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Andreas Schönhals

Other affiliations: Universidade Nova de Lisboa
Bio: Andreas Schönhals is an academic researcher from Bundesanstalt für Materialforschung und -prüfung. The author has contributed to research in topics: Glass transition & Dielectric. The author has an hindex of 37, co-authored 175 publications receiving 7373 citations. Previous affiliations of Andreas Schönhals include Universidade Nova de Lisboa.


Papers
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BookDOI
01 Jan 2003
TL;DR: Kremer et al. as mentioned in this paper proposed the theory of dielectric relaxation and analyzed the spectral properties of polymeric systems using NMR spectroscopy and NNMR spectra.
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

3,050 citations

Journal ArticleDOI
TL;DR: By measuring the complex dielectric function over 15 decades in frequency, Elmroth et al. as mentioned in this paper evaluated the scaling of the α-relaxation for several glass-forming liquids including propylene carbonate.
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)]

185 citations

Journal ArticleDOI
TL;DR: In this article, a broad frequency (10−1−106 Hz) and temperature range (178−423 K) was used to probe the polymer/silica interface and two relaxation processes were identified for the composites: one corresponding to the bulklike polymer and a second one related to polymer chains close to the silica surface.
Abstract: Samples with different amounts of poly(vinyl acetate) adsorbed on silica particles were prepared in order to study their interfacial interactions and dynamics. The interface of adsorbed polymers to a substrate plays an important role in many applications such as polymer nanocomposites, thin films, and coatings. Characterization of such interfaces is thus of high importance since they were found to differ from bulk properties. Thermogravimetric analysis (TGA) is applied to analyze the amounts of polymer adsorbed on silica particles. Broadband dielectric spectroscopy (BDS) is employed in a broad frequency (10–1–106 Hz) and temperature range (178–423 K) in order to probe the polymer/silica interface. Two relaxation processes can be identified for the composites: one corresponding to the bulklike polymer and a second one related to polymer chains close to the silica surface. For the latter the dynamic glass transition is shifted to higher temperature due to reduced mobility. This effect is investigated in dep...

180 citations

Journal ArticleDOI
TL;DR: In this article, the segmental dynamics and glass transition temperature of polystyrene (PS) thin films were investigated by alternating current (AC) calorimetry and dielectric spectroscopy (BDS).
Abstract: We investigate the segmental dynamics and glass transition temperature (Tg) of polystyrene (PS) thin films. The former is investigated by alternating current (AC) calorimetry and dielectric spectroscopy (BDS). The Tg, underlying the equilibrium to out-of-equilibrium crossover from the supercooled liquid to the glass, is obtained by differential scanning calorimetry (DSC) and capacitive dilatometry (CD). We show that the intrinsic molecular dynamics of PS are independent of the film thickness both for the freestanding and supported films, whereas Tg decreases with film thickness from several microns down to 15 nm. This result is found for complementary methods and in a simultaneous measurement in BDS and CD. This questions the widespread notion that segmental mobility and the equilibrium to out-of-equilibrium transition are, under any experimental conditions, fully interrelated. For thin films, it appears that the molecular mobility and Tg are affected differently by geometrical factors.

177 citations

Journal ArticleDOI
TL;DR: The strong tendency of ibuprofen to form hydrogen bonded aggregates such as dimers and trimers either cyclic or linear which seems to control in particular the molecular mobility of ib uprofen was confirmed by IR spectroscopy, electrospray ionization mass spectrometry, and MD simulations.
Abstract: The molecular mobility of amorphous ibuprofen has been investigated by broadband dielectric relaxation spectroscopy (DRS) covering a temperature range of more than 200 K. Four different relaxation processes, labeled as R, � , γ, and D, were detected and characterized, and a complete relaxation map was given for the first time. The γ-process has activation energy Ea ) 31 kJ ·mol -1 , typical for local mobility. The weak � -relaxation, observed in the glassy state as well as in the supercooled state was identified as the genuine Johari-Goldstein process. The temperature dependence of the relaxation time of the R-process (dynamic glass transition) does not obey a single VFTH law. Instead two VFTH regimes are observed separated by a crossover temperature, TB ) 265 K. From the low temperature VFTH regime, a Tg diel (τ )100 s) ) 226 K was estimated, and a fragility or steepness index m ) 93, was calculated showing that ibuprofen is a fragile glass former. The D-process has a Debye-like relaxation function but the temperature dependence of relaxation time also follows the VFTH behavior, with a Vogel temperature and a pre-exponential factor which seem to indicate that its dynamics is governed by the R-process. It has similar features as the Debye-type process observed in a variety of associating liquids, related to hydrogen bonding dynamics. The strong tendency of ibuprofen to form hydrogen bonded aggregates such as dimers and trimers either cyclic or linear which seems to control in particular the molecular mobility of ibuprofen was confirmed by IR spectroscopy, electrospray ionization mass spectrometry, and MD simulations.

154 citations


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Journal ArticleDOI
31 Mar 1995-Science
TL;DR: The onset of a sharp change in ddT( 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.
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.

4,016 citations