The molecular weight and temperature dependence of polymer surface tension: Comparison of experiment with interface gradient theory

Abstract

The Cahn-Hilliard density gradient theory was used in conjunction with the Flory, Orwoll, and Vrij (FOV) and the Sanchez and Lacombe (SL) equations of state theories to predict surface tensions (γ) with bulk pressure-volume-temperature data. Wide temperature and molecular weight (MW) ranges for each polymer were studied in an attempt to clarify previous discrepancies reported between polymer and low MW liquids and the predictions of density gradient theory. Surface tensions were measured using the modified Wilhelmy technique for both nonpolar and highly polar polymers including poly(dimethyl siloxanes), polyethylenes, polystyrenes, poly(ethylene oxides) (PEO), and poly(propylene oxides) as a function of temperature. Included in the study were PEOs with different chain end functionalization. With one adjustable parameter for a given polymer series, the agreement of FOV theory was better than 1% in terms of the temperature and MW dependence indicating that bulk liquid properties govern surface tension and that the density gradient formalism adequately describes the phenomena. With no adjustable parameters the FOV theory predicts γ to better than 8% for all polymers and oligomers. From the comparison between experiment and theory it is found that the contribution of surface configurational entropy is negligibly small. Some deficiencies of the SL equation of state are traced to its description of the entropy of the system based on the lattice construction.