Spin Diffusion with ¹³C Selection and Detection for the Characterization of Morphology in Labeled Polymer Blends with MAS NMR

Abstract

A pulse sequence has been implemented for the determination of domain sizes in 13 C enriched organic materials with two-dimensional high-resolution magic angle spinning (MAS) NMR correlation spectroscopy. The method integrates the high resolution of 13 C MAS for selection and detection with an efficient polarization transfer across domain boundaries via strong 1 H dipolar interactions. A one-dimensional version with spectral editing allows for improved efficiency with respect to measurement time. With this MAS NMR and labeling approach heterogeneity at length scales from 1 to 100 nm can be determined for systems without long-range order. As an example, proton spin diffusion times up to 500 ms were used to study the morphology of a phase-separated semi-interpenetrating network of poly(styrene-co-acrylonitrile) and cross-linked poly(styrene-co-maleic anhydride). From the spin diffusion behavior a characteristic domain diameter d ) 63 ( 4 nm was calculated.

2D13C–13C MAS NMR Correlation Spectroscopy with Mixing by True 1H Spin Diffusion Reveals Long-Range Intermolecular Distance Restraints in Ultra High Magnetic Field

“…This value for the diffusivity has also been used for experiments employing moderate MAS frequencies (14)(15)(16)(19)(20)(21). The favorable polarization transfer properties of 1 H can be combined with the superior spectral resolution of 13 C nuclei in a 2D 13 C-13 C MAS CP 3 correlation spectroscopy experiment (14,15). In an early application of this method, the morphology and phase separation of 13 C-labeled semi-interpenetrating networks were investigated (15,20).…”

“…The favorable polarization transfer properties of 1 H can be combined with the superior spectral resolution of 13 C nuclei in a 2D 13 C-13 C MAS CP 3 correlation spectroscopy experiment (14,15). In an early application of this method, the morphology and phase separation of 13 C-labeled semi-interpenetrating networks were investigated (15,20). In the solid state NMR of complex solid-type biological assemblies, for example membrane proteins, the same principles could be applied to probe shorter range intra-and intermolecular distances for structure determination.…”

“…Prior versions of the CP 3 experiment use a 13 C lock pulse after the first CP, which allows the residual 1 H signal to decay during a spin lock time τ > T 2 (14,15). In practice, this yields a considerable loss of the 13 C signal, in particular for materials with a long 1 H T 2 .…”

“…The residual 13 C transverse magnetization after the second CP period vanishes only for a long τ m T 2 (15). For shorter τ m , the phase of the residual 13 C signal can be cycled relative to the phase of the 13 C signal detected during t 2 (14).…”

“…Solid circles indicate the carbons involved in the intermolecular correlations. 13 C- 13 C correlations (14,15). A modified CP 3 experiment is presented, optimized for short 1 H mixing times 0.1 ms < τ m < 1 ms. Correlations spanning distances between 0.1 and 1.0 nm are easily generated.…”