Synthesis and Spinning of PAN-based Polymer with Controlled Cross-sectional Shape for Manufacturing High-performance Carbon Fiber.

Abstract

1. Introduction

Polyacrylonitrile (PAN) based Carbon fiber, having high thermal stability, strong physical, mechanical, and non-corrosive properties, is one of the main reinforcing fibers in lightweight, high-strength and rigid composite materials [1]. Recently, the possibility of using such carbon fiber as a gas diffusion layer of a hydrogen fuel cell is currently being aggressively studied. In order to manufacture a high-quality gas diffusion layer, the performance of the composed carbon fiber must also be high. For manufacturing high-performance carbon fiber, the quality of the precursor fiber greatly contributes to the properties of the carbon fiber. Also, there are three main requirements for the PAN precursor polymer: high molecular weight, co-polymer and high purity [2-3]. In this study, polymer of poly (acrylonitrile-co-methacrylic acid) (Poly (AN-co-MAA), MW>400,000 g/mol) was polymerized, and precursor fiber spinning was performed by dry-jet-wet spinning. In particular, by controlling the coagulation bath parameters during fiber spinning, the cross-sectional shape of the precursor fiber and changes in mechanical properties were confirmed to prepare a precursor fiber for carbon fiber with high physical properties.

2. Experimental

2.1 PAN dope preparation Acrylonitrile (AN), methacrylic acid (MAA) as monomers and Azobisisobutyronitrile (AIBN) as initiator was dissolved into the Dimethyl Sulfoxide (DMSO), and the solution was synthesized into a polymer by free radical solution polymerization. After washing the polymerized solution with a large amount of DI water, it was dried sufficiently to obtain a polymer powder. After that, the synthesized Poly (AN-co-MAA) powder was dissolved in a Dimethyl Formamide (DMF) solvent as a spinning dope solution (Fig. 1).

2.2 PAN Precursor fiber manufacturing The spinning was conducted at a temperature at which the solution was not gelated. The coagulation bath used a mixed solvent of methanol and DMF at a temperature of 0℃ or less. Precursor fiber production was carried out through a dry-jet-wet spinning and a post-drawing (Fig. 2).

3. Results and Discussion

The synthesized Poly (AN-co-MAA) was evaluated by intrinsic viscosity and NMR measurement, and the molecular weight (over the 400,000 g/mol) and mole fraction of the copolymer (under the 4 mol%) have been confirmed. The polymer powder was mixed with DMF to prepare a spinning solution at various concentrations, and rheological properties were measured for verifying spinnability and solution homogeneity analysis. After then, the solid concentration was set to 15 g/dL. When the cross-sectional shape was analyzed, it was confirmed that concentration of the coagulation bath mixed with water and methanol among the experimental process variables acted as the most important factor. This is because there is a difference in the rate at which the solvent in the dope solution diffuses into the coagulation bath during fiber spinning due to the difference in solvent exchange rate between solvents (DMF) and poor solvent (DI water). Therefore, it was possible to manufacture precursor fibers of various shapes from irregular to flat shapes depending on the coagulation bath conditions. Overall, the tensile properties of the fibers have an average strength of about 0.8 GPa and a tensile modulus of about 15 GPa. This tensile result has the potential to be improved depending on the elongation, which is expected to increase through additional experiments.

4. Conclusions and Future Works

In this study, in order to control the cross-sectional shape of fibers with high mechanical properties of PAN-based fibers, which are carbon fiber precursors, various process variable conditions were controlled in the polymer synthesis and dry-jet-wet spinning process optimi