POLYMER-STABILIZED PD SOLS - KINETICS OF SOL FORMATION AND STABILIZATION MECHANISM

The effects of different water-soluble polymers such as polyvinyl alcohol (PVA), polyvinylpyrrolidone (PVP), and poly-2-vinylpyridine (P2VP) on the mechanism of sol formation and stabilization of aqueous nanometer-sized Pd sols have been studied using TEM and UV/VIS photospectroscopy. In the case of UV/VIS spectroscopy an (empirical) relation between the absorbance at high wavelength and the particle size is found, A weakly adsorbing polymer (PVA) does not affect the sol formation, On the other hand, strongly adsorbing polymers like PVP and P2VP retard the growth of the particles. Two other parameters that affect the particle size are the reducing agent (H3PO2) concentration and the oxygen content of the solution. The effect of the polymer concentration and the molecular weight of the polymer on the stability and sedimentation of the Pd sol indicates that the weakly adsorbing PVA does not adapt its conformation during adsorption onto the very small Pd particles. Therefore, adsorption of Pd particles on a PVA coil is probably a more realistic model for this system. On the other hand, strongly adsorbing polymers like PVP and P2VP will adapt their conformation, resulting in a kind of ''beads on a string'' structure, The sol formation/stabilization mechanism of P2VP sols is complex compared to that of PVP-stabilized sols, because of the polyelectrolyte character of P2VP. (C) 1995 Academic Press, Inc.