Tuning the Tg-confinement effect in thin polymer films via minute levels of residual surfactant which “cap” the free surface

Abstract

We compare the effect of confinement on the glass transition temperature (Tg) of ultrathin supported films of polystyrene produced via emulsion polymerization (E-PS) and anionic polymerization (A-PS). Using spectroscopic ellipsometry and fluorescence to characterize Tg, we find that E-PS films supported on silica exhibit Tg-confinement effects that are suppressed and even eliminated when compared to A-PS films on silica. We prepared E-PS samples with varying dodecyl sulfate surfactant concentration made via sequential purification. Using Epton’s Method, we determined the amount of surfactant present in bulk unwashed E-PS sample and in several washed samples. Films made from E-PS containing as little as 0.023 wt% surfactant show elimination of the Tg-confinement effect within error down to a thickness of 14 nm. Films made from E-PS containing 0.0058 wt% surfactant show suppression of the Tg-confinement effect relative to that observed by both ellipsometry and fluorescence in A-PS films. Using X-ray photoelectron spectroscopy, we observe that the sulfur atom in the surfactant is localized at the top 2–4 nm of the film next to the polymer-air interface. The residual surfactant left from emulsion polymerization at or very near the free surface restricts the perturbation of Tg by the free surface, thereby suppressing and even eliminating the Tg-confinement effect in ultrathin E-PS films relative to that in A-PS films.