Cracking the Mystery of Polyelectrolyte Coacervate Structure and Dynamics

聚电解质复合物是两种带相反电荷的带电长聚合物分子的混合物。它们的电荷相反，导致两种不同的聚电解质相互吸引，形成一种紧密的混合物或复合物。多电解质复合物的一个重要例子是染色体，它含有DNA， DNA是带负电荷的聚合物，因此会自我排斥。因此，大自然使用带正电的蛋白质与DNA结合，并将其捆绑成染色体的紧密形状。多电解质复合物的许多其他例子包括生物膜、水下粘合剂、药物输送载体和食品加工试剂。尽管它们的重要性和对它们的高级应用越来越感兴趣，但如何控制它们的结构和力学性能的基本理解仍然没有发展。建议的工作重点是测量这些配合物的机械性能，例如它们的粘度和刚度，以及计算机模拟，以确定这些性能是如何由组合物（包括聚电解质、盐和ph）控制的。了解组合物和机械性能之间的关系将提供深入了解为什么配合物具有它们所做的性能，以及如何为未来的应用设计这些性能。也可能与生物功能有关，这些功能受到这些复合物的微观力学特性的影响。这项研究将与研究生和本科生的教育、推广和软件开发相结合。PI和他的团队将测量具有广泛变化组成和含有不同盐的聚电解质凝聚体的线性流变学，并试图测试时间-温度，时间-盐，时间-水合作用和时间- ph叠加，从而使数据可以折叠到近似的“主曲线”上。具体来说，他们将测量聚阳离子聚（N，N-二甲氨基乙基甲基丙烯酸酯）和聚（二烯基二甲基铵）与聚阴离子聚（丙烯酸）和聚（苯乙烯磺酸），以及盐离子Na+或K+， Cl-或Br-的线性流变性能。他们将探索凝聚体之间的惊人差异和异常，这取决于所使用的聚电解质、盐和链长度，包括通过改变聚阴离子与聚阳离子的分子量而产生的弛豫时间的不对称性，粘度对聚合程度的异常依赖，以及一些（但不是全部）凝聚体的低频平台模量。系统的方法将提供对流变行为的关键决定因素的全面理解，并克服现有数据集的局限性。他们还将进行分子动力学模拟，以确定聚电解质和盐离子之间局部相互作用的性质，这些相互作用决定了弛豫速率。这部分将通过开发新的时间相关函数来完成，以确定局部单体扩散是由离子配对动力学控制，还是由集体“玻璃”动力学控制，或者是两者的混合控制。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

PART 1: NON-TECHNICAL SUMMARYPolyelectrolyte complexes are a mixture of two kinds of electrically charged, long polymer molecules, of opposite charge. Their opposite charge causes the two kinds of polyelectrolytes, which can be very different from each other, to nevertheless attract each other and form an intimate mixture, or complex. An important example of a polyelectrolyte complex is the chromosome, which contains DNA, which is a negatively charged polymer and hence repels itself. Nature therefore uses positively charged proteins to bind to the DNA and bundle it into the compact shape of the chromosome. Among the many other examples of polyelectrolyte complexes are biological membranes, underwater adhesives, drug delivery vehicles, and food processing agents. Despite their importance and the growing interest in them for advanced applications, basic understanding of how to control their structure and mechanical properties remains undeveloped. The proposed work focuses on measurements of the mechanical properties of these complexes, such as their viscosity and stiffness, and computer simulations to determine how these properties are controlled by the composition, including the polyelectrolytes, salts, and pH. Understanding the relationship between the composition and mechanical properties will provide deep insight into why complexes have the properties they do, and how to design these properties for future applications. There may also be connections to biological function, which are affected by the micro-mechanical properties of these complexes. The research will be integrated with education of graduate and undergraduate students, outreach, and creation of software.PART 2: TECHNICAL SUMMARYThe PI and his group will measure the linear rheology of polyelectrolyte coacervates of widely varying composition and containing different salts, and seek to test time-temperature, time-salt, time-hydration, and time-pH superpositions that allow data to be collapsed onto approximate “master curves.” Specifically, they will measure the linear rheological properties of polycations poly(N,N-dimethylaminoethyl methacrylate) and poly(diallyldimethylammonium) with polyanions poly(acrylic acid) and poly(styrene sulfonate), and with salt ions Na+ or K+, and Cl- or Br-. They will explore the surprising differences and anomalies among coacervates, depending on the polyelectrolytes, salts, and the chain lengths used, including an asymmetry in relaxation time produced by changing the molecular weight of polyanion vs. polycation, anomalous dependence of viscosity on degree of polymerization, and a low-frequency plateau modulus for some, but not all, coacervates. The systematic approach will provide a comprehensive understanding of the key determinants of rheological behaviour and overcome the limitations of existing data sets. They will also carry out molecular dynamics simulations to determine the nature of the local interactions among polyelectrolytes and salt ions that determine the rates of relaxation. This will be done in part through development of novel time correlation functions to determine whether local monomer diffusion is governed by ion-pairing dynamics, or by collective “glassy” dynamics, or some mixture of the two..This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.