Abstract:

We investigated the phase behavior of thin film, thickness h  100 nm, mixtures of a polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) diblock copolymer with star-shaped polystyrene (SPS) molecules of varying functionalities f, where 4 < f < 64, and molecular weights per arm M_arm. The miscibility of the system and the surface composition varied appreciably with M_arm and f. For large values of M_arm, regardless of f, the miscibility of the system was qualitatively similar to that of linear chain PS/PS-b-P2VP mixtures – the copolymer chains aggregate to form micelles, each composed of an inner P2VP core and PS corona, which preferentially segregate to the free surface. On the other hand, for large f and small M_arm, SPS molecules preferentially resided at the free surface. Moreover, blends containing SPS molecules with the highest values of f and lowest values of Marm were phase separated. These observations are rationalized in terms of competing entropic interactions and the dependence of the surface tension of the star shaped molecules on M_arm and f.

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