NMR Technique Development

Program Scope: Development and application of advanced solid-state nuclear magnetic resonance (NMR) methods and scattering simulations for elucidating the nanometer-scale structure and dynamics of heterogeneous polymer-based materials under the following types of conditions: Ionomers as used, for instance, in all-solid H2/O2 fuel cells; Biological and biomimetic apatite-polymer nanocomposites; Polymers intercalated in layered silicates. 

Major Program Achievements:

Structure of Nafion:  A definitive new model of the nanometer-scale structure of the Nafion ionomer, used in fuel-cell proton-exchange membranes, has been developed based on NMR and quantitative scattering analysis.  High-resolution 13C NMR of fluorinated polymers such as Nafion and PTFE (Teflon) was achieved for the first time, by fast magic-angle spinning and pulsed 19F decoupling; it revealed a high degree of conformational order of the Teflo-like backbone of Nafion.  Advanced NMR experiments show that Nafion backbones form rotating, conformationally ordered helices that pack with limited orientational order.  The branch points are relatively immobile, while the anionic sidegroups move significantly in the presence of water.   In order to characterize the supramolecular structure, we have developed an algorithm for simulating small-angle scattering data using multidimensional numerical Fourier transformation as familiar from 2D NMR.  On this basis, we can exclude all specific models of Nafion previously proposed; we prove that instead of spherical clusters, bilayers, or polymer bundles, the ionic sidegroups form long, parallel water channels of ~ 2.5 nm diameters, which explain the salient properties of Nafion.

Structure of Biological and Biomimetic Nanocomposites:  The apatite-collagen nanocomposite in bone has been elucidated in terms of the composition of both components and their distances from the organic-inorganic interface.  Several new NMR approaches for studying nanocomposites, including HeteronucleAr Recoupling with Dephasing by Strong Homonuclear Interactions of Protons (HARDSHIP), have been introduced.  They have revealed the thickness of apatite nanocrystals in bone, the concentration and location of carbonate and hydroxide ions, the presence of bound and viscous water layers at the interface, and the COO- and CH-OH apatite-binding sites of collagen. These methods have then been applied to biomimetic polymer-apatite nanocomposites.

Improved Characterization of Heterogeneous Polymers:  New 1H spin diffusion NMR methods for characterizing heterogeneities in multicomponent polymer materials, on the 0.5 - 50 nm scale, have been introduced.  They provide improved contrast through 13C evolution and detection, and improved accuracy by calibration of local spin diffusion coefficients.  Sensitivity enhancement (4 - 10-fold) has been achieved in various NMR experiments by indirect 1H detection, signal refocusing, or multiple alternating depolarization.

Heterogeneous para/ferromagnetic polymer materials.  Many polymer-based materials contain para- or ferromagnetic particles.  New insights into their effects on the NMR of the polymer matrix have been obtained.

Program Impact: This work has provided insights into the microscopic origins of macroscopic properties of heterogeneous polymers; hopefully, this will eventually lead to improved materials. The NMR techniques developed by our group have been and will be used worldwide by other NMR groups.

Contact Information:
Klaus Schmidt-Rohr

Selected Publications