Shear-induced order effects in bi-soft segment urethane/urea elastomers

In this work we study shear-induced order effects, observed on thin films (∼20-60 μm) of urethane/urea elastomers prepared from a polypropylene oxide based isocyanate terminated triol prepolymer (PU) and polybutadiene diol (PBDO) in the amount of 25% (PU/25PBDO), 50% (PU/50PBDO), 60% (PU/60PBDO) and 75% (PU/75PBDO) by weight of polybutadiene. Optical microscopy and light scattering as a function of temperature were used in this study. The initial isotropic elastomers, PU/60PBDO and PU/75PBDO, after cessation of a uniaxial mechanical deformation relax to a band texture, consisting of equidistant bright and black lines perpendicular to the shear direction. If the uniaxial mechanical stress is applied again perpendicular to the band structure in PU/60PBDO, a new periodicity with wave vector perpendicular to the axis of the field develops, while for PU/75PBDO two new periodicities can be observed, one with wave vector perpendicular to the axis of the field and the other transverse. The process of band formation is completely reversible in cycles of increasing and decreasing strain ratios. In samples PU/25PBDO and PU/50PBDO this kind of effect is not observed. The bands periodicity is temperature independent. The temperature dependence of the scattered intensity produced by the bands is different for bands perpendicular and transverse to the axis of the stress field. We show evidence that the urethane linkage is responsible for the relaxation instabilities observed, in this kind of material.