Two model solutions, one containing organic salt and the other none, of poly(N-isopropylacrylamide) (PNIPAM) in DMF solvent with the same concentration of 17 wt% were electrospun to investigate the extension rate profile in the straight jet as well as the subsequent jet bending (whipping) behavior. Both model solutions have identical rheological properties, while the solution with added salt possesses electrical conductivity 16 fold higher than the other. At a given flow rate of Q, light scattering experiments were carried out to derive the jet diameters of the straight segment dj at different jet positions z so that the z-dependence of jet velocity (vj, = 4Q/πdj2) was feasibly calculated, from which the profile of extension rate (ε˙) was obtained by dvj/dz. Using high speed video imaging, the onset of jet bending was observed, and the motion of the first bend was traced, from which the extension rate of the bending jet could be also estimated. Compared to the neat solution, the solution with salt exhibits a bigger Taylor cone and a shorter straight segment with a larger jet-end diameter to proceed the subsequent jet bending process. Moreover, the magnitudes of ε˙ in the straight jet and in the bending jet are all dramatically enhanced with solution conductivity, yielding PNIPAM fibers with a smaller diameter on the grounded collector. Our results show that it is the extension rate developed in the spin line plays an important role in determining the final fiber diameter. The onset of bending instability is attributed to the local buckling in the end section of the straight jet, which is unambiguously observed by video imaging with a high frame rate of 104 fps. The formation of jet buckling is due to the compressive forces associated with the air drag to resist the electric stretching force induced by the setup electric field. The dynamics of bending jet was elucidated in this work.
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