Surface tension of polystyrene blends: Theory and experiment

• Published in: Journal of polymer science. Part B, Polymer physics Polymer physics, 2009-09, Vol.47 (17), p.1666-1685
• Main Authors: Qian, Zhenyu, Minnikanti, Venkatachala S, Sauer, Bryan B, Dee, Gregory T, Kampert, William G, Archer, Lynden A
• Format: Article
• Language: English
• Subjects: additives; Applied sciences; Blends; Computer simulation; Differential scanning calorimetry; Exact sciences and technology; Interaction parameters; Organic polymers; Physicochemistry of polymers; Polymer blends; Polystyrene resins; Properties and characterization; Segregations; Stars; Surface properties; Surface tension; surfactants
• ISSN: 0887-6266; 1099-0488; 1099-0488
• DOI: 10.1002/polb.21771

Surface tension of linear-linear and star/linear polystyrene blends were measured using a modified Wilhelmy method. Our results show that for both polystyrene blend systems, the surface tension-composition profile is convex, indicating a strong surface excess of the component with lower surface energy. Star/linear blends display more convex surface tension profiles than their linear-linear counterparts, indicative of stronger surface segregation of the branched-component relative to linear chains. As a first step toward understanding the physical origin of enhanced-surface segregation of star polymers, self-consistent field (SCF) lattice simulations (both incompressible and compressible models) and Cahn-Hilliard theory were used to predict surface tension-composition profiles. Results from the lattice simulations indicate that the highly convex surface tension profiles observed in the star/linear blend systems are only possible if an architecture-dependent, Flory interaction parameter (χ = 0.004) is assumed. This conclusion is inconsistent with results from bulk differential scanning calorimetry (DSC) measurements, which indicate sharp glass transitions in both the star/linear and linear/linear homopolymer blends and a simple linear relationship between the bulk glass transition temperature and blend composition. To implement the Cahn-Hilliard theory, pressure-volume-temperature (PVT) data for each of the pure components in the blends were first measured and the data used as input for the theory. Consistent with the experimental data, Cahn-Hilliard theory predicts a larger surface excess of star molecules in linear hosts over a wide composition range. Significantly, this result is obtained assuming a nearly neutral interaction parameter between the linear and star components, indicating that the surface enrichment of the stars is not a consequence of complex phase behavior in the bulk.