(336g) Nanoconfined Self-Diffusion of Poly(isobutyl methacrylate)

Even though the physics of nanoconfined polymers have been extensively studied for years with respect to properties such as glass transition temperature (T_g), viscosity, elastic modulus and others, diffusion of polymer chains along confining interfaces has not been widely studied. This is likely due to the lack of experimental techniques available for these measurements. Here we present a study of self-diffusion of poly(isobutyl methacrylate) (PiBMA) nanoconfined to thin films via a technique based on fluorescence recovery after patterned photobleaching (FRAPP). PiBMA is an ideal polymer for this study because it exhibits a film thickness-independent T_g down to ∼7R_g, as measured by spectroscopic ellipsometry for both fluorophore and non-fluorophore containing polymers. Since the diffusion coefficient of polymers depends strongly on the proximity of diffusion temperature to T_g, this attribute allows a straightforward measure of nanoconfined diffusion without superimposed influence from T_g related nanoconfinement effects.

This FRAPP approach uses an epifluorescence microscope that allows for direct, in situ, visualization of polymer diffusion over several periods of a photobleached array. This visualization approach is more robust compared to measuring fluorescence intensity alone and also significantly increases the experimental throughput. The self-diffusion of PiBMA parallel to the confining interfaces was found to be film thickness independent down to ~14R_g, where R_g is the radius of gyration. The diffusion coefficient agreed well with the bulk diffusion coefficient predicted by the Rouse model for unentangled polymer diffusion. Furthermore, the diffusion coefficient is independent of the location of the fluorophore on the polymer backbone.