Interfacial mobility scale determines the scale of collective motion and relaxation rate in polymer films

Hanakata, Paul Z.; Douglas, Jack F.; Starr, Francis W.

doi:10.1038/ncomms5163

Cited by 211 publications

(282 citation statements)

References 58 publications

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“…Simulation has led the way in defining the existence and precise nature of cooperative motion in cooled liquids. In particular, several studies have established that the activation freeenergy barrier ΔG for structural relaxation is proportional to the average length of string-like clusters involving particle exchange motion (20,21,(34)(35)(36)(37)41). These well-defined string clusters provide a concrete realization of the CRRs.…”

Section: Resultsmentioning

confidence: 99%

“…Fig. 1 shows the applicability of the VFT equation to all our simulation data for the relaxation time τ of the coherent intermediate scattering function (density-density correlations), including data for the pure polymer melt, polymer nanocomposites (35), and thin polymer films (36,37). This description of our data is uniformly excellent from a numerical standpoint, but the freevolume model provides little insight into the magnitude of η 0 , D, and T 0 , so that it is hard to predict trends with molecular structure.…”

Section: Resultsmentioning

confidence: 99%

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Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials

Betancourt

Hanakata

Starr

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

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182

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The study of glass formation is largely framed by semiempirical models that emphasize the importance of progressively growing cooperative motion accompanying the drop in fluid configurational entropy, emergent elasticity, or the vanishing of accessible free volume available for molecular motion in cooled liquids. We investigate the extent to which these descriptions are related through computations on a model coarse-grained polymer melt, with and without nanoparticle additives, and for supported polymer films with smooth or rough surfaces, allowing for substantial variation of the glass transition temperature and the fragility of glass formation. We find quantitative relations between emergent elasticity, the average local volume accessible for particle motion, and the growth of collective motion in cooled liquids. Surprisingly, we find that each of these models of glass formation can equally well describe the relaxation data for all of the systems that we simulate. In this way, we uncover some unity in our understanding of glass-forming materials from perspectives formerly considered as distinct.glass formation | elasticity | cooperativity | free volume | strings T here are numerous theoretical approaches aiming to describe the universal liquid dynamics approaching the glass transition. One class of theories emphasizes the importance of the congested nature of the local atomic environment in cooled liquids, focusing on the amount of "free volume" available to facilitate molecular rearrangement (1). This free-volume approach is also linked to the more modern jamming model of glass formation (2). Older treatments of glass formation based on this perspective can be traced back to Batchinski (3), Doolittle (4), and Hildebrand (5) for small liquids, and to Williams and coworkers (6) and Duda and Vrentas (7, 8) for polymer materials. There is also more recent work based on the free-volume perspective, for example, positron lifetime measurements (9) that probe the cavity structure of glass-forming (GF) liquids. DebyeWaller measurements (9, 10), based on neutron, X-ray, or other scattering measurements, emphasize another type of free volume that is associated with the volume explored by particles as they rattle about their mean positions in a condensed material. This type of free-volume modeling has also been refined to take into account the shape of these "rattle" volumes (11,12).Another family of glass-formation models emphasizes the emergent elasticity in glassy materials (13). These approaches build on the idea that the solid-like nature of glasses is one of their most conspicuous, and perhaps defining, properties. Dyre (13) and Nemilov (14) have argued that the activation energy for transport should grow in proportion to the shear modulus. The models of Hall and Wolynes (15) and Leporini and coworkers (10, 16) can also be included in this class if the Debye-Waller factor is taken as a measure of local material stiffness.Approaches emphasizing the underlying complex potential energy surface have also found consider...

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials

Betancourt

Hanakata

Starr

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

Self Cite

182

306

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“…This BS model is very similar to other models recently used in simulation studies of polymers in confinement. 27,31,39 All simulation results are given in LJ units. The model contains two intrinsic length scales, corresponding to the minima of the bond and LJ potentials.…”

Section: Simulation Protocolmentioning

confidence: 99%

“…This dynamical behaviour is in agreement with previous simulation studies in confined liquids. 6,27,63 We further proceeded to the examination of the height profiles of films with a square surface pattern supported on both substrates. For reasons of computational efficiency, we considered a system with a 1:1 aspect ratio as illustrated in fig.8.…”

Section: Appendix Hydrodynamic Boundary Condition: Control and Charamentioning

confidence: 99%

“…23,24,26 Molecular dynamics (MD) simulation has been a valuable tool for the elucidation of the mechanisms that determine the behaviour of polymers in confinement. 6,9,[27][28][29][30][31][32][33][34] Given the wide spread of length and time scales that govern the structure and dynamics of such systems, different approaches and certain simplifications have been developed in order to meet the requirements of the system under study. The dramatic increase of the relaxation time upon approaching the glassy state is common to all polymers regardless of the underlying chemistry.…”

Section: Introductionmentioning

confidence: 99%

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Molecular dynamics simulation of the capillary leveling of viscoelastic polymer films

Tanis

Meyer

Salez

et al. 2017

The Journal of Chemical Physics

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Abstract. Surface tension-driven flow techniques have recently emerged as an efficient means of shedding light into the rheology of thin polymer films. Motivated by experimental and theoretical approaches in films bearing a varying surface topography, we present results on the capillary relaxation of a square pattern at the free surface of a viscoelastic polymer film, using molecular dynamics simulations of a coarse-grained polymer model. Height profiles are monitored as a function of time after heating the system above its glass-transition temperature and their time dependence is fitted to the theory of capillary leveling. Results show that the viscosity is not constant, but time dependent. In addition to providing a complementary insight about the local inner mechanisms, our simulations of the capillary-leveling process therefore probe the viscoelasticity of the polymer and not only its viscosity, in contrast to previous experimental approaches.

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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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Interfacial mobility scale determines the scale of collective motion and relaxation rate in polymer films

Cited by 211 publications

References 58 publications

Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials

Quantitative relations between cooperative motion, emergent elasticity, and free volume in model glass-forming polymer materials

Molecular dynamics simulation of the capillary leveling of viscoelastic polymer films

Entropy Theory of Polymer Glass‐formation in Variable Spatial Dimension

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