Analysis of cross-polarization dynamics between 1H and 19F in Viton fluoroelastomer using solid-state 19F magic angle spinning and 1H → 19F cross-polarization magic angle spinning NMR

The cross-polarization dynamics between H-1 and F-19 in a Viton-type fluoroelastomer were studied using F-19 magic angle spinning (MAS) and H-1 --> F-19 cross-polarization (CP)/MAS NMR at 188.29 MHz. The phenomenological theory of spin thermodynamics based on the spin temperature hypothesis was employed to describe the polarization transfer between the H-1 and F-19 spin baths. Simultaneous fitting of the evolution of magnetization in the standard CP and a modified CP(TORQUE) experiment, using the exact solutions of the equations for the spin thermodynamics, gave unique sets of the parameters T-HF, T-1 rho(H) and T-1 rho(F) for five separate peaks in the F-19 spectra. The values of T-1 rho(H) and T-1 rho(F) obtained are consistent with those independently measured by spin-locking experiments. The true values of THF are significantly larger (by a factor of ca. 4) than those derived from a simple two-parameter fit of the CP behaviour with variable contact time. The temperature dependences of T-1(F) and T-1 rho(F) were measured between -49 and 86 degrees C. The relationship between the CP dynamics parameters and the local molecular motion in the polymer chains is discussed. Copyright (C) 1999 John Wiley & Sons, Ltd.