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 () shear plane and 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.