Advancing Processability and Material Performance of Synthetic Polyamino Acids with Transformable Secondary Structures

非技术综述：许多生物材料的性质和性能取决于其组成分子的结构和组织，这些分子可以是螺旋或片状的形式。有趣的是，纤维蛋白中从螺旋到片状的动态转变促进了强度、硬度和能量耗散能力的显著增加。通过非常简单的结构块和它们的转换就可以实现先进和多样化的材料性能，这一事实激励了研究人员基于同样的概念开发新的聚合物材料。多氨基酸(PaaS)，也称为合成多肽，可以采用类似的结构。然而，在高相对分子质量(MWs)的固体PAA中诱导结构转变在很大程度上是一个尚未解决的挑战。因此，许多PAA材料要么具有较差的热机械性能，要么与挤出和压缩模塑等聚合物加工技术不兼容。本项目旨在开发一种通用策略，通过利用PAAS的亚稳、可变形结构并控制其原位转变和层次化组织，显著改善合成PAAS的热机械性能和加工性能。这项研究的成功完成将打破溶液工艺的限制，利用标准的聚合物加工技术，在熔体中制备新的PAA材料，用于大规模生产形态和性能可控的薄膜/棒/纤维。研究生和本科生将接受生物启发的聚合物材料方面的培训，并获得聚合物合成、材料表征、力学和计算机模拟方面的技能。该计划将把研究整合到新的和现有的课程中，促进跨学科教育和高质量的研究体验，并通过讲座和研讨会向公众展示材料研究的应用。技术摘要：通过氨基酸N-羧酸酐单体的开环聚合制备的高分子PAAS是形成稳定的α-螺旋或β-折叠结构的多肽模型体系。近年来，随着活性协同聚合等策略的发展，具有复杂结构和可控分子结构的PAAS以高产率和高纯度得到了大规模的合成。尽管如此，由纤维蛋白制成的结构生物材料具有更先进的材料性能，而且它们比有机材料具有更多样化的力学行为。这项计划中的研究配备了如何合成能够从α-螺旋转化为β-折叠的PAA共聚物的新知识，旨在提高固体PAA材料的机械性能和加工性。对PAAS有序二级结构的控制、其在固态中的原位转化和层次化组织是本研究的核心。经过精心设计的含有可变形二级结构的PAA共聚物将被用于制造硬质热固性材料、半硬质应变硬化材料，以及在低应力下柔软但具有高抗拉强度和延伸性的材料。这个项目的发现可能会使聚合物系统的产生接近一些自然生物材料中的复杂程度和多功能性。这项研究还提供了具有内在二级结构的合成聚合物的模型系统，其中分子内和分子间网络的不同划分决定了材料的宏观性质，从而能够将实验结果与模拟和建模的预测进行比较。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

NON-TECHNICAL SUMMARY:The properties and performance of many biological materials rely on the structures and organization of their constituent molecules, which can be in the form of helices or sheets. Interestingly, dynamic transition from helices to sheets in fibrous proteins facilitates a remarkable increase in the strength, stiffness, and energy dissipation capacity. The fact that advanced and diverse material performance can be achieved by remarkably simple structural blocks and their transformations has inspired researchers to develop new polymeric materials based on the same concept. Polyamino acids (PAAs), also known as synthetic polypeptides, can adopt analogous structures. However, inducing the structural transitions in the solid PAA of high molecular weights (MWs) is a largely unmet challenge. As a result, many of the PAA materials either have poor thermomechanical properties or are incompatible with polymer processing techniques such as extrusion and compression molding. This project aims to develop a general strategy to significantly improve the thermomechanical properties and processability of synthetic PAAs by taking advantage of metastable, transformable structures of PAAs and control over their in-situ transition and hierarchical organization. Successful completion of the research will break the constraint of solution process and prepare new PAA materials in the melt for large-scale production of films/bars/fibers with morphological and properties control, using standard polymer processing techniques. Graduate and undergraduate students will be trained on bioinspired polymeric materials and acquire skills in polymer synthesis, material characterization, mechanics, and computer simulations. The program will integrate the research into the new and existing course, promote interdisciplinary education and high-quality research experiences, and demonstrate the application of materials research through lectures and workshops for the public. Emphasis will be given to involving underrepresented students at all levels.TECHNICAL SUMMARY:Prepared by ring-opening polymerization of amino acid N-carboxyanhydride monomers, high-MW PAAs are polypeptide model systems that form stable alpha-helix or beta-sheet structures. With the recent development of living cooperative polymerization and other strategies, PAAs with complex architectures and controlled molecular structures have been synthesized on large scales at high yield and purity. Still, structural biological materials made from fibrous proteins have far more advanced material performance, and they have more diverse mechanical behaviors than their organic counterparts. Equipped by a new knowledge on how to synthesize PAA copolymers capable of transforming from alpha-helices to beta-sheets, the planned research seeks advancements in the mechanical properties and processability of solid PAA materials. Control over ordered secondary structures of PAAs, their in-situ transformation and hierarchical organization in the solid state are central to this study. Carefully designed PAA copolymers containing transformable secondary structures will be utilized to make hard thermoset materials, semi-hard strain-stiffening materials, and materials that are soft at low stress but have high tensile strength and extensibility. The findings from this project may enable the generation of polymeric systems that will approach the level of sophistication and versatility found in some of nature’s biomaterials. The research also provides a model system of synthetic polymers with intrinsic secondary structures in which the different partitioning of intramolecular and intermolecular networks determines the macroscopic properties of materials, enabling comparison of the experimental results with predictions from simulations and modeling.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.