Complex Materials
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.
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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.
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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.
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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.