Vitrification of thin polymer films: from linear chain to soft-colloid like behavior

The glass transition temperature $T_{\mathrm{g}}$ of sufficiently thin, supported, polymer films is dependent on the film thickness. Based on the nature of the polymer substrate interactions $T_{g}$ may increase, $\Delta T_{\mathrm{g}}$ \textgreater 0, or decrease, $\Delta T_{\mathrm{g}}$ \textless 0, in relation to the bulk. We show that for star-shaped macromolecules the value of $\Delta T_{\mathrm{g}}$ depends on the functionality $f$ of the molecule, for polymer films supported by the same substrate. Specifically in the case of polystyrene (PS) macromolecules, with arms of molecular weight M$_{\mathrm{arm}}$\textless 10 kg./mol., supported by silicon oxide substrates, $\Delta T_{\mathrm{g}} $\textless 0, when f\textless 4. For much higher functionalities, f $\ge $ 32, where the polymer exhibits soft-colloid like behavior $\Delta T_{\mathrm{g}}$ $\sim$ 0. For values of 4\textless f\textless 32, $\Delta T_{\mathrm{g}} $\textgreater 0. The transition from the linear-chain to the soft-colloid behavior is gradual and occurs with increasing $f$ and/or decreasing M$_{\mathrm{arm}}$. With the help of molecular dynamics simulations we rationalize this behavior in terms of competing entropic effects, associated with changes in $f$ and $M_{\mathrm{arm}}$, which drives the ability of these molecules to efficiently pack at interfaces.