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Molecular Mobility of Amorphous N-Acetyl-α-Methylbenzylamine and Debye Relaxation evidenced by Dielectric Relaxation Spectroscopy and Molecular Dynamic Simulations

Abstract

The present work focusses on the molecular mobility characterization of amorphous N-acetyl-α-methylbenzylamine (Nac-MBA) by Broadband Dielectric Spectroscopy (BDS) coupled to Fast Scanning Calorimetry (FSC) and Molecular Dynamic (MD) Simulations covering over 12 decades in the frequency range. This study revealed four relaxation processes: from the fast γ secondary process, to a slow βJG followed by the structural α relaxation and a very intense Debye contribution (Almost 90% of the global dielectric intensity). The relaxation time of both secondary processes displays an Arrhenius temperature dependence behavior while α and Debye (D) relaxations follow a Vogel Fulcher Tammann Hesse (VFTH) evolution. The cooling rate dependency of the fictive temperature Tf obtained from (FSC) mimics with an excellent agreement the VFTH behavior of structural relaxation. The relaxation times derived from MD simulations are consistent with the coaxial-line reflectometry measurements. Furthermore the dielectric glass transition Tg (τ=100 s)=239.65 ±0.20 K is in good accordance with the calorimetric value Tg=240.65 ±0.50 K, and the calculated fragility index m=48±4 permitted to classify Nac-MBA as an intermediate glass forming liquid. The Vogel temperature and the pre-exponential factor derived from both VFTH laws are quite similar; indicating some correlations between the two dynamics even though their microscopic origins are obviously different. It is confirmed from MD simulations that the exponential intense D process emanates from the formation of linear intermolecular H-bonding aggregates (precursor of the crystalline structure) leading to parallel neighboring dipole orientation. The relaxational motion of the resulting effective dipole in a cooperative environment is expressed as the exponential D process.

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Publication details

The article was received on 31 Jul 2018, accepted on 03 Dec 2018 and first published on 03 Dec 2018


Article type: Paper
DOI: 10.1039/C8CP04880K
Citation: Phys. Chem. Chem. Phys., 2018, Accepted Manuscript
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    Molecular Mobility of Amorphous N-Acetyl-α-Methylbenzylamine and Debye Relaxation evidenced by Dielectric Relaxation Spectroscopy and Molecular Dynamic Simulations

    B. Atawa, N. T. T. Correia, N. Couvrat, F. Affouard, G. Coquerel, E. Dargent and A. Saiter, Phys. Chem. Chem. Phys., 2018, Accepted Manuscript , DOI: 10.1039/C8CP04880K

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