Mass DSC/TG and IR ascertained structure and color change of polyacrylonitrile fibers in air/nitrogen during thermal stabilization

Abstract

The structure evolution was studied by mass spectrum (MS), differential scanning calorimetry (DSC) and thermogravimetry (TG), Fourier transform infrared (FTIR) spectroscopy. The results indicated that the C¼ ¼N and C¼ ¼C groups appeared gradually with the increase of the temperature in air and nitrogen. The C¼ ¼O groups appeared because of oxidative reaction in air. The C¼ ¼N, C¼ ¼C and C¼ ¼O groups were all chromophores. The effect of conjugated C¼ ¼N and C¼ ¼C on the absorption of the visible light was shifted to longer wavelengths and indicated p-p* transition. There was a strong bathochromic effect as the number of C¼ ¼C bonds were increased. The effect of C¼ ¼O and -NH 2 on the absorption of the visible light was shifted to longer wavelengths and indicated n-p* transition. Oxygen could facilitate chemical reactions in air. Hence, the color of PAN in air was deeper than in nitrogen at the same temperature. The structural change of PAN in air was faster and more complex than in nitrogen. PAN fibers treated in air turned black after 230 C. However, PAN fibers turned black at 350 C in nitrogen. The MS and FTIR indicated that cyclization occurred before dehydrogenation during stabilization in air and nitrogen.

UV-Based Dual Mechanism for Crosslinking and Stabilization of PAN-Based Carbon-Fiber Precursors

“…The peaks located at 2940, 2865, 2242, 1730, 1454, and 1055 cm À1 have been assigned to CH 2 asymmetric stretching, CH 2 symmetric stretching, CN stretching, C¼O stretching (due to the presence of methyl acrylate), CH 2 scissor vibration, and -C-C-C-backbone bending, respectively. 8,25,31,38,39 These results confirm that simply raising the temperature in the UV chamber has no discernible effect on the chemical modification of the nonirradiated samples. Figure 13.5 displays FTIR spectra obtained from samples UV treated at 100 1C for 300 s, with and without photoinitiators.…”

“…The presence of the 1520, 1600, 1630, and 1680 cm À1 peaks is a clear indicator of the formation of the ladder structure after the UV treatment. 8,[38][39][40] In contrast, due to the absence of photoinitiator in pure samples, a dehydrogenation reaction is believed to be the primary pathway during UV treatment. Hydrogen radicals produced during the UV treatment combine to form hydrogen, with carbon-carbon double-bond formation in the backbone of the precursor or even crosslinking and scission of the polymer chains, presumably in the amorphous region.…”

“…All these peaks are related to the cyclized structure produced during the thermal oxidation treatment. 8,25,31,38,39 Based on FTIR spectra displayed in Figure 13.9, the FTIR conversion indices for pure, non-UV-treated, thermally stabilized samples are presented in Figure 13.10 as a function of the thermal stabilization time. Also included is the temperature profile followed during these thermal stabilization experiments.…”

“…Peaks located in the double-bond region between 1450 and 1700 wave numbers (cm À1 ) are associated with the ladder structure. 8,[38][39][40] The scans were conducted from 400 to 4000 cm À1 and the transmittance intensities were normalized against the intensity of the 2940 cm À1 peak (CH 2 asymmetric stretching) for comparison. The extent of the cyclization reaction was estimated from the following conversion index: 11,41,42 Conversion index ð%Þ ¼…”

Preparation and thermal stability of initiator fragments end-capped oligomers/silica nanocomposites

“…On the other hand, it is well known that polyacrylonitriles (PANs) are widely used as precursors for fabricating high performance carbon fibers [8][9][10][11][12][13][14][15]. The formation of PAN precursor fibers possessing the heatresistance ability by heating to 200 ∼ 300°C is due to the change of the chemical structure from the linear molecular chain into ladder structures through a variety of chemical reactions including cyclization of nitrile groups leading to hydronaphthiridine rings, dehydrogenation leading to a certain degree of aromatization, and oxidation reaction leading mainly to acridone and some other structure [9,11,12].…”