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

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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.