carbon fiber precursors based on polyethylene graft pitch copolymer
Open Access
- Author:
- Li, Houxiang
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- January 27, 2021
- Committee Members:
- Tze-Chiang Chung, Dissertation Advisor/Co-Advisor
Tze-Chiang Chung, Committee Chair/Co-Chair
Ralph H Colby, Committee Member
Michael Anthony Hickner, Committee Member
Semih Eser, Outside Member
John C Mauro, Program Head/Chair - Keywords:
- Carbon Fiber Precursors
Polyolefins - Abstract:
- Today, both of the mainstream precursors of carbon fibers (CFs)—PAN and pitch—have many downsides, which are limiting the cost reduction and improvement on production efficiency of CFs. With a rapidly increasing global demand for this type of high-performance material, there is growing interest in finding a better precursor. However, despite numerous studies, researchers have yet to develop a new polymeric precursor that is less expensive, melt-spinnable, and offers a high carbon yield. In this thesis, I discuss our research approach and the experimental results that may address this important technological issue. We have developed a new class of hydrocarbon polymer precursors, called 'PE-g-Pitch', based on polyethylene (PE) polymer chain with grafted pitch-like side groups with the structure of polycyclic aromatic hydrocarbon (PAH). Some ungrafted pitch is also present in the precursor to reduce the overall melt viscosity during the melt-spinning process. In addition to its low-cost raw materials (PE-copolymer and petroleum pitch), the resulting PE-g-Pitch precursors with some suitable compositions also show good melt processability and high carbon yield under a simple one-step thermal conversion process in N2 atmosphere. Additionally, PE-g-Pitch can also be solution-spun as an alternative option. In Chapter 1, I present the background information on CFs and the mainstream CF precursors applied in the industry today. Some recent research studies about CF precursors are also discussed, along with their scientific considerations and the challenges hindering them from further development. This chapter also includes the studies that inspired us in designing the new CF precursor PE-g-Pitch. Chapter 2 is about the research at the early stage of our study before the invention of PE-g-Pitch. I explain our research approach for designing new synthetic hydrocarbon polymers, as well as the experimental results showing the desirable hydrocarbon structures that can be thermally transformed into a carbon crystal structure with high carbon yield. Specifically, I focus on a new hydrocarbon polymer system based on poly(phenylacetylene) derivatives that have a π-electrons conjugated molecular structure and the active side groups for Diels-Alder-type cycloaddition reaction. One with a para-substituted acetylene group shows an exceptionally high carbon yield of more than 90%. The other one with a para-substituted phenylacetylenyl group also offers a high carbon yield of 75%, as well as good solubility in common organic solvents. This polymer is suitable for electro-spinning processes to form the corresponding precursor fibers. In Chapter 3, I expand the structure-designing strategy to the PE copolymer and discuss the preparation of the melt-spinnable PE-g-Pitch precursor. To prepare the precursor, a semi-crystalline PE copolymer containing diphenylacetylenyl side groups was applied to undergo a Diel-Alders cycloaddition reaction with petroleum pitch to allow the PAH molecules to be grafted onto the PE backbones. A systematic study was conducted on the blend samples of PE-g-Pitch and unreacted petroleum pitch that serve as a plasticizer to reduce melt viscosity for the melt-spinning process. Furthermore, the investigation shows the detailed condition of precursor preparation and the thermal conversion process forming the corresponding CFs with a high carbon yield of 73% and the desirable polymorphous CF morphology. In Chapter 4, to understand the stabilization mechanism of PE-g-Pitch, this material was obtained by using the toluene-soluble portion of petroleum pitch as the reactant that can be completely removed by Soxhlet extractor after the cycloaddition reaction. I discuss the crucial role that PAH side groups play in forming cross-linkages via polycondensation reactions during the stabilization at low temperature, and subsequently in promoting the dehydrogenation of PE chains to form a more extensively conjugated structure that facilitates the carbon conversion. In Chapter 5, I focus on the PE-g-Pitch precursor prepared by the Diels-Alder cycloaddition reaction between petroleum pitch and a lower-cost and more easily-prepared PE copolymer with 4-bromophenyl side groups (PE-co-4-bromostyrene), which was synthesized by the copolymerization of ethylene and 4-bromostyrene. A systematic study is discussed to provide important information about the effect of the size of pitch-like side groups on the solution-spinnability and carbon yield of PE-g-Pitch. The enlarged side groups generate strong π-interaction and form physical cross-linking between polymer chains in the solution. They also facilitate a higher carbon conversion by creating a more conjugated structure. Additionally, the preparation of precursor fibers by both solution- and melt-spinning methods, as well as the procedures for CF conversion, are discussed. Some resulting CFs show mechanical properties suitable for general applications even without optimizing on the stabilization and carbonization conditions. In Chapter 6, I discuss an alternative method for preparing the poly(ethylene-4-bromostyrene) copolymer via the bromination of a poly(ethylene-styrene) copolymer that has been widely commercialized at a very low cost. The brominated poly(ethylene-styrene) copolymer has a more random microstructure than the previous PE-co-4-bromostyrene synthesized by direct copolymerization of ethylene and 4-bromostyrene. PE-g-Pitch prepared from this PE copolymer with a more random structure has good solution-spinnability and can achieve a carbon yield of about 60%. Overall, this thesis provides details on several strategies for preparing a PE-g-Pitch precursor for the economic and efficient production of high-performance CFs. The new precursor exhibits a combination of advantages, including (i) Low raw materials cost (ii) Less energy consumption (iii) Melt-spinnable or solution-spinnable in a common solvent (iv) High carbon yield of >50% (v) More thorough structural stabilization (vi) Easily scaled-up production