Effect of adding nanometre-sized heterogeneities on the structural dynamics and the excess wing of a molecular glass former.

S Gupta,J K H Fischer,M Ohl,N Jalarvo,P Lunkenheimer,E Novak,A Loidl

doi:10.1038/srep35034

Abstract

We present the relaxation dynamics of glass-forming glycerol mixed with 1.1 nm sized polyhedral oligomeric silsesquioxane (POSS) molecules using dielectric spectroscopy (DS) and two different neutron scattering (NS) techniques. Both, the reorientational dynamics as measured by DS and the density fluctuations detected by NS reveal a broadening of the α relaxation when POSS molecules are added. Moreover, we find a significant slowing down of the α-relaxation time. These effects are in accord with the heterogeneity scenario considered for the dynamics of glasses and supercooled liquids. The addition of POSS also affects the excess wing in glycerol arising from a secondary relaxation process, which seems to exhibit a dramatic increase in relative strength compared to the α relaxation.

Highlights

It should be noted that the JG relaxation and the excess wing (EW) are distinctively different from the well-known fast β process as predicted by mode-coupling theory (MCT), which has proven to be successful in explaining some of the most interesting phenomena of glassy dynamics[20]
The added hybrid-plastic polyhedral oligomeric silsesquioxane (POSS) molecules consist of a silicon-oxygen cage (Si8O12) in which the Si atoms occupy the corners of a cube and are bonded to oxygen atoms which are located on the edges
V is the volume of the POSS molecule, r is the hard sphere radius, and Δρ =ρPOSS −ρsolvent is the contrast difference between POSS and the solvent

Summary

Introduction

It should be noted that the JG relaxation and the EW are distinctively different from the well-known fast β process as predicted by mode-coupling theory (MCT), which has proven to be successful in explaining some of the most interesting phenomena of glassy dynamics[20]. It gave birth to the concept of dynamic heterogeneities[22,23,24,25] or spatially heterogeneous dynamics[26] It leads to a distribution of relaxation times, i.e., each molecule relaxes with exponential time dependence as in the Debye theory, but the relaxation times are different for different molecules. This explains one of the hallmark features of glassy dynamics, the non-exponentiality, which shows up, e.g., by a broadening of the α peaks in dielectric-loss spectra. The influence of added POSS molecules on glassy dynamics has been investigated in several previous works[28,29,33,34]

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Results

Conclusion