Thermally induced morphology transition from cylindrical to lamellar microdomains wan found for a poly(styrenablock-butediene-block-s~ene) (SBS) triblock copolymer having a 0.66 weight fraction of polystyrene blocka by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Polybutadiene (PB) cylinders with hexagonal c l w packing were formed in a polystyrene (PS) matrix when the SBS wan cast from a methyl ethyl ketone solution. It wan found that the PB cylinders were transformed into lamellae on annealing at 160 OC. This is a transition from thermodynamically quasi-stable to stable morphologies. On the baais of SAXS and TEM, the transition turned out to occur via coalescence of cylinders without their translational movementa. It ia propoeed that an undulation of the interface induced by the inatabiIity of the interface might play an important role for the coalescence of the cylinders. It wan also found by TEM that the coherency of lamellar microdomains thus formed became higher with further annealing;i.e., the persistence length of the undulating lamellae became longer.

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