1.6 Advanced Fiber Characteristics
1.6.1 Ribbons, Wrinkles, Branching, and Netting
Electrospinning generally produces fibers with circular cross sections. However, other morphologies have been observed, e.g. flat ribbons, wrinkled surfaces, and branched fibers (Figure 1.4) (Koombhongse et al. 2001; Huang et al. 2000). The morphology affects the specific surface area as well as the mechanical properties of the resulting fiber mats (Camposeo et al. 2013).
Ribbon structures can have two forms, flat ribbon and a ribbon with two tubular ends (Figure 1.4) (Koombhongse et al. 2001). With the use of a high‐speed camera, Reneker and coworkers determined that this morphology formed prior to deposition. The fibers emerged from the tailor cone with a circular cross section. A thin skin formed on the fiber as the outer layer of the fiber dried and thinned. The solvent inside the skin diffused out rapidly due to the high surface area to volume of the fiber during thinning, and with atmosphere pressures, the skin deforms. The fibers changed from circular to elliptical to either flat ribbons or ribbon with two tubular ends when the fiber skin collapses (Koombhongse et al. 2001). The final shape, i.e. flat ribbon or ribbon with two tubular ends, is dictated by the stiffness of the skin and self‐repulsion charges. Ribbon structures were observed with fibers made with 30 wt% PS in DMF, 10% poly(ether imide) in hexafluoro‐2‐propanol (Koombhongse et al. 2001), and 81 kDa recombinant protein (Huang et al. 2000).
Figure 1.4 Overview of interesting electrospun structures: (A (a–d)) ribbons
(Source: Reprinted from Koombhongse et al. (2001). Copyright (2001). Wiley.),
(B) wrinkled fiber
(Source: Reprinted (adapted) with permission from Pai et al. (2009). Copyright (2009). American Chemical Society.),
(C (a, b)) branched fibers
(Source: Reprinted from Koombhongse et al. (2001). Copyright (2001). Wiley.),
and (D) fiber nets
(Source: Reprinted from Wang et al. (2017). Copyright (2017), with permission from Elsevier.).
Alternatively, when the skin collapses, the fiber surface can wrinkle due to the buckling instability associated with the skin pulling inward as the solvent evaporates. The final shape, i.e. wrinkled fibers or a flat fiber, depends on the ratio of the Young's modulus of the core to that of the shell as well as the ratio of the radius of the fiber to thickness of the shell. The wrinkles are less deep when there are more wrinkles and the cross section is closer to circular. The morphology can also be affected by solvent volatility. Higher volatility tends to result in ribbons, whereas lower volatility tends to lead to wrinkling. For example, polystyrene/tetrahydrofuran (high volatility) formed ribbons, whereas polystyrene/DMF (low volatility) produced wrinkled fibers at the same polymer concentration (Koombhongse et al. 2001; Wang et al. 2009).
Another morphology that can occur in electrospun fibers is branching. Branched fibers are a small fiber splitting from the main fiber, and these splits are often times formed at the bends of the fibers. Similarly, split fibers occur when a primary fiber splits into smaller fibers. This phenomenon is due to jet instability. Nanofiber/net structures have also been reported. Din et al. electrospun a polyvinyl alcohol/H2O/formic acid mixture and observed typical electrospun nanofibers ~200 nm and a 2D‐nanonet of interconnected nanofibrils with average fiber diameter ~25 nm. This morphology was attributed to the formic acid. The authors posit that the ionized formic acid causes excess charge and a number of charged droplets. Charged droplets form thin films in contact with the liquid jet. Upon solvent evaporation and thermal‐induced phase separation (TIPS) in the thin film, the solidification of the polymer‐rich phase results in a Steiner network of 2D nanonet/nanofibrils. The structure affected transport properties through the electrospun membrane as well as the membrane wettability. Similar structures have been observed with polyamide 6, polyacrylic acid, polyurethane, chitosan, and poly(methyl methacrylate) (Wang et al. 2017).
1.6.2 Porous Fibers
Porous electrospun nanofibers (Figure 1.5) are of increasing interest in filtration, energy storage, tissue engineering, and catalysis (Li and Xia 2004; Katsogiannis et al. 2016; Wei et al. 2013; Natarajan et al. 2014; Hou et al. 2014). The pores increase surface area of the fibers (Katsogiannis et al. 2016; Natarajan et al. 2014). In tissue engineering, porous scaffolds better mimic extracellular matrix and improve cell attachment (Katsogiannis et al. 2015, 2016).
Figure 1.5 Overview of advanced electrospun nanofiber cross sections: (a) porous fibers
(Source: Dayal et al. (2007).),
(b, c) core–shell fibers
(Source: (b) Reprinted from Dicks and Heunis (2010). Copyright (2010). T.D.J. Heunis and L.M.T. Dicks;
(c) Reprinted from Jalaja et al. (2016), Copyright (2015), with permission from Elsevier.),
and (d, e) nanochannels
(Source: Zhao et al. (2007)).
One approach to making porous nanofibers is electrospinning multicomponent fibers and selectively leaching one of the components (Wei et al. 2013; Gupta et al. 2009). For example, polycaprolactone and sodium chloride can be electrospun from a solvent mixture of methanol and chloroform. The subsequent fibers were submerged in water to selectively dissolve the salt to produce porous fibers (Wang et al. 2009). Polymers have also been used as porogens (Wei et al. 2013) and lead to interconnected pores throughout the fiber. Leeching of salt with water is advantageous because it avoids introducing toxic substances into the system (Hou et al. 2014). However, the salt may not be fully mixed resulting in uneven pore sizes and distributions. Further, this method is time‐consuming as complete leeching of the salt can sometimes be difficult (Wei et al. 2013).
Porous fibers can also be achieved during electrospinning by inducing phase separation (Dayal et al. 2007; Dayal and Kyu 2006). Several variations for inducing phase separation have been explored including TIPS/vapor‐induced phase separation (VIPS) in which water condenses on the surface of the fibers as the solvent evaporates and reduces temperature leading to pores (Natarajan et al. 2014), and nonsolvent‐induced phase separation (NIPS) in which solvent mixtures are used to induce phase separation (Dayal and Kyu 2006). In NIPS, a mixture of solvents of varying volatilities is used. In this approach, the electrospinning solution is a stable, one‐phase system. As solvent evaporates and the polymer concentration increases, the system traverses across the metastable and unstable, two phase region of the phase diagram. Liquid–liquid phase separation into a polymer‐rich phase and a polymer‐poor phase occurs by spinodal decomposition or nucleation and growth. The polymer‐rich phase secures the bulk foundation of the fiber. With further increase in polymer concentration, an interconnected pore structure forms due to spinodal decomposition in the unstable, two‐phase region. The interconnected pore structure becomes the porous fiber as the polymer poor phase evaporates. The porous fibers tend to form if the polymer/solvent