Viscoelastic Behavior of Stimuli-Responsive Metallo-Supramolecular Polymers

刺激响应聚合物（SRPs）在施加外部刺激时表现出性质的变化，例如温度、离子强度、pH值、电场、磁场或机械场或化学或生物分析物的变化。例子包括液晶聚合物（lcp），聚合物溶液和凝胶，它们经历相形态的变化，电和磁流变流体和电活性聚合物（EAPs）。SRPs作为智能薄膜在传感器、致动器、电光器件等领域具有潜在的应用，其中粘弹性特性变得非常重要。近年来，基于金属与配体结合的金属超分子聚合物和网络的发展取得了新的进展。这个项目要求支持研究一个新的金属超分子聚合物家族的粘弹性行为的分子起源，由S. J. Rowan和他的同事开发，它可以溶解在各种有机溶剂中，并且可以通过加入光子或离子传导部分方便地功能化。了解这些材料的粘弹性和热力学行为的分子起源是迈向合理分子设计的关键一步，旨在优化其性能，并了解如何将其加工成功能薄膜。到目前为止，还没有系统地进行这种研究，这是本提案的主题。Rowan等人通过添加镧系元素和过渡金属离子（它们分别作为交联剂和扩链剂），从双配基和功能化单体中创建了超分子聚合物和可控结构网络。因此，分子量和网络结构是由离子浓度和组成、离子结合强度和动力学决定的。这些材料将金属离子的功能与动态聚合物网络的可加工性和机械强度相结合。我们建议首先研究单体与过渡金属离子混合形成的超分子聚合物的性质。粘度，光散射和小角度中子散射研究将作为单体：金属离子比的函数，在不同的溶剂中，并作为温度的函数进行。探讨了金属离子与溶剂配位力的作用。结果将在平衡链和/或动力学冻结链理论的背景下，用表观链分子量来解释。这些研究将扩展到研究添加镧系元素作为交联剂的效果。凝胶化和/或相分离的开始将通过散射和流变测量来探测。凝胶相的结构和形态将通过光、x射线和中子散射技术以及光学和透射电子显微镜进行检查。这些研究将为这种重要的新型刺激响应材料提供前驱体结构和金属离子结合特性与热力学和粘弹性行为之间关系的第一个全面图景。这一建议的智力价值源于这样一个事实，即所获得的结果将对一类重要的新的刺激反应材料产生重要的新的基础知识。此外，它们将用于测试现有的自组装聚合物和网络理论，更重要的是，它们有望发现新的现象，这将为理论家提供新的挑战。此外，该项目将产生更广泛的影响，因为它将培养两名博士研究生，并有望为本科生提供在聚合物研究的重要领域的研究机会。研究结果将在国内和国际会议上传播，并将在主要科学期刊上发表。此外，预计我们的发现将产生经验知识，这将是迈向这些材料作为功能性，刺激响应涂层和薄膜的实际商业应用的重要一步。

Stimuli-responsive polymers (SRPs) exhibit a change in properties upon application of an externalstimulus, such as a change in temperature, ionic strength, pH, electric, magnetic or mechanical fields or bychemical or biological analytes. Examples include liquid crystal polymers (LCPs), polymer solutions andgels, which undergo a change in phase morphology, electro- and magnetorheological fluids and electroactivepolymers (EAPs). SRPs have potential applications as smart films in sensors, actuators, electro-opticdevices, etc., where the viscoelastic properties become important. A recent advance has been thedevelopment of metallosupramolecular polymers and networks based on metal:ligand binding. This projectrequests support to investigate the molecular origin of the viscoelastic behavior of a novel family ofmetallosupramolecular polymers, developed by S. J. Rowan and coworkers, which dissolve in variousorganic solvents, and which can be conveniently functionalized by incorporation of photonic or ionconductingmoieties. Knowledge of the molecular origin of the viscoelastic and thermodynamic behavior ofthese materials is a key step towards rational molecular design, aimed at optimizing their properties, and tounderstanding how to process them into functional films. Such studies have not been systematicallypursued to date, and are the subject of this proposal. Rowan et al. create supramolecular polymers and networks of controlled architectures from a bisligandfunctionalized monomer via addition of lanthanide and transition metal ions, which act, respectively,as crosslinkers and as chain extenders. Thus, the molecular weight and network architecture is determinedby the ionic concentration and composition, and by the ion-binding strength and kinetics. These materialscombine the functionality of the metal ions with the processability and mechanical strength of a dynamicpolymeric network. We propose studies to first investigate the properties of the supramolecular polymersformed by mixing the monomer with transition metal ions. Viscometric, light scattering and small-angleneutron scattering studies will be performed as a function of monomer:metal ion ratio, in different solvents,and as a function of temperature. The role of the coordinating power of the metal ion and solvent will beexplored. The results will be interpreted in terms of apparent chain molecular weight, in the context oftheories of equilibrium chains and/or kinetically frozen chains. These studies will then be extended to studythe effects of adding lanthanide as crosslinker. The onset of gelation and/or phase separation will be probedby scattering and rheological measurements. The structure and morphology of the gel phase will beexamined by light, x-ray and neutron scattering techniques and by optical and transmission electronmicroscopy. These studies will provide the first comprehensive picture of the relationship betweenprecursor structure and metal ion binding characteristics versus thermodynamic and viscoelastic behaviorfor this important new class of stimuli-responsive materials. The intellectual merit of the proposal derives from the fact that the results obtained will generateimportant new fundamental knowledge on an important new class of stimuli-responsive materials. Inaddition, they will serve to test existing theories of self-assembling polymers and networks, and moreimportantly are expected to identify novel phenomena, which will provide new challenges for theorists. Moreover, the project will have broader impact, in that it will serve to train two PhD graduatestudents, and is expected to provide research opportunities for undergraduate students in a vital area ofpolymer research. The results will be disseminated at national and international meetings and will bepublished in leading scientific journals. In addition, it is expected that our findings will generate empiricalknowledge, which will be an important step toward practical commercial application of these materials asfunctional, stimuli-responsive coatings and films.