OXIDATIVE STABILIZATION OF ACRYLIC FIBERS .3. MORPHOLOGY OF POLYACRYLONITRILE

A new model for the structure of oriented acrylic fibres is presented. The polyacrylonitrile molecules (or the acrylic sequences in a co-polymer) are suggested to form two distinct regions within a fibril: amorphous (disordered) and partially ordered. In the partially ordered regions, the polymer molecules assume a contorted helical shape to form rods with a diameter averaging about 6.0 Å in which the nitrile units are oriented at various angles to the rod axis, but are spaced irregularly on or near the surface of the rod. The nitrile groups of adjacent rods can interpenetrate to form dipole pairs. The rods are ordered into a liquid crystal-type array, giving in some cases a lamellar-like texture oriented perpendicular to the fibril axis, with the ordered lamellae regions interspersed with amorphous regions. Evidence for the structure is obtained from transmission electron microscopy observations, a transient peak observed in small-angle X-ray scattering when fibres are thermally treated, as well as wide-angle X-ray diffraction patterns. The proposed model is consistent with the absence of a periodic repeat unit along the chain direction, with the h k 0 reflections seen in wide-angle X-ray and electron diffraction, with the spherulitic morphology reported in some studies, and with the platelike morphologies obtained under some conditions of precipitation from dilute solution. © 1979 Chapman and Hall Ltd.