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Research Highlights

Introduction


Methods

High Resolution Solid State NMR Spectroscopy, Pulsed EPR Spectroscopy 

Quantum Chemical Calculation of Bulk and Solvent Shifts

NMR-Spectroscopy with Hyperpolarized Systems


Systems

Supramolecular Structures
Surfaces and Interfaces


Phenomena And Function

Chain Organization and Mobility in Semicrystalline and Amorphous Polymers
Structure and Dynamics of New Polymeric Proton Conductors
Columnar Stacking and Photoconductivity
Nanostructured Hybrids Composed of Synthetic and Biological Components
Functional Stimuli-Responsive Polymer Networks


Applications

Chain Branching
Fourier Transform Rheology



Introduction

Polymer behavior and function depends on both the molecular structure and the organization of the macromolecules in the solid state, in the melt, in solution or on surfaces. Our group focuses on structure-property relationships as revealed by probing the structure and dynamics of polymers and supramolecular systems over wide length- and time scales. Our interdisciplinary activities profit from collaborations with the other groups of the institute as well as partners in academia and industry. We are engaged in chemistry and physics, in experiment and simulation, as well as in fundamental and applied research. Moreover, we maintain a balance of activities and expertise encompassing methods, systems, phenomena and applications.

Remarkably, local mobility and chain diffusion of polymers such as poly(ethylene) or poly(carbonate) is still under discussion today, but in general the emphasis in macromolecular science has shifted from established polymer materials to supramolecular structures and hybrid materials. This provides new challenges for physical characterization which must distinguish different mechanisms of assembly, e.g. H-bonds, π-π interactions, ionic forces or surfaces and describe cooperative behavior on nanometer length scales. Last, but not least, insight into structure and dynamics should be related to the desired function of the materials. Here, elucidating the proton mobility in new types of proton conductors is particularly challenging.

This report describes the advances achieved by our group in these different areas and emphasizes that magnetic resonance techniques in combination with computer simulation as well as new approaches to study the macroscopic behavior, such as Fourier transform rheology, are well suited to cope with these challenges. Moreover, a laboratory for state-of-the-art NMR imaging with hyperpolarized gases has been built up. Recent developments as well as applications of NMR and pulsed EPR are described in comprehensive reviews.

  • H. W. Spiess: "NMR Spectroscopy", in Macromolecular Engineering, edited by K. Matyjaszewski, Y.Gnanou, L. Leibler, WILEY-VCH, Weinheim, Vol. 3, 1937-1965 (2007).

  • G. Jeschke, H. W. Spiess: "Distance Measurements in Solid-State NMR and EPR Spectroscopy", Lect. Notes Phys. 684, 21-63 (2006).

  • H. Steininger, M. Schuster, K. D. Kreuer, A. Kaltbeitzel, B. Bingöl, W. H. Meyer, S. Schauff, G. Brunklaus, H. W. Spiess: "Intermediate Temperature Proton Conductors for PEM Fuel Cells Based on Phosphonic Acid as Protogenic Group: A Progress Report", Phys. Chem. Chem. Phys. 9, 1764-1773 (2007).


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Methods

High Resolution Solid State NMR Spectroscopy, Pulsed EPR Spectroscopy

Due to the lack of full crystalline order in many systems of current interest in solid state chemistry, materials science and biology, structures are not known with atomic resolution. By developing suitable solid-state NMR techniques we aim at providing the missing pieces of information. We exploit the distance- and angle-dependent dipole-dipole couplings between nuclear spins, in particular 1H-1H and 1H-13C. Coupled with the remarkably large 1H-chemical shifts in the solid state, e.g. due to hydrogen bonds or ring current effects in aromatic moieties, unique structural and dynamic information is available. Since the NMR linewidths in the solid state are usually much broader than in liquids, the highest available magnetic fields should be applied. In Fig. 1 we exemplify the substantial gain in spectral resolution through our new 850 MHz spectrometer installed in fall of 2006. The compound is a chromophoric barbituric acid, which serves as a model of hydrogen bond mediated molecular recognition. Clearly, only the 850 MHz spectrum resolves two peaks in the hydrogen bonded region, proving asymmetry of the binding site.


Fig 1

Comparison of a 2D double quantum NMR spectrum of a chromophoric barbituric acid

in the solid state at 850 and 500 MHz

Moreover, motional averaging of 1H-13C dipole-dipole couplings can be quantified in terms of dynamic order parameters S = (1/2 (3cos2θ - 1)), where θ is the angle between the instantaneous C-H direction and the residue-fixed axis around which the motion occurs. In this way the amplitude of the motion can be quantified providing information about the packing of the different moieties within a supramolecular structure. In addition to the dipole-dipole coupling, site-selective quadrupole couplings unravel site-selective molecular dynamics, Fig. 2. Readily exchangeable sites like OH or NH are easily deuterated by dissolution in deuterated protic solvents. Moreover, 1H-2H-couplings, e.g. between a protonated additive and a deuterated polymer matrix, provide unprecedented information on spatial proximity on an atomic level or e.g. polymer/solute interactions and augment the knowledge on dynamics by structural parameters

Somewhat longer distances than with NMR can be accessed by pulsed EPR techniques. Sensitivity improvements have extended the proven ranges of such measurements to more than 7.5 nm for electron-electron distances. Moreover, coarse-grained modeling was implemented to separate the contribution due to different conformations of the spin label from the contribution due to bending of rod-like molecular backbones. The asymmetric distance distributions determined in end-labeled oligo(para-phenyleneethynylene) model systems could thus be analyzed in terms of the worm-like chain model and the persistence length could be obtained, Fig. 3. As information on the residual flexibility of shape-persistent molecules is a key requirement for the rational design of well-defined nanostructures, this approach should find applications well beyond the determination of the persistence length of semiflexible polymers.


image description


Fig. 2 Site-selective molecular dynamics in Histidine-HCl-H2O


Fig. 3 Distance distribution of chain ends in oligo(para-phenyleneethynylene) from pulsed EPR experiments


  • A. Godt, M. Schulte, H. Zimmermann, G. Jeschke. "How flexible are oligo(para-phenylene-ethynylene)s?" Angew. Chem. Int. Ed. 45, 7560-7564 (2006).


Quantum Chemical Calculation of Bulk and Solvent Shifts

Hydrogen bonds are a particularly important source of supramolecular organization in synthetic as well as biological systems. They can conveniently be studied by 1H NMR, as the chemical shift of protons in hydrogen bonds is very sensitive to the strength of this bond. Quantitative analysis of such data, however, requires combination of the NMR experiment with state-of-the-art quantum chemical calculations. In order to check this approach, bulk chemical shifts in crystalline amino acids resulting from the packing were studied. As shown in Fig. 4, the bulk shifts are of the same magnitude as the whole chemical shift differences of different functional groups in organic compounds, but can quantitatively be accounted for by the calculations based on the known crystal structures. In reverse, solid-state 1H NMR combined with quantum chemical calculations can then be used to yield detailed structural information of hydrogen-bonded non-crystalline systems, including intra- and intermolecular packing effects.

The determination of the detailed microscopic structure and dynamics of complex aqueous solutions is a challenge for modern physics and chemistry. The nature of the interaction of water with dissolved ions and molecules is crucial for a broad range of chemical, biological, and physical processes. While liquid-state NMR experiments are routinely used to characterize reaction products in solution, the quantum-chemical prediction of NMR solvent shifts is a complex problem, since the ultrafast dynamics of the hydrogen bonding network requires a combination of statistical phase-space sampling with the accurate calculation of a large number of spectra. The calculation of spectra for such fluctuating systems were done using a fully quantum-mechanical description for a HCl solution and a hybrid quantum-mechanical/mechanical-modeling technique (QM/MM) for a solvated adenine molecule. In both cases, the microscopic description of the solvation structure was obtained via Car-Parrinello molecular dynamics simulations. In Fig. 5, the resulting concentration-dependent proton NMR chemical shift for hydrochloric acid is shown to be in excellent agreement with experiment, thus confirming the accuracy of the simulated structural and dynamical properties.

Fig. 4 Observed and calculated bulk chemical shifts due to hydrogen bonding in crystalline amino acids

Fig. 5 Snapshot from the HCl molecular dynamics simulation, illustrating a "frozen" situation of the hydrogen bond network and NMR chemical shifts in HCl(aq)



  • J. Schmidt, A. Hoffmann, H. W. Spiess and D. Sebastiani: "Bulk Chemical Shifts in Hydrogen Bonded Systems from First Principles Calculations and Solid-State-NMR", J. Phys. Chem. B 110, 23204-23210 (2006).

  • T. Murakhtina, J. Heuft, E.J. Meijer and D. Sebastiani: "First-principles and Experimental 1H NMR Signatures of Solvated Ions: the case of HCl(aq)", ChemPhysChem 7, 2578-2584 (2006).

  • S. Komin, C. Gossens, I. Tavernelli, U. Rothlisberger and D. Sebastiani: "NMR Solvent Shifts of Adenine in Aqueous Solution from Hybrid QM/MM Molecular Dynamics Simulations", J. Phys. Chem. B 111, 5225-5232 (2007).

NMR-Spectroscopy with Hyperpolarized Systems

NMR-Imaging has become a well established tool in medical diagnostics (MRI). The low signal intensity, however, remains a serious limitation, in particular for gas filled volumes. This problem can be overcome by the use of hyperpolarized (HP) gases by which signal enhancement of several orders of magnitude can be achieved. In this context a lab for HP-gases has been built up in our group. Hyperpolarized 129Xe can be produced with our home-built apparatus and 3He is available from our collaborators at the physics department of the University of Mainz (Prof. W. Heil). Together with the radiology department of the University-Clinics (Prof. W. Schreiber) our work on improving and developing new methods in polarizing, handling and applying hyperpolarized gases already led to promising results in non-invasive investigations of lung diseases in three dimensions, see Fig. 6.

Fig. 6 3He NMR Image of a lung under otherwise natural conditions.

Fig. 7 Pseudo spin echoes of 3He at constant pulse delay and increasing diffusivity. Note the shift of the echo position.

The interesting physiological properties of Xe e.g. its ability to pass the blood/brain barrier, can only be exploited if the Xe gas can be dissolved in bio-compatible liquids for subsequent injection into the blood stream. In collaboration with the company MEMBRANA we have developed a method using hollow-fiber membranes to load blood or other liquids with Xe in continuous flow. This procedure solves many practical problems (e.g. bubbles and foams, lifetime and linewidth of NMR-signals) and allows application in NMR Imaging, high resolution biomolecular NMR as well as probing nanoporous structures alike.

Triggered by the importance of gas diffusion in lung imaging we studied the effect of diffusion of 3He on the NMR echo formation. For rapid diffusion of the gas the formation of the regular Hahn-echo is largely suppressed, but substantial refocusing of the signal still occurs, yet at considerably shorter times, Fig. 7. The formation of these pseudo-echoes can quantitatively be modeled taking into account the diffusive motion throughout the experiment, a case never considered in the NMR literature before. This now makes it possible to measure diffusion constants in a single shot and offers new possibilities avoiding susceptibility artifacts in lung imaging.


           Fig. 8 Parahydrogen induced polarization of 13C in 1-hexyne (75 MHz ALTADENA PHIP-NMR, single pulse). Note the large negative enhancement of specific signals marked in blue.

In order to make hyperpolarization a versatile new tool for NMR in materials science, chemistry, biology and medicine, laser-induced polarization of noble gases is not sufficient. Additional methods are needed to be able to polarize molecules at will. Here, introducing polarization via hydrogenation by parahydrogen (Para-Hydrogen Induced Polarization, PHIP) or transferring polarization from electrons to nuclei via Dynamic Nuclear Polarization, DNP can be applied and considerable activity exists in this area worldwide. While the first approach requires expertise in chemical synthesis, the second approach requires EPR spectroscopy. All this know-how is available in our group and we take advantage of that. First results showing successful polarization of 13C nuclei in the 1-hexyne model compound are shown in Fig. 8.

  • D. Baumer, E. Brunner, P. Blümler, P.P. Zänker, and H.W. Spiess: "NMR Spectroscopy of Laser-polarised 129Xenon under Continuous Flow: New Route to Study Aqueous Solutions of Biomolecules", Angew. Chem. Int. Ed. 45, 7282-7284 (2006).

  • R. H. Acosta, P. Blümler, L. Agulles-Pedrós, A. E. Morbach, J. Schmiedeskamp, A. Herweling, U. Wolf, A. Scholz, W. G. Schreiber, W. Heil, M. Thelen, and H. W. Spiess: "Controlling Diffusion of 3He by Buffer Gases: A Structural Contrast Agent in Lung MRI", J. Magn. Reson. Imaging 24, 1291-1297 (2006).

  • Paul P. Zänker, Jochen Schmidt, Jörg Schmiedeskamp, Rodolfo H. Acosta, and Hans W. Spiess: "Spin Echo Formation In Presence of Fast Stochastic Dynamics", Phys. Rev. Lett. 99, 263001 (2007).

  • M. Roth, J. Bargon, H. W. Spiess, A. Koch: "Parahydrogen Induced Polarization of Barbituric Acid Derivatives. 1H Hyperpolarization Studies", Magn. Reson. Chem., available online (2008).


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Systems

Supramolecular Structures

The solid-state NMR techniques developed in our group provide access to the key elements of supramolecular organization, in particular hydrogen bonds, ionic forces, and π-π interactions. The aromatic character of a molecule determines the experimentally observed π-shift in the NMR resonance of neighboring atoms, which is due to the local attenuation of external magnetic fields by electronic orbital currents of the delocalized aromatic electrons. Nucleus Independent Chemical Shift maps (NICS) represent a numerical and graphical way to illustrate aromaticity of a molecular or condensed-phase system, and allow qualitative interpretation of the experimental π-shifts. The dependence of the NMR response to aromaticity is a function of the electronic structure, and small changes in the latter may result in a dramatic alteration of the π-shifts. In Fig. 9, the electronic ring currents of a hexapyrrolohexaazacoronene molecule in its dicationic and neutral states are displayed, along with their induced NICS maps. It could be shown that the main difference of the (aromatic) dication with respect to the (antiaromatic) neutral form is the presence of a strong ring-current on the outer rim of the molecule, causing a considerable π-shift in the vicinity of the dications.

Fig. 9 Current densities of hexapyrrolohexaazacoronene when placed in a magnetic field normal to the molecular plane, along with the induced NICS maps. The orientation of the currents is indicated by the curved arrows.


  • M. Takase, V. Enkelmann, D. Sebastiani, M. Baumgarten and K. Müllen: "Annularly-fused Hexapyrrolohexaazacoronenes: A Multiple Interior Nitrogen-containing pi-System with Stable Oxidation States", Angew. Chem. Int. Ed. 46, 5524-5527 (2007).


Molecular self assembly provides a strategy to build non-biological systems with nanometer dimensions to create tailor-made robust system geometries which are resistant against impurity incorporation. Consequently, encapsulation of small molecules inside cavities and hosting of suitable guest species is feasible and shows high potential for applications within nanotechnology. As an example we studied organic nanotubes composed of non-tubular, bowl-shaped subunits of calix[4]hydroquinone (CHQ), see Fig. 10. The CHQ molecule consists of four hydroquinone rings which are linked by CH2 spacers, where four hydroxyl groups at one side form an array of strong OH---O hydrogen bonds imposing a bowl-like shape. These molecules are able to self-assemble into an organic nano-tubular structure built from four in-line stacks of CHQ molecules, mutually linked by a hydrogen-bond network involving additional water molecules (Fig. 10). As a consequence, one proton per bridging water molecule can undergo additional hydrogen bonding. The tube dimension as well as this functionality offers the possibility of selective guest-host interaction. By combining solid-state proton NMR spectroscopy and quantum chemical calculations the inclusion of guest molecules can be monitored, as molecules located inside the complexes experience a frequency shift caused by screening effects from the aromatic electrons of the host superstructure. Indeed, the CHQ superstructures are able to recognize specific molecules which fit well into their nanostructures. Successful adsorption has been achieved for three-component mixtures of water and acetone with either 2-methyl-2-propanol or 2-propanol. In both cases, the alcohols were superior to acetone in filling the CHQ tubes. In the case of ethanol, however, no clear preference was found. Also, pyridine (and its derivatives) are not incorporated.

Fig. 10 Recognition of solvent molecules by nanotubes composed of calixarenes.


  • G. Brunklaus, A. Koch, D. Sebastiani, H. W. Spiess: "Selectivity of guest-host interactions in self-assembled hydrogen-bonded nanostructures observed by NMR", Phys. Chem. Chem. Phys. 9, 4545-4551 (2007)


Surfaces and Interfaces

The superior mechanical and heat resistance properties of polymer-clay nanocomposites are governed by interface effects. By substituting a small fraction of the compatibilizing surfactants with spin-labeled analogues, the molecular structure of the interface and its dynamics can be studied by EPR. The accessible temperature range corresponds to the one used in nanocomposite production by melt intercalation and in application of the materials. Complementary results are obtained by selectively addressing phosphonium surfactant head groups via 31P solid-state NMR. Both techniques reveal a slowly moving fraction of strongly clay-attached surfactant molecules as well as a fast moving, liquid-like fraction. Polymer intercalation into organoclays causes a decrease of surfactant mobility only if the glass transition temperature of the polymer is above the order-disorder transition temperature of the surfactant layer. On the EPR time scale of a few nanoseconds, the liquid-like surfactant fraction is then observed only above the glass transition temperature of the polymer. Electron spin echo envelope modulation spectroscopy (ESEEM) indicates a closer contact of the polymer with the middle part of the surfactant tail rather than with the end of the tail. From the obtained data a picture was deduced, where surfactants lie almost flat on the clay platelets with a mobility gradient along their alkyl chains, Fig. 11.

Fig. 11

Structure scheme for organically modified clay. The surfactant tails are almost parallel to the clay surface and exhibit a dynamic gradient along the alkyl chain.


  • G. Panek, S. Schleidt, Q. Mao, M. Wolkenhauer, H.W. Spiess, G. Jeschke: "Heterogeneity of the Surfactant Layer in Organically Modified Silicates and Polymer/Layered Silicate Composites", Macromolecules 39, 2191-2200 (2006).

  • S. Schleidt, H. W. Spiess and G. Jeschke: "A site-directed spin-labeling study of surfactants in polymer-clay-nanocomposites", Colloid. Polym. Sci. 284, 1211-1219 (2006).


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Phenomena And Function

Chain Organization and Mobility in Semicrystalline and Amorphous Polymers

The conformation of the chains at the interface between crystalline and non-crystalline regions in semi-crystalline polymers is crucial for their dynamics. In our collaboration with Dr. S. Rastogi (Dutch Polymer Institute Eindhoven) and Prof. Do Yoon, Seoul National University, we studied the dynamic behavior of ultrahigh molar mass linear polyethylene in the solid state comparing the behavior of solution crystallized (SC) and melt crystallized (MC) samples, Fig. 12, as well the effect of incorporating comonomers. While the local mobility as probed via 1H-13C dipole-dipole couplings was not affected significantly by the incorporation of comonomer units, the higher conformational order in the SC samples indeed leads to a significantly reduced local mobility compared with the MC sample. In contrast, chain diffusion, where on a much longer time scale the all-trans stems in the crystals diffuse to the gauche-containing non-crystalline regions and vice versa is considerably faster in the MC samples. As the temperature dependence of the diffusive motion is similar in SC and MC samples, the difference in timescale directly gives the entropy difference between SC and MC systems. The comparison of the diffusion constant of the chain diffusion and the local motion in the crystallites, also available from NMR, is even more interesting. Substantial acceleration of the local mobility with increasing temperature as compared to the chain diffusion is observed which can be attributed to the formation of defects in the expanded lattice. These defects, however, are not effective in moving the whole stem, despite the fact that their mobility is detected by mechanical relaxation, Fig. 13.

Fig. 12 Chain organization and chain motion in semicrystalline polyethylene

Fig. 13 Chain diffusion vs. local jump rate of melt crystallized polyetheylene


While many techniques are available for studying the time scale of molecular motions, only NMR spectroscopy and neutron scattering can provide detailed information on the geometry of such processes. Since the two techniques are most effective at probing dynamics at different time scales, direct comparisons of probing the geometry and the dynamics in the same system are scarce. We were recently able to provide such a comparison for the complex rotational motion of the phenylene groups in amorphous polycarbonate based on published 2H- and newly recorded 13C-NMR data covering a wide temperature range, and recent quasielastic neutron scattering (QENS) data. The results of the two techniques were found to be in remarkable agreement. No evidence was found for additional motions characterized by 90° flips recently deduced from QENS data alone. Instead, the phenylene motion in the glassy state displays a heterogeneous distribution of rotational angles, about 80° in width, centered at a flip angle of 180°, which stays essentially constant over a wide temperature range. Thus, the phenylene motion that can consistently be observed in NMR and neutron scattering experiments is sensitive to the details of the local packing.

  • T.-Y. Cho, E. Ji Shin, W. Jeong, B. Heck, R. Graf, G. Strobl, H. W. Spiess, D. Y. Yoon: "Effects of Comonomers on Lamellar and Noncrystalline Microstructure of Ethylene Copolymers", Macromol. Rapid Commun. 27, 322-327 (2006).

  • R. Graf, B. Ewen, H. W. Spiess: "Geometry of phenylene motion in polycarbonate from NMR spectroscopy and neutron scattering", J. Chem. Phys. 126, 041104 (2007).

  • Y.-F. Yao, R. Graf, H. W. Spiess, D. R. Lippits, S. Rastogi: "Morphological Differences in Semi-crystalline Polymers: Its Implications on Local Dynamics and Chain Diffusion", Phys. Rev. E 76, 060801(R) (2007).


Structure and Dynamics of New Polymeric Proton Conductors

The growing necessity for clean energy sources to substitute fossil energy has created high demands for batteries and fuel cells. Therefore, various approaches have been proposed, aiming at developing new classes of proton conducting membranes for high temperature applications. In a project together with the groups of Prof. Wegner, Prof. Müllen, and several groups in Germany new polymer proton conductors based on phosphonic acid are studied. Such materials are considered very promising, due to their high charge carrier concentration, thermal stability and oxidation resistivity of the protogenic group. From 1H, 2H, 13C, and 31P NMR combined with computer simulation detailed information on the proton mobility, water content, and the -unwanted- condensation of the phosphonic acid groups can be obtained. High mobility is found for the protons, whereas on the same timescale no mobility associated with reorientation of the phosphonic acid groups or the polymer backbone is observed. The 1H chemical shifts of P-OH protons provide evidence for the presence of a hydrogen bond network, Fig. 14, which allows for proton transport via a Grotthus-type mechanism along a given chain as well as between adjacent chains. The MD simulations further show that proton vacancies can be trapped in anhydride defects produced by condensation. Thus, we proposed that the formation of phosphonic acid anhydride reduces conductivity in two ways: first, by the reduction of the number of charge carriers (mobile P-OH protons), and second, by trapping the proton vacancies. This picture is in good agreement with the observed proton conductivity, which show a drastic decrease for annealed PVPA as a function of degree of condensation. Hence, the minimization of condensation of phosphonic acid groups together with the presence of strong hydrogen bonds is crucial to the charge transport in phosphonic acid based proton conductors.


Fig. 14 Left: Distribution of the CP-OH angle within a given phosphonic acid group of poly(vinyl phosphonic acid) PVPA, predicted from first-principles molecular dynamics simulations . Right: Experimental 2H NMR lineshapes as function of temperature, illustrating motional narrowing due to the hydrogen bond dynamics, exhibiting an effectively isotropic motion.


  • A. Kaltbeitzel, S. Schauff, H. Steininger, B. Bingöl, G. Brunklaus, W.H. Meyer, H. W. Spiess: "Water Sorption of Polyvinyl Phosphonic Acid and its Influence on Proton Conductivity", Solid State Ionics 178, 469-474 (2007).

  • Y. J. Lee, B. Bingöl, T. Murakhtina, D. Sebastiani, W.H. Meyer, G. Wegner and H.W. Spiess: "High Resolution Solid State NMR Studies of Poly(vinyl phosphonic acid) Proton Conducting Polymer: Molecular Structure and Proton Dynamics", J. Phys. Chem. B 111, 9711-9721 (2007).

  • Y. J. Lee, T. Murakhtina, D. Sebastiani and H.W. Spiess: "2H Solid State NMR of Mobile Protons: It is Not Always the Simple Way", J. Am. Chem. Soc. 129, 12406-12407 (2007).


Columnar Stacking and Photoconductivity

Columnar structures based on hexabenzocoronene (HBC) mesogens remain in the focus of our studies, where we enjoy a long-standing and successful collaboration with the Müllen-group. Enhanced supramolecular order of such systems generally enhances their desired function, i. e. photoconductivity. The perfection of the π-stacking as well as the processability of the systems strongly depends on the architecture of the side chains attached. As shown in Fig. 15 branched side groups, which lower the transition temperatures, lead to a high order parameter of the discs within the columns. These columns are embedded in a liquid-like environment generated by the disordered and highly flexible side groups. Remarkably, recent computer simulations in the Kremer group nicely confirm these findings and are able to directly relate these structural features to the observed photoconductivity.

The nature of the periphery, symmetry, size, and topology of building blocks for supramolecular assemblies have a distinct impact upon the electronic properties and the organization into columnar superstructures. Polyaromatic hydrocarbons, synthesized in various flavors in the group of K. Müllen are promising candidates for such superstructures. In a recent collaboration, we computed the electronic excitation energies of these systems, where similar techniques had to be applied as illustrated above for liquid systems. While the calculation of the energy levels of the static equilibrium structure leads to misleading results, the combination of first-principles simulation of thermal motion with quantum-chemical calculations of the excited electronic states along the molecular dynamics trajectories yields a very good agreement with experiment. Both computational and experimental UV/Vis spectra, shown in Fig. 16, emphasize the dependence of the characteristic α-, p-, and β-bands on the overall size and topology of the extended nanographenes.

Both, experimental data and quantum-chemical predictions, show a linear dependence of the absorption maximum of the α- and the β-band, respectively, versus the number of carbon atoms in the aromatic system. Surprisingly, the p-band, which corresponds to the transition between the HOMO and the LUMO, did not reveal a consistent dependence upon the size of the aromatic system. The calculation revealed that this band is not only due to a HOMO-LUMO transition, but also a transition between HOMO-1 and LUMO+1.


Fig. 15 Organization of hexabenzocoronene, HBC in the liquid crystalline phase from 1H-13C dipole-dipole couplings

Fig. 16 Molecular structures of the extended nanographenes and their excitation energies (experimental and calculated) as a function of the number of aromatic carbon atoms.

  • W. Pisula, M. Kastler, D. Wasserfallen, M. Mondeshki, J. Piris, I. Schnell, K. Mullen: "Relation between supramolecular order and charge carrier mobility of branched alkyl hexa-peri-hexabenzocoronenes", Chem. Mater. 18, 3634-3640 (2006).

  • M. Kastler, J. Schmidt, W. Pisula, D. Sebastiani, and K. Müllen: "From Armchair to Zigzag Peripheries in Nanographenes", J. Am. Chem. Soc. 128, 9526-9534 (2006).


Nanostructured Hybrids Composed of Synthetic and Biological Components

The interplay between rigid core and side groups also governs the self organization of hybrids consisting of polyphenylene dendrimers and poly-L-lysines. Such systems, that we study together with the Müllen group and Prof. George Floudas, University of Ioannina, Greece, have attracted interest for the possible development of multiple antigen peptides or as nonviral gene delivery systems. In bulk, the dendron cores form columns which are separated by the polypeptide side groups. The self-assembly mechanism is governed by correlations between the polyphenylene cores and between the α-helical poly-L-lysines for the shorter and longer oligopeptide chains, respectively. Therefore the predominant factor controlling the self assembly is the polypeptide length. These packing requirements have consequences on the peptide secondary structures: poly-L-lysine-substituted polyphenylenes with n<16 lysine residues form ill-defined secondary structures, yet well defined hexagonal arrays of the columns, Fig. 17, left: Intermediate poly-L-lysines stabilize predominantly α-helical structures in contrast to their linear analogues, interfering with the hexagonal order, Fig. 17, right, and longer poly-L-lysines (n>54) side groups display mixed secondary structures. Thus constrained poly-L-lysines of intermediate length can adopt secondary structures that differ from their linear analogues. This fact is of particular importance in the design of multiple antigen peptides, where knowledge of the peptide secondary structure is essential.

Fig. 17 Schematic representation of the self assembly in the functionalized polyphenylene dendrimers. Left: with short polypeptides (N<16), Right: with longer polypeptides (N>20)


For the desired biomedical application of such systems the diffusive motion of the individual functionalized dendrimers is of interest. The size of the particle will depend on the secondary structure of the polypeptide, which in turn will change the diffusivity. As the dendrimers also bear a fluorescent perylenediimide core, this question was addressed in a collaboration with the Butt-group, determining the single molecule diffusivity by fluorescence correlation spectroscopy (FCS) in the focus of a confocal microscope. As shown in Fig. 18 the hydrodynamic radius depends on the number of lysine residues in the corona. An abrupt change occurs around n=16 residues, marking the transition from coil/β-sheet conformations to α-helical poly-Z-L-lysines as detected by 13C NMR spectroscopy.


Fig. 18 Size of polypeptide functionalized polyphenylene dendrimer from FCS (left) and change in polypeptide conformation form 13C NMR (right)


  • M. Mondeshki, G. Mihov, R. Graf, H. W. Spiess, K. Müllen, P. Papadopoulos, A. Gitsas, G. Floudas: "Self Assembly and Molecular Dynamics of Peptide Functionalized Polyphenylene Dendrimers", Macromolecules 39, 9605-9613 (2006).

  • K. Koynov, G. Mihov, M. Mondeshki, C. Moon, H.W. Spiess, K. Muellen, H-J. Butt, G. Floudas: "Diffusion and conformation of peptide functionalized polyphenylene dendrimers studied by fluorescence correlation and 13C NMR spectroscopy", Biomacromolecules 8, 1745-1750 (2007).


Functional Stimuli-Responsive Polymer Networks

Polymer networks that can take up many times their own mass in water attract a lot of attention in materials science. Such networks can be chemically fine tuned to expel water and collapse from a swollen state upon an external stimulus (temperature, pH, salt content etc.). This sensitivity towards external triggers makes polymer networks interesting for use as functional materials, e.g. in molecular sensors or as drug delivery agents. Many materials - e.g. polyelectrolytes, poly(N-isopropylacrylamide), or block copolymers of poly(ethylene oxide)-poly(propylene oxide) - can be used to form networks. Since the crosslinking of polymer chains can be chemical or physical (e.g. by electrostatic or hydrophobic forces, or hydrogen bonding), a vast variety of structures and properties are possible.

In collaboration with Dr. Ulrich Jonas from the Knoll-group we have started a thorough EPR-spectroscopic investigation on such temperature-responsive polymeric networks based on poly(N-isopropyacrylamide), PNIPAAM. We studied the release of small molecules applying CW EPR and advanced pulse EPR methods on spin probes as tracers for carried molecules. EPR spectroscopy on these reporter molecules shows high selectivity and site-specificity and delivers a large variety of information on local guest-host interaction, the distribution of guest molecules (on the nanometer scale), and accessibility by solvent. In our systematic study it was possible to obtain a picture of the thermally induced collapse on the molecular scale, which proceeds over a substantially broader temperature range than indicated by the sharp macroscopic volume transition. Furthermore, the sampling of hydrophilic and hydrophobic environments by the spin probes suggests a discontinuous collapse mechanism with a coexistence of collapsed and expanded network regions. These structural inhomogeneities on the nanoscale - swollen hydrophilic network cavities coexisting with collapsed hydrophobic regions - also lead to an inhomogeneity in chemical reactivity, which could be revealed by careful analysis of the acid-catalyzed spin probe decay inside the gels. Due to spatial confinement and availability of active acid protons, the hydrophilic regions form nanoreactors, which strongly accelerate the reaction while the hydrophobic regions act as nanoshelters, in which enclosed spin probes are protected from the decay. From hydrogels with varying content of carboxylic acid and amide functionalities, the importance of locally activated protons for the catalytic activity can be deduced. The results show that our simple system consisting of a statistical, binary or tertiary, copolymer can display remarkably complex behavior that mimics spatial and chemical inhomogeneities observed in functional biopolymers such as enzymes, see Fig. 19.

Fig. 19

Left: Temperature-dependent CW EPR spectra of TEMPO in an aqueous solution with PNIPAAM-based hydrogels.

Right: It was shown that a simple statistical copolymer network shows remarkable complexity on the nanoscale. These hydrogels show structural inhomogeneities that lead to the coexistence of network regions that significantly enhance a chemical reaction (nanoreactors) and shield molecules from the reaction (nanoshelters).

  • M. J. N. Junk, U. Jonas, D. Hinderberger: "EPR Spectroscopy Reveals Nano-Inhomogeneities in Structure and Reactivity of Thermoresponsive Hydrogels", Small, in press (2008)


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Applications

Chain Branching

The macroscopic properties of polyolefins strongly depend on the chain microstructure. In recent years new single site catalysts have enabled much greater synthetic control over the polydispersity, type of branch and branch content. In polyethylene, the physical properties of both the solid and the melt can be tuned by the presence of branches of various lengths in the polymer backbone. Such branches form structural defects during crystallization and thus strongly affect crystallization rates, ultimate crystallinity, and other bulk mechanical properties. For instance, long-chain branching is known to influence the zero-shear viscosity even at concentrations of 2 branches per 100 000 CH2 groups. Thus, it is very important to quantify the degree of chain branching with 13C NMR in solution being the method of choice. In 13C NMR the chemical shifts of branch points as well as adjacent carbon positions can be distinguished from the backbone resonances. However, when applied to polyethylene, exceedingly long measurement times are needed, because of the low solubility of polyethylene, even at high temperatures. Solid state type NMR, under magic-angle spinning (MAS), on the other hand, can be used to overcome these limitations, despite the substantial loss of spectral resolution as compared to solution NMR. Under optimized conditions about two orders of magnitude reduction of measuring times where achieved, allowing quantification of 7-9 branches in 100 000 CH2 groups in a single overnight run, see Fig. 20.


Fig. 20

Quantification of low branch content in optimized melt state 13C-NMR under MAS



  • K. Klimke, M. Parkinson, C. Piel, W. Kaminsky, H. W. Spiess, M. Wilhelm: "Optimisation and Application of Polyolefin Branch Quantification by Melt-State 13C NMR Spectroscopy", Macromol. Chem. Phys. 207, 382-395 (2006).


Fourier Transform Rheology

During the production and processing of materials the non-linear mechanical regime is omnipresent at large scale. Additionally complex fluids show a great variety of rheological properties. For the processing it is important to understand the relation between the shear induced structures, e.g. of dispersions, and the non-stationary non-linear mechanical properties. Here the concept of Fourier-transform rheology, introduced by Manfred Wilhelm, provides a new way of quantifying such non-linearity in terms of amplitude and phase of overtones. The individual higher harmonics appearing in the shear stress response, however, do not have a simple physical interpretation. Instead, we recently showed that the whole overtone spectra can be described as a superposition of different overtone spectra of typical non-linear rheological effects, like strain hardening (triangle), strain softening (rectangle) and shear bands or wall slip (saw tooth). This novel analysis of FT-Rheology experiments thus separates the non-linear mechanical response into the underlying physical phenomena and opens up new applications of this technique in different fields, such as processing of polymers, dispersions and food. Fig. 21 shows an example how a rather complex non-linear response leading to a complex overtone spectrum with harmonics with alternating intensities can nicely be analyzed in terms of these characteristic functions.

Fig. 21 Top: Experimental and reconstructed toque response and resulting FT rheology spectrum. Bottom: Characteristic functions for linear response, strain softening and shearbands/wallslip used in the reconstruction of the experimental time data.


  • C. O. Klein, H. W. Spiess, A. Calin, C. Balan, M. Wilhelm: "Separation of the Nonlinear Oscillatory Response into a Superposition of Linear, Strain Hardening, Strain Softening, and Wall Slip Response", Macromolecules 40, 4250-4259 (2007).


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