FRACTAL STRUCTURE OF A CROSS-LINKED POLYMER RESIN - A SMALL-ANGLE X-RAY-SCATTERING, PULSED FIELD GRADIENT, AND PARAMAGNETIC RELAXATION STUDY

The fractal distribution of pore sizes in a poly(4-vinylpyridine) resin cross-linked by diamagnetic (Cd2+) or paramagnetic (VO2+, Cu2+) divalent metallic ions has been characterized by three complementary techniques. Small-angle x-ray scattering has provided a direct determination of the surface fractal dimension D(f) approximately 2.4-2.6 for the pore system between 4 and 44.5 nm as well as the mass fractal of dimension 2.65-2.75 for the polymer network below 4 nm. Proton-pulsed-field-gradient experiments on adsorbed methanol have been performed at variable delays between the gradient pulses, covering the free and bounded diffusion regimes. They have evidenced the same fractal distribution of pore radii in the 1-50-mu-m range, providing an estimate of the solvent diffusion coefficient of the order of 5 X 10(-10) m2/s. The pores of radii between 3 nm and 1-mu-m have been sampled by the methanol H-2 and C-13 relaxations in resins cross-linked by paramagnetic ions. In this range, the paramagnetically enhanced relaxation is strongly dependent on the pore surface-to-volume ratio and exhibits a multiexponential behavior at short times. The analysis of the data confirms the D(f) value given by the two other methods and shows that the pores are surrounded by a solvent adsorption shell of thickness about ten molecular diameters. This is significant of the roughness of the interface between the bulk solvent and the polymer network suggested also by the reduction of the methanol diffusion coefficient, as compared with the neat liquid, as well as by the mass fractal dimension of this network. Thus, three quite different methods concur to show that this organic disordered material has a fractal distribution of pore sizes over 4 orders of magnitude. This result is of particular importance for studies on diffusion processes in cross-linked resins used in catalysis or chemical separation.