**4. Electrospinning of PU nanofibers**

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,

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

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

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,

foams, and medical applications because of their good biocompatibility [34].

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

sometimes lower impact strength [33].

20 Aspects of Polyurethanes

abrasion, and excellent hydrolytic stability [36, 37].

There are many parameters that affect the electrospinning process and the resultant fiber morphology. Several authors investigated the effects of solution and process parameters such as material composition, concentration, rheological properties, applied voltage, tip-to collector distance, collector types on the resultant PU nanofibers [26, 27, 47–49]. Among these parameters, polymer solution properties have the most significant influences on the process and the resultant fiber morphology [9], since viscoelastic and gravitational forces play a major role. Viscoelastic force depends on polymer solution concentration, average molecular weight of the polymer, final viscosity, and surface tension of the solution [47].
