Extending the timescale of molecular simulations by using time–temperature superposition: rheology of ionic liquids,Soft Matter

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Extending the timescale of molecular simulations by using time–temperature superposition: rheology of ionic liquids
Soft Matter ( IF 2.9 ) Pub Date : 2021-06-28 , DOI: 10.1039/d1sm00701g
Adegbola Balogun ₁ , Daria Lazarenko ₂ , Fardin Khabaz ₃ , Rajesh Khare ₁

Affiliation

Molecular dynamics simulations are used to determine the temperature dependence of the dynamic and rheological properties of a model imidazolium-based ionic liquid (IL). The simulation results for the volumetric properties of the IL are in good agreement with the experimental results. The temperature dependence of the diffusion coefficient of anions and cations follows the Vogel–Fulcher–Tammann equation over the range of the temperatures studied. The shear viscosity of the IL shows a Newtonian plateau at low shear rates and shear-thinning behavior at high shear rates. The dynamic modulus values indicate that the IL behaves like a viscous liquid at high temperatures and low frequencies, while its viscoelastic response becomes similar to that of an elastic solid at low temperatures and high frequencies. Using the time–temperature superposition (TTS) principle, the dynamic moduli, shear viscosity, and mean squared displacement of cations and anions in the diffusive regime can be collapsed onto master curves by applying a single set of shift factors. Due to the large mismatch in the timescale investigated by the atomistically detailed simulations and experiments, the glass transition temperature predicted in simulations shifts to higher values. When this timescale mismatch is accounted for by using appropriate shift factors, the master curves of the dynamic moduli obtained in simulations closely match those obtained in experiments. This result demonstrates the exciting ability of TTS to overcome the large timescale disparity between simulations and experiments which will enable the use of molecular simulations for quantitatively predicting the rheological property values at frequencies of practical interest.

中文翻译：

通过使用时间-温度叠加来扩展分子模拟的时间尺度：离子液体的流变学

分子动力学模拟用于确定基于咪唑鎓的离子液体 (IL) 模型的动态和流变特性的温度依赖性。IL 的体积特性的模拟结果与实验结果非常吻合。在所研究的温度范围内，阴离子和阳离子的扩散系数的温度依赖性遵循 Vogel-Fulcher-Tammann 方程。IL 的剪切粘度在低剪切速率下显示牛顿平台，在高剪切速率下显示剪切稀化行为。动态模量值表明，IL 在高温和低频下表现得像粘性液体，而其粘弹性响应在低温和高频下变得类似于弹性固体。使用时间-温度叠加 (TTS) 原理，通过应用一组移动因子，可以将扩散区域中阳离子和阴离子的动态模量、剪切粘度和均方位移折叠到主曲线上。由于原子详细模拟和实验研究的时间尺度存在很大的不匹配，模拟中预测的玻璃化转变温度会转移到更高的值。当通过使用适当的移位因子来解释这种时间尺度不匹配时，模拟中获得的动态模量的主曲线与实验中获得的曲线非常匹配。

更新日期：2021-07-16

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