Behavior of a stationary jet of concentrated polyacrylonitrile solution

The fiber formation process via the uniaxial stretching jet of concentrated polyacrylonitrile solutions in dimethyl sulfoxide has been investigated. Data on viscoelastic properties obtained by oscillatory shear rheometry have been compared with the evolution of jet thinning during stretching. It was found that at a fixed temperature and moderate air humidity (20%), the jet thinning process involves an initial rapid viscous thinning followed by phase separation and the formation of solid fibers coated with solvent droplets. Changes in morphology as the solution transforms into solid fiber were analyzed using cryo-microscopy. It was demonstrated that at the final thinning stages there exists an increase in a solvent concentration in the near-surface layers of the jet/fiber. Finally, the role of air humidity was analyzed. Two possible scenarios of fiber formation were considered: predominantly due to uniaxial deformation or through coagulation of solution with moisture from the air humidity. Comparative morphological analysis of prepared fiber cross sections showed that monolithic fibers with homogeneous transverse morphology are obtained in the case of strain-induced formation. In contrast, fibers obtained due to phase separation induced by air humidity have gradient porosity. The mechanisms underlying the observed behavior are discussed.