Yu Boyuan, Rauscher Phillip M, Jackson Nicholas E, Rumyantsev Artem M, de Pablo Juan J
Pritzker School of Molecular Engineering, University of Chicago, Chicago, Illinois 60637, United States.
Center for Molecular Engineering and Materials Science Division, Argonne National Laboratory, Lemont, Illinois 60439, United States.
ACS Macro Lett. 2020 Sep 15;9(9):1318-1324. doi: 10.1021/acsmacrolett.0c00522. Epub 2020 Aug 26.
Considerable interest in the dynamics and rheology of polyelectrolyte complex coacervates has been motivated by their industrial application as viscosity modifiers. A central question is the extent to which classical Rouse and reptation models can be applied to systems where electrostatic interactions play a critical role on the thermodynamics. By relying on molecular simulations, we present a direct analysis of the crossover from Rouse to reptation dynamics in salt-free complex coacervates as a function of chain length. This crossover shifts to shorter chain lengths as electrostatic interactions become stronger, which corresponds to the formation of denser coacervates. To distinguish the roles of Coulomb interactions and density, we compare the dynamics of coacervates to those of neutral, semidilute solutions at the same density. Both systems exhibit a universal dynamical behavior in the connectivity-dominated (subdiffusion and normal diffusion) regimes, but the monomer relaxation time in coacervates is much longer and increases with increasing Bjerrum length. This is similar to the cage effect observed in glass-forming polymers, but the local dynamical slowdown is caused here by strong Coulomb attractions (ion pairing) between oppositely charged monomers. Our findings provide a microscopic framework for the quantitative understanding of coacervate dynamics and rheology.
聚电解质复合凝聚层的动力学和流变学因其作为粘度调节剂的工业应用而备受关注。一个核心问题是经典的劳斯（Rouse）模型和蛇行（reptation）模型在多大程度上可以应用于静电相互作用对热力学起关键作用的系统。通过分子模拟，我们直接分析了无盐复合凝聚层中从劳斯动力学向蛇行动力学的转变与链长的函数关系。随着静电相互作用变强，这种转变会移向更短的链长，这对应于更致密凝聚层的形成。为了区分库仑相互作用和密度的作用，我们将凝聚层的动力学与相同密度下中性半稀溶液的动力学进行比较。在连通性主导（亚扩散和正常扩散）区域，这两个系统都表现出普遍的动力学行为，但凝聚层中单体的弛豫时间要长得多，并且随着比耶鲁姆（Bjerrum）长度的增加而增加。这类似于在玻璃形成聚合物中观察到的笼效应，但这里局部动力学减慢是由带相反电荷的单体之间强烈的库仑吸引力（离子配对）引起的。我们的研究结果为定量理解凝聚层动力学和流变学提供了一个微观框架。
