A Novel Technique for Fiber Formation: Mechanotropic Spinning-Principle and Realization

Abstract

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.

Keywords: aromatic polyamide; fiber spinning; phase separation; polyacrylonitrile; polymer solutions; uniaxial extension.

Figure A1

The instability area (shaded) in the coordinates $(U,\chi )$. Point (1) corresponds to the initial extension rate close to the die in the fiber spinning process and (2) is the final point where the external tension force is applied.

Figure A2

Schematic picture for the fiber spinning.

Figure 1

Experimental device for stretching a jet up to a definite length. 1—fiber-optic illuminator along the jet axis; 2—back lightening; 3—syringe with solution; 4—lens to focus the light into the center of an extended jet; 5—camera; 6—jet.

Figure 2

Experimental devices for fiber spinning with constant speed. Spinning is carried out from a solution drop (A) or syringe orifice (B). 1—rheometer with rotating cylinder which is used for measuring torque at fiber winding; 2—a camera moving along a fiber with a lens of high magnification; 3—syringe with a polymer solution; 4—monofilament under spinning; 5—back lightening; 6—drop of a solution; 7—syringe orifice; 8—constant-rate supply engine.

Figure 3

General view of the experimental setup for continuous mechanotropic spinning.

Figure 4

Viscous properties (non-Newtonian flow curves) of CPABI and PAN solutions in different solvents (shown at the curves).

Figure 5

Frequency dependences of the storage (left) and the loss moduli (right) for PAN solutions in different solvents and the CPABI solution (shown at the curves).

Figure 6

Phase separation at constant draw ratio for 7 wt. % of CPABI in DMAc+3% LiCl (A) and PAN solutions in DMSO (B). Appearing wavy jet shape and glow reflect inhomogeneity obliged to phase separation.

Figure 6

Phase separation at constant draw ratio for 7 wt. % of CPABI in DMAc+3% LiCl (A) and PAN solutions in DMSO (B). Appearing wavy jet shape and glow reflect inhomogeneity obliged to phase separation.

Figure 7

Removal of unit separated droplets from surface of the PAN (A) and from the Taylor cone in CPABI fiber spinning (B).

Figure 8

Enlarged images of solvent droplets on fiber surfaces for CPABI in DMAc solution (A) and PAN in DMSO solution (B).

Figure 9

Stress development in extension of drops of PAN solutions in different solvents (upper rank) and of CPABI solution in DMAc + LiCl.

Figure 10

Extension of jets in PAN solution spinning at different rates (indicated in photos).

Figure 11

Continuous spinning of fibers from 7% solution of CPABI. Frames correspond to different distance from the spinneret.

Figure 12

Images of continuous spinning of PAN fibers at different magnification: (A)—general view; (B)—enlarged images of the jet section.

Figure 13

Stresses during stationary stage of the continuous spinning of PAN from DMSO solutions.

Figure 14

Dependences of the average stress (left) and calculated apparent elongation viscosity (right) on the average deformation rate in continuous spinning process of PAN solution.

Figure 15

Photos of PAN—web in 5 (A) and 10 min (B) after electrospinning from DMSO solution.