Capillary rise infiltration (CaRI) enables the fabrication of polymer nanocomposite films (PNCFs) with high nanoparticle loading (> 50 vol%). The process involves generating a bilayer of nanoparticle and polymer film, and thermally annealing the film above the glass transition temperature (Tg) of the polymer to induce polymer imbibition into the voids in the nanoparticle packing. Upon CaRI, polymer experiences strong physical confinement within the nanoparticle packing, which may lead to changes in the polymer properties and the infiltration dynamics, subsequently affecting the macroscopic PNCF structure and properties. As such, understanding polymer behavior under confinement is crucial to enable optimal process and nanocomposite design. In this work, we study the effect of physical confinement, polymer-nanoparticle interactions, and undersaturation on the polymer CaRI dynamics. We utilize in situ spectroscopic ellipsometry to determine the effective polymer viscosity based on the Lucas-Washburn analysis, and to determine the polymer Tg when confined in the nanoparticle packing. We observe increased polymer viscosity and Tg with confinement, until a threshold confinement ratio is reached. Furthermore, under extreme nanoconfinement, the polymer-nanoparticle interaction is negligible relative to the confinement effect. In undersaturated CaRI (UCaRI), such that a bilayer film with insufficient polymer to completely fill the void space in the nanoparticle packing is annealed, there is a two-stage filling process – a rapid capillary rise with a clear invading front, and a gradual polymer spreading likely via surface diffusion. As such, the UCaRI process enables the fabrication of nanoporous polymer-infiltrated nanoparticle films with uniform or gradient composition, depending on the annealing time and polymer volume fraction. These UCaRI films also have tunable optical and mechanical properties with polymer composition. Finally, we characterize the fracture toughness of UCaRI films based on a nanoindentation-based pillar splitting method. We show that confinement-induced polymer capillary bridges and chain bridging of nanoparticles to drastically toughen the UCaRI film, even upon infiltrating small amounts of polymer. Thus, this work provides insights to the processing-structure-property relationships of the CaRI process to generate functional nanocomposite films with high nanoparticle loadings.