Effects of structure and blending calcium salts on fatigue crack propagation behavior of model polyurethanes

The fatigue crack propagation (FCP) of model polyurethanes having different chemical structure and molecular weight of soft segment was studied in terms of their micro-structural changes. In addition, the effects of calcium salts, i.e., calcium chloride (CaCl₂) and hydroxyapatite (HAP), blended into the polyurethanes on the FCP behavior were investigated under the same cyclic loading conditions.

The model polyurethanes, the copoly(ether-urethane-urea)s (PEUU) with different molecular weight and chemical structures of soft segment were synthesized in two-stage step polymerization. The microstructure and deformation behavior of the model polyurethanes were characterized using Fourier-Transform Infrared Spectroscopy (FT- IR), Small Angle X-ray Scattering (SAXS), Dynamic Mechanical Analysis (DMA), and polarized FT-IR microscope. The cyclic loading was performed by an automated cyclic tensile machine.

The empirical equations were derived from the relationship between the tearing energy and the FCP rate of model polyurethanes. The FCP rates at the same level of tearing energy were applied to compare the FCP resistance of the model PEUU under constant strain amplitude condition. The exponent m' of the empirical equations can be used to estimate the fatigue life of the PEUU. The FCP behavior of model PEUU was discussed from molecular level aspects which most probably effected the FCP properties; initial morphology, crosslink density, domain disruption property, and deformation behavior at the crack tip.

It was found that all model polymers, including the PEUU-salt blends, exhibited properties of domain-matrix morphology. The differences in molecular weight and chemical structure of soft segments and the introduction of calcium salts into the polyurethanes effected the FCP behavior. The strain energy density term was likely related to the event of crack growth rather than the FCP resistance. Finally, the FCP rates were most sensitive to the nature of the domain for the model PEUU. Particularly, blending CaCl₂ into the model PEUU reduced the fatigue resistance due to weakening in the cohesive force of domain.