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 ( v̄ ∇) shear plane and v̄ 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.