CAREER: Dynamic Polymer Materials with Advanced Polymer Architecture and Carbon Nanotube Reinforcements

本项目旨在通过明智地选择分子结构和纳米级增强来开发具有增强机械性能和导电性的聚合物材料。在聚合物材料中引入“动态键”（即可以在不同基团之间交换的化学键）可以导致自愈特性，并在材料受到外部损坏时增强稳定性。然而，当施加外力时，这也会导致永久性的形状变化，这可能会限制应用中的效用。本提案中开发的聚合物将不同类型的动态键结合到同一材料中，以实现自愈性和长期稳定性的结合。有必要了解底层聚合物结构如何影响动态交联聚合物的性能。底层聚合物的结构将为此进行调整，并通过加入碳纳米管来实现额外的增强，这将提高机械性能和导电性。由于动态键合的愈合性能，该研究有望生产出具有增强寿命和可再加工性的材料。这将使下一代聚合物材料能够延长使用寿命，降低更换成本，提高机械性能，增强导电性。本研究中开发的材料合成策略可以转化为增材制造，使一系列具有复杂结构的定制材料成为可能。该项目还将包括将动态交联聚合物放在K-12学生手中的外展活动，并为K-12教师开展继续教育活动。该项目还将开发教育工具，促进批判性思维和课堂参与，培养下一代科学家，推动他们从事富有成效的科学事业。此外，该项目将吸引不同背景的学生参与研究生和本科生的研究，以促进传统上代表性不足的人群参与STEM领域。本研究将开发定义明确的包含双动态交联的聚合物，其中在一种聚合物材料中使用快速交换的非共价连接剂和在外部刺激下交换的动态共价连接剂。这将创造出结合韧性、延展性、可再加工性和自愈特性的材料，同时保持传统热固性的机械稳定性和抗蠕变性。该研究将结合双动态交联与可控结构聚合物的新材料，以及碳纳米管增强材料与基体结合的新策略。聚合物的合成可以控制链长、交联剂沿链的位置和交联剂的密度。Diels-Alder化学将用于碳纳米管增强材料的动态粘接。将引入互穿网络和分段聚合物等先进结构来改善机械性能。每种材料的性能将通过一系列机械和电气测试进行评估，并导致宏观性能与底层聚合物结构的相关性。本研究的具体目标是：1。通过精确控制链长和聚合物骨架上交联剂的位置，创建双动态单穿透和互穿网络。利用碳纳米管和基质之间的动态键来增强双动态交联聚合物。通过对聚合物微观结构的精确控制，制备互穿网和单网碳纳米管复合材料。阐明聚合物的结构和微观结构与材料的力学和电学性能之间的结构-性能关系。教育活动集中在三个重点领域：1)拓展中学师生；2)新教学法的发展；3)增加STEM的多样性。第一个领域将包括向K-12学生推广非共价和动态共价交联剂聚合物材料，以及为教师提供继续教育活动。第二个领域将侧重于让学生反思自己学习的工具，并在实验和课堂活动之间建立联系。第三个领域将增加传统上代表性不足群体的学生参与STEM学习和研究。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识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 SUMMARYThis project aims to develop polymeric materials with enhanced mechanical properties and electrical conductivity through judicious choice of molecular structure and nanoscale reinforcement. Introducing "dynamic bonds" (i.e., chemical bonds that can be exchanged between different groups) into a polymeric material can lead to self-healing properties and enhanced stability in case of external damage to the material. However, this can also lead to permanent changes in shape when external forces are applied, which could limit utility in applications. The polymers developed in this proposal will incorporate different types of dynamic bonds into the same material to enable the combination of self-healable properties with long-term stability. There is a need to understand how the underlying polymer structure impacts the properties of dynamically cross-linked polymers. The structure of the underlying polymer will be tuned for this purpose and additional reinforcement will be enabled by incorporation of carbon nanotubes, which will improve mechanical properties and electrical conductivity. This research is anticipated to produce materials with enhanced lifetime and reprocessability due to the healing properties enabled by dynamic bonding. This will enable the next generation of polymeric materials to have extended useful lifetime, with reduced replacement costs, improved performance against mechanical challenges, and enhanced conductivity. The material synthesis strategies developed in this research could be translated to additive manufacturing enabling a range of tailor-made materials with complex structures. This project will also include outreach activities that place dynamically cross-linked polymers in the hands of K-12 students and develop continuing education activities for K-12 teachers. This project will also develop educational tools to promote critical thinking and involvement in classrooms to train the next generation of scientists and propel them to productive careers in science. Further, this project will engage students of diverse backgrounds in graduate and undergraduate research, to foster participation in STEM fields from populations that are traditionally underrepresented.PART 2: TECHNICAL SUMMARY This research will develop well-defined polymers containing dual-dynamic crosslinking, where both a non-covalent linker that exchanges rapidly and a dynamic covalent linker that exchanges under external stimulus are used in one polymer material. This will create materials that combine the toughness, malleability, reprocessability and self-healing character imparted by dynamic bonding while retaining mechanical stability and resistance to creep of traditional thermosets. The research will lead to new materials by combining dual dynamic crosslinking with polymers of controlled structure and new strategies for bonding carbon nanotube reinforcements to the matrix. Polymers will be synthesized with control over chain-length, placement of crosslinkers along the chain, and density of crosslinkers. Diels-Alder chemistry will be used to dynamically bond the carbon nanotube reinforcements. Advanced structures such as interpenetrating networks and segmented polymers will be introduced to improve mechanical properties. Performance of each material will be evaluated through a number of mechanical and electrical tests, and lead to correlation of macroscopic properties with the underlying polymer structure. Specific objectives of this research are:1. Create dual dynamic single and interpenetrating networks with precise control over chain length and crosslinker placement along the polymer backbone.2. Reinforce dual dynamically crosslinked polymers using dynamic bonds between carbon nanotubes and the matrix.3. Make interpenetrating network and single network carbon nanotube composite materials with precise control over polymer microstructure.4. Elucidate structure-property correlations between the polymer architecture and microstructure and the mechanical and electrical properties of the materials.Educational activities focus on three key areas: 1) Outreach to secondary school teachers and students; 2) Development of new pedagogies; 3) Increasing diversity in STEM. The first area will include outreach to K-12 students on polymeric materials with non-covalent and dynamic covalent crosslinkers, as well as continuing education activities for teachers. The second area will focus on tools to engage students to reflect on their own learning, and develop connections between experiments and classroom activities. The third area will increase involvement of students from traditionally underrepresented groups in STEM studies and research.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.