(751c) Glassy Behavior of Polymers Confined in Unique Geometries

Polymers confined to the nanometer length-scale are important in applications ranging from active layers in photovoltaic cells to membranes in separation technologies to vehicles in drug delivery.  As shown extensively in the past, the glass transition temperature (T_g) (and its associated dynamics) of confined polymers can deviate substantially from the bulk.  Due to ease of processing, most of these studies have focused on the 1-dimensional thin film geometry, while few have extended investigations to unique geometries and to higher dimensionalities.  Here, we utilize a 3-dimensional nanoparticle geometry to explore confinement-induced effects on the T_g and the dynamic fragility of polymers.  In the case of bare polystyrene (PS) and poly(4-methylstyrene) (P4MS) nanoparticles, the T_g is observed to decrease systematically with decreasing nanoparticle diameter.  On the contrary, by capping the nanoparticles with a rigid silica shell, i.e., a polymer/silica core-shell geometry, the T_g remains invariant as a function of diameter.  Furthermore, using variable cooling rate differential scanning calorimetry, we measure the isochoric fragility (m_v) of silica-capped PS and P4MS nanoparticles, which decreases significantly with confinement.  In the case of P4MS, we show that the ratio of the isobaric fragility to the isochoric fragility, i.e., m_p/m_v, remains constant with diameter, indicating that the relative effects of thermal activation and volume contribution on the dynamics near the glass transition remain unchanged under confinement.  Lastly, we utilize a novel confining geometry, i.e., thin polymer films coated onto silica nanoparticles, to extend our understanding of confinement effects on the T_g and the dynamic fragility.  We explain the observed T_g-confinement and fragility-confinement results in terms of interfacial effects perturbing glass transition dynamics.