Nature of dynamic gradients, glass formation, and collective effects in ultrathin freestanding films

Abstract

Molecular, polymeric, colloidal, and other classes of liquids can exhibit very large, spatially heterogeneous alterations of their dynamics and glass transition temperature when confined to nanoscale domains. Considerable progress has been made in understanding the related problem of near-interface relaxation and diffusion in thick films. However, the origin of “nanoconfinement effects” on the glassy dynamics of thin films, where gradients from different interfaces interact and genuine collective finite size effects may emerge, remains a longstanding open question. Here, we combine molecular dynamics simulations, probing 5 decades of relaxation, and the Elastically Cooperative Nonlinear Langevin Equation (ECNLE) theory, addressing 14 decades in timescale, to establish a microscopic and mechanistic understanding of the key features of altered dynamics in freestanding films spanning the full range from ultrathin to thick films. Simulations and theory are in qualitative and near-quantitative agreement without use of any adjustable parameters. For films of intermediate thickness, the dynamical behavior is well predicted to leading order using a simple linear superposition of thick-film exponential barrier gradients, including a remarkable suppression and flattening of various dynamical gradients in thin films. However, in sufficiently thin films the superposition approximation breaks down due to the emergence of genuine finite size confinement effects. ECNLE theory extended to treat thin films captures the phenomenology found in simulation, without invocation of any critical-like phenomena, on the basis of interface-nucleated gradients of local caging constraints, combined with interfacial and finite size-induced alterations of the collective elastic component of the structural relaxation process.

Confinement effects on the spatially inhomogeneous dynamics in metallic glass films

“…( 4) recovers to the formula in Ref. [4], which is In previous works [22,27], authors found that the collective elastic barrier characterizing for effects of cooperative motions on the glass transition is almost zero in bulk metallic glasses. We suppose the conclusion remains unchanged in the metallic glass films.…”

“…[8]. The flattening of dynamical gradients in the midfilm region has been predicted by both simulation and theory for polymer free-standing films [4]. In thicker films, the interference becomes smaller and the mobility gradient approximately behaves as if an oneinterface system.…”

“…From these, we determine how mobility changes and explain why the relaxation time gradient for free-standing and supported polymer films obeys a "double-exponential" form suggested by simulations few decades ago [5,6]. While finite-size effects on the inhomogeneous glassy dynamics in free-standing polymer films have been recently investigated [4], problems for capped films have not been addressed yet, particularly when material is metallic glass. It is still unclear how two solid interfaces cause nonadditive interference effects to modify mobility gradients and invalidate the "double-exponential" form, how strong the interfacial coupling is when the films are thinner, how good an ansatz superposition approximation [5] is when modelling a normalized alpha relaxation time gradient of metallic glass films at different thicknesses.…”

“…After several decades of continuous debate, the activated dynamics of thin glass-forming films remains poorly understood due to complexities of confinement effects and diverse applications [1][2][3][4][5][6][7][8][9][10][11]. Vapor interfaces universally accelerates relaxation nucleated from free surfaces towards the interior.…”

“…We extended the ECNLE theory to predict the spatial gradient of the alpha relaxation time and glass transition temperature in polymer film systems having vapor and solid surface [1][2][3][4]. Our new treatment describes the spatial propagation of caging constraints from the surface into the film interior.…”