S. B. Warner

Oxidative stabilization of acrylic fibres - Part 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.

Oxidative stabilization of acrylic fibres - Part 1 Oxygen uptake and general model

The mechanism of oxidative stabilization of acrylic fibres is characterized by two limiting cases which are determined by the fibre chemistry, the reaction conditions, and the diameter of the filament. These limiting cases correspond to diffusion-limited and reaction-limited kinetic processes. Although the chemistry of stabilization is too complex to specify, the various reactions are separated into two categories: those which occur prior to or concurrently with polymerization of the nitrile groups, called “prefatory reactions”; and those which occur subsequent to nitrile polymerization, called “sequent reactions”. Under conditions which allow the prefatory reactions to occur significantly before the sequent reactions, the diffusion of oxygen to reactive sites is limited by previously oxidized material; and the fibre shows a typical two-zone morphology. Under conditions where the prefatory and sequent reactions occur sequentially, the overall stabilization process is limited by the rate of the prefatory reactions; but a skin is established at the fibre surface which acts as an oxygen barrier. Data from a variety of sources, including oxygen analysis, microscopic examination, fibre residue after etching, tension developed in fibres held at constant length, and small-angle X-ray patterns, are cited as evidence for the two limiting cases.

Oxidative stabilization of acrylic fibres: Part 2 Stabilization dynamics

As acrylic fibres are heated in air to induce the stabilization reactions, the tension developed when stabilized at constant length and the instantaneous velocity of stabilizing fibres undergoing continuous processing both depend on the chemical composition, diameter, and orientation of the precursor fibre. An orientated fibre will tend to shrink when heated in the range 130 to 160° C, and hence will develop tension if restrained at constant length. Although this process has no direct relation to the stabilization process, it will influence the instantaneous velocity of the fibre during the later stages of continuous processing. As a fibre held at constant length is heated above 160° C the tension developed by entropic relaxation decreases and the fibre starts to undergo the prefatory and sequent reactions of stabilization described in a previous paper. If the prefatory reactions are rapid, a rigid structure is quickly established in the fibre and tension again increases rapidly. However, if the prefatory reactions are slow, select portions of the fibre react preferentially and the unreacted portions tend to relax to maintain a temporary quasi-equilibrium tension level. In both cases the fibres shrink at the later stages of stabilization because of chemical reactions. The shape of the tension-time curve is similar to the oxygen-uptake curves: The diffusion-limited mechanism of stabilization produces parabolic curves whereas the reaction-limited mechanism produces linear curves. Because each element of a fibre undergoing processing is subjected to the same tension at all times, previously orientated fibres first shrink, then stretch, and finally shrink again. These competing processes give rise to a changing instantaneous velocity. Data are presented for fibres of varying chemical composition, diameter, and initial orientation as well as for different conditions of stabilization.

Ion etching of amorphous and semicrystalline fibres

Ion etching of amorphous and semicrystalline fibres produces structures which can be observed in either the transmission electron microscope or the scanning electron microscope. The structures so produced have previously been identified as resulting from the etching process (artifacts), or as representing characteristics of the material, or both. The artifacts can be eliminated or minimized by rotating the sample during irradiation, using a low angle of incidence, and ensuring that the temperature of the sample surface remains low. When such precautions are used, amorphous fibres and semicrystalline fibres which arenot oriented remain featureless after ion etching. Oriented semicrystalline fibres, however, develop a striated structure which is oriented perpendicular to the stretch direction. The spacing between the striations is in the range of 500 to 5000 å, an order of magnitude larger than the characteristic lamellar spacing in the materials. These transverse structural features reflect characteristic features of drawn fibres; but the relation between these features and the lamellar spacing is unclear.

Heterogeneities in carbon fibers

Abstract Acrylic fibers, stabilized acrylic fibers, and graphite fibers have been selectively etched by ion bombardment. After ion etching, the fibers are characterized by structures oriented transverse to the fiber axis with an average spacing ranging from 500 to 3000 A. These transverse structures are considered to be representative of structural inhomogeneities in the fibers, which are transmitted from the precursor fiber through the stabilization treatment to the final carbon fibers. The relation between these heterogeneities and the standard microstructura! models of carbon fibers remains to be elucidated satisfactorily.