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Behavior of gradient copolymers at liquid/liquid interfaces.

The behavior of styrene/acrylic acid gradient and diblock copolymers at liquid/liquid interfaces was investigated by using drop shape analysis to measure the interfacial tension. Copolymers were dissolved in chloroform, and pendant drops of these solutions were created in water. Molecular conformations at the interface were inferred by measuring changes in the interfacial tension as the interface was contracted and expanded through control of the drop volume. In this way, we were able to independently determine the interfacial pressure and area modulus of the adsorbed layer. Gradient copolymers showed the largest interfacial pressure, a result that is attributed to kinetic factors associated with the nature of the micellar aggregates that form in the chloroform phase. The area modulus of the adsorbed layer depended on the processing history and was not directly related to the interfacial pressure. This result is attributed to a local segmental desorption process where portions of the molecules reversibly desorb while the number of copolymer molecules at the interface remains fixed.