![]() Krüger, Jan-Kristian ![]() ![]() E-print/Working paper (2008) Based on experimental evidences we present a phenomenological description of the thermal glass transition as a dynamical phase transition. Different susceptibilities calculated on the basis of this ... [more ▼] Based on experimental evidences we present a phenomenological description of the thermal glass transition as a dynamical phase transition. Different susceptibilities calculated on the basis of this description are in good qualitative agreement with experimental data. As an essential feature this novel view of the glass transition reflects the kinetic as well as the transition aspect of the thermal glass transition. [less ▲] Detailed reference viewed: 71 (5 UL)![]() Müller, Ulrich ![]() ![]() in New Journal of Physics (2008), 10(2), 023-031 Reactive network forming polymer systems like epoxies are of huge technological interest because of their adhesive properties based on specific interactions with a large variety of materials. These ... [more ▼] Reactive network forming polymer systems like epoxies are of huge technological interest because of their adhesive properties based on specific interactions with a large variety of materials. These specific interactions alter the morphology of the epoxy within areas determined by the correlation length of these interactions. The changed morphology leads to interphases with altered (mechanical) properties. Besides these surface-induced interphases, bulk interphases do occur due to segregation, crystallization, diffusion, etc. A new experimental technique to characterize such mechanical interphases is μ-Brillouin spectroscopy (μ-BS). With μ-BS, we studied interphases and their formation in epoxies due to segregation of the constituent components and due to selective diffusion of one component. In the latter case, we will demonstrate the influence of changing the boundary conditions of the diffusion process on the shape of the interphase. [less ▲] Detailed reference viewed: 116 (0 UL)![]() Philipp, Martine ![]() ![]() ![]() in New Journal of Physics (2008), 10 A commercial physical gel composed of gelatine, water and glycerol shows a sol–gel transition which has been resolved by optical rotation measurements by step-wise heating the gel. This transition is not ... [more ▼] A commercial physical gel composed of gelatine, water and glycerol shows a sol–gel transition which has been resolved by optical rotation measurements by step-wise heating the gel. This transition is not observable in the longitudinal acoustic mode measured at hypersonic frequencies with Brillouin spectroscopy. Depending on the thermal treatment of the investigated material during the sol–gel transition and within the gel state, Brillouin spectroscopy reflects tremendously different hypersonic dynamics. These distinct dynamics are responsible for the formation of different glassy states at low temperatures including that of a glass-ceramic. The large variety of super-cooled and glassy states is attributed to distinct distributions of the gel's constituents within the samples. Surprisingly, the same gel state can be produced either by annealing the gel over months or by the non-equilibrium effect of thermo-diffusion (Soret effect) in the course of some minutes. [less ▲] Detailed reference viewed: 104 (2 UL)![]() Philipp, Martine ![]() ![]() in eXPRESS Polymer Letters (2008), 2(8), 546-552 High performance refractometry has been proven to be a useful tool to elucidate the isothermal curing process of nanocomposites. As a model system an amine-hardening epoxy filled with non-surface-treated ... [more ▼] High performance refractometry has been proven to be a useful tool to elucidate the isothermal curing process of nanocomposites. As a model system an amine-hardening epoxy filled with non-surface-treated alumina nanoparticles was selected. The tremendous resolution of this experimental technique is used to study morphological changes within nanocomposites via the refractive index. It is shown that these morphological changes are not simply due to the curing process but also depend on the sequence of mixing the nanoparticles either first into the resin or first into the hardener. Independent of the resin/hardener composition, the type of the mixing sequence discriminates systematically between two distinct refractive index curves produced by the curing process. The difference between the two refractive index curves increases monotonically with curing time, which underlines the importance of the initial molecular environment of the nanoparticles. [less ▲] Detailed reference viewed: 117 (2 UL)![]() Krüger, Jan-Kristian ![]() ![]() in Henkel, M.; Pleimling, M.; Sanctuary, Roland (Eds.) Aging of the Glass Transition (2007) The nature of the glassy state and of the glass transition of structural glasses is still a matter of debate. This debate stems predominantly from the kinetic features of the thermal glass transition ... [more ▼] The nature of the glassy state and of the glass transition of structural glasses is still a matter of debate. This debate stems predominantly from the kinetic features of the thermal glass transition. However the glass transition has at least two faces: the kinetic one which becomes apparent in the regime of low relaxation frequencies and a static one observed in static or frequency-clamped linear and non-linear susceptibilities. New results concerning the so-called ?-relaxation process show that the historical view of an unavoidable cross-over of this relaxation time with the experimental time scale is probably wrong and support instead the existence of an intrinsic glass transition. In order to prove this, three different experimental strategies have been applied: studying the glass transition at extremely long time scales, the investigation of properties which are not sensitive to the kinetics of the glass transition and studying glass transitions which do not depend at all on a forced external time scale. [less ▲] Detailed reference viewed: 86 (3 UL)![]() Philipp, Martine ![]() ![]() ![]() in Beiträge zur Experimentalphysik, Didaktik und computergestützten Physik (2007) Detailed reference viewed: 89 (5 UL)![]() Sanctuary, Roland ![]() ![]() ![]() in Thermochimica Acta (2006), 445(2), 111-115 Thermal investigations on two selected model-nanocomposites have been made. They differ with regard to the type of the anorganic nanoparticles that have been filled into an organic oligomer matrix. The ... [more ▼] Thermal investigations on two selected model-nanocomposites have been made. They differ with regard to the type of the anorganic nanoparticles that have been filled into an organic oligomer matrix. The properties of nanocomposites may vary between those of a simple mixture of independent components and those of a system, where specific interfacial interactions between the constituting parts lead to ‘new’ properties. Depending on the type of the nanoparticles filled into the matrix, the resulting properties might be closer to one or to the other extreme. We used temperature modulated differential scanning calorimetry (TMDSC) to investigate a matrix of the oligomer diglycidyl ether of bisphenol A (DGEBA) filled either with SiO2- or Al2O3-nanoparticles. The dependence of the complex specific heat capacity () on the concentration of nanoparticles shows a clear difference between the two systems as far as the glass transition of the oligomer is concerned. The SiO2 composite seems to behave more like a simple mixture, whereas the Al2O3 composite shows ‘new’ properties. [less ▲] Detailed reference viewed: 146 (2 UL)![]() ; ; Baller, Jörg ![]() in Journal of Physics. Conference Series (2006), 40 The elastic properties of the isotropic state of condensed matter are given by the elastic constants ell and c44. In the liquid state the static shear stiffness c44 vanishes whereas at sufficient high ... [more ▼] The elastic properties of the isotropic state of condensed matter are given by the elastic constants ell and c44. In the liquid state the static shear stiffness c44 vanishes whereas at sufficient high probe frequencies a dynamic shear stiffness may appear. In that latter case the question about the existence of a Cauchy relation appears. It will be shown that a pure Cauchy relation can appear only under special conditions which are rarely fulfilled. For all investigated materials, including ceramics, liquids and glasses, a linear relation between ell and c44 called generalized Cauchy relation is observed, which, surprisingly, follows a linear transformation. [less ▲] Detailed reference viewed: 151 (9 UL)![]() ; ; et al in Physical Review Letters (2005), 94(12), 129603-129604 Detailed reference viewed: 99 (1 UL)![]() Krüger, Jan-Kristian ![]() ![]() in Physical Review Letters (2005), 94 Detailed reference viewed: 116 (3 UL)![]() Krüger, Jan-Kristian ![]() ![]() in Adhesion - Current Research and Application (2005) Detailed reference viewed: 81 (6 UL)![]() Krüger, Jan-Kristian ![]() in New Journal of Physics (2003), 5(80), 1-11 Using Brillouin spectroscopy as a probe for high-frequency clamped acoustic properties, a shear modulus can be measured in addition to the longitudinal modulus already well above the thermal glass ... [more ▼] Using Brillouin spectroscopy as a probe for high-frequency clamped acoustic properties, a shear modulus can be measured in addition to the longitudinal modulus already well above the thermal glass transition. On slow cooling of the liquid through the thermal glass transition temperature Tg, both moduli show a kink-like behaviour and the function follows a generalized Cauchy relation (gCR) defined by the linear relation , which completely hides the glass transition. In this work we show experimentally that on fast cooling this linear transformation becomes violated within the glassy state, but that thermal ageing drives the elastic coefficients towards the gCR, i.e. towards a unique glassy state. [less ▲] Detailed reference viewed: 104 (5 UL)![]() Krüger, Jan-Kristian ![]() ![]() in Physical Review. B, Condensed Matter and Materials Physics (2002), 66(1), 012206-012210 It is well known that in addition to the longitudinal modulus, viscoelastic liquids show a shear stiffness at sufficiently high probe frequencies due to structural relaxations. For probe frequencies that ... [more ▼] It is well known that in addition to the longitudinal modulus, viscoelastic liquids show a shear stiffness at sufficiently high probe frequencies due to structural relaxations. For probe frequencies that are large compared to the structural relaxation frequency, the measured elastic longitudinal and shear moduli become so-called clamped properties (c11∞ and c44∞, respectively). During freezing or polymerization of amorphous liquids, these clamped moduli behave in a strongly nonlinear fashion as a function of temperature or polymerization time. Based on Brillouin spectroscopy data we will show that there exists a linear relation between c11∞ and c44∞ over a large temperature or polymerization time range. Surprisingly, the parameters of this linear relation between the elastic moduli vary only little for different materials. Implications for the nonlinear elastic behavior at the glass transition will be discussed on the basis of mode Grüneisen parameters. [less ▲] Detailed reference viewed: 104 (1 UL)![]() Krüger, Jan-Kristian ![]() ![]() in Physical Review Letters (2002), 89 For the low molecular weight fragile liquid diglycidyl ether of bisphenol A we report, based on Brillouin and dielectric spectroscopy, on a thermal glass transition where the relaxation time of the α ... [more ▼] For the low molecular weight fragile liquid diglycidyl ether of bisphenol A we report, based on Brillouin and dielectric spectroscopy, on a thermal glass transition where the relaxation time of the α process does not go to infinity. Instead, the structural α relaxation disappears spontaneously at the transition point. That discontinuity in relaxation time coincides with a kink in the longitudinal hypersonic velocity and determines unambiguously the transition from the liquid to the glassy state. [less ▲] Detailed reference viewed: 87 (0 UL)![]() Krüger, Jan-Kristian ![]() in Ferroelectrics (2001), 259(1-4), 27-36 Within this paper we present mono-disperse dendrimers as a new class of ideal canonical glass formers where the frustration mechanism originates from the self similar intramolecular order. Because of ... [more ▼] Within this paper we present mono-disperse dendrimers as a new class of ideal canonical glass formers where the frustration mechanism originates from the self similar intramolecular order. Because of their fractal intramolecular structure these dendrimers are hardly able to build up a three dimensional translation symmetry necessary to form the crystalline state. Instead they show an unconventional dynamic and static freezing behavior at rather low temperatures. Based on temperature modulated differential scanning calorimetry (TMDSC) and Brillouin spectroscopy (BS) we give further evidences for the existence of an ideal primary glass transition with a thermal glass transition temperature which increases with the generation index of the fractal molecule. However, the primary glass transitions of higher generations do not completely erase the memory of the fractal intramolecular mobility but allow the survival of dynamic processes significant for the freezing of the constituting molecular unit. [less ▲] Detailed reference viewed: 99 (0 UL)![]() Krüger, Jan-Kristian ![]() ![]() in Vide -Paris- (2001), 56(300), 375-393 Detailed reference viewed: 40 (4 UL)![]() Krüger, Jan-Kristian ![]() ![]() in Proceedings of Frontiers in Science and Technology, Advanced Physics Studies Press (2000),7th FSRC FERROELECTRICS-2000 (2000) Detailed reference viewed: 49 (8 UL)![]() Baller, Jörg ![]() ![]() in Journal of Physics : Condensed Matter (2000), 12(25), 5403-5409 High resolution Brillouin spectroscopy was used to characterize the elastic stiffness properties of consolidated nano-crystalline yttrium oxide as well as of the related single-crystalline state. Defect ... [more ▼] High resolution Brillouin spectroscopy was used to characterize the elastic stiffness properties of consolidated nano-crystalline yttrium oxide as well as of the related single-crystalline state. Defect enriched grain boundaries are discussed as sources for the extremely soft elastic properties of the nano-crystalline state. [less ▲] Detailed reference viewed: 78 (0 UL)![]() Krüger, Jan-Kristian ![]() ![]() in Physical Review. B, Condensed Matter and Materials Physics (1999), 60(14), 10037-10041 Time-domain temperature-modulated differential scanning calorimetry and high-performance Brillouin spectroscopy have been used as experimental techniques in order to investigate the effect of the thermal ... [more ▼] Time-domain temperature-modulated differential scanning calorimetry and high-performance Brillouin spectroscopy have been used as experimental techniques in order to investigate the effect of the thermal glass transition on the specific heat as well as on the acoustic properties, quantities which are intimately related by thermodynamics, but measured at completely different frequencies. Using a siloxane as a model substance, we demonstrate the efficiency of hypersonic relaxations until thermal freezing of the material. Introducing the optoacoustic dispersion function as a sensitive measure for structural relaxations, we show that the high-frequency α relaxations are truncated by the thermal glass transition instead of dying out at much higher temperatures. Moreover, the extrapolated static compliance of the rubbery state goes to zero close to but below the thermal glass transition temperature Tg, indicating the existence of an acoustic instability which is reminiscent of the so-called Kauzmann paradox. [less ▲] Detailed reference viewed: 76 (0 UL)![]() Krüger, Jan-Kristian ![]() in Nanostructured Materials (1999), 12(1), 519-522 The nature of the glass transition is still a matter of debate. We used temperature modulated DSC (TMDSC) and Brillouin spectroscopy ( BS ) to investigate the freezing behaviour of canonical glasses ... [more ▼] The nature of the glass transition is still a matter of debate. We used temperature modulated DSC (TMDSC) and Brillouin spectroscopy ( BS ) to investigate the freezing behaviour of canonical glasses ( salol and di-n-butyl-phtalate (DBP)) confined to controlled pore glasses (CPG) with pore diameters between 2.5 and 20 nm. The temperature behaviour of the heat capacity of the composite system deviates from the bulk fluid properties and we interpret the remaining kink-like anomaly at Tg as a static glass transition. In smaller pores Tg is only slightly shifted to lower temperatures. This operative Tg is in good agreement with sound velocity data showing a sharp kink at Tg. The working hypothesis of the existence of a static glass transition is supported by the fact that the -relaxation process disappears with decreasing pore diameter. [less ▲] Detailed reference viewed: 107 (1 UL) |
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