Structure and Dynamics in Complex Materials:
polymer networks: structure, dynamics, and swelling
phase structure and dynamics in liquid crystals and dendrimers
polymer crystallization: kinetics and molecular origin
dynamics and order in confined media
hydrogen bonds and supramolecular structure in polymers
 
Solid-State NMR Methodology:
magic-angle spinning (MAS) methods: dipolar couplings and internuclear distances
multiple-quantum spectroscopy
exchange spectroscopy
 
Self-Diffusion Measurements:
pulsed field-gradient NMR diffusometry
fluorescence correlation (in preparation)
 
Current Collaborations:
J.-U. Sommer, A. Vidal, ICSI (CNRS), Mulhouse, France (network structure and swelling)
R. Yerushalmi-Rozen and M. Gottlieb, Beer Sheva, Israel (polymer crystallization and gelation phenomena)
Michael Börsch, Freiburg and Stuttgart (fluorescence correlation spectroscopy)
Ingo Schnell, MPI-P, Mainz (MAS methodology)
E. Bourgeat-Lami, LCPP (CNRS), Villeurbanne, France (synthesis of nano-composites)
R. Simonutti and P. Sozzani, Milano, Italy (inclusion compounds)
 

Polymer Networks: Structure, Dynamics, and Swelling
Networks are commonly treated as homogeneous systems down to the molecular level. Yet, many recent studies have indicated the existence of heterogeneities on very different length scales, starting with molecular-scale distributions of the length of elastically active network chains, and even reaching the micron scale for swollen systems, where concentration fluctuations frozen-in by the complex topological structure are believed to become amplified. Such heterogeneities may have substantial effects on the macroscopic properties, and a deeper insight into their nature will help to optimize structure-property relationships.

We have developed and apply advanced multiple-quantum NMR methods, which are suited to characterize the stretching of individual network chains on the molecular level and even yield information on distributions. The figure shows the detection of a broadening of the dynamic chain order parameter distribution upon swelling, indicating significant swelling heterogeneity.

Diffusion methods are further applied for the investigation of motional heterogeneities of guest molecules in the networks. Here we compare results from a bulk method, pulsed-field gradient NMR diffusometry, with spatially resolved measurements using fluorescence correlation spectroscopy.


Biaxial Nematic Liquid-Crystalline Side-Chain Polymers
Biaxial nematic liquid crystals have been theoretically predicted as early as 1970. Yet, before we started our work, a genuine thermotropic biaxial nematic phase has not yet been discovered. Recent investigations of Finkelmann et al. indicate that the attachment of the mesogenic unit to a polymer chain might provide the necessary stabilization of a biaxial phase.

We have constructed an NMR probe capable of rapid sample flips perpendicular to the magnetic field, with which the symmetry properties of the alignment tensor of the sample under investigation can be determined. Results on a specific side-chain LC polymer indicate that a side-on attachment of the mesogenic group helps to stabilize a biaxial nematic phase.


Rigid Polyphenylene Dendrimers

Dendrimers with phenyl rings as their only building block are unique in that they are shape persistent. They are of interest as nano-sized carriers in catalysis or for light-harvesting applications. A variety of static and MAS NMR techniques is well suited for the characterization of fast processes as well as slow dynamics. By application of CODEX NMR, we could for example identify slow and restricted flips of terminal phenyl rings as the dominating type of motion in these otherwise rigid macromolecules.

When comparing generation-dependent measurements of the rate of the slowly moving terminal phenyl rings, as well as a small fraction of rapidly moving rings, it is indicated that the dynamics is governed by the packing of arm ends at the dendrimer periphery. Polyphenylene dendrimers of higher generation therefore qualify as one of the few examples of dense-shell dendrimers.


Chain Order and Dynamics in Inclusion Compounds
A variety of organic compounds, e.g. cyclodextrin, are capable of accommodating guest molecules in hydrophobic cavities. Crystalline, channel-like inclusion compounds of such hosts with polymers are interesting model systems for the study of the conformational dynamics of isolated polymer chains.

The formation of inclusion compounds can be readily followed by 13C CP MAS spectroscopy. Using two-dimensional dipolar recoupling methods under fast MAS conditions (30 kHz), carbon-proton dipolar couplings associated with the polymer chain can be determined. These provide information on the amplitude of chain motion in the channel, as characterized by the order parameter, Sb.


For the sample under investigation, fast-MAS proton spectra are also sufficiently resolved. Using double-quantum correlation spectroscopy (the solid-state analogue of the INADEQUATE method), guest-host dipolar contacts are detectable. Thereby, fast diffusion of the polymer chains along the channels can be excluded. An estimate of the upper limit of the translational diffusion coefficient of the chain is 5⋅10-15 m2/s.


Magic-Angle Spinning (MAS)
The most important aspect in solid-state NMR is the dominance of orientation-dependent interactions, which are usually averaged out in NMR spectra of solutions. They lead to substantial broadening of the spectra and a loss of spectral resolution.

High resolution can be re-gained by rapid sample rotation about the magic angle (54.7°), which, much like in the case of solutions, effectively averages all orientation-dependent interactions.

While almost liquid-like resolution is achievable for 13C already at moderate spinning frequencies, very-fast MAS in excess of 25 kHz is needed in order to narrow dipolar-broadenend proton spectra. This is due to the strong multi-spin dipolar-coupling network among the abundant protons, which is also responsible for the smearing of the typical features of powder (Pake) spectra in the static limit.


Dipolar Couplings and Internuclear Distances
Dipolar coupling constants are dependent on the distance between the coupled spins. Yet, their measurement is precluded when MAS is applied, because this valuable information is simply averaged away. The solution to this paradox is the application of recoupling pulse sequences of a specific duration as part of a two- or higher dimensional experiment. The variation of the recoupling time while monitoring the signal intensity provides access to the dipolar coupling. The experiment might embody the temporary excitation of higher-quantum coherences. A special variant of this approach comprises a second Fourier transformation over an indirect time domain, which leads to the appearance of coupling-specific sideband patterns.

The spectra shown here are special in that they were obtained on samples naturally abundant in 15N (!) by using inverse detection. I.e., the detection of the signal takes place on protons instead of the weakly sensitive heteronucleus.

The distances which are extracted from the spinning sideband patterns are in good agreement with results from other methods. In fact, only neutron scattering or X-ray scattering involving substantial refinement is able to provide distance information with comparable accuracy.


Proton Multiple-Quantum Spectroscopy
The measurement of inter-proton dipolar couplings is particular attractive in that it provides information on dynamics in polymers, and has the advantage of being applicable to as-synthesized samples. Polymers, and other soft matter, are, unlike normal liquids, characterized by anisotropic molecular motions. This leads to the appearance of weak, residual dipolar couplings, which is proportional to the order parameter of the polymer chain. This order parameter provides a link to macroscopic (e.g mechanical) properties, and can be used to put theories of polymer dynamics to a test.

While line shape analysis might not be very reliable, due to the problem of multiply coupled spins and featureless spectra, multiple-quantum spectroscopy is able to provide faithful information. The technique resembles the recoupling experiments mentioned above, but can also be applied in static samples. Information on dipolar couplings is extracted by observing the signal intensity as a function of the duration of a double-quantum pulse sequence.
We have developed an experimental strategy, with which it is possible to not only determine the amount of liquid-like components and the average order parameter in a given network sample, but also to characterize the order parameter distribution. The method was tested on a series of bimodal network samples, which were obtained by end-linking mixtures of short and long precursor polymers.


Exchange Spectroscopy under MAS
The centerband-only detection of exchange (CODEX) experiment, recently introduced by Schmidt-Rohr et al., is a powerful
MAS method which allows the elucidation of molecular rotations on the ms to s time scale. It probes reorientations of the chemical-shift anisotropy tensor of 13C or other heteronuclei by means of recoupling. It is convenient in that exchange is detected by the appearance of signals in highly resolved MAS spectra.

From the variation of signal intensities as a function of the mixing time, tm, or the recoupling time, NrτR, it is possible to obtain the correlation time of the slow process, tc, and the jump angle, respectively.

The long-time intensity plateau is further dependent on the number of sites accessible to the jump motion. A decrease of this plateau intensity at higher temperatures is indicative of short correlation times reaching the experimental time scale (intermediate motional regime).

We have made extensive use of the CODEX technique in our investigations of polyphenylene dendrimers, where we could identify a rather peculiar small-angle phenyl flip by about 24°. As is seen in the figure, it is even possible to obtain information on the imprecision of the jump.