(327d) Influence of Polymer Diffusivity in Nanoconfinement on the Onset of Viscous Fingering

The onset of nanoscale viscous fingers within nanoporous media is a function of polymer concentration and molecular weight and the media permeability. Comparing the onset of these fingers to prior literature, it suggests the polymer effective viscosity and diffusivity must vary greatly in nanoconfinement as compared to in bulk solutions. Aiming to understand how confinement influences polymer dynamics, this work uses Fluorescent Recovery After Photobleaching (FRAP) via laser scanning confocal microscopy to measure the mobility of fluorescent poly(vinyl alcohol) (PVA) in both bulk solutions and in colloidal crystals. PVA used in this study has an average molecular weight of 67,000 Da and the colloidal crystal is fabricated from 1 micron silica microspheres. Diffusivity of PVA in both cases is extracted by fitting a model to the fluorescent intensity recovery in the bleached region. In the bulk, fluorescence recovery matches the model well. In the colloidal crystal, the model describes the initial fluorescence recovery, suggesting the effective diffusivity is two orders of magnitude lower than in the bulk solution, but overestimates the long-time polymer fluorescence. This deviation from the predicted longer recovery times within the colloidal crystal suggests either a partitioning of polymer by molecular weight or more complex interactions of polymer chains in confinement. Molecular dynamics simulations using coarse-grained models for polymer chains and explicit solvent in bulk (i.e., no confinement effects) and in nanoporous colloidal crystal describe the primary influence of nanoconfinement on the polymer chain conformations and diffusion. It is theorized that a combination of pore size and molecular crowding, which is a function of polymer concentration and molecular weight, slows the polymer dynamics considerably. This effective decrease in polymer diffusivity and increase in effective viscosity results in the conditions that support continuous fingering in these films.