Although it has long been recognized that dynamics in supercooled liquids might be spatially heterogeneous, only in the past few years has clear evidence emerged to support this view. As a liquid is cooled far below its melting point, dynamics in some regions of the sample can be orders of magnitude faster than dynamics in other regions only a few nanometers away. In this review, the experimental work that characterizes this heterogeneity is described. In particular, the following questions are addressed: How large are the heterogeneities? How long do they last? How much do dynamics vary between the fastest and slowest regions? Why do these heterogeneities arise? The answers to these questions influence practical applications of glass-forming materials, including polymers, metallic glasses, and pharmaceuticals.

Spatially Heterogeneous Dynamics in Supercooled Liquids

In this article, the effects of size and confinement at the nanometre size scale on both the melting temperature, Tm, and the glass transition temperature, Tg, are reviewed. Although there is an accepted thermodynamic model (the Gibbs–Thomson equation) for explaining the shift in the first-order transition, Tm, for confined materials, the depression of the melting point is still not fully understood and clearly requires further investigation. However, the main thrust of the work is a review of the field of confinement and size effects on the glass transition temperature. We present in detail the dynamic, thermodynamic and pseudo-thermodynamic measurements reported for the glass transition in confined geometries for both small molecules confined in nanopores and for ultrathin polymer films. We survey the observations that show that the glass transition temperature decreases, increases, remains the same or even disappears depending upon details of the experimental (or molecular simulation) conditions. Indeed, different behaviours have been observed for the same material depending on the experimental methods used. It seems that the existing theories of Tg are unable to explain the range of behaviours seen at the nanometre size scale, in part because the glass transition phenomenon itself is not fully understood. Importantly, here we conclude that the vast majority of the experiments have been carried out carefully and the results are reproducible. What is currently lacking appears to be an overall view, which accounts for the range of observations. The field seems to be experimentally and empirically driven rather than responding to major theoretical developments.

https://iopscience.iop.org/article/10.1088/0953-8984/17/15/R01/pdf

Effects of confinement on material behaviour at the nanometre size scale

Dynamics of supercooled liquids and glassy solids

Nanophase separation on length scales of 1–5 nanometres has been reported previously for small-molecule liquids1, metallic glasses and also for several semicrystalline, liquid-crystalline2,3 and amorphous4 polymers. Here we show that nanophase separation of incompatible main and side-chain parts is a general phenomenon in amorphous side-chain polymers with long alkyl groups. We conclude from X-ray scattering and relaxation spectroscopy data for higher poly(n-alkyl acrylates) (PnAA) and poly(n-alkyl methacrylates) (PnAMA) that alkyl groups of different monomeric units aggregate in the melt and form self-assembled alkyl nanodomains with a typical size of 0.5–2 nm. A comparison with data for other polymer series having alkyl groups reveals that important structural and dynamic aspects are main-chain independent. A polyethylene-like glass transition within the alkyl nanodomains is observed and discussed in the context of a hindered glass transition in self-assembled confinements. This is an interesting link between central questions in glass-transition research and structural aspects in nanophase-separated materials.

Nanophase separation and hindered glass transition in side-chain polymers

Near their glass transition temperature T(g), supercooled liquids display dramatic changes regarding the dynamics if subject to geometrical restrictions on the scale of 2 to 200 nm. Confinement-induced shifts of T(g) of 25 K have been reported, equivalent to relaxation times that differ by several orders of magnitude compared with the bulk liquid at the same temperature. Both acceleration and frustration of structural relaxations have been observed, and the effects can depend strongly on the physical and chemical properties of the interface, on soft versus hard confinement, and on the size and dimensionality of the confining topology. This review attempts to extract a unifying picture from the past 20 years of diverse observations that involve experiments, simulations, and model considerations.

Dynamics of Nanoconfined Supercooled Liquids

Molecular Dynamics in Confining Space: From the Single Molecule to the Liquid State

Broad-band dielectric spectroscopy is employed to study the molecular dynamics of (dielectrically active) glass-forming liquids which are confined to (dielectrically inactive) zeolites and nanoporous glasses. For the H-bond-forming liquid ethylene glycol (EG) embedded in zeolites of different sizes and topologies one observes a sharp transition from a single-molecule dynamics (with an Arrhenius-type temperature dependence) to that of a liquid (with a temperature dependence of the mean relaxation rate following a Vogel-Fulcher-Tammann (VFT) law): while EG in silicalite (showing a single-molecule relaxation) has a coordination number of four, EG in zeolite beta or -5 has a coordination number of five and behaves like a bulk liquid. For the H-bonded liquid propylene glycol confined to (uncoated and silanized) nanopores (pore sizes: 2.5 nm, 5.0 nm and 7.5 nm), a molecular dynamics is observed which is comparable to that of the bulk liquid. Due to surface effects in uncoated nanopores, the relaxation time distribution is broadened on the long-term side and the mean relaxation rate is decreased by about half a decade. This effect can be counterbalanced by lubricating the inner surfaces of the pores. That causes the molecular dynamics of the molecules inside the pores to decouple from the solid walls and the resulting relaxation rate becomes slightly faster compared to that for the bulk liquid. For the `quasi'-van der Waals liquid salol confined to silanized nanopores, the molecular dynamics is completely different to that for the H-bonded systems: over a wide temperature range, the dynamics of the confined system is identical to that of the bulk liquid. But with decreasing temperature, a sharp pore-size-dependent transition is found from a VFT-type to an Arrhenius-type temperature dependence. This reflects the inherent length scale of cooperativity of the dynamic glass transition.

https://iopscience.iop.org/article/10.1088/0953-8984/11/10A/013/pdf

How many molecules form a liquid

Broadband dielectric spectroscopy (10−2 Hz - 109Hz) is employed to study the molecular dynamics of low-molecular-weight glassforming liquids being confined to nanopores. For the H-bond forming liquid propylene glycol being confined to (uncoated and silanized) nanopores (pore size: 2.5 nm, 5.0 nm and 7.5 nm) a molecular dynamics is observed which is comparable to that of the bulk liquid. Due to surface effects in uncoated nanopores the relaxation time distribution is broadened on the long term side and the mean relaxation rate is decreased by about half a decade. This effect can be counterbalanced by lubricating the inner surfaces of the pores resulting in a relaxation rate which is slightly faster compared to the bulk liquid. For the H-bonded liquid ethylene glycol (EG) embedded in zeolites of different pore size and topology one observes a sharp transition from a single-molecule dynamics to that of a liquid depending on the coordination number of the confined molecules. While EG in silicalite (showing a single molecule relaxation) has four neighboring molecules, EG in zeolite beta or AIPO4-5 has a coordination number of five and behaves like a bulk liquid.

W. Schwieger

Papers

Molecular Dynamics in Confining Space: From the Single Molecule to the Liquid State

How many molecules form a liquid

Glass Transition in Sub-nanometer Confinement