectroscopy (EIS) were used to monitor the corrosion. The optimal gap distance between the electrodes was determined for corrosion protection. The best protective efficiency reached more than 97% of the total protection as measured at room temperature.
Polymers at interfaces play a major role in a broad variety of applications ranging from engineering purposes (for instance polymer based nanocomposites) to high tech implications (for instance light emitting diodes). Here the glass transition and glassy dynamics is considered for epoxy-based nanocomposite with Layered Double Hydroxide nanofiller and for thin films of a misicble polymer blend of PVME/PS with thicknesses down to 7 nm. The materials are investigated by spectroscopic techniques (broadband and specific heat spectroscopy), as well as by fast scanning calorimetry and small- and wide-angle X-ray scattering.