Enhanced high-frequency molecular dynamics in the near-surface region of polystyrene thin films observed with β-NMR

Abstract

β-detected nuclear spin relaxation of (8)Li(+) has been used to probe the depth dependence of molecular dynamics in high- and low-molecular-weight deuterated polystyrene. The average nuclear spin-lattice relaxation rate, 1/T(avg)(1), is a measure of the spectral density of the polymer motion at the Larmor frequency (41 MHz at 6.55 T). In both samples, 1/T(avg)(1) is depth independent below ∼200 K but above this temperature it decreases approximately exponentially with distance from the free surface, returning to bulk behavior for depths greater than ∼10 nm. This is direct evidence for a region near the free surface with enhanced molecular dynamics compared with the bulk. The effective thickness of the surface region increases with increasing temperature and is finite even above the glass transition. These results present challenges for the current understanding of dynamics near the surface of polymer glasses.

Exploring the Dynamics of Glasses Using Beta Detected NMR

“…2 for four different temperatures at an applied field of 6.55 Tesla. The relaxation is quite fast, typical of the organic polymer glasses we have studied [12,13]. At 100 K, there is not much difference between the two glasses.…”

“…As in polystyrene [12,13], we expect the implanted 8 Li + in TPD is rather strongly bound to one or more of the phenyl rings of the molecule. This binding is sufficiently strong that one can consider the 8 Li + as bound, and not diffusing.…”

“…1) has a molecular weight of 516 amu, a glass transition temperature of 342.1 K, and a band gap of 3.1 eV. Due to the polyaromatic structure of TPD, we expect 8 Li to bind strongly with the phenyl rings as has been observed for polystyrene [12,13], and this will inhibit 8 Li diffusion.…”

“…Previous β-NMR experiments on polymers [12,13], demonstrated that the 8 Li spin lattice relaxation (SLR) is sensitive to molecular dynamics through the fluctuating quadrupolar interaction. In this paper, we report a preliminary exploration into the molecular dynamics in an ultrastable and an ordinary molecular glass thin film.…”

Developments in μSR and β NMR: Beyond a Muon Lifetime

“…The small quadrupole moment of 8 Li and resulting quadrupolar relaxation leads to high sensitivity to the molecular dynamics that depend on the distance to the surface. 8 Li turns out to be an excellent probe of these dynamics that occur on a wide range of time scales [43]. This also led to a collaboration with Joerg Rottler who is an expert in molecular dynamics within the UBC Stewart Blusson Quantum Matter Institute (SBQMI).…”