This proposal unites academic research groups at the University of Virginia, Cornell University, and Florida State University with a leading polyolefin industrial scientist at ExxonMobil Research and Engineering Corporation. The research focuses on the development of novel polypropylene synthetic chemistry and an exploration of the fundamental physical phenomena underlying nucleation and growth in quiescent and flow-induced crystallization of semicrystalline polymers. Specifically, the PIs will use branching architecture as a tool to control nucleation and thereby manipulate the final crystalline morphology and macroscopic material properties. The team assembled to achieve this goal is skilled in novel polyolefin synthesis, crystallization kinetics and structural characterization, rheology and flow-induced crystallization, and industrial polymer processing. Model isotactic polypropylene (iPP) materials, including narrow molecular weight distribution linear, star, H-, and comb polymers, will be synthesized with precisely controlled stereoregularity and location of branch points. Quiescent crystallization experiments will principally seek to ascertain: (1) the influence of increasing chain irregularity due to branching on the level of crystalline organization and relative content of the alpha and gamma phases in homopolymer samples; and (2) the type and conformation of branching architecture that enhances nucleation in blends with linear chains. Flow-induced crystallization of linear and branched iPP blends will seek to determine: (1) how crystallization kinetics, nucleation density, degree of crystallinity, and crystalline structure are influenced by branching for fixed longest relaxation time; (2) if molecular architecture alters the local segmental orientation to promote nucleation; and (3) how polymorphism and morphology depend upon the number of arms (stars), ratio of branch to main chain molecular weight (H-polymers), and number of branch points (combs). NON-TECHNICAL SUMMARY Over 43 million tons of thermoplastic resins are produced in the U.S. each year with an estimated market value of over $65 billion. Much processing is performed in an ad hoc manner without the benefit of modeling or coherent blending strategies. Since the raw materials are often not renewable, waste in processing has a significant environmental impact. Moreover, the ability to exert better control over crystallinity and crystalline morphology will lead to better films, lighter weight parts, and also inject inexpensive PP materials into novel applications due to extended material properties. By providing quiescent and flow-induced crystallization data on well-defined material systems, theoretical tools allowing quantitative predictions of semicrystalline morphology are expected to result from this work. Students in Chemistry and Chemical Engineering will be not only be exposed to modern polymer synthesis and characterization, rheology, and material characterization techniques (e.g., X-ray scattering, birefringence, optical and transmission electron microscopy), but they will also be able to participate in industrial research experiences at ExxonMobil. The PIs will also combine their diverse talents and perspectives to assemble a K12 educational program on "Plastics" to be adopted in their respective communities. The PIs also have a record of including underrepresented groups in their research efforts (e.g., undergraduates from Ghana and Panama and several female undergraduates, graduates, and postdocs). Additionally, the FAMU-FSU College of Engineering is a jointly managed program of FAMU, a historically black college and university, and FSU with 40% minority and 25% female enrollment, and numerous African-American undergraduates have conducted undergraduate research in the laboratory of the PI at that institution.
This project is a collaboration between scientists in industry and in universities who, together, formed a focused research group (FRG) sponsored by the U.S. National Science Foundation (NSF) program to provide Grant Opportunities for Academic Liaisons with Industry (GOALI). Three universities—California Institute of Technology, Cornell University and Florida State University—teamed with one of the world’s major polyolefin producers—ExxonMobil—to explore molecular approaches to improve this important class of materials (Figure 1). NSF funding is leveraged by the additional support provided by the industry collaborator, enabling ambitious multidisciplinary research. With their low cost and wide diversity of material properties, polyolefins are the most widely used family of synthetic polymers today. In their solid form they are neither fully crystalline nor amorphous; instead they are "semicrystalline" with a crystal fraction that depends strongly on how the material was processed. The microscopic arrangement of crystallites and noncrystalline material profoundly affects their physical properties. This research program was devoted to understanding the ways that a polymer’s molecular structure and processing conditions can be used to create new combinations of properties. This type of research requires the combined expertise of scientists and engineers from different fields. Chemists at Cornell developed new ways to synthesize polyolefins and used them to produce molecules with unusual arrangements of their side groups (Figure 2). Advanced characterization methods available at ExxonMobil were applied to obtain essential information regarding the molecular structures new polymers. Scientists at Florida State carefully analyzed the spectra from ExxonMobil to solve for the chain structure and characterize its relationship to the types of crystallites that formed. Together they made discoveries of scientific interest and technological significance, leading to both publications in scientific journals and a patent. To clarify the role of polymer processing in governing the material properties of polyolefin fibers, films and injection molded parts, the Caltech team applied unique instrumentation to examine polymers with carefully controlled molecular structure that were synthesized by ExxonMobil specifically for this research. The experiments revealed a very strong interaction between the local molecular structure (short side branches that contain just a few atoms) and the global molecular topology. Polymers that contain just a few percent of short side branches inhibit the formation of highly oriented structure, even if the polymer is subjected to strong shearing. To understand this more deeply, Caltech researchers developed a new method for analyzing the sequence of structure formation in polymers that have short side branches. Using a mathematical method to correlate changes in structure at different length scales, the Caltech-ExxonMobil collaboration provided new insight into the sequential formation of crystallites, with each subsequent population forming under the constraints created by crystallites that formed before them (Figure 3). Through this project, all three universities fostered the development of young scientists and engineers. Graduate students engaged in the research as part of their training; and the multi-institution collaboration gave them valuable experience coordinating their activities with collaborators across the country and across different scientific fields. Working with scientists in industry gave the students insight into careers beyond academia. Professors involved in this research also helped to educate future students about the opportunities available in science and technology careers. To foster a diverse workforce, the professors met with young people who are from groups that are underrepresented in today’s science and technology workforce. Therefore, the professors made special efforts to ensure that women and minority students were included in their outreach programs (Figure 4).
