Dissertation Defense – Huibin Chang
Dr. Satish Kumar, Advisor, MSE
Dr. Yulin Deng, CHBE
Dr. Kyriaki Kalaitzidou, ME
Dr. Robert Moon, MSE
Dr. Paul S. Russo, MSE
"Studies on polyacrylonitrile/cellulose nanocrystals composite precursor and carbon fibers"
Carbon fibers exhibit lower density and higher specific strength and modulus as compared to other structural materials such as steel, and are therefore gaining more attention as a light-weight material for high performance applications. Currently polyacrylonitrile (PAN), a petroleum derivative, is the predominant precursor for carbon fibers. In order to make “green” carbon fibers, renewable materials have been proposed as precursor for carbon fibers. Cellulose nanocrystals (CNCs) have attracted significant interest due to biorenewability, sustainability, high aspect ratio, and high mechanical properties. This research adopts PAN as the matrix polymer and CNCs as nanofillers. PAN fibers containing 1, 5, 10, 20 and 40 wt% CNC were spun by gel-spinning technology. The structure, morphology, as well as mechanical and thermal properties of the precursor and carbon fibers were studied.
The work includes five parts: (i) Developing a co-solvent based method to disperse CNCs in organic solvents. (ii) Using this co-solvent based CNC dispersion approach, PAN fibers containing up to 40 wt% CNC were obtained. (iii) In these high CNC loaded polymer fibers, the orientation and stress transfer in CNC composite fibers were characterized by Raman spectroscopy. (iv) Investigating the effect of CNC on the stabilization kinetics of PAN fibers. (v) PAN/CNC composite fibers were successfully converted to carbon fibers. The results show that H2O/dimethylformamide (DMF) co-solvent can more effectively disperse individual CNC than pure H2O or pure DMF. The addition of CNCs can improve the tensile modulus and result in higher glass transition temperature of PAN fibers. Strain to failure of nanocomposites typically goes down with an increase in filler content. However, remarkably, fully drawn PAN/CNC fibers containing 40 wt% CNC exhibit same strain to failure as fully drawn PAN fibers. The high orientation of CNC in PAN fibers was confirmed by Raman spectroscopy, where 1095 cm−1 Raman band in CNC show a two-fold symmetry under vertical/vertical(VV) mode and a four-fold symmetry under vertical/horizontal(VH) mode. For stabilized fibers, the addition of CNCs in PAN fibers improves the orientation of the ladder polymer when fibers are stabilized either in air or in N2 followed by air. Also, the activation energy of cyclization and crosslinking reactions of PAN is reduced by 17.5 and 19 %, respectively, with the addition of 40 wt% CNC. Under the same processing conditions, carbon fibers made from PAN fibers containing 20 wt% CNC show tensile strength of 2.3 GPa and tensile modulus of 252 GPa, which are comparable to PAN based carbon fiber properties (tensile strength of 1.9 GPa and tensile modulus of 251 GPa).