Self-Organization in Electrospun Polymer Solutions: From Dissipative Structures to Ordered Fiber Structures through Fluctuations

Self-organization in nonequilibrium systems is a research topic in the statistical physics universally important for formation of patterns or orders in various systems, including nature. In this work, we investigated the self-organization processes of ordered structures via cascade evolutions of various dissipative structures through fluctuations in electrospun poly(vinyl alcohol) aqueous solutions in both mesoscopic and macroscopic length scales. The flowing jets were investigated both in situ, macroscopically with a high speed video imaging and ex situ, mesoscopically under optical and electron microscopies as a function of the polymer concentration. In the semidilute unentangled solutions, the main jet evolved a self-similar, cascade network composed of a set of the bulge and the branched subjets issued from the bulge as a building block of the network. In the semidilute entangled solutions, the main jet evolved a series of bulges elongated along the jet axis without formation of the branched subjets. We propose that the bulges were formed in the jet by the orientation-fluctuations-induced concentration fluctuations (OFICF) and the resultant phase separation, triggered by the chain stretching in the jet. The ex situ investigations unveiled the deformed bulges contained remarkable internal structures driven by phase separation into polymer-rich stringlike structures, which comprised the finer elongated bulges interconnected with bundles of oriented chains (finer jets), in the matrix of polymer-lean solution. These bulges and their internal structures are dissipative structures which were developed as a consequence of the dissipation of the increased free energy due to the conformational entropy loss of the chains in the solutions. Further stretching of the jet solution resulted in creation of the finer deformed bulge(s) and the finer jet(s): the finer jet(s) in turn further developed even finer deformed bulges via the OFICF through the consequent cascade energy dissipation mechanism. The cascade transformation of the dissipative structures from the micro- to nanosized scales, during which solvents were squeezed out to the jet surface and removed later, eventually yielded the beaded fibers deposited on the collectors. At higher concentrations, the elongated bulges were self-assembled into highly elongated strings, i.e., fine fibers composed of bundles of the stretched chains, embedded in the uniform nonbeaded fibers.