Uniaxial elongational viscosity of poly(styrene-block-butadiene-block-styrene) melts

Experimental studies about the effect of block copolymerized structures on the uniaxial elongational viscosity were performed. Two poly(styrene-block-butadiene-block-styrene) (SBS, S/B weight ratio = 4/6) samples with linear and star (4 arms) structures were used for the elongational viscosity measurement at constant strain rates. The block copolymer samples exhibited the strain-softening elongational viscosity. This property was in contrast to the strain-hardening elongational viscosity which is observed for homopolymer melts. The cause of the strain-softening property was analyzed from the Structural change by small angle X-ray (SAX) and transmission electron microscope (TEM). The lamellar structures were more oriented and became longer along the elongational direction by the elongation, while the lamellar thickness remained almost constant. The observation indicates that the lamellar structures, instead of the SBS molecules, were oriented and stretched. The theological discussion on the strain-softening elongational viscosity was also carried out by the BKZ model. The damping functions of the block copolymers from step-shear relaxation experiments showed larger decrease than that of the Doi-Edwards prediction. The structural change observation and the BKZ analysis suggested that the largely decreased damping functions are due to the lamellar orientation and cause the strain-softening elongational property.