A Solvent System Involved Fabricating Electrospun Polyurethane Nanofibers for Biomedical Applications

A novel Trichloromethane (TCM)/2,2,2-Trifluoroethanol (TFE) solvent system was developed for fabricating electrospun thermoplastic polyurethane (TPU) nanofibers. TPU solution stability made from this novel solvent system was improved compared to that from the traditional N, N-Dimethylformamide (DMF)/Tetrahydrofuran (THF) solvent system. The minimum TPU solution concentration that can be electrospun was decreased to 0.5% w/v. The conductivity and viscosity of the TPU solution increased with the increasing ratio of TFE in the solvent system. The obtained electrospun TPU nanofibers fabricated from this novel solvent system showed smooth morphology and uniform diameter distribution. Mechanical strength of TPU nanofibers was improved using this new solvent system. Young’s modulus and tensile strength of the electrospun TPU nanofiber meshes first decreased and then increased, while the strain elongation ratio first increased and then decreased. The new solvent system significantly improves the fiber elongation ratio while maintaining the modulus and tensile strength. The chemical structure of the TPU was not affected by the TCM/TFE solvent system. Electrospun TPU nanofiber meshes prepared by using the TCM/TFE solvent system showed better cytocompatibility, which means the electrospun TPU fibrous scaffold has great potential in biomedical application.


Introduction
Nanofibers have been widely used in tissue engineering and regenerative medicine [1][2][3], and electrospinning is one of the most convenient and controllable methods to manufacture nanofibers. Thermoplastic polyurethane (TPU) is one type of polymer with excellent properties, such as high strength, high toughness, durable wear resistance and good oil resistance that has been used in textile, food, national defense and other industries [4][5][6]. Because of their excellent biodegradability and biocompatibility, TPU nanofibers have been widely used in biomedical applications [7,8]. Similar to other typical electrospun fibrous scaffolds, many factors have an effect on the application of TPU or TPU-based electrospun fibrous scaffolds, such as the morphology and structure, mechanical properties, and so on. There are many parameters (e.g., rheological properties [9], collection distance and electric field strength [10]) that affect the electrospinning process and the resultant TPU nanofiber morphology. Electrospun solution properties (special for viscoelastic forces) have the most significant influences among these parameters [11].

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect the resultant TPU nanofibers properties [13]. Previous studies have demonstrated that TPU can be easily dissolved in polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The polarity of the mixed solvent depends on the ratio of the two solvents. The solubility of TPU in the TCM/TFE solvent system with different solvent ratios is shown in Table 1. When the concentration of TPU was 5% (w/v), TPU only swelled at V TCM /V TFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and form a uniform and stable TPU solution when V TCM / V TFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solutions keep clear and transparent during long-term storage, and this is beneficial to industrial production. Table 1. Solubility of TPU (5% w/v) in TCM/TFE solvent system at different solvent ratios.

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect the resultant TPU nanofibers properties [13]. Previous studies have demonstrated that TPU can be easily dissolved in polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The polarity of the mixed solvent depends on the ratio of the two solvents. The solubility of TPU in the TCM/TFE solvent system with different solvent ratios is shown in Table 1. When the concentration of TPU was 5% (w/v), TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and form a uniform and stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solutions keep clear and transparent during long-term storage, and this is beneficial to industrial production. DMF and THF were common solvents of TPU in previous studies [21][22][23]. In this study, the DMF/THF (5/5) solvent system was used as the control group. Compared with the TPU solution made from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was much more stable. As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing in the DMF/THF solvent system, and the solution gradually turned orange-red. It can be concluded that TPU is unstable in the DMF/THF solvent system. However, TPU can quickly dissolve in the TCM/TFE solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneous and stable. No precipitates or color change happened in this solvent system, which suggests that the chemical structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maintaining the stability of electrospun jet [9]. When the concentration of the solution is lower than the optimum concentration, polymer chains in the solution do not have enough crosslinks. This will make it difficult to form a Taylor cone or get electrospun fibers with beads while performing the electrospinning. Figure 2 shows SEM images of nanofibers prepared by electrospinning from TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). Electrospun TPU nanofibers from TCM/TFE solvent show a smooth surface morphology and uniform diameter distribution, while electrospun TPU nanofibers from DMF/THF show many beads. There are several Polymers 2020, 12, x FOR PEER REVIEW 4 of 12

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect the resultant TPU nanofibers properties [13]. Previous studies have demonstrated that TPU can be easily dissolved in polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The polarity of the mixed solvent depends on the ratio of the two solvents. The solubility of TPU in the TCM/TFE solvent system with different solvent ratios is shown in Table 1. When the concentration of TPU was 5% (w/v), TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and form a uniform and stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solutions keep clear and transparent during long-term storage, and this is beneficial to industrial production. DMF and THF were common solvents of TPU in previous studies [21][22][23]. In this study, the DMF/THF (5/5) solvent system was used as the control group. Compared with the TPU solution made from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was much more stable. As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing in the DMF/THF solvent system, and the solution gradually turned orange-red. It can be concluded that TPU is unstable in the DMF/THF solvent system. However, TPU can quickly dissolve in the TCM/TFE solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneous and stable. No precipitates or color change happened in this solvent system, which suggests that the chemical structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maintaining the stability of electrospun jet [9]. When the concentration of the solution is lower than the optimum concentration, polymer chains in the solution do not have enough crosslinks. This will make it difficult to form a Taylor cone or get electrospun fibers with beads while performing the electrospinning. Figure 2 shows SEM images of nanofibers prepared by electrospinning from TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). Electrospun TPU nanofibers from TCM/TFE solvent show a smooth surface morphology and uniform diameter distribution, while electrospun TPU nanofibers from DMF/THF show many beads. There are several

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect the resultant T nanofibers properties [13]. Previous studies have demonstrated that TPU can be easily dissolved polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The polarity of the mix solvent depends on the ratio of the two solvents. The solubility of TPU in the TCM/TFE solv system with different solvent ratios is shown in Table 1. When the concentration of TPU was 5% (w TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and form a uniform a stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solutions keep clear a transparent during long-term storage, and this is beneficial to industrial production. DMF and THF were common solvents of TPU in previous studies [21][22][23]. In this study, DMF/THF (5/5) solvent system was used as the control group. Compared with the TPU solution ma from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was much more stab As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing in the DMF/T solvent system, and the solution gradually turned orange-red. It can be concluded that TPU unstable in the DMF/THF solvent system. However, TPU can quickly dissolve in the TCM/T solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneous and stable. precipitates or color change happened in this solvent system, which suggests that the chemi structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maintaining the stabi of electrospun jet [9]. When the concentration of the solution is lower than the optimu concentration, polymer chains in the solution do not have enough crosslinks. This will make difficult to form a Taylor cone or get electrospun fibers with beads while performing electrospinning. Figure 2 shows SEM images of nanofibers prepared by electrospinning fr TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). Electrospun T nanofibers from TCM/TFE solvent show a smooth surface morphology and uniform diame distribution, while electrospun TPU nanofibers from DMF/THF show many beads. There are seve Polymers 2020, 12, x FOR PEER REVIEW

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect the resulta nanofibers properties [13]. Previous studies have demonstrated that TPU can be easily diss polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The polarity of th solvent depends on the ratio of the two solvents. The solubility of TPU in the TCM/TFE system with different solvent ratios is shown in Table 1. When the concentration of TPU was 5 TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and form a unif stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solutions keep cl transparent during long-term storage, and this is beneficial to industrial production.
DMF and THF were common solvents of TPU in previous studies [21][22][23]. In this stu DMF/THF (5/5) solvent system was used as the control group. Compared with the TPU solutio from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was much mor As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing in the DM solvent system, and the solution gradually turned orange-red. It can be concluded that unstable in the DMF/THF solvent system. However, TPU can quickly dissolve in the TC solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneous and sta precipitates or color change happened in this solvent system, which suggests that the c structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maintaining the of electrospun jet [9]. When the concentration of the solution is lower than the o concentration, polymer chains in the solution do not have enough crosslinks. This will difficult to form a Taylor cone or get electrospun fibers with beads while perform electrospinning. Figure 2 shows SEM images of nanofibers prepared by electrospinnin TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). Electrosp nanofibers from TCM/TFE solvent show a smooth surface morphology and uniform d distribution, while electrospun TPU nanofibers from DMF/THF show many beads. There are Polymers 2020, 12, x FOR PEER REVIEW

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect the r nanofibers properties [13]. Previous studies have demonstrated that TPU can be easily polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The polarity solvent depends on the ratio of the two solvents. The solubility of TPU in the TCM system with different solvent ratios is shown in Table 1. When the concentration of TPU TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and form a stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solutions ke transparent during long-term storage, and this is beneficial to industrial production.
DMF and THF were common solvents of TPU in previous studies [21][22][23]. In th DMF/THF (5/5) solvent system was used as the control group. Compared with the TPU s from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was much As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing in t solvent system, and the solution gradually turned orange-red. It can be concluded unstable in the DMF/THF solvent system. However, TPU can quickly dissolve in t solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneous an precipitates or color change happened in this solvent system, which suggests that structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maintainin of electrospun jet [9]. When the concentration of the solution is lower than t concentration, polymer chains in the solution do not have enough crosslinks. This difficult to form a Taylor cone or get electrospun fibers with beads while pe electrospinning. Figure 2 shows SEM images of nanofibers prepared by electrosp TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). Elec nanofibers from TCM/TFE solvent show a smooth surface morphology and unifo distribution, while electrospun TPU nanofibers from DMF/THF show many beads. The

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect nanofibers properties [13]. Previous studies have demonstrated that TPU can be polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The pol solvent depends on the ratio of the two solvents. The solubility of TPU in the system with different solvent ratios is shown in Table 1. When the concentration of TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and fo stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solution transparent during long-term storage, and this is beneficial to industrial productio DMF and THF were common solvents of TPU in previous studies [21][22][23]. DMF/THF (5/5) solvent system was used as the control group. Compared with the T from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing solvent system, and the solution gradually turned orange-red. It can be concl unstable in the DMF/THF solvent system. However, TPU can quickly dissolve solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneo precipitates or color change happened in this solvent system, which suggests structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maint of electrospun jet [9]. When the concentration of the solution is lower th concentration, polymer chains in the solution do not have enough crosslinks. difficult to form a Taylor cone or get electrospun fibers with beads whil electrospinning. Figure 2 shows SEM images of nanofibers prepared by elec TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). nanofibers from TCM/TFE solvent show a smooth surface morphology and distribution, while electrospun TPU nanofibers from DMF/THF show many beads Polymers 2020, 12, x FOR PEER REVIEW

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that a nanofibers properties [13]. Previous studies have demonstrated that TPU ca polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The solvent depends on the ratio of the two solvents. The solubility of TPU in system with different solvent ratios is shown in Table 1. When the concentratio TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved a stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These sol transparent during long-term storage, and this is beneficial to industrial prod DMF and THF were common solvents of TPU in previous studies [21 DMF/THF (5/5) solvent system was used as the control group. Compared with from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of sta solvent system, and the solution gradually turned orange-red. It can be unstable in the DMF/THF solvent system. However, TPU can quickly dis solvent system. Moreover, the TPU (TCM/TFE) solution system is homog precipitates or color change happened in this solvent system, which sugg structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in m of electrospun jet [9]. When the concentration of the solution is low concentration, polymer chains in the solution do not have enough crossl difficult to form a Taylor cone or get electrospun fibers with beads electrospinning. Figure 2 shows SEM images of nanofibers prepared by TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% ( nanofibers from TCM/TFE solvent show a smooth surface morphology distribution, while electrospun TPU nanofibers from DMF/THF show many b Polymers 2020, 12, x FOR PEER REVIEW

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters t nanofibers properties [13]. Previous studies have demonstrated that TP polar solvents [13,17], so we chose two polar solvents, TCM and TFE solvent depends on the ratio of the two solvents. The solubility of T system with different solvent ratios is shown in Table 1. When the concen TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be disso stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. Thes transparent during long-term storage, and this is beneficial to industria DMF and THF were common solvents of TPU in previous studie DMF/THF (5/5) solvent system was used as the control group. Compared from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent sy As shown in Figure 1, TPU flocculent precipitate appeared after 48 h o solvent system, and the solution gradually turned orange-red. It can unstable in the DMF/THF solvent system. However, TPU can quickl solvent system. Moreover, the TPU (TCM/TFE) solution system is ho precipitates or color change happened in this solvent system, which structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important rol of electrospun jet [9]. When the concentration of the solution is concentration, polymer chains in the solution do not have enough c difficult to form a Taylor cone or get electrospun fibers with be electrospinning. Figure 2 shows SEM images of nanofibers prepare TCM/TFE and DMF/THF solvent systems with TPU concentration of nanofibers from TCM/TFE solvent show a smooth surface morpho distribution, while electrospun TPU nanofibers from DMF/THF show m Polymers 2020, 12, x FOR PEER REVIEW

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important param nanofibers properties [13]. Previous studies have demonstrated t polar solvents [13,17], so we chose two polar solvents, TCM an solvent depends on the ratio of the two solvents. The solubility system with different solvent ratios is shown in Table 1. When the c TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6 transparent during long-term storage, and this is beneficial to ind DMF and THF were common solvents of TPU in previous DMF/THF (5/5) solvent system was used as the control group. Com from DMF/THF, TPU solution made from the TCM/TFE (5/5) solv As shown in Figure 1, TPU flocculent precipitate appeared after solvent system, and the solution gradually turned orange-red. unstable in the DMF/THF solvent system. However, TPU can solvent system. Moreover, the TPU (TCM/TFE) solution system precipitates or color change happened in this solvent system, structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an importa of electrospun jet [9]. When the concentration of the solut concentration, polymer chains in the solution do not have eno difficult to form a Taylor cone or get electrospun fibers w electrospinning. Figure 2 shows SEM images of nanofibers p TCM/TFE and DMF/THF solvent systems with TPU concentrat nanofibers from TCM/TFE solvent show a smooth surface mo distribution, while electrospun TPU nanofibers from DMF/THF sh

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect the resultant TPU nanofibers properties [13]. Previous studies have demonstrated that TPU can be easily dissolved in polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The polarity of the mixed solvent depends on the ratio of the two solvents. The solubility of TPU in the TCM/TFE solvent system with different solvent ratios is shown in Table 1. When the concentration of TPU was 5% (w/v), TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and form a uniform and stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solutions keep clear and transparent during long-term storage, and this is beneficial to industrial production.
DMF and THF were common solvents of TPU in previous studies [21][22][23]. In this study, the DMF/THF (5/5) solvent system was used as the control group. Compared with the TPU solution made from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was much more stable. As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing in the DMF/THF solvent system, and the solution gradually turned orange-red. It can be concluded that TPU is unstable in the DMF/THF solvent system. However, TPU can quickly dissolve in the TCM/TFE solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneous and stable. No precipitates or color change happened in this solvent system, which suggests that the chemical structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maintaining the stability of electrospun jet [9]. When the concentration of the solution is lower than the optimum concentration, polymer chains in the solution do not have enough crosslinks. This will make it difficult to form a Taylor cone or get electrospun fibers with beads while performing the electrospinning. Figure 2 shows SEM images of nanofibers prepared by electrospinning from TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). Electrospun TPU nanofibers from TCM/TFE solvent show a smooth surface morphology and uniform diameter distribution, while electrospun TPU nanofibers from DMF/THF show many beads. There are several Polymers 2020, 12, x FOR PEER REVIEW 4 of 12

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect the resultant TPU nanofibers properties [13]. Previous studies have demonstrated that TPU can be easily dissolved in polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The polarity of the mixed solvent depends on the ratio of the two solvents. The solubility of TPU in the TCM/TFE solvent system with different solvent ratios is shown in Table 1. When the concentration of TPU was 5% (w/v), TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and form a uniform and stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solutions keep clear and transparent during long-term storage, and this is beneficial to industrial production.
DMF and THF were common solvents of TPU in previous studies [21][22][23]. In this study, the DMF/THF (5/5) solvent system was used as the control group. Compared with the TPU solution made from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was much more stable. As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing in the DMF/THF solvent system, and the solution gradually turned orange-red. It can be concluded that TPU is unstable in the DMF/THF solvent system. However, TPU can quickly dissolve in the TCM/TFE solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneous and stable. No precipitates or color change happened in this solvent system, which suggests that the chemical structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maintaining the stability of electrospun jet [9]. When the concentration of the solution is lower than the optimum concentration, polymer chains in the solution do not have enough crosslinks. This will make it difficult to form a Taylor cone or get electrospun fibers with beads while performing the electrospinning. Figure 2 shows SEM images of nanofibers prepared by electrospinning from TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). Electrospun TPU nanofibers from TCM/TFE solvent show a smooth surface morphology and uniform diameter distribution, while electrospun TPU nanofibers from DMF/THF show many beads. There are several

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect the resultant T nanofibers properties [13]. Previous studies have demonstrated that TPU can be easily dissolved polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The polarity of the mix solvent depends on the ratio of the two solvents. The solubility of TPU in the TCM/TFE solv system with different solvent ratios is shown in Table 1. When the concentration of TPU was 5% (w TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and form a uniform a stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solutions keep clear a transparent during long-term storage, and this is beneficial to industrial production. DMF and THF were common solvents of TPU in previous studies [21][22][23]. In this study, DMF/THF (5/5) solvent system was used as the control group. Compared with the TPU solution ma from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was much more stab As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing in the DMF/T solvent system, and the solution gradually turned orange-red. It can be concluded that TPU unstable in the DMF/THF solvent system. However, TPU can quickly dissolve in the TCM/T solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneous and stable. precipitates or color change happened in this solvent system, which suggests that the chemi structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maintaining the stabi of electrospun jet [9]. When the concentration of the solution is lower than the optimu concentration, polymer chains in the solution do not have enough crosslinks. This will make difficult to form a Taylor cone or get electrospun fibers with beads while performing electrospinning. Figure 2 shows SEM images of nanofibers prepared by electrospinning fr TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). Electrospun T nanofibers from TCM/TFE solvent show a smooth surface morphology and uniform diame Polymers 2020, 12, x FOR PEER REVIEW

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect the resulta nanofibers properties [13]. Previous studies have demonstrated that TPU can be easily diss polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The polarity of th solvent depends on the ratio of the two solvents. The solubility of TPU in the TCM/TFE system with different solvent ratios is shown in Table 1. When the concentration of TPU was 5 TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and form a unif stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solutions keep cl transparent during long-term storage, and this is beneficial to industrial production.
DMF and THF were common solvents of TPU in previous studies [21][22][23]. In this stu DMF/THF (5/5) solvent system was used as the control group. Compared with the TPU solutio from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was much mor As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing in the DM solvent system, and the solution gradually turned orange-red. It can be concluded that unstable in the DMF/THF solvent system. However, TPU can quickly dissolve in the TC solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneous and sta precipitates or color change happened in this solvent system, which suggests that the c structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maintaining the of electrospun jet [9]. When the concentration of the solution is lower than the o concentration, polymer chains in the solution do not have enough crosslinks. This will difficult to form a Taylor cone or get electrospun fibers with beads while perform electrospinning. Figure 2 shows SEM images of nanofibers prepared by electrospinnin TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). Electrosp nanofibers from TCM/TFE solvent show a smooth surface morphology and uniform d Polymers 2020, 12, x FOR PEER REVIEW

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect the r nanofibers properties [13]. Previous studies have demonstrated that TPU can be easily polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The polarity solvent depends on the ratio of the two solvents. The solubility of TPU in the TCM system with different solvent ratios is shown in Table 1. When the concentration of TPU TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and form a stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solutions ke transparent during long-term storage, and this is beneficial to industrial production.
DMF and THF were common solvents of TPU in previous studies [21][22][23]. In th DMF/THF (5/5) solvent system was used as the control group. Compared with the TPU s from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was much As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing in t solvent system, and the solution gradually turned orange-red. It can be concluded unstable in the DMF/THF solvent system. However, TPU can quickly dissolve in t solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneous an precipitates or color change happened in this solvent system, which suggests that structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maintainin of electrospun jet [9]. When the concentration of the solution is lower than t concentration, polymer chains in the solution do not have enough crosslinks. This difficult to form a Taylor cone or get electrospun fibers with beads while pe electrospinning. Figure 2 shows SEM images of nanofibers prepared by electrosp TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). Elec nanofibers from TCM/TFE solvent show a smooth surface morphology and unifo Polymers 2020, 12, x FOR PEER REVIEW

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect nanofibers properties [13]. Previous studies have demonstrated that TPU can be polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The pol solvent depends on the ratio of the two solvents. The solubility of TPU in the system with different solvent ratios is shown in Table 1. When the concentration of TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and fo stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solution transparent during long-term storage, and this is beneficial to industrial productio  [21][22][23]. DMF/THF (5/5) solvent system was used as the control group. Compared with the T from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing solvent system, and the solution gradually turned orange-red. It can be concl unstable in the DMF/THF solvent system. However, TPU can quickly dissolve solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneo precipitates or color change happened in this solvent system, which suggests structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maint of electrospun jet [9]. When the concentration of the solution is lower th concentration, polymer chains in the solution do not have enough crosslinks. difficult to form a Taylor cone or get electrospun fibers with beads whil electrospinning. Figure 2 shows SEM images of nanofibers prepared by elec TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). nanofibers from TCM/TFE solvent show a smooth surface morphology and Polymers 2020, 12, x FOR PEER REVIEW

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that a nanofibers properties [13]. Previous studies have demonstrated that TPU ca polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The solvent depends on the ratio of the two solvents. The solubility of TPU in system with different solvent ratios is shown in Table 1. When the concentratio TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved a stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These sol transparent during long-term storage, and this is beneficial to industrial prod DMF and THF were common solvents of TPU in previous studies [21 DMF/THF (5/5) solvent system was used as the control group. Compared with from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of sta solvent system, and the solution gradually turned orange-red. It can be unstable in the DMF/THF solvent system. However, TPU can quickly dis solvent system. Moreover, the TPU (TCM/TFE) solution system is homog precipitates or color change happened in this solvent system, which sugg structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in m of electrospun jet [9]. When the concentration of the solution is low concentration, polymer chains in the solution do not have enough crossl difficult to form a Taylor cone or get electrospun fibers with beads electrospinning. Figure 2 shows SEM images of nanofibers prepared by TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% ( nanofibers from TCM/TFE solvent show a smooth surface morphology Polymers 2020, 12, x FOR PEER REVIEW

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters t nanofibers properties [13]. Previous studies have demonstrated that TP polar solvents [13,17], so we chose two polar solvents, TCM and TFE solvent depends on the ratio of the two solvents. The solubility of T system with different solvent ratios is shown in Table 1. When the concen TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be disso stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. Thes transparent during long-term storage, and this is beneficial to industria DMF and THF were common solvents of TPU in previous studie DMF/THF (5/5) solvent system was used as the control group. Compared from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent sy As shown in Figure 1, TPU flocculent precipitate appeared after 48 h o solvent system, and the solution gradually turned orange-red. It can unstable in the DMF/THF solvent system. However, TPU can quick solvent system. Moreover, the TPU (TCM/TFE) solution system is h precipitates or color change happened in this solvent system, which structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important rol of electrospun jet [9]. When the concentration of the solution is concentration, polymer chains in the solution do not have enough c difficult to form a Taylor cone or get electrospun fibers with b electrospinning. Figure 2 shows SEM images of nanofibers prepare TCM/TFE and DMF/THF solvent systems with TPU concentration of nanofibers from TCM/TFE solvent show a smooth surface morpho distribution, while electrospun TPU nanofibers from DMF/THF show m Polymers 2020, 12, x FOR PEER REVIEW

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important param nanofibers properties [13]. Previous studies have demonstrated t polar solvents [13,17], so we chose two polar solvents, TCM an solvent depends on the ratio of the two solvents. The solubility system with different solvent ratios is shown in Table 1. When the c TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6 transparent during long-term storage, and this is beneficial to ind DMF and THF were common solvents of TPU in previous DMF/THF (5/5) solvent system was used as the control group. Com from DMF/THF, TPU solution made from the TCM/TFE (5/5) solv As shown in Figure 1, TPU flocculent precipitate appeared after solvent system, and the solution gradually turned orange-red. unstable in the DMF/THF solvent system. However, TPU can solvent system. Moreover, the TPU (TCM/TFE) solution system precipitates or color change happened in this solvent system, structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an importa of electrospun jet [9]. When the concentration of the solut concentration, polymer chains in the solution do not have eno difficult to form a Taylor cone or get electrospun fibers w electrospinning. Figure 2 shows SEM images of nanofibers p TCM/TFE and DMF/THF solvent systems with TPU concentrat nanofibers from TCM/TFE solvent show a smooth surface m distribution, while electrospun TPU nanofibers from DMF/THF sh DMF and THF were common solvents of TPU in previous studies [21][22][23]. In this study, the DMF/THF (5/5) solvent system was used as the control group. Compared with the TPU solution made from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was much more stable. As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing in the DMF/THF solvent system, and the solution gradually turned orange-red. It can be concluded that TPU is unstable in the DMF/THF solvent system. However, TPU can quickly dissolve in the TCM/TFE solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneous and stable. No precipitates or color change happened in this solvent system, which suggests that the chemical structure of TPU is stable in this novel solvent system.

Solution Properties of TPU in TCM/TFE Solvent Systems
The TPU solution properties are the most important parameters that affect the resultant TPU nanofibers properties [13]. Previous studies have demonstrated that TPU can be easily dissolved in polar solvents [13,17], so we chose two polar solvents, TCM and TFE. The polarity of the mixed solvent depends on the ratio of the two solvents. The solubility of TPU in the TCM/TFE solvent system with different solvent ratios is shown in Table 1. When the concentration of TPU was 5% (w/v), TPU only swelled at VTCM/VTFE = 10/0, 9/1, 3/7 and 0/10. TPU can be dissolved and form a uniform and stable TPU solution when VTCM/ VTFE is 8/2, 7/3, 6/4, 5/5, 4/6. These solutions keep clear and transparent during long-term storage, and this is beneficial to industrial production.
DMF and THF were common solvents of TPU in previous studies [21][22][23]. In this study, the DMF/THF (5/5) solvent system was used as the control group. Compared with the TPU solution made from DMF/THF, TPU solution made from the TCM/TFE (5/5) solvent system was much more stable. As shown in Figure 1, TPU flocculent precipitate appeared after 48 h of standing in the DMF/THF solvent system, and the solution gradually turned orange-red. It can be concluded that TPU is unstable in the DMF/THF solvent system. However, TPU can quickly dissolve in the TCM/TFE solvent system. Moreover, the TPU (TCM/TFE) solution system is homogeneous and stable. No precipitates or color change happened in this solvent system, which suggests that the chemical structure of TPU is stable in this novel solvent system.

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maintaining the stability of electrospun jet [9]. When the concentration of the solution is lower than the optimum concentration, polymer chains in the solution do not have enough crosslinks. This will make it difficult to form a Taylor cone or get electrospun fibers with beads while performing the electrospinning. Figure 2 shows SEM images of nanofibers prepared by electrospinning from TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). Electrospun TPU nanofibers from TCM/TFE solvent show a smooth surface morphology and uniform diameter distribution, while electrospun TPU nanofibers from DMF/THF show many beads. There are several

Electrospun TPU Nanofibers
It is well-known that polymer concentration plays an important role in maintaining the stability of electrospun jet [9]. When the concentration of the solution is lower than the optimum concentration, polymer chains in the solution do not have enough crosslinks. This will make it difficult to form a Polymers 2020, 12, 3038

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Taylor cone or get electrospun fibers with beads while performing the electrospinning. Figure 2 shows SEM images of nanofibers prepared by electrospinning from TCM/TFE and DMF/THF solvent systems with TPU concentration of 5% (w/v). Electrospun TPU nanofibers from TCM/TFE solvent show a smooth surface morphology and uniform diameter distribution, while electrospun TPU nanofibers from DMF/THF show many beads. There are several reasons for the formation of beads. One is the low solubility of TPU in the DMF/THF solvent, which results in the low crosslinks among the TPU polymer chains in the solution. The other is from the low volatility of DMF (saturated vapor pressure of 3.7 mmHg) in the DMF/THF solvent system. In this case, the solvent cannot completely volatilize, while the drops fly from the needle tip to the collector. Compared to DMF, TCM has a much higher volatility (saturated vapor pressure is 160 mmHg) [17]. Even if the concentration of TPU in the solvents of TCM/TFE is decreased to a minimum electrospinnable concentration of 0.5% (w/v), the solvent can volatilize in time and facilitate nanofibers formation. Therefore, TCM/TFE solvents improved the electrospinnability of TPU, which is beneficial for reducing the production cost and its use as a biological scaffold.
Polymers 2020, 12, x FOR PEER REVIEW 5 of 12 volatility of DMF (saturated vapor pressure of 3.7 mmHg) in the DMF/THF solvent system. In this case, the solvent cannot completely volatilize, while the drops fly from the needle tip to the collector. Compared to DMF, TCM has a much higher volatility (saturated vapor pressure is 160 mmHg) [17].
Even if the concentration of TPU in the solvents of TCM/TFE is decreased to a minimum electrospinnable concentration of 0.5% (w/v), the solvent can volatilize in time and facilitate nanofibers formation. Therefore, TCM/TFE solvents improved the electrospinnability of TPU, which is beneficial for reducing the production cost and its use as a biological scaffold. When the TPU concentration was increased to 8% (w/v), TPU solutions from both TCM/TFE and DMF/THF solvent systems could be electrospun into nanofibers with a smooth surface and uniform diameter distribution (Figure 3). The TPU fiber diameter from DMF/THF solvent is (549 ± 117) nm and the TPU fiber diameter from TCM/TFE solvent is (1155 ± 161) nm. The diameter of the nanofibers produced by the DMF/THF solvent system is significantly smaller than that of TCM/TFE. This is mainly because the viscosity of TPU-DMF/THF solution is much lower than that of TPU-TCM/TFE. Figure 3a shows SEM images of electrospun TPU nanofibers from TCM/TFE and DMF/THF solvent systems (TPU concentration is 8% w/v). TPU nanofibers prepared by electrospinning with different solvent ratios of TCM/TFE have smooth and bead-free morphology. As the TFE component in the solvent system increases, the surface roughness of the fiber first decreases and then increases. TPU nanofibers have the lowest surface roughness when the ratio of TCM/TFE is VTCM/VTFE = 5/5. Diameters of the nanofibers were (1032 ± 380) nm, (997 ± 226) nm, (1155 ± 161) nm, (2339 ± 628) nm for VTCM/VTFE = 8/2, 7/3, 5/5, 4/6, respectively (Figure 3b). The diameter of TPU nanofibers increased the most when the TCM/TFE ratio changed from 5/5 to 4/6. The viscosity of TPU solution in DMF/THF and TCM/TFE solvent systems are shown in Figure 3c. It can be concluded that the viscosity of TPU solution in TCM / TFE solvent system increased with the increase of TFE ratio. During the formation of electrospun nanofibers, the most critical factor affecting the fibers' diameter is the splitting of droplets after leaving the nozzle in the electric field [3]. If the droplet can be easily split, the nanofibers with fine fiber diameters can be obtained. On the contrary, the more difficult it is for droplets to be split, the larger the diameter of the nanofibers obtained. Previous research [24] reported that the solution viscosity, volatility of solvent system and conductivity have a great influence on the diameter and morphology of nanofibers. It can be inferred that when VTCM/VTFE = 4/6, the extremely significant increase in the diameter of the nanofiber is caused by the increase in the viscosity of the solution, which means the viscosity of the solution becomes the main factor affecting the fiber diameter. When the TPU concentration was increased to 8% (w/v), TPU solutions from both TCM/TFE and DMF/THF solvent systems could be electrospun into nanofibers with a smooth surface and uniform diameter distribution (Figure 3). The TPU fiber diameter from DMF/THF solvent is (549 ± 117) nm and the TPU fiber diameter from TCM/TFE solvent is (1155 ± 161) nm. The diameter of the nanofibers produced by the DMF/THF solvent system is significantly smaller than that of TCM/TFE. This is mainly because the viscosity of TPU-DMF/THF solution is much lower than that of TPU-TCM/TFE. Figure 3a shows SEM images of electrospun TPU nanofibers from TCM/TFE and DMF/THF solvent systems (TPU concentration is 8% w/v). TPU nanofibers prepared by electrospinning with different solvent ratios of TCM/TFE have smooth and bead-free morphology. As the TFE component in the solvent system increases, the surface roughness of the fiber first decreases and then increases. TPU nanofibers have the lowest surface roughness when the ratio of TCM/TFE is V TCM /V TFE = 5/5. Diameters of the nanofibers were (1032 ± 380) nm, (997 ± 226) nm, (1155 ± 161) nm, (2339 ± 628) nm for V TCM /V TFE = 8/2, 7/3, 5/5, 4/6, respectively (Figure 3b). The diameter of TPU nanofibers increased the most when the TCM/TFE ratio changed from 5/5 to 4/6. The viscosity of TPU solution in DMF/THF and TCM/TFE solvent systems are shown in Figure 3c. It can be concluded that the viscosity of TPU solution in TCM/TFE solvent system increased with the increase of TFE ratio. During the formation of electrospun nanofibers, the most critical factor affecting the fibers' diameter is the splitting of droplets after leaving the nozzle in the electric field [3]. If the droplet can be easily split, the nanofibers with fine fiber diameters can be obtained. On the contrary, the more difficult it is for droplets to be split, the larger the diameter of the nanofibers obtained. Previous research [24] reported that the solution viscosity, Polymers 2020, 12, 3038 6 of 12 volatility of solvent system and conductivity have a great influence on the diameter and morphology of nanofibers. It can be inferred that when V TCM /V TFE = 4/6, the extremely significant increase in the diameter of the nanofiber is caused by the increase in the viscosity of the solution, which means the viscosity of the solution becomes the main factor affecting the fiber diameter. The molecular structure of electrospun TPU nanofibers from DMF/THF and TCM/TFE solvent system was characterized by Fourier transform infrared spectrometer (FTIR), as shown in Figure 4. The repeated amide group is a characteristic group of TPU [25]. It can be seen from the infrared absorption spectra that there is no significant difference in the chemical groups between TPU nanofibers from DMF/THF and TCM/TFE solvent systems. In general, the typical absorption peaks of the amide bond in the TPU chemical structure can be detected at 1700 cm −1 (stretching vibration of C=O in amide bond), 1530 cm −1 (bending vibration of amide bond N-H) and 1223 cm −1 (C-N stretching vibration of amide bond) [20]. It can be concluded that the solvent in the solution system will not change the chemical structure of TPU. In addition, other chemical groups of TPU can also be observed in the infrared absorption spectrum, such as the asymmetric stretching vibration of CH2 at 2929 cm −1 [15], the stretching vibration of symmetrically distributed CH2 at 2830 cm −1 , and the C=C skeleton vibration of aromatic benzene ring at 1600 cm −1 . In the infrared absorption spectrum, the stretching vibration of C=O bond from the ester group at 1730 cm −1 and the stretching vibration of C-O-C at 1100 cm −1 were also observed [26]. However, due to vibration coupling, there was a peak before and after 1100 cm −1 . The molecular structure of electrospun TPU nanofibers from DMF/THF and TCM/TFE solvent system was characterized by Fourier transform infrared spectrometer (FTIR), as shown in Figure 4. The repeated amide group is a characteristic group of TPU [25]. It can be seen from the infrared absorption spectra that there is no significant difference in the chemical groups between TPU nanofibers from DMF/THF and TCM/TFE solvent systems. In general, the typical absorption peaks of the amide bond in the TPU chemical structure can be detected at 1700 cm −1 (stretching vibration of C=O in amide bond), 1530 cm −1 (bending vibration of amide bond N-H) and 1223 cm −1 (C-N stretching vibration of amide bond) [20]. It can be concluded that the solvent in the solution system will not change the chemical structure of TPU. In addition, other chemical groups of TPU can also be observed in the infrared absorption spectrum, such as the asymmetric stretching vibration of CH 2 at 2929 cm −1 [15], the stretching vibration of symmetrically distributed CH 2 at 2830 cm −1 , and the C=C skeleton vibration of aromatic benzene ring at 1600 cm −1 . In the infrared absorption spectrum, the stretching vibration of C=O bond from the ester group at 1730 cm −1 and the stretching vibration of C-O-C at 1100 cm −1 were also observed [26]. However, due to vibration coupling, there was a peak before and after 1100 cm −1 . Figure 5 shows the mechanical properties of electrospun TPU nanofibers manufactured from TCM/TFE and DMF/THF systems. With the increase of TFE in the solvent system, the tensile strength of TPU nanofibers (Figure 5b system, the strain elongation ratio of TPU fibers from TCM/TFE solvent system first increased and then decreased. The strain elongation ratio reached the highest value when the ratio of V TCM /V TFE was 5/5. The strain elongation ratio of the TPU fibers from each solvent system showed an opposite trend to the tensile strength of the fiber mats. The strain elongation ratios were 104.51%, 183.65%, 331.79% and 155.70%for V TCM /V TFE = 8/2, 7/3, 5/5 and 4/6, respectively. Mechanical properties of TPU nanofibers are highly related to the TCM and TFE combinations. TCM is highly volatile, but TFE is less volatile than TCM. With the increase of the TFE ratio in the solvent system, the volatility of TCM/TFE solution prepared for electrospinning became weaker. Therefore, there is still a little solvent left when the fiber reached the collecting plate, and this promoted more crosslinking of the fiber, thus enhancing the maximum elongation of the fiber.  With the increase of TFE in the system, the strain elongation ratio of TPU fibers from TCM/TFE solvent system first increased and then decreased. The strain elongation ratio reached the highest value when the ratio of VTCM/VTFE was 5/5. The strain elongation ratio of the TPU fibers from each solvent system showed an opposite trend to the tensile strength of the fiber mats. The strain elongation ratios were 104.51%, 183.65%, 331.79% and 155.70%for VTCM/VTFE = 8/2, 7/3, 5/5 and 4/6, respectively. Mechanical properties of TPU nanofibers are highly related to the TCM and TFE combinations. TCM is highly volatile, but TFE is less volatile than TCM. With the increase of the TFE ratio in the solvent system, the volatility of TCM/TFE solution prepared for electrospinning became weaker. Therefore, there is still a little solvent left when the fiber reached the collecting plate, and this promoted more crosslinking of the fiber, thus enhancing the maximum elongation of the fiber. Figure 6a shows the thermal properties of TPU nanofibers. The specific solvent evaporation temperature, melting temperature and glass transition temperature are listed in Table 2. All samples show a broad endothermal peak from 10 to 70 °C. This peak is attributed to both the evaporation of the solvent (TCM, TFE and THF) residue and the melting of the soft segment of TPU [27]. The broad endothermal peak around 162 °C was from the melting of the hard segment in TPU nanofiber meshes from the TCM/TFE solvent system, while the counterpart of the TPU nanofibers mesh from the DCM/THF solvent was around 150 °C. This suggests that TPU nanofiber meshes from the new solvent system show better thermal stability. TPU nanofiber meshes from the TCM/TFE solvent system showed a glass transition of the hard segment of TPU around 102 °C, while the Tg of the TPU nanofiber from DMF/THF solvent showed a less weak glass transition around 103 °C. This suggests that the TPU nanofiber from TCM/TFE solvent system had more hard segments [28], and thus the TPU nanofiber mesh from TCM/TFE solvent system had a higher Young's modulus. However, the sample from the TCM/TFE (5/5) solvent system showed the weakest glass transition around 102 °C,  Figure 6b shows the XRD spectra of electrospun TPU nanofibers. TPU nanofibers from TCM/TFE solvent systems all display a strong diffraction peak at 25.9° with two shoulders at 20.4° and 40.9°. The peak at 20.4° represents the existence of a mixed ordered structure of both hard and soft domains and an amorphous phase of the TPU matrix [29]. The peaks at 25.9° and 40.9° represent the mixing part of hard and soft components of TPU, respectively [30]. There is no shoulder at 20.4° for the TPU nanofibers from DMF/THF solvent system. This implies that the crystalline properties of TPU may be modified by different solvent systems, and this is consistent with the results of DSC.  Figure 6a shows the thermal properties of TPU nanofibers. The specific solvent evaporation temperature, melting temperature and glass transition temperature are listed in Table 2. All samples show a broad endothermal peak from 10 to 70 • C. This peak is attributed to both the evaporation of Polymers 2020, 12, 3038 8 of 12 the solvent (TCM, TFE and THF) residue and the melting of the soft segment of TPU [27]. The broad endothermal peak around 162 • C was from the melting of the hard segment in TPU nanofiber meshes from the TCM/TFE solvent system, while the counterpart of the TPU nanofibers mesh from the DCM/THF solvent was around 150 • C. This suggests that TPU nanofiber meshes from the new solvent system show better thermal stability. TPU nanofiber meshes from the TCM/TFE solvent system showed a glass transition of the hard segment of TPU around 102 • C, while the T g of the TPU nanofiber from DMF/THF solvent showed a less weak glass transition around 103 • C. This suggests that the TPU nanofiber from TCM/TFE solvent system had more hard segments [28], and thus the TPU nanofiber mesh from TCM/TFE solvent system had a higher Young's modulus. However, the sample from the TCM/TFE (5/5) solvent system showed the weakest glass transition around 102 • C, which suggests that it had the lowest hard segments among all the samples from the new solvent system. This also accounts for the lowest Young's modulus of the TCM/TFE (5/5) sample. This finding should inspire researchers, as it means that the ratio of soft segments and hard segments is tunable through modifying the new solvent system.  Figure 6b shows the XRD spectra of electrospun TPU nanofibers. TPU nanofibers from TCM/TFE solvent systems all display a strong diffraction peak at 25.9° with two shoulders at 20.4° and 40.9°. The peak at 20.4° represents the existence of a mixed ordered structure of both hard and soft domains and an amorphous phase of the TPU matrix [29]. The peaks at 25.9° and 40.9° represent the mixing part of hard and soft components of TPU, respectively [30]. There is no shoulder at 20.4° for the TPU nanofibers from DMF/THF solvent system. This implies that the crystalline properties of TPU may be modified by different solvent systems, and this is consistent with the results of DSC.   Figure 6b shows the XRD spectra of electrospun TPU nanofibers. TPU nanofibers from TCM/TFE solvent systems all display a strong diffraction peak at 25.9 • with two shoulders at 20.4 • and 40.9 • . The peak at 20.4 • represents the existence of a mixed ordered structure of both hard and soft domains and an amorphous phase of the TPU matrix [29]. The peaks at 25.9 • and 40.9 • represent the mixing part of hard and soft components of TPU, respectively [30]. There is no shoulder at 20.4 • for the TPU nanofibers from DMF/THF solvent system. This implies that the crystalline properties of TPU may be modified by different solvent systems, and this is consistent with the results of DSC.
The ternary phase diagram can clearly show the ratio range of the solvent, which can produce uniform and smooth fibers without liquid beads in theTCM/TFE solvent system [31]. As shown in Figure 7, smooth, uniform and non-adhesive nanofibers can be obtained with TPU concentrations ranging from 0.5% to 8% and TCM accounting for 80% to 40% of the solvent system. According to previous research, when DMF and THF mixed solution is used as solvent, the required concentration of polyurethane for electrospinning is 6%-18% [22,23]. As shown in Figure 7, compared with the DMF/THF solvent system, smooth and uniform nanofibers can be obtained by electrospinning with smaller TPU polymer concentration and more solvent ratio by using the TCM/TFE solvent system. Figure 7, smooth, uniform and non-adhesive nanofibers can be obtained with TPU concentrations ranging from 0.5% to 8% and TCM accounting for 80% to 40% of the solvent system. According to previous research, when DMF and THF mixed solution is used as solvent, the required concentration of polyurethane for electrospinning is 6%-18% [22,23]. As shown in Figure 7, compared with the DMF/THF solvent system, smooth and uniform nanofibers can be obtained by electrospinning with smaller TPU polymer concentration and more solvent ratio by using the TCM/TFE solvent system. Figure 7. Mapping of the region for the production of bead free TPU fibres with: (a) 0.05% w/v, (b) 5% w/v, (c) 8% w/v. The area enclosed by the three colored lines indicates the ternary mixture compositions that can lead to the production of bead-free TPU nanofibers. Figure 8a shows SEM images of the rMSCs on the electrospun TPU nanofiber scaffolds. At day 4 of culture, the TPU nanofiber scaffolds from both DMF/THF and TCM/TFE solvent systems show close cell viability. It can be clearly seen that the TPU nanofiber scaffolds provide solid support for the rMSCs and that the cell adhered well to the scaffold. At day 7 of culture, rMSCs distributed uniformly on the TPU nanofiber scaffold from TCM/TFE at VTCM/VTFE = 5/5 solvent while gathering close to each other on other samples. The quantitative cell viability of rMSCs was measured by CCK-8 method. In particular, at day 7, there was nearly 1.3-fold cell growth on the TPU nanofiber scaffold from TCM/TFE at VTCM/VTFE = 5/5 than that from DCM/THF at VDCM/VTHF = 5/5. We can also easily see that the TPU nanofiber scaffold from TCM/TFE at VTCM/VTFE = 5/5 solvent showed the highest OD values (Figure 8b). This is because the TPU nanofibers from VTCM/VTFE = 5/5 solvent have a smaller diameter. In this case, the scaffold has a higher porosity, and this is beneficial to providing sufficient culture nutrition for the rMSCs. These data suggest that the TPU scaffolds from the solvent system of TCM/TFE (5/5) show excellent biocompatibility. Figure 7. Mapping of the region for the production of bead free TPU fibres with: (a) 0.05% w/v, (b) 5% w/v, (c) 8% w/v. The area enclosed by the three colored lines indicates the ternary mixture compositions that can lead to the production of bead-free TPU nanofibers. Figure 8a shows SEM images of the rMSCs on the electrospun TPU nanofiber scaffolds. At day 4 of culture, the TPU nanofiber scaffolds from both DMF/THF and TCM/TFE solvent systems show close cell viability. It can be clearly seen that the TPU nanofiber scaffolds provide solid support for the rMSCs and that the cell adhered well to the scaffold. At day 7 of culture, rMSCs distributed uniformly on the TPU nanofiber scaffold from TCM/TFE at V TCM /V TFE = 5/5 solvent while gathering close to each other on other samples. The quantitative cell viability of rMSCs was measured by CCK-8 method. In particular, at day 7, there was nearly 1.3-fold cell growth on the TPU nanofiber scaffold from TCM/TFE at V TCM /V TFE = 5/5 than that from DCM/THF at V DCM /V THF = 5/5. We can also easily see that the TPU nanofiber scaffold from TCM/TFE at V TCM /V TFE = 5/5 solvent showed the highest OD values (Figure 8b). This is because the TPU nanofibers from V TCM /V TFE = 5/5 solvent have a smaller diameter. In this case, the scaffold has a higher porosity, and this is beneficial to providing sufficient culture nutrition for the rMSCs. These data suggest that the TPU scaffolds from the solvent system of TCM/TFE (5/5) show excellent biocompatibility.

Conclusions
A novel solvent system with TCM and TFE at different component ratios were developed for fabricating electrospun TPU nanofibers. The selected TCM and TFE component ratios are VTCM: VTFE = 8:2, 7:3, 5:5, 4:6. The concentration of TPU ranges from 0.5% to 8%. The obtained electrospun TPU nanofibers fabricated from this novel solvent system showed smooth morphology and uniform diameter distribution. The viscosity of the TPU solution increased with the increasing ratio of TFE in the solvent system. The Young's modulus and tensile strength of the electrospun TPU nanofiber meshes first decreased and then increased. The maximum tensile strength of the electrospun TPU nanofiber first increased and then decreased. FTIR data showed that the chemical structure of the TPU was not affected by the TCM/TFE solvent system when compared to that of the DMF/THF solvent system. The TPU solution stability from the TCM/TFE solvent system was improved in contrast with the DMF/THF solvent system. The new solvent system can significantly improve the fiber tensile elongation ratio while maintaining the modulus and tensile strength. Electrospun TPU nanofiber meshes prepared using the novel TCM/TFE solvent system did not show cytotoxicity, which means the electrospun TPU meshes have excellent biocompatibility.

Conclusions
A novel solvent system with TCM and TFE at different component ratios were developed for fabricating electrospun TPU nanofibers. The selected TCM and TFE component ratios are V TCM :V TFE = 8:2, 7:3, 5:5, 4:6. The concentration of TPU ranges from 0.5% to 8%. The obtained electrospun TPU nanofibers fabricated from this novel solvent system showed smooth morphology and uniform diameter distribution. The viscosity of the TPU solution increased with the increasing ratio of TFE in the solvent system. The Young's modulus and tensile strength of the electrospun TPU nanofiber meshes first decreased and then increased. The maximum tensile strength of the electrospun TPU nanofiber first increased and then decreased. FTIR data showed that the chemical structure of the TPU was not affected by the TCM/TFE solvent system when compared to that of the DMF/THF solvent system. The TPU solution stability from the TCM/TFE solvent system was improved in contrast with the DMF/THF solvent system. The new solvent system can significantly improve the fiber tensile elongation ratio while maintaining the modulus and tensile strength. Electrospun TPU nanofiber meshes prepared using the novel TCM/TFE solvent system did not show cytotoxicity, which means the electrospun TPU meshes have excellent biocompatibility.