monitoring the impact of annealing on the dynamic glass transition of nanometres-thick polymer layers provides new insights into the mechanisms behind the tremendous changes in the performance of macromolecular materials in close proximity to an interface. Here we present results revealing a correlation between deviations from bulk behaviour, manifesting in changes to the glass transition temperature, the reduction of dielectric strength and the growth of an irreversibly adsorbed layer (Guiselin brushes). The non-universal behaviour of polymers under confinement could be explained in terms of a dimensionless number given by the ratio between the timescale of adsorption and the annealing time. In particular, in the case of slow adsorption kinetics, such as for polystyrene on aluminium, deviations from bulk behaviour correspond to metastable states with an extremely long lifetime.

When cooled or pressurized, polymer melts exhibit a tremendous reduction in molecular mobility. If the process is performed at a constant rate, the structural relaxation time of the liquid eventually exceeds the time allowed for equilibration. This brings the system out of equilibrium, and the liquid is operationally defined as a glass-a solid lacking long-range order. Despite almost 100 years of research on the (liquid/)glass transition, it is not yet clear which molecular mechanisms are responsible for the unique slow-down in molecular dynamics. In this review, we first introduce the reader to experimental methodologies, theories, and simulations of glassy polymer dynamics and vitrification. We then analyse the impact of connectivity, structure, and chain environment on molecular motion at the length scale of a few monomers, as well as how macromolecular architecture affects the glass transition of non-linear polymers. We then discuss a revised picture of nanoconfinement, going beyond a simple picture based on interfacial interactions and surface/volume ratio. Analysis of a large body of experimental evidence, results from molecular simulations, and predictions from theory supports, instead, a more complex framework where other parameters are relevant. We focus discussion specifically on local order, free volume, irreversible chain adsorption, the Debye-Waller factor of confined and confining media, chain rigidity, and the absolute value of the vitrification temperature. We end by highlighting the molecular origin of distributions in relaxation times and glass transition temperatures which exceed, by far, the size of a chain. Fast relaxation modes, almost universally present at the free surface between polymer and air, are also remarked upon. These modes relax at rates far larger than those characteristic of glassy dynamics in bulk. We speculate on how these may be a signature of unique relaxation processes occurring in confined or heterogeneous polymeric systems.

The behavior of polymers confined in ultrathin films (thickness < 200 nm) can sensitively differ from that observed in macroscopic samples. Based on the simple arguments of finite size and interfacial effects, film thickness, and surface interactions should be sufficient to univocally determine the deviation from bulk behavior. However, recent models suggest that a third key parameter, namely, the interfacial free volume, should also be considered. We describe a novel methodology that quantifies the volume available for structural relaxation at the interface between a thin polymer layer and its supporting substrate. Experiments performed at different annealing conditions verified that the shift in glass transition temperature, measured in thin films upon confinement, is proportional to the degree of adsorption and, thus, to the interfacial free volume.

Recent experimental evidence showed a strong correlation between the behavior of polymers under confinement and the presence of a layer irreversibly adsorbed onto the supporting substrate, hinting at the possibility to tailor the properties of ultrathin films by controlling the adsorption kinetics. At the state of the art, however, the study of physisorption of polymer melts is mainly limited to theory and simulations. To overcome this gap, we present the results of an extensive investigation of the kinetics of irreversible adsorption of entangled melts of polystyrene onto silicon oxide. We show that the process of chain pinning proceeds via a first order reaction mechanism, which slows down at large surface coverage, and the adsorbed amount scales with the predictions of reflected random walk. We propose an analytical form of the time evolution of the thickness of the adsorbed layer with two well-defined regimes: linear at short times and logarithmic at longer times, separated by a temperature independent crossover thickness and a molecular weight independent crossover time, in line with simulations and theory.

The structural dynamics of polymers and simple liquids confined at the nanometer scale has been intensively investigated in the last two decades in order to test the validity of theories on the glass transition predicting a characteristic length scale of a few nanometers. Although this goal has not yet been reached, the anomalous behavior displayed by some systems--e.g. thin films of polystyrene exhibit reductions of Tg exceeding 70 K and a tremendous increase in the elastic modulus--has attracted a broad community of researchers, and provided astonishing advancement of both theoretical and experimental soft matter physics. 1D confinement is achieved in thin films, which are commonly treated as systems at thermodynamic equilibrium where free surfaces and solid interfaces introduce monotonous mobility gradients, extending for several molecular sizes. Limiting the discussion to finite-size and interfacial effects implies that film thickness and surface interactions should be sufficient to univocally determine the deviation from bulk behavior. On the contrary, such an oversimplified picture, although intuitive, cannot explain phenomena like the enhancement of segmental mobility in proximity of an adsorbing interface, or the presence of long-lasting metastable states in the liquid state. Based on our recent work, we propose a new picture on the dynamics of soft matter confined in ultrathin films, focusing on non-equilibrium and on the impact of irreversibly chain adsorption on the structural relaxation. We describe the enhancement of dynamics in terms of the excess in interfacial free volume, originating from packing frustration in the adsorbed layer (Guiselin brush) at t(*) ≪ 1, where t(*) is the ratio between the annealing time and the time scale of adsorption. Prolonged annealing at times exceeding the reptation time (usually t(*) ≫ 1 induces densification, and thus reduces the deviation from bulk behavior. In this Colloquium, after reviewing the experimental approaches permitting to investigate the structural relaxation of films with one, two or no free surfaces by means of dielectric spectroscopy, we propose several methods to determine gradients of mobility in thin films, and then discuss on the unexploited potential of analyses based on the time, temperature and thickness dependence of the orientational polarization via the dielectric strength.

Confined at the nanoscale level, polymers crystallize much slower than in bulk, and in some cases, the formation of ordered structure is inhibited within extremely long experimental time scales. Here, we report on the thickness dependence of the conversion rate of the amorphous fraction of ultrathin films of poly(ethylene terephthalate) during isothermal cold crystallization. We present a new analytical method assessing the impact of irreversible chain adsorption and permitting to disentangle finite size and interfacial effects. From the μm range down to a few tens of nm, we observed an increase in crystallization time scaling with the inverse of the film thickness, which is a fingerprint of finite size effects. Films thinner than ∼20 nm did not crystallize, even after prolonged annealing in the temperature range where the crystallization rate reaches its maximum value. Noticing that this threshold corresponds to the total thickness of the layer irreversibly adsorbed within our investigation time, we explain these findings considering that chain adsorption increases the entropic barrier required for the formation of crystalline structures.

We employ a fluorescence bilayer method to directly measure the glass transition temperature (T g) of the irreversibly adsorbed layer of polystyrene (PS) buried in bulk films as a function of adsorption time, t ads. This bilayer geometry allows for the examination of interfacial effects on T g of the adsorbed nanolayer. In the presence of a free surface, we observe a substantial reduction in T g from bulk that lessens with t ads as a result of increased chain adsorption at the substrate. Submerging the adsorbed layer and effectively removing the free surface results in a suppression of the T g deviation at early t ads, suggesting chain adsorption dictates T g at long t ads. Annealing in the bilayer geometry promotes recovery of bulk T g on a time scale reflecting the degree of adsorption. Our data are quantitatively rationalized via the free volume holes diffusion model, which explains adsorbed nanolayer T g in terms of the diffusion of free volume pockets toward interfacial sinks.

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