Interfacial Tension in Binary Polymer Blends and the Effects of Copolymers as Emulsifying Agents

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Abstract

The structure and the thermodynamic state of polymeric interfaces are important features in many materials of technological interest. This is especially true for multiconstituent systems such as blends of immiscible polymers, where the interface structure can affect greatly their morphology and, thus, their mechanical properties. In this article, we first present a review of the experimental and theoretical investigations of the interfacial tension in phase-separated homopolymer blends. We emphasize the effects of temperature and molecular weight on the behavior: interfacial tension γ decreases with increasing temperature (for polymer systems exhibiting upper critical solution temperature behavior) with a temperature coefficient of the order of 10–2 dyn/(cm °C), whereas it increases with increasing molecular weight. The increase follows a $\gamma = \gamma _\infty \left( {1 - k_{\operatorname{int} } M_{\text{n}}^{ - z} } \right)$ dependence (with z ≈ 1 for high molecular weights), where γ is the limiting interfacial tension at infinite molecular weight and M n the number average molecular weight. Suitably chosen block or graft copolymers are widely used in blends of immiscible polymers as compatibilizers for controlling the morphology (phase structure) and the interfacial adhesion between the phases. The compatabilitizing effect is due to their interfacial activity, i.e., to their affinity to selectively segregate to the interface between the phase-separated homopolymers, thus reducing the interfacial tension between the two macrophases. The experimental and theoretical works in this area are reviewed herein. The effects of concentration, molecular weight, composition, and macromolecular architecture of the copolymeric additives are discussed. An issue that can influence the efficient utilization of a copolymeric additive as an emulsifier is the possibility of micelle formation within the homopolymer matrices when the additive is mixed with one of the components. These micelles will compete with the interfacial region for copolymer chains. A second issue relates to the possible trapping of copolymer chains at the interface, which can lead to stationary states of partial equilibrium. The in-situ formation of copolymers by the interfacial reaction of functionalized homopolymers is also discussed.