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

Abstract

Phase separation processes following high‐rate extension in unentangled polymer solutions are studied theoretically. The flow‐induced demixing is associated with the coil–stretch transition predicted in high‐molecular‐weight polymer solutions at high‐enough Weissenberg numbers. The developed mean‐field theory is valid in the dilute/semidilute solution regime, where the stretched coils overlap strongly. We elucidate and discuss the main kinetic stages of the polymer/solvent separation process including (i) growth of concentration fluctuations and formation of oriented protofibrils by anisotropic spinodal decomposition; (ii) development of well‐defined highly oriented and stiff fibrils forming an anisotropic network (cross‐linked fiber); (iii) microphase separation and lateral collapse of the network yielding dense oriented fiber. These novel predictions are in qualitative agreement with the experimental data. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 623–637

Orientation and Aggregation of Polymer Chains in the Straight Electrospinning Jet

“…As shown in earlier studies [ 31 , 32 , 33 ], the flow-induced orientation of the polymer chains reduces their steric repulsion and shifts their interactions from repulsive to attractive. The attraction between the polymer segments weakly depends on their orientation [ 36 ].…”

“…When the parameter becomes negative, the polymer solution starts to separate into polymer-rich and solvent phases. The volume fraction of the polymer in the polymer-rich phase is obtained from the equality of osmotic pressure to zero: (it is assumed that ) [ 31 , 32 , 33 ].…”

“…Obviously, inequality is valid. To estimate , consider the dynamics of concentration fluctuations ( is the wave vector, is time), which in the linear approximation obeys the equation [ 32 , 33 ] …”

“…As a result, a network of highly-oriented fibrils with diameter of emerges (the number of chains in the domain remains fixed) ( Figure 7 d). The characteristic collapse time is of the order of and is much shorter than [ 32 ], therefore the network of fibrils could be fully developed in the straight jet during the stretching regime. The latest stages that include aggregation of fibrils in bundles ( Figure 7 e), and collapse of the network accompanied by the solvent release were studied for threads of polymer solutions in [ 32 , 33 ].…”

“…A polymer/solvent demixing in polymer solutions arises due to flow-induced orientation of the polymer chains, leading to a subsequent reversal of their effective interactions from repulsive to attractive. As a result, the elongated chains tend to micro-separate and form a network of fibrils compressing laterally by squeezing and releasing the solvent out to the jet surface [ 32 , 33 ].…”