Cyclization propagation path during oxidation of polyacrylonitrile fibre

Abstract

To obtain carbon fibres of high tensile strength and Young's modulus, the first major step is the low temperature thermal stabilization of polyacrylonitrile (PAN) fibres. Stability in the structure of PAN is achieved through the conversion of an open chain structure into a closed chain or aromatic structure. The aromaticity so developed can be determined by X-ray diffraction (XRD) [1], orientation distribution pattern [2] or differential scanning calorimetry (DSC) [3]. Watt [4] reported that, to obtain carbon fibres with the best mechanical properties, the oxidation of PAN should be carried out to such an extent that about 50% cyclization takes place. It was also confirmed by Bahl and Manocha [5] that carbon fibres with 50% cyclization possess the maximum tensile strength. The PAN structure is initially cyclized to about 100% by increasing the residence time, and excess oxygen-containing groups are subsequently reduced by treating the stabilized or oxidized fibre with a reducing liquid, namely a solution of pyrogallol [6]. The introduction of itaconic acid comonomer into PAN precursor can lower the cyclization temperature, because the itaconic acid comonomer is a potential initiator of cyclization, and acrylate comonomers such as 2-ethylhexyl acrylate in PAN precursor may prevent the cyclization propagation of nitrile groups during oxidation [7]. The amorphous phase has a larger free volume, which leads oxygen to easily attack the AN unit of the amorphous phase to form a ladder polymer [8]. Since the itaconic acid comonomer can initiate cyclization of nitrile groups, the introduction of itaconic acid comonomer, which exists in the amorphous phase, into PAN precursor would lower the cyclization temperature. The transformation of ladder polymer from AN units initiates in the disordered phase, and then forms the boundaries of the ordered phase [9]. Although we know that the cyclization reaction begins in the amorphous phase, we do not know whether the crystal region on the surface of the PAN fibre or the amorphous region in the centre of the fibre is cyclized first. This question is discussed in this letter. PAN fibres (12 K, 1.2 denier per filament) were stabilized in a continuous oxidizing furnace with 3.4 m min -1 speed in air. The oxidized PAN fibres of different colours were cut down as testing samples. A Scintage DMS 200 X-ray diffractometer with CuKo~ radiation as the source was used to study the aromatization index (AI value) of the oxidized fibre. The AI value was calculated from