Reduction of the Interfacial Tension between Poly(dimethylsiloxane) and Poly(ethylene oxide) by Block Copolymers: Effects of Molecular Architecture and Chemical Composition

Interfacial tensions σ were measured by means of the sessile drop method for mixtures of poly(ethylene oxide) [PEO, M w = 41 kg/mol] with five samples of poly(dimethylsiloxane)s [PDMS, M w ranging from 4 to 177 kg/mol] in the temperature interval 70−130 °C. Within experimental error σ does not depend on T and the influences of molar mass can be well described by a linear dependence on M n -2/3 . The ability of block copolymers to reduce σ when added in small amounts was studied for two families of these substances. With type I all blocks are made of either DMS or EO and the members differ in the molecular architecture only (diblocks, triblocks, and “bottle-brushes”). Type II comprises diblock copolymers, and the members differ in the chemical composition of the blocks which consist of EO, styrene, or methyl methacrylate. The reduction of σ upon an increase in x add, the base mole fraction of the additive in the PEO phase, is substantial for all additives under consideration; it can be well described by an equation of Tang−Huang containing two characteristic parameters: σ s, the saturation interfacial tension approached in the limit of large x add, and x char, the additive concentration required to achieve 1/ e of the maximum reduction. In the case of type I block copolymers, v, the total number of DMS segments, turns out to be more decisive for their efficiency than their architecture, in contrast to the theoretical expectation. A lower limit of σ s ≅ 1 mN/m is reached for v > 30. At least the present representatives of type II additives are less efficient than those of type I; an increase in the number of segments does not change σ s in a noteworthy way, whereas there exist indications that it reduces x char .