Thermodynamics of swelling in unfilled and filler-loaded networks

Within the framework of the van der Waals-network model a consistent interpretation of swelling and simple extension in differently crosslinked networks is presented. It is observed that the excess parameters in the Staverman-Koningsveld-Kleintjens version do not depend on the degree of crosslinking. Swelling of filler-loaded rubbers shows universal features because of not depending on the type and the properties of the filler. By introducing the Einstein-Smallwood modification in an adequate manner one understands this phenomenon without any further parameter adjustments. It is the consequence of having quasi-permanent filler-to-matrix contacts that are not modified in presence of solvent molecules. The excess-parameters in the swollen matrix are not affected. The entropy elastic stress due to the swelling induced deformation of the matrix is apparently too small as to enforce chain-slippage. The strength of the adhesion of the polymer inhibits filler-to-solvent contacts. These results defend the mean-field treatment of the boundary problem as presented by Einstein-Smallwood, and allows a valuable proof of the thermodynamics of swelling in networks.Dedicated to Prof. H. H. Kausch on the occasion of his 60th birthday