We have examined the direct effect of manipulating the number of free surfaces on the measured glass transition temperature $T_{\mathrm{g}}$ of thin polystyrene films. Thin films in the range $35\mathrm{nm}<h<114\mathrm{nm}$ with molecular weights of $592\mathrm{kg}/\mathrm{mol}$ and $1144\mathrm{kg}/\mathrm{mol}$ were studied. Ellipsometry was used to determine the temperature dependence of the thickness and refractive index of freestanding films. By noting the change in slope in each of these quantities, a $T_{\mathrm{g}}$ value can be assigned in quantitative agreement with previously reported results. For thin freestanding films this value is reduced from that of the bulk. The exact same films are then transferred to a Si substrate and the $T_{\mathrm{g}}$ of the resulting supported film was determined. The $T_{\mathrm{g}}$ values of the now supported films are the same as the bulk value and the same as previous reports of similar supported films. These experiments unambiguously show that free interfaces are the dominant cause of the $T_{\mathrm{g}}$ reductions for the film thicknesses studied.

• Received 8 May 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.055701