Cracking the Mystery of Polyelectrolyte Coacervate Structure and Dynamics

第一部分：非技术性聚合物电解质络合物是两种带电的长聚合物分子的混合物，它们的电荷相反。它们相反的电荷导致这两种可能截然不同的聚电解质相互吸引，形成亲密的混合物或络合物。聚电解质复合体的一个重要例子是染色体，它含有DNA，DNA是一种带负电荷的聚合物，因此会排斥自己。因此，大自然使用带正电的蛋白质与DNA结合，并将其捆绑成紧凑的染色体形状。聚电解质复合体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剂。尽管它们很重要，而且人们对它们在高级应用中的兴趣也越来越大，但对如何控制它们的结构和机械性能的基本了解仍然没有发展起来。拟议的工作重点是测量这些络合物的机械性能，如它们的粘度和硬度，并通过计算机模拟来确定这些性质如何受组成的控制，包括聚电解质、盐和pH。了解组成和机械性能之间的关系将有助于深入了解为什么络合物具有它们的性质，以及如何为未来的应用设计这些性质。还可能与生物功能有关，这些功能受到这些复合体的微观机械性能的影响。这项研究将与研究生和本科生的教育、推广以及软件的开发相结合。第2部分：技术总结Pi和他的团队将测量组成差异很大、含有不同盐的聚电解质凝聚体的线性流变性，并寻求测试时间-温度、时间-盐、时间-水合和时间-pH叠加，使数据能够折叠到近似的“主曲线”上。具体地说，他们将测量聚阳离子聚(N，N-二甲氨基甲基甲基丙烯酸乙酯)和聚二烯丙基二甲基铵与聚阴离子聚丙烯酸和聚苯乙烯磺酸，以及与盐离子Na+或K+，以及Cl-或Br2-的线性流变性。他们将探索凝聚体之间令人惊讶的差异和异常，这取决于所使用的聚电解质、盐和链长，包括通过改变聚阴离子和聚阳离子的分子量而产生的松弛时间不对称，粘度与聚合度的异常依赖，以及一些但不是所有凝聚体的低频平台模数。系统的方法将提供对流变行为的关键决定因素的全面了解，并克服现有数据集的限制。他们还将进行分子动力学模拟，以确定决定松弛速率的聚电解质和盐离子之间局部相互作用的性质。这将在一定程度上通过开发新的时间关联函数来确定局部单体扩散是由离子配对动力学、还是由集体的“玻璃”动力学或两者的某种混合来管理的。该奖项反映了NSF的法定使命，并已通过使用基金会的智力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.