Collaborative Research: Synthesis and Rigidity Quantification of Ladder Polymers with Controlled Structural Defects
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With funding from the Macromolecular, Supramolecular and Nanochemistry Program of the Chemistry Division, Professors Lei Fang of Texas A&M University and Xiaodan Gu of University of Southern Mississippi are investigating how chain flexibility influences the physical properties of conjugated ladder polymers. Conjugated polymers are a unique class of non-metallic polymers with semiconducting properties resembling that of the chemical element silicon. These long chain carbon-based molecules can become conductive when external voltage is applied (like in transistors) or light shines on them (like in photovoltaic cells). Conjugated polymers are extensively used in solar cells, LED screens and other applications that utilize the conversion of electricity to light. Ladder polymers, on the other hand, are a type of double stranded polymers with the connectivity of a ladder. This is achieved by interconnecting repeating units along the main polymer chain by four chemical bonds, instead of the two bonds typically seen in conventional plastics. In this research, conjugated ladder polymers with varied structural features are synthesized. Control polymers are prepared with deliberately introduced backbone defects that consist of small molecules. Detailed studies are then conducted to correlate backbone composition and length with the flexibility of the polymer chains. These studies are enabled by employing neutron and light scattering techniques which provide accuracy at lengths ranging from sub 1 nanometer (1/100,000 of human hair diameter) to well beyond 10 micrometers (the width of cotton fiber). Correlations established as a result of this work may provide knowledge that could lead to the development of materials with better optical and electronic performance. Educational innovation tackles the problems associated with outdated contents in undergraduate organic chemistry laboratory courses. The newly designed ?Nobel Prize Reactions? are first implemented at both universities and then disseminated at national and international scales. Outreach activities expose undergraduate and high school students to modern chemical research. This work is specifically designed to benefit a large number of underrepresented minorities and economically disadvantaged students in Mississippi. The primary goal of this research is to establish the fundamental correlations between backbone constitution and the chain rigidity for rigid ladder conjugated polymers. The research is conducted through the design and synthesis of model ladder polymers with varied structural features, and controls with deliberately introduced backbone defects, followed by quantitative evaluation of their chain persistence length and correlation with structural features. Chain rigidity of the ladder polymer models and controls is quantified using combined modern characterization tools with an emphasis on neutron and light scattering. Studies are also performed to understand the influence of chain bending energy on persistent length for ladder polymers. Comprehensive structural-rigidity correlation through iterative design-synthesis-measurement-design cycles is established. Results associated with this award have the potential to advance knowledge on how the chain rigidity (or flexibility) determines the fundamental properties and practical applications of a wide range of polymers. 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.
