Influence of shear processing on morphology orientation and mechanical properties of styrene butadiene triblock copolymers

The influence of ram extrusion on structure and mechanical properties of a triblock copolymer consisting of polystyrene (S) outer blocks and poly(styrene-stat-butadiene) (S/B) middle block is studied for a wide range of shear rates. Structural features on the mesoscale (10-100 nm) are investigated by small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Transition Moment Orientation Analysis (TMOA) is applied to quantify the orientation on the molecular (segmental) scale (<1 nm). All extruded samples microphase-separate and show a lamellar morphology with periodicities of about 33 nm. Significant orientation is observed on the mesoscale where the surface normals of the lamellae are preferentially perpendicular to the extrusion direction. The corresponding degree of orientation drops slightly at elevated shear rates of about 600 s(-1). Interestingly, Chevron-like pattern with two preferred orientations of the lamellae are observed in cross-sections probably due to shear velocity gradients in the rectangular die. In contrast, significant orientation on the molecular scale is absent for styrene and butadiene units indicating basically random orientation of the chain segments. The mechanical properties are, however, strongly anisotropic. Uniaxial tensile tests performed parallel and perpendicular to the extrusion direction reveal higher E-parallel to moduli (1.1 - 0.6 GPa) along with yielding but significantly smaller E-perpendicular to moduli (100-250 MPa) without pronounced yielding. Main trends in both moduli, E-parallel to and E-perpendicular to, can be explained based on mesoscale orientation using the analytical composite model. In general, the results demonstrate that orientation effects on the mesoscale have a strong influence on the mechanical properties and must be considered during the optimization of extruded or injection-molded components made from microphase-separated block polymers. (C) 2014 Elsevier Ltd. All rights reserved.