Measurement of the local 1H spin-diffusion coefficient in polymers

Area of Science:

• Solid-state Nuclear Magnetic Resonance (NMR) Spectroscopy
• Polymer Science
• Materials Science

Background:

• Proton spin diffusion is crucial for determining domain sizes in heterogeneous organic solids.
• Accurate spin diffusion coefficients are often lacking, as they are typically derived from model systems.
• Quantifying the effects of experimental conditions like magic-angle spinning (MAS) and spin-lock fields on spin diffusion remains challenging.

Purpose of the Study:

• To develop a method for the direct measurement of local proton ((1)H) spin-diffusion coefficients in rigid polymers.
• To investigate the influence of proton density and partial mobility on spin diffusion.
• To quantify the effects of transverse and magic-angle spin-lock fields on spin diffusion.

Main Methods:

• Utilized heteronuclear dephasing of (1)H magnetization, followed by a mixing time for (1)H spin diffusion, and (13)C detection via cross-polarization.
• Employed (1)H homonuclear decoupling and (13)C 180-degree pulse recoupling to create 'holes' in the proton magnetization.
• Analyzed (1)H spin diffusion into these holes using (13)C-detected REDOR curves and numerical simulations.

Main Results:

• Successfully measured local (1)H spin-diffusion coefficients in rigid polymers, ranging from 0.2-0.5 nm(2)/ms on the 0.5-nm scale.
• Demonstrated the ability to create and probe spherical holes of 0.4-0.8 nm diameter in the proton magnetization.
• Observed that these short-range coefficients are smaller than previously reported values for larger scales (10-nm).

Conclusions:

• The developed method allows for direct and accurate measurement of local spin-diffusion coefficients in polymers.
• This technique provides a convenient way to explore the dependence of spin diffusion on local proton density and mobility.
• The method is effective for quantifying the impact of various experimental conditions, such as spin-lock fields, on spin diffusion.