IFF
Scientific Report 1999/2000

Apart of routine synthesis of homo- and diblockcopolymers, the synthesis of partially deuterium labelled branched polymers like H-polymers for SANS studies in connection with a BRITE EURAM project was one of the major activities. This synthesis encountered more difficulties than anticipated in particular because of tiny amounts of impurities in the deuterated butadiene. On the basis of sophisticated, purification techniques it became possible to prepare well defined H-polymers with labels at different positions. According to conventional analytical procedures like GPC, membrane osmosis and light scattering these materials were up to specifications. At present the H-polymers are studied with temperature gradient interaction chromatography (TGIC), a technique providing a much higher resolution than conventional GPC. First results seem to indicate that the H-polymers still contain higher or a lower branched structures which were invisible by conventional analytical techniques

In order to study partial dynamic structure factors, partially labeled 1-4 and 1-2 polybutadienes were synthesized starting from the corresponding modified butadiene monomers. As a step stone to further well defined branched structures the synthesis of an organo lithium initiator bearing a protected OH group was undertaken. This initiator can be used for the preparation of OH terminated linear and branched telechilic and heterotelechilic polymers. Finally, I like to mention a patent application together with the biotechnology department on organic catalysts which are fixed at polymer backbones.

The core activity of the institute focuses on dynamical properties of soft condensed matter systems including polymers and complex fluids. In this context, dynamical features from soft vibrations through secondary relaxations and the glass process up to large scale motions are investigated.

On the high frequency side, combining X-ray and neutron Brillouin scattering, a fast coherent quasielastic component inside the Brillouin line was identified and correlated to strain relaxation in polymer melts. (report Buchenau). In the regime of the classical relaxation processes a thorough investigation of polymer blends was undertaken. On the system PI/PVE previous studies by local probes displayed heterogeneous dynamic behavior of the two components, though the blend is perfectly miscible. Covering a Q-range of nearly two orders of magnitude by quasielastic neutron scattering it could be shown that this apparent local heterogeneities relate to a purely dynamic chain specific cross over from homogeneous entropy driven modes to chain specific local relaxation (report Hoffmann). In the merging regime of the a - and b -relaxations in polyvinylacetat a quasielastic neutron scattering study on the Q dependent relaxation behavior was performed. Other than for polymers studied so far, only a weak Q dependence of the characteristic relaxation rate was found. Rubbery electrolytes were studied both with dielectric spectroscopy and quasielastic neutron scattering. Thereby, the interplay between the polymer and ionic motion was investigated. It was found that the glass dynamics is significantly reduced upon addition of ions.

A series of neutron spin echo experiments on polyethylene melts with different molecular weights was performed, in order to study the reptation dynamics as a function of molecular weight. While the experiments confirm the model of De Gennes for high molecular weights, it was found that with decreasing chain length a significant increase of the main parameter of the De Gennes theory, namely tube diameters d occurs. Furthermore, for chains shorter than about 7 entanglement lengths the experimental data cannot be described in terms of the reptation model and a crossover to another dynamic regime occurs. These findings are in agreement with the packing model of entanglement formation which predicts a transition from entangled to nonentangled dynamics at a critical chain length of about 7 entanglement lengths. Experiments on diblockcopolymers with hydrogen labels at the chain connection point followed up recent investigations on the overall dynamics of such molecules. The NSE experiments revealed fast interface motions which are driven by the surface tension and display all characteristics of ripplon excitations. (report Monkenbusch)

Last year we reported the discovery of strong amphiphilicity boosting effects which occur upon addition of small amounts of amphiphilic diblockcopolymers into microemulsions. Performing careful two-dimensional contrast matching experiments, it became possible to identify the position and state of these polymers within the microemulsion system. The polymers are placed within the interface and stay there as single chains exhibiting mushroom like conformations (report Endo). In this way the elasticity properties of the surfactant layers are significantly altered, such as to change the phase boundery for the microemulsion phase to lower surfactant concentrations. Aside of structural properties also the dynamics of bicontinuos microemulsions containing polymers was systematically evaluated. At high momentum transfers the theoretically predicted relationship between characteristic relaxation time and momentum transfer (W » Q³) was observed. The presence of the polymer introduces qualitative changes into the microemulsion dynamics. The aggregation behavior of PEP/PEO amphiphilic diblocks was studied in water as a function of molecular weight. Upon reduction of chain length a transition from spherical to cylindrical micelles was observed. Finally the interplay of paraffin crystallization and random copolymers containing crystallizable parts in oil was investigated. The crystallization of C₂₄ in decane is changed massively in the presence of the statistical copolymer PEB12. Instead of macroscopic wax crystals PEP12 induces two-dimensional large platelets. Using contrast variation the wax and the polymers were made visible separately. In both cases two-dimensional structures are observed, the thickness of the wax platelets agrees with that of a monomolecular layer of C₂₄.

The SANS studies on three component mixtures of two homopolymers and the corresponding diblockcopolymer were continued on the system PB/PS. Similarly, as for the system PEE/PDMS at small diblock concentrations a cross over from a 3d-Ising to the isotropical Liftshitz critical behavior was found. The critical range as well as the Ginsburg number are significantly smaller than in PEE/PDMS. Furthermore, close to the Liftshitz critical point a phase behavior similar to that of bicontinuos microemulsions was found.

The ongoing research on rubbers now focuses on networks filled with commercial activated silica fillers. It became possible, to produce such networks with a good dispersion of the filler. By SANS the growth and the deformation dependence of the fractal filler clusters could be studied as a function of volume fraction and strain. Phase matched diblockcopolymers with cylindrical morphology of the microdomains were synthesized and allow now an observation of the overstrain of the matrix for anisotropic structures. First experiments have been performed. Rheological experiments on the universality of the exponents for the relaxation time spectrum in the flow and glass region of homopolymer melts were performed as a function of packing length. In particular, experiments on PVCH, the polymer with the largest packing length which can be synthesized anionically, was carried out. These investigations critically inspect whether the standard determination of the plateau modulus on monodisperse homopolymers from the peak height in the imaginary part of the complex modulus agree with that from the Kramers-Kronig relation.