STRUCTURE AND THERMODYNAMICS OF THE LIQUID-VAPOR INTERFACE OF FLUOROCARBONS AND SEMIFLUORINATED ALKANE DIBLOCKS - A MOLECULAR-DYNAMICS STUDY

We use molecular dynamics simulations to predict the equilibrium liquid-vapor interface structure and surface tension of two liquids, one comprised of short fluorocarbon-hydrocarbon bon diblock chains and the other of short fluorocarbon chains. Larger Lennard-Jones diameters and shallower well depths distinguish the perfluoromethyl segments from the methyl ones. In this model, realistic bond angle potentials, torsional potentials, and bond lengths describe the intramolecular interactions. At high temperatures, the density profile of the copolymer melt decays monotonically from the bulk liquid density to the vapor density and the structure of the free surface is similar to that of homopolymer melts. Increasing the chain length or decreasing the temperature causes the fluorocarbon segments to segregate to the free surface. Consequently, the constraint of connectivity between the two blocks results in oscillatory density profiles and a rich structure. Our model predicts that a copolymer can have a lower surface tension than either homopolymer of similar length. We also find that the simple Lennard-Jones based model is deficient in that it fails to explain the surface tension differences between decane and perfluorodecane.