Force field induced heterogeneous molecular orientation within polyacrylonitrile monofilaments.

The molecular orientation evolution in the radial direction of PAN precursor fiber is investigated by polarized Raman spectroscopy. Samples with different drawing ratios during the steam stretching process are prepared. The corresponding crystallinity distribution in the cross section is mapped by confocal Raman spectroscopy. Nano-TA is also applied to measure the thermal stability evolution along the radial direction. The results showed that the skin-core difference in terms of molecular orientation became more obvious as drawing ratio increased. The Raman mapping and nano-TA results are also consistent with this trend. Finally, a model is proposed to explain the evolution of chains with increasing drawing ratio.