**3. Polyurethanes**

PUs contains urethane group (–NH–(C=O)–O–) in common (**Figure 2a**) [30, 31], while most PUs are thermosetting polymers, in contrast to thermosetting polymers, thermoplastic polyurethanes (TPUs) melt when they are heated (**Figure 2b**) and are easy to use in manufacturing processes. Varying the structure of PUs, their properties can be varied in a wide range [32]. PUs are formed by reaction of polyisocyanates with hydroxyl-containing compounds.

Reneker [6], and Reneker and Chun [7], spun various kinds of polymers including polyethylene oxide, nylon, polyimide, DNA, polyaramid, and polyaniline, and were able to characterize their properties. Afterward this old technology rediscovered, refined, and expanded into non-textile applications. Electrospinning process is unique among other nanofiber fabrication techniques in terms of ease of use, vast possibilities of material selection and combination, and it has a potential for scale-up. This has led to electrospinning being considered as a key platform technology that will be studied to develop products for a wide range including drug delivery, tissue scaffolding, wound dressing, electronics, chemical sensors, filtration, and so on [8]. This rediscovery and attention are partly the results of leading-edge technology, especially, scanning probe microscopy and high-resolution electron microscopy, which enable the

The electrospinning process simply constituted of a high voltage power supply, a spinneret and a deposition area named as collector generally covered with an aluminum foil. The potential difference between the spinneret and the collector leads to the stretch of the polymeric solution and creates a thin nanofiber jet from solution toward to the deposition area. During this electrospinning process, the solvent evaporates and ultrafine fibers are collected [9, 10].

The electrospinning process has attracted a great deal of attention due to the ability to fabricate fibers with diameters on the nanometer scale [11, 12], vast possibilities for surface functionalization [13, 14] with high surface area to volume or mass ratio, small inter-fibrous pore size and high porosity [11, 15, 16]. Technically, almost any soluble polymer with a sufficiently high molecular weight can be electrospun, and [17] various polymers have been successfully

The method can be applied to synthetic and natural polymers, polymer blends, and polymers loaded with chromophores, nanoparticles, or active agents, as well as to metals and ceramics [18]. A large number of inorganic salts, inorganic and organic particles, and carbon nanotubes (CNTs), can also be immobilized in polymer fibers [19]. More than 100 different polymers have been successfully electrospun into ultrafine fibers using this technique [10] including

PUs contains urethane group (–NH–(C=O)–O–) in common (**Figure 2a**) [30, 31], while most PUs are thermosetting polymers, in contrast to thermosetting polymers, thermoplastic polyurethanes (TPUs) melt when they are heated (**Figure 2b**) and are easy to use in manufacturing processes. Varying the structure of PUs, their properties can be varied in a wide range [32]. PUs are formed by reaction of polyisocyanates with hydroxyl-containing compounds.

Main process equipments and setup are presented in **Figure 1**.

electrospun into nanofibers cost-effectively compared to the other methods.

exploration of the "nanodimension."

synthetic polymers such as PU [20–29].

**3. Polyurethanes**

**2. Electrospinning**

18 Aspects of Polyurethanes

**Figure 1.** Electrospinning setup (a), Taylor cone (b), whipping (c), electrospining jet (d), collectors (e), single syringe pump (f), polymer solutions (g), and high voltage supply (h).

**Figure 2.** (a) Urethane group, (b) thermoplastic polyurethane general formula [35].

Desired properties can be tailored by selecting the type of isocyanate and polyols, or combination of isocyanates and combination of polyols [32, 33]. Strong intermolecular bonds make polyurethanes useful for diverse applications in adhesives and coatings, also in elastomers, foams, and medical applications because of their good biocompatibility [34].

The factors determining properties of a polyurethane elastomer are: structure of the polyol, type of diisocyanate, type of the chain extender, molar ratio NCO/OH, soft-segment concentration, molecular weight of the polyol and filler. In polyurethane elastomers, chains are linear, and cross-linking was achieved by physical bonds and hard domain formation. They flow when they are melted and harden by cooling (thermoplastic behavior). Displaying reversible cross-linking, domains are destroyed above the melting point of the hard phase, but are reformed when they get cooled. These materials are called "thermoplastic urethanes" (TPUs) [32]. In addition to the linear TPUs, obtained from difunctional monomers, branched or crosslinked thermoset polymers are made with higher functional monomers. Linear polymers have good impact strength, good physical properties, and excellent processibility, but limited thermal stability. On the other hand, thermoset polymers have higher thermal stability, but sometimes lower impact strength [33].

The vast selection of polyols, isocyanates, and chain extenders allows PUs to be varied from soft thermoplastic elastomers to adhesives, coatings, flexible foams, and rigid thermosets [25]. TPU elastomers are segmented block copolymers, comprising of hard- and soft-segment blocks. The soft-segment blocks are formed from long-chain polyester or polyether polyols and 4,4′,-methylenebis(phenyl isocyanate) (MDI); the hard segments are formed from shortchain diols, mainly 1,4-butanediol and MDI [33]. The unique properties of linear TPUs are attributed to their long-chain structure. TPUs are resilient elastomers of significant industrial importance, which possess a range of desirable properties such as elastomeric, resistant to abrasion, and excellent hydrolytic stability [36, 37].

PU is often chosen as a material for composing a nanoweb due to its chemical stability, mass transport, good mechanical properties, and also excellent nanofiber forming characteristic [24, 38]. Electrospun PU nanofiber mats exhibiting good mechanical properties may have a wide variety of potential applications in high-performance air filters, protective textiles, wound dressing materials, sensors, in biomedical applications, drug delivery, etc. [21, 37–44]. It is also frequently used in wound dressing studies because of its good barrier properties and oxygen permeability. It has reported that semipermeable dressings, many of which are PU, enhance wound healing [21, 39]. Degradable and biocompatible aliphatic PU could also be formed into scaffolds via melt electrospinning [42]. Beside these biomedical applications, the PU nanofiber filters were prepared by electrospinning process based on 3D particle filtration modeling and some theoretical predictions were obtained for the filtration efficiency [45]. In another study, PU cationomers (PUCs) containing different amounts of quaternary ammonium groups were synthesized and electrospun into non-woven nanofiber mats for use in antimicrobial nanofilter applications [46].

Varying the structure of PUs, the properties of PUs can vary in a wide range [32]. The flexibility to tailor the structure during processing is one of the main advantages of PUs over other types of polymers. A lot of different types of PU used in electrospinning process, some of them were synthesized before electrospinning according to the researchers intended use and some of them were used as they received.

This chapter deals with the electrospinning of PU nanofibers including TPUs and their potential applications. The nanofiber morphology and influence of experimental parameters including the solution concentration, flow rate, collection distance, and electric voltage are discussed in terms of electrospinning of PU nanofibers and potential applications of PU nanofibers are reviewed.
