Determination of the glass transition temperature of thin unsupported polystyrene films using interference fringes

Controversy exists as to why nanometer-sized polymer films have thermal properties that differ from bulk samples. This paper presents a simple method of determining the glass transition temperature (T-g) of unsupported ultrathin polymer films based on the determination of the refractive index from Fabry-Perot interference fringes observed in the infrared or UV-visible regions. T-g is extrapolated from a plot of the variation of refractive index with temperature. The plot of T-g as a function of the thickness of polystyrene films presents a sharp decrease when the thickness reaches values below 60nm. Results also show variations in the slope of the glassy line, alpha(g), with decreasing thickness, supporting the hypothesis that an interface with a different mobility does exists close to the polymer-air interface. However, this decrease was not as sharp as expected indicating that the decrease in T-g with thickness could not be solely explained by the presence of an interface of different mobility. Published by Elsevier B.V.