Glassy dynamics of soft matter under 1D confinement: How irreversible adsorption affects molecular packing, mobility gradients and orientational polarization in thin films

Abstract

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.

Cooling Rate Dependent Glass Transition in Thin Polymer Films and in Bulk

“…This is likely due to the fact that films deposited on a soft substrate, in the case of the work of Gao et al [57], Krytox oil, exhibit weak degree of adsorption. As mentioned, this constitutes a crucial parameter in determining the magnitude of T g deviations [75][76][77].…”

“…In such a case, differently from the vast majority of other reports, increase of the T g with decreasing thickness was found. Later the effect of the interfacial interaction polymer/substrate on the T g was highlighted by several studies [35,48,77,84,90,94,107,120] and summarized in a review [77]. Interestingly, at a given interfacial interaction polymer/substrate, the film T g can be tuned by annealing at temperatures considerably larger than the polymer bulk T g [75,76].…”

Probing the rheological properties of supported thin polystyrene films by investigating the growth dynamics of wetting ridges

“…The introduction of boundary effects, [18][19][20][21][22] finite size effects, 23,24 conformation and chain packing effects [25][26][27][28] and other associated new mechanisms, [29][30][31][32][33] due to thinning of the nanometer films, usually results in sufficiently more complex rheological behaviors that are not easily predictable using the bulk rules. The introduction of boundary effects, [18][19][20][21][22] finite size effects, 23,24 conformation and chain packing effects [25][26][27][28] and other associated new mechanisms, [29][30][31][32][33] due to thinning of the nanometer films, usually results in sufficiently more complex rheological behaviors that are not easily predictable using the bulk rules.…”

Effect of Confinement Geometry on Out-of-Equilibrium Glassy Dynamics

“…Several works systematically studied how the interaction between the polymer and substrate affects the magnitude of T g deviation from bulk behaviour. More recently the effect of interfacial interaction was studied in several experiments [47,70,115,126,139,149,167,185] and simulations [173,184]. By mean of ellipsometry, they showed that, depending on whether thin poly(methyl methacrylate) (PMMA) films were supported on gold or native silicone oxide wafers, depression or enhancement of T g was observed respectively.…”

“…For instance, it has been shown how films with the same thickness can exhibit more or less pronounced decrease and, in some cases, even increase of T g (see e.g. A glance on the effect of interfacial energy on T g deviations from bulk behaviour is provided by Napolitano et al [115]. 12.3).…”