VISCOELASTIC PROPERTIES OF A STYRENE-ISOPRENE-STYRENE TRIBLOCK COPOLYMER AND ITS BLENDS WITH POLYISOPRENE HOMOPOLYMER AND STYRENE-ISOPRENE DIBLOCK COPOLYMER

The effects of added polyisoprene homopolymer (PI) and polyisoprene-block-styrene diblock copolymer (SI) on the viscoelastic properties of a polystyrene-block-polyisoprene-block-polystyrene triblock copolymer (SIS) having a spherical domain morphology have been examined. The former two species are used to model the effects of imperfections in structures that result from triblock copolymers missing, respectively, two or one polystyrene end blocks. The results indicate that the loss modulus and tangent delta in the plateau region can be dominated by imperfections in the copolymer structure. The interpretation of viscoelastic data in the rubbery plateau region as an indication of interface structure in block copolymers is therefore greatly complicated. Small angle X-ray scattering (SAXS) and rheological tests were also used to determine the order-disorder transition temperature for an SIS triblock copolymer and its blend with an SI diblock. The SAXS data are consistent with Han's rheological criteria for determining the order-disorder transition temperature. However, the complex viscosity does exhibit Newtonian behavior for low rates at the highest temperatures, even though the domain morphology persists. Stress relaxation and dynamic modulus data at high temperatures clearly show a secondary ''rubbery'' plateau at long times, and we offer a qualitative explanation for this feature based on a proposed relaxation mechanism.