Molecular simulation of structural relaxation in ultrathin polymer films.
Molecular simulation of structural relaxation in ultrathin polymer films.
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DOI：
10.1039/c3cp53555j
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发表时间：
2013-11
期刊：
影响因子：
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通讯作者：
Q. Tang;Wenbing Hu
中科院分区：
文献类型：
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作者：
Q. Tang;Wenbing Hu
玻璃态聚合物薄膜的结构弛豫和物理老化与聚合物基纳米器件的寿命密切相关，在过去的二十年里引起了人们的广泛关注。目前，在聚合物薄膜中观察到的物理老化是基于自由体积扩散模型（FVDM）解释的，该模型尚未在分子水平上通过模拟验证。在这里，我们进行了Monte Carlo模拟，通过引入空位扩散机制（类似于FVDM），研究在聚合物薄膜的结构弛豫。结果表明，链段的局域平均密度随时间的对数线性增加，与实验中荧光强度和介电强度的测量结果相似。实验结果表明，薄膜弛豫速率对温度的响应与玻璃态聚合物薄膜一致。弛豫速率随温度降低而出现的峰值可以归因于两种机制（链段迁移率和精确的初始空位浓度）从分子水平上的竞争。我们的研究结果表明，与空位扩散模型的模拟确实可以提供新的见解，从分子水平上理解的结构弛豫和物理老化的聚合物薄膜。
Structural relaxation and physical aging in glassy polymer films have attracted much attention in the past two decades due to their strong correlation with the lifetime of polymer-based nano-devices. Currently, the observed physical aging in polymer films was explained on the basis of a free volume diffusion model (FVDM) that has not yet been validated by simulations at the molecular level. Here we performed a Monte Carlo simulation by introducing the vacancy diffusion mechanism (similar to FVDM) to investigate the structural relaxation in ultrathin polymer films. The results show that the local average density of segments increases linearly with the logarithm of time, similar to the fluorescence intensity and dielectric strength measurements in experiments. The responses of relaxation rates in ultrathin films to temperatures are consistent with that for glassy polymer thin films in experiments. The emergence of a peak of relaxation rates with decreasing temperature can be attributed to the competition of two mechanisms (segment mobility and accurate initial vacancy concentration) from the molecular levels. Our results demonstrate that the simulations with a vacancy diffusion model could indeed provide new insights from the molecular levels to understand the structural relaxation and physical aging in ultrathin polymer films.