Theory of Bound Rubber

Abstract

Abstract A theory of bound rubber formation has been developed which treats the effect as random adsorption of structural units of polymer on reactive sites which are assumed to exist on the surface of filler particles. Equations are derived for the fraction of bound rubber and for the molecular weight distribution of free (unbound) rubber. The theory contains only one adjustable parameter, the filler surface area per reactive site, A0. It is shown for the case of the Schulz distribution that the amount of bound rubber depends but slightly on the dispersion parameter of the polymer and is determined essentially by the adsorption index MwcP/A0NA, where c is the filler concentration, P is its specific surface area, and NA is the Avogadro number. All of the experimentally observed features of the bound rubber effect, including preferential adsorption of large molecules, are correctly predicted, the quantitative agreement of the theoretical equations with available experimental data being satisfactory. This supports the underlying assumption that the processes involved in the polymer-filler interaction may be approximated by a random-process model.