Abstract
The 1H–13C cross-polarization (CP) kinetics in poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) was studied under moderate (10 kHz) magic-angle spinning (MAS). To elucidate the role of adsorbed water in spin diffusion and proton conductivity, PMETAC was degassed under vacuum. The CP MAS results were processed by applying the anisotropic Naito and McDowell spin dynamics model, which includes the complete scheme of the rotating frame spin–lattice relaxation pathways. Some earlier studied proton-conducting and nonconducting polymers were added to the analysis in order to prove the capability of the used approach and to get more general conclusions. The spin-diffusion rate constant, which describes the damping of the coherences, was found to be strongly depending on the dipolar I–S coupling constant (DIS). The spin diffusion, associated with the incoherent thermal equilibration with the bath, was found to be most probably independent of DIS. It was deduced that the drying scarcely influences the spin-diffusion rates; however, it significantly (1 order of magnitude) reduces the rotating frame spin–lattice relaxation times. The drying causes the polymer hardening that reflects the changes of the local order parameters. The impedance spectroscopy was applied to study proton conductivity. The activation energies for dielectric relaxation and proton conductivity were determined, and the vehicle-type conductivity mechanism was accepted. The spin-diffusion processes occur on the microsecond scale and are one order faster than the dielectric relaxation. The possibility to determine the proton location in the H-bonded structures in powders using CP MAS technique is discussed.
Highlights
The cross-polarization (CP) technique, often combined with magic-angle spinning (MAS), has been widely used in solidstate NMR studies for several decades.[1−4] Typically, CP is used to enhance signals of less abundant spins (S) by using magnetization of abundant ones (I) with a larger gyromagnetic ratio
As CP is promoted by the dipolar I−S interactions that are intrinsically sensitive to internuclear distances, it plays a major role for probing short-range ordering and local dynamics.[5,6]
In the presence of strong heteronuclear dipolar interactions, e.g., between 1H and 13C, the kinetics of CP is described by the socalled I−I*−S model (I = 1H and S = 13C) combining a coherent term of the isolated I*−S spin pair and an incoherent term related to interactions with other protons in a thermal spin-bath in a phenomenological way.[7−9] This model can qualitatively explain different build-up constants and general relaxation rates for different chemical groups
Summary
The cross-polarization (CP) technique, often combined with magic-angle spinning (MAS), has been widely used in solidstate NMR studies for several decades.[1−4] Typically, CP is used to enhance signals of less abundant spins (S) by using magnetization of abundant ones (I) with a larger gyromagnetic ratio. CP MAS kinetics, i.e., the dependence of the NMR signal intensity on the contact time for interacting spins, has revealed its capability to resolve fine structural effects as well as dynamics at the atomic level.[7,8] The studies on spin diffusion and relaxation processes are very useful for understanding fine details of materials suitable for quantum information processing and of supramolecular aggregates for molecular electronics.[9,10]. It is well-known that the spin-diffusion processes are dominant in solid-state NMR dynamics. This signifies that combination of these two methods are sufficiently developed to be used for new series of polymer conductors
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