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

Abstract

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.

21st Century Advances in Fluorescence Techniques to Characterize Glass‐Forming Polymers at the Nanoscale

“…To measure self‐diffusion from FRAP, portions of polymer films covalently labeled with a fluorescent dye, such as nitrobenzofurazan, were exposed to bright light to permanently bleach the fluorescent dyes within the exposed portion of the film. Commonly, a grating is used to create a pattern of alternating bleached and unbleached lines, as shown in Figure a . Over time, the intensity of the bleached region recovers as polymers with unbleached fluorophores diffuse into the region and bleached fluorophores diffuse out.…”

“…Over time, the intensity of the bleached region recovers as polymers with unbleached fluorophores diffuse into the region and bleached fluorophores diffuse out. The intensity across the image is monitored as a function of time (Figure b) and the diffusion coefficient can be extracted from the decay in amplitude vs. time (Figure c) from the relationwhere D is the in‐plane diffusivity, λ is the wavelength of the patterned photobleach, and τ is the characteristic relaxation time measured for several values of λ …”

“…b) Normalized intensity vs position before bleaching (red), just after bleaching (green), and 17 h after bleaching (blue). Adapted with permission . Copyright 2012, American Chemical Society.…”

“…Over the past few decades, there has been substantial investigation into the deviation from bulk properties observed in polymers when confined to the nanoscale . By restricting one or more dimensions to less than hundreds of nanometers, researchers have observed dramatic changes in T g , physical aging, mobility, fragility, diffusion, viscosity, elastic modulus, and rubbery stiffening, among others, with very few exceptions . The magnitude and direction of these deviations, as well as the confinement length scale necessary to induce changes varies for specific polymer systems .…”

Surface energy gradient driven convection for generating nanoscale and microscale patterned polymer films using photosensitizers

“…Chemicals used in this study were purchased from Fisher Scientific or Sigma‐Aldrich, and all chemicals were used as received unless otherwise noted. PiBMA ( M n = 10.0 kg/mol, PDI = 1.06) was synthesized by activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) according to a previously described procedure . Polished, 4 in diameter silicon wafers were purchased from Silicon Quest International.…”