Dynamics of poly-nipam chains in competition with surfactants at liquid interfaces: from thermoresponsive interfacial rheology to foams

Abstract

We report results on the interfacial viscoelasticity and foaming of solutions of a thermoresponsive polymer (poly-n-isopropylacrylamide) with and without added surfactants, and as a function of temperature. With pure polymer solution, a clear transition is evidenced: both interfacial shear and dilational rheology shift from a fluid-like to a solid-like behavior at a well-defined temperature. The high temperature regime shows that the layer shares many features with soft glassy systems. At all temperatures, the foaming is low and the foam produced is unstable. Adding a surfactant not only helps to foam and to stabilize the foam, but also removes the thermal responsivity of the interfacial viscoelasticity. Under the conditions used here, we observe that the surfactant concentration threshold for altering the high temperature interfacial viscoelasticity is low, and is of the order of 1% of the surfactant critical micelle concentration. It turns out to be very different from critical values for the polymer–surfactant association found previously by structural studies (in bulk and at interface), and also below the threshold value required to stabilize the foam.