POLYMER MATRIX INFLUENCE ON THE KINETICS OF SOME FUNDAMENTAL INORGANIC COLLOIDAL REACTIONS

Among the most useful methods for the preparation of pure metal powder s is the thermolysis of metal carbonyl complexes in hydrocarbon soluti ons. Zero-valent cobalt particles are obtained by the decomposition of Co-2(CO)(8). The reaction is primarily governed by diffusion, which i s strongly dependent on the viscosity of the solution. In a solution c ontaining polystyrene, the viscosity is directly proportional to the c oncentration of the polymer. To study the variation of the rate consta nts of this colloidal reaction as a function of the solution viscosity , we examined various polystyrene solutions of different molecular wei ghts and concentrations. The reaction rate increases considerably at p olymer content below and at the critical coil overlap concentration. A bove this concentration, as the polymer coils become entangled and con tracted, the decreased mobility of the molecules due to higher viscosi ty and lower diffusion rate lowers the reaction rate. The influence of the molecular weight of the polystyrene on the reaction kinetics had a ''catalytic'' effect on reaction rates and was most dramatic with a MW(avg) of similar to 120 000. We find that in the dilute polymer regi me, below the PS coil overlap threshold, the polymer chains can provid e the necessary support for the aggregation of the cobalt particles. A lso, in the dilute regime, the mobility of the cobalt molecules is not hindered due to the low solution viscosity. However, there is evident ly a critical molecular weight at which the ''catalytic'' effect of th e polymer is at its maximum. These results will be discussed, and poss ible mechanisms for the polymer-enhanced colloid reactions will be off ered.