Hartley Austin D, Drayer William F, Ghanekarade Asieh, Simmons David S
Department of Chemical, Biological, and Materials Engineering, The University of South Florida, Tampa, Florida 33620, USA.
J Chem Phys. 2023 Nov 28;159(20). doi: 10.1063/5.0165650.
Glass-forming liquids exhibit long-lived, spatially correlated dynamical heterogeneity, in which some nm-scale regions in the fluid relax more slowly than others. In the nanoscale vicinity of an interface, glass-formers also exhibit the emergence of massive interfacial gradients in glass transition temperature Tg and relaxation time τ. Both of these forms of heterogeneity have a major impact on material properties. Nevertheless, their interplay has remained poorly understood. Here, we employ molecular dynamics simulations of polymer thin films in the isoconfigurational ensemble in order to probe how bulk dynamic heterogeneity alters and is altered by the large gradient in dynamics at the surface of a glass-forming liquid. Results indicate that the τ spectrum at the surface is broader than in the bulk despite being shifted to shorter times, and yet it is less spatially correlated. This is distinct from the bulk, where the τ distribution becomes broader and more spatially organized as the mean τ increases. We also find that surface gradients in slow dynamics extend further into the film than those in fast dynamics-a result with implications for how distinct properties are perturbed near an interface. None of these features track locally with changes in the heterogeneity of caging scale, emphasizing the local disconnect between these quantities near interfaces. These results are at odds with conceptions of the surface as reflecting simply a higher "rheological temperature" than the bulk, instead pointing to a complex interplay between bulk dynamic heterogeneity and spatially organized dynamical gradients at interfaces in glass-forming liquids.
玻璃形成液体表现出寿命长、空间相关的动力学非均匀性，其中流体中的一些纳米尺度区域比其他区域弛豫得更慢。在界面的纳米尺度附近，玻璃形成体在玻璃化转变温度Tg和弛豫时间τ方面也表现出大量界面梯度的出现。这两种非均匀性形式都对材料性能有重大影响。然而，它们之间的相互作用仍然知之甚少。在这里，我们采用等构型系综中聚合物薄膜的分子动力学模拟，以探究本体动力学非均匀性如何被玻璃形成液体表面动力学的大梯度改变以及如何改变这种大梯度。结果表明，尽管表面的τ谱向更短时间偏移，但其比本体中的更宽，且空间相关性更低。这与本体不同，在本体中，随着平均τ增加，τ分布变得更宽且空间组织性更强。我们还发现，慢动力学中的表面梯度比快动力学中的延伸到薄膜中更远——这一结果对界面附近不同性质如何受到扰动具有启示意义。这些特征均未随笼蔽尺度非均匀性的变化而局部跟踪，强调了界面附近这些量之间的局部脱节。这些结果与将表面简单地视为反映比本体更高“流变温度”的概念不一致，而是指向玻璃形成液体中本体动力学非均匀性与界面处空间组织的动力学梯度之间的复杂相互作用。
