A new transition point of polyacrylonitrile

A polyacrylonitrile sample of molecular weight 78,000 was dissolved into a 15% solution in 70% aqueous HNO₃, spun into an undrawn model filament with diameter 0.3 mm. using a 30% aqueous HNO₃ as the coagulation bath. The fresh filament was drawn 4–16× in water at various temperatures (40, 65, and 75°C.) and then steamed at 90–180°C. Dynamic mechanical tests were carried out on these filaments at 138 cps and it was found that the steamed filaments had a new transition at about 160°C. in addition to the two well-known transitions at 60–70°C. (β) and 110°C. (α_a), although the unsteamed samples had only the two absorptions, α_a and β. This newly found transition was designated α_x. The α_x was measured at 3.5, 11, and 110 cps from which the activation energy of this relaxational motion was estimated to be 95 kcal./mole. The α_x peak height was lowered by stretching and recovered on resteaming; the change in the peak height was paralleled by the changes in the x-ray crystallinity produced by the stretching and the resteaming. These phenomena lead us to a conclusion that the region contributing to the α_x absorption is of considerably higher order than that of the α_a transition with an activation energy about 50 kcal./mole and intimately related to the crystallinity, although it cannot be connected directly to the crystalline region itself because a single crystal mat did not show the α_x.