Symmetric-to-asymmetric transition in triblock copolymer-homopolymer blends

In blends of a symmetric poly(styrene-ethylene/butylene-styrene) tri-block-copolymer with a polystyrene homopolymer, small-angle x-ray scattering and cryotransmission electron microscopy measurements reveal a microstructure consisting of a disordered arrangement of poly(ethylene/butylene) membranes suspended in polystyrene. For triblock volume fractions less than 0.22, the membranes form an asymmetric sponge or L-4 phase, consisting predominantly of equilibrium vesicles. For volume fractions greater than 0.22, they form a symmetric sponge-phase (L-3 phase), separated from the L-4 phase by a first-order transition.