Calorimetric study of blend miscibility of polymers confined in ultra-thin films

Abstract.

Miscibility in blends of polystyrene and poly(phenylene oxide) (PS/PPO) confined in thin films (down to 6 nm) was investigated using a recently developed sensitive differential alternating current (AC) chip calorimeter. Comparison of composition dependence of glass transition in thin films with common models should provide information on miscibility. This study focuses on the blend system polystyrene and poly(phenylene oxide) (PS/PPO) because it is thought as a miscible model system in the whole composition range. Furthermore, its local dynamic heterogeneity is already identified by dynamic mechanic thermal analysis (DMTA) and solid state NMR techniques. For this blend, we find that even for the thinnest films (6 nm, corresponding to about half of PPO’s radius of gyration R _g) only one glass transition is observed. The composition dependence of T _g is well described by the Fox, Couchman or Gordon-Taylor mixing law that are used for the miscible bulk blends. Although there is a contradicting result on whether T _g decreases with decreasing film thickness between our calorimetric measurements and Kim’s elipsometric measurements on the same blend (Kim et al. Macromolecules 2002, 35, 311–313), the conclusion that the good miscibility between PS and PPO remains in ultrathin films holds for both studies. Finally, we show that our chip calorimeter is also sensitive enough to study the inter-layer diffusion in ultrathin films. PS chain in a thin PS/PPO double layer that is prepared by spin coating PPO and PS thin film in tandem will gradually diffuse into the PPO layer when heated above T _g of PS, forming a PS_xPPO_100−x blend. However, above the PS_xPPO_100−x blend, there exists an intractable pure PS like layer (∼30  nm in our case) that does not diffuse into the blend beneath even staying at its liquid state over 10 hours.