Mechanical and swelling behaviour of well characterized polybutadiene networks

Endlinking of hydroxyl-terminated polybutadiene with the appropriate isocyanate has been used to prepare well characterized networks. Two networks have been studied with molecular weights of the prepolymers being 6100 and 2400 g/mole by g.p.c. Cylindrical specimens were prepared and the derivatives of the stored energy function with respect to the stretch invariants were determined by torque and normal force measurements in torsion. From these data the Valanis-Landel stored energy function derivatives w′(λ) were determined for both networks. The stored energy function for the junction constraint model of Flory, which is a special form of the Valanis-Landel function, has been fitted to that determined from the experiments. The contributions, ΔA_ph and ΔA_c, to the stored energy function from the phantom network and from the junction constraints respectively do not agree with predictions from the topologies of the networks. In spite of this the form of w′(λ) for the junction constraint model gives an excellent 'curve fit' to the data. Comparison is also made with equilibrium swelling.