Microstructural transformations and fatigue behavior of NDI-based polyurethane in response to extreme dynamic loads: A focus on hard segment content

The microstructure of casting polyurethane (CPU) affects its performance under high dynamic loads, which is critical for developing high-performance materials. Herein, NDI-based casting polyurethane materials with varying hard segment contents (20%, 30%, 40%) were synthesized, and their behaviors under tensile fatigue (a strain amplitude of 100%, a frequency of 10 Hz, and up to 50,000 cycles) was investigated. As the number of fatigue cycles increased, the applied load required to achieve the same strain decreased, primarily due to stress softening and plastic deformation in the CPU. Microstructural analysis revealed a reduction in hydrogen bonding degree, leading to a decrease in structural regularity and an increase in defects. Atomic force microscopy (AFM) images revealed that the hard segments gradually integrated into the soft segment matrix, which reduced the content of high- and medium-modulus phase regions and correspondingly increased the low-modulus phase region. Transmission electron microscopy (TEM) images clearly suggested the destruction process of the spherulites, in which the crystallization structure deteriorated more significantly after fewer cycles in CPUs with higher hard segment content. The crystallinity, long period, and transition layer thickness of the CPUs demonstrated a decreasing trend with increasing fatigue cycles. This study provides valuable insights into the performance of CPUs under dynamic loading, laying a theoretical foundation for designing durable, and fatigue-resistant materials.