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

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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.