The dynamics of freestanding films: predictions for poly(2-chlorostyrene) based on bulk pressure dependence and thoughtful sample averaging
The dynamics of freestanding films: predictions for poly(2-chlorostyrene) based on bulk pressure dependence and thoughtful sample averaging
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独立式薄膜的动力学：基于体积压力依赖性和深思熟虑的样本平均的聚（2-氯苯乙烯）预测
DOI：
10.1039/d1sm01175h
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发表时间：
2021
期刊：
影响因子：
3.4
通讯作者：
Lipson, Jane E.
中科院分区：
文献类型：
--
作者：
White, Ronald P.;Lipson, Jane E.
在本文中，我们在聚（2-氯苯乙烯）18 nm的独立膜的节段弛豫模型，仅使用散装样品的数据来表征系统，并预测薄膜弛豫时间（τ）作为温度的函数，在半定量协议与薄膜数据。翻译成电影的预测散装表征的能力是我们以前的工作的直接结果，连接自由表面的影响，在电影中的散装变化的压力。我们的方法结合了局域相关晶格（LCL）状态方程，用于预测任何给定密度（ρ）下的自由体积值（Vfree），然后将其用于合作自由体积（CFV）速率模型来预测τ（T，Vfree）。这项工作的一个关键特征是，我们使用CFV预测的长度尺度Lcoop（z）计算局部平均密度分布，作为与表面距离的函数，ρav（z），重排分子片段在其中合作。正如我们在过去所展示的，ρav（z）比局域分布ρ（z）要宽得多，局域分布ρ（z）转化为弛豫分布τ（z），其宽度反映了实验和模拟结果。此外，我们还讨论了对整个薄膜样品的位置相关弛豫时间的对数（<log τ（z）>）进行平均的重要性，而不是对弛豫时间本身进行平均，以便最好地近似整个样品的平均值，可以直接与实验进行比较。
In this paper we model the segmental relaxation in poly(2-chlorostyrene) 18 nm freestanding films, using only data on bulk samples to characterize the system, and predict film relaxation times (τ) as a function of temperature that are in semi-quantitative agreement with film data. The ability to translate bulk characterization into film predictions is a direct result of our previous work connecting the effects of free surfaces in films with those of changing pressure in the bulk. Our approach combines the Locally Correlated Lattice (LCL) equation of state for prediction of free volume values (Vfree) at any given density (ρ), which are then used in the Cooperative Free Volume (CFV) rate model to predict τ(T, Vfree). A key feature of this work is that we calculate the locally averaged density profile as a function of distance from the surface, ρav(z), using the CFV-predicted lengthscale, Lcoop(z), over which rearranging molecular segments cooperate. As we have shown in the past, ρav(z) is significantly broader than the localized profile, ρ(z), which translates into a relaxation profile, τ(z), exhibiting a breadth that mirrors experimental and simulated results. In addition, we discuss the importance of averaging the log of position dependent relaxation times across a film sample (<log τ(z)>), as opposed to averaging the relaxation times, themselves, in order to best approximate a whole sample-averaged value that can be directly compared to experiment.