J. K. Krüger

Concentration and temperature dependence of the refractive index of ethanol-water mixtures: Influence of intermolecular interactions

The temperature and concentration dependence of the refractive index, nD(x, T) , in ethanol-water mixtures agrees with previous data in the ethanol-rich concentration range. The refractive index versus concentration x determined at 20 °C shows the expected maximum at about 41 mol% water (22 mass% water). The temperature derivative of the refractive index, dnD/dT, shows anomalies at lower water concentrations at about 10 mol% water but no anomaly at 41 mol% water. Both anomalies are related to intermolecular interactions, the one in nD seems to be due to molecular segregation and cluster formation while the origin of the second one in dnD/dT is still not clear.

Brillouin scattering of semicrystalline poly(4-methyl-1-pentene): study of surface effects of bulk and film material

Abstract Partially crystalline but highly transparent poly(4-methyl-1-pentene) has been investigated as bulk and film material by Brillouin scattering. Special surface effects have been demonstrated in terms of elastic compressional and shear constants. The temperature dependence of the hypersonic velocity has been studied between 17 and 350K indicating a glass transition temperature of 288K.

Simultaneous determination of elastic and optical properties of polymers by high performance Brillouin spectroscopy using different scattering geometries

General aspects of high performance Brillouin spectroscopy in polymers using special scattering geometries such as 90A-scattenng geometry are discussed. Technical improvements are reported resulting in absolute accuracies up to 0.05 % for sound velocity determination. A method of data analysis is presented delivering simultaneously the complete set of elastic stiffness constants. The influence of birefringence on the Brillouin line shifts in anisotropic polymeric systems is estimated and techniques to reduce this influence are proposed. The determination of the principal refractive indices by Brillouin spectroscopy is discussed. Furthermore, a quantityDX, which is sensitive to hypersonic relaxation processes, is introduced.

Ferroelectricity in VF2 based copolymers

Abstract Introduction of monomers of tri-fluoro or tetra-fluoro-ethylene (F3E or F4E) along the chain of polyvinylidene fluoride (PVF2 is able to induce spontaneous crystallization in a ferroelectric structure. Such copolymers exhibit a ferroelectric transition to a disordered para-electric phase. After a short review concerning the phase diagrams and the piezo and pyroelectric properties, we present recent results on the ferroelectric behaviour of VF2-F3E copolymers and on their first order ferroelectric transition. This investigation is based on dielectric measurements, second harmonic generation of light, × ray diffraction, thermal expansion measurements and Brillouin spectroscopy. Interpretation emphasizes the composite structure of these semi crystalline materials.

Evidence for structural transformations in polymer melts

Abstract Crystallizing and non-crystallizing polymers have been investigated by Brillouin spectroscopy in the liquid state. The temperature gradient of the sound velocity of crystallizing polymers shows a discontinuity at ∼60–110K above the melting transition. The non-crystallizing polymers investigated show no uniform behaviour. We interpret the phase between the melt temperature and the temperature of the additional transformation as a phase of locally nematic structure. This interpretation is also supported by a study of density, refractive index, viscosity and hypersonic attenuation.

GRADIENT OF THE MECHANICAL MODULUS IN GLASS–EPOXY–METAL JOINTS AS MEASURED BY BRILLOUIN MICROSCOPY

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.

Influence of Al(2)O(3) nanoparticles on the isothermal cure of an epoxy resin.

The influence of Al(2)O(3) nanoparticles on the curing of an epoxy thermoset based on diglycidyl ether of bisphenol A was investigated using temperature-modulated differential scanning calorimetry (TMDSC) and rheology. Diethylene triamine was used as a hardener. TMDSC not only allows for a systematic study of the kinetics of cure but simultaneously gives access to the evolution of the specific heat capacities of the thermosets. The technique thus provides insight into the glass transition behaviour of the nanocomposites and hence makes it possible to shed some light on the interaction between the nanoparticles and the polymer matrix. The Al(2)O(3) fillers are shown to accelerate the growth of macromolecules upon isothermal curing. Several mechanisms which possibly could be responsible for the acceleration are described. As a result of the faster network growth chemical vitrification occurs at earlier times in the filled thermosets and the specific reaction heat decreases with increasing nanoparticle concentration. Rheologic measurements of the zero-shear viscosity confirm the faster growth of the macromolecules in the presence of the nanoparticles.

Phase transition mechanisms revealed by optical spectroscopy

Light scattering provides a powerful tool for analysing the spectrum of lattice vibrations with respect to structural transformations. Characteristic aspects are reviewed shortly. Recent results of Brillouin and Raman investigations referring to the transitions of Rochelle salt and ammonium sulfate are presented. The elastic anomalies of these systems in the frequency range of acoustical phonons are discussed and the coupling of the order parameters to lattice vibrations is investigated.

Nonlinear elastic properties of solid polymers as revealed by Brillouin spectroscopy

In order to study the anharmonic elastic behaviour of solid polymers, we introduce a Brillouin technique, including the appropriate theoretical background, to obtain the complete tensor of elastic stiffness constants of third-order. First results are reported for glassy polycarbonate.

Imaging of microwave-induced acoustic fields in LiNbO3 by high-performance Brillouin microscopy

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.