Friedrich Kremer

Bio: Friedrich Kremer is an academic researcher from Max Planck Society. The author has contributed to research in topics: Ferroelectricity & Liquid crystal. The author has an hindex of 30, co-authored 96 publications receiving 5676 citations.

Broadband dielectric spectroscopy

Abstract: A. Schoenhals, F. Kremer: Theory of Dielectric Relaxation.- F. Kremer, A. Schoenhals: Broadband Dielectric Measurement Techniques.- A. Schoenhals, F. Kremer: Analysis of Dielectric Spectra.- F. Kremer, A. Schoenhals: The Scaling of the Dynamics of Glasses and Supercooled Liquids.- P. Lunkenheimer, A. Loidl:Glassy Dynamics Beyond the a-Relaxation.- F. Kremer, A. Huwe, A. Schoenhals, S. Rozanski: Molecular Dynamics in Confining Space.- A. Schoenhals: Molecular Dynamics in Polymer Model Systems.- G. Floudas: Effect of Pressure on the Dielectric Spectra of Polymeric Systems.- J. Mijovich: Dielectric Spectroscopy of Reactive Polymeric Systems.- F. Kremer, A. Schoenhals: Collective and Molecular Dynamics of (Polymeric) Liquid Crystals.- L. Hartmann, K. Fukao, F. Kremer: Molecular Dynamics in thin Polymer Layers.- F. Kremer, S. Rozanski: The Dielectric Poperties of Semiconducting Disordered Solids.- P.A.M. Steeman, J. v. Turnhout: The Dielectric Properties of Inhomogeneous Media.- R. Boehmer, G. Diezemann: Principles and Applications of Pulsed Dielectric Spectroscopy and Nonresonant Dielectric Hole Burning.- R. Richert: Local Dielectric Relaxation by Solvation Dynamics.- T. Pakula: Dielectric and Dynamic Mechanical Spectroscopy-A Comparison.- R. Boehmer, F. Kremer: Dielectric and (Multidimensional) NMR Spectroscopy-A Comparison.- A. Arbe, J. Colmenero, D. Richter: Polymer Dynamics by Dielectric Spectroscopy and Neutron Scattering-A Comparison

Anomalies in the scaling of the dielectric alpha -relaxation.

Abstract: By measuring the complex dielectric function over 15 decades in frequency we evaluate the scaling of the α-relaxation for several glass-forming liquids including propylene carbonate. The temperature dependence of the mean relaxation time and of the relaxation strength of the relaxation function displays two dynamical regions being separated by a crossover temperature. The observed findings are essentially not in 'accordance with predictions of the mode-coupling theory and light scattering results for propylene carbonate [M. Elmroth et al., Phys. Rev. Lett. 68, 79 (1992)]

Scaling of the α relaxation in low-molecular-weight glass-forming liquids and polymers

Abstract: By measuring the complex dielectric susceptibility over 15 decades in frequency we compare the \ensuremath{\alpha} relaxation in low-molecular-weight glass-forming liquids and polymers. To describe the scaling behavior of the \ensuremath{\alpha} relaxation, a minimal set of four parameters is necessary, the mean relaxation time, the dielectric strength, and two shape parameters, describing the low- and high-frequency wings of the relaxation function, in pronounced contrast to a recently published scaling function.

Influence of concentration fluctuations on the dielectric alpha-relaxation in homogeneous polymer mixtures

Abstract: The dynamics of the α-relaxation in homogeneous mixtures of polystyrene (PS) and poly(cyclohexyl acrylate-stat-butyl methacrylate) (P(CHA-stat-BMA)) is analyzed within the framework of a concentration fluctuation model. The shape and widths of the dielectric relaxation spectra as well as the range of calorimetric glass transition in the mixtures are respectively associated with distributions of relaxation time and T arising from the presence of concentration fluctuations. The magnitude of these fluctuations is obtained by fitting the dielectric loss curves to a model function obtained as a convolution of a relaxation time distribution for the mixture with a function describing the dielectric loss of the dielectrically active component. The relaxation time distribution is calculated from William-Landel-Ferry (WLF) or Vogel-Fulcher-Tammann (VFT) free volume scaling with the assumption of a Gaussian concentration distribution in the samples. Fits of the measured α-relaxation spectra which are dominated by P(CHA-stat-BMA) offer as a reset the mean square concentration fluctuations in the mixtures. The fluctuations are interpreted in terms of the random phase approximation, and it is shown that is correlated to the length scale of cooperativity governing the relaxation process near the glass transition. The temperature and composition dependence of both and the size of the domains of cooperativity in PS/P(CHA-stat-BMA) are determined

Broad-band dielectric spectroscopy on the molecular dynamics of bulk and adsorbed poly(dimethylsiloxane)

Abstract: The molecular dynamics of poly(dimethylsiloxane) (PDMS) and its mixtures with hydrophilic and hydrophobic Aerosil (fumed silica) was investigated. After quenching from ambient temperature and subsequent reheating to about 150 K, pure PDMS exhibits the α-relaxation in the amorphous state. At higher temperatures the α c -relaxation process, which is assigned to the relaxation of the amorphous fraction in the semicrystalline polymer, is observed. Filled PDMS shows in contrast three distinct relaxation processes which originate from the varying degree of interaction with the filler surface

Formation of glasses from liquids and biopolymers.

Abstract: Glasses can be formed by many routes. In some cases, distinct polyamorphic forms are found. The normal mode of glass formation is cooling of a viscous liquid. Liquid behavior during cooling is classified between "strong" and "fragile," and the three canonical characteristics of relaxing liquids are correlated through the fragility. Strong liquids become fragile liquids on compression. In some cases, such conversions occur during cooling by a weak first-order transition. This behavior can be related to the polymorphism in a glass state through a recent simple modification of the van der Waals model for tetrahedrally bonded liquids. The sudden loss of some liquid degrees of freedom through such first-order transitions is suggestive of the polyamorphic transition between native and denatured hydrated proteins, which can be interpreted as single-chain glass-forming polymers plasticized by water and cross-linked by hydrogen bonds. The onset of a sharp change in d dT( is the Debye-Waller factor and T is temperature) in proteins, which is controversially indentified with the glass transition in liquids, is shown to be general for glass formers and observable in computer simulations of strong and fragile ionic liquids, where it proves to be close to the experimental glass transition temperature. The latter may originate in strong anharmonicity in modes ("bosons"), which permits the system to access multiple minima of its configuration space. These modes, the Kauzmann temperature T(K), and the fragility of the liquid, may thus be connected.

Nonexponential relaxations in strong and fragile glass formers

Abstract: Deviations from thermally activated and from exponential response are typical features of the vitrification phenomenon and previously have been studied using viscoelastic, dielectric, calorimetric, optical, and other techniques. Linear response data from literature on about 70 covalent glass formers, ionic melts, supercooled liquids, amorphous polymers, and glassy crystals are surveyed. Except for orientational glasses and monohydric aliphatic alcohols a distinct but broad correlation of non‐Debye behavior with non‐Arrhenius relaxations is found. Within the broad trend several groups of materials, distinguished by their respective molecular complexity, can be identified and are shown to exhibit narrow correlations. At a given degree of deviation from Arrhenius behavior externally imposed stresses are relaxed with a departure from exponential behavior which is stronger the more the molecular or atomic subunits of the glassforming material are interconnected with each other.

Relaxation in glassforming liquids and amorphous solids

Abstract: The field of viscous liquid and glassy solid dynamics is reviewed by a process of posing the key questions that need to be answered, and then providing the best answers available to the authors and their advisors at this time. The subject is divided into four parts, three of them dealing with behavior in different domains of temperature with respect to the glass transition temperature, Tg , and a fourth dealing with ‘‘short time processes.’’ The first part tackles the high temperature regime T.Tg ,i n which the system is ergodic and the evolution of the viscous liquid toward the condition at Tg is in focus. The second part deals with the regime T;Tg , where the system is nonergodic except for very long annealing times, hence has time-dependent properties ~aging and annealing!. The third part discusses behavior when the system is completely frozen with respect to the primary relaxation process but in which secondary processes, particularly those responsible for ‘‘superionic’’ conductivity, and dopart mobility in amorphous silicon, remain active. In the fourth part we focus on the behavior of the system at the crossover between the low frequency vibrational components of the molecular motion and its high frequency relaxational components, paying particular attention to very recent developments in the short time dielectric response and the high Q mechanical response. © 2000 American Institute of Physics.@S0021-8979~00!02213-1#