A unifying framework to quantify the effects of substrate interactions, stiffness, and roughness on the dynamics of thin supported polymer films

Hanakata, Paul Z.; Betancourt, Beatriz A. Pazmiño; Douglas, Jack F.; Starr, Francis W.

doi:10.1063/1.4922481

Cited by 130 publications

(198 citation statements)

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“…Small values of K result in large fluctuations of the substrate beads around their position on the lattice sites, which corresponds to small substrate stiffness (soft substrates), whereas large values of K result in strong tethering of the substrate beads to their lattice sites resulting in a large stiffness (hard substrate) [9]. The stiffness gradient is implemented along the x-direction (see Fig.…”

Section: The Modelmentioning

confidence: 99%

“…To this end, many methods have been explored, for example heterogeneous surface chemistries for different patterns [7][8][9], temperature and electric potential gradients [2,6], and surface topography [10][11][12][13][14]. Other techniques for moving nanoscale objects are based on electrical current [15][16][17][18], charge [19][20][21], thermal energy (selective heating) [22][23][24], simple stretch [25], and complicated chemical reactions (e.g.…”

Section: Introductionmentioning

confidence: 99%

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Stiffness-guided motion of a droplet on a solid substrate

Theodorakis

Егоров

Milchev

2017

The Journal of Chemical Physics

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A range of technologies require the directed motion of nanoscale droplets on solid substrates. A way of realizing this effect is durotaxis, whereby a stiffness gradient of a substrate can induce directional motion without requiring an energy source. Here, we report on the results of extensive molecular dynamics investigations of droplets on a surface with varying stiffness. We find that durotaxis is enhanced by increasing the stiffness gradient and, also, by increased wettability of the substrate, in particular, when droplet size decreases. We anticipate that our study will provide further insights into the mechanisms of nanoscale directional motion.

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Section: The Modelmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Stiffness-guided motion of a droplet on a solid substrate

Theodorakis

Егоров

Milchev

2017

The Journal of Chemical Physics

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“…Changes in the cohesive interaction strength in the interfacial region are important because they alter the activation free energy ∆µ, which effects even the liquid regime far above the glass transition temperature. 29,34 We find that many important aspects of the dynamics can be understood from interfacial changes of ∆µ, rather than changes in the scale of collective motion. Specifically, we find (i) a greatly enhanced mo- Of course, the general validity of this extended string model of glass-formation requires further confirmation in polymer nanocomposites and other types of non-uniform glassforming materials to test the validity of this model.…”

Section: Discussionmentioning

confidence: 93%

“…34 Note that both L A and τ A depend on film thickness h, polymer-substrate interaction strength ε, as well as in the different regions of the film with, but the range of value is not large, L A = 1.40 ± 0.02 and τ A = 2.3 ± 1.0. 29 We emphasize that τ 0 is not a free fitting parameter, but τ 0 rather is determined 34 Although the approximate invariance of string size to location within the film does not explain the large variations in relaxation time within films, we may use Eq. 2 to understand the dynamics of thin polymer film for both film as a whole and local regions within it, and the relation of film relaxation to that of the bulk material.…”

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confidence: 94%

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Collective Motion in the Interfacial and Interior Regions of Supported Polymer Films and Its Relation to Relaxation

2019

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To understand the role of collective motion in the often large changes in interfacial molecular mobility observed in polymer films, we investigate the extent of collective motion in the interfacial regions of a thin supported polymer film and within the film interior by molecular dynamics simulation. Contrary to commonly stated expectations, we find that the extent of collective motion, as quantified by string-like molecular exchange motion, is similar in magnitude in the polymer-air interfacial layer as the film interior, and distinct from the bulk material. This finding is consistent with Adam-Gibbs description of the segmental dynamics within mesoscopic film regions where the extent of collective motion is related to the configurational entropy of the film as whole rather than a locally defined extent of collective motion or configurational entropy.It is generally appreciated that thin supported polymer films, and other polymeric nanoconfined materials (nanocomposites, spherical polymer nanoparticles, polymer nanotubes, etc.), 1 arXiv:1902.04632v2 [cond-mat.soft] 2 May 2019 exhibit large gradients of mobility in their interfacial regions that can greatly influence their end-use properties. 1-5 Typically, depending on the type of interface and the nature and magnitude of the interaction strength and the material properties of the surrounding medium, the scale of the interfacial regions 6 with altered mobility is on the order of a few nm, and the relaxation time in the interfacial region of glass-forming materials can differ from the overall relaxation time of the film by a factor 7-11 as large as 10 7 . Since mobility gradients in thin films can evidently be quite large, it is not surprising that this phenomenon has elicited significant research interest from both theoretical and practical perspectives.Changes in mobility of this magnitude can be rationalized within a widely utilized framework for understanding the slowing down of relaxation in bulk glass-forming liquids introduced by Adam and Gibbs (AG). 12 Specifically, a dramatic enhancement of mobility might be interpreted in terms of a reduction of the scale of collective motion in thin films, and some evidence for a reduced degree of collective motion has been reported based on molecular dynamics simulations. 13,14 However, this former work did not consider how collective motion is altered in the interfacial region, but only for the film as a whole. In particular, Riggleman et al. 15 observed the scale of collective motion in thin polymer films to decrease somewhat as the films were made thinner, a trend notably contrary to what one might naively expect from the Gibbs-DiMarzio model of glass-formation 16 where a reduction of system dimensionality should lead to a reduction of the configurational entropy S c of fluid, 17 and a corresponding increase of the glass transition temperature. The fact that many experimental studies indicate an apparent depression of the glass transition temperature T g is often taken as a point against the configurational entropy...

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Entropy Theory of Polymer Glass‐formation in Variable Spatial Dimension

Douglas

Freed

2016

Advances in Chemical Physics

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We explore the nature of glass-formation in variable spatial dimensionality (d) based on the generalized entropy theory, a synthesis of the Adam-Gibbs model with direct computation of the configurational entropy of polymer fluids using an established statistical mechanical model. We find that structural relaxation in the fluid state asymptotically becomes Arrhenius in the d → ∞ limit and that the fluid transforms upon sufficient cooling above a critical dimension near d = 8 into a dense amorphous state with a finite positive residual configurational entropy. Direct computations of the isothermal compressibility and thermal expansion coefficient, taken to be physical measures of packing frustration, demonstrate that these fluid properties strongly correlate with the fragility of glass-formation.PACS numbers: 64.70.QGrowing evidence indicates that molecular packing plays an essential role in both the physics of crystallization [1][2][3][4][5] and glass-formation [6][7][8][9][10][11][12][13][14][15][16][17], prompting a consideration of variable spatial dimensionality (d) as a conceptual probe of these solidification processes. In particular, the "decorrelation principle", recently proposed by Torquato and Stillinger [18,19], states that unconstrained correlations in hard hyperspheres diminish with increasing d and vanish in the d → ∞ limit. Likewise, recent simulations [7][8][9] have indicated that certain often cited aspects of glass-formation, such as the dynamic heterogeneity and the decoupling between structural relaxation and diffusion, become diminished at elevated d. While prior studies [11][12][13][14][15] have focused mainly on how critical packing fractions associated with glass-formation in hard hyperspheres vary with d, little is known concerning the d dependence of the characteristic temperatures and fragility of glass-formation in molecular fluids. No prior work has considered complex fluids, such as polymeric liquids, where many molecular parameters may be tuned to control the nature of glass-formation.In this Letter, we consider the glass-formation of a model polymer glass-forming (GF) liquid in d dimensions. Our approach is based on the generalized entropy theory (GET) [20], which is a combination of the lattice cluster theory (LCT) [21] and the Adam-Gibbs (AG) theory [22] linking the configurational entropy s c (i.e., the entropy devoid of the vibrational component) to the structural relaxation time τ α . The LCT employs a ddimensional hypercubic lattice model, whose use facilitates the development of an expansion about the meanfield limit of infinite d. The LCT thus naturally provides * Contribution of the National Institute of Standards and Technology -Work not subject to copyright in the United States. † wsxu@uchicago.edu ‡ jack.douglas@nist.gov § freed@uchicago.edu a framework for considering the d dependence of polymer thermodynamic properties, thereby making the GET suitable for exploring glass-formation in d dimensions.As a primary result, we find that structural relaxation in the fluid st...

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A unifying framework to quantify the effects of substrate interactions, stiffness, and roughness on the dynamics of thin supported polymer films

Cited by 130 publications

References 113 publications

Stiffness-guided motion of a droplet on a solid substrate

Stiffness-guided motion of a droplet on a solid substrate

Collective Motion in the Interfacial and Interior Regions of Supported Polymer Films and Its Relation to Relaxation

Entropy Theory of Polymer Glass‐formation in Variable Spatial Dimension

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