Deviations from Bulk Glass Transition Dynamics of Small Molecule Glass Formers: Some Scenarios in Relation to the Dimensionality of the Confining Geometry

Abstract

Deviations from the bulk behavior of the temperature dependence of the structural relaxation time $\tau$(T), typically well described by the Vogel-Fulcher-Tammann (VFT) relation, are the most obvious signature for “confinement effects,” which comprise various scenarios ranging from slight changes in the VFT parameters to cases where the VFT behavior completely breaks down to an Arrhenius law. An attractive idea that has finally stimulated many studies on nanoconfined glass formers is the concept of cooperativity with a cooperative length scale $\xi$ that dates back to Adams Gibbs and is also a subject of modern concepts for the description of glasses and supercooled liquids. In this chapter, we discuss the question in how far experiments on glass-forming liquids, confined to nanometer-sized geometries, can prove the cooperative nature and of the dynamic glass transition and provide quantitative information about its characteristic length scale $\xi$. Here, we have particularly focused on experimental evidence regarding deviations of $\tau$(T) from the bulk behavior. To classify experimental findings from very diverse systems, we introduce three major “deviation scenarios,” which range from a complete breakdown of the VFT-law (type I) via a VFT-Arrhenius cross-over scenario (type II) to a scenario (III) representing a perturbed VFT behavior in terms of an overall accelerated or retarded glass transition dynamics. Four cases are discussed in detail, all of them referring to H-bonding molecules or molecular groups. EG confined in various zeolitic hosts revealed either bulk dynamics or the scenario I, while EG mixed with amylopectine (starch) showed an evolution through the cross-over scenario (II) toward genuine Arrhenius behavior (I). A third system, representing a H-bonding “liquid” confined to a self-assembled (smectic) layer structure, yielded a clear-cut scenario II along with a linear relation between the layer thickness and the cross-over frequency. Subsequently, the dynamics of ultrathin films of glycerol having one free surface was discussed as an example for a perturbated VFT-scenario (type III). Apart from all molecular details and the diversity of confining host structures, it is evident that the strength of the confinement, expressed by the number of spatially restricted dimensions (1D $=$ layer, 3D $=$ cavities), should be linked to the severeness of changes in the dynamics. Inventarization of various experimental findings reported in the literature including them described above indeed confirm a correlation between the “deviation scenario” and the number of constrained axes. Finally, the effect of mechanisms other than cooperativity arguments, e.g., surface interactions, density, packing, and orientation effects, will briefly be discussed to rationalize obvious exceptions from the proposed correlation.