Scientific research:
1. Anionic polymerization
The construction of an high-vacuum anionic polymerization line in the chemistry laboratoies of Prof. H. W. Spiess allows for the synthesis of polymers with different topologies, including block copolymers of different symmetries and chain end functionalities that are used in subsequent physical experiments, see below.
2. Synthesis and characterization of model a ,w -macrozwitterionic block copolymers
In these model systems with tandem interactions two opposing self assembly mechanisms are combined in one molecule: microphase separation which tends to separate the block copolymer chain ends, and ionic aggregation which tends to bring the chains ends together. The resulting frustration leads to interesting structural as well as dynamical properties of the material. In solution using a selective solvent these materials form micelles and can be treated as model macromolecular surfactants. Their study allows to quantitatively understand the effects of additional charges at specific sites along the polymer chain to the aggregation behavior in solution. In the bulk using additional low molecular salt to screen the coulomb interactions morphological parameters, as e.g. the lamellar long period, can be fine tuned. Ultimately, this should lead to salt induced phase transitions between different block copolymer mesophases as a result of changes in interface curvature. Mainly scattering techniques (light, X-rays, neutrons) for the structural investigations as well as spectroscopic techniques (EPR, NMR) to elucidate dynamical properties are employed.
3. Complex fluids under shear flow: block copolymers
In these investigations instead of an additional intrinsic self assembly mechanism the interaction of block copolymers with an additional external mechanical field is studied. The specific aim is to understand the coupling mechanisms governing the orientation behavior of lamellar diblock copolymers under shear flow. This involves interpretation of the orientation behavior obtained in the nonlinear regime of the viscoelastic response as revealed by 2-dimensional small angle X-ray scattering (SAXS) on the basis of the linear viscoelastic properties as measured, e.g., by dynamic mechanical spectroscopy. In this way an orientation diagram for the orientation of lamellar styrene-isoprene diblock copolymers under large amplitude oscillatory shear flow (LAOS) in the vicinity of the phase transition to the isotropic phase can be revealed (Fig.1). Since it is ve ry similar to the orientation diagram, e.g., observed for lyotropic lamellar phases under shear it is likely that insights gained by studying these phenomena will be of wide applicability in material science of soft matter.
Fig.1: Orientation diagram for PS-b-PI under LAOS in the vicinity of TODT
4. Organically modified silica mesostructures from block copolymer mesophases
These studies aim at using block copolymer mesophases for specific functions. In the present case the function is to act as structure directing molecules for the synthesis of organically modified ceramic materials. After the synthesis the inorganic phases have symmetry properties and long range order as known from the phase diagrams of block copolymer mesophase (Fig. 2). The work uses an approach often observed in nature where the final morphology of an inorganic species is determined by the preorganization of organic molecules through self assembly. The same strategy has already successfully been employed in the preparation of inorganic mesoporous materials using low molecular weight surfactants or ordered liquid crystalline mesophases as templates. The combination of inorganic siliceous components in a hybrid material with block copolymers is appealing for various reasons. First, a blend of desirable macroscopic properties in the final product is expected, which might lead to e.g. easily processable materials with interesting mechanical properties. Since the block copolymer chemistry (architecture, chain length, composition, etc.) can be varied substantially, one is able to fine-tune the properties of the composite as it is known from the study of polymers for many years. Furthermore, the length scale of the microstructures of block copolymers is on the order of the characteristic length scale of the chains, i.e., ranging from 5-100 nm, which may make mesoporous materials with larger pore sizes accessible. In these studies mainly X-ray scattering and transmission electron microscopy are employed for the structural investigations.
Fig.2: Schematic representation of the synthesis of organic-inorganic hybrid materials
5. Translational and rotational dynamics in heterogeneous polymers as revealed by FRS, NMR- and EPR-spectroscopy
For these purposes a new Forced Rayleigh Scattering (FRS) apparatus was constructed at the MPI for polymer research in order to investigate the transport properties in e.g. selforganizing, microstructured materials. In particular, study of the tracer diffusion of small dye molecules in microphase separated block copolymers for which the transport properties are particularly interesting since the supramolecular structure of these materials imposes topological constraints on the diffusing moieties that can lead to anisotropic behavior. Besides transport properties the local dynamics in heterogeneous polymer materials is investigated using advanced NMR and EPR techniques. Here, special attention is attributed to the dynamic properties of the interfaces that are responsible to a large extend for the macroscopic behavior of such materials. To this end model systems are synthesized and characterized using, e.g., EPR-spectroscopy.
List of selected publications
1.) M. Templin, A. Franck, A. Du Chesne, H. Leist, Y. Zhang, R. Ulrich, V. Schädler, U. Wiesner, Organically Modified Aluminosilicate Mesostructures from Block Copolymer Phases, Science 278 (1997), 1795-1798
2.) U. Wiesner, Feature Article: Block coplymers under large amplitude oscillatory shear flow: order and dynamics , Macromol. Chem. Phys. 198 (1997), 3319-3352
3.) V. Schädler, V. Kniese, T. Thurn-Ahlbrecht, U. Wiesner, H. W. Spiess, Self-Assembly of Ionically End-Capped Diblock Copolymers, Macromolecules 31 (1998), 4828-4837
4.) V. Schädler, P. Lindner, U. Wiesner, E. Mendes, Ionic and Zwitterionic Model Macromolecular Surfactants, J. Phys. Chem. B 102 (1998), 7316-7318