Studies of Polymer, Copolymer, and Associating Liquids by Melt Surface Tension Methods and Cahn-Hilliard Density-Gradient Theory

Abstract

Abstract The purpose of the this paper is twofold. The molecular weight dependence of polymer and oligomer surface tension is carefully examined both theoretically and experimentally. Also, a series of measurements on a variety of materials including polymers, associating liquids (water, ethylene glycol, and others), and copolymers is used to examine the effect of both bulk and surface structure on surface tension. Pressure-volume-temperature (PVT) data have been obtained for each material for which surface tensions were measured. The Cahn-Hilliard interface theory in conjunction with an equation-of-state theory is used to predict the surface tensions (γ) of these systems in order to understand how the bulk properties affect surface structure. Theoretical predictions clearly reproduce the M -1 n dependence for high molecular weights and the M -2/3 n dependence for the lower molecular weights and the temperature dependence of all materials studied. Data for several other polymers as a function of molecular weight are also reviewed in terms of theoretical modeling with minimal adjustable parameters as a function of chemical structure. Due to the surface structure caused by the hydrogen bonding, the surface tensions of water and ethylene glycol could not be adequately described by density-gradient theory. For higher molecular weight dialcohols and other linear alkanes and linear polyethers, the theory can adequately reproduce the experimental results. For one polymer, poly(hexafluoropropylene oxide), the data suggest that orientation of the CF 3 side-groups toward the surface occurs. This was substantiated by studies of fluorocarbon/hydrocarbon and PDMS/PEO block copolymers.