Department of Chemistry and Research Institute for Basic Science, Sogang University, Seoul 121-742, Republic of Korea.
Soft Matter. 2017 Feb 8;13(6):1190-1199. doi: 10.1039/c6sm02468h.
We investigate the glass transition and interfacial dynamics of single strand fibers of flexible polymers by employing molecular dynamics (MD) simulations along with a coarse grained model. While the polymer fiber has drawn significant attention due to its applicability in tissue engineering and stretchable electronics, its dynamic properties, especially the glass transition temperature (T), are yet to be understood at the molecular level. For example, there has been a controversy on the effect of the polymer fiber radius (R) on T: T decreased with a decrease in R for some polymer fibers, whereas T of other polymer fibers was not sensitive to R. In this article, we estimate the bond relaxation time of polymers and evaluate both T and fragility (m) as a function of R. We illustrate that T of the polymer fiber decreased with a decrease in R monotonically and also that the values of T follow faithfully the empirical equation proposed by Keddie et al. as a function of R, which was successfully employed to fit the values of T of both polyvinyl alcohol (PVA) fibers and polyethylene (PE) fibers. We also find that the dynamics of polymers at the interface between a polymer fiber and air is faster than that of polymers at the center. By employing Adam-Gibbs theory, we show that the fast interface dynamics of polymer fibers should influence the cooperative motion of monomers, which should be responsible for the decrease in T for smaller values of R. Near the interface there are more mobile monomers that participate in the cooperative motions of polymers. Interesting is that due to the curved surface (unlike flat polymer films) the cooperative motion of monomers is anisotropic in polymer fibers.
我们通过分子动力学（MD）模拟和粗粒化模型研究了柔性聚合物单链纤维的玻璃化转变和界面动力学。由于聚合物纤维在组织工程和可拉伸电子产品中的应用，它已经引起了广泛的关注，但其动态特性，特别是玻璃化转变温度（T），在分子水平上仍有待理解。例如，聚合物纤维半径（R）对 T 的影响存在争议：对于一些聚合物纤维，T 随 R 的减小而降低，而对于其他聚合物纤维，T 对 R 不敏感。在本文中，我们估计聚合物的键弛豫时间，并评估 T 和脆性（m）作为 R 的函数。我们说明聚合物纤维的 T 随 R 的减小而单调下降，并且 T 的值忠实地遵循 Keddie 等人提出的经验方程作为 R 的函数，该方程成功地用于拟合聚乙烯醇（PVA）纤维和聚乙烯（PE）纤维的 T 值。我们还发现聚合物纤维与空气之间界面处的聚合物动力学比中心处的聚合物动力学更快。通过采用 Adam-Gibbs 理论，我们表明聚合物纤维的快速界面动力学应该会影响单体的协同运动，这应该是 R 值较小导致 T 降低的原因。在界面附近，有更多的可移动单体参与聚合物的协同运动。有趣的是，由于曲面（与聚合物薄膜不同），聚合物纤维中单体的协同运动具有各向异性。
