Papers by Friedrich Kremer
Polymer, 2006
The dielectric response of 1,4-cis-polyisoprene under applied mechanical oscillatory shear with v... more The dielectric response of 1,4-cis-polyisoprene under applied mechanical oscillatory shear with various shear amplitudes was investigated. For this purpose, a special setup was constructed which enables to measure dielectric spectra under the influence of large amplitude oscillatory shear (LAOS); the setup is explained in detail. A strong influence of the shear amplitude on the dielectric relaxation strength was observed if the dielectric normal mode was mechanically affected. With increasing amplitude the relaxation strength decreased while the mean relaxation time, the width and asymmetry basically remained unchanged. We interpret this process as an orientational phenomenon of the end-to-end-vectors which results in a decreasing fluctuation amplitude of the polarization fluctuations.
Characterization of Polymer Surfaces and Thin Films
Thin polystyrene films are prepared by spin-coating and annealed above the calorimetric glass tra... more Thin polystyrene films are prepared by spin-coating and annealed above the calorimetric glass transition temperature Tg alternatively in vacuum, in a pure nitrogen atmosphere, in the presence of water vapor or in ambient air. It is experimentally shown that these preparative conditions have a pronounced impact on the stability of the samples, on their surface topography and on the molecular mobility. Our
Physica B-Condensed Matter, 1992
We have used neutron spin echo and dielectric spectroscopy to investigate the relaxational proper... more We have used neutron spin echo and dielectric spectroscopy to investigate the relaxational properties of polyisoprene. NSE data reveal a relaxation similar to what was found previously in polybutadiene. The comparison to the dielectric relaxation and rheological data exhibits that all time scales shift with the same temperature dependent factor. Neutron spin echo spectroscopy (NSE) recently has proved to be a valuable tool in the investigation of the slow dynamics in glassforming systems [l-3]. Here,
Solid State Communications, 1988
ABSTRACT
Optics and Lasers in Engineering, 2012
On the basis of a brief review of four common image recognition algorithms for microspheres made ... more On the basis of a brief review of four common image recognition algorithms for microspheres made of polystyrene or melamine resin, we present a new microsphere localization method for low-contrast silica beads under white light illumination. We compare both the polystyrene and silica procedures with respect to accuracy and precision by means of an optical tweezers setup providing CMOS video microscopy capability. By that we demonstrate that our new silica algorithm achieves a relative position uncertainty of less than 7 1 nm for micron-sized microspheres, significantly exceeding the precision of the other silica approaches studied. Second, we present an advancement of our single microsphere tracking method to scenarios where two polystyrene, melamine resin or silica microspheres are in close-to-contact proximity. While the majority of the analysis algorithms studied generate artefacts due to interference effects under these conditions, we show that our new approach yields accurate and precise results.
Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 1997
Molecular Crystals and Liquid Crystals Science and Technology. Section A. Molecular Crystals and Liquid Crystals, 1999
While confined nematogens have been extensively investigated in adsorbents of a large variety of ... more While confined nematogens have been extensively investigated in adsorbents of a large variety of cavity sizes, comparably little is known about smectogens under geometrical restraints. We study ferroelectric smectogens in porous material from nanometer to micrometer cavity sizes. No influences on the molecular dynamics but dramatic changes in the collective reorientation behaviour, in particular a disappearance of the collective modes
Macromolecules, 2009
Broadband dielectric spectroscopy, pulsed field gradient nuclear magnetic resonance (PFG NMR), an... more Broadband dielectric spectroscopy, pulsed field gradient nuclear magnetic resonance (PFG NMR), and differential scanning calorimetry are combined to study charge transport and dipolar relaxations in novel hyperbranched polyamide amines. The dielectric spectra are dominated by conductivity contributions at higher temperatures (masking out the structural R-relaxation process), whereas two secondary dipolar relaxation processes are observed at lower temperatures for the two samples investigated. Based on Einstein and Einstein-Smoluchowski relations, the diffusion coefficient is extracted from the dielectric spectrasin quantitative agreement with independent PFG NMR measurements. It exhibits a Vogel-Fulcher-Tammann temperature dependence, while the effective number density of the charge carriers varies only weakly with temperature. Charge transport and structural R-relaxation are found to be characterized by a decoupling index of ∼7 for the hyperbranched polymers studied.
Macromolecular Chemistry and Physics, 2008
Cover: The schematic representation on the cover shows the behavior of the dielectric loss and dy... more Cover: The schematic representation on the cover shows the behavior of the dielectric loss and dynamic glass transition of ultra‐thin polymer films during annealing above the glass transition temperature. In the early stages of annealing metastable states of the glassy dynamics are detected, which ‐ similarly to confinement effects ‐ exhibit pronounced deviations from the bulk glass transition. Further details can be found in the article by A. Serghei* and F. Kremer on page 810.
Macromolecular Chemistry and Physics, 2005
Liquid Crystals, 1989
Broadband dielectric spectroscopy on side group liquid crystal polymers ... Dielectric studies on... more Broadband dielectric spectroscopy on side group liquid crystal polymers ... Dielectric studies on three liquid-crystalline side group polymers were performed in the temperature range from 100 K to 450 K and in the frequency range from lo-' Hz to lo7 Hz. All samples exhibit an CI ...
Liquid Crystals, 2000
New hydrogen bonded blends of LC copolymers containing functional carboxyl groups with a low mole... more New hydrogen bonded blends of LC copolymers containing functional carboxyl groups with a low molecular mass pyridine-containing dopant were obtained and the orientational, optical and elastic properties of the blends were measured using the Fréedericksz method of threshold transitions in a magnetic field. The averaged order parameter S of the hydrogen bonded blends is found to be lower than that
Journal of the American Chemical Society, 2002
Interference microscopy and FTIR microscopy are applied to study intracrystalline concentration p... more Interference microscopy and FTIR microscopy are applied to study intracrystalline concentration profiles of methanol in CrAPO-5 zeolite crystals. By using both techniques, the high spatial resolution of interference microscopy is complemented by the ability of FTIR spectroscopy to pinpoint adsorbates by their characteristic IR bands. For the first time two-dimensional concentration profiles of an unprecedented quality are reported which show a nonhomogeneous distribution of adsorbate in zeolite crystal under equilibrium with the adsorbate vapor. These nonhomogeneous profiles are attributed to regular intergrowth effects in CrAPO-5. A possible internal structure of CrAPO-5 crystals is suggested.
Journal of Polymer Science Part B: Polymer Physics, 2006
The dynamic glass transition and the dilatometric glass transition temperature are simultaneously... more The dynamic glass transition and the dilatometric glass transition temperature are simultaneously characterized in thin films of hyperbranched aromatic polyesters by broadband dielectric spectroscopy and capacitive scanning dilatometry. A diverging thickness dependence is detected: while the temperature position of the alpha relaxation peak Tα decreases by ∼30 K, the dilatometric Tg increases by ∼10 K with decreasing film thickness. This emphasizes the subtle character of the glass transition phenomenon—as manifested in the molecular dynamics and in the (structural) thermal expansion—and proves that, in contrast to the bulk, different experimental techniques do not necessarily deliver similar results in confinement. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3006–3010, 2006
Journal of Polymer Science Part B: Polymer Physics, 2010
Broadband dielectric spectroscopy is used to investigate molecular dynamics and charge transport ... more Broadband dielectric spectroscopy is used to investigate molecular dynamics and charge transport in three hyperbranched polyester amides with hydroxyl, phenyl, and stearate terminal groups. At higher temperatures, the dielectric spectra are interpreted in terms of hopping conduction in a spatially randomly varying energy landscape, whereas two secondary dipolar relaxations attributed to librations of the terminal and amide groups dominate the low temperature regime. Despite a shift of more than 3 decades in the dc conductivity upon variation of the end groups, the Barton–Nakajima–Namikawa relation is shown to hold. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1651–1657, 2010
Journal of Optics, 2011
ABSTRACT An optical tweezers setup with optical detection in three dimensions and active height r... more ABSTRACT An optical tweezers setup with optical detection in three dimensions and active height regulation has been developed. The presented novel method to determine the relative height of a microparticle from its microscopic image is based on the analysis of the integrated light intensity of the main maximum of the diffraction pattern. After the determination of a master curve as reference, the height can be detected with an accuracy of up to 2 nm. The method is applicable under microscopic white light illumination and is simple to implement. As an example of measurements where active height regulation is indispensable, force–distance curves are discussed. Furthermore, the colloid height is calculated geometrically. In the range where the geometrical estimation provides reliable results, the values are found to be in quantitative agreement with the suggested algorithm.
Journal of Molecular Biology, 2006
Optical tweezers are employed to study the action of the histone-like protein from Thermotoga mar... more Optical tweezers are employed to study the action of the histone-like protein from Thermotoga maritima (TmHU) on DNA at a single molecule level. Binding and disruption of TmHU to and from DNA are found to take place in discrete steps of 4-5 nm length and a net binding enthalpy of about 16k B T. This is in reasonable agreement with a microscopic model that estimates the extension of the binding sites of the protein and evaluates the energetics mainly for bending of the DNA in the course of interaction.
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Papers by Friedrich Kremer