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Scientific Report 1998/1999

In 1998 the research reactor FRJ-2 was delivering neutrons for 151 days. Altogether 191 experiments were performed at the different neutron scattering instruments. 143 or 75% of them with external participation. These numbers include experiments performed within the Institute for Scattering Techniques, the Institute for Neutron Scattering (INS) and the different external Collaborating Research Groups. The external users came from 53 different institutions. Fig.1 displays the distribution of the external users with respect to their origin. As may be seen, the FRJ-2 attracted large interest not only in the national but also in the international community. At this point I like to thank in particular the reactors operation department for the very good cooperation between users and operators.

In 1998 a neutron laboratory course was hold for the second time. This course comprised 20 hours of lectures and one week of practical experiments at seven different neutron instruments. Among the more than 100 applicants from different Universities in Germany and beyond 35 participants were accepted. The neutron laboratory course was very well received by the students. In 1999 a third course is foreseen.

In 1998 the construction of the new thermal time of flight instrument was completed. First test experiments revealed a high thermal flux 2 ´ 10⁸ integral at sample position from this number a pulsed intensity of about 2 ´ 10⁵ n/cm²s may be estimated. Unfortunately, due to manufacturing problems of the very delicate high speed background chopper, some delays in the commissioning of the instrument occurred.

With respect to this chopper, we like to acknowledge the cooperation with the IGV within the Forschungszentrum - provision of the magnetic bearings - and the jet engine manufacturer MTU - manufacturing of the chopper.

The double crystal diffractometer was furnished with new channel cut perfect silicon crystals which improved the resolution characteristics of this instrument significantly. In particular the broad wings of the resolution function are now strongly reduced. Finally, in 1998 we commenced with design studies for a new back scattering instrument at the Munich reactor FRM-II, which is supported by the federal government. In particular the properties of the phase space transformation by a fast turning wheel was studied in detail (report Kirstein).

In 1999 the main activities will be concentrated on the construction of the back scattering instrument mentioned above. We will participate in the development of neutron time of flight spin echo, where we develop broad band flippers and are actively engaged in the test experiments at the instrument IN15 in Grenoble. Furthermore, it is planned to consider also the zero field NSE technique as a complementary way to perform NSE experiments.

The research activities within the INS are focussed in the field of polymer physics and complex fluids. In order to be successful here, a close cooperation between polymer synthesis which by itself is an object of research, and physics investigation is necessary. In the following I discuss the different main topics.

In 1998 the synthesis of branched polymers and blockcopolymers were the backbone of the activity. Star polymers of different functionalities and very high molecular weight were prepared as well as H-shaped polymers which are important in the frame of a European Brite-Euram project for polymer processing. In order to investigate aspects of the reptation model, high molecular weight PVCH polymers were synthesized M_W > 10⁶. In order to deepen our understanding of mixed polymer and surfactant systems, for the first time an organic synthesis of fully deuterated nonionic surfactants was accomplished in our laboratory. In addition quite a large number of homopolymers or simple diblockcopolymer were synthesized. Finally I would like to emphasize the collaboration between the Institute for Biotechnology and the polymer laboratory on the development of organic cathalysts which are fixed at a polymer backbone.

For 1999 it is foreseen to synthesize in particular H-shaped and comb like polymers for the BRITE-EURAM project. In addition further amphiphilic polymers and star polymers are on the agenda.

Finally experiments on the living polymerization with SANS are going to be extended to in situ experiments of the polymerization kinetics and the associated aggregation states.

The core activity of the institutes relates to dynamical properties of soft condensed matter systems. Thereby we cover dynamical features from the large scale motions of long chains in the melt to the local relaxation processes and the soft vibrational properties. Starting from the high frequency side, X-ray and neutron Brillouin scattering experiments were performed on polybutadiene melts. From a direct comparison of the results it is clear that above 1.5 meV energy transfer the X-rays are superior. Below this energy the better resolution of neutron experiments favors neutron scattering.

In this low energy transfer regime new dynamical features were observed, which are proportional to the density of states and visible down to a momentum transfer of 0.1Å^-1.

Exploiting the possibilities of IN15, the large scale motions of long chain polymer melts was systematically investigated varying the chain molecular weight. Furthermore, studies were undertaken, where labeled sections were placed at different positions along the chain. Over all a very good agreement with the reptation model is found - actually no other formulated theory is able to explain the data. Furthermore, apparently contour length fluctuation processes are directly visible in the dynamic response. Large scale motion was also investigated on diblockcopolymers around the order disorder transition to the lamellar phase. Thereby the different aspects of the dynamic response were fully studied in using different chain contrasts.

The experimental results were compared with the most advanced dynamical random phase approximation theory. Severe deviations were found with respect to the predicted collective response. The experiments reveal strong indications for the importance of surface excitations in the dynamic response of the microphase separated domains.

In 1999 most of these activities are going to be continued. New emphasis will be placed on the study of dynamic form factors in the high frequency regime, where the new time of flight instrument SV29 will be very helpful. This instrument will also allow to address again the question of the dynamical heterogeneity. Furthermore, the dynamics of polymer containing microemulsions will be addressed.

Aside of the dynamic experiments this field is another stronghold of the activities within the institute. The most important discovery of 1998 was the realization that small additions of amphiphilic polymers to microemulsions increase the emulsification properties significantly. This feature was documented in terms of phase diagrams. Neutron small angle scattering experiments on the structure of polymer containing microemulsions and the confirmations of the polymers within these systems were just started (report Willner).

The micellarization properties of blockcopolymers were investigated on the amphiphilic systems as well as on polymers with architecture. I would like to mention particularly new experiments on the exchange kinetics of unimers between polymer micelles, which can be uniquely accessed by contrast variation in small angle neutron scattering.

In this frame also the aggregation behavior of carbo hydride copolymers, which are used as fuel additives should be mentioned. Polymers of only two blocks of polyethylene and polyethylenepropylene aggregate into platelet shaped micelles, which act as nucleation centers for wax crystals in diesel fuel and suppress the cold filter plugging point. In 1998 we studied random copolymers of these two components, which aggregate to form loose networks with a large number of crystalline connecting points. Unexpectedly we found a characteristic mesoscopic length scale in these structures which indicates some order in the chaos.

In 1999 the study of the amphiphilic blockcopolymers both with respect to the micellarization behavior as well as concerning their action in microemulsions will be continued.

In this field in particular ternary systems of two homopolymers and a compatibility providing diblockcopolymer were undertaken. The phase transitions and the phase diagrams in such systems were explored; of particular interest is a composition range were complete compatibility in a homogeneous phase is observed (report Schwahn).

After having identified the effect of topological constraints on rubber elasticity, the focus of the ongoing research is the effect of filler particles in rubbers. Here experiments on SiO₂ fillers are undertaken and a systematic characterization of the systems is performed.

This concerns both, the surface properties of the SiO₂ filler as well as the exploration of different cross linkers. Aside of the work on rubbers SANS experiments on the deformation of branched polymers under external stress are performed.

These experiments are connected to rheological studies and aim on an exploration of how the non linear viscosity can be influenced by specific branching, in order to improve polymer processing conditions.

Aside of the polymer research, three other topics are investigated by neutron scattering.

1. Dynamics of microemulsion: With the investigation of mixed polymer microemulsion systems this field is now conncected to the polymer research.
   
2. Tunneling phenomena and molecular crystals (see report M. Prager)
   
3. Dynamics of small molecules in porous media. In 1998 the dynamics of ethane propane and benzene in MCM 41, a micro crystalline material with tubes of 5 nm size, were studied. Both translational diffusion as well as reorientational motion were identified. The main conclusion of these experiments is that the molecules adsorbed at the channel walls have a mobility similar to the liquid phase. These experiments are accompanied by computer simulations which are performed by the collaborating chemical institutes