Study of Polyacrylonitrile/Cellulose Nanocrystal Composite Fibers via In-situ Polymerization, and Super Engineering Plastic Based Nano Composite Fibers

Abstract

The polymer nanocomposite is composed of a nanoscale reinforcing filler inside a polymer matrix. There are differences in the properties of composite materials depending on the shape and size of the filler. As the size of the filler decreases, the surface area-to-volume ratio increases and the interaction between the nanoparticles and the polymer matrix increases, and thus the enhanced properties of the composite also can be obtained. Cellulose nanocrystal (CNC) is a crystalline nano material after removing amorphous segments from cellulose. CNC is widely studied to be used as nano reinforcing fillers of composite materials due to its one-dimensional shape, high mechanical properties, many functional groups, high surface area, and eco-friendly characteristics. Based on these features, the polymer nanocomposite fibers containing CNC have been mainly studied. In chapter 2, polyacrylonitrile (PAN) was synthesized by in-situ polymerization method in the solvent where the cellulose nanocrystal (CNC) was dispersed. Then, the efficiency of the spinning process is increased by using a polymerization solution that has not undergone additional purification directly as a spinning dope. As-spun fiber was fabricated using dry-jet wet spinning method, and in-situ PAN/CNC fiber having TDR 15, 16.5, 18, 19.5 was manufactured through a post-drawing process. The mechanically stirred ex-situ PAN/CNC fiber and control PAN fibers were prepared and compared with the behavior of mechanical properties of the in-situ PAN/CNC fibers. In the case of as-spun, TDR 15, 16.5, 18 fibers, in-situ PAN/CNC fibers had the 0.96 GPa of tensile strength and 19.8 GPa of tensile modulus which are the highest values. This is because the interaction between the PAN chain and CNC inside the in-situ PAN/CNC fiber was the greatest, and this maximized the reinforcing effect of the CNC on composite fibers. Hereafter, three types of PAN precursor fibers (control PAN, in-situ PAN/CNC, and ex-situ PAN/CNC at TDR 18) were stabilized at 260 ℃ for 3 hours and carbonized at 1300 ℃ and 1400 ℃ to produce carbon fibers. Carbon fibers using in-situ PAN/CNC as a precursor showed the outstanding tensile properties. By analyzing the structural evolutions through the Raman spectra, it was confirmed that the IG/ID ratio and degree of decreasing amorphous part of carbonized in-situ PAN/CNC fibers were the largest. It means that the efficiency of forming a graphitic structure is changed by the low molecular weight material contained in the in-situ PAN/CNC precursor fibers. This research suggested that the CNC and low molecular weight polymers remained inside the fibers after direct spinning affect the mechanical properties of fibers. In chapter 3, Poly(Arylene Ether Sulfone)(PAES)/cellulose nanocrystal (CNC) nanocomposite fibers were fabricated using dry-jet wet spinning and post-drawing method at CNC concentrations of 0, 1, 5, 10 wt% with respect to the polymer. TDR of control PAES and PAES/CNC nanocomposite fibers were 4.8, 5.76, and 6.4, respectively. The tensile strength and modulus of control PAES fibers were 122.0 MPa and 3.2 GPa, respectively, at TDR 6.4. The tensile strength of PAES/CNC1 at TDR 6.4 is 170.8 MPa which is the highest value and the highest tensile modulus value is 6.1 GPa of PAES/CNC10 at TDR 6.4. The tensile modulus continuously increased according to the CNC content and draw ratio. This is due to the increase in CNC alignment and the high modulus of the CNC itself. However, the tensile strength increases with the draw ratio, but tends to decrease as the CNC content increases from 1 wt% to 10 wt%. From the above trend, it was found that CNC acts as an element that inhibits the alignment of the PAES chains. This study suggested that the maximization of mechanical properties of amorphous polymer nanocomposite fibers can be achieved by optimizing the CNC content and fiber manufacturing process.