Influence of entanglements on the viscoelastic relaxation of polyurethane melts and gels

The frequency dependence of the shear modulus was studied for cross-linked polyurethane melts and gels at various cross-link densities. The polyurethane used in this study was based on mixtures of bifunctional linear poly(propylene oxide) (PPO) and trifunctional star PPO end-linked with diisocyanate. The connectivity extent was varied by varying the stoichiometric ratio of hydroxyl groups and isocyanate groups. The results were compared with mean field theory predictions. Relaxation of trapped entanglements was observed for almost fully end-linked systems containing a very low concentration of trifunctional PPO.