Modification of Thin Film Phase Separation by a Surfactant
Li Piin Sung, Alamgir Karim, Jack F. Douglas, Charles C. Han
Phase separation in thin films leads to the formation of surface patterns (holes, bumps, and bicontinuous surface features) which depend on the film composition. These features are interpreted to arise from surface tension variations within the film that accompany phase separation. The introduction of surfactants is expected to cause profound changes in these phase separation induced patterns since the driving force for pattern formation should be greatly diminished. We investigate model polystyrene/polybutadiene (PS/PB) films with and without a block copolymer (PS-b-PB) surfactant additive to check this possibility. A strong suppression in the development of organized surface patterns is indeed found upon addition of block copolymer. Moreover, the surfaces of these surfactant-modified films remain smooth indefinitely, which has evident practical importance for the creation of defect free films. An unanticipated feature of these surfactant-modified films is the observation of large-scale intensity fluctuations in their optical micrographs. These patterns have the appearance and spectral characteristics of critical fluctuations in a fluid mixture near its critical point for phase separation.