On the nature of phase separation of polymer solutions at high extension rates

Abstract

Experiments with stretching moderately concentrated polymer solutions have been carried out. Model experiments were carried out for poly(acrylonitrile) solutions in dimethyl siloxane. Just the choice of concentrated solutions allowed for a clear demonstration of a demixing effect with the formation of two separate phases—an oriented polymer fiber and solvent drops sitting on its surface. An original experimental device for following all subsequent stages in the demixing process was built. It combined two light beams, one transverse to the fiber and a second directed along (inside) the fiber, the latter played the role of an optical line. This gives a unique opportunity to observe processes occurring inside a fiber. The process of demixing starts from the volume phase separation across the whole cross section of a fiber at some critical deformation and the propagation of the front of demixing along the fiber. Then a solvent cylindrical skin appears which transforms into a system of separate droplets. New experimental data are discussed based on a comparison of the current different points of view on the phenomenon of deformation‐induced phase separation: thermodynamic shift of the equilibrium phase transition temperature, growth of stress‐induced concentration fluctuations in two‐component fluids, and mechanically pressing a solvent out from a polymer network. The general belief is that a rather specific (so‐called “beads‐on‐a‐string”) structure of a filament is realized in stretching dilute solutions: beads of a polymer solution connected by oriented polymer bridges forming a single object. The situation in stretching moderately concentrated solutions appears quite different: real phase separation was observed. So, the alternative phenomenon to the formation of the “beads‐on‐a‐string” structure has been experimentally proven. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 559–565

From capillary to elastic instability of jets of polymeric liquids: Role of the entanglement network of macromolecules

“…After that, small droplets of the solvent, as beads on a string (frame 110), are joined into larger droplets and roll down over the fiber under the action of the gravitational force (frames 114, 121, and 125). This is due to elastic instability manifested in the phase separation of the solution with the formation of a fiber of a rubbery oriented polymer with unlimited "lifetime" (see [7]). Its deformations are completely similar to rubbery elastic deformations of a melt at high extension rates.…”

Phase separation kinetics in unentangled polymer solutions under high‐rate extension

“…The nematic transition is not considered here: it is assumed that concentration or less.It may also be useful to comment that the SCC effect for the lateral demixing in extensional flows is always weak compared to the orientation‐induced thermodynamic (osmotic) driving force for (i.e., in the regime of stretched chains): To see this, the osmotic pressure must be compared to the lateral viscous polymer stress , where the relevant viscosity and is the lateral chain size for ( is the chain tension). Hence, the typical is roughly independent of Wi for , so the lateral SCC driving force is not expected to increase with Wi. The results of this study can be qualitatively applied to several polymer systems like solutions of PAN in DMSO, and PAAA or PEO water solutions . However, in most of the previous experimental studies on extension of very dilute solutions, the jet dynamics are strongly affected by the capillary effects related to the Plateau–Rayleigh instability whose interference with possible demixing and solvent release processes requires a special consideration.…”

“…However, in most of the previous experimental studies on extension of very dilute solutions, the jet dynamics are strongly affected by the capillary effects related to the Plateau–Rayleigh instability whose interference with possible demixing and solvent release processes requires a special consideration. That is why we rather draw attention to semidilute PAN in DMSO solutions, where the Plateau–Rayleigh instability is apparently suppressed. One such system is a PAN solution in DMSO with concentration ( ) falling in the semidilute, , and unentangled, , regime.…”

“…) does not depend either on concentration or on polymer molecular weight: depends mainly on T and the chain stiffness parameter l / d . For the PAN solutions, we expect as and (cf. the Appendix).…”

“…Most of the experimental evidence on phase separation in polymer solutions under extension concerns unentangled or moderately entangled systems . The underlying rheological phenomena are complicated and are not understood well theoretically; hence, we chose to start their analysis with a simpler unentangled regime.…”