On the glass transition and physical aging in nanoconfined polymers

The glass transition temperature (T_g) and physical aging behavior of nanoconfined polymer films were investigated by novel fluorescence methods. These studies have revealed that there are large modifications in both T_g and physical aging behavior due to interfacial effects. For example, T_g was observed to decrease compared to bulk in polymers where free-surface effects dominate (e.g., for polystyrene (PS) on silica substrates), while T_g was observed to increase compared to bulk for polymers where strong attractive substrate interactions dominate (e.g., for poly(2-vinylpyridine) (P2VP) on silica substrates). Similar interfacial effects were observed for physical aging, where attractive substrate effects retarded physical aging compared to bulk. Furthermore, the T_g-nanoconfinement effect was observed to be widely tunable by small variations on the repeat unit structure of PS or by the addition of low molecular weight diluents or plasticizers. Finally, T_g and enthalpy relaxation behavior were investigated by differential scanning calorimetry for PS-silica and P2VP-silica nanocomposites. As in the nanoconfined film studies, interfacial interactions were key in dictating the ultimate properties of the nanocomposite, but it was also observed that preparation method plays a significant role.