Uniaxial drawing of isotactic poly(acrylonitrile): Development of oriented structure and tensile properties

Uniaxial drawing of isotactic poly(acrylonitrile) (iso-PAN, isotactic triad fraction of 68%) and the resultant structure and tensile properties of drawn products were studied. The results were compared to those of atactic-PAN (at-PAN). Dried gel films prepared from 2 to 10 wt % solutions in N,N'-dimethylformamide were initially drawn by Solid-state coextrusion (first-stage draw) to an extrusion draw ratio of 16, followed by a further tensile draw at 100-200 degrees C (second-stage draw). The ductility of iso-PAN increased rapidly above 100 degrees C, due to the onset of molecular motion in crystalline regions, as found by WAXD at elevated temperatures. In contrast, the ductility of at-PAN increased above the first-order crystal/crystal transition at around 150 degrees C. Thus, the temperature for optimum second-stage draw of iso-PAN, 130-140 degrees C, was significantly lower than that (160-180 degrees C) of at-PAN, reflecting their crystal softening temperatures. The maximum achieved total draw ratios (DRt), after the two-stage draw, were comparable for these PANs. The shapes of stress/strain curves for highly drawn products recorded at room temperature were significantly different between iso- and at-PAN. The meridional WAXD patterns of these samples revealed that the difference is ascribed to their chain conformations which change with the applied tensile stress. The iso-PAN likely takes a predominantly 3/1 helical chain conformation, whereas at-PAN seems to consist of both planar zigzag and helical sequences, as previously suggested. However, upon increasing the tensile stress on oriented fibers, the helical sequences progressively transform into a planar zigzag conformation which shows a higher modulus. Such an effect of the stress was more prominent in at-PAN than in iso-PAN fibers. Thus, the maximum achieved tensile modulus, as well as the modulus at a given DRt, was slightly higher for iso-PAN than for at-PAN (28.5 +/- 1.0 vs 23.0 +/- 1.0 GPa). However, the maximum tensile strength at the break was comparable for each PAN, at 0.90 +/- 0.05 GPa.