The Effect of Extreme Spatial Confinement and Interfacial Interactions on the Glass Transition of Polymers in Polymer-infiltrated Nanoparticle Packings*

Chain and segmental confinement of polymer and polymer-particle interfacial interactions in polymer nanocomposites have been known to cause deviation in polymer properties relative to the bulk. Capillary Rise Infiltration (CaRI) enables polymer infiltration into nanoparticle (NP) packings, confining polymer within the small pores and increasing the interfacial area. In this study, we investigate the influence of spatial confinement and interfacial interactions on the glass transition temperature (T_g) of polymers in SiO₂ NP packings. The degree of confinement is tuned by varying the polymer molecular weight as well as the size of NPs (11~100 nm), producing 3~30 nm pore diameter. Interfacial interaction is controlled by comparing polystyrene (PS) and poly(2-vinylpyridine) (P2VP), which have different interactions with SiO₂. We show that small pore size is a major factor that significantly increases T_g of polymers in these NP packings. Stronger interfacial interaction further increases T_g in highly confined conditions. For example, for 8 kg/mol polymers, in 11 nm NP packings, T_g of PS and P2VP increases by 50 K and 93 K, respectively, while T_g is close to bulk in 100 nm NP packings for both polymers.