Surface tension of polystyrene blends: Theory and experiment

Abstract

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 enhancedsurface 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 (v ¼ 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.

Surface segregation driven by molecular architecture asymmetry in polymer blends

“…Segregation driven by star branching has also been demonstrated experimentally by Greenberg et al [15]. Archer and co-workers [23] did not directly measure surface segregation but measured how the surface energy of starlinear blend melts varied with the molecular architecture of the star-branched chains. They interpreted the variation in surface energy with blend composition using both a Cahn-Hilliard model and self-consistent field theory (SCFT).…”

Entropic segregation of short polymers to the surface of a polydisperse melt

“…This has a wide range of implications in regards to surface tension [15,16], wall slip [17,18], the glass transition of thin films [19,20], and the effective force between polymer surfaces [21], to name a few. The phenomenon also has similar implications for mixtures of chemically identical molecules of different architecture [22][23][24][25][26].…”

Reduction of polymer surface tension by crystallized polymer nanoparticles

“…As such, it would be expected to disperse more uniformly in the poly mer melt 20 and would be better modeled as a star polymer or at least a linear polymer with greater contour volume (longer) than the host polymer (it should be of equal or greater contour volume since the crystallization could join several polymers together). The groups of Russell, 21 Fredrickson, 22 Archer, 23,24 and Sauer 25 have discussed theo retical and experimental results for polydisperse polymer melts and linear/star mixtures. In such cases, these authors have shown that the longer polymers (here representing loosely crystallized particles) would avoid the surface and, since the average molecular weight would be higher than a monodisperse polymer system (noncrystallized system), the surface tension would increase rather than decrease.…”

“…10 This makes sense since in this picture the "particles" (longer poly mers or star polymers) would have more conformational en tropy than the noncrystallized polymers-they would have fewer free ends after crystallizing together. It is known that the architecture of a polymer affects its localization with re spect to a surface with molecules having fewer free ends avoiding the surface; [21][22][23][24][25][26] free ends have less configurational entropy to lose near a surface. The experimental situation of Wei et al 2 is modeled in this paper by nanoparticles that are an extreme limit of this effect: a single segment "polymer" that is purely an end-segment and that therefore has a strong entropic affinity to the surface.…”