Institute for Neutronscattering
General Overview
User program at the FRJ-2 reactor
From October 1st, 2001 the user operation at the FRJ-2 is funded by the European Large Facility Access Program, which supports the European use of the Jalich reactor in supporting foreign users to perform experiments at our facility. A first call for proposals was placed with a deadline of November 30, 2001 - 21 proposals for 180 days beamtime were received, which compares very favorable with the 45 days available within the program. An international review panel has been established and following the first reviewing process 89 days of beamtime were allocated to European users for the first four month of 2002.
In 2001 the FRJ-2 operation went smoothly delivering 129 days of beam time with good reliability. During that time altogether 149 experiments were performed, 64.43% with outside users. Figure 1 displays the origin of the different user groups, who partly came several times during 2001 to Jiilich. Research groups from German and European universities took the majority of the beam time. In addition, we had users from industry and from national and international research centers.
In 2001 the 5th International Neutron Laboratory Course took place. This training course for students was supported by the European Neutron Round Table. Again the number of applicants (73) surpassed largely the available places. From the 40 accepted participants 14 came from outside Germany.
Neutron instrumentation (F&E-Nr. 23.89.1)
The development of neutron instrumentation and the construction of new neutron instruments is one of the focal points of the institute comprising activities in three areas.
• Preparation of instrumentation for 3rd generation neutron sources like the European Spallation Source (ESS).
• Construction of instrumentation for the FRM-2 reactor in Munich.
• Upgrading of existing instrumentation in Rilich.
The main endeavor in the field of spallation source instrumentation is the ongoing feasibility study (hardware) for a very high resolution neutron spin echo (NSE) spectrometer for the American spallation source (SNS) in Oak Ridge. The study is performed together with the HMI and should lead to the best of its class neutron spin echo instrument. Such an instrument will allow us to gain experience in the instrumentation of Megawatt spallation sources and at the same time will offer access to the use of SNS. The instrument was presented at the EFAC (Experimental Facilities Advisory Committee) in Oak Ridge and received very high marks. An increase of the time resolution to 1 its combined with a significant increase of the solid angle are the ambitious design goals. The feasibility study aims on the design of the magnetic fields using superconducting technology and the development of correction elements. For that purpose a test stand has been build facilitating very precise magnetic field measurements using NMR probes. Also computer simulations on the instrument performance are carried out.
Furthermore, a design study for a very high resolution backscattering instrument to be operated at the ESS was undertaken. The design goal is the combination of a large dynamic range, high resolution and significant intensity.
The work on the backscattering spectrometer for the FRM-2 .in Munich proceeded according to plan. The main activities focused on the manufacturing of the analyzers and their mechanics. Furthermore, the design of the sample position and the concept of the spectrometer housing was finalized.
In Jülich, the upgrade of the small angle scattering machines was continued. In particular together with the ZEL a new two dimensional scintillation detector was developed allowing for an one order of magnitude higher counting rate. A number of sample environment devices like an elongational rheometer, a shear cell, a stretching machine for melts or rubbers and a cell for time resolved SANS studies of mineralization processes were constructed.
The focusing small angle scattering machine (KWS-3) was commissioned and inelastic fixed windows scans at the backscattering spectrometer as a new technique to monitor quasielastic scattering was developed.
Polymers, membranes and complex fluids (F&E-Nr. 23.30.0)
Most of the research in the institute concentrates 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. Theoretical guidance is available through a close collaboration with the Institute Theory II of the IFF.
Polymer synthesis
Aside of standard preparations of homo- and blockcopolymers for particular physics problems, a number of items need to be emphasized:
• In the field of branched polymers, the synthesis of so called porn-porn polymers - these are star polymers which are joined by a cross bar was persued. The synthesis of a half-porn-porn - a ASB 1-star - was achieved.
• Alkoxy-silanes as linking agents for branched polymers of the A7B1 or A,4B2 type were explored. For an industrial collaboration with DSM nylons-6 star polymers were created.
• Polydienes and polyalkanes which are soluble in silicon oil were developed and likewise amphiphilic blockcopolymers with silicon oil soluble blocks were synthesized with the aim to boost silicon oil microemulsions.
• Polyalkane-PEO blockcopolymers as compatibilizers for nanocomposites with isotactic polypropylene were achieved.
• A number of triblockcopolymers for the study of interfaces in microphase separated diblockcopolymers as well as for the investigation of the reptation dynamics were created.
• Finally, the GPC triple detector for molecular weight analysis was tested and the limits of universal calibration using viscosity and light detection, were explored. In the molecular weight range from 1000g/mol to 500.000g/mol a molecular weight analysis on an absolute scale with a maximum error of 10% was found to be possible.
Polymer dynamics
We continued our studies on the dynamics of polymer chains covering all scales from the macroscopic motion to local dynamics including the motions of side groups.
• Large scale motion
One of this years highlights was the study of the single chain dynamic structure factor of polymer melts as a function of molecular weight. In a collaboration with Prof. T. McLeish from the University of Leeds these experiments facilitate a molecular identification of the often proposed tube length fluctuation process in the reptation model as the limiting mechanism to local reptation (see report A. Wischnewski). A study on the molecular weight dependence of the polymer melt viscosity to very high degrees of polymerization led to an observation of the viscosity cross over to so called pure reptation behavior (m ~ M3,4 -> M3) for several polymers (PVCH, PI, PIB, PS).
In the momentum transfer range Q below the first structure factor maximum the collective and self motion of polyisobutylene (PIB) melts were studied as a function of temperature. Unexpectedly and still not yet fully understood, we found that the relaxation of the short range order or the structural relaxation observed at the first structure factor maximum exhibits a stronger temperature dependence as the collective dynamics in the low Q regime - with decreasing temperature the short range order lives increasingly longer than the collective modes at low Q. The corresponding collective relaxation times observed with neutron spin echo agree quantitatively with the relaxation time of the shear stress. The self motion follows the temperature dependence of the collective modes (see report U. Buchenau).
In the same spatial range but at picosecond time scales, the fast dynamics of polybutadiene (PB) was investigated. Comparing deuterated and protonated samples of the same scattering power, it was possible to show that the low Q inelastic scattering is purely coherent. The experiment demonstrated that this inelastic scattering is not due to large amplitude motion but rather due to a correlated small amplitude motion of many atoms. This finding supports strongly the concept of a strong coupling of the so called fast process to the longitudinal sound waves.
Earlier studies of the fast process of glass forming materials in porous media have shown that the confinement reduces strongly the low frequency part of the Boson peak. At a first sight, this phenomenon appeared to be related to a cut off of long wave length sound waves due to the confining media. New studies using larger pore sizes displayed a similar effect invalidating the sound wave argument. The also observed slowing down and broadening of the a-relaxation could be explained by a model of an inhomogeneous distribution of relaxation times within the pores.
• Local dynamics
On polyisoprene melts, the methyl group dynamics was investigated using a combination of time of flight and backscattering experiments. The measurement validated a model of an Gaussian energy barrier distribution and led to a detailed determination of the corresponding parameters.
In polymeric electrolytes (polypropyleneoxide/LiCl04) measurements at the structure factor maximum displayed a slowing down and broadening of the structural relaxation depending on salt concentration. Studies of the self motion at small momentum transfers showed a temperature shift of the relaxational processes upon salt addition, while the characteristics of the entropy driven Rouse dynamics prevailed.
Using dielectric techniques, the extend of the dipole fluctuations within the ß-relaxation was studied in some detail. For PVAc the dipole fluctuation [dm2ß] of the ß-relaxation was correlated with the mean squared displacement <u2> from backscattering experiments. A linear relation was found.
Cocrystallisation on polymeric and mineralisation on biomimetic templates
The study of cocrystallization processes of polymers, organic and nonorganic materials are a new field in our institute. Crystallization processes were studied first in connection with wax crystal modifiers for middle distillate fuels, where crystalline-amorphous diblockcopolymers were investigated. At present two lines are followed.
• Cocrystallization of paraffin with random crystalline-amorphous copolymers
The self assembly of PEB-n with n = 7.5 and 11 (n gives the number of ethyl side branches per 100 backbone carbons) copolymers in dilute decane solution and their effect on paraffin crystal formation (C24H50 and C36H74) were studied. Using contrast variation the structure of the polymers and the paraffin associates could be separately identified. At low temperature the two PEB-n copolymers by themselves selfassemble into needle shaped objects. For n = 7.5 and n = 11 the selfassembling process starts at different temperatures (60°C and 0°C respectively). The temperature of selfassembly strongly correlates with the wax crystal modification efficiency of the polymeric additives. From the SANS data, we could conclude that the paraffin and the polymers cocrystallize into common two-dimensional structures (collaboration with Prof. R.K. Prud'homme, Univ. Princeton).
• Mineralization in the presence of associating polymers and on biomimetie templates
Recently, it has been found, that double hydrophilic blockcopolymers which have an electrolytic and an neutral block, serve as efficient builders for Calcium binding and precipitation in CaCO3 solutions. Despite of this extraordinary polymer effect, microscopic information on the conformation and the configuration of the polymer in the process is still not available. Using small angle neutron scattering (SANS) exploratory investigations of the nucleation and the growth processes as well as the final crystalline products were performed. First evidence for a nanometer internal structure of the minerals was found (collaboration with Prof. M. Antonietti, MPI Golm). As the second system, CaCO3 mineralization on gold colloids (10nm size) which were covered with 4 dimethylaminopiridene was investigated (collaboration with Prof. W. Tremel, Univ. Mainz).
Branched polymers and rubbers
The study of the matrix deformation in filled elastomers was continued. An ideal cylindrical filler was prepared by microphase separation of a blockcopolymer and dispersed in homopolymer matrixes. Matching conditions were employed such, that only the matrix chains were visible. The SANS result indicate that rod like fillers do not lead to visible reinforcement effects of the polymer matrix reinforcement. Industrial silica filler were investigated by a combination of SAXS and SANS techniques, in order to observe the matrix deformation (SANS) and the deformation of the filler network (SAXS). The polymer chain deformation in the samples is an agreement with the hydrodynamic reinforcement effect, though, the dependence on the filler content is irregular (collaboration with Prof. E. Straube, Univ. Halle)
Real time SANS experiments combined with measurements of the dynamic moduli and the sol fraction allowed an identification of scission reactions during the cure of rubbers. It was found that due to the immobility and the recrosslinking of broken chains, random degradation processes do not effect the quality of the rubber.
Complex fluids
Complex fluids of three different types were investigated. Studies on the polymer efficiency boosting effect in microemulsions were continued on droplet phase systems. Bicontinuous polymeric microemulsions were characterized and star polymers/colloidal mixtures were investigated. Furthermore, dynamic experiments on microemulsions with three different morphologies were performed.
• Droplet phase microemulsions containing amphiphilic diblockcopolymers
Droplet phase oil in water microemulsions consisting of water, decane and CIOE4 surfactants containing amphiphilic diblockcopolymers of polyethylenepropylene-polyethyleneoxide PEP5-PEOy with the molecular weights (in 1000) y = 15 and 80 were investigated. The polymer concentration was chosen such that on the average one polymer was attached to one droplet. Systematic contrast variation experiments allowed the determination of the different partial structure functions. They could be described by a spherical core-shell-shell model. The core consists of oil, the first shell contains the surfactant, the next outer shell is a depletion zone, where the polymer is almost not present but beyond this shell the polymer is homogenously distributed. The observation of the depletion zone is in particularly clear for the larger chain system (collaboration with Prof R. Strey, Univ. Koln).
• Microemulsion dynamics
Extensive small angle scattering and neutron spin-echo experiments on a microemulsion system that exhibits boosting of its oil-in-water solubilization capability on the addition of polymeric cosurfactants have been performed. The dynamics in the bicontinuous phase and in the lamellar phase can be described consistently in the high wavevector regime by applying the model of free membrane patches of Zilman & Granek [PRL 77, 4788 (1996)] if proper evaluation (partly) numerical is performed. The curvature elasticity K extracted by this means from the dynamics is consistent with the expectations from other methods. The influence of the polymer cosurfactant on x extracted from the dynamic scattering is only marginal. At larger length scales where the interference of membranes contributes to S(Q) the dynamics exhibits deGennes narrowing. The investigations clearly showed the range of validity of the most recent theoretical concepts to describe the local and medium scale dynamics of these microemulsion phases and the conditions for and limits of their applicability.
• Microemulsion phases in a three component polymer blend
Ternary polymer blends containing two partly incompatible homopolymers and the corresponding symmetric diblockcopolymer exhibit similar features as mixtures of amphiphilics with oil and water. The structure factor of a
ternary mixture of dPB/PS/dPB-PS polymers was investigated in the neighborhood of the Lifshitz line (LL). There, we observed a so called reentrance phase behavior with a two phase region limited by an upper and lower critical solution temperature Tu and Tl respectively. The temperature distance of the immiscibility DT= Tu - Tl decreased from 15K at a diblockcopolymer volume fraction p= 6.3% to zero at the double critical point (DCP) at p= 7.05%. Approaching the DCP we observe an increase of the critical exponents to twice their values. If, however, the reduced temperature field tul = I (T - Tu) (Tl - T) I / T2 instead of t = I T - Tu I / T is used, the predicted Lifshitz critical exponents are evaluated. There is experimental evidence, that the observation of a reentrance phase behavior with a lower phase boundary is related to the formation of a droplet microemulsion phase which beyond the Lifshitz line becomes bicontinuous.
• Star polymer/colloid mixtures
The influence of polymer branching on depletion forces was investigated by studying mixtures of hard sphere colloids and star branched polymers with increasing arm numbers f = 2/32 but constant Rg =~ 50nm. The position of the glass liquid demixing transition was found to be related to the amount of branching. Using SANS and SAXS partial structure factors in the star polymer colloid mixtures could be measured. The scattering data reflect the relative distance to the demixing transition (collaboration with Prof. P.N. Pusey, Univ. Edinburgh and Prof. H. Lowen, Univ. Dasseldorf).
Further research activities (F&E-Nr. 23.15.0)
Tunneling systems
The interpretation of rotational tunneling spectra, the phonon density of states and internal excitations on the basis of measured crystal structures and fundamental interactions - either transferable pair interaction potentials or united force fields - is successfully extended to the various isomers of xylene and the methyl halides.
Electron transfer between metal organic molecules containing methyl groups is related to the mechanism of hydrogen bond formation. The prominent example is trimethyl-aluminum. Studies have been extended to the respective gallium and indium alkyls.
Rotational tunneling as a probe of disorder is applied to a case - partially deuterated methane III - where the symmetry of the molecule is incompatible with the site symmetry. Large line broadenings allow an estimate of the distribution of barrier heights.
Biological macromolecules
The study on the interaction of DNA with its water hull was extended to a first example of a DNA-drug complex (planar aromatic complex). Experimental basis is the determination of the self correlation of hydrogen at the DNA on humidification with D2O. Those spectra delver as typical information the ubiquitous onset of conformational fluctuations at around 200K for biomolecules and the characteristic sublinear relaxation. On this basis, no qualitative difference to pure DNA was observed. This absence of a difference speaks in favor of the intercalation of the drug between the bases - a position which hardly affects the conformational freedom.
Dieter Richter