IFF
Scientific Report 2000/2001

The year 2000 was a normal year of reactor operation. The FRJ-2 delivered neutrons for 177 days with good reliability. During that time all together 201 experiments were carried out, 114 of them with external participation. These experiments were performed out with the support of the Institute for Neutronscattering, the Institute for Scattering Techniques and external collaborating research groups. In 2000 the 4^th Neutron Laboratory Course took place. This time, the participation was opened for European students and supported financially by the European Neutron Round Table. From the 36 accepted participants 12 came from outside Germany. Seven students performed the Laboratory Course as part of the advanced practical course in materials science in the frame of the physical chemistry curriculum at the University of Münster.

In 2000 at the NSE spectrometer in the ELLA laboratory a double spin echo set up was implemented. This device increased the accessible dynamic range to four orders of magnitude in time. Setting up a collimator system in front of the detector reduced the instrumental background significantly, allowing now the study of very weak incoherent scattering which is always a problem for spin echo spectroscopy. Here, the Jülich instrument provides now unique capabilities (see report A. Wischnewski).

The backscattering spectrometer (BSS 1) was equipped with a new set of analyzers featuring large perfect silicon crystals. In this way, the instrumental resolution was significantly improved. In particular the line shape of the resolution function was transformed from a Lorenzian to a Gaussian shape, enabling the instrument to detect minute amounts of quasielastic scattering close to the central peak (see report H. Grimm).

At the triple axis spectrometer (IN 12), by a number of instrumental improvements in particular using a big vacuum tank around the sample, the instrumental background in forward direction was significantly decreased. In this way e. g inelastic spectroscopy on glass forming liquids at very small Q-values became possible.

The design and construction of the backscattering spectrometer for the FRM-II in Munich proceeded according to the management plan. The design of the phase space transformation chopper and the analyzer system was completed; both items are now under construction. A new system for glueing perfect silicon crystals on the analyzer spheroids was developed and implemented. The doppler drive which uses aircushions was finally constructed. One remaining problem relates to the surface treatement of the guiding rails and grooves, which most likely is solved by atmospheric plasma spraying. An intermediate solution was chosen for the construction of the prototype.

Most of the research in the institute focuses on the study of soft condensed matter systems, emphasizing polymers and complex fluids. Such research requires a close collaboration between synthetic chemists and physicists. In the institute this is realized by a synthetic laboratory which provides the samples for the physics research.

Aside of the standard preparation of homo- and block-copolymers two items deserve to be highlighted:

Dynamic properties of polymers are investigated from large scale motions like reptation, reaching to local relaxation and the b -process in glass forming polymers.

• Large scale motion

• Local dynamics

In a collaboration with the group of Prof. Gompper (Theorie II), we exploited the ability to tune the surface elasticity of bicontinuous microemulsions by the addition of minute amounts of amphiphilic block copolymers. We showed that the bending elasticity can be directly read off from structural data. For the first time, we measured the spatial renormalization of the bending elasticity and determined the prefactor, which was under discussion for more than ten years (see report G. Gompper/Theorie II).

The dynamics of bicontinuous microemulsions was studied over a large range of composition and momentum transfer combining NSE and dynamic light scattering. In the range of the structural relaxation the data agree qualitatively with the prediction of the time dependent Landau-Ginzburg theory of Gompper. At more local scales the data assume asymptotically the behavior predicted by the Zilman and Granek theory.

Our studies on the micellarization properties of PEO-PEP amphiphilic diblockcopolymers were continued by measurements on symmetric species of the PEOx-PEPx type. A shape transition from spherical to rod like micelles was detected. The experimental results could be interpreted satisfactorily by a mean field model of Nagaraian and Ganesh (see report H. Kaya).

The short range order of polymers in the melt was investigated on a series of differently labeled Polyisoprene materials. The experimental structure factors were quantitatively compared with computer simulations (group of Prof. Colmenero/Univ. San Sebastian) achieving in general good agreement (see report R. Zorn). Now the simulations are used in order to identify specific partial structure factors contributing to the observed experimental spectra.

In a collaboration with the theory group of Prof. Löwen in Düsseldorf, careful studies on the phase diagram of star polymer colloid mixtures were undertaken. Thereby, the star arm number was varied from 2 to 32 and different star colloid size ratios were employed. A detailed comparison of experiment and theory lead to a consistent explanation of the experimental findings in terms of a soft star potential-colloid (see report J. Stellbrink).

The phase diagram of a three component polymer mixture consisting of a dPB/PS (deuterated Polybutadiene/Polystyrene) homopolymer blend of critical composition and different amounts of the corresponding dPB-PS diblock copolymer was explored. Beyond the Lifshitz line (at 6% diblock content) a micro emulsion phase was observed. First attempts were undertaken to determine the phase boundaries between disordered, bicontinuous, droplet, and ordered phases by applying theoretical approaches for the micro emulsion phases. Furthermore, as a function of temperature the Lifshitz line depends on the diblock concentration, an observation contradicting present mean field theories.

The work on rubbers concentrated on the role of fillers. Spherical silica particles aggregating in clusters and cylindrical fillers arising from microphase separations in a triblockcopolymer of the type Polybutadiene-Polystyrene-Polybutadiene were investigated. A careful analysis of the overstrain of the polymer matrix yielded a reinforcement factor for the silica fillers agreeing with a Padé-formulation of the strain amplification. To our surprise an analysis of the rod like filler does not reveal any strain amplification effect at all. A combination of SAXS and SANS results is consistent with an unaffected matrix network and limited strain orientation of the rods with subsequent breaking of the cylinders.

The work in the Brite Euram project on "The science and technology of long chain branching in Polyolefines and their process control", concentrated on the analysis of the relaxation behavior of partially labeled H-blockcopolymers. The theory development was performed in collaboration with Prof. Straube from Halle and Prof. McLeish from Leeds. An inclusion of the reduction of topological constraints due to dynamic dilution processes and the consideration of incompressibility leads now to a workable approach which gives a relatively good description of the SANS pattern as well as the rheological results.

Further work was carried out in an industrial collaboration with DSM, where the unperturbed chain dimensions of Nylon 6 were studied. These experiments were extremely difficult and needed the elimination of polyelectrolyte as well as intrachain hydrogen bonding effects.

The study of rotational tunneling concentrated on the evaluation of defect structures and their influence on the tunneling spectra. Comparison of the tunneling spectra in glassy and crystalline toluene were performed in collaboration with the group of Prof. Colmenero in San Sebastian. A nice example of controlled defect structures and their effect on the rotational tunneling is reported in the contribution by M. Prager. Other than in glasses controlled defect structures allow to observe the subtle effects of environmental changes on the rotational tunneling properties of methyl groups.

We studied the temperature – and humidity dependence of the self correlation of the hydrogen atoms in DNA as probe for the onset of configurational fluctuations. The measurements were extended to three different degrees of hydration and – in frequency – down to the GHz regime. The onset of relaxational motion at about 200K relates to a large extent to the presence of water and it is absent for dry DNA.

Dieter Richter