Date of Award

12-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Advisor

Dr. Amod A. Ogale

Committee Member

Dr. Douglas E. Hirt

Committee Member

Dr. Christopher L. Kitchens

Committee Member

Dr. David P. Anderson

Abstract

Due to their high strength and stiffness coupled with low density, carbon fibers are the preferred reinforcing fibers used in high-performance polymer matrix composites. Consequently, the demand for carbon fibers is expected to grow from 40,000 metric tons to about 150,000 metric tons over the next decade. A large fraction of the increase is anticipated in the industrial sector that is cost-sensitive. Therefore, there is a need for the development of novel processes and precursors to reduce the carbon fiber production cost and expand the use of carbon fibers. This research focuses on the development of an alternative, rapid stabilization route for polyacrylonitrile (PAN) precursor-based carbon fibers. The main goal was to investigate the stabilization reactions induced by the addition of photo-initiators and UV-treatment of polyacrylonitrile (PAN) based precursors. Also, the role of this external photoinitiator on the fiber spinnability, properties of the spun precursor fibers, and the properties of the resulting thermal stabilized and carbonized PAN-based fibers were systematically studied.

Two mechanisms of photo-initiation were investigated: homolytic cleavage and hydrogen abstraction. Solution-cast PAN copolymer samples containing both types of photo-initiators were irradiated for different durations (100 - 600 s) and temperatures (65 - 100°C, i.e., below and above glass transition temperature). FTIR spectra show the formation of carbon-oxygen, carbon-nitrogen, and carbon-carbon double bonds attributed to the development of cyclized structure. Samples containing hydrogen abstraction photoinitiator show higher extents of cyclization among the different set of samples. This observation was also confirmed by higher gel contents. FTIR conversion indices of samples UV treated above glass transition temperature were higher compared with that for the same specimens UV treated below glass transition temperature, as expected. DSC results show that samples containing hydrogen abstraction photoinitiator enable a higher extent of post-UV thermal stabilization. FTIR spectra of the UV treated samples, when compared with only thermally stabilized specimens, confirm that the addition of 1 wt% photoinitiator to PAN followed by 5 minutes of UV treatment increases the rate of the cyclization reaction and reduces the thermal oxidation time by over an hour, which could significantly reduce the conventional stabilization time by half.

Rheology measurements show no adverse effect on the viscosity of solutions by the addition of the photoinitiator. Fibers containing photoinitiator were successfully wet-spun from PAN-DMSO solution. SEM micrographs show no deterioration of the post-stretched fiber microstructure due to the presence of photoinitiator. After UV treatment, fibers that contained 4,4’-bis(diethylamino)benzophenone display a higher tensile modulus as compared with that of other sets. Wide-angle X-ray diffraction results show no significant decrease in interplanar spacing and size of the crystals within the fibers containing photoinitiator, but such fibers retain a higher extent of molecular orientation after being UV treated. Conversion indices were measured from the WAXD spectra and compared with those for fibers that were only thermally stabilized. These results agree with previous FTIR ones, which established that only 5 minutes of UV treatment leads to a conversion index that is observed in control fibers after more than an hour of only thermo-oxidative stabilization.

After a short UV treatment that induced cyclization and crosslinking, precursor fibers could be rapidly thermo-oxidatively stabilized and successfully carbonized. SEM micrographs show no deterioration of the microstructure or significant skin-core formation of the fibers due to UV treatment and presence of photoinitiator. Fast-thermal stabilized pure polyacrylonitrile carbon fibers contained core-hollow fiber defects due to inadequate thermal stabilization, but such defects were not observed for fast-thermal stabilized fibers that were UV treated and contained photoinitiator. Tensile testing results show fibers containing 1 wt% photoinitiator and UV treated for 5 minutes display higher tensile modulus than all the other set of thermal stabilized and carbonized fibers. WAXD results show a higher development of the aromatic structure and molecular orientation in thermal stabilized fibers. Although no significant increase in interplanar spacing and size of the crystals was observed within the UV-assisted-stabilized carbon fibers containing photoinitiator, but such fibers retain a higher extent of molecular orientation when compared with control pure fibers.

In summary, these results establish for the first time the positive effect of the addition of photoinitiator and UV treatment on the properties of the polyacrylonitrile-based fibers. This novel, dual UV-thermal mechanism can be used to develop processes that can reduce the thermal oxidation time (faster fiber production) while retaining the mechanical properties of the carbon fibers thus produced.

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