Measurement of spin-lattice relaxation times in multiphase polymer systems

Solid-state Nuclear Magnetic Resonance (NMR) has emerged as a pivotal technique for unraveling the microstructure and dynamics of intricate polymer and biological materials. Within this context, site-specific proton spin-lattice relaxation times in the laboratory frame (T₁) and rotating frame (T_1ρ) have become indispensable tools for investigating phase separation structures and molecular dynamics in multiphase polymer systems. Notably, the site-specific measurement of proton T₁ and T_1ρ is usually achieved via ¹³C detection in polymers, where ¹H polarization is typically transferred to ¹³C via cross polarization (CP). Nevertheless, CP relies on the ¹H-¹³C heteronuclear dipolar couplings, and thus it does not work well for the mobile components. In this study, via the integration of CP and RINEPT (refocused insensitive nuclei enhanced by polarization transfer), we propose a robust approach for the measurement of site-specific proton T₁ and T_1ρ in multiphase polymers. It overcomes the limitation of CP on transferring ¹H polarization to ¹³C in mobile components, and thus enables simultaneous determination of site-specific proton T₁ and T_1ρ in rigid and mobile components in multiphase polymers in a single experiment. Such experiment can also be used for dynamics-based spectral editing due to the dynamic selectivity of CP- and RINEPT-based polarization transfer process. The proposed experiments are well demonstrated on three typical multiphase polymer systems, poly(methyl methacrylate)/polybutadiene (PMMA/PB) polymer blend, polyurethane (PU) and polystyrene-polybutadiene-polystyrene (SBS) elastomers. We envisage the proposed experiments can be a universal avenue for structural and dynamic elucidation of multiphase polymers containing both rigid and mobile components.