NUCLEAR-RELAXATION AND FRACTAL STRUCTURE OF A CROSS-LINKED POLYMER

We have investigated the longitudinal 2H and 13C relaxations of methanol and nitromethane adsorbed on poly(4-vinylpyridine) (P4VP) cross-linked by paramagnetic VO2+ and Cu2+ ions as well as by diamagnetic Cd2+ ion. The aim of the proposed method is to gain dynamical as well as structural information on a disordered organic system. Electron spin resonance, scanning electron microscopy, and small-angle X-ray scattering give complementary information on the rigid network (resin) whose nodes are the paramagnetic ions. The nonexponential decay of the longitudinal nuclear magnetization observed in paramagnetic systems has been modeled, following a theory of Mendelson, by a superposition of exponential decays associated to a fractal distribution of quasi disconnected spherical pores. The power law ∼t3-df found in this theory at short time has allowed a determination of the fractal dimension df of the surface as well as the surface contribution of the spin-lattice relaxation rate. The frequency and temperature dependences of the surface relaxation of methanol are explained by a reorientation of the adsorbed molecule in the coordination sphere of the paramagnetic ion. The bulk relaxation is due to a translational diffusion of the solvent relaxed by a distribution of paramagnetic probes. © 1990 American Chemical Society.