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.
