Experiments carried out on a series of seven different polymers with molecular weights varying over a wide range have allowed us to confirm that stable jets can be obtained at concentrations much below the crossover point. A jet was considered as stable if its lifetime exceeds the Plateau–Rayleigh time by several orders of magnitude. The systematic study carried out for poly­(ethylene oxide) solutions in a wide range of high molecular weight showed that the lowest concentration at which a stable fiber can still be formed is scaled by [η]−2.14±0.3 or M –1.63±0.29. However, for the domain of not so high M, the spinnability concentration corresponds to the onset of entanglements and scales as M –0.70±0.14, which is the same as the dependence of the crossover concentration on molecular weight. The difference in the scaling exponents reflects two possible regimes of stable fiber formation in fiber spinning. These exponents are close to those obtained by Palangetic et al. [Polymer2014554920] for other polymer solutions in the electrospinning experiments. Several examples of spinnability at very low concentrations for other polymer solutions are demonstrated. A possibility of the formation of stable jets from dilute solutions is explained by an increase of the intermolecular interactions of extended macromolecular chains, resulting in the phase separation and leading to the formation of fibers created by oriented macromolecules. The theoretical considerations show that there are two sources of jet stabilization at low concentrations (high M), namely, the coil–stretch transition and demixing of the polymer solution.

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We present basic experimental data and the theoretical background of a novel technique for fiber spinning from polymer solutions. The principal feature of the advanced process is realization of phase separation with detachment of a solvent, accompanied by the orientation of macromolecules, under the action of high extension rates. This is similar in some respects to dry spinning, though the driving force is not diffusion with subsequent evaporation of a solvent but redistribution of polymer-solvent interactions in favor of polymer-polymer and solvent-solvent ones governed by mechanical stresses. A promise of this approach has been demonstrated by experiments performed with polyacrylonitrile solutions in different solvents and solutions of the rigid-chain aromatic polyamide. We examined mechanotropic fiber spinning in model experiments with stretching jets from a drop of polymer solution in different conditions, and then demonstrated the possibility of realizing this process in the stable long-term continuous mode. During extension, phase separation happens throughout the whole section of a jet, as was confirmed by visual observation. Then a solvent diffuses on a jet surface, forming a liquid shell on the oriented fiber. Instability of this cover due to surface tension leads either to formation of separate solvent drops “seating” on the fiber or to the flow of a solvent down to the Taylor cone. The separate liquid droplets can be easily taken off a fiber. The physics underlying this process is related to the analysis of the influence of macromolecule coil-to-stretched chain transition on the intermolecular interaction.