Tricontinuous double gyroid cubic phase in triblock copolymers of the ABA type

We report the synthesis and morphological characterization of two triblock copolymers of the ABA type, where A is polystyrene (PS) and B polyisoprene (PI). The volume fraction of the minority component, PS or PI, is approximately 1/3. Cubic microdomain morphologies, already found in diblock and star block copolymers with the same composition range, are observed for the first time in the case of linear triblock copolymers. The two ABA triblocks are on opposite sides of the phase diagram, which signifies that both the A end blocks and the B midblock are capable of forming the interconnected double network structure. Investigation of the morphology was done via birefringence, small-angle X-ray scattering (SAXS), and transmission electron microscopy(TEM). Birefringence measurements showed each triblock structure to have isotropic optical properties. The characteristic ratio of the observed Bragg peaks, q(2)/q(1), was approximately root 4/root 3 for each sample, indicating a set of eight possible cubic space groups. TEM data showed an interconnected tricontinuous microdomain structure. Since the two triblocks have essentially complementary structures (PS = 0.32 in one and PI = 0.36 in the other), the TEM images of OsO4-stained thin sections are complementary and the diffraction patterns of the images are approximately equal according to Babinet's principle. Examination of high-symmetry projections demonstrated p6mm, p4mm, and c2mm symmetry present in the TEM images. Comparison with the [111], [100], and [100] projections of the eight cubic space groups satisfying the SAXS data eliminated all but the <Fm(3)over bar m> and <Ia(3)over bar d> groups as possible structures. Due to the observed connectivity of the structure, the <Fm(3)over bar m> structure could be eliminated by inspection of possible network structures and the resultant symmetries. Computer simulations of a model structure (double gyroid) based on level surfaces with <Ia(3)over bar d> symmetry and their Fourier transforms showed excellent agreement with the high-symmetry projections and their respective optical transforms.