Surface Tension of Symmetric Star Polymer Melts

• Published in: Macromolecules 2008-07, Vol.41 (13), p.5007-5013
• Main Authors: Qian, Zhenyu, Minnikanti, Venkatachala S, Sauer, Bryan B, Dee, Gregory T, Archer, Lynden A
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Surface properties
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma8002888

To evaluate the effect of polymer architecture on surface tension, glass transition, and other thermodynamic properties, we synthesized a series of 4-arm and 11-arm symmetric star polystyrenes. Surface tension was measured as a function of molecular weight of the stars and temperature in the melt using a modified Wilhelmy plate technique. We find that architectural effects play a significant role in determining the molecular weight dependence of polymer melt surface tension. A variable density lattice model that considers effects of entropic attraction of polymer chain ends to surfaces, compressibility and density gradients in the region near the surface is used to determine the origin of this observation. This analysis is complemented with surface tension calculations using more classical thermodynamic models that consider only bulk property changes with polymer architecture and molecular weight. Bulk thermodynamic properties for selected stars were derived from pressure−volume−temperature (PVT) measurements. These data are used to calculate the cohesive energy density (CED). This was then used to determine surface tension of the stars using a recently developed theory. Possible effects of the chemical differences of the sec-butyl terminal groups versus the backbone segments are also discussed in terms of bulk property modification and surface segregation of end groups.