Theory of spin diffusion in liquid-phase polymer systems

A general theory of spin diffusion in condensed media is constructed by the method of Zwanzig-Mori projection operators using the superpositional approximation to decouple the many-particle correlation functions. The spin diffusion coefficient is expressed in the form D-sp = D-tr + D-f, where D-tr is the contribution associated with translational displacements of the molecules and Df is the contribution caused by intermolecular flip-flop processes. The expression for D-tr differs from the well-known Kubo-Green formula for the self-diffusion coefficient D-sd in that the integrand contains an additional factor P-f(t), which is the probability of the molecular spins not participating in intermolecular flip-flop transitions over the time t. A microscopic expression is obtained for Df in the form of a time integral of the intermolecular dipole-dipole dynamic correlation functions. For liquid-phase polymer system with fairly high molecular mass the condition D-sp much greater than D-sd is satisfied. (C) 1998 American Institute of Physics. [S1063-7761(98)01308-0].