Investigation of the Mechanical Behaviors of Waterborne Polyurethane: The Critical Influence of Hard Segment Content Over Various Strain Rates

In protective applications, polyurethane (PU) is a key material, yet the microstructural mechanisms governing its dynamic mechanical properties are not well understood. This study investigates the influence of hard segment content on the low strain rate compression and high strain rate impact properties of waterborne polyurethane (WPU) by modulating the NCO/OH ratio. Mechanical responses are characterized using a universal testing machine and a split Hopkinson pressure bar (SHPB) system. Additionally, the hydrogen bonding and microphase separation structure are analyzed using FTIR, DSC, DMA, and SAXS. These findings reveal that the glass transition temperatures (T_g^DSC and T_g^DMA) shift toward higher temperatures with increasing hard segment content, which is attributed to the intensified hydrogen bonding cross-linked network, as corroborated by FTIR and SAXS analyses. The WPU demonstrates a pronounced strain rate sensitivity across a broad range of strain rates (10⁻⁴–10⁴ s⁻¹). Notably, the 45 wt.% hard segment WPU523 sample shows heightened sensitivity, attributed to complex hydrogen bonding heterogeneity and a higher Herman's orientation factor during loading, the key to WPU's dynamic mechanical response.