Phase separation of polymer blends in thin films

Abstract

AbstractFilms of polystyrene‐poly(vinylmethyl ether) blends of various compositions are formed by a dip‐coating procedure, the thickness of the film being controlled by the concentration of the solution. The substrates used are glass and gold. The phase separation process is followed by a laser light scattering experiment in which the total forward scattering intensity is monitored as a function of temperature. Morphological examination shows that phase separation occurs by a spinodal decomposition mechanism. A thickness effect on the phase separation temperature is noticeable when film thickness is smaller than 1 μm. This effect is substrate dependent. In all films formed on gold the spinodal temperature increases as film thickness decreases. Films formed on glass exhibit a destabilizing effect on decreasing film thickness. This effect is slight in films of composition poorer in polystyrene than the critical composition, and is enhanced in films richer in polystyrene. The stabilizing effect of decreasing the thickness of films formed on the gold substrate is considered to reflect mainly a purely geometrical effect. The decreasing dimensionality is shown by simple theoretical considerations to increase the phase‐separation temperature. However, the phase separation behavior of thin films on glass appears to be the result of two kinds of substrate‐polymer interactions in addition to the geometrical effect: (a) electrostatic interaction of the charged glass surface (a destabilizing effect at all film compositions) and (b) selective adsorption of polystyrene on glass.