Enhanced Thermal Stability of Polymers under Extreme Nanoconfinement*

It has been shown that the properties of polymers in weakly-interacting polymer nanocomposites (PNCs), such as Polystyrene(PS) and SiO₂ nanoparticles (NP), can deviate from bulk due to segmental and chain confinement. Capillary Rise Infiltration (CaRI) of polymers into densely-packed NP films can produce PNCs with ultra-high NP loadings. Depending on the degree of confinement, the glass transition temperatures (T_g) of PS/SiO₂ CaRI films can increase up to ~57 K. In this study, unentangled PS/SiO₂ CaRI films were used to study the thermal degradation of polymers under extreme nanoconfinement. The degree of confinement was controlled by using NPs with different diameters, thus adjusting the effective pore diameter by 3~30 nm. We show that the thermal stability of PS is significantly increased in smaller pores under both thermo-oxidative and pyrolytic conditions. The characteristic degradation time during isothermal degradation at high temperatures is proportional to the viscosity of PS at lower temperatures, indicating an entropic origin of the improved stability. The details of the kinetics of the degradation process through surface or bulk is also explored.