Glycerol confined in zeolitic imidazolate frameworks: The temperature-dependent cooperativity length scale of glassy freezing

Abstract

In the present work, we employ broadband dielectric spectroscopy to study the molecular dynamics of the prototypical glass former glycerol confined in two microporous zeolitic imidazolate frameworks (ZIF-8 and ZIF-11) with well-defined pore diameters of 1.16 and 1.46 nm, respectively.The spectra reveal information on the modified  relaxation of the confined supercooled liquid, whose temperature dependence exhibits clear deviations from the typical super-Arrhenius temperature dependence of the bulk material, depending on temperature and pore size. This allows assigning well-defined cooperativity length scales of molecular motion to certain temperatures above the glass transition. We relate these and previous results on glycerol confined in other host systems to the temperature-dependent length scale deduced from nonlinear dielectric measurements. The combined experimental data can be consistently described by a critical divergence of this correlation length as expected within theoretical approaches assuming that the glass transition is due to an underlying phase transition.

Anormal Thermal History Effect on the Structural Dynamics of Probucol Infiltrated into Porous Alumina

“…One can recall that this deviation occurring at T g,interfacial has been rationalized in terms of, i.e., the limited length scale of dynamic cooperativity of molecular motions, 67,68 or the vitrification of molecules strongly interacting with the interface (the interfacial layer). 62 Although the results obtained previously for glycerol infiltrated in metal−organic frameworks (MOF) supported the first hypothesis, 69,70 there are many reports, also coming from our group, confirming the latter explanation that was validated based on the combined dielectric and calorimetric investigations. Note that the data collected for various glass formers infiltrated into AAO templates of different d revealed that the deviation of τ α (T)dependences occurring at T g,interfacial corresponds well with the calorimetric high glass transition temperature recorded in thermograms.…”

Supercooled water confined in a metal-organic framework

“…Using hosts with different pore diameters then can provide information about the temperature-dependent cooperativity length. 11 Therefore it seems natural to explain our results in confined water in the same way: Process 3 is the  relaxation of confined supercooled water (however, differing significantly from bulk water as discussed below) with T g  159 K. With decreasing temperature, at T cr  175 K its cooperativity length has grown to about 1.9 nm and, without confinement, would exceed the pore size under further cooling. Below T cr , the detected relaxation still reflects cooperative  dynamics, but with temperature-independent cooperativity because  is limited by the pore diameter d.…”

“…Such behavior is often found in supercooled liquids and can be used to estimate the size of the cooperativity length scale in dependence of temperature (see, e.g., Refs. 1,11,47). Within the framework of a cooperativity-driven glass transition, 72,73,74,75 this would imply that, for the temperature range where this process is detected (about 136 -180 K), the cooperativity length in bulk water always exceeds the pore size of about 1.2 nm.…”

“…MOFs have outstanding design versatility: For example, by using different organic linkers the pore sizes and the apertures between them can be varied. In two recent reports, 11,56 we have demonstrated that MOFs are well-suited host materials for confinement studies of the glass transition. These works concentrated on the cooperativity length scale of molecular motions in glycerol, a good glass former that can be easily supercooled even in its bulk form.…”

“…The confinement of liquids in spaces of nanometer size has proven a useful tool for the investigation of the glass transition. 1,2,3,4,5,6,7,8,9,10,11 Such experiments can provide valuable information about the increasingly cooperative nature of molecular motion assumed to arise during the gradual transition from a liquid into a glass under cooling.1 , 2 ,3, 6 ,11 Moreover, confinement studies enable to investigate the glassy and supercooled states of liquids that cannot be easily supercooled in their bulk form, avoiding crystallization when encaged in pores holding a limited number of molecules only. One of the most prominent examples in this respect is water, 12,13,14,15,16,17,18,19 a liquid of obvious relevance in numerous fields.…”