K. Schäler, M. Roos, P. Micke, Y. Golitsyn, A. Seidlitz, T. Thurn-Albrecht, H. Schneider, G. Hempel, K. Saalwächter.
Basic principles of static proton low-resolution spin diffusion NMR in
nanophase-separated materials with mobility contrast.
Solid State Nucl. Magn. Reson. 72, 50-63 (2015).
We review basic principles of low-resolution proton NMR spin diffusion experiments, relying on mobility
differences in nm-sized phases of inhomogeneous organic materials such as block-co- or semicrystalline
polymers. They are of use for estimates of domain sizes and insights into nanometric dynamic inhomogeneities.
Experimental procedures and limitations of mobility-based signal decomposition/filtering
prior to spin diffusion are addressed on the example of as yet unpublished data on semicrystalline
poly(ε-caprolactone), PCL. Specifically, we discuss technical aspects of the quantitative, dead-time free
detection of rigid-domain signals by aid of the magic-sandwich echo (MSE), and magic-and-polarizationecho
(MAPE) and double-quantum (DQ) magnetization filters to select rigid and mobile components,
respectively. Such filters are of general use in reliable fitting approaches for phase composition determinations.
Spin diffusion studies at low field using benchtop instruments are challenged by rather
short 1H T1 relaxation times, which calls for simulation-based analyses. Applying these, in combination
with domain sizes as determined by small-angle X-ray scattering, we have determined spin diffusion
coefficients D for PCL (0.34, 0.19 and 0.032 nm2/ms for crystalline, interphase and amorphous parts,
respectively). We further address thermal-history effects related to secondary crystallization. Finally, the
state of knowledge concerning the connection between D values determined locally at the atomic level,
using 13C detection and CP- or REDOR-based “1H hole burning” procedures, and those obtained by calibration
experiments, is summarized. Specifically, the non-trivial dependence of D on the magic-angle
spinning (MAS) frequency, with a minimum under static and a local maximum under moderate-MAS
conditions, is highlighted.