Nanoconfined Self-Diffusion of Poly(isobutyl methacrylate) in Films with a Thickness-Independent Glass Transition

Even though the physics of nanoconfined polymers have been extensively studied for years, diffusion of polymer chains along confining interfaces has not been widely studied, likely because there are few experimental techniques available for these measurements. Here a fluorescence recovery after patterned photobleaching (FRAPP) technique is developed using 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. Using this technique, self-diffusion of poly(isobutyl methacrylate) (PiBMA) was investigated at 80 °C (29 °C above the glass transition temperature, T g) and was found to be film thickness independent down to 30 nm (∼14R g, where R g is the radius of gyration) with a diffusion coefficient well predicted by the Rouse model (1.05 × 10–12 cm2/s). PiBMA is an ideal polymer for this study because it exhibits a film thickness-independent T g down to 15 nm (∼7R g) as measured by spectroscopic ellipsometry. 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 nanoconfinement effects.