Morphologies of Cross-Linked Segmented Polyurethanes. Evolution during Maturation and Consequences on Elastic Properties and Thermal Compressive Fatigue

The solid state morphology evolution of a segmented polyurethane (SPU), with a cross-linked soft phase has been studied. Materials were cast with a small excess of isocyanate (6%) and kept in time under atmospheric humidity at ambient temperature. Several analyses were performed on 2 mm thick plates in order to determine structural evolutions with time (DSC, DMTA, AFM, TEM, SAXS) and their effects on static mechanical properties. SAXS experiments have been performed for mature samples and their analysis through different models enable us to describe quantitatively the morphologies of hard segments with concomitant nodules and lamellae. The evolution of this morphology was monitored during maturation and heating ramps. For 2 mm thick plates, a 15% modulus increase and a decrease in tan δ at 120 °C were obtained when the final properties were reached, 4 weeks after cure. It is presumably the result of the hydrolysis of free NCO groups to produce urea linkages at the border of the hard domains. Moreover as this SPU was developed for applications in fatigue, a specific compressive fatigue test at constant load was also designed to study the behavior of 39 mm diameter SPU cylinders under such stress (between 0.33 and 3.3 MPa at 27 Hz). Because of the viscoelastic behavior, a self-heating phenomenon was observed that could induce failure if the temperature reached the melting temperature of the hard segments. The fatigue behavior was improved during maturation, thanks to a decrease in the damping and a simultaneous increase in modulus. Unlike 2 mm thick plates, the final properties for 39 mm diameter cylinders were obtained only after a 13-week maturation, presumably due to a thickness effect which delays the moisture diffusion. This maturation time was characterized by the stability of the structure and thus of fatigue properties.