Improvements in the stress‐strain behavior of urethane rubbers by bimodal network formation

Urethane rubbers find increasing application as binders for composites. Typical examples are solid propellants. The rubber networks are formed by end linking hydroxy‐terminated prepolymers with tri‐ or higher functional isocyanates. A recent trend in solid propellant technology is the replacement of the traditional low‐energy binders with “energetic” binders containing nitro, nitrato, or azido groups. Since these energetic polymers create relatively short interchain lengths between the cross‐link points, the binders give notoriously poor mechanical properties. Our study demonstrates that significant improvements in the stress‐strain behavior are attained with bimodal modifications of the energetic binders, that is, by blending these energetic short chains with very long chains prior to curing into rubbers. Molecular aspects of the improvement have been examined in terms of polymer types, crosslinkers, viscoelastic factors, and solid filler content. Results indicate that the improvement is primarily due to the extent of nonaffine deformation of the bimodal rubber network.