Network structure and mechanical properties of allophanate‐cured polyurethane rubber

An allophanate‐cured polyurethane (PU) has been prepared. The relationship between trifunctional network structure and mechanical properties is discussed. Mooney‐Rivlin constants C₁ and C₂ are determined from stress‐strain isotherms. C₂ is approximately constant. The average value of structure factor A₃ in the limit at large deformation is 0.432, in fair agreement with the theoretical prediction of 1/3. The structure factor A′₃ of all PU samples becomes roughly unity in the limit at small deformation. The experimental results are interpreted by the recent Flory theory (a fluctuation‐constraint model of junctions). The κ parameter decreases with increasing crosslink density; the ξ parameter is small and approaches zero. Increasing crosslink density tends to cause phantom deformation for extended PU networks. However, the absolute value of the reduced stress in the limit at large deformation (or for the equivalent phantom network) [f ph*] is believed to be overestimated for PU networks with high crosslink density. The origin of this behavior is discussed in detail.