Date of Award

Fall 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

School

Polymer Science and Engineering

Committee Chair

Jeffrey S. Wiggins

Committee Chair School

Polymer Science and Engineering

Committee Member 2

Sarah E. Morgan

Committee Member 2 School

Polymer Science and Engineering

Committee Member 3

Sergei I. Nazarenko

Committee Member 3 School

Polymer Science and Engineering

Committee Member 4

Derek L. Patton

Committee Member 4 School

Polymer Science and Engineering

Committee Member 5

Robson F. Storey

Committee Member 5 School

Polymer Science and Engineering

Committee Member 6

None

Abstract

Basic research to control the morphology of polyacrylonitrile (PAN)-based carbon fiber is crucial for next generation composites as it determines their mechanical properties and final use. Poor molecular design of PAN-based precursors and fiber processing causes morphological defects and mechanical limitations.1,2 This research focused on utilizing the controlled polymerization technique, reversible addition-fragmentation chain transfer (RAFT), of novel acrylamide comonomers to afford well-defined precursors with precisely controlled molecular design. This controlled RAFT technique improved the overall precursor graphitic structure as evident by the increased extent of stabilization and reduced activation energy as compared to precursors prepared by traditional free radical polymerization.

The effect of increasing N-ethyl acrylamide (NEAA), N-isopropylacrylamide (NIPAM), and N-tert-butylacrylamide (NTAA) comonomer concentration on copolymer architecture and PAN ring closure was evaluated. Reactivity ratio calculations confirmed that all acrylamide comonomers would cross-propagate with acrylonitrile to yield the desired alternating PAN copolymer architecture. Increased comonomer concentration reduced the amount of cyclization sites, which resulted in an overall decrease in PAN ring closure upon heating as evident by reduced extent of stabilization and exothermic behavior. The knowledge gained on the interdependencies of precursor design on PAN copolymer architecture and ring closure was used to down-select three precursors, two RAFT-based precursors that displayed promising graphitic structure and one free radical precursor, for white fiber spinning.

Circular white fibers were spun at Deakin University through the careful selection of white fiber spinning parameters, where fiber diameters of ~ 12 µm or less were observed by scanning electron microscopy. RAFT-based white fibers exhibited more consistent break stress values than free radical-based white fibers and suggested that controlling precursor design and fiber processing afforded a more regular white fiber morphology. The amount of white fiber spun was hindered by the limited amount of RAFT precursor; therefore, attempts to synthesize several grams of high molecular weight PAN precursors were performed via a continuous reactor technique and only yielded a molecular weight of ~ 45,000 g/mol. Ultimately, this research provided new knowledge on the effect of controlling precursor molecular design and fiber processing on fiber morphology.

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