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

The cross-polarization dynamics between H-1 and F-19 in a Viton-type fluoro elastomer 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 theo ry of spin thermodynamics based on the spin temperature hypothesis was empl oyed to describe the polarization transfer between the H-1 and F-19 spin ba ths. 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. Th e temperature dependences of T-1(F) and T-1 rho(F) were measured between -4 9 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.