Department of Polymer Science and Engineering , University of Massachusetts , Amherst , Massachusetts 01003 , United States.
J Am Chem Soc. 2019 Apr 10;141(14):5886-5896. doi: 10.1021/jacs.9b00562. Epub 2019 Apr 1.
Using dynamic light scattering technique, we address the role of added salt at higher concentrations on the "ordinary-extraordinary" transition in solutions of charged macromolecules. The "ordinary" behavior has previously been associated with a "fast" diffusion coefficient which is independent of salt concentration C and polymer concentration C if the ratio C/ C is above a threshold value. The "extraordinary" transition is associated with formation of aggregates, with a "slow" diffusion coefficient, formed from similarly charged macromolecules. By investigating aqueous solutions of sodium poly(styrenesulfonate) and sodium chloride with variations in C, C, and polymer molecular weight, M, we report the emergence of a new diffusive "fast" relaxation mode at higher values of C, C, and M, in addition to the previously known "fast" and "slow" relaxation modes. Furthermore, we find that M plays a crucial role on the collective dynamics of polyelectrolyte solutions with salt, instead of just the C/ C ratio as previously postulated. As M is progressively decreased, the salty solution exhibits dynamical transitions from three modes to two modes and then to one mode of relaxation. The emergence of the new fast mode and the dynamical transitions are in marked departure from the general premise of the ordinary-extraordinary transition developed over several decades. In an effort to rationalize our experimental findings we present a theory for the collective dynamics of polyelectrolyte solutions with salt by addressing the coupling between the relaxations of polyelectrolyte chains, counterions from the polymer and added salt, and co-ions from the salt. The predictions are in qualitative agreement with experimental findings. The present combined work of experiments and theory forms the basis for accurately characterizing dynamics of charged macromolecules in salty solutions, which are ubiquitous in biological systems and polyelectrolyte-based technologies.
利用动态光散射技术，我们研究了在带电高分子溶液中，高浓度添加盐对“普通-超常”转变的作用。“普通”行为以前与“快速”扩散系数相关联，该系数与盐浓度 C 和聚合物浓度 C 无关，如果 C/C 的比值高于阈值。“超常”转变与聚集物的形成有关，聚集物由带相同电荷的高分子形成，具有“缓慢”的扩散系数。通过研究不同 C、C 和聚合物分子量 M 的水溶液中的聚（苯乙烯磺酸钠）和氯化钠，我们报告了在更高的 C、C 和 M 值处出现新的扩散“快速”弛豫模式，除了以前已知的“快速”和“缓慢”弛豫模式。此外，我们发现 M 在带盐的聚电解质溶液的集体动力学中起着至关重要的作用，而不仅仅是以前假设的 C/C 比值。随着 M 的逐渐降低，含盐溶液表现出从三种模式到两种模式再到一种模式的动力学转变。新快速模式的出现和动力学转变与几十年来发展起来的普通-超常转变的一般前提明显不同。为了使我们的实验发现合理化，我们通过解决高分子链、聚合物和添加盐的抗衡离子以及盐的共离子之间的弛豫耦合，提出了一种带有盐的聚电解质溶液的集体动力学理论。预测与实验结果定性一致。本实验与理论的综合工作为准确描述带电高分子在生物系统和聚电解质基技术中普遍存在的含盐溶液中的动力学奠定了基础。
