Soft/hard segment contents dependent strain rate sensitivity mechanism under tensile loading in Polyurethane

Polyurethane (PU), as a block copolymer with soft and hard segments, demonstrates broad application potential in impact resistance owing to the outstanding mechanical properties. Measurement techniques are improved for soft materials like PUs with low strength and large deformability, enabling systematic investigation of macroscopic mechanical responses. Molecular dynamics simulations are employed to reveal the atomic-scale microstructural evolution during tension. Results indicate that the logarithmic stress increases linearly with the logarithmic strain rate and the hard segment content in PUs. Under low strain rate, the deformation is mainly concentrated within soft segments with superior coordinated deformation ability and providing excellent ductility. The deformation of hard segments gradually synchronizes with that of soft segments as the strain rate increases. More numerous void initiation sites emerge and the void regions expand with increasing hard segment content. Statistical analysis shows that the hydrogen bond count decreases approximately linearly wsssith strain, and the reduction rate increases linearly with the logarithmic strain rate. The hydrogen bond reduction rate coefficient, defined as the dependence of the reduction rate on the logarithmic strain rate, shows an initial decrease followed by an increase with increasing hard segment content, which aligns well with the correlation between the strain rate sensitivity coefficient. Both coefficients reach the values of inflection points in PU with a hard segment content around 45%, after which they rebound slightly with a further increase in hard segment content.