The variation of morphology of triblock copolymers of the ABC type, poly(isoprene-b-styrene-5-2-vinylpyridine), with composition was studied by transmission electron microscopy. Four types of morphology were observed for the microphase-separated structures of the samples whose volume fractions of the middle-block polymer, polystyrene, range from 0.3 to 0.8, while keeping the volume fractions of the end-block polymers, i.e., polyisoprene and poly(2-vinylpyridine), equal. They are a three-phase four-layer lamellar structure, an ordered tricontinuous double-diamond structure, a cylindrical structure composed of two kinds of cylindrical domains formed by two end-block polymers in a polystyrene matrix, and a spherical structure composed of two kinds of spherical domains formed by two end-block polymers in a polystyrene matrix. The variation of morphology with composition is discussed in comparison with that of diblock copolymers.

Four types of morphologie, lamellar, ordered tricontinuous double-diamond (OTDD), cylindrical, and spherical structures, of isoprene (I)-styrene (S)-2-vinylpyridine (P) triblock copolymers having I and P block polymers with the same volume fractions, were studied by transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). These studies revealed that the spherical structure has the packing manner of the CsCl type consisting of two spherical domains of I and P end-block polymers and the cylindrical structure, which consists of two kinds of cylindrical domains of end-block polymers, has the tetragonallycoarrayed pattern. Moreover, the OTDD structure proposed previously was confirmed by TEM observation of one (P domains) of the double-diamond frameworks in a film specimen stained with phosphotungstic acid. From these results, together with the TEM and SAXS studies on three-phase four-layer lamellar structure, we conclude that the morphologies of the triblock copolymers used in this work have the characteristics of superlattice structures.

A complicated three-dimensional equilibrium morphology, named an ordered tricontinuous double-diamond (OTDD) structure, similar to the ordered bicontinuous double-diamond structure for diblock copolymers was found for the microphase-separated structure of poly (isoprene-6-styrene-b-2-vinylpyridine). When the contents of both end-block polymers are equal, this structure occurs from 0.48 to 0.66 for the volume fractions of middle-block polymer, i.e., polystyrene. The transmission electron micrographs at the two different directions, i.e., the [111] and [001] directions, are in good agreement with the corresponding images formed by assuming the OTDD structure proposed here.

The radii of gyration of a polystyrene block along the directions parallel and perpendicular to the lamellae in the lamellar structure of poly(styrene-6-2-vinylpyridine) were measured by small-angle neutron scattering. It was found that the block polymer is extended along the perpendicular direction, while it is contracted along the parallel direction. The extension is in accord with the theories of microphaseseparated structure, but no theory predicts the contraction. From the molecular weight dependences of the radius of gyration along the directions parallel and perpendicular to the lamellae, it is concluded that the block polymer is contracted along the parallel direction to keep the volume occupied by the block chain constant when the microphase separation occurs.

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