Size Independent Glass Transition of Polystyrene Nanospheres

The effect of confinement on the glass transition temperature (T$_{\mathrm{g}})$ has been studied in thin films, nanocomposites, and nanoparticles. Keddie et al studied T$_{\mathrm{g}}$ as a function of film thickness and found that while T$_{\mathrm{g}}$ decreased with decreasing film thickness when the film was on a gold substrate, there was an increase in T$_{\mathrm{g}}$ with decreasing film thickness when the film was on silicon substrate. Thin films are a popular system to study; however understanding other systems is crucial to understanding the fundamental effects of confinement. Polymer nanoparticles are an attractive system due to their potential as drug delivery agents, however the studies on them are limited and the results are in poor agreement. However most researchers have claimed to find that Tg is dependent on the size of the nanoparticles. Here temperature-varied fluorescence spectroscopy was used to study the glass transition temperature of polystyrene nanospheres of varying sizes (including sizes far below what has been actively investigated in the literature) and anomalous size independent T$_{\mathrm{g}}$ for particles below a certain size was found.