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

The static and dynamic properties of polymer-layered silicate nanocomposites are discussed, in the context of polymers in confined spaces and polymer brushes. A wide range of experimental techniques as applied to these systems are reviewed, and the salient results from these are compared with a mean field thermodynamic model and non-equilibrium molecular dynamics simulations.

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Polymer-silicate nanocomposites : Model systems for confined polymers and polymer brushes

Despite the decade-long study of the effect of nanoconfinement on the glass-transition temperature (Tg) of amorphous materials, the quest to probe the distribution of Tgs in nanoconfined glass formers has remained unfulfilled. Here the distribution of Tgs across polystyrene films has been obtained by a fluorescence/multilayer method, revealing that the enhancement of dynamics at a surface affects Tg several tens of nanometres into the film. The extent to which dynamics smoothly transition from enhanced to bulk states depends strongly on nanoconfinement. When polymer films are sufficiently thin that a reduction in thickness leads to a reduction in overall Tg, the surface-layer Tg actually increases with a reduction in overall thickness, whereas the substrate-layer Tg decreases. These results indicate that the gradient in Tg dynamics is not abrupt, and that the size of a cooperatively rearranging region is much smaller than the distance over which interfacial effects propagate.

The distribution of glass-transition temperatures in nanoscopically confined glass formers

We have measured the thickness dependence of the glass-transition temperature (Tg) of thin films of poly(methyl methacrylate)(PMMA) by using spectroscopic ellipsometry to detect the discontinuity in thermal expansivity occurring at Tg. We studied films on two surfaces: the native oxide of silicon, and evaporated gold. The Tg of PMMA on a gold surface decreases with decreasing film thickness, in accordance with previous results for polystyrene on silicon. We suggest that at the air surface a liquid-like layer exists whose size diverges as Tg is approached from below. For films of PMMA on the native oxide of silicon, however, we find a slight increase in Tg with decreasing thickness. We speculate that hydrogen bonding at the interface restricts mobility and leads to an increase in Tg, outweighing the effect of the free surface.

Interface and surface effects on the glass-transition temperature in thin polymer films

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

The glass transition temperature of thin polystyrene films has been measured as a function of film thickness. It is found that the glass transition decreases in temperature as the thickness of the film is reduced. The effect is not strongly molecular-weight dependent, ruling out chain confinement as the major cause; instead we suggest that at the surface of the glassy film a liquidlike layer exists whose size diverges as the glass transition temperature is approached from below.

https://iopscience.iop.org/article/10.1209/0295-5075/27/1/011/pdf

Size-Dependent Depression of the Glass Transition Temperature in Polymer Films

The Controlled Evolution of a Polymer Single Crystal

Spectroscopic ellipsometry has been used to measure the glass transition temperature of thin polymer films. Polystyrene films on a silicon substrate show a substantial molecular weight independent depression of glass transition temperature for films of decreasing thickness. Poly (methyl methacrylate) films show an increase when prepared on a silicon native oxide surface, but a decrease on gold substrates. The competing roles of the free surface, interactions with the substrate and finite size effects in determining the dynamics of polymers in confined geometries are discussed.

R. A. Cory

Papers

Size-Dependent Depression of the Glass Transition Temperature in Polymer Films

The Controlled Evolution of a Polymer Single Crystal

Polymer Dynamics in Thin Films