CP MAS kinetics and impedance spectroscopy studies of local disorder in low-dimensional H-bonded proton-conducting materials

Abstract

The ¹H–¹³C cross-polarization magic angle spinning (CP MAS) kinetics was studied in poly(vinyl phosphonic acid) (pVPA), i.e. material with high degrees of freedom of proton motion along H-bonded chains. It has been shown that the CP kinetic data for the adjacent ¹H–¹³C spin pairs can be described in the frame of the isotropic spin-diffusion approach. The rates of spin diffusion and spin-lattice relaxation as well as the parameters accounting for spin coupling and the effective size of spin clusters have been determined. The local order parameter S ≈ 0.63±0.02, determined as the ratio of the measured dipolar ¹H–¹³C coupling constant and the calculated static dipolar coupling constant, is significantly lower than the values deduced for related sites in other polymers and in series of amino acids. This means that the local disorder of the C–H bonds in pVPA is between those for rather rigid C–H bond configurations having S = 0.8–1.0 and highly disordered –CH₃ groups (S ~ 0.4). This effect can be attributed to the presence of the proton transfer path where proton motion is easy to activate. The activation energy for the proton motion E_a = 59±7 kJ/mol was determined from the impedance spectroscopy data analysing the temperature and frequency dependences of the complex dielectric permittivity of pVPA. The rates of proton spin-lattice relaxation and spin diffusion are of the same order and both run in the time scale of milliseconds.