Morphology and micromechanical deformation behavior of SB-block copolymers. II. Influence of molecular architecture of asymmetric star block copolymers

The influence of molecular architecture on morphology, mechanical properties, and micromechanical deformation behavior of asymmetric styrene/butadiene star block copolymers is investigated by transmission electron microscopy (TEM) and uniaxial tensile testing. In contrast to diblock copolymers, star block copolymers having 74% polystyrene (PS) show a lamellar morphology. Furthermore, tapered asymmetric star block copolymers having a PS core reveal additional PS domains inside the PB lamellae not observed in untapered star block copolymers. The introduction of tapered sequences in star block copolymers results in a significant improvement of tensile properties due to the increased interfacial width between the phases as well as differences in morphology. The investigated star block copolymers show a homogeneous plastic deformation of PS lamellae. This mechanism is called thin-layer yielding of PS lamellae. The lamellae are locally deformed via inhomogeneous necking and drawing. Lamellae which are not oriented parallel to the load direction are twisted to the deformation direction. Neat star block copolymers (without tapered chains) exhibit a premature failure of PS lamellae arising from the decreased interfacial width, which results in a decrease of their ultimate strain. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 701–713, 2002