Tuning Hard-Segment Regularity for Microphase Control and Property Enhancement in Hydroxyl-Terminated Polybutadiene-Based Polyurethane Elastomers

Polyurethane elastomers (PUs) with nonpolar soft segments exhibit excellent flexibility and hydrophobicity, but the poor compatibility between nonpolar soft segments and polar hard segments limits their overall performance. In this work, hydroxyl-terminated polybutadiene (HTPB) and p-phenylene diisocyanate (PPDI) were combined with chain extenders differing in symmetry and side methyl substitution to tune hard-segment regularity. Symmetric extenders promoted excessive aggregation and heterogeneous microstructures, whereas asymmetric or methyl-substituted extenders suppressed ordered microphase separation, yielding more uniform morphologies and superior properties. The optimized sample, C3-1, achieved high tensile strength (16.9 MPa), toughness (34.3 MJ·m–3), and thermal stability (320.8 °C), representing the most flexible heat-resistant HTPB-based PU reported to date. These results demonstrate that hard-segment regularity governs both microphase morphology and macroscopic properties. Furthermore, two effective design strategies, reducing extender symmetry and introducing side methyl groups, are established for developing advanced nonpolar PUs with balanced strength, flexibility, and stability.