Polyurethane network structure from tensile tests

The mechanical behavior of polyurethane networks based on polyether (PE-PU) and polyester (PES-PU) diols is studies in light of recent molecular theories of rubber elasticity. The relationship between reduced stress (or modulus), extension ratio α, and network structure is discussed. In the range of low extensions, the deformation behavior of PESPU appears to be more affinelike than PE-PU networks. When crosslinking agents with higher functionality are used, behavior closer to the affine limit is observed. Data in the low deformation range (α → 1) allow the estimation of network molecular weights predicted by the limiting phantom and affine networks. However, an exact determination of the true chain length cannot be obtained. The uncertainty in the molecular weight is due to the uncertainty in the theoretical structural parameter A'_?, which can assume values 1–2/? ≤ A'_? ≤ 1. The range of possible network chain molecular weights can be narrowed when the real network, which always presents defects, is formed starting from building blocks of known molecular weight M_s, as in our case. In these real cases the molecular weight is between the value M_a Predicted by the affine model and the stoichiometric value M_s.