A Direct Quantitative Measure of Surface Mobility in a Glassy Polymer

Chai, Yi; Salez, Thomas; McGraw, Joshua D.; Benzaquen, Michael; Dalnoki-Veress, Kari; Raphaël, Élie; Forrest, James A.

doi:10.1126/science.1244845

Cited by 213 publications

(264 citation statements)

References 38 publications

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“…The behavior of thin glassy films has often been interpreted in terms of a two-population model, in which there is a glassy, immobile layer near the center of the film and a liquid-like mobile layer near the free surfaces [1,[24][25][26]. This interpretation is supported by observations of probe molecules embedded in films [27,28].…”

Section: {S} {Smentioning

confidence: 99%

Disconnecting structure and dynamics in glassy thin films

Sussman

Schoenholz

Cubuk

et al. 2017

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

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Nanometrically thin glassy films depart strikingly from the behavior of their bulk counterparts. We investigate whether the dynamical differences between bulk and thin film glasses can be understood by differences in local microscopic structure. We employ machine-learning methods that have previously identified strong correlations between local structure and particle rearrangement dynamics in bulk systems. We show that these methods completely fail to detect key aspects of thin-film glassy dynamics. Furthermore, we show that no combination of local structural features drawn from a very general set of two-and multi-point functions is able to distinguish between particles at the center of film and those in intermediate layers where the dynamics are strongly perturbed.Confinement of glassy materials to nanometric length scales leads to striking changes to their microscopic dynamics and consequently to their material properties [1]. Direct observations in both experiments and simulations find exponentially more particle rearrangements near the free surface of a film [2][3][4][5]. A key question is whether enhanced dynamics near a free surface (or suppressed dynamics near a substrate) are connected with structural changes. So far, all structural features studied decay too rapidly into the bulk [1] to explain the altered dynamics.Until recently, the study of bulk glassy systems has also been plagued by the inability to connect dynamical changes with structural ones. However, machine learning methods have proven remarkably successful in identifying a local structural quantity, termed "softness" and denoted S i for particle i, that is strongly correlated with particle rearrangements [6][7][8]. Softness is over an order of magnitude more predictive of rearrangements than measures such as the local potential energy or coordination number [9]. The average softness, S , is directly predictive of the relaxation time of a bulk supercooled liquid [7] or aging bulk glass [8], with higher values of S corresponding to shorter relaxation times at higher temperatures. In bulk systems, it is therefore now clear that dynamical slowing down near the glass transition is intimately associated with structural changes. Here we ask whether the enhancement of dynamics near the surface of free glassy films can similarly be understood.The answer is no. Not only does softness fail to predict the enhanced dynamics near the surface of free glassy films, we find that for a very general set of quantities that characterize the local structural environment surrounding a particle, there is no combination of these quantities that can distinguish between parts of the film with very different dynamics. The enhanced dynamics near a free glassy surface therefore appear to be fundamentally different from the enhanced dynamics that result from heating bulk glassy systems. Although we cannot rule out the possibility that structural quantities that we have not considered might tell a different story, our results suggest that near glassy free surfaces, relaxat...

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Section: {S} {Smentioning

confidence: 99%

Disconnecting structure and dynamics in glassy thin films

Sussman

Schoenholz

Cubuk

et al. 2017

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

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“…Using the calibrated stepped-film technique, it is then possible to study the capillary relaxation of polystyrene films below their glass-transition temperature T g [3]. The conclusion is striking: the surface evolves!…”

Section: Measure Of Surface Mobility In a Glassmentioning

confidence: 99%

Glass transition at interfaces

et al. 2017

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“…The first factor might be investigated in greater detail with atomistic simulations and surface sensitive spectroscopy (33), whereas a number of new experimental methods are available to probe dynamics at the surface of glasses (25,34). Structure at liquid surfaces is an active area of study (35,36), and our results indicate that physical vapor deposition might be a useful tool in this endeavor.…”

mentioning

confidence: 91%

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

Dalal

Walters

Lyubimov

et al. 2015

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

201

351

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Physical vapor deposition is commonly used to prepare organic glasses that serve as the active layers in light-emitting diodes, photovoltaics, and other devices. Recent work has shown that orienting the molecules in such organic semiconductors can significantly enhance device performance. We apply a high-throughput characterization scheme to investigate the effect of the substrate temperature (T substrate ) on glasses of three organic molecules used as semiconductors. The optical and material properties are evaluated with spectroscopic ellipsometry. We find that molecular orientation in these glasses is continuously tunable and controlled by T substrate /T g , where T g is the glass transition temperature. All three molecules can produce highly anisotropic glasses; the dependence of molecular orientation upon substrate temperature is remarkably similar and nearly independent of molecular length. All three compounds form "stable glasses" with high density and thermal stability, and have properties similar to stable glasses prepared from model glass formers. Simulations reproduce the experimental trends and explain molecular orientation in the deposited glasses in terms of the surface properties of the equilibrium liquid. By showing that organic semiconductors form stable glasses, these results provide an avenue for systematic performance optimization of active layers in organic electronics.glasses | organic semiconductors | molecular orientation | physical vapor deposition | high-throughput characterization

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A Direct Quantitative Measure of Surface Mobility in a Glassy Polymer

Cited by 213 publications

References 38 publications

Disconnecting structure and dynamics in glassy thin films

Disconnecting structure and dynamics in glassy thin films

Glass transition at interfaces

Tunable molecular orientation and elevated thermal stability of vapor-deposited organic semiconductors

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