Tensile properties and equilibrium swelling of plasticized amorphous polyurethane rubbers

Incorporation of plasticizers or other inert, liquid diluents causes a profound weakening of rubbers. The modulus of elasticity and tensile strength decrease approximately with the square of the gel fraction v₀ of rubber which was present at the time of formation of the three-dimensional network. Equlibrium swelling of plasticized rubbers is well represented by v_2m = kv₀, where v_2m is the volume fraction of rubber network in the swollen state, at equlibrium, and k a constant equal to v_2m for the unplasticized rubber. This swelling law is independent of the way in which v₀ is obtained; inert liquid diluents, comprising molecularly dispersed compounds, as well as suspensions of insoluble liquids, gases, and chemically inert, solid fillers yield the same relation. Further, it is independent of the chemical nature of solvent and rubber. It appears that also the mechanical properties of foams, liquid-filled foams, and plasticized elastomers containing the same volume fraction of network rubber are very similar.