EFFECT OF ADDED BLOCK COPOLYMER ON THE PHASE-SEPARATION KINETICS OF A POLYMER BLEND .2. OPTICAL MICROSCOPIC OBSERVATIONS

The Ostwald ripening occurring in the very late stage of the phase-separation process has been investigated by observation under an optical microscope. Samples studied are blends of low molecular weight polystyrene and polybutadiene to which small amounts of styrene-butadiene block copolymer have been added. The presence of the block copolymer depresses the rate of growth of the average particle size of the dispersed phase. The extent of the retardation increases with an increase in the amount of added copolymer and with an increase in the molecular weight of the copolymer. A copolymer of much smaller molecular weight, having its microphase-separation temperature below the temperature of the experiments, exhibits no such retardation effect. The effect arises as a result of the reduction in the interfacial tension due to an accumulation of the block copolymer at the interface. Application of the Lifshitz-Slyozov theory to the particle growth rate data then enables evaluation of the interfacial tension as a function of the concentration of the added copolymer. Comparison of this result against the theory by Leibler suggests that the copolymer molecules adsorbed at the interface are highly swollen with homopolymer molecules and packed rather loosely with each other. The possibility of the formation of thermodynamically stable droplets stabilized by a layer of adsorbed block copolymer, recently proposed from theoretical considerations, does not appear to arise in the systems studied here.