Diaz-Vela Daniel, Hung Jui-Hsiang, Simmons David S
The University of Akron, 250 South Forge Street, Akron, Ohio 44325, United States.
The University of South Florida, 4202 East Fowler Avenue, ENB 118, Tampa, Florida 33620, United States.
ACS Macro Lett. 2018 Nov 20;7(11):1295-1301. doi: 10.1021/acsmacrolett.8b00695. Epub 2018 Oct 11.
Near-interface alterations in dynamics and glass formation behavior have been the subject of extensive study for the past two decades, both because of their practical importance and in the hope of revealing underlying correlation lengths underpinning glass transition more generally. Here we employ molecular dynamics simulations of thick films to demonstrate that these effects emerge, for segmental-scale translational dynamics at low temperature, from a temperature-independent rescaling of the local activation barrier. This rescaling manifests as a fractional power law decoupling relationship of local dynamics relative to the bulk, with a transition from a regime of weak decoupling at high temperatures to a regime of strong decoupling at low temperatures. The range of this effect saturates at low temperatures, with 90% of the surface perturbation in the barrier lost over a range of 12 segmental diameters. These findings reduce the phenomenology of nanoconfinement effects to two properties-a position-dependent, temperature independent, barrier rescaling factor and an onset time scale-while substantially constraining the predictions required from any theoretical explanation of this phenomenon.
在过去二十年中，动力学和玻璃形成行为的近界面变化一直是广泛研究的主题，这既是因为它们的实际重要性，也是希望更普遍地揭示支撑玻璃化转变的潜在关联长度。在这里，我们采用厚膜的分子动力学模拟来证明，对于低温下的链段尺度平移动力学，这些效应源于局部活化能垒与温度无关的重新标度。这种重新标度表现为局部动力学相对于本体的分数幂律解耦关系，从高温下的弱解耦状态转变为低温下的强解耦状态。这种效应的范围在低温下达到饱和，在12个链段直径的范围内，能垒中90%的表面扰动消失。这些发现将纳米限域效应的现象学简化为两个特性——一个与位置有关、与温度无关的能垒重新标度因子和一个起始时间尺度——同时极大地限制了对该现象的任何理论解释所需的预测。
