Development of high ductility and tensile properties by a two-stage draw of poly(acrylonitrile): Effect of molecular weight

Ultradrawing of atactic poly(acrylonitrile) (PAN) was investigated for a M_v series, ranging 8.0 × 10⁴–2.3 × 10⁶. Samples for the draw were prepared from 0.5–30 wt % solutions of PAN in N,N′-dimethylformamide. The solutions were converted to a gel by quenching from 100 to 0°C. The dried gel films were initially drawn uniaxially by solid-state coextrusion (first-stage draw) to an extrusion draw ratio (EDR) of 16, followed by further tensile draw at 100–250°C (second-stage draw). The maximum total draw ratio (DR_t,max) and tensile properties achieved by two-stage draw increased remarkably with sample M_v. Other factors affecting ductility were the solution concentration from which gel was made and the second-stage draw temperature. The effects of these variables became more prominent with increasing M_v. The temperature for optimum second-stage draw increased with sample M_v. Both the initial gel and the drawn products showed no small-angle X-ray long period scattering maximum, suggesting the absence of a chain-folded lamellae structure, which had been found in our previous study on the drawing of nascent PAN powder. The chain orientation function (f_c) and sample density (ρ_s) increased rapidly with DR_t in the lower range (DR_t < 30) and approached constant values of f_c = 0.980–0.996 and ρ_s = 1.177–1.181 g/cm³, respectively, at higher DR_t > 30–100. The tensile modulus also showed a similar increase with DR_t. The tensile strength increased linearly with DR_t, reaching a maximum, and decreased slightly at yet higher DR_t. The highest modulus of 28.5 GPa and strength of 1.6 GPa were achieved with the highest M_v of 2.3 × 10⁶. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 629–640, 1998