Fragility and glass transition temperature of polymer confined under isobaric and isochoric conditions

It is now well established that the glass transition temperature (T _g) of polymers can deviate substantially from the bulk with nanoscale confinement. Understanding the impact of confinement on the T_g of polymers is important but it does not provide information about the temperature dependence of the cooperative segmental dynamics near the T_g, a phenomenon commonly referred to as the dynamic fragility. Here, we measure the dynamic fragility index (m) as well as the T_g of confined poly(4-methylstyrene) (P4MS) under isobaric and isochoric conditions. We accomplish this via variable cooling rate differential scanning calorimetry (DSC) studies on aqueous-suspended and silica-capped P4MS nanoparticles. We observe that both the isobaric (m_p) and isochoric (m_v) fragilities decrease with confinement. However, T_g decreases and remains constant with isobaric and isochoric confinement, respectively. The importance of interfaces in the observed trends as well as comparisons to prior studies is discussed.