Department of Chemistry, Key Laboratory of Surface & Interface Science of Polymer Materials of Zhejiang Province, National Engineering Lab for Textile Fiber Materials and Processing Technology (Zhejiang), Zhejiang Sci-Tech University, Hangzhou, China.
Department of Chemical, Biological, and Materials Engineering, University of South Florida, Tampa, FL, USA.
Nature. 2021 Aug;596(7872):372-376. doi: 10.1038/s41586-021-03733-7. Epub 2021 Aug 18.
Many emerging materials, such as ultrastable glasses 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 enhancement 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 gradient. 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 properties.
许多新兴材料，如用于手机显示屏和 OLED 电视屏幕的超稳定玻璃，其特性都归因于玻璃形成液体表面增强的迁移率梯度。这种表面迁移率增强的发现改变了我们对玻璃形成体行为的理解，以及如何将其塑造成改进的材料。在聚合物玻璃中，这些界面改性由于存在第二个长度尺度——聚合物链的大小，以及界面迁移率梯度的长度尺度而变得复杂。在这里，我们通过模拟、理论和时间分辨的表面纳米蠕变实验来揭示，即使在由短链组成的亚缠结聚合物中，玻璃状聚合物表面的这种两尺度性质也会导致瞬态橡胶状、缠结状表面行为的出现。我们发现，这种效应源自于片段动力学和链构象统计的叠加梯度。这种橡胶状行为的寿命将对包括摩擦学、粘附和聚合物玻璃表面愈合在内的应用中涉及的表面松弛有广泛的影响，随着材料的冷却而延长。表面层在时间-温度叠加（TTS）中普遍失效，这是聚合物物理和流变学的一个基本原理。这一发现可能需要重新评估具有高界面面积的聚合物玻璃中长时间性质预测的策略。我们预计，这种界面瞬态弹性体效应和 TTS 失效通常会在从纳米复合材料到薄膜的高分子系统中发生，其中界面主导着材料的性质。
