The effects of soft segment structure on the fatigue crack propagation of model polyurethanes.

The present work is a study of the effects of soft segment molecular weight and chemical structure on the fatigue crack propagation of model copoly (ether-urethane-urea)s (PEUU). The PEUU were synthesized using polypropylene glycol (PPG), polytetramethylene glycol (PTMG), and polyethylene glycol (PEG) as the soft segment component. The number average molecular weights of the polyethers were within the range of 1000-2000. Methylene bis (4-phenylisocyanate) (MDI) and ethylene diamine were used as the diisocyanate and the chain extender, respectively. The cyclic loading experiments were carried out using a computerized film stretcher that can conduct sinusoidal operation at a constant strain amplitude, strain rate, and frequency. The Rivlin-Thomas tearing energy, T, and the fatigue crack propagation (FCP) rate were selected to characterize the fatigue behavior of the model polyurethanes. An empirical equation was applied to define the fatigue properties of model polyurethanes and to evaluate the fatigue resistance. To investigate the effect of molecular variables on the FCP, the morphological changes caused by structural differences and cyclic stress were determined using dynamic viscoelastometer (Rheovibron), Small Angle X-ray Scattering (SAXS), and Fourier-Transform Infrared (FT-IR) spectroscopy. Mooney-Rivlin plot was used to determine the crosslink density variation. In addition the orientation behavior at the crack tip was characterized by IR dichroism technique using a polarized FT-IR microscope. The results indicated a reasonable relationship between the FCP rates and the hard segment content, crosslink density, and deformation property at the crack tip. However, the initial stage of phase separation and domain disruption behavior did not show a good correlation with the FCP properties of model polyurethanes. Among the model polyurethanes tested, the PEUU with PTMG (Mn = 1000) exhibited the best fatigue resistance at given test condition.