Mechanical-Fatigue-Driven Hierarchical Structural Evolution in Polyurethane Elastomers with Different Hard Segment Contents

Fatigue-induced degradation of polyurethane elastomers (PUEs) significantly affects their long-term performance, yet the effect of the distribution state of hard domains on their fatigue durability remains poorly understood. In particular, the microstructural evolution under compression fatigue, especially when thermal effects are minimized, is scarcely studied. This study investigates how adjusting the hard segment content (HSC) regulates the distribution and hierarchical organization of hard domains. Low-frequency compression fatigue was employed to isolate purely mechanical damage mechanisms. This allows us to elucidate their influence on the fatigue behavior of PUEs. Characterization results show that the high-HSC sample (PU-H25) forms a continuous and highly ordered spherulitic hard segment network that carries most of the compressive load. However, this rigid architecture is susceptible to stress concentration, leading to progressive degradation of the hard network, and pronounced permanent deformation. In contrast, the low-HSC material (PU-H17) contains hard segments dispersed as isolated physical cross-links within the soft-segment matrix. Under cyclic loading, deformation is primarily accommodated by the soft phase, producing a progressive softening behavior. Although PU-H17 exhibits a larger initial strain, it demonstrates superior elastic recovery. The medium-HSC sample (PU-H21) develops a semicontinuous hard domain morphology that enables cooperative load transfer between hard and soft phases, resulting in the highest structural stability and the slowest fatigue-induced damage evolution. Overall, the results demonstrate that HSC is a key factor governing the fatigue response of PUEs by tailoring their microphase-separated morphology. As HSC increases, the dominant fatigue mechanism shifts from soft-phase-controlled stress dissipation, to cooperative load sharing between hard and soft phases, and finally to hard-phase-dominated load bearing and fracture. These mechanistic insights provide a basis for designing PUEs with tailored fatigue resistance for specific service conditions.