| Abstract: | Acrylic fibers containing methyl acrylate copolymer have been thermally treated in the temperature range 473°–573°K in an inert, oxygen-free environment. The optical properties and an x-ray diffraction orientation parameter have been studied in these fibers, with special attention to correlation with the essential chemical reactions and to the behavior of polymer chain orientation. Densification of the fibers, illustrated by specific gravity and refractive index changes during the thermal treatment, is interpreted as a more efficient packing of the polymer chains in addition to the development of more highly refractive and dense chemical species. An extrapolated value of refractive index of 1.70 for 100% nitrile polymerization supports the presence of naphthyridine structures. Birefringence changes from ?0.003 to +0.27 imply a high degree of retention of the preferred orientation of polymer chains, despite the severe change in the chemical identity of the chains. The contrary behavior of the orientation parameter from the 100 arc of the x-ray diffraction of polyacrylonitrile as the thermal treatment proceeds indicates that the residual polyacrylonitrile sequences have reduced orientation. |
