Dynamic Load Response of 1,5-Naphthalene Diisocyanate (NDI) Based Casting Polyurethane: A Study of Microstructural and Mechanical Evolution

Studies on structural and performance changes under dynamic loads, such as cyclic tension, are limited. We synthesized a casting polyurethane based on 1,5-naphthalene diisocyanate with exceptional fatigue resistance, capable of withstanding up to 700,000 cycles of flexural fatigue. Tensile fatigue tests were conducted at a stretching strain amplitude of 100%, a frequency of 10 Hz, and cycles ranging from 10,000 to 100,000 to investigate the microstructures and properties. Atomic force microscopy revealed a gradual decrease in the high modulus region content, while the medium modulus region initially increased and then decreased, and the low modulus region showed an increasing trend. Transmission electron microscopy images clearly depicted the destruction process of spherulites: crystal bundles initially separated from the spherulites, and with increasing cycles, these crystal bundles became smaller, eventually evolving into “small black dots” after 100,000 cycles. Wide-angle and small-angle X-ray diffraction analyses indicated that crystallinity, grain size, and long period decreased, while the transition layer thickness increased. We further explored the correlation between structural evolution and performance, elucidating the underlying mechanisms. This study provides crucial insights into the performance of casting polyurethanes under dynamic loading, valuable for designing durable, fatigue-resistant materials.