Development of High Ductility and Tensile Properties upon Two-Stage Draw of Ultrahigh Molecular Weight Poly(acrylonitrile)

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Abstract

Solutions (1−5 wt %) of ultrahigh-molecular-weight atactic poly(acrylonitrile) (UHMW-PAN) in N,N‘-dimethylformamide were crystallized to gel by quenching from 100 to 0 °C. The dried gel films exhibited poor ductility on straight tensile draw. However, they were ductile on solid-state coextrusion to an extrusion draw ratio (EDR) of 8−16 (first-stage draw). Thus, the initially coextrusion drawn films were further drawn by a tensile force (second-stage draw). Drawability of the gel film was strongly influenced by several factors, including the solution concentration from which gel was made, first-stage EDR, and second-stage draw temperature. With increasing prior solution concentration, the ductility of a gel film decreased. The maximum achieved total draw ratio (DRt,max) by the two-stage draw and the uniformity of drawn products increased with first-stage EDR. Further, the ductility increased rapidly above ∼150 °C, reaching a maximum at ∼190 °C, and decreased at yet higher temperatures. A DRt,max of ∼110 was achieved under optimum conditions. The efficiency of draw was lower for a gel film prepared from a lower solution concentration, as evaluated by the crystalline orientation function (fc) and tensile properties vs DR. The highest fc of 0.998, and tensile modulus of 26 GPa and strength of 1.3 GPa were obtained by ultradrawing of the gel films, with a prior solution concentration of 1−2 wt %. Further, the modulus is close to the uncertain X-ray crystal modulus of atactic PAN (28 GPa). The wide-angle X-ray diffraction patterns measured at room temperature showed an orthorhombic chain packing with significant disorder along the chain axis, independent of draw temperature and DR. Further, the temperature variations of the orthorhombic (200)o and (110)o spacings revealed that the initially orthorhombic cell transforms to the hexagonal unit cell around 150 °C. Thus, the rapid increase in ductility above ∼150 °C is ascribed to the existence of this reversible crystal/crystal transition, above which the molecular motion in crystalline regions is activated.

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