High molecular weight improves microstructure and mechanical properties of polyacrylonitrile based carbon fibre precursor

This work presents the influence of polyacrylonitrile (PAN) molecular weight (MW) on the microstructure and macrostructure of PAN based carbon fibre precursor while fixing the PAN solution viscosity to achieve identical spinning. The impact of molecular weight on macrostructure was reported by cross sectional shape, fibre density, shear viscosity, mechanical properties and microstructure evolution was reported by void size, shape, and crystallinity. The study was carried out in the PAN MW range between 141 and 503 Kgmol-1. The PAN fibre strength improvement of 5–9 cNdtex−1 with increasing PAN MW is caused by the drop in PAN fibril size from 24 nm at 141 Kgmol-1 to 18 nm at 503 Kgmol-1. The improvement in PAN fibre density from 1.14 to 1.18 gcm−3 with increasing PAN MW is caused by the improved microstructure alignment from 15o to 8o and is the driving force for increased mechanical modulus from 115 to 150 cNdtex−1.