Electrocapillary Wave Diffraction Measurement of Interfacial Tension Reduction between Two Oligomers (Poly(dimethylsiloxane) and Poly(ethylene Glycol)) by a Block Copolymer and the Mean-Field Theory Prediction

Abstract

With the aid of a technique that induces electric-field-generated capillary waves on a polymeric liquid surface and the resulting wave propagation characteristics detected by an optical diffraction method, we examined the behavior of a diblock copolymer (poly(dimethylsiloxane-b-ethylene oxide)) at the interface between two oligomeric ethers, low molecular weight poly(dimethylsiloxane) (PDMS) and poly(ethylene glycol) (PEG). The interfacial tension values in the presence of the diblock copolymer at the interface were obtained from the dispersion equation. Upon addition of increasing amounts of the copolymer, the interfacial tension dropped rapidly and leveled off as the concentration surpassed some value. The reduction in the interfacial tension with an increasing amount of the block copolymer calculated by a self-consistent mean-field (SCMF) model showed good agreement with the experimental data, quantitatively as well as qualitatively. The SCMF calculations predicted that the effects of the copolymer on the interfacial properties in the strong segregation regime were governed by the structure of the copolymer (not only by the degree of polymerization of both the longer block and the short block but also by their relative length ratio) as well as by the degree of polymerization of the homopolymers. Depending on the relative length ratio between the two blocks, micelle formation competed with surface segregation of the block copolymer molecules.