Date of Award

Fall 12-11-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

Committee Chair

Dr. Jeffrey Wiggins

Committee Chair Department

Polymers and High Performance Materials

Committee Member 2

Dr. Sarah Morgan

Committee Member 2 Department

Polymers and High Performance Materials

Committee Member 3

Dr. Charles McCormick

Committee Member 3 Department

Polymers and High Performance Materials

Committee Member 4

Dr. Robson Storey

Committee Member 4 Department

Polymers and High Performance Materials

Committee Member 5

Dr. Derek Patton

Committee Member 5 Department

Polymers and High Performance Materials

Abstract

Controlled polyacrylonitrile (PAN)-based carbon fiber precursors with defined molecular weights, polydispersities, compositions, and architectures have been prepared for their study on thermal ring-closing stabilization behavior. PAN and its copolymers of number average molecular weights exceeding 170,000 g/mol were successfully synthesized via low temperature reversible addition-fragmentation chain transfer (RAFT) polymerization. RAFT polymerizations of PAN-based precursors were compared to conventional free radical solution polymerizations with a focus on the effects of molecular weight and polydispersity on structural evolution and cyclization efficiency. When RAFT polymerization was extended to copolymers, it was found that RAFT copolymers achieved greater cyclization intensities and improved thermal stability as compared to analogous uncontrolled free radical copolymers. The greater thermal stability was attributed to the more controlled polymerization method and the reduction of chain transfer and small molecule defects.

New comonomers were introduced for PAN-based precursors and explored in relation to traditional comonomers. N-isopropylacrylamide (NIPAM) was found to be a promising comonomer by simultaneously serving as a mediator to thermal cyclization as well as a plasticizer to facilitate processing and spinning. Utilizing RAFT polymerization in combination with a semibatch reaction technique the copolymer sequencing of p(AN-co-NIPAM) was systematically investigated. Results suggest that adjusting the feed rate of each comonomer affects the comonomer distribution along the backbone by offering tunable cyclization behaviors.

Attempts were made to mediate tacticity of PAN to study effects of tacticity on cyclization. A series of Lewis acids and fluoroalcohols were employed as additives in the polymerization, but no changes in tacticity were observed.

A 98/2 p(acrylonitrile-co-NIPAM) fiber was prepared from conventional free radical solution polymerization. The fiber morphology, characterized by Transmission Electron Microscopy (TEM), displayed minimal defects at the nanoscale with a characteristic ribbon-like wavy pattern. The degree of orientation in the fibers was found to exceed that of a commercial-grade PAN-based precursor. The thermo-oxidative stability of the lab-produced fiber shared similar characteristics to commercial fibers and has set the benchmark for future designs of PAN-based carbon fiber precursors.

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