Elongation of Triblock Copolymer Melt:  Elongation Flow Opto-Rheometry and Small-Angle X-ray Scattering Study

The elongational flow-induced morphological change of a triblock copolymer melt of polystyrene-b-poly(ethylene butylene)-b-polystyrene (SEBS) with polystyrene (PS) weight fraction of 30% has been investigated using elongational flow opto-rheometry (EFOR) and small-angle X-ray scattering (SAXS). The alignment of the PS cylindrical domains in a preferred direction is achieved by roll processing at 200 °C. In the EFOR measurement, the uniaxial elongation with a Hencky strain rate, ε<INF>0</INF>, of 0.01 s^-1 is applied at 180 °C in the direction either parallel (case 1 elongation) or perpendicular (case 2 elongation) to the cylinder axis. Transient elongational viscosity, η(ε<INF>0</INF>;t), measured during the case 1 elongation exhibits an initial increase, followed by a gradual decrease to almost a steady state before rupture. One measured during the case 2 elongation shows three different regions with elongation time:  an initial increase followed by a gradual decrease and a final, rapid increase. The SAXS measurement on the elongated films demonstrates that the alignment of the cylindrical domains persists during the case 1 elongation, whereas the cylindrical domains are rotated by 90° to be aligned parallel to the elongation direction during the case 2 elongation, indicating that the final orientation of the cylinders is not determined by the initial alignment of the domain, but by the Hencky strain rate and the tensile stress level applied on the specimen.