Five polystyrene−poly(dimethylsiloxane) di- and triblock copolymers were prepared by two different methods, a classical anionic polymerization and a cationic polymerization of styrene in the presence of a new macromolecular initiator. Spherical, cylindrical, and lamellar phase-separated morphologies were identified. Their sizes were measured by transmission electron microscopy (TEM) on films obtained from bulk and solvent-cast samples. DSC and TEM show that the phase-separated morphologies are strongly segregated with no long range ordering. The specific microphase surface area, obtained by TEM, is strongly correlated with the broadening of the polystyrene glass transition.

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