Does fragility of glass formation determine the strength of T g-nanoconfinement effects?

Abstract

Nanoscale confinement has been shown to alter the glass transition and associated mechanical and transport properties of glass-forming materials. Inspired by expected interrelations between nanoconfinement effects, cooperative dynamics in supercooled liquids, and the "fragility" (or temperature-abruptness) of the glass transition, it is commonly expected that nanoconfinement effects on T should be more pronounced for more fragile glass formers. Here we employ molecular dynamics simulations of glass formation in the bulk and under nanoconfinement of model polymers in which we systematically tune fragility by several routes. Results indicate that a correlation between fragility and the strength of nanoconfinement effects is weak to modest at best when considering all systems but can appear to be stronger when considering a subset of systems. This outcome is consistent with a reanalysis of the Adam-Gibbs theory of glass formation indicating that fragility does not necessarily track in a universal way with the scale of cooperative motion in glass-forming liquids. Finally, we find that factors such as composition gradients or variability in measurement sensitivity to different parts of the dynamic gradient have the potential to significantly confound efforts to identify trends in T-nanoconfinement effects with variables such as fragility, emphasizing the importance of employing diverse data sets and multiple metrologies in the study of this problem.

A structural interpretation of the two components governing the kinetic fragility from the example of interpenetrated polymer networks

“…In others cases, the fragility appears to be mostly dominated by the rigidity of the system (which is also the case of the present study, where the rigidity of the samples is varied through the degree of crosslinking). This work further confirms that fragility does not necessarily provide the same structural information as the study of cooperative motions in glass‐forming liquids, and that these approaches should be considered as complementary rather than interchangeable …”

String-like collective motion in the α- and β-relaxation of a coarse-grained polymer melt

“…3) is relevant to the approach of Adam and Gibbs since L has been observed to increase in direct proportion to the activation free energy G(T ) of the structural relaxation time determined from F coh (q, t) (or other relaxation functions). [35][36][37][38]60,61 Accordingly, the string model of glassformation 38 generalizes the Adam-Gibbs model [Eq. 3], where ⌧ ↵ can be quantitatively described by…”

“…38 and 60, which has also been validated for a range of model systems at various thermodynamic conditions. [36][37][38]53,[60][61][62][63] ) Equation (4) describes ⌧ ↵ quantitatively, where the high temperature activation free energy µ (the only free parameter) is determined by fitting Eq. (4) to simulation data.…”

Mechanical and viscoelastic properties of confined amorphous polymers

“…There is appreciable evidence from both experiments and simulations that the properties of confined polymers are actually a distribution of properties due to gradients that extend from the interfaces. The reported property for a given polymer film is an average of this distribution, which has been demonstrated to be technique dependent for T g .…”

“…As fragility is related to the glass forming process, there are some general correlations between the stiffness of the polymer and fragility . However, fragility also can be impacted by nanoconfinement with experiments and simulations indicating a decrease in fragility with confinement and recent simulations demonstrate only a weak correlation between fragility and the strength of the nanoconfinement effect . Experimentally, the relationship between fragility of the bulk polymer and nanoconfinement effects appears to break down for mechanical properties when examining polymers with nonlinear architectures .…”