More Like this

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

   https://doi.org/10.1073/pnas.2104398118  

   Ghanekarade, Asieh; Phan, Anh D.; Schweizer, Kenneth S.; Simmons, David S. (July 2021, Proceedings of the National Academy of Sciences)

   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. 

   more » « less
2. Interplay between dynamic heterogeneity and interfacial gradients in a model polymer film

   https://doi.org/10.1063/5.0165650  

   Hartley, Austin D; Drayer, William F; Ghanekarade, Asieh; Simmons, David S (November 2023, The Journal of Chemical Physics)

   Glass-forming liquids exhibit long-lived, spatially correlated dynamical heterogeneity, in which some nm-scale regions in the fluid relax more slowly than others. In the nanoscale vicinity of an interface, glass-formers also exhibit the emergence of massive interfacial gradients in glass transition temperature Tg and relaxation time τ. Both of these forms of heterogeneity have a major impact on material properties. Nevertheless, their interplay has remained poorly understood. Here, we employ molecular dynamics simulations of polymer thin films in the isoconfigurational ensemble in order to probe how bulk dynamic heterogeneity alters and is altered by the large gradient in dynamics at the surface of a glass-forming liquid. Results indicate that the τ spectrum at the surface is broader than in the bulk despite being shifted to shorter times, and yet it is less spatially correlated. This is distinct from the bulk, where the τ distribution becomes broader and more spatially organized as the mean τ increases. We also find that surface gradients in slow dynamics extend further into the film than those in fast dynamics—a result with implications for how distinct properties are perturbed near an interface. None of these features track locally with changes in the heterogeneity of caging scale, emphasizing the local disconnect between these quantities near interfaces. These results are at odds with conceptions of the surface as reflecting simply a higher “rheological temperature” than the bulk, instead pointing to a complex interplay between bulk dynamic heterogeneity and spatially organized dynamical gradients at interfaces in glass-forming liquids. 

   more » « less
3. Glass formation and dynamics of model polymer films with one versus two active interfaces

   https://doi.org/10.1039/D3SM00719G  

   Ghanekarade, Asieh; Simmons, David S (November 2023, Soft Matter)

   Polymers and other glass-forming liquids can exhibit profound alterations in dynamics in the nanoscale vicinity of interfaces, over a range appreciably exceeding that of typical interfacial thermodynamic gradients. The understanding of these dynamical gradients is particularly complicated in systems with internal or external nanoscale dimensions, where a gradient nucleated at one interface can impinge on a second, potentially distinct, interface. To better understand the interactions that govern system dynamics and glass formation in these cases, here we simulate the baseline case of a glass-forming polymer film, over a wide range of thickness, supported on a dynamically neutral substrate that has little effect on nearby dynamics. We compare these results to our prior simulations of freestanding films. Results indicate that dynamical gradients in our simulated systems, as measured based upon translational relaxation, are simply truncated when they impinge on a secondary surface that is locally dynamically neutral. Altered film behavior can be described almost entirely by gradient effects down to the thinnest films probed, with no evidence for finite-size effects sometimes posited to play a role in these systems. Finally, our simulations predict that linear gradient overlap effects in the presence of symmetric dynamically active interfaces yield a non-monotonic variation of the whole free standing film stretching exponent (relaxation time distribution breadth). The maximum relaxation time distribution breadth in simulation is found at a film thickness of 4–5 times the interfacial gradient range. Observation of this maximum in experiment would provide an important validation that the gradient behavior observed in simulation persists to experimental timescales. If validated, observation of this maximum would potentially also enable determination of the dynamic gradient range from experimental mean-film measurements of film dynamics. 

   more » « less
4. Narrow acoustic impedance matching layer accelerates long-range coupling of local modulus between glassy and rubbery polymer domains

   https://doi.org/10.1063/5.0320844  

   Couturier, Alexander A; Cai, David B; Burton, Justin C; Roth, Connie B (April 2026, Journal of Chemical Physics)

   We test and demonstrate that the fundamental origin for long-range ∼200 nm local property gradients between glassy and rubbery polymer domains is an acoustic impedance matching framework of boson peak wavelengths. Recent research has suggested that the transmission of λ ∼ 5 nm acoustic waves across glassy-rubbery interfaces is the cause of dynamical gradients leading to the broad ≈250 nm glass transition Tg(z) and modulus G̃(z) profiles observed between polymer domains. These acoustic waves with wavelength λ ∼ 5 nm are of comparable energy to the boson peak and the collective vibrations associated with precursors of structural rearrangements. In this work, we directly test this proposed conceptual framework by inserting a 5 nm impedance-matching random copolymer layer at the interface between glassy polystyrene (PS) and rubbery polybutadiene (PB) domains. The resulting viscoelastic changes of PS/PB bilayer films are studied using a quartz crystal microbalance, where an acoustic transfer-matrix continuum mechanics model is used to determine the change in depth-dependent modulus G̃(z) imparted by the added 5 nm styrene-butadiene copolymer P(S-r-B). We find the emergence of the broad modulus gradient to be accelerated by the added 5 nm copolymer layer. These results demonstrate a direct correlation between compositional interfacial widths wI ≈ 5–10 nm and the broad ≈100–300 nm G̃(z) modulus gradient that emerges between glassy and rubbery domains, supporting the idea that it is the transmission of λ ∼ 5 nm acoustic waves across dynamically distinct domains that is key to the mechanism behind the coupling of local properties between them. More broadly, these findings provide new insight into the fundamental nature and mechanisms behind the length scales impacting local dynamical heterogeneity of glasses. 

   more » « less
5. Comparison of Physical Aging and Glass Transition in Glassy–Rubbery Polymer Bilayer Films

   https://doi.org/10.1021/acs.jpcb.4c07902  

   McGuire, Jennifer A; Merrill, James H; Couturier, Alexander A; Thees, Michael F; Roth, Connie B (March 2025, Journal of Physical Chemistry B)

   In the present work, we use ellipsometry to extract the physical aging response of thin glassy polystyrene (PS) layers from rubbery–glassy bilayer films of poly(n-butyl methacrylate) (PnBMA) atop PS. How the soft interface between rubbery and glassy polymer domains can impact the physical aging response of glassy domains is unclear. Measurements in the literature have shown that the local glass transition temperature Tg of PS is strongly reduced near a PnBMA/PS interface with a magnitude twice as large compared to that imparted by a free surface. As the free surface is known to reduce physical aging, we anticipated large changes in the physical aging response of PS within PnBMA/PS bilayer films. However, surprisingly the aging response remained equivalent to bulk down to 75 nm PS layer thicknesses that were the thinnest we found could be accurately measured given the optical limits of dispersion. With complementary fluorescence measurements, we show that the average Tg(hPS) of such PS layers within 150 nm PnBMA/75 nm PS bilayer films are also still bulk. These findings demonstrate that films with finite domain sizes have interfacial dynamical gradients that are significantly altered from those previously measured in systems with semi-infinite domain sizes. 

   more » « less