Solvent-Free Preparation of Thermally Stable Poly(urethane-imide) Elastomers

Polyurethanes (PUs) are widely used in many applications due to their versatile chemical and physical properties. To meet the increasing demands on PU with respect to intrinsic mechanical and thermal properties, they can be modified with thermally stable aromatic imide structures to form poly(urethane imide) (PUI). However, the preparation of PUI materials has been limited by the need for strong polar solvents, which hinder their industrial scale development. In this work, we prepared imide-containing isocyanate-terminated prepolymers via the reaction of polymeric aromatic or aliphatic isocyanates and pyromellitic dianhydride in completely solvent-free conditions. The subsequent PUI elastomers made from these prepolymers exhibited enhanced char formation and stiffness in comparison to the reference elastomers without imide structures, among which the aromatic PUI elastomer shows improved flame retardancy according to the initial cone calorimetry measurement. This work demonstrates the potential use of imide prepolymers in other PU applications where high thermal stability and flame retardancy are required. The solvent-free synthesis also paves a way for the large-scale production of PUI materials.