**4.4. Electrospinning of PU/blends and PU nanocomposite nanofibers**

*4.2.1. Melt electrospinning*

28 Aspects of Polyurethanes

and 1,4-butanediol, (CH2

) 4

erties were analogous in vivo soft tissues.

**4.3. Collectors for electropsun PU nanofibers**

Although most electrospinning researches are based on polymer solutions, there are also several researchers that use polymer melts. Generally, most electrospinning conditions also affect the molten polymer in electrospinning [9]. Melt electrospinning is especially attractive for tissue engineering scaffold manufacturing process, because it is possible to eliminate potentially cytotoxic solvents in contrast to electrospinning from a polymer solution. However, there are some technical challenges related to the need for a well-controlled high-temperature setup and there is a difficulty in developing an appropriate polymer. In the study of Karchin et al., a biodegradable and thermally stable PU, produced from 1,4-butanediamine

**Figure 5.** Schematic diagram of laser-electrospinning system. Modified from Takasaki et al. [63].

formulations were used in the melt electrospinning [42]. A catalyst-purified PU based on 1,4-butane diisocyanate, polycaprolactone, and 1,4-butanediol in a 4/1/3 M ratio respectively, yielded a non-toxic polymer that could be electrospun from the melt. The authors concluded that this electrospun polymer contained point bonds between fibers and its mechanical prop-

In most electrospinning setup, the collector plate is made out of a conductive material and covered by a piece of aluminum foil, which is electrically grounded so that there is a stable potential difference between the source and the collector [9]. Rotating drum with different diameters [47, 59, 64–69], parallel electrodes [70], rotating wire drum collector [71], rotating tube collector with knife-edge electrodes below [66], disc collector [72–74] array of counter-electrodes [75], rotating drum with sharp pin inside [76], ring collector placed in parallel [77] could be also used for different purposes [13]. Despite the random and centered collection of nanofibers on the collector plate, the most basic form of getting aligned nanofiber deposition is through the


In electrospinning, it is sometimes useful to combine the properties of two or more polymers to achieve a new structure. This can be obtained either by physical mixing to form a blend or through polymerization to form a copolymer [9]. Addition of a second component could also facilitate the electrospinning process. For example, it was difficult for most of the natural polymers to be electrospun; however, addition of a synthetic polymer could improve the

**Figure 6.** (a) Schematic representation of the reactive electrospinning apparatus with UV light using a rotating mandrel. Modified from Theron et al. [65]. (b) Electrospun vascular graft for regeneration of blood vessel [78].

**Figure 7.** SEM images of self-assembled polyurethane electrospun nanofibers [81] .

processability of these polymers [82]. PUs are easy to electrospin and they can be mixed either with a natural polymer or with a synthetic polymer [83, 84, 87] for special applications such as collagen [88], dextran [43], and hydroxypropyl cellulose (HPC) [41].

Buruaga et al. [83] dissolved a water-soluble polymer [poly(ethylene oxide) (PEO)] in the PU dispersion and fibers were obtained from electrospinning of the resulting mixture. The template polymer (PEO) was removed from electrospun fibers by water extraction, then they obtained pure PU fibers. The authors offered a new perspective for the preparation of micro and nanofibers by using aqueous dispersions for the preparation of water-insoluble PU fibers by electrospinning. Lee et al. [84] dissolved various polyblends of poly(vinyl chloride) and PU (Pellethane 2363-80AE) in a mixture of THF and DMF. They produced nanofibers in different ratios, with several electrospinning conditions and investigated the relationship between morphology and mechanical behavior of the resulting fiber mats. Point-bonded structures in the PU fiber mats increased with increasing PU composition and the mechanical properties of the fiber mats. In the study of Hong et al. [44], PU/organically modified MMT (O-MMT) nanocomposites were prepared via a solution intercalation method and electrospun. The authors investigated the effect of O-MMT on the morphology and physical properties of the PU/O-MMT nanofiber mats. To prepare the PU and PU/O-MMT nanofibers, 11 wt% of PU and PU/O-MMT solutions in a mixed solvent of DMAc/THF (7/3 w/w) were electrospun. Increasing the content of O-MMT resulted in the linearly increase of conductivities of the PU/O-MMT solutions and this decreased the average diameters of the PU/O-MMT nanofibers. Produced PU and PU/O-MMT nanofibers were uniform and not microphase separated. They achieved a well distributed and oriented MMT layers within the PU/O-MMT nanofibers. When the PU/O-MMT nanofibers were annealed, the exfoliated MMT layers hindered the microphase separation of the PU. Incorporation of MMT layers into PU nanofibers improved Young's modulus and tensile strength of PU/O-MMT nanocomposites. Akçakoca Kumbasar et al. [85] loaded β-CD into TPU nanofibers (**Figure 8**). They observed that TPU/CD nanofibers had higher fiber diameters compared to pure TPU nanofibers and fiber diameters increased with the increase in β-CD concentration. The authors also proved the inclusion complex formation capability of TPU/CD nanofibers by the phenolphthalein test method.

Hu and Yu [86] prepared shape stabilized bio-phase change material (PCM) by encapsulating the wax inside the PU nanofibers using coaxial electrospinning. The encapsulated bio-PCMs

**Figure 8.** 30% β-CD loaded (a) 8% TPU, (b) 10% TPU nanofibers.

processability of these polymers [82]. PUs are easy to electrospin and they can be mixed either with a natural polymer or with a synthetic polymer [83, 84, 87] for special applications such as

Buruaga et al. [83] dissolved a water-soluble polymer [poly(ethylene oxide) (PEO)] in the PU dispersion and fibers were obtained from electrospinning of the resulting mixture. The template polymer (PEO) was removed from electrospun fibers by water extraction, then they obtained pure PU fibers. The authors offered a new perspective for the preparation of micro and nanofibers by using aqueous dispersions for the preparation of water-insoluble PU fibers by electrospinning. Lee et al. [84] dissolved various polyblends of poly(vinyl chloride) and PU (Pellethane 2363-80AE) in a mixture of THF and DMF. They produced nanofibers in different ratios, with several electrospinning conditions and investigated the relationship between morphology and mechanical behavior of the resulting fiber mats. Point-bonded structures in the PU fiber mats increased with increasing PU composition and the mechanical properties of the fiber mats. In the study of Hong et al. [44], PU/organically modified MMT (O-MMT) nanocomposites were prepared via a solution intercalation method and electrospun. The authors investigated the effect of O-MMT on the morphology and physical properties of the PU/O-MMT nanofiber mats. To prepare the PU and PU/O-MMT nanofibers, 11 wt% of PU and PU/O-MMT solutions in a mixed solvent of DMAc/THF (7/3 w/w) were electrospun. Increasing the content of O-MMT resulted in the linearly increase of conductivities of the PU/O-MMT solutions and this decreased the average diameters of the PU/O-MMT nanofibers. Produced PU and PU/O-MMT nanofibers were uniform and not microphase separated. They achieved a well distributed and oriented MMT layers within the PU/O-MMT nanofibers. When the PU/O-MMT nanofibers were annealed, the exfoliated MMT layers hindered the microphase separation of the PU. Incorporation of MMT layers into PU nanofibers improved Young's modulus and tensile strength of PU/O-MMT nanocomposites. Akçakoca Kumbasar et al. [85] loaded β-CD into TPU nanofibers (**Figure 8**). They observed that TPU/CD nanofibers had higher fiber diameters compared to pure TPU nanofibers and fiber diameters increased with the increase in β-CD concentration. The authors also proved the inclusion complex for-

collagen [88], dextran [43], and hydroxypropyl cellulose (HPC) [41].

**Figure 7.** SEM images of self-assembled polyurethane electrospun nanofibers [81] .

30 Aspects of Polyurethanes

mation capability of TPU/CD nanofibers by the phenolphthalein test method.

Hu and Yu [86] prepared shape stabilized bio-phase change material (PCM) by encapsulating the wax inside the PU nanofibers using coaxial electrospinning. The encapsulated bio-PCMs can potentially be used for thermal storage and thermal protection areas, which has appealing environmental advantages. The authors encapsulated the soy wax into PU fibers without being miscible with PU fibers and adjusted the wax content either by the concentration of wax/chloroform solution or flowing rate. Results of thermal analysis showed that the enthalpy increased as the wax content increase. The authors conducted 100 heating-cooling cycles and the thermal properties of the fibers were unaltered. The outer PU layer prevented the leakage of the bio-wax like a reservoir and it also enhanced the modulus and lowered the tensile strain. They produced uniform fiber morphology with a core-shell structure and a homogeneous wax distribution throughout the core of the fibers.

Vlad et al. [41] synthesized PU from hexamethylene diisocyanate (HDI), polytetramethylene ether glycol (PTMEG), and butanediol (BD). They mixed PU with different proportion of hydroxypropyl cellulose (HPC) for biomedical applications. Increase of HPC amount of sample provoked a decrease of contact angles. Similar behavior was observed for fibrinogen adsorption, which confirms that PU/HPC nanofibers are suitable in biomedical applications. Unnithan et al. [87] combined PU with two biopolymers, cellulose acetate (CA) and zein to produce an antibacterial electrospun nanofibrous scaffolds. In another study, they used a solution composed of dextran, PU, and ciprofloxacin HCl (CipHCl) drug for wound dressing applications [43]. They investigated the viability, proliferation, and attachment of fibroblasts to the PU-dextran and PU-dextran-drug scaffolds. Their results indicated that the composite mat has a good bactericidal activity and the cells especially interacted with the drug containing scaffolds.

Chen et al. [88] produced collagen functionalized-TPU nanofibers (TPU/collagen) by coaxial electrospinning technique (**Figure 9**) with a goal to develop biomedical scaffold. 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) was used as solvent for collagen and TPU. The authors carried out the cross-linking process by placing electrospun membrane (collagen (shell)/TPU (core) (8 wt%/3 wt%) together with a supporting aluminum foil in a desiccator using glutaraldehyde (GTA) (25% water solution) with different process time. Feasibility of PU/collagen core-shell construct as an optimal tissue engineering scaffold materials was supported by its high porosity and adequate pore size. Pig illiac endothelial cells proliferation in vitro demonstrated the feasibility and efficacy of using TPU/collagen composite nanofibers for improving cell-scaffold

**Figure 9.** (a) Transmission electron microscopy image of the core shell nanofibers, (b) coaxial nozzle for electrospinning [89].

interactions pore size and these composite nanofibers had the characters of native extracellular matrix and may be used effectively as an alternative material for tissue engineering and functional biomaterials.

Huang et al. [90] electrospun random and aligned nanofibrous scaffolds based on collagenchitosan-TPU blend to mimic the componential and structural aspects of the native extracellular matrix. They also investigated the optimal proportion to keep the balance between biocompatibility and mechanical strength. The scaffolds were cross-linked by GTA vapor to prevent them from being dissolved in the culture medium. The mechanical properties of the scaffolds were found to be flexible with a high tensile strength. Cell viability studies with endothelial cells and Schwann cells demonstrated that the blended nanofibrous scaffolds had good biocompatibility and aligned fibers could regulate cell morphology by inducing cell orientation. Chen and Chiang grafted collagen to PU fiber surface by low temperature oxygen plasma treatment, which could improve surface hydrophilicity to promote wound healing and facilitate covalent binding of collagen molecules to the plasma-treated PU surface [91]. After modification, the nanofibrous membrane's antimicrobial activity improved to ∼100% inhibition of bacterial growth. Water absorption ability of membrane was increased, which facilitates its use as a functional wound dressing. Also, their results demonstrated that the nanofibrous membrane was better than gauze and commercial collagen sponge wound dressing in wound healing rate.

### **4.5. Mechanical properties of electrospun PU nanofibers**

The electrospinning technique have the advantages of being simple, convenient, and inexpensive in comparison with conventional methods such as wet, dry, and melt spinning. Unfortunately, the practical applications have been limited because produced electrospun fiber mats have poor mechanical properties, low molecular orientation, and broad distribution of fiber diameter. Hence, an enhancement in both mechanical and physical properties of the electrospun fiber mats is very important from an industrial point of view [92]. TPUs present a class of polymers that possess a range of very desirable properties: they are elastomeric, resistant to microorganisms and abrasion, and have excellent hydrolytic stability and many commercially available TPUs can be used to make good electrospinning solutions [64]. Earlier studies on the electrospinning process have been more focused on the basic principles [2–4] and processing parameters like the voltage applied, tip-to collector distance, and viscosity of solution [20, 26–28, 48]. The future use of electrospun PU materials in practical applications will require good mechanical properties. Thus, several authors recently investigated the mechanical properties of electrospun PU fiber mats [19, 40, 64, 65, 92–94].

Lee et al. [92] used Pellethane 2363-80AE and solved it in a mixture of THF and DMF (60/40, v/v) at room temperature at a concentration of 8 wt%. The authors investigated the mechanical behaviors of TPU by cyclic tensile tests. Produced electrospun TPU mats were composed of randomly oriented sub-micron fibers, where each fiber was restricted by physical netting and entanglements. They have seen almost linear elastic behavior until the fiber mats undergo breaking. Major cause of energy loss and stress softening at relatively low strains was the slippage of the electrospun fibers. At higher strains, the breaking of electropsun fiber at pointbonding junctions, as well as the slippage crossed fibers occurred as a further source of the dissipation energy. Pedicini and Farris [64] prepared the Pellethane 2103-80AE in DMF at room temperature at a concentration of 7% by weight and bulk samples by thermally processing plaques from polymer pellets. The uniaxial tensile tests results indicated that the behavior of the electrospun Pellethane 2103-80AE to be distinctly different from the bulk. Qualitatively, the electrospun mat is also elastomeric in nature, but the shape of the stress-strain curve for the electrospun material is not sigmoidal in contrast to bulk material. The curve is monotonic and its slope has not got an inflection. When they applied a strain to the electrospun mat, they got oriented fibers. They also concluded that the apparent molecular orientation in the electrospun fibers leads to the pronounced reduction in elongation to failure of the electrospun mat, relative to the bulk.

interactions pore size and these composite nanofibers had the characters of native extracellular matrix and may be used effectively as an alternative material for tissue engineering

**Figure 9.** (a) Transmission electron microscopy image of the core shell nanofibers, (b) coaxial nozzle for electrospinning [89].

Huang et al. [90] electrospun random and aligned nanofibrous scaffolds based on collagenchitosan-TPU blend to mimic the componential and structural aspects of the native extracellular matrix. They also investigated the optimal proportion to keep the balance between biocompatibility and mechanical strength. The scaffolds were cross-linked by GTA vapor to prevent them from being dissolved in the culture medium. The mechanical properties of the scaffolds were found to be flexible with a high tensile strength. Cell viability studies with endothelial cells and Schwann cells demonstrated that the blended nanofibrous scaffolds had good biocompatibility and aligned fibers could regulate cell morphology by inducing cell orientation. Chen and Chiang grafted collagen to PU fiber surface by low temperature oxygen plasma treatment, which could improve surface hydrophilicity to promote wound healing and facilitate covalent binding of collagen molecules to the plasma-treated PU surface [91]. After modification, the nanofibrous membrane's antimicrobial activity improved to ∼100% inhibition of bacterial growth. Water absorption ability of membrane was increased, which facilitates its use as a functional wound dressing. Also, their results demonstrated that the nanofibrous membrane was better than gauze and commercial collagen sponge wound dress-

The electrospinning technique have the advantages of being simple, convenient, and inexpensive in comparison with conventional methods such as wet, dry, and melt spinning. Unfortunately, the practical applications have been limited because produced electrospun fiber mats have poor mechanical properties, low molecular orientation, and broad distribution of fiber diameter. Hence, an enhancement in both mechanical and physical properties of the electrospun fiber mats is very important from an industrial point of view [92]. TPUs present a class of polymers that possess a range of very desirable properties: they are elastomeric, resistant to microorganisms and abrasion, and have excellent hydrolytic stability and many commercially available TPUs can be used to make good electrospinning solutions [64]. Earlier studies on the electrospinning process have been more focused on the basic principles [2–4] and processing parameters like the voltage applied, tip-to collector

and functional biomaterials.

32 Aspects of Polyurethanes

ing in wound healing rate.

**4.5. Mechanical properties of electrospun PU nanofibers**

Cha et al. [93] synthesized shape-memory PU block copolymers to prepare electrospun non-wovens. The authors prepared PU solutions in a mixture of DMF and THF, and electrospun PU non-wovens with hard-segment concentrations of 40 and 50 wt%. The average diameter of low viscosity (ca. 130–180 cPs) beaded electrospun fibers was about 800 nm. In contrast, the average diameter of high viscosity (ca. 530–570 cPs) electrospun fibers was about 1300 nm. The mechanical properties of the electrospun PU non-wovens were investigated and found the increase in the hard-segment concentration increased the tensile strengths as well as the viscosities. Also, because of a difference in the velocities of the drum collectors, the tensile strength in the machine direction was higher than that in the transverse direction. Prepared PU non-wovens have a shape recovery of more than 80% that included hard-segment concentrations of 40 and 50 wt%.

The main weakness of electrospun nanofibrous membrane structures seems to be their poor mechanical properties caused by relaxation processes occurring immediately after fiber formation, at which a certain degree of molecular orientation is lost. With the aim to overcome this problem, some researchers combined single-walled carbon nanotubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs) (**Figure 10a**) [94] to prepare PU nanocomposite fibers and achieve a significantly enhanced Young's modulus [19, 40, 95].

Sen et al. [19] demonstrated the effect of the chemical functionalization of SWNTs on the mechanical properties of SWNT-reinforced composites of electrospun PU nanofibers. The tensile strength of ester functionalized and as-prepared SWNT-PU membranes is enhanced

**Figure 10.** (a) TEM images of MWNTs/PU nanofibers and multi-wall carbon nanotube (MWCNT) [94]. (b) Schematic diagrams of single-wall carbon nanotube (SWCNT) [95].

by 104 and 46%, respectively as compared to electrospun pure polyurethane membranes. The tangent moduli of as-prepared and ester functionalized SWNT-PU membranes were also higher than the control PU membranes and were 215 and 250%, respectively.

Liu and Pan [95] produced MWNTs/PU composite nanofibrous membranes (**Figure 10b**). They used a thermal treatment method to improve the comprehensive properties of the membranes. They reported that the proposed thermal treatment method effectively improved the conductivity and mechanical properties of the nanomaterials, with 3 wt% of MWNTs, the conductivity of the membranes reached 8.29 × 10−5 Scm−1, which was nearly three orders higher than that of the untreated sample and the tensile strength and the modulus increased more than 50% after thermal treatment. PU and PU/multi-walled carbon nanotube (MWCNT) nanocomposite nanofibers, both with diameters of 350 nm, were prepared by Kimmer et al. as well [40]. The appearance of smaller nanowebs in PU/MWCNT nanofiber structures with diameters of 20–40 nm was observed. They attributed the existence of these structures to the occurrence of strong secondary electric fields, which were created between individual conducting MWCNTs (distributed in the PU/MWCNT nanocomposites). From the TEM images individual and very well aligned MWCNTs can be seen in the outer surface of the main nanofiber from which nanoweb fiber was created. However, the authors did not investigate the mechanical properties of the prepared electrospun fiber mats.

Cross-linking the nanofibrous mats is another way to improve the mechanical properties. Theron et al. [65] modified a medical standard TPU (Pellethane 80A) with latent cross-linkable groups; and they determined the effect of subsequent cross-linking on viscoelastic properties and degradation resistance. They confirmed the successful cross-linking by insolubility of the materials. Pellethane was cross-linked with containing an increasing number of pentenoyl groups. Cross-linking decreased (up to 42%, p < 0.01) the hysteresis and creep (44%, p < 0.05), and significantly improved the degradation resistance in vitro. Modified Pellethane was also electrospun into tubular grafts and with UV irradiation they were cross-linked to make them insoluble. Prototype grafts had a burst pressure of >550 mmHg, and for uncross-linked and cross-linked samples, it was 12.1 ± 0.8 and 6.2 ± 0.3%/100 mmHg, respectively. They concluded that the modification of Pellethane with PCL occurred on the carbamate nitrogen and cross-linking resulted in improved viscoelastic properties. Although there were some decrease in tensile strength and strain, the polymers had sufficient strength and extensibility. They suggested to use these cross-linked materials such as vascular grafts where repetitive and relatively low stresses is encountered.
