Surface Structures and Segregation of Polystyrene/Poly(methyl methacrylate) Blends Studied by Sum-Frequency (SF) Spectroscopy

Molecular structures at the surface and interface of polystyrene (PS)/poly(methyl methacrylate) (PMMA) blend films were obtained by sum-frequency (SF) spectroscopy. Using this technique, both the surface of the blend and the internal interface between the two blend polymers are simultaneously probed. Two types of blend film structures were obtained depending on sample treatment. The as-cast film (dried under vacuum at room temperature) gave a PMMA-on-PS bilayer structure whose SF spectrum was dominated by the C−H symmetric stretch of the PMMA ester methyl groups. When the bulk PMMA concentration was higher than 13% w/w, the SF features were identical to that of the pure PMMA films. The lack of PS SF features in this blend film suggests that the PS phenyl rings may be randomly oriented at the PS−PMMA interface. Upon annealing at 150 °C for 20 h, the PS segregated to the film surface as a result of its lower surface free energy, and a PS-rich phase was formed near the polymer−air interface, as evidenced by the SF spectroscopy, contact angles, and XPS measurements. However, the SF peak of the ester methyl group was still observed, indicating that the PMMA was still oriented at the PMMA−PS interface.