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A β-NMR study of the depth, temperature and molecular-weight dependence of secondary dynamics in polystyrene. Entropy-enthalpy compensation and dynamic gradients near the free surface McKenzie, Iain; Fujimoto, Derek; Karner, Victoria L.; Li, Ruohong; MacFarlane, W. Andrew; McFadden, Ryan M. L.; Morris, Gerald D.; Pearson, Matthew R.; Raegen, Adam N.; Stachura, Monika; Ticknor, John O.; Forrest, James A.

Abstract

We investigated the depth, temperature and molecular-weight (MW) dependence of the γ-relaxation in polystyrene glasses using implanted 8Li+ and β-detected NMR (β-NMR). Measurements were performed on thin films with MW ranging from 1.1 to 641 kg/mol. The temperature dependence of the average 8Li spin-lattice relaxation time (Tavg1) was measured near the free surface and in the bulk. Spin-lattice relaxation is caused by phenyl ring flips, which involve transitions between local minima over free-energy barriers with enthalpic and entropic contributions. We used transition state theory to model the temperature dependence of the γ-relaxation, and hence Tavg1. There is no clear correlation of the average entropy of activation (∆‡S) and enthalpy of activation (∆‡H) with MW, but there is a clear correlation between ∆‡S and ∆‡H, i.e. entropy-enthalpy compensation. This results in the average Gibbs energy of activation, ∆‡G, being approximately independent of MW. Measurements of the temperature dependence of Tavg1 as a function of depth below the free surface indicate the inherent entropic barrier, i.e. the entropy of activation corresponding to ∆‡H = 0, has an exponential dependence on distance from the free surface before reach ing the bulk value. This results in ∆‡G near the free surface being lower than the bulk. Combining these observations results in a model where the average fluctuation rate of the γ-relaxation has a “double-exponential” depth dependence. This model can explain the depth dependence of 1/Tavg1in polystyrene films. The characteristic length of enhanced dynamics is ∼6 nm and approximately independent of MW near room temperature.

Item Metadata

Title
A β-NMR study of the depth, temperature and molecular-weight dependence of secondary dynamics in polystyrene. Entropy-enthalpy compensation and dynamic gradients near the free surface
Creator
McKenzie, Iain; Fujimoto, Derek; Karner, Victoria L.; Li, Ruohong; MacFarlane, W. Andrew; McFadden, Ryan M. L.; Morris, Gerald D.; Pearson, Matthew R.; Raegen, Adam N.; Stachura, Monika; Ticknor, John O.; Forrest, James A.
Contributor
The Stewart Blusson Quantum Matter Institute
Date Issued
2022-01-31
Description
We investigated the depth, temperature and molecular-weight (MW) dependence of the γ-relaxation in polystyrene glasses using implanted 8Li+ and β-detected NMR (β-NMR). Measurements were performed on thin films with MW ranging from 1.1 to 641 kg/mol. The temperature dependence of the average 8Li spin-lattice relaxation time (Tavg1) was measured near the free surface and in the bulk. Spin-lattice relaxation is caused by phenyl ring flips, which involve transitions between local minima over free-energy barriers with enthalpic and entropic contributions. We used transition state theory to model the temperature dependence of the γ-relaxation, and hence Tavg1. There is no clear correlation of the average entropy of activation (∆‡S) and enthalpy of activation (∆‡H) with MW, but there is a clear correlation between ∆‡S and ∆‡H, i.e. entropy-enthalpy compensation. This results in the average Gibbs energy of activation, ∆‡G, being approximately independent of MW. Measurements of the temperature dependence of Tavg1 as a function of depth below the free surface indicate the inherent entropic barrier, i.e. the entropy of activation corresponding to ∆‡H = 0, has an exponential dependence on distance from the free surface before reach ing the bulk value. This results in ∆‡G near the free surface being lower than the bulk. Combining these observations results in a model where the average fluctuation rate of the γ-relaxation has a “double-exponential” depth dependence. This model can explain the depth dependence of 1/Tavg1in polystyrene films. The characteristic length of enhanced dynamics is ∼6 nm and approximately independent of MW near room temperature.
Genre
Article; Postprint
Type
Text
Language
eng
Date Available
2025-04-23
Provider
Vancouver : University of British Columbia Library
Rights
Attribution-NonCommercial-NoDerivatives 4.0 International
DOI
10.14288/1.0448525
URI
http://hdl.handle.net/2429/90739
Affiliation
Science, Faculty of; TRIUMF; Non UBC; Chemistry, Department of; Physics and Astronomy, Department of
Citation
McKenzie, I., Fujimoto, D., Karner, V. L., Li, R., MacFarlane, W. A., McFadden, R. M. L., Morris, G. D., Pearson, M. R., Raegen, A. N., Stachura, M., Ticknor, J. O., & Forrest, J. A. (2022). A β-NMR study of the depth, temperature, and molecular-weight dependence of secondary dynamics in polystyrene: Entropy–enthalpy compensation and dynamic gradients near the free surface. The Journal of Chemical Physics, 156(8).
Publisher DOI
10.1063/5.0081185
Peer Review Status
Reviewed
Scholarly Level
Faculty; Postdoctoral; Graduate
Rights URI
http://creativecommons.org/licenses/by-nc-nd/4.0/
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Attribution-NonCommercial-NoDerivatives 4.0 International