Thermodynamics of linear and star polymers at fluid interfaces.

Performing molecular dynamics simulations on model systems we study the structural changes and thermodynamic stability of polymers of varying topology (linear and star-shaped) at interface between two liquids. We find that homopolymers are attracted to the interface in both good and poor solvent conditions showing that they are surface active molecules even though not amphiphilic. In most cases changing polymer topology had only a minor effect on the desorption free energy. A noticeable dependence on polymer topology is only seen for relatively high molecular weight polymers at interface between two good solvents. Examining separately the enthalpic and entropic components of the desorption free energy suggests that its largest contribution is the decrease in the enthalpic part of interfacial free energy caused by the adsorption of the polymer at the interface. Finally we propose a simple method to qualitatively predict the trend of the interfacial free energy as a function of the polymer molecular weight.