Properties of PAN Fibers Solution Spun into a Chilled Coagulation Bath at High Solvent Compositions
In this work, multifilament, continuous polyacrylonitrile (PAN) fiber tow was solution spun mimicking industrial processing at the small pilot scale (0.5 k tow), while carefully altering the composition of the coagulation bath, in order to determine the effect on the resulting fiber shape, density, orientation, and tensile properties at varying points in the spinning process. Novel here are the abnormally high coagulation bath solvent compositions investigated, which surpass those often reported in the literature. In addition, the coagulation bath was maintained at a slightly chilled temperature, contrary to reported methods to produce round fibers. Further, by altering the composition of the bath in a step-wise fashion during a single spinning run, variations in all other process parameters were minimized. We found that with increasing solvent composition in the coagulation bath, the fibers not only became round in cross section, but also became smaller in diameter, which persisted down the spin line. With this decrease in diameter, all else equal, came an accompanying increase in apparent fiber density via a reduction in microvoid content. In addition, molecular orientation and tensile properties also increased. Therefore, it was found that inadequate understanding of the coagulation bath effects, and spinning at low coagulation bath solvent compositions, can hinder the ability of the fiber to reach optimum properties.
Changes in the Structure of Wet-Spun Acrylic Fibers During Processing
The structural changes that occur during processing of a wet-spun 93% polyacrylonitrile -7% polyvinyl acetate co polymer are discussed. Stress-strain, x-ray diffraction, shrinkage, and electron microscopy data are explained in terms of a model in which the molecules are in an irregular helical conformation in the relaxed state and can extend during de formation. The molecules are arranged in fibrils which, in turn, form a three-dimensional network. The fibrillar ele ments remain intact through the coagulation, hot-stretching, and drying, although network junctures break and reform. The model proposed is an extension of that used by Rosenbaum to describe the behavior of acrylic fibers after processing and is in agreement with previous work on the structural characterization of polyacrylonitrile.
Rheo‐optical Behavior of Polyacrylonitrile: Creep and Creep Recovery
The rheo‐optical behavior of PAN homopolymer film has been investigated by means of creep and creep recovery experiments with simultaneous measurements of birefringence changes. Creep experiments were carried out at temperatures of 70°, 100°, 120°, 140°, and 160°C, at a series of stress levels. This temperature range spanned the 90° and 140°C transition temperatures. A progressive change in shape of the creep curve in a log‐log plot of compliance vs time is seen: from very flat and gradual to more S‐shaped both with increasing temperature and with increasing stress at the higher temperatures. This S‐shaped curve can be identified with the cold‐drawing phenomenon, even though no neck formation was seen. Creep recovery is more complete at lower temperatures and after lower final strains in the creep experiment. Most of the recovery is instantaneous; only a very small amount of further time‐dependent recovery is observed. At higher temperatures and higher strains, creep seems to involve permanent changes in ...