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The interfacial dynamics of amorphous materials as revealed by beta-NMR measurements and molecular simulations Fujimoto, Derek Jun


The free surface is important for developing a fundamental understanding of dynamical length scales in glasses. We first investigate the relaxation of freestanding atactic polystyrene (aPS) thin films with molecular dynamics simulations. As in previous coarse-grained simulations, the surface modification on the relaxation times for backbone segments and phenyl rings may be expressed as a power law relation, wherein the bulk dynamics fully encapsulate the temperature-dependence. Variation of the coupling exponent with distance from the surface is consistent with depth-dependent activation barriers. We also quantify a reduction of dynamical heterogeneity, transient spatial fluctuations of the dynamics, at the interface which can be interpreted in the framework of cooperative models for glassy dynamics. Capable of depth-resolved measurements near the surface, implanted-ion beta-detected nuclear magnetic resonance (𝛽-NMR) has been a powerful technique for the study of dynamics in aPS thin films. We have completed and commissioned an upgrade to the 𝛽-NMR spectrometer, extending the accessible upper temperature, and enabling a direct comparison between this experimental technique and the molecular dynamics simulations. We demonstrate that the modified spectrometer is now capable of operation to at least 400 K, an improvement of more than 80 K. We also demonstrate the application of 𝛽-NMR as a probe of ionic liquid molecular dynamics through the measurement of ⁸Li⁺ spin-lattice relaxation (SLR) and resonance in 1-ethyl-3-methylimidazolium acetate. The motional narrowing of the resonance, and the local maxima in the SLR rate, 1/T₁, imply a sensitivity to sub-nanosecond Li⁺ solvation dynamics. From an analysis of 1/T₁, we extract an activation energy and Vogel-Fulcher-Tammann constant in agreement with the dynamic viscosity of the bulk solvent. Near the melting point, the lineshape is broad and intense, and the form of the relaxation is non-exponential, reflective of our sensitivity to heterogeneous dynamics near the glass transition. We also employ the depth resolution capabilities of this technique to probe the subsurface dynamics with nanometer resolution. We show modified dynamics near the surface in, and above, the glassy state.

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