Department of Materials Science and Engineering, University of Fukui, Fukui 9108507, Japan.
J Phys Chem B. 2023 Jun 1;127(21):4896-4904. doi: 10.1021/acs.jpcb.3c00841. Epub 2023 May 18.
The anomalous dynamics and glass transition behaviors of supercooled liquids under nanoconfinement, such as ultrathin polymer films, have attracted much attention in recent decades. However, a complete elucidation of this mechanism has not yet been achieved. For the dynamics of bulk materials without confinement, we previously proposed a dynamically correlated network (DCN) model, which was found to agree well with the experimental data. The model assumes that segments with thermal fluctuations are dynamically correlated to their neighbors to form string-like clusters, which eventually grow into networks as temperature decreases. In this study, we applied the DCN model to nanoconfined free-standing films by using a simple cubic lattice sandwiched between two free surface layers consisting of virtual "uncorrelated" segments. The average size of DCNs at lower temperatures decreased with decreasing thickness because of confinement. This trend was associated with a decrease in the percolation temperature at which the size of DCN diverges. It was also revealed that the fractal dimension of the generated DCNs exhibits a peak with respect to temperature. The segmental relaxation time for free-standing polystyrene films was evaluated, and the predicted thickness dependence of the glass transition temperature qualitatively agreed with the experimental data. The results suggest that the concept of DCN is compatible with the dynamics of free-standing thin films.
超冷液体在纳米受限条件下的异常动力学和玻璃化转变行为，如超薄膜聚合物，近几十年来引起了广泛关注。然而，这种机制尚未得到完全阐明。对于无受限的块状材料动力学，我们之前提出了一个动态相关网络（DCN）模型，该模型与实验数据吻合得很好。该模型假设具有热波动的片段与它们的邻居动态相关，形成线状簇，随着温度的降低最终形成网络。在这项研究中，我们通过使用由虚拟“不相关”片段组成的两个自由表面层之间夹着的简单立方晶格，将 DCN 模型应用于纳米受限的自由站立薄膜。由于受限，较低温度下 DCN 的平均尺寸随厚度的减小而减小。这种趋势与 DCN 尺寸发散的渗透温度降低有关。还揭示了生成的 DCN 的分形维数随温度呈现峰值。评估了自由站立聚苯乙烯薄膜的分段弛豫时间，预测的玻璃化转变温度对厚度的依赖性与实验数据定性一致。结果表明，DCN 的概念与自由站立薄膜的动力学是兼容的。
