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 system is partially miscible with an upper critical solution temperature envelope at the electrospinning temperature (Dayal et al. 2007; Dayal and Kyu 2006).
Electrospinning process parameters can affect the properties of the porous fiber, i.e. fiber diameters, pore diameters, and pore distributions. Katsogiannis et al. postulate that increasing the tip‐to‐collector distance increases the percentage of area covered with pores due to increased solvent evaporation. Experimentally, the area percent of fibers covered in pores could be increased from 10% to 30% by increasing the tip‐to‐collector distance. Increasing the voltage also increased the pore area from ~10% to 20–45% which was attributed to the voltage increasing the rate of stretching and solvent evaporation. Increasing the flow rate also increased the area percent covered in pores (Katsogiannis et al. 2016). Pore size and distribution is affected by polymer solution properties, namely, the molecular weight of the polymer. The relative humidity of the environment also affects pore size and distribution. For example, Natarajan et al. increased the humidity from 30% to 70% and observed the average pore size of polylactic acid (PLA) fibers to increase from 85 to 135 nm. A relative humidity above 30% was required to observe porous fibers (Natarajan et al. 2014).
Foam‐assisted electrospinning is another single‐step method to produce mesoporous fibers. A PVP template, with titanium precursor and foaming agent diisopropyl azodiformate are electrospun using ethyl alcohol and acetic acid as the solvents. The release of vapor from the foaming agent created pores throughout the polymeric precursor fibers. The fibers were subsequently calcined to achieve mesoporous TiO2 fiber (Hou et al. 2014).
1.6.3 Core–Shell Fibers
1.6.3.1 Coaxial Electrospinning
Hierarchically structured nanofibers, e.g. core–shell fibers have been of interest in a wide range of applications such as catalysis, drug delivery, and self‐healing materials (Li and Xia 2004; Crespy et al. 2012; Li et al. 2010; Moghe and Gupta 2008; Park and Braun 2010). One approach to making core–shell fibers has been coaxial electrospinning. In coaxial electrospinning, two polymer solutions are electrospun through a spinneret of two coaxial capillaries. Under steady operation, continuous, coaxial streams of both core and shell fluids are observed as they exit the nozzle. Following coaxial electrospinning, the core can be removed to achieve hollow fibers (Figure 1.5). Alternatively, the sheath can be removed to achieve smaller nanofibers than can be achieved with conventional electrospinning. Generally, the electrospinning solution selected for the sheath is electrospinnable. The core fluid may be electrospinnable or one that does not readily form fibers when electrospun alone. The shell fluid envelops the core fluid and prevents the core fluid from breaking up into droplets. Stabilization occurs due to viscoelasticity of the shell solution and reduced surface tension at the core–shell interface. Selecting a common solvent results in particularly low interfacial tension. Since electrospinning is a relatively fast process, the core and shell solutions may or may not be miscible; the two fluids do not mix significantly over the short duration of the electrospinning process. One important consideration is solubility of the polymer solutions, i.e. the polymers must not precipitate at the fluid interface (Moghe and Gupta 2008; Yu et al. 2004).
Analogous to conventional electrospinning, solution properties of both solutions and process parameters, e.g. applied electric field strength, flow rates, affect fiber quality. The flow rates of the core and shell solutions are of particular practical importance. If the flow rate of the core is too high, the core fluid breaks up into droplets. If the flow rate of the shell is too high, the spinning of the core is not continuous and the fibers form without a continuous thread of core material. Generally, having the core flow rate lower than the sheath flow rates promotes stable jetting of both fluids. Further, the ratio of the diameter of the core to the shell and core loading are dictated by the flow rates (Moghe and Gupta 2008; Yu et al. 2004).
Coaxial extrusion enables electrospinning of fluids that are difficult to process. Typically, the nonelectrospinnable polymer is the core with an electrospinnable shell solution (Yu et al. 2004). For example, to produce superhydrophobic materials, Teflon amorphous fluoropolymer (AF) has been processed using polyvinylidene fluoride (PVDF) as the shell polymer. Teflon AF coating has also been applied to PVDF fibers by coaxial electrospinning PVDF as the core and Teflon AF as the shell (Muthiah et al. 2010).
