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Mobility gradients yield rubbery surfaces on top of polymer glasses

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Abstract

Many emerging materials, such as ultrastable glasses1,2 of interest for phone displays and OLED television screens, owe their properties to a gradient of enhanced mobility at the surface of glass-forming liquids. The discovery of this surface mobility enhancement3,4,5 has reshaped our understanding of the behaviour of glass formers and of how to fashion them into improved materials. In polymeric glasses, these interfacial modifications are complicated by the existence of a second length scale—the size of the polymer chain—as well as the length scale of the interfacial mobility gradient6,7,8,9. Here we present simulations, theory and time-resolved surface nano-creep experiments to reveal that this two-scale nature of glassy polymer surfaces drives the emergence of a transient rubbery, entangled-like surface behaviour even in polymers comprised of short, subentangled chains. We find that this effect emerges from superposed gradients in segmental dynamics and chain conformational statistics. The lifetime of this rubbery behaviour, which will have broad implications in constraining surface relaxations central to applications including tribology, adhesion, and surface healing of polymeric glasses, extends as the material is cooled. The surface layers suffer a general breakdown in time−temperature superposition (TTS), a fundamental tenet of polymer physics and rheology. This finding may require a reevaluation of strategies for the prediction of long-time properties in polymeric glasses with high interfacial areas. We expect that this interfacial transient elastomer effect and TTS breakdown should normally occur in macromolecular systems ranging from nanocomposites to thin films, where interfaces dominate material properties5,10.

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Fig. 1: Formation of wetting ridge and its topological profile.
Fig. 2: Polymer nano-rheology and surface chain dynamics.
Fig. 3: Failure of TTS at the glassy polymer surface.
Fig. 4: Emergence of rubbery dynamics at the surface of the unentangled polymer.

Data availability

The data that support the findings of this study are available within the article and its Supplementary Information. Raw simulation trajectories are available upon request from D.S.S.

Code availability

Simulations employ standard codes (LAMMPS) and methods that are freely available or documented in the literature.

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Acknowledgements

We thank N. L. Yamada for assisting with the neutron reflectivity measurements and O. K. C. Tsui and J. J. Zhou for discussions. B.Z. acknowledges financial support from the Natural Science Foundation of China (grant numbers 21973083 and 21504081), and R.D.P. and K.R. acknowledge support from the National Science Foundation (NSF) Materials Research Science and Engineering Center Program through the Princeton Center for Complex Materials (grant numbers DMR-1420541 and DMR-2011750) and the NSF through grant number CBET-1706012. D.S.S. and A.G. acknowledge support from the National Science Foundation through grant number CBET-1854308. X.W. thanks the Natural Science Foundation of China (grant numbers 21674100 and 21873085), and K.T. acknowledges the JST-Mirai Program (JPMJMI18A2). We also acknowledge the BL-16 line at J-PARC (programme no.2017L2501) for providing beam time.

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Contributions

B.Z. and R.D.P. conceived and supervised the experiments. Z.H., N.Z. and K.R. performed experiments. D.S.S. and A.G. conceived and analysed all simulations and theories. A.G. performed all simulations under the supervision of D.S.S. D.K. and K.T. provided neutron reflectivity data. All authors discussed the results and wrote the manuscript.

Corresponding authors

Correspondence to David S. Simmons, Rodney D. Priestley or Biao Zuo.

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Supplementary Information

This file contains notes on the theoretical development, Supplementary Simulation Methods, Supplementary Data, Supplementary Table 1, Supplementary Figs 1-16 and Supplementary References.

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Hao, Z., Ghanekarade, A., Zhu, N. et al. Mobility gradients yield rubbery surfaces on top of polymer glasses. Nature 596, 372–376 (2021). https://doi.org/10.1038/s41586-021-03733-7

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