THE GYROID - A NEW EQUILIBRIUM MORPHOLOGY IN WEAKLY SEGREGATED DIBLOCK COPOLYMERS

We report the identification of a new equilibrium microdomain morpholo gy in an intermediate to weakly segregated diblock copolymer melt. A p olystyrene-polyisoprene (SI) diblock copolymer consisting of 37 wt % s tyrene and of total M(w) = 27 400 was observed to transform from the l amellar morphology (in equilibrium at low annealing temperatures) to a new morphology at annealing temperatures approximately 50-degrees-C b elow the order-disorder transition (ODT). The transformation was obser ved to be thermoreversible. investigation of the new morphology via sm all-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) revealed the new structure to have remarkable three-dimensional long-range order, to belong to the cubic space group Ia3d, and to pos sess a bicontinuous cubic microstructure. From computer simulations of model structures and comparison with microscopy results, we propose m odels for the new morphology based on the triply periodic G minimal su rface (gyroid) discovered by Schoen;1 similar morphologies have been o bserved in a variety of microphase-separated surfactant-water systems. Blends of this diblock with various short-chain homopolymers were use d to investigate the compositional extent of the region of Ia3d stabil ity on the phase diagram; the results indicate that the Ia3d phase is stable over a wide range of minority component volume fractions.