| Abstract: | On a large industrial scale, segmented polyurethanes are processed from solution in the dry spinning process to produce textile fibres. Rheo-optical investigations of the flow behaviour of polyurethane solutions enable new material functions to be determined and provide important information for processing. Proportional increases with shear rate were observed for the flow birefringence, Δn', and the orientation, ϕ. The polymer segments were more easily aligned in the direction of the shear field in more concentrated solutions than in dilute solutions. The same tendency was observed for samples with differing molar masses. An ideal standardisability for the temperature (in a window of 20 K) was found over the entire range of shear rate and, hence, the change in the Newtonian and non-Newtonian flow behaviour was also observed to be completely identical. Using the stress-optical rule, it was possible to determine the first normal stress difference. The stress-optical coefficient, C, was 2.6·10–9 Pa–1. The normal stress values lie in the range of accessible shear rates below the shear stress, but do, however, rapidly approach this value as the strain increases. Even at a shear rate of 100 s–1 the viscoelasticity of a 17 wt.-% polyurethane solution is already significant. At a high shear rate the Weissenberg number, We, which is a measure of the viscoelasticity, has a constant limiting value that only depends on the power law exponent, n. Its values in the range of high shear rates mostly lie between 2 and 3 and are rarely (for the smallest n) above 3. |
