Shear-induced morphologies of cubic ordered block copolymer micellar networks studied by in situ small-angle neutron scattering and rheology

Triblock copolymers in a solvent selective for their middle blocks provide the basis for the formation of novel physical networks in which cross-links are formed by self-assembled domains of the end blocks. Triblock copolymers of poly(styrene)-poly(ethylene butylene)-poly(styrene),(SEBS) dissolved in oil constitute such a network system of thermoplastic elastomer. We present combined mechanical and structural data on such SEBS gels as studied using a Rheometrics RSA-2 instrument modified for in situ measurements of small-angle neutron scattering and rheology. The self-association of the PS blocks not only promotes the formation of highly interconnected end-block domains but, within a given temperature range, constitutes a network with body-centered cubic (bcc) microstructure. The texture of the bcc phase can, within a given temperature window, be controlled in great detail by applying large-amplitude oscillatory shear of specific amplitude,and frequency. We map out the shear-induced morphology and show that both {111}/<112> and {110}/<111> domain texture can be made, where {hkl} and {hkl} represent the crystal plane and crystal axis parallel to the (upsilon) over bar(V) over bar shear plane and (upsilon) over bar shear direction, respectively. The thermodynamically stable cubic phase is limited at high-temperature by an order-disorder transition and at low temperature by the glass transition of polystyrene.