Fan Jinpeng, Emamy Hamed, Chremos Alexandros, Douglas Jack F, Starr Francis W
Department of Physics, Wesleyan University, Middletown, Connecticut 06459-0155, USA.
Materials Science and Engineering Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA.
J Chem Phys. 2020 Feb 7;152(5):054904. doi: 10.1063/1.5135731.
While glass formation of linear chain polymer melts has widely been explored, comparatively little is known about glass formation in star polymer melts. We study the segmental dynamics of star polymer melts via molecular dynamics simulations and examine the cooperative nature of segmental motion in star melts. In particular, we quantify how the molecular architecture of star polymers, i.e., the number of arms and the length of those arms, affects the glass transition temperature T, the non-Gaussian nature of molecular displacements, the collective string-like motion of monomers, and the role of chain connectivity in the cooperative motion. Although varying the number of arms f and the molecular mass M of the star arms can significantly influence the average star molecular shape, all our relaxation data can be quantitatively described in a unified way by the string model of glass formation, an activated transport model that derives from the Adam-Gibbs model, where the degree of cooperative motion is identified with the average length L of string-like particle exchange motions observed in our simulations. Previous work has shown the consistency of the string model with simulations of linear polymers at constant volume and constant pressure, as well as for thin supported polymer films and nanocomposites with variable polymer-surface interactions, where there are likewise large mobility gradients as in the star polymer melts studied in the present paper.
虽然线性链聚合物熔体的玻璃形成已得到广泛研究，但对于星型聚合物熔体中的玻璃形成却知之甚少。我们通过分子动力学模拟研究星型聚合物熔体的链段动力学，并考察星型熔体中链段运动的协同性质。特别地，我们量化星型聚合物的分子结构，即臂数和臂长，如何影响玻璃化转变温度(T)、分子位移的非高斯性质、单体的集体串状运动以及链连接性在协同运动中的作用。尽管改变星型臂的臂数(f)和分子量(M)会显著影响平均星型分子形状，但我们所有的弛豫数据都可以用玻璃形成的串模型以统一的方式进行定量描述，这是一种源自亚当 - 吉布斯模型的活化输运模型，其中协同运动的程度与我们模拟中观察到的串状粒子交换运动的平均长度(L)相关。先前的工作表明，串模型与恒容和恒压下线性聚合物的模拟一致，也与具有可变聚合物 - 表面相互作用的薄支撑聚合物膜和纳米复合材料一致，在这些体系中同样存在如本文所研究的星型聚合物熔体那样大的迁移率梯度。
