Interfacial Tension of a Polystyrene−Poly(ethylene oxide) Diblock Copolymer at the Water−Toluene Interface

Abstract

This paper investigates the interfacial properties of a polystyrene−poly(ethylene oxide) (PS−PEO) diblock copolymer adsorbed at the water−toluene interface using axisymmetric drop shape analysis-profile (ADSA-P). The molecular weight of the PS block was 231 400 while the PEO block had a molecular weight of 20 670. The diblock copolymer was dissolved in toluene to form a 0.131 mg/mL solution and was allowed to adsorb onto a pendant drop of water at 20 °C. The variation in interfacial tension with time during the equilibration process was similar to that of a surfactant, beginning at the water−toluene interfacial tension and decreasing to an equilibrium value of 27.8 mJ/m2 after a period of 2 h. The interface was then compressed by decreasing the volume of the drop in a linear manner. The shape of the resulting interfacial tension−area isotherm can be rationalized by a schematic model of the interfacial molecular conformation. According to the proposed model, the average molecular spacing is determined by the PS block at the start of compression and by a combination of the PEO block and the PS brush at the end of compression. These results show that the use of ADSA-P to measure interfacial tensions is a useful way to obtain indirect evidence of the interfacial molecular conformation.