Fu Yao, Bocharova Vera, Ma Mengze, Sokolov Alexei P, Sumpter Bobby G, Kumar Rajeev
Department of Aerospace Engineering and Engineering Mechanics, University of Cincinnati, Cincinnati, OH-45220, USA.
Phys Chem Chem Phys. 2017 Oct 18;19(40):27442-27451. doi: 10.1039/c7cp04249c.
It is well-known that the nature and size of the counterions affect the ionic conductivity and glass transition temperature of ionic polymers in a significant manner. However, the microscopic origin of the underlying changes in the dynamics of chains and counterions is far from completely understood. Using coarse-grained molecular dynamics simulations of flexible and semi-flexible ionic polymers, we demonstrate that the glass transition temperature of ionic polymeric melts depends on the size of monovalent counterions in a non-monotonic manner. The glass transition temperature is found to be the highest for the smallest counterions and decreases with an increase in the counterion radii up to a point, after which the glass transition temperature increases with a further increase in the radii. This behavior is because the counterions have significant effects on the coupled dynamics of the charges on the chains and counterions. In particular, increase in the radii of the counterions leads to strongly coupled dynamics between the charges on the chains and the counterions. The static dielectric constant of the polymer melts also has a significant effect on the coupling and the glass transition temperature. The glass transition temperature is predicted to decrease with an increase in the dielectric constant. This, in turn, leads to an increase in the diffusion constant of the counterions at a given temperature. Backbone rigidity is shown to increase the glass transition temperature and decrease the coupling. Furthermore, faster counterion dynamics is predicted for the melts of semi-flexible chains in comparison with flexible chains at the same segmental relaxation time. As the semi-flexible chains tend to have a longer segmental relaxation time, semi-flexible polymers with high dielectric constants are predicted to have diffusion constants of counterions comparable with flexible polymers.
众所周知，抗衡离子的性质和大小会显著影响离子聚合物的离子电导率和玻璃化转变温度。然而，链和抗衡离子动力学潜在变化的微观起源仍远未被完全理解。通过对柔性和半柔性离子聚合物进行粗粒化分子动力学模拟，我们证明离子聚合物熔体的玻璃化转变温度以非单调方式依赖于单价抗衡离子的大小。发现对于最小的抗衡离子，玻璃化转变温度最高，并且随着抗衡离子半径的增加，玻璃化转变温度会下降，直到某一点，之后随着半径的进一步增加，玻璃化转变温度又会上升。这种行为是因为抗衡离子对链上电荷与抗衡离子的耦合动力学有显著影响。特别是，抗衡离子半径的增加会导致链上电荷与抗衡离子之间的强耦合动力学。聚合物熔体的静态介电常数也对这种耦合和玻璃化转变温度有显著影响。预计玻璃化转变温度会随着介电常数的增加而降低。这反过来又会导致在给定温度下抗衡离子的扩散常数增加。主链刚性被证明会提高玻璃化转变温度并降低耦合。此外，预计在相同的链段弛豫时间下，半柔性链熔体的抗衡离子动力学比柔性链更快。由于半柔性链往往具有更长的链段弛豫时间，预计具有高介电常数的半柔性聚合物的抗衡离子扩散常数与柔性聚合物相当。
