Thermal stabilization of polyacrylonitrile fibres

A polyacrylonitrile (PAN) fibre, Dralon T (DT), and a copolymer manufacture d as a carbon fibre precursor, Special Acrylic Fibre (SAF), were studied as received and after heat-treatment in air at various temperatures up to 300 degrees C. Wide angle X-ray scattering (WAXS) was undertaken on single fib res, fibre bundles and films, utilizing both conventional and synchrotron s ources. With a conventional source, the normal WAXS pattern was observed fo r both polymers, i.e. two main equatorial reflections, a diffuse equatorial reflection and a diffuse off-equatorial reflection. With a synchrotron sou rce, seven equatorial WAXS reflections could be detected in DT. Also, a sha rp, meridional reflection was observed in SAF as received and in DT after s tretching at 100 degrees C. Unpolarized and polarized infra-red spectra wer e obtained from single fibres and films by FTIR microscopy. A measure of th e extent of reaction (EOR) following heat-treatment was derived from the in tensity of the nitrile absorption and the intensity of absorption at 1600 c m(-1): EOR = [I-1600/(I-CN + I-1600)] TWO complementary measures of orienta tion were used to follow changes on heat-treatment: the peak width at half- height of an azimuthal plot of the most intense WAXS reflection and the nit rile dichroic ratio from FTIR microscopy. For fibres held under tension dur ing heat-treatment, the degree of orientation by either measure did not dec rease until the EOR exceeded 0.5. Solid-state C-13 NMR, together with IR re sults, indicated that at least three chemical processes occurred on heat-tr eatment: nitrile reaction, conjugated C = C formation and oxidation. (C) 19 99 Elsevier Science Ltd. All rights reserved.