References of "Alnot, Patrick"
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See detailSecond order elasticity at hypersonic frequencies of reactive polyurethanes as seen by generalized Cauchy relations.
Philipp, Martine UL; Vergnat, Christelle; Müller, Ulrich UL et al

in Journal of Physics : Condensed Matter (2009), 21

The non-equilibrium process of polymerization of reactive polymers can be accompanied by transition phenomena like gelation or the chemical glass transition. The sensitivity of the mechanical properties ... [more ▼]

The non-equilibrium process of polymerization of reactive polymers can be accompanied by transition phenomena like gelation or the chemical glass transition. The sensitivity of the mechanical properties at hypersonic frequencies—including the generalized Cauchy relation—to these transition phenomena is studied for three different polyurethanes using Brillouin spectroscopy. As for epoxies, the generalized Cauchy relation surprisingly holds true for the non-equilibrium polymerization process and for the temperature dependence of polyurethanes. Neither the sol–gel transition nor the chemical and thermal glass transitions are visible in the representation of the generalized Cauchy relation. Taking into account the new results and combining them with general considerations about the elastic properties of the isotropic state, an improved physical foundation of the generalized Cauchy relation is proposed. [less ▲]

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See detailAcoustic Profilometry Of Interphases In An Epoxy Due To Segregation and Diffusion Using Brillouin Microscopy.
Müller, Ulrich UL; Bactavatchalou, R.; Baller, Jörg UL et al

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 ▲]

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See detailChemically induced transition phenomena in polyurethanes as seen from generalized mode Grüneisen parameters.
Müller, Ulrich UL; Philipp, Martine UL; Bactavatchalou, R. et al

in Journal of Physics : Condensed Matter (2008), 20(20), 205101-205108

Many phenomenological properties of reactive polymers like polyurethanes increase or decrease continuously in the course of the curing process before saturating at the end of the chemical reaction. This ... [more ▼]

Many phenomenological properties of reactive polymers like polyurethanes increase or decrease continuously in the course of the curing process before saturating at the end of the chemical reaction. This holds true for instance for the mass density, the refractive index, the chemical turnover and the hypersonic properties. The reason for this monotone behaviour is that the chemical reaction behaves like a continuous succession of irreversible phase transitions. These transitions are superposed by the sol–gel transition and possibly by the chemically induced glass transition, with the drawback that the latter two highlighted transitions are often hidden by the underlying curing process. In this work we propose generalized mode Grüneisen parameters as an alternative probe for elucidating the polymerization process itself and the closely related transition phenomena. As a model system we use polyurethane composed of a diisocyanate and varying ratios of difunctional and trifunctional alcohols. [less ▲]

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See detailAbout the nature of the structural glass transtion: an experimental approach
Krüger, Jan-Kristian UL; Alnot, Patrick; Baller, Jörg UL et al

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 ▲]

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See detailComplex specific heat capacity of two nanocomposite systems
Sanctuary, Roland UL; Baller, Jörg UL; Krüger, Jan-Kristian UL et al

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 ▲]

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See detailThe generalized Cauchy relation: a probe for local structure in materials with isotropic symmetry.
Bactavatchalou, R.; Alnot, Patrick; Baller, Jörg UL et al

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 ▲]

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See detailMapping of Microwave-Induced Phonons by µ-Brillouin Spectroscopy: Hypersons in ZnO on Silicon
Krüger, Jan-Kristian UL; Mainka, J.; Alnot, Patrick et al

in Journal de Physique IV (2005), 129

From the structural point of view a simple Cauchy relation is not expected to hold for isotropic materials. Such a Cauchy relation would imply the reduction of independent elastic stiffness constants for ... [more ▼]

From the structural point of view a simple Cauchy relation is not expected to hold for isotropic materials. Such a Cauchy relation would imply the reduction of independent elastic stiffness constants for the isotropic state from two to one. However, high frequency elastic data of glasses and viscous liquids show a linear transformation between the shear and the longitudinal elastic stiffness which is called a generalized Cauchy relation. It seems, that the parameters of the linear transformation are related to the global and local symmetry and/or order. Brillouin investigations on the elastic stiffness coefficients of a consolidated nano-crystalline material (CeO2) and of DGEBA/SiO2 nano-composites are used in order to elucidated the role of the discrepancy between local and global symmetry and/or order. [less ▲]

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See detailImaging of microwave-induced acoustic fields in LiNbO3 by high-performance Brillouin microscopy
Vincent, B.; Krüger, Jan-Kristian UL; Elmazria, Omar et al

in Journal of Physics : D Applied Physics (2005), 38(12), 2026-2030

High performance Brillouin microscopy (BM) has been used to characterize the spatial distribution of piezoelectrically induced acoustic fields excited at microwave frequencies in a LiNbO3 single crystal ... [more ▼]

High performance Brillouin microscopy (BM) has been used to characterize the spatial distribution of piezoelectrically induced acoustic fields excited at microwave frequencies in a LiNbO3 single crystal. It is demonstrated that under suitable conditions BM is able to detect microwave-induced bulk as well as surface acoustic waves. Brillouin spectroscopy is able to probe sound wave intensities of induced phonons, which are as small as those of thermal phonons. [less ▲]

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See detailGradient of the Mechanical Modulus in Glass-Epoxy-Metal Joints as Measured by Brillouin Microscopy.
Krüger, Jan-Kristian UL; Possart, Wulff; Bactavatchalou, R. et al

in Journal of Adhesion (2004), 80(7), 585-599

The newly developed Brillouin microscopy is used for the first time to measure in situ the longitudinal elastic stiffness coefficient in the GHz-range inside of glass-epoxy-metal joints as a function of ... [more ▼]

The newly developed Brillouin microscopy is used for the first time to measure in situ the longitudinal elastic stiffness coefficient in the GHz-range inside of glass-epoxy-metal joints as a function of distance from the substrates. Interphases with a local variation of mechanical properties are quantitatively characterized. These interphases possess unexpected widths of tens to hundreds of microns. Inside the interphases, the spatial variation of the longitudinal stiffness coefficient depends on the type of substrate, on the curing conditions for the epoxy and probably on the distribution of internal stresses. The obtained spatial mechanical profiles provide valuable insight into the morphology-driven mechanics of the interphase, but additional information is needed for a full understanding of their physical and chemical origin. The presented results prove the sensitivity of the Brillouin microscopy; the elastic stiffness coefficients are detected with an accuracy in the subpercentage range. The spatial resolution is better than 10 µm. [less ▲]

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See detailDifferent glassy states, as indicated by a violation of the generalized Cauchy relation.
Krüger, Jan-Kristian UL; Britz, Th; le Coutre, A. et al

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 ▲]

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See detailAcoustic profilometry within polymers as performed by Brillouin microscopy
Sanctuary, Roland UL; Bactavatchalou, R.; Müller, Ulrich UL et al

in Journal of Physics : D Applied Physics (2003), 36(21), 2738-2742

Using high performance Brillouin spectroscopy we present a new technique, which enables us to perform acoustic microscopy with a spatial resolution of about 1 μm. This technique, called Brillouin ... [more ▼]

Using high performance Brillouin spectroscopy we present a new technique, which enables us to perform acoustic microscopy with a spatial resolution of about 1 μm. This technique, called Brillouin microscopy, is tested on several bulk- and film-like polymer samples. [less ▲]

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See detailUniversal relaxation behavior of classical liquid crystals at hypersonic frequencies.
Grammes, C.; Krüger, Jan-Kristian UL; Bohn, K.-P. et al

in Physical Review. E : Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics (1995), 51

Within this work we present comparative Brillouin and ultrasonic investigations of p-methoxybenzylidene p-(n-butylaniline), p-azoxyanisol, and 4-cyano-4’-pentyl-biphenyl performed in the vicinity of the ... [more ▼]

Within this work we present comparative Brillouin and ultrasonic investigations of p-methoxybenzylidene p-(n-butylaniline), p-azoxyanisol, and 4-cyano-4’-pentyl-biphenyl performed in the vicinity of the nematic-isotropic transition. We confirm the existence of a universal hypersonic relaxation mechanism of weak activation energy and a related significant splitting of the two principal longitudinal acoustic modes within the nematic phase. Brillouin refractometry is proposed as a method to determine the magnitude of the nematic order parameter simultaneously with the second-order elastic stiffness constants. The origin of the hypersonic anisotropy is discussed in terms of thermal relaxations and the evolution of the nematic order parameter. [less ▲]

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See detailSecond-order elasticity of liquid crystals within their nematic state at high frequencies
Krüger, Jan-Kristian UL; Grammes, C.; Jiménez, R. et al

in Physical Review. E : Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics (1995), 51(3), 2115-2128

Within this work we present Brillouin and ultrasonic investigations performed on the liquid crystals p-methoxybenzylidene p-(n-butylaniline) (MBBA), p-azoxyanisol (PAA), 4-cyano-4-n-alkylbiphenyles (nCB ... [more ▼]

Within this work we present Brillouin and ultrasonic investigations performed on the liquid crystals p-methoxybenzylidene p-(n-butylaniline) (MBBA), p-azoxyanisol (PAA), 4-cyano-4-n-alkylbiphenyles (nCB), and 4-n-pentoxybenzylidene-4'-n-octoaniline (5O.8). Special attention has been paid to the evolution of a significant splitting of the two longitudinal polarized acoustic modes within the nematic phase. Angle-resolving Brillouin spectroscopy allowed the determination of the complete stiffness tensors and hence a discussion of the propagation of quasitransverse polarized acoustic modes in the nematic state of liquid crystals. We analyze the behavior of the eigenvectors of the relevant acoustic modes and discuss their significance for the propagation of quasishear modes. For 5O.8 we report an acoustic instability within its nematic state. We confirm the existence of a universal thermal relaxation mechanism of weak activation energy for the nematic state of classical liquid crystals. The observed hypersonic anisotropy is discussed in terms of that relaxation mechanism and is compared with that of glass-forming liquid crystals. [less ▲]

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