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

We compare the effect of confinement on the glass transition temperature (T-g) of ultrathin supported films of polystyrene produced via emulsion polymerization (E-PS) and anionic polymerization (A-PS). Using spectroscopic ellipsometry and fluorescence to characterize T-g, we find that E-PS films supported on silica exhibit T-g-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 T-g-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 T-g-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 T-g by the free surface, thereby suppressing and even eliminating the T-g-confinement effect in ultrathin E-PS films relative to that in A-PS films. (C) 2016 Elsevier Ltd. All rights reserved.