Self-Organization in Electrospun Polymer Solutions: From Dissipative

May 22, 2018 - through the PTFE tubing into the stainless needles (Hamilton, outer diameter = 1.47 mm). High electrical voltage (Bertan, 205B) was app...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Self-Organization in Electrospun Polymer Solutions: From Dissipative Structures to Ordered Fiber Structures through Fluctuations Chi Wang*,† and Takeji Hashimoto*,‡ †

Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC Kyoto University, Kyoto 606-8501, Japan

‡

S Supporting Information *

ABSTRACT: 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.

1. INTRODUCTION Following the seminal works of Zeleny1 and Taylor,2 Reneker’s group3 revisited the electrospinning process in the early 1990s to further work on producing polymer fibers with submicrometer diameter. These fibers are generally difficult to obtain by the conventional fiber spinning processes. Since then, the development of electrospinning has considerably progressed, and the widespread applications of the electrospun nanofibers have been realized in many different fields. Semidilute solutions with a sufficient entanglement density are generally required to yield bead-free fibers.4 Despite the widespread interest in this process, the mechanism of fiber formation is not completely disclosed to date. Electrospinning involves a sophisticated interplay of hydrodynamic with solution viscoelaticity, electrostatics with the setup electric field, and the fast solvent removal during jet stretching. Disclosing these coupled factors to rationalize these phenomena completely is difficult. In addition, real-time observations on spinning jet are challenging due to the fast processing time (ca. several milliseconds) and small diameter (ca. several micrometers) for observations. To © XXXX American Chemical Society

compensate for the experimental observation gap, theoretical simulations on the electrodynamic jet flow have gained successes to provide useful insights into the jet behavior.5−11 The majority of simulation studies is based on the equations of continuum mechanics, which consider the flowing jet as a single-phase solution, and the concentration in the flying jet is gradually increased due to the solvent evaporation. However, the extensional-flow-induced phase separation in the spinline is likely to occur, when the jet stretching rate induced by the electric field is higher than the intrinsic relaxation rate of polymer chains in the solution.12,13 Generally, in dynamically asymmetric mixtures of polymer and solvent, the coupling14−19 between stress and diffusion may lead to the stress-induced concentration fluctuations,14,16,18,20,21 in order to dissipate the extra free energy caused by deformation of the entangled network chains, and eventually to the phase Received: March 26, 2018 Revised: May 22, 2018

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DOI: 10.1021/acs.macromol.8b00647 Macromolecules XXXX, XXX, XXX−XXX

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different concentrations were prepared at 95 °C under constant stirring for several hours to ensure homogeneity. When the prepared solutions with PVA concentration (2−10 wt %) were cooled down to ambient temperature, the solution viscosity was repeatedly measured at 25 °C. The measured viscosity remained unchanged until 6 h. Thus, all measurements of solution properties were taken at 25 °C within 6 h after the hot solution was cooled down. Surface tension and conductivity were measured using Face surface tension meter (CBVP-A3) and consort conductivity meter (C832), respectively. The linear viscoelastic properties of the solutions were determined in a rheometer (ARES) by using a cup-and-bob feature. The oscillatory shear mode was used to determine the storage modulus G′(ω) and loss modulus G″(ω) over a range of angular frequencies; in this mode, the complex viscosity η* was calculated using the equation [G′(ω)2 + G″(ω)2]0.5/ω.37 The zero-shear viscosity η0 was subsequently determined from the Newtonian region at the low frequency. The intrinsic viscosity [η] at 25 °C was 1.065 dL/g, and the determined Huggin’s constant and Kraemer’s constant were 0.336 and 0.158, respectively (Figure S1, Supporting Information). The overlap concentration ϕ* determined by using ∼1/[η] was 0.93 wt % (0.72 vol %). 2.2. Electrospinning Processing and Fiber Morphology. The homogeneous polymer solution at 25 °C was delivered by a syringe pump (Cole−Parmer) at a controlled flow rate (Q) of 0.2 mL/h through the PTFE tubing into the stainless needles (Hamilton, outer diameter = 1.47 mm). High electrical voltage (Bertan, 205B) was applied to the needle spinneret. To construct a needle-plate electrode configuration, we used a steel net (30 × 30 cm2) to collect electrospun fibers at a tip-to-collector distance (H) of 21 cm below the needle tip. The dynamics of electrospinning jet was examined by using a highspeed camera (Fastec IL4) equipped with a telescope lens to keep a distance from the charged flowing jet. In general, the frame rate of 3000−5000 frames/s and shutter speed of 100−300 μs were applied to catch the motion of the straight jet. In addition, the cone height (Hc) and the jet length (Lj) measured from the needle end to the cone apex and to the straight jet end, respectively, were determined. The morphology of as-spun fibers was observed by using a scanning electron microscope (SEM, Hitachi SU8010) and a transmission electron microscope (TEM, Jeol JEM1400). For TEM observations, the electrospun products were collected by placing the carbon-film coated 200 mesh copper grids on top of the grounded collector for a short period of time, and no sample staining was applied. According to the electron microscope images, the dimensions of beads, barbs, and fibers were obtained. Ex situ observations of the collected jets were carried out by using a polarized optical microscope (POM, Leica DMLP).

separation,22,23 driven by the flow-induced thermodynamic instability18,24 upon increasing the flow rate. The flow-induced concentration fluctuation occurs in the spinline via the solvent squeeze mechanism25 from the regions rich in polymer to those lean in polymer against the osmotic pressure. Consequently, solvent-rich and solvent-lean phases are formed along the jet. This process breaks down the general assumption of the singlephase solution for the electrospinning jet. Hence, a new scenario for the fiber formation during electrospinning should be further explored, as explored in the present study. The extension-induced phase separation may also occur in the filament stretching conducted for elongation viscosity measurement when the stretching rate reaches a critical high value.26,27 Poly(vinyl alcohol) (PVA) is a considerably remarkable biocompatible polymer due to its desirable characteristics for many biomedical applications. With its high specific surface area, PVA nonwoven fabrics composed of electrospun nanofibers are widely used as drug-releasing carriers and tissue scaffolding. Previous studies28−31 on electrospinning of PVA mainly focused on the correlations between the processing parameters and fiber diameter. Despite all these efforts, a knowledge gap still exists in understanding the detailed mechanism of fiber formation. The existing gap is due to the lacking knowledge on the jet behavior from the viewpoint that formation of internal structures in the flowing jet prior to formation of the fiber morphology is scarcely investigated.32−34 By contrast, the fiber morphology on the collector has been intensively examined to reveal many distinct features, such as the beads-on-a-string structure (beaded fibers), the ribbons, and the branched fibers.35,36 The morphology of solid fibers should also be relevant with the jet morphology under the given processing parameters. On this basis, in situ observations of the flowing jet behavior are desirable and crucial to understand the abundant and diversified fiber morphology obtained, such as the formation of barbs along with the PVA nanofibers,29 which will be termed as barbed fibers in this work. In the present work, aqueous solutions of an almost completely hydrolyzed PVA (∼99% hydrolysis) with different concentrations were electrospun. First, we studied the concentration dependence of rheological properties to determine the overlap and entanglement concentrations, which are important in understanding the chain conformation in the solution for electrospinning. Afterward, we performed electrospinning to reveal the spinnability of PVA solutions in a wide concentration range. Emphasis was given on the in situ observation of the flowing jet behavior by using high-speed video imaging. According to our flowing jet observations and solid fiber examinations, the existing gap described above were clarified, and a new scenario for fiber formation was proposed as follows. In the electrospun polymer solutions, fiber formation is a self-organization process via formation of dissipative structures and the transformation from the dissipative structures to ordered fiber structures through the orientation-induced concentration fluctuations. Therefore, extension-induced phase separation of the single-phase PVA solution in a quiescent state occurs in the spinning line and triggers the self-organization process to produce the solid fibers with internal string structures.

3. RESULTS Sufficient entanglement density in the polymer solution is a prerequisite for electrospinning to prepare nanofibers.4,12 Deducing the entanglement concentration (ϕe) prior to electrospinning is important in a given polymer/solvent system. Figure 1 shows the double-logarithmic plots of specific viscosity (ηsp) of the solution versus solution concentration, from which ϕe (= 4.6 wt %, 3.6 vol %) can be deduced. ηsp was calculated by using (η0 − ηs)/ηs, where ηs is the solvent viscosity and η0 is the zero-shear viscosity of PVA/water solution. The determination of η0 of PVA solutions with different concentrations is presented in Figure S2. As shown in Figure 1, ηsp increases almost linearly with ϕ in the double-logarithmic plot in the dilute solution regime (ηsp ∼ ϕ1.12). The increase in ηsp with ϕ becomes rapid in the “semidilute unentangled” regime (defined as the regime satisfying ϕ* < ϕ < ϕe) where the power-law exponent α in ηsp ∼ ϕα gradually cross-overed from α ∼ 1.12 (in the “dilute” solution regime) to α ∼ 4.76 (in the “semidilute entangled” solution regime). At concentration higher than that of ϕe, ηsp rises steeply at a constant slope of 4.76 (i.e., ηsp ∼

2. EXPERIMENTAL SECTION 2.1. Solution Preparation and Properties. PVA powders with a 99% degree of hydrolysis were obtained from Sigma-Aldrich Co. The average molecular weight is 166 000 g/mol. Aqueous solutions with B

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ϕ4.76) in the double-logarithmic plot. The incipient of the final slope domain marked by the arrow indicates the ϕe of 4.6 wt %. Notably, the solution conductivity and surface tension change with the PVA concentration as shown in the inset. When the PVA concentration increases from 5 to 10 wt %, the solution conductivity increases from 0.8 to 1.2 mS/cm. However, the surface tension decreases from 56 to 47 dyn/cm. 3.1. Effect of Solution Concentration on Eletrospinnability. Among the solution properties, solution viscosity plays the dominant role in determining the final fiber diameter. Figure 2 illustrates the collected products from the electrospinning of PVA solutions with different concentrations. When solutions with a concentration higher than ϕ* but lower than ϕe are used for electrospinning, beaded fibers are readily observed (parts a and b). To obtain bead-free fibers, the minimum concentration required for the solution (ϕmin) is 7 wt % (part d). Consequently, the ϕmin/ϕe ratio is ca. 1.5, which is in agreement with previous findings.4 In solutions with a low PVA concentrations (ϕ* < ϕ < 4 wt %), beads and barbs are readily produced to form the mixture of beaded and barbed structures, respectively. The barb formation was reported and analyzed previously.29 With the decreased PVA concentration, the barb amount is increased, and the bead population is reduced. The inset of Figure 2a shows that the barbs are densely produced from the 2 wt % solution with the barbs extended perpendicular to the fiber axis. By contrast, the beads are approximately symmetric around the fiber direction. In addition, some regions appear featureless in the absence of any fiber-like structure. Such characteristic may be associated with the dissolution of the barbed fibers as-deposited on the

Figure 1. Concentration dependence of specific viscosity of PVA/ water solutions at 25 °C. ϕe is the entanglement concentration, and ϕ* is the overlap concentration determined from the reciprocal of the intrinsic viscosity. Inset shows the concentration dependence of solution conductivity and surface tension at 25 °C.

Figure 2. SEM images of fibers electrospun from solutions with different PVA concentrations of (a) 2, (b) 5, (c) 6, and (d) 7 wt % observed on the grounded collector Inset is the higher magnification to reveal the detailed morphology of the fibers. The arrows in (a) point out the regions, where no fiber is clearly seen because droplets of solvent, which result from the flow-induced phase separation in the liquid jet as will be detailed later, were detached from the jet, fell onto the collector, and then dissolved piles of the barbed fibers as-deposited on the collector into a thin featureless amorphous solid layer. White circles and yellow circles in the inset indicate the barbs and spindle beads along the fibers, respectively. The lower is the concentration of the electrospun solution, the more barbed fibers are developed. C

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Figure 3. Snapshots of a high speed video showing bulges and branched jets in the electrospun 2 wt % PVA aqueous solution. The dark dots in the dotted circles and other similar dots are artifacts which are commonly observed at the same positions in all the snapshots (a) to (d). Frame rate of 3000 frames/s and the shutter speed of 100 μs are used for the snapshots.

Figure 4. Snapshots of a high speed video of 2 wt % PVA solution to show the breaking of the main jet (upper row a) and branched jet (bottom row b). The arrow points out the breaking point. For each row, five consecutive images with time interval of 0.2 ms from left to right.

% PVA solution was electronspun at a fixed Q = 0.2 mL/h in the range of V from 10.5 to 12 kV which provides a stable conejet electrospinning mode. The results led us to conclude that the concentration plays a more influential role in determining the fiber morphology than the applied voltage. Thus, in this work, we focused primarily on the effects of the solution concentration on the electronspun jets and fibers obtained on the grounded collector under the given processing condition of V = 11 kV, Q = 0.2 mL/h, and H = 21 cm. 3.2. Observed Jet Features in Air and Corresponding As-Spun Fibers. High-speed imaging with a frame rate of 2000−5000 frames/s and short exposure period of 100−300 μs is generally required to follow and observe the real-time motion of jet with a traveling time of several tens of milliseconds from the tip of the Tailor cone to the grounded collector. Some

collector by the collision of solvent droplets detached from the flowing jet. These distinct features should be further investigated and will be discussed in the latter section. To study the effect of concentration on electrospinning, only entangled solutions yielding bead-free fibers were studied. Prior to electrospinning, the processing window for stable cone-jet electrospinning mode was constructed to determine the common processing variables of H = 21 cm, the applied voltage V = 11 kV, and Q = 0.2 mL/h. When the solution concentration ϕ changes from 7 to 10 wt %, both the cone height (Hc) and straight jet length (Lj) increases as summarized in Table S1 of the Supporting Information. The electrospun fiber diameter remarkably increases from 210 to 512 nm. The morphology of as-spun fibers examined by SEM is presented in Figure S3. To elucidate the effect of applied voltage V, the 7 wt D

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Figure 5. (a) A cascade network composed of the bulges and the branched jets, in which the bulges and branched jets become finer as the cascade level increases from 1, 2, 3, ..., etc. The dark dots in the dotted circles are the artifacts as described earlier in Figure 3. (b) SEM image of as-spun PVA fibers with the barb feature on the fiber. (c) TEM image showing the typical spacing between spindle beads and barbs. The barb length normal to the fiber is 1.5 μm, the interbarb distance is 8.3 μm, and the fiber diameter between the barbs is 87 ± 17 nm. Note that tiny spindle beads with a size of 205 nm along the fiber axis exist between barbs. (d) TEM image of tiny beaded fiber with the bead diameter of 115−130 nm and the fiber diameter of 20−30 nm. The diameter modulation of fiber section between beads is evidently seen. All SEM and TEM images were obtained from the 2 wt % PVA aqueous solution and for the fibers collected on the grounded collector and for those collected on the carbon-film coated cupper grids on top of the grounded collector, respectively.

Figure 6. TEM images showing the beaded fiber and barbed fibers collected on a carbon-film-coated copper grid placed on top of the grounded collector for the electrospun 2 wt % PVA aqueous solution. (a) 10 beads and a barb in total with different diameters from 1020 to 185 nm are connected by a fiber with diameter of 98 nm. (b) Micrometer-sized barbs with a tiny protruded string (fiber) at the barb tip (TB1 and TB2). The barb length is ca. 1.5−3.0 μm. The red arrows show the tip of the barb (TB1 and TB2). The black arrows point out a memory of the solvent-rich droplets (i.e., the droplets of the solution lean in polymer) with diameter of 110 nm, which were detached from the jet surface, subsequently fallen onto the collector, and evaporated. (c) Enlarged image of the region enclosed by the dashed square (c) in part (b) to show the tiny fibers with diameter of 15−30 nm. (d) Enlarged image of the region marked by the solid square (d) in part (b) to show the deposit of the PVA-lean solution detached from the jet surface, subsequently fallen and spread onto the collector, and eventually dried through evaporation of the solvent. (e) A nanosized barb with a length of 267 nm and the fiber diameter of barb tip (TB1) of 13 nm. (f) The PVA fiber with a diameter of 30 nm between barbs and/or beads.

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Figure 5d, the TEM showing a part of the fiber with a spatial variation of the diameter between the two spindle beads may reflect a memory effect of the bulge being extensively stretched along the jet axis. Figure 6 presents TEM images of the beaded fibers and the barbed fibers collected on a carbon-film-coated copper grid placed on top of the grounded collector for the electronspun 2 wt % PVA aqueous solutions. Figures 6c and 6d are the enlarged images of the squared regions (c) and (d) in Figure 6b, respectively: Figure 6c reveals the tiny fibers with diameter of 15−30 nm, while Figure 6d reveals the deposition of polymer-lean solution on the grid. Notably, a tiny fiber shown in Figures 6c protrudes from the tip of the barb (shown by the arrow marked by TB1 in Figures 6b and 6e). The nanosized barb with the protruded tiny fiber TB1 is also displayed in Figure 6e; the tip of the barb reveals the residual structure of the protruded tip end frozen after shrinking into the round tip end driven by the surface tension. The protruded tip is anticipated to have been linked the tiny fiber shown in Figure 6c before the break, freezing, and shrinking. In Figure 6f, TEM shows a part of the single fiber with ∼30 nm in diameter with some ordered structures (Figure 6f) with the characteristic length of ∼1 nm along the fiber axis. The PVA aqueous solution with a high polymer concentration (7 wt %) forms a relatively stable straight jet, which is free from the branched subjets, and a subsequent normal whipping jet, as shown in Figure 7a. (i) The straight jet exhibited the bulges deformed and elongated along the jet axis. Nevertheless, (ii) the electronspun fibers observed on the collector by SEM show nonbeaded, uniform fibers, as shown in Figure 7b. On the other hand, (iii) OM images display stringlike structures inside the straight jets as will be clearly shown later in Figure 16a. The three pieces of evidence (i) to (iii) described above may imply that the observed electrospun fibers are composed of fine fibers designated as “strings” as their internal structures. This expectation is confirmed by the TEM image shown in Figure 7c, which shows the two strings with dark contrast marked by the white arrows with a string diameter of ca. 110−125 nm. To reveal the internal structure variation, a TEM tracing along the fiber direction was performed. Interestingly, the two strings separately embedded in the spun fibers may locally merge as shown in the portion marked by the red arrows in Figure 7d to eventually form the core−shell fiber with a core diameter of 137 ± 3 nm and an outer diameter of 230 ± 16 nm (Figures 7e and 7f). 3.3. Off-Line OM and POM Observations of Internal Structures of the Flowing Jets. 3.3.1. Sampling Methods of the Flowing Jets for Off-Line OM and POM Observations. Figure 8 shows a special method of sampling the flowing jet and fixing the sampled jet as a whole and its internal structure (designated as a “sampling/fixing” method). The flowing jet was rapidly obtained by using a collecting device equipped with a cavity glass. The detached jet was directly received in a reservoir filled with a nonsolvent for PVA, as shown in Figure 8. 1-Propanol was used as the nonsolvent. 3.3.2. Off-Line OM Observation. Figure 9 displays the bright field optical microscope (OM) image, which shows the jet with bulges and branches. The OM images are taken from pieces of the flowing jets (Figure 3) sampled by using the sampling/ fixing method shown in Figure 8. The images in parts a and b show those of the same jet with low and high magnifications, respectively. A part of the image shows the jets, which contain a series of bulges. Large bulges are observed in the thick jet,

typical snapshots of the electrospinning jet are illustrated in Figure 3 for the PVA solution of 2 wt %, which is higher than ϕ* but lower than ϕe. The dark dots in the dotted circles and other similar dots in Figure 3a show the detached droplets depositing on the camera lens, which is irrelevant with the bulge formation in the flowing jet. To the best of our knowledge, the electrified jet exhibits several unreported jet morphologies. The jets may develop a bulge, bulges, or a series of bulges that are interconnected by main jets (parts a−c) or branched jets (designated as subjets) (part d) approximately equidistant (part b). The bulge serves to develop a branched jet with or without bulges (parts c and d). The branched jet or the subjet is fine with a smaller diameter than that of the main jet. The bulge developed in the branched jet is smaller than that in the main jet. The jets with bulges and branched jets are rotated and vibrated similar to a pendulum. They are deformed by the electric force imposed on charges induced on the jet surface, as shown in Figures 3c and 3d, and broken at a point as shown in Figure 4; the jet breaking moment is captured in the main jet (upper panel, from the snapshots a3 to a5 in Figure 4) and in the branched jet (lower panel, from the snapshots b2 to b3 in Figure 4). The broken jets are detached from the main/ branched jets, and they fall into the grounded collector. The bulges issue the branched subjet(s) based on the essentially same principle that the Taylor-cone-apex issues the main jet. A set of the branches and the bulge can travel either upward or downward to the grounded collector, as shown in Figure S4; this movement reflects the relative positions between the surface-charged main jet and the surface-charged set of the branches and bulge. The movement also depends mainly on a balance of the following two forces: (1) net electric force acting on the surface charges of both the main jet and the set of the branches and bulge and (2) the viscoelastic force acting on the stretched chains in the flowing main jet liquid and subjet liquids. The traveling speed of the bulge is ca. 0.15−0.27 m/s. The movement of the branch and bulge along the jet implies the existence of interface between the main jet and the set of branches, thereby indicating that the internal structure of the moving bulge is different from that in the main jet and the subjets. The jets develop a series of branched jets and bulges, starting from a bulge in the main jet, which results in a cascade network composed of the bulge and the branched jet, as a building block of the cascade, as shown in Figure 5a. The bulges and branches become fine with the increasing cascade level from to 1, 2, and 3, as shown in Figure 5a. The jets shown in Figures 3 to 5 yielded the barbed fibers on the grounded collector as elucidated by SEM (Figure 5b) and TEM (parts c and d in Figure 5 and in Figure 6b to be described later). The barbed fibers consist of large barbs connected by thick fibers. These barbs may be created when the large bulges and jets, which still contain a sufficient amount of the solvent, are detached from the main jets and subsequently fallen onto and collided with the collector. The large bulge and thick jet are expected to be flattened with their edges thickened on the collector. The small bulges and thin jets, which contain a small amount of solvent and consequently high PVA concentration, may yield small barbs and thin fibers, as shown in Figures 5b and 6b. Figure 5c displays that small spindle beads extended along the fiber axis with a size of 205 nm along the fiber axis exist between two large barbs with a barb length of 1.5 μm normal to the fiber axis. The interbarb distance is 8.3 μm, and the fiber diameter between the barbs and the spindle beads is 87 ± 17 nm. In F

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Figure 8. Sampling/fixing method of the flowing jet invented for the off-line observations with OM and POM. (a) The top and side views of the nonsolvent reservoir used to collect the flowing jet in the middle of straight jet. (b) The top and side views of the sampling device with a fast moving arm rotating perpendicular to the straight jet. The glass slide containing the nonsolvent reservoir is attached to the moving arm.

Figure 7. (a) Three consecutive snapshots of the straight jet, followed by jet whipping at the jet end (marked by the arrow) shown by a high speed video with time interval of 0.4 ms. (b) SEM image showing the nonbeaded, uniform fibers on the grounded collector. (c), (d), and (e) are TEM images of a selected as-spun fiber traced along the fiber direction to reveal a variation of the internal structure of the spun fibers. The arrows in (c) show the two strings with dark contrast separated each other in air or amorphous matrix as a consequence of the solvent evaporation of the polymer-lean region or phase. The string diameter is ca. 110−125 nm. In (d), the two strings move closer to each other and merge at the positions shown by the red arrows. In (e), the two strings merge to form a core/shell fiber with the core diameter of 137 ± 3 nm (shown by solid line) and the outer diameter of 230 ± 16 nm (by dotted line). The dashed rectangular region in (e) is enlarged in (f) for a clear observation of core/shell fiber. All images were obtained for the 7 wt % PVA aqueous solution. Q = 0.2 mL/h, H = 14 cm, V = 9.6 kV, Tr = 24 °C, and RH = 51−60%. Shutter speed = 10 μs for the image in (a).

Figure 9. Off-line OM image of the jet obtained from the electrospun 2 wt % PVA aqueous solution taken by using the sampling/fixing method shown in Figure 8. The image of (b) is the enlargement of the dotted rectangular region in (a). The dotted rectangular region in (b) is enlarged in Figure 14 for a detailed discussion in section 4.4.

whereas small bulges are observed in the thin jet. The small bulges in the thin jet should be developed in the branched jet. The large bulges tend to imply the existence of internal structures, while the straight jet between the bulges seem to be relatively homogeneous and composed of stretched PVA chains, which will be detailed in section 4.4.

across the interface of the jet along the radial direction of the jet, respectively. The mutual diffusion of 1-propanol and water develops the interfacial region, where the volume fraction ϕK (K = propanol or water, ϕpropanol + ϕwater = 1) along the radial direction of the jet changes during the early stage diffusion, as schematically illustrated from Figures 10a to 10b. In the interfacial region facing the nonsolvent where ϕpropanol is high, the nonsolvent-induced phase separation between PVA and the mixed solvent occurs via spinodal decomposition (SD) to form a skin layer rich in PVA both in the jet and in the bulge, as shown in Figures 10c and 10d, respectively. The skin layer is vitrified because of the high Tg of PVA (85 °C). The vitrified or solidified skin layer may suppress or retard the mutual diffusion of the nonsolvent across the interface and serve to conserve the

4. DISCUSSION 4.1. Principles of Fixing the Structure of the Sampled Jet with a Nonsolvent. We discuss the basic principles of the method, as described in section 3.3.1; this method is used in fixing both the sampled jet itself and its internal structure with the nonsolvent. When the piece of the sampled jet is quickly immersed in the nonsolvent, the nonsolvent (1-propanol) and the solvent (water) for PVA immediately interdiffuse in and out G

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the mixed solvent of 1-propanol and water becomes higher than the critical concentration for the gelation ϕgel but smaller than that for the SD ϕSD (Figure 11). Notably, at point A, the system is brought at a deep quench from the spinodal point and consequently into a large thermodynamic instability, which yields a considerably large rate of the spinodal decomposition. 4.2. Formation of Bulges as Dissipative Structures. The straight jet is in a nonequilibrium state where PVA chains are oriented as a consequence of the jet flow induced by the electric stress imposed on the jet. The jet displays a higher free energy than the corresponding quiescent solution primarily due to the reduction in the conformational entropy of the polymer chains outweighing the gain of the enthalpy reduction due to the interchain associations. Thermal orientation fluctuations of the chains in the jet from the average chain orientation will exist under the external stress. These fluctuations will induce phase separation into polymer-rich and polymer-deficient regions via the solvent-squeezing mechanism23,24 inherent in dynamically asymmetric systems;15,17 this phenomenon is schematically illustrated by the change of the jet in Figures 12a and 12b,

Figure 10. Changes in the spatial distribution of 1-propanol and water immediately (a) and sometimes (b) after immersing the detached jet with nonsolvent (ϕproponal + ϕwater = 1). The cross section of the jet normal (c) and parallel (d) to the jet axis showing the skin layer of the amorphous PVA chains formed through the nonsolvent-induced phase separation.

jet and bulge structures and their internal structures, if they exist. Moreover, ϕpropanol inside the skin layer increases above zero with further time elapse. Consequently, PVA gelation occurs in the solution inside the skin of the jets and bulges when ϕpropanol becomes higher than the critical concentration of 1-propanol for the gelation in the mixed solvent (ϕgel). This gelation may effectively suppress a further progress on the phase separation of the solution enclosed by the skin layer involved by increasing ϕpropanol with time. Thus, this gelation process may serve to conserve the existing internal structures of the jet and bulge and even the orientation of PVA chains within them, if they exist. Figure 11 shows schematically a phase diagram for the aqueous PVA solution in the parameter space of temperature

Figure 12. Jet before (a) and after (b) the bulge formation as a consequence of a dissipation process of the energy stored in the jet.

where the thick arrows in part (a) indicate the solvent-squeeze directions. The hypothesis for the orientation-fluctuationinduced concentration fluctuations (OFICF) and phase separation via the squeezing of solvent from regions with high chain orientation to regions with low chain orientation is presented in the Appendix. The polymer-rich regions are the straight jet part with a diameter thinner than that of the bulge, as shown in Figure 12b. This thin diameter is a consequence of the solvent-squeezing toward the bulge, as shown by the arrows in Figure 12a. The straight jet part exhibits a chain orientation and concentration higher than the average orientation and average concentration, respectively. Furthermore, the polymer-lean regions form the bulge where the average diameter is larger; the chain orientation is lower than those of the average as shown in part (b). The bulge has more heterogeneous structure and a larger scattering volume than the straight jet. Thus, the former scatters light more strongly than the latter; thereby the former appears to be dark in contrast to the latter in the high speed video image. The orientation relaxation of the chains in the bulge reduces the cost of the conformational entropy loss. The enhanced intermolecular interactions between the orientated PVA chains induced by the increased polymer concentration and chain orientation in the straight jet part can also reduce the net

Figure 11. Schematic phase diagram of the PVA aqueous solution in the parameter space of temperature (T) or ϕproponal and ϕPVA. The solid curve is the sol−gel transition curve, and the dashed curve is the spinodal transition curve.

(T) and PVA concentration (ϕPVA).38 The increase in ϕpropanol in the mixed solvent is essentially equivalent to the decrease in T for the PVA in pure water. We postulated that in the interfacial region of the jet and bulge ϕpropanol is high. Thus, the system at a given concentration of PVA in water, i.e., ϕPVA = ϕPVA,1, is placed at point A in the phase diagram, where SD starts to occur rapidly. In the interior of the jet or bulge, the gelation is considered to start to occur effectively at point B, where the nonsolvent (1-propanol) concentration (ϕpropanol) in H

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micrometer, submicrometer, and eventually to nanometer. This cascading transformation of the dissipative structure, which enables a cascading reduction in the free energy barriers for ordering into the fiber, is evidenced with the existence of the micrometer-, submicrometer-, and nanosized beaded (or barbed) fibers on the collector, as shown in Figures 5 and 6. 4.4. Internal Structures of the Flowing Jets. Figure 14 illustrates an enlarged image of a part of the jet image encompassed by the red rectangle shown in Figure 9b. The OM image shows a series of bulges and deformed bulges connected with the straight jet. Parts (a) and (b) present the OM image and the dark field OM image of the same jet, respectively. The dark field image (POM) was taken under crossed polarizers with their polarization directions oriented in vertical and horizontal directions in the figure. The smallest bulges and the thinnest jet observed in the bright field OM image are 2.8 and 1.0 μm, respectively. The jet and bulge show a skin layer in dark blue or blue color in their peripheries, as clearly shown in the enlarged images of a1, a3, and a4 in the insets of Figure 14a. The large bulges a3 and a4 are elongated along the jet direction by the electric force. The jet and the bulge display optical anisotropy so that they become bright in the POM images, except for parts of the jets oriented parallel to the polarization direction of one of the crossed polarizers (e.g., the parts marked by a2 in Figure 14a) and for those with considerably thin thickness (parts marked by a5 and a6 in Figure 14a). The POM image shows no skin layer so that the skin presents no birefringence and consequently is amorphous. The bulges a3 and a4 appear to have fine internal structures, as revealed by the enlarged images a3 and a4 presented in the insets of Figure 14a and by those a3 and a4 presented in the insets of Figure 14b. The inset a4 in Figures 14a and 14b reveals that the given elongated bulge is composed of two strings aligned parallel to each other; each string is anticipated to be composed of a series of fine bulges (demixed domains with a low chain orientation) interconnected by the fine subjets (bundles of stretched chains), as schematically illustrated in part a of Figure 15. The left edge of the elongated bulge in the inset a4 in Figure 14a (shown by the part around the arrow marked by a5) does not show up any strings; it rather shows up only the skin layer (a sheath of coaxial cylindrical structure) in the OM image but almost nothing in the POM image. This piece of evidence may reveal that in this part of the elongated bulge the fine bulges in the stringlike structures inside the elongated bulge (as shown by part a in Figure 15) are further stretched out, so that the fine bulges may be transformed to the fine subjets (bundles of the stretched chains) to result in the bulge-free stringlike structures, as schematically drawn in part b of Figure 15: the strings existing inside the sheath (skin) of the coaxial cylindrical structure is remarkably thin, so that they may be unable to be resolved under the OM and POM. This fiber may be captured by TEM, which should be confirmed in future works. Figure 15 may schematically present a plausible mechanism, by which the nonbeaded fiber composed of the assembly of the fine strings is formed as revealed by Figures 7b and 7c. In a piece of the jet marked by a6 in the OM shown in Figure 14a, the internal structure enclosed by the sheath of the jet cannot be observed under the POM (Figure 14b). Thus, the jet looks like a coaxial cylinder with its sheath and core corresponding to the skin and an empty space, respectively. This characteristic appearance may also be interpreted in terms of the existence of the thin fiber(s) (the fine bundle of stretched chains which is unable to be resolved by OM and

enthalpy of the straight jet part via the transformation of the intermolecular associations from PVA/water to PVA/PVA and water/water. Both of the physical factors described above partially dissipate the stored elastic energy in the flowing jets. Thus, the bulge is considered as a dissipative structure developed during the energy dissipation process under the imposed external electric force on the flowing jet. According to our scenario, the solvent removal during electrospinning may be due to the solvent evaporation and stress-induced syneresis of the solvent from the stretched jet which results from the stress-induced phase separation between solvent and polymer along the radial direction of the jet. The former yields the gradual reduction of jet diameter to the final vitrification, while the latter produces the solvent-rich layer or phase in the periphery of the jet, which may be detached from the main jet when the jet rapidly vibrates, particularly in the whipping process. The latter mechanism works even for solvents with relatively high boiling points and outweighs the evaporation mechanism in this case. The detached liquid droplets rich in solvent finally deposit on the collector. They may also subsequently dissolve dry fibers on the collector to form the featureless dark region, as shown in Figure 2a. 4.3. Cascade Evolution of Finer Bulge and Finer Jet. The charges developed on the surface of the bulge and straight jet under the applied electric field further stretch the bulge and straight jet due to the electric force acting on the surface charges. The stretched and deformed bulges are transformed into the jet with the characteristic internal structures, as evidenced later in Figure 14, unless they are not broken. The stretched straight parts of the jets and bulges stretched under the electric field are found to be occasionally broken and fall onto the collector. The stretching of the bulge and jet in Figure 13a deforms the bulge to further develop finer straight jet(s) and finer bulge(s)

Figure 13. A bulge in the jet (a) is stretched into a finer bulge and finer jet (b-1), (b-2), and (b-3) and creates a branch of a fine branched jet with or without the bulge (c).

as schematically shown in Figure 13b. New bulges may evolve during the stretching the finer jet. This process will be shown later in Figure 14. The chain stretching in the jet and the partial orientation relaxation of the chains in the jet induced by solvent squeezing may result in new bulges in the jet. Further stretching of the bulge into finer jet and the formation of finer bulges in the jet repeatedly occur, as schematically shown in the changes from parts (a) to (b-1), (b-1) to (b-2), and (b-2) to (b-3), etc. This repeated stretching and chain relaxation causes a cascading dissipation of the stored energy coupled with a cascade reduction in the size of the straight jets and the bulges from I

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Figure 14. Off-line OM image (a) and POM image (b) of the jet taken by using the sampling/fixing method. The images are taken on a part of the same jet as shown by the dotted rectangular region in Figure 9b.

POM) in the center of the jet. In this case, the solution of PVA coexisting with the bundle of the oriented chains will form the skin due to the nonsolvent-induced phase separation described previously. The less elongated bulge a3 is composed of many fine bulges (demixed domains), which are polymer-lean domains formed by the stress-induced demixing within the bulge originally formed in the jet, as shown in Figures 3 to 5. The PVA aqueous solution with a high polymer concentration (7 wt %) formed a relatively stable jet free from the branched jets but with the bulges deformed and elongated along the jet axis as already shown in Figure 7a. Corresponding off-line OM images of the flowing jet of the 7 wt % solution

revealed the demixed domains within the highly elongated bulges, as shown in Figure 16. The minimum concentration for PVA solutions to produce evidently uniform fibers, in which one or two strings are embedded, is 7 wt % (Figures 7b−e). The straight jet exhibits a rapid and small fluctuation of jet diameter from the cone apex to the straight jet end (Figure 7a), thereby indicating the presence of highly elongated bulges with a lateral dimension only slightly larger than that of the main jet. Tracing the displacement of these highly elongated bulges with elapse time was attempted but failed. The off-line OM image of the collected jet shown in the inset of Figure 16a clearly demonstrates four parallel strings. Along each string, a series of J

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entangled system, respectively, with τR being the longest Rouse relaxation time). In other words, we can observe the bulges in the straight jet section or in the whipping jet, without restriction to the region near the Taylor cone. The initial appearance of bulges is approximately 1 mm away from the Taylor cone for the 2 wt % PVA solution (Figure 3) or barely observed in the straight jet, except for highly elongated bulges along the jet at ∼4 mm away from the Taylor cone, for the 7 wt % PVA solution (Figure 7a). The droplet (i.e., bulge) formation in the whipping jet is clearly observed in the polystyrene/DMF solution (Figure 3b in ref 43) during electrospinning to produce beads-on-a-string structure on the collector. Greenfeld et al.39 also provided a notable finding, which is based on their theoretical calculations, that the solvent molecules are forced to squeeze out of the entire network driven by the contraction of the jet under high-stretching force. Hence, a solvent layer may be inferred at the jet surface due to stress-induced phase separation.44

Figure 15. Elongated bulge which is composed of the skin and the internal structure: the stringlike structures composed of a series of fine bulges (demixed domains) interconnected by the fine subjets (part a) and the bundle of the stretched chains (bulge-free strings) (part b) stretched out from the stringlike structures. As shown schematically, the two bundles are independently formed from the two stringlike structures.

fine bulges with a diameter of 0.84 μm are linked together by the fine jets. According to these OM observations, the originally formed bulge in the straight jet is driven by the stress-induced phase separation within the flowing jet, while the demixed domains within the bulge are driven with stress-induced phase separation in the confined space of the deformed bulge. Thus interestingly, a dual stress-induced phase separation exists at the dual length scales in the self-assembly of this system. We also determined that the stringlike structures are pulled out from both edges of the elongated bulge (for example a3, as observed in both the OM and POM images in Figure 14). These stringlike structures are eventually transformed into the bulge-free strings, i.e., the bundles of stretched chains. Greenfeld et al.39 indirectly inferred the likeliness of the flowinduced phase separation in the electrospinning jet. They modeled a small section of jet near the Taylor cone with a lattice model of beads and linear springs to represent the entangled chains subjected to a considerable deformation of the entire network as a whole. Their result is in contrast with our scenario that the coupled orientation fluctuations and concentration fluctuations yielding bulges may occur in the local environment in the jet when the stretching rate induced by the electric stresses is higher than the intrinsic orientation relaxation rate of polymer chains (i.e., the chain retraction rate40−42 τe−1, where τe = τR and 2τR for nonentangled and

5. CONCLUSIONS Understanding the rheological properties of electrospinning solution and its related phase diagram is important to realize good morphology control of the as-spun fibers. The formation of beaded fibers with the beads-on-a-string structures on the collector is due to the extensional-flow-induced phase separation in the flowing jet. Specifically, orientation fluctuations of dynamically stretched chains are developed in the flowing jet subjected to the electrical force induced by the external field: the electrical force is counterbalanced with resistive forces of inertial, surface tension, and rheological forces of the polymer solution. The resultant force to stretch the jet varies along the spinning jet: thus, the extension rate varies along the spinning line. Given that the extension rate is higher than the chain retraction rate, the local inhomogeneity originating from orientation fluctuations induces the flow of the solvent from the regions with high chain orientation to the regions with low chain orientation driven by the chemical potential difference of solvent in these two regions. This solvent flow develops the concentration fluctuations against osmotic pressures, thereby developing polymer-rich regions with high chain orientation (the thinner jet in Figure 12b) and solvent-

Figure 16. Off-line OM image of the jet obtained from the electrospun 7 wt % PVA aqueous solution taken by using the sampling/fixing method. (a) Phase contrast optical image; inset is the enlargement for clear observation of internal jet structure. Four strings with a series of the fine bulges and the fine subjets interconnected together align parallel one another along the jet axis. A typical bulge with a diameter of 0.84 μm is displayed. (b) POM image. The directions of polarizer and analyzer are displayed by the arrows. Parts (c) and (d) are the enlarged portions of the dashed boxes marked by (c) and (d) in part (b), respectively. K

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Macromolecules ⎛ ⎞ ⎛ ∂ΔF ⎞ ∂ΔF ⎟ ⎛ ∂ϕp ⎞ ⎟⎟ μ1 = ⎜ = ⎜⎜ ⎟ ⎜⎜ ⎝ ∂n1 ⎠T , V ⎝ ∂ϕp ⎠⎟ ⎝ ∂n1 ⎠ n2 , T , V T ,V

rich regions with low chain orientation (the bulge in Figure 12b), the process of which effectively dissipates the energy stored in the jet due to the external energy imposed on it. The thinner jet and the bulges, which are developed as dissipative structures, will be further kept under the uniaxial stretching force field, which will repeatedly create the next-generation dissipative structures, i.e., the finer jets and the finer bulges. The obtained morphologies of the beads-on-a-string and the barbson-a-string on the grounded collector by electrospinning of the semidilute solutions are attributed to a cascade transformation of bulges and branched jets at different length scales. With the sufficient increase in polymer concentration, the cascade evolution of the dissipative structures in the flowing jet as described above (but free from the branched subjets) develops the stringlike structures as internal structures of the jet: the strings are composed of a series of the (deformed) bulges interconnected by the fine subjets (Figure 16). These strings and fine jets are eventually transformed into the stretched out stringlike structures free from the bulges (part b in Figure 15) which exist as the internal structures (fine fibers) in the nonbeaded fibers on the collector (Figures 7b and 7c). Further work is needed to determine the general applications of our fiber formation scenario to other polymer solutions with weak segment−segment interactions, e.g., polystyrene solutions.

fz ≡

=

ϕp−2/3 5V

(A4)

(αz − 1/αz 2)

⟨cos2 θi⟩ =

(A5)

∫0

π

P(θi) cos2 θi sin θi dθi /

∫0

π

P(θi) sin θi dθi (A6)

Thus, fz increases with αz asymptotically according to αz. Moreover, μ1 increases with increasing fz as predicted from eqs A3 and A5. The flow may induce locally inhomogeneous stretching of the chains and thereby evolve the orientation fluctuations with the regions having higher and lower chain orientations which locally coexist. The flow-induced fluctuations in turn induce the flow of the solvent from the regions having higher chain orientation with a larger μ1 to the regions having the lower chain orientation with a smaller μ1 against osmotic pressure and thereby develop also such concentration fluctuations that make the regions having a higher and lower fz are more and less concentrated than average, respectively. In semidilute unentangled solutions in the concentration range of ϕ* < ϕ < ϕe, thermal concentration fluctuations may locally develop the regions with and without chain overlaps. A few transient clusters made of lightly entangled chains are likely developed in the overlapping regions: chains in these clusters behave differently from the single chains in the nonoverlapping regions. This phenomenon is especially significant for polymer chains with a poor solvent, i.e., with relatively large segment− segment attractive interaction, such as the present PVA/water solutions. When the jet is subjected to a strong uniaxial stretching by the electrical forces, the orientation fluctuations and/or concentration fluctuations in local environments may also occur to give rise to the bulge formation, as described earlier. In dilute solutions (ϕ < ϕ*), electrospinning is degenerated to electrospraying to produce nanoparticles, and no string structure is observed. Single-chain particles and clusters of several chains can be produced because of the agglomeration of random coils during jet stretching.46

(A1)

where kB is the Boltzmann constant, Nc is the total number of network chains, and ϕp is the polymer volume fraction in the solution. The Helmholtz free energy ΔF at temperature T for the network chains satisfying the ideal rubber elasticity is given by ⎛ ⎞ 1 2 kBTNcϕp−2/3⎜αz 2 + − 3⎟ 2 αz ⎝ ⎠

1 [3⟨cos2 θi⟩−1] 2

where θi is the orientation angle that the ith segment of the chain makes with respect to the stretching axis z, and ⟨cos2θi⟩ is the statistical average of cos2θi with respect to the orientation distribution function of P(θi).

APPENDIX. ORIENTATION-FLUCTUATION-INDUCED CONCENTRATION FLUCTUATIONS We discuss here the coupling between orientation fluctuations and concentration fluctuations for a polymer solution with chain entanglements subjected to an elongation rate larger than the disentanglement rate and the chain retraction rate. In such elongated polymer solutions, the thermal orientation fluctuations may be built up, so that there are regions having higher and lower chain orientation than the average one. We anticipate that the regions having the higher chain orientation may result in having a larger concentration than those having the lower chain orientation via the solvent squeezing from the regions with the higher chain orientation to those with the lower chain orientation against osmotic pressure. In order to prove the expectation described above, we simplify our system by replacing the entanglement points with chemical cross-linking points under the imposed condition of the elongation rate outweighing the disentanglement rate and the chain retraction rate. The entropy change ΔS accompanied by a uniaxial stretching of the unstretched swollen cross-linked polymer along the z direction under the affine deformation of the constant volume with the elongation ratio αz is given by45

ΔF = −T ΔS =

(A3)

where n1 and n2 are number of moles of the solvent and polymers, respectively, V is the volume of the swollen crosslinked polymer, and V1 is the molar volume of the solvent. Thus, μ1 asymptotically increases with αz according to αz2. The second-order orientation factor of the chain segments fz defined below also increases with αz as follows:

■

⎛ ⎞ 1 2 ΔS = − kBNcϕp−2/3⎜αz 2 + − 3⎟ αz 2 ⎝ ⎠

⎛ ⎞ 1 2 − 3⎟ kBTNcV1ϕp1/3⎜αz 2 + αz 3 ⎝ ⎠

=

(A2)

The chemical potential of the solvent in the stretched swollen cross-linked polymer chains μ1 is given by L

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(13) Reneker, D. H.; Yarin, A. L.; Zussman, E.; Xu, H. Advances in Applied Mechanics; Aref, H., Van Der Giessen, E., Eds.; Elsevier/ Academic: London, 2007; Vol. 41, pp 43−195. (14) Helfand, E.; Fredrickson, G. H. Large fluctuations in polymer solutions under shear. Phys. Rev. Lett. 1989, 62, 2468−2471. (15) Doi, M.; Onuki, A. Dynamic coupling between stress and composition in polymer solutions and blends. J. Phys. II 1992, 2, 1631−1656. (16) Milner, S. T. Dynamical theory of concentration fluctuations in polymer solutions under shear. Phys. Rev. E: Stat. Phys., Plasmas, Fluids, Relat. Interdiscip. Top. 1993, 48, 3674−3691. (17) Onuki, A. Dynamic scattering and phase separation in viscoelastic two-component fluids. J. Non-Cryst. Solids 1994, 172-174, 1151−1157. (18) Onuki, A. Phase transitions of fluids in shear flow. J. Phys.: Condens. Matter 1997, 9, 6119−6157. (19) Toyoda, N.; Takenaka, M.; Saito, S.; Hashimoto, T. Experimental studies of stress-diffusion coupling in semi-dilute polymer solutions. I. ’Viscoelastic length’ and viscoelastic effects on early stage spinodal decomposition. Polymer 2001, 42, 9193. (20) Wu, X.-L.; Pine, D. J.; Dixon, P. K. Enhanced concentration fluctuations in polymer solutions under shear flow. Phys. Rev. Lett. 1991, 66, 2408−2411. (21) Hashimoto, T.; Fujioka, K. Shear-enhanced concentration fluctuations in polymer solutions as observed by light scattering. J. Phys. Soc. Jpn. 1991, 60, 356−359. (22) Kume, T.; Hattori, K.; Hashimoto, T. Time Evolution of Shearinduced Structures in Semidilute Polystyrene Solutions. Macromolecules 1997, 30, 427−434. (23) Hashimoto, T. Light scattering from multicomponent polymer systems in shear fields: real-time, in situ studies of dissipative structures in open non-equilibrium systems. In Soft Matter Characterization; Borsali, R., Pecora, R., Eds.; Springer: Berlin, 2008; Vol. 1, Chapter 8, pp 373−462. (24) Saito, S.; Hashimoto, T. Critical conditions for structure formation in semidilute solutions induced under continuous shear flow. J. Chem. Phys. 2001, 114, 10531−10543. (25) Saito, S.; Matsuzaka, K.; Hashimoto, T. Structures of a semidilute solution under oscillatory shear flow. Macromolecules 1999, 32, 4879−4888. (26) Sattler, R.; Gier, S.; Eggers, J.; Wagner, C. The final stages of capillary break-up of polymer solutions. Phys. Fluids 2012, 24, 023101. (27) Semakov, A. V.; Kulichikhin, V. G.; Tereshin, A. K.; Antonov, S. V.; Malkin, A. Y. On the nature of phase separation of polymer solutions at high extension rates. J. Polym. Sci., Part B: Polym. Phys. 2015, 53, 559−565. (28) Tang, C.; Saquing, C. D.; Harding, J. R.; Khan, S. A. In situ crosslinking of elecctrospun poly(vinyl alcohol) nanofibers. Macromolecules 2010, 43, 630−637. (29) Holzmeister, A.; Yarin, A. L.; Wendorff, J. H. Barb formation in electrospinning: experimental and theoretical investigations. Polymer 2010, 51, 2769−2778. (30) Rošic, R.; Pelipenko, J.; Kristl, J.; Kocbek, P.; Bešter-Rogač, M.; Baumgartner, S. Physical characteristics of poly(vinyl alcohol) solutions in relation to electrospun nanofiber formation. Eur. Polym. J. 2013, 49, 290−298. (31) Liu, F.; Nishikawa, T.; Shimizu, W.; Sato, T.; Usami, H.; Amiya, S.; Ni, Q. Q.; Murakami, Y. Preparation of fully hydrolyzed polyvinyl alcohol electrospun nanofibers with diameters of sub-200 nm by viscosity control. Text. Res. J. 2012, 82, 1635−1644. (32) Baumgarten, P. K. Electrostatic spinning of acrylic microfibers. J. Colloid Interface Sci. 1971, 36, 71−79. (33) Shin, Y. M.; Hohman, M. M.; Brenner, M. P.; Rutledge, G. Experimental characterization of electrospinning: the electrically forced jet and instabilities. Polymer 2001, 42, 09955−09967. (34) Yarin, A. L.; Kataphinan, W.; Reneker, D. H. Branching in electrospinning of nanofibers. J. Appl. Phys. 2005, 98, 064501. (35) Fong, H.; Chun, I.; Reneker, D. H. Beaded nanofibers formed during electrospinning. Polymer 1999, 40, 4585−4592.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00647. Figure S1: determination of intrinsic viscosity of PVA aqueous solution; Figure S2: determination of zero-shear viscosity of solutions with different concentrations; Table S1: effects of solution concentration on the solution properties, cone height Hc, jet length Lj, and fiber diameter deposited on the collector df; Figure S3: morphologies of fibers electrospun from solutions of different concentrations; Figure S4: measurements of moving speeds of branched jets (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(C.W.) E-mail: [email protected]. *(T.H.) E-mail: [email protected]. ORCID

Chi Wang: 0000-0003-0627-2341 Notes

The authors declare no competing financial interest. T.H. Professor Emeritus, Kyoto University, Kyoto, Japan.

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ACKNOWLEDGMENTS We thank C. W. Tsai, H. Y. Lai, and T. Y. Chou for their careful performance of electrospinning experiments to validate the present findings. This research has been supported by the Ministry of Science and Technology of Taiwan (MOST 1062221-E-006-211-MY3).

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N

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