INTERFACIAL-TENSION REDUCTION IN POLYSTYRENE POLY(DIMETHYLSILOXANE) BLENDS BY THE ADDITION OF POLY(STYRENE-B-DIMETHYLSILOXANE)/

The effects of diblock copolymer addition on the interfacial tension o f immiscible homopolymer blends are examined for the ternary system co mprising polystyrene (PS), poly(dimethylsiloxane) (PDMS), and poly(sty rene-b-dimethylsiloxane) [P(S-b-DMS)]. Interfacial tension is measured , as a function of the diblock copolymer concentration and the molecul ar weight of PDMS, using an automated pendant drop tensiometer. The in terfacial tension of the blend initially decreases upon an increase in the copolymer concentration and then attains a constant value above a certain critical concentration. A maximum interfacial tension reducti on of 82% is achieved at a critical concentration of 0.002% diblock co polymer. At a fixed PS molecular weight, the reduction in interfacial tension increases and the critical concentration decreases with an inc rease in the PDMS molecular weight. These results are compared to the predictions of Leibler's theory for copolymer brushes wherein the crit ical concentration can be attributed either to saturation of the inter face (i.e., for the dry brush case) or to the onset of copolymer micel le formation (i.e., for the wet brush case). The degree of interfacial tension reduction is found to be dependent on the sample preparation procedure. When the copolymer is mixed into the PS phase, the amount o f interfacial tension reduction is much less than the reduction when i t is blended into the PDMS phase. This behavior suggests that the poly mer blend interface may act as a kinetic trap that limits the attainme nt of global equilibrium in these systems.