Jan K. Krüger

A new Brillouin scattering technique for the investigation of acoustic and opto-acoustic properties: application to polymers

Within this paper we propose a versatile Brillouin technique which allows an easy determination of acoustic and opto-acoustic properties of thin films. This new technique is especially suitable for studying acoustic dispersion behaviour at hypersonic frequencies. Typical applications in the field of polymers have been used to demonstrate the efficiency of the proposed scattering technique.

Effect of nitrogen concentration on plasma reactivity and diamond growth in a H2CH4N2 microwave discharge

Abstract The effect of nitrogen on the growth of diamond by microwave plasma-assisted chemical vapour deposition, when added in a H2CH4 gas mixture was investigated by micro-Raman spectroscopy and by scanning electron microscopy. A clear improvement in the surface morphology and quality of the diamond films indicates the beneficial effect of adding nitrogen to the gas mixture. The concomitant study of plasma parameters and diamond films properties leads to a better understanding of the effect of nitrogen in the diamond growth process and to the determination of optimal concentrations of nitrogen and methane in the mixture. We report in this paper the effect of varying the concentration of nitrogen introduced in different H2CH4 mixtures. The plasma reactivity is studied by optical emission spectroscopy during diamond film growth, and we focus our experiments on the variation of the emission line of the CH and CN radicals. These species can be considered as key species to explain reactions of the nitrogen in the gaseous phase and its interaction with the surface of the film in growth.

Epoxy and Polyurethane Networks in Thin Films On Metals—Formation, Structure, Properties

Thin adhesive films on metals are used as an appropriate model for the characterization of the interphase in polymer‐metal compounds. This interphase is a result of the specific interactions between the adhesive partners. It is particularly relevant for the performance, and the long‐term stability of adhesively bonded structures and coatings. The influence of the metal substrate affects the formation of the polymer network during the polymerization, as well as its resulting structure and properties significantly as compared to the bulk. As examples, a thermosetting epoxy (EP) and a polyurethane (PU) elastomer are chosen for the investigation of the morphology and the chemical structure of the thin films (thickness between 20 nm and 3 μm) on native surfaces of gold, aluminium, and copper. The results for the two types of polymers reveal characteristic differences with respect to reaction rate and final degree of cure as a function of the film thickness on the different metals. The final structure turns out to be a consequence of competing processes such as polyaddition and side reactions including their kinetics, catalysis or inhibition, complexation and dissolution of the complexes, preferential adsorption and phase separation, and the changes in molecular mobility.

From Molecular Dehydration to Excess Volumes of Phase-Separating PNIPAM Solutions

For aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions, a structural instability leads to the collapse and aggregation of the macromolecules at the temperature-induced demixing transition. The accompanying cooperative dehydration of the PNIPAM chains is known to play a crucial role in this phase separation. We elucidate the impact of partial dehydration of PNIPAM on the volume changes related to the phase separation of dilute to concentrated PNIPAM solutions. Quasi-elastic neutron scattering enables us to directly follow the isotropic jump diffusion behavior of the hydration water and the almost freely diffusing water. As the hydration number decreases from 8 to 2 for the demixing 25 mass % PNIPAM solution, only a partial dehydration of the PNIPAM chains occurs. Dilatation studies reveal that the transition-induced volume changes depend in a remarkable manner on the PNIPAM concentration of the solutions. The excess volume per mole of H2O molecules expelled from the solvation layers of PNIPAM during phase separation probably strongly increases from dilute to concentrated PNIPAM solutions. This finding is qualitatively related to the immense strain-softening previously observed for demixing PNIPAM solutions.

Combined effect of nitrogen and pulsed microwave plasma on diamond growth

Abstract Deposition of diamond layers from a CH4-H2 microwave discharge operating in pulse mode was achieved. The resulting diamond layers showed less graphite contamination as compared with that obtained in continuous discharge. Furthermore the addition of nitrogen to the discharge operating in this pulse mode was shown to further decrease the graphite contamination and to improve crystallisation due to reduction of secondary germination. However using nitrogen in the gas mixture resulted also in a decrease in nucleation density and in discontinuous formation of diamond layers.

Hypersonic properties of nano- and microstructured CVD diamond

Basic information related to the acoustic dispersion, local sound velocity distribution and influence of the grain size on the elastic properties of white plasma-assisted chemical vapour-deposited diamond has been obtained by a new Brillouin scattering technique. The obtained results show no acoustic dipersion and the existence of internal stresses that influence the elastic properties of the diamond sample.

Combined effect of nitrogen and pulsed microwave plasma on diamond growth using CH4–CO2 gas mixture

Abstract Deposition of diamond layers in a CH4–CO2 microwave discharge operating in pulsed mode was achieved. The effects of nitrogen addition and microwave power modulation on the plasma reactivity were studied by time resolved optical emission spectroscopy. An enhancement of the molecular dissociation during the early stage of the pulse has been observed. The resulting diamond layers showed less graphite contamination as compared with that obtained in continuous discharge. Furthermore, the addition of nitrogen to the discharge operating in this pulsed mode further decreases the graphite contamination and improves crystallization due to reduction of secondary germination.

On the interplay between matter transport and structure formation at epoxy–hardener interfaces visualized by scanning Brillouin microscopy

Structural developments are investigated in network-forming reactive polymers by time- and space-resolved scanning Brillouin microscopy. Hypersonic properties are probed to reveal the subtle interplay between molecular transport, dissolution, polymerization and network defects in the vicinity of the interface between reactants, which are either pure epoxy resin or various epoxy resin–hardener mixtures, topped by a layer of pure hardener. The trans-interfacial polymerization produces heterogeneous epoxy structures of either gelatinous or glassy nature. Interestingly, the hardener can easily penetrate and swell these networks and epoxy network fragments can be transported over several millimetres by convective flow. The observed features may be used to form interpenetrating networks during self-healing procedures.

Phase Instability and Molecular Kinetics Provoked by Repeated Crossing of the Demixing Transition of PNIPAM Solutions

The demixing process of aqueous poly(N-isopropylacrylamide) (PNIPAM) solutions can occur either via a nucleation and growth process or via spinodal decomposition. The ensuing self-assembly, leading to heterogeneous morphologies within the PNIPAM solution, is codetermined by kinetic processes caused by molecular transport. By subjecting PNIPAM solutions to cyclic changes in temperature leading to repeated crossing of the demixing transition, we are able to assess the importance of kinetics as well as of overheating and supercooling of the phase transition within the metastable range delimited by the binodal and spinodal lines. First indications about the location of these stability limits for the low- and high-temperature phases, separated by about 1.6 K, could be gained by detailed kinetic studies of the refractive index. These investigations are made possible due to the novel technique of temperature-modulated optical refractometry.

Kinetic processes at the demixing transition of PNIPAM solutions

Kinetic processes, which are joined with mass transport, are studied in the vicinity of the sharp LCST-type demixing transition of semi-dilute aqueous poly(N-isopropyl acrylamide) (PNIPAM) solutions. These processes are slow as compared to the highly cooperative collapse of individual polymer chains. Purely elastic properties, that are particularly sensitive to this phase transition, are addressed depending on the temperature, space and time by Brillouin spectroscopy. Above the demixing temperature Tc, we discriminate between kinetics related to the phase separation into PNIPAM-rich and PNIPAM-poor domains and kinetics connected to the impact of gravitation on the on-going phase separation. Using shallow temperature jumps of 0.3 °C, the growth of compact PNIPAM-rich agglomerates with identical gel-like mechanical consistency is provoked independently of temperature and position within the sample above Tc. Astonishingly, the transition temperature does not vary while heating or cooling the solutions across the phase transition, although the elastic properties depend strongly on space and time during the equilibration of PNIPAM concentration gradients following the re-entrance into the low-temperature phase.