Collaborative Research: Robust General Methods for Determination of Polyelectrolyte Molecular Weight and Polydispersity

在化学系大分子、超分子和纳米化学(MSN)计划的支持下，宾夕法尼亚州立大学的Ralph H.Colby教授和弗吉尼亚理工学院和州立大学的Louis A.Madsen教授正在开发测量聚电解质分子量的方法。聚电解质是一种长链大分子，其重复单元包含一个电解质基团。这个基团在水中解离，使聚合物带电。与盐类似，聚电解质溶液具有导电性。许多生物和人工大分子是聚电解质，在生物医学、技术和工业科学中得到了应用。一些值得注意的例子包括植入物涂层、受控药物输送、净水系统、食品涂层和化妆品。虽然有许多已开发的技术用于准确确定中性聚合物的分子量，但针对离子或带电聚合物的方法相对较少。主要原因是缺乏足够的理论方法来描述聚电解质的性质和在溶液中非常复杂的行为。本研究旨在为包括天然和合成体系在内的具有单一电荷类型的离子聚合物(聚阴离子或聚阳离子)的分子量测定提供依据。这里正在开发的方法，如果成功，将大大加快聚电解质的设计和合成。此外，拟议的理论和实验方法的开发和验证预计将进一步影响聚合物科学的物理基础，并使人们能够对离子聚合物有新的理解。这项工作还有望促进许多生物聚合物的分子量表征，如透明质酸、肝素、DNA和RNA。培训机会和接触聚合物化学的概念和技术将提供给研究生和本科生在这两个机构。研究活动还将被用来招收大分子专业的学生，并为他们在聚合物科学领域的职业生涯做好准备。弗吉尼亚理工大学的外展活动将包括重点介绍离子聚合物凝胶、大分子链运动和能量储存方面的概念。这项亲身实践的K-12推广计划旨在拓宽思维，鼓励小学生和高中生考虑从事科学和技术职业。宾夕法尼亚州立大学正在进行的REU(本科生研究经验)计划将被利用，以包括从代表性不足的群体中招收学生。聚电解质越来越多地用于替代能源技术、工程人体组织、药物输送和许多其他应用。聚电解质的广泛开发和应用的一个关键障碍是需要极端努力来表征它们的绝对分子量。此外，沿着每条链的强烈静电斥力与拉伸链条的熵惩罚之间的平衡是微妙的，还没有详细了解，使得简单的模型只在特定情况下适用。本项目将专注于聚电解质在溶液中行为的详细标度理论，该理论是测量数均分子量(MN)的四种不同方法(使用测量的末端模数、松弛时间、链扩散、相关长度和粘度)的基础。这四种方法将采用溶液流变学、X射线散射和核磁共振扩散法进行测量。重点将放在开发一个工具箱，为这些方法在半稀疏无纠缠状态下的实际应用建立明确的操作策略和参数空间。测量范围将包括一系列广泛的聚电解质化学物质。此外，使用基于扩散的核磁共振波谱测定分子量方法可靠地量化聚电解质的多分散性的技术正在开发中。最后，对中性聚合物的分析方法的实用性也进行了探讨。如果成功，这项研究不仅将提供一个其他人可以使用的实验技术工具箱，而且还将提供一个理论框架，用于描述转化为分子量和多分散性的数据，这些数据在表征带电聚合物时至关重要。该奖项反映了NSF的法定使命，并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。

With the support of the Macromolecular, Supramolecular, and Nanochemistry (MSN) Program in the Division of Chemistry, Professors Ralph H. Colby from Pennsylvania State University and Louis A. Madsen from Virginia Polytechnic Institute and State University are developing methods for measuring the molecular weight of polyelectrolytes. Polyelectrolytes are long chain macromolecules whose repeating units contain an electrolyte group. This group dissociates in water, making the polymer charged. Similar to salts, polyelectrolyte solutions are electrically conductive. Many biological and artificial macromolecules are polyelectrolytes and find application in biomedical, technological and industrial science. Some notable examples include implant coatings, controlled drug delivery, water-purification systems, food coatings and cosmetics. While there are many developed techniques used to accurately determine the molecular weight of neutral polymers, methodologies for ionic or charged polymers are comparatively scarce. The principal reason is the lack of adequate theoretical approaches to describe polyelectrolyte properties and very complex behavior in solution. This research aims to provide a basis for molecular weight determination for ionic polymers having a single type of charge (polyanions or polycations), including both natural and synthetic systems. The methods being developed here, if successful, could significantly accelerate the design and synthesis of polyelectrolytes. Additionally, the development and validation of the proposed theoretical and experimental methods is expected to further impact the physical underpinnings of polymer science and enable new understanding of ionic polymers. This work also promises to facilitate the characterization of molecular weight for many biopolymers such as hyaluronic acid, heparin, DNA and RNA. Training opportunities and exposure to polymer chemistry concepts and techniques will be provided to graduate and undergraduate students at both institutions. Research activities will also be utilized to recruit students into macromolecular programs and prepare them for careers in polymer science. Outreach activities at Virginia Tech will include presentations that highlight concepts in ionic polymer gels, macromolecular chain motions and energy storage. This hands-on K-12 outreach program seeks to broaden thinking and encourage elementary and high school students to consider pursuing science and technology careers. The ongoing REU (research experiences for undergraduates) program at Pennsylvania State University will be leveraged to include recruitment of students from underrepresented groups. Polyelectrolytes are increasingly used in alternative energy technologies, engineered human tissues, drug delivery, and a host of other applications. A critical barrier to wider development and application of polyelectrolytes lies in the extreme effort required to characterize their absolute molecular weight. Furthermore, the balance between strong electrostatic repulsions along each chain with the entropic penalty for stretching the chain is delicate and not yet understood in detail, making simple models only work under specific circumstances. This project will focus on a detailed scaling theory for polyelectrolyte behaviors in solution that underlies four distinct methods (using measured terminal modulus, relaxation time, chain diffusion, correlation length, and viscosity) for measuring number-average molecular weight (Mn). These four methods will employ measurements by solution rheology, X-ray scattering, and NMR diffusometry. A strong emphasis will be placed on developing a toolbox that establishes well-defined operating strategies and parameter spaces for practical applications of these methods in the semi-dilute unentangled regime. The scope of measurements will include a wide array of polyelectrolyte chemistries. Furthermore, a technique to reliably quantify polydispersity for polyelectrolytes using the diffusion-based molecular weight determination method by NMR spectroscopy is under development. Lastly, the utility of the methods for analyzing neutral polymers will also be explored. If successful, this research will not only provide a toolbox of experimental techniques that others can use, but also a theoretical framework for scaling relationships to describe data that translate into molecular weight and polydispersity, parameters of critical importance in characterizing charged polymers.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.