Quantitative analysis of entropic stress effect on the structural rearrangement during pre-stabilization of PAN precursor fibers

•The morphological indices of PAN fiber decrease due to entropic relaxation during Free Shrinkage thermal treatment.•Rubbery flow behavior increases segmental motions followed by increase of amorphous and crystallites orientation indices.•The morphological perfection results from incorporation of straight and oriented amorphous chains to crystallites.

The stress and temperature dependency of structural rearrangements of Polyacrylonitrile (PAN) fibers has been investigated during pre-stabilization step. Continuous pre-stabilization of a special grade PAN fiber has been carried out in four different temperatures under three distinct imposed stresses, namely Free Shrinkage, Zero Shrinkage and Under Stress treatments. Due to entropic relaxation during Free Shrinkage treatment, the crystallinity sharply decreases from 38.6% to 31.50% and the average orientation angle of amorphous segments along the fiber axis increases from 25.7° to 31.06°. Imposing dimensional constraints in Zero Shrinkage treatment shows improvement in structural perfection with an increase in crystallinity and crystallite thickness, while average orientation angle of amorphous segments decreases to 22.92°. The contribution of temperature and higher imposed stress in Under Stress treatment causes a reduction of average orientation angle of amorphous segments to 19.33°, leading to incorporation of straight and oriented amorphous chains in the paracrystalline region followed by an increase of crystallinity up to 47.61%.

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