Using a fluorescence method based on Forster resonance energy transfer or FRET, we studied the impact of confinement on the translational diffusivity of the small-molecule dye 9,10-bis(phenylethynyl)anthracene (
Ddye) in supported, thin films of polystyrene (PS). Relative to bulk films and near the PS glass transition temperature (
Tg),
Ddye is reduced by 80 - 90% in 100-nm-thick, high molecular weight (MW) PS (400 kg/mol) films. However, at this film thickness, the
Tg of the film is unchanged from the bulk value. With further nanoconfinement at thicknesses below 50 nm, the PS film
Tg decreases with decreasing thickness. These results are associated with fragility-confinement effects, with fragility decreasing in PS films with decreasing thickness below ~ 200 nm. Fragility values reflect the breadth of the cooperative segmental relaxation distribution in the polymer, thus indicating that the breadth of that relaxation distribution narrows with confinement, i.e., sufficient confinement can result in a loss of a portion of the fast relaxation regions and of the slow relaxation regions. The thickness-dependence of
Ddye reflects the time scales associated with the fast relaxation tail of that distribution, which can be many orders of magnitude faster than the slow regions. At PS film thickness below ~200 nm, the relaxation distribution begins to narrow, with the shortest relaxation times shifting to longer relaxation times, leading to a reduction in
Ddye. In contrast, the thickness-dependence of
Tg reflects the average cooperative segmental relaxation time and thus the slow relaxation regions of that distribution. In sufficiently confined PS films (< 50 nm thick), the narrowing of the relaxation distribution begins to impact the long relaxation time side of the distribution, with the slow relaxation regions become faster. As the
Tg value reflects the average cooperative segmental relaxation time, which is fully dominated by the slow relaxation regions, this narrowing leads to a reduction in
Tg with nanoconfinement below 50 nm in thickness.
To provide critical support for the fragility-confinement effect being the origin of the dye diffusivity-confinement effect, we also performed studies on the effect of confinement on Ddye in low MW PS (6 kg/mol) PS films. Because 6 kg/mol PS exhibits a much lower bulk fragility than high MW PS, confinement has a much-reduced effect on low MW PS fragility, with both fragility and Ddye in 100-nm-thick low MW PS films being unchanged from bulk values. Thus, at thickness ranges where fragilty-confinement effects are observed (with high MW PS), Ddye values also exhibit confinement effects. However, by suppressing those confinement effects via choice of polymer MW (using sufficiently low MW PS), the impact of confinement on Ddye values are also suppressed. There are important technological implications for these results as photolithography used in microelectronics manufacturing employs thin films of low MW polymer which contain photoacid dyes that must not diffuse beyond some nanolength scales during processing.
