Effect of Physical Nanoconfinement on the Viscosity of Unentangled Polymers during Capillary Rise Infiltration

We investigate the role of physical confinement on the polymer viscosity and the glass transition temperature (T-g) of unentangled polymers undergoing capillary rise infiltration (CaRI). CaRI thermally drives polymer infiltration into the voids of densely packed nanoparticle films via capillarity, inducing extreme nanoconfinement of the polymer. We tune the confinement ratio (CR), defined as the ratio of the polymer radius of gyration to the average pore radius in the nanoparticle packing, by using different polymer molecular weights and by varying the nanoparticle size constituting the packing, respectively. We show that physical confinement of unentangled polymers in the interstices of weakly interacting nanoparticles leads to increased viscosity by more than 2 orders of magnitude relative to the bulk viscosity and to increased polymer T-g by 32 K. The increase in both viscosity and T-g increases with CR and saturates at CR similar to 1. The correlation between the viscosity and T-g increase suggests that the slowdown in translational chain dynamics is directly correlated to the decreased polymer segmental motion under nanoconfinement. These findings emphasize the importance of understanding the effect of extreme nanoconfinement on the transport and thermal properties of polymers, even in weakly interacting systems, which in turn will provide guidelines in optimizing processing parameters and properties of the resulting CaRI nanocomposite films.