Solventannealing induced phase separation and dewetting in PMMA/SAN blend films: composition dependence

The competition between “dewetting” and “phase separation” behaviors in polymer blend films attracts significant attention. The simultaneous phase separation and dewetting in PMMA/SAN [poly(methyl methacrylate) and poly(styrene-ran-acrylonitrile)] blend ultrathin films upon solvent annealing have been observed in our previous work. The composition dependence of phase behaviors in the same system has been investigated using atomic force microscopy and grazing incidence small-angle X-ray scattering. Both phase separation and dewetting were observed across the entire composition range investigated. The evolution of film structures during solvent annealing was dependent on the blend composition. Solvent annealing led to complex structures, in which the upper (small spots, bicontinuous structures or large continuous droplet) and lower (mimic-film) parts were composed of phase-separated SAN- and PMMA-rich phases, respectively. With increasing blend SAN weight fraction, the diameter of the lower PMMA-rich mimic-film decreased. The upper SAN-rich phase structure transformed depending on the SAN content; to small spots (<50% SAN content), a bicontinuous structure (50% SAN content), or a large continuous droplet (>50% SAN content), on top of the PMMA mimic-film.