Solvent-free thermocuring electrospinning to fabricate ultrathin polyurethane fibers with high conductivity by in situ polymerization of polyaniline

Abstract

A new solvent-free thermal assisted electrospinning (e-spinning) process for the preparation of polyurethane (PU) microfibers has been developed. Based on the synthetic process of conventional PU, the synthesis of the PU precursor solution in this work was improved for the solventless e-spinning process. Low molecular weight (low-M_w) polyol (PTMG-500) was employed for preparing the prepolymer, and another low-M_w polyol (PEG-400) was used as the chain extender. During the chain extension, e-spinning was conducted and assisted by thermal radiation (∼50 °C), and PU microfibers with an average diameter of 25–60 μm were fabricated. The spinnable viscosity range in this e-spinning process was 1000–2500 dPa s. In addition, 2,2-bis(hydroxymethyl)propionic acid (DMPA) was introduced into the precursor solution to partially take the place of the chain extender (PEG-400), and thus the hydrophilicity of the as-spun fibrous mat was improved, which was helpful for in situ polymerization of polyaniline (PANi) on the fibers' surface to obtain conductive PU/PANi composite fibers. This thermal assisted e-spinning process is a solvent-free and eco-friendly process, which may become an interesting approach to fabricate a variety of PU-based functional fibers with applications in protective and anti-corrosive coatings, antistatic fabrics, and sensors.

---

1. Introduction

Thanks to simple operation and flexible setup, electrospinning (e-spinning) has become an effective means to fabricate ultrathin fibers.^1–5 The as-spun fibers (or fibrous mats) can be utilized in ultrafine filtering,^6,7 protective textile,⁸ desalination,⁹ ultrasensitive sensors,^10–12 medical dressings and tissue engineering, etc.^13–15 Traditional solution e-spinning uses a polymer solution as a precursor, and the dosage of solvent (except for water) is usually more than 80 wt%. During the e-spinning process, the volatile solvent is hard to recycle, which may lead to serious environmental or safety problems and limit its industrial-scale applications.

In order to solve these problems, solvent-free e-spinning techniques have been paid more and more attention. For instance, Liu et al. reported a solventless e-spinning based on quick curing of ethyl cyano-acrylate (ECA, 502 glue) initiated by moisture in air. Because of low viscosity of ECA, polymethyl methacrylate (PMMA) was added into the ECA to improve spinnability, but there was still more than 10 wt% loss of ECA monomer in the e-spinning process.¹⁶ A typical solventless e-spinning process is melt e-spinning, which has been studied for many years, but its apparatus is relatively complicated and the as-spun fibers are thick and coarse.^17–22 Recently, a solvent-free and highly efficient way to prepare polyurethane acrylate (PUA) fibers was developed by our group, in which the e-spinning was assisted by UV light and the jet was in situ polymerized completely.²³ This UV-curing e-spinning exhibits wide applicability in fabrication of functional composite fibers. However, it needs modified e-spinning device and inert gas such as N₂ atmosphere to overcome strong polymerization inhibition of oxygen on the e-spinning jet.²³ In addition, a supercritical CO₂-assisted e-spinning of polydimethylsiloxane (PDMS) and poly(D,L-lactic acid) (PLA) without using any solvent was also reported.²⁴ In fact, the supercritical CO₂ acted as a solvent, and the as-spun fibers were very short (∼4 mm) and thick (hundreds of microns in diameter).

Polyurethane (PU) is a significant synthetic material and widely used in plastics, coatings, adhesives, elastic and foaming materials, and biomedical materials such as artificial blood vessel²⁵ and artificial esophagus.²⁶ PU fibers, also well-known as Spandex, were industrialized in 1959 by DuPont company.^27,28 Because of good elasticity and wearing comfort, PU fibers are applied widely in the clothing fabric, which are produced mainly by conventional wet spinning or melt spinning in plants. However, the equipment is huge and the process is complex. In recent years, fabrication of thermoplastic PU ultrathin fibers by melt e-spinning or conventional solution e-spinning has also attracted much attention.^29–32 The as-spun ultrathin fibers or composite ones can be applied in wound dressing,²⁹ water-proof fabrics,³⁰ regenerative medicine,³¹ etc.

Generally, the preparation of PU is divided into two stages. Firstly, multi-isocyanate reacts with middle molecular weight (mid-M_w) polyols (M_w = 1000–2000) to prepare prepolymers, and solvent such as N,N-dimethylformamide (DMF) is indispensable. Then the chains are extended by feeding small molecule polyalcohols or amines such as butanediol (BDO). If there is not any solvent employed, the viscosities of prepolymer and its mixture with extenders are too high to e-spin directly. In this paper, we optimized the recipe of PU precursor in which the prepolymer was given by isocyanate and low-M_w polyol, and then chain was extended by low-M_w polyol as well. The e-spinning setup was modified and assisted by a thermal radiation source. In this case, e-spinning was conducted successfully without any solvent and the jet was solidified into microfibers. This solvent-free e-spinning provides an eco-friendly process to fabricate PU microfibers or PU-based functional ones.

2. Experimental

2.1 Materials

4,4-Methylenediphenyldiisocyanate (MDI, Yantai Wanhua Co., Ltd) was melted at 45 °C and the clear liquid utilized after removing precipitates. Polytetramethylene glycol (PTMG, M_w = 250 and 650, Sigma-Aldrich, labeled as PTMG-250 and PTMG-650, respectively), polyethylene glycol (PEG, M_w = 400, Sigma-Aldrich, labeled as PEG-400), and butanediol (BDO, Sigma-Aldrich) were dehydrated before used in a vacuum at 90 °C and 65 °C, respectively. PTMG (M_w = 500, labeled as PTMG-500) was not commercial product, and formulated with 37.5 wt% of PTMG-250 and 62.5 wt% of PTMG-650. 2,2-Bis(hydroxymethyl)propionic acid (DMPA, Sigma-Aldrich) was dried in a 100 °C oven in vacuum for at least 4 h. Aniline, ammonium peroxysulfate (APS) and camphorsulfonic acid (CSA) were purchased from TCI (Shanghai) and not refined further before used.

2.2 E-spinning apparatus

As shown in Fig. 1, the homemade e-spinning device consists of a high-voltage power supply (Tianjin Dongwen), a modified rotating disk collector, a syringe pump and a thermal radiation lamp. The distance of tip-to-collector was ∼25 cm, and that of thermal source to the spinneret was 30–40 cm to keep temperature of the spinning area about 50 °C. The temperature near the spinneret and surface of disk collector was lower than 30 °C.

Fig. 1 Schematic illustration of the thermally assisted solventless e-spinning setup.

2.3 Preparation of solutions and e-spinning process

Synthesis of e-spinning precursor (PU prepolymer). 95.0 g (0.38 mol) of MDI and 55.0 g (0.11 mol) of PTMG-500 were mixed in a flask (500 mL), stirred by a magnetic stirrer and heated by an oil-bath (80 °C) under nitrogen protecting for 4 h and then the reaction was terminated to afford the prepolymer.

Synthesis of e-spinning precursor (chain extending). 108.0 g (0.27 mol) of PEG-400 as chain extender was added into the prepolymer. After well-mixed by a magnetic stirrer for 5 min under protection of N₂, ∼10 g of the solution was taken out and loaded in a syringe for e-spinning under a high voltage of 25 kV.

The residual solution, ∼250 mL, was utilized for measuring viscosity by a Viscotester (VT-04F, RION Co., LTD) during chain extending. DMPA was introduced as a part of chain extender to improve the polarity of PU chain. For comparison, BDO was employed as small molecular extender and the changes in viscosity were recorded as well.

2.4 Characterization

The morphologies of as-spun fibers were characterized by using an optical microscope (Olympus BX51) and a scanning electron microscope (SEM; JEOL JSM-6390). Contact angles of the e-spun fibrous mats were examined by static water contact angle measurement (DSA100). The e-spinning precursors and resultant fibers were identified by Fourier transform infrared spectroscopy (FT-IR; Nicolet AVATAR 370DTGS). Molecular weights of PU prepolymer and as-spun fibers were determined with a gel permeation chromatography (GPC) system (Tosoh EcoSEC HLC-8320GPC) including a refractive index detector, column 1 (TSKgel SuperHZ3000/2000/2000/2000/1000 for low M_w measurement) and column 2 (TSKgel G5000/4000/3000/2000HXL for high M_w measurement). The electrical conductivities of resultant PU and PU/PANi fibers were measured by a Keithley 6487 high resistance meter system at room temperature.

3. Results and discussion

3.1 Recipe of PU e-spinning precursor

As mentioned above, ultrathin PU fibers can be produced by melt or solution e-spinning. To reduce the viscosity of prepolymer without using any solvent, a conventional recipe of prepolymer for PU is improved to meet this requirement. The prepolymer has lower viscosity due to its lower average M_w. As presented in Table 1, the prepolymer is chain-extended by adding the low-M_w polyol, which takes the place of conventional small molecular polyalcohol. In addition, the polymerization speed can be decreased because polyol has lower reactivity than small molecular alcohol. So, the viscosity of e-spinning precursor solution will not be increased so fast. Namely, reduction of the speed of chain extending (viscosity increase) may win more time for e-spinning. Moreover, conventional catalyst is not included in this improved recipe.

Table 1 Comparison between two kinds of PU synthesis process

	Conventional PU	Improved PU for e-spinning
Prepolymers	–NCO + –OH (middle Mw)	–NCO + –OH (low Mw)
Extenders	Small molecules: BDO, NPG, etc.	Polyols (low Mw)
Solvents	Yes	No
Catalyst used	Yes	No

---

3.2 Viscosity of e-spinning solution and morphologies of e-spun fibers

As presented in Table 2, the prepolymers given by MDI and PTMG-500 (molar ratio = 3.5 : 1) were similar and the chain extenders (2.5 equivalent of MDI), BDO (small molecular polyalcohol), PEG-400 (low-M_w polyol), and PEG-400 containing DMPA were introduced, respectively. When the BDO was used as a chain extender (Run. 1, Table 2), the viscosity was increased quite quickly from 5 to 3000 dPa s in 30 min, as shown in Fig. 2. In this case, e-spinning is hardly carried out. While the PEG-400 was used as a chain extender (Run. 2, Table 2), the viscosity was increased slowly, and e-spinning could be conducted. The morphologies of resulting fibers at 40 min, 60 min, 80 min and 100 min are shown in Fig. 3. When the chain-extending polymerization continued for 40 min, the viscosity was up to 100 dPa s (Fig. 2). The e-spinning precursor was sprayed and only liquid beads were formed on the collector (Fig. 3a). 20 min later, beaded fibers were afforded (Fig. 3b). When the viscosity rose to over 500 dPa s at 80 min, fibers were observed on the collector. However, the fibers were crosslinked (Fig. 3c and e), indicating the fiber solidification was not completed. 100 min later, the viscosity of precursor increased to over 1000 dPa s, smooth and isolated fibers were obtained with 60 μm in average diameter (Fig. 3d and f). The e-spinning was merely kept less than 20 min because the jet was hardly dragged from the spinneret even at a higher voltage of 30 kV due to the rapidly increased viscosity (over 2500 dPa s).

Table 2 The viscosity of PU polymerizing process

	Run. 1	Run. 2	Run. 3
Prepolymer/g	224.1	149.4	180.0
Chain extender/g	BDO/36.1	PEG-400/108.0	PEG-400/96.0 + DMPA/11.3

---

Fig. 2 The changes and comparison in viscosity when BDO, PEG-400, and mixture of PEG-400 + DMPA were used as chain extender, respectively.

Fig. 3 Morphologies of e-spun PU fibers, extender of PEG-400 was added for (a) 40 min, (b) 60 min, (c) 80 min, and (d) 100 min. Insets (e) and (f) are SEM pictures for (c) and (d), respectively. The e-spinning voltage was 25 kV and the rotating speed of collector was 10 rpm.

PU fibers are hydrophobic and unfavorable for applications in coating, dyeing, etc. DMPA is an ordinarily used modifier in waterborne system and the –COOH group introduced to improve hydrophilicity of PU chain.^33–35 In this work, DMPA was employed as a part of extender (PEG-400), as shown in Run. 3 (Table 2), and 4 wt% of DMPA was introduced. As indicated in Fig. 2, the viscosity was increased more quickly than that of Run. 2 of Table 2 because the small molecular polyalcohol has higher reactivity. The e-spinning time was kept only for 10 min, shorter than that (∼20 min) of pure PEG-400 used. However, the as-spun fibers, as shown in Fig. 4, have similar morphologies (Fig. 3c and d).

Fig. 4 Optical images of e-spun PU fibers, a mixed extender of PEG-400 and DMPA was added for chain extending: (a) 70 min, (b) 95 min, respectively. The e-spinning voltage was 25 kV and the rotating speed of collector was 10 rpm.

3.3 Solidification mechanism of e-spun fibers

As shown in Scheme 1, the prepolymer was synthesized with MDI (–NCO) and PTMG-500 (–OH). The chain extending was also the reaction between –NCO and –OH (Scheme 2). This reaction speed is affected by temperature. A higher temperature will lead to a faster reaction speed.³⁶ When the polymer jet ejects from the spinneret and enters into the hot spinning area (50 °C) radiated by a thermal source, the reaction between –NCO and –OH will accelerate and the M_w of PU will increase quickly (please see the following context). So, the jet becomes solidified and deposits on the disk collector, as illustrated in Fig. 5.

Scheme 1 Synthesis of PU prepolymer.

Scheme 2 Chain-extending, the prepolymer and extender mixed and stirred for about 5 min at 25 °C, then loaded in spinneret. The jet was in situ solidified quickly by thermal assistance.

Fig. 5 Solidification mechanism of the e-spun PU microfibers under thermal radiation.

3.4 Influence of rotating speed on e-spun fibers

The influence of rotating speed of disk collector on average diameter of the resultant fibers was investigated. As shown in Fig. 6a1 and b1, when the rotating speed increased from 10 rpm to 20 rpm, the average fiber diameter decreased from 60 μm to 25 μm, because the increase of rotating speed of disk collector could increase tensile force of the jet or fiber between collector and spinneret, and thus decrease the fiber diameter.

Fig. 6 Images of as-spun PU fibers under 10 rpm of rotating speed of collector, (a1) and (a2) are optical and SEM pictures, respectively. (b1) and (b2) are optical and SEM pictures, respectively, for fibers e-spun under 20 rpm. The extender was PEG-400 and chain-extending time was 110 min. The-spinning voltage was 25 kV.

However, the increase of rotating speed may decrease thermal radiation time on the jet in the hot spinning area and thus affect the fiber solidification. If the thermal radiation on the jet is not enough, the fiber solidification may be not perfect, and cross-linked fibers can be observed (Fig. 6b2).

3.5 FT-IR and GPC analysis

As shown in Fig. 7, the strong absorption peak at 2270 cm⁻¹ belongs to NCO group, and stretching vibration of C=O carbonyl of urethane is at 1725 cm⁻¹. The breathing vibration of benzene ring asymmetrically substituted is assigned at ∼1600 cm⁻¹ and ∼1510 cm⁻¹, which is a medium intensity peak. The absorption band near to 3400 cm⁻¹ is attributed to –OH of PEG-400. Compared with the raw materials, the NCO group was disappeared completely in the as-spun fibers, and –OH group also consumed up, which demonstrates that the jet was solidified into fibers. The average molecular weight of the as-spun PU fiber was about 50 000 based on the GPC spectrum (Fig. S2†).

Fig. 7 The FT-IR spectra of PEG-400, PU prepolymer and e-spun PU fibers.

Compared with that of prepolymer (Fig. S1†), it can be also concluded that the oligomer and remained MDI in the prepolymer reacted with chain extender completely and converted into solid PU fiber.

3.6 Application: preparation of conductive PU/PANi fibers

PU/PANi is an interesting organic composite material, which can be applied in anticorrosion,³⁷ antistatic applications,³⁸ electromagnetic shielding,³⁹ and others.⁴⁰ In this article, COOH group (DMPA) is introduced into PU chain (PU–COOH) to improve the polarity and hydrophilic capacity of the resultant fibers. The contact angle of fibrous mats was measured in static water contact angle measurement.

As shown in Fig. S3,† the contact angle is 110.9° for pure PU fibers and 92.7° for PU–COOH fibers, indicating that –COOH groups on the surface of PU–COOH fibers can improve hydrophilicity of the fibers. On the other hand, –COOH groups on the fibers' surface can be used as an organic acid dopant for in situ polymerization of aniline, as shown in Scheme 3, which is different from the previous application as blends of PU and PANi.^37–39 Thanks to COOH groups on the surface of PU–COOH fibers, PANi via in situ polymerization may be anchored onto the surface to form a nanostructured PANi coating (Fig. S4†), which supports the electrical conductivity of composite fibers.

Scheme 3 Preparation of conductive PU/PANi fibers: 4 wt% of DMPA was employed as a part of extender to afford PU–COOH fibers, and then the as-spun fibers were immersed into the water solution of aniline, APS and CSA to fabricate PU/PANi composite fibers via in situ polymerization of aniline.

The PU/PANi fibers containing –COOH groups were fabricated and taken from solution of aniline via in situ polymerization. As a control experiment, the e-spun pure PU fibers were also immersed into in situ polymerization solution of aniline under similar conditions. After washed and dried, the appearance of PU and PU–COOH fibers became grey and deep green, respectively. As presented in Table 3, the conductivity of pure PU fibers bearing PANi is about 10⁻⁸ S cm⁻¹. For comparison, the deep green PU–COOH fibers bearing PANi (PU/PANi) exhibit high conductivity of 10⁻² S cm⁻¹, over six orders of magnitude increased. As shown in Fig. 8b, less amount of PANi is deposited on the pure PU fibers than PU–COOH fibers (Fig. 8a).

Table 3 The color and conductivity of as-prepared fibers

Type of fibers	Before bearing PANi		After bearing PANi
	Color	Cond./S cm^−1	Color	Cond./S cm^−1
Pure PU	White	10^−14	Grey	10^−8
PU–COOH	White	10^−14	Deep green	10^−2

---

Fig. 8 Morphologies of e-spun PU fibers after in situ oxidative polymerization of aniline, SEM pictures for (a): PU–COOH bearing PANi (PU/PANi) and (b) pure PU bearing PANi.

The structure of the PU/PANi composite fibers was further identified by FT-IR spectroscopy, as shown in Fig. 9. The stretching vibration of carbonyl group (C=O) in DMPA (COOH) is at 1689 cm⁻¹.⁴¹ The peak of 1718 cm⁻¹ (Fig. 9a) is attributed to the carbonyl group (C=O) in PU (–NH–COO–), which is overlapped with COOH in PU–COOH (Fig. 9b) at 1685 cm⁻¹ and further with acid dopant of CSA at 1687 cm⁻¹ (Fig. 9d). There are broad absorbing bands at 3200 to 2400 cm⁻¹ in the FT-IR spectrum of PU/PANi fibers (Fig. 9d), which are assigned to stretching vibration of OH, NH and CH.

Fig. 9 FT-IR spectra of e-spun fibers of (a) pure PU (PEG-400 as extender), (b) PU–COOH (PEG-400 + DMPA as extender), (c) pure PANi and (d) PU/PANi (PU–COOH bearing PANi).

4. Conclusion

In summary, PU microfibers were produced by a solvent-free thermal assisted e-spinning process, in which the precursor solution was prepared by a homemade PU prepolymer and chain extender. Differing from traditional PU recipes that mid-M_w polyol and isocyanate are used to produce prepolymer and then small molecular polyalcohols are used as chain extender, in this work, PU e-spinning precursor was prepared by prepolymer that low-M_w polyols (PTMG-500) reacted with isocyanate and other low-M_w polyols (PEG-400) acted as chain extender. The viscosity of precursor was lowered in absence of any solvent and thus was suitable for solvent-free thermal assisted e-spinning. When the viscosity rose to 1000 dPa s, the resultant fibers were solidified completely. While the viscosity increased to more than 2500 dPa s, the e-spinning process couldn't continue. The increase of rotating speed of disk collector (from 10 rpm to 20 rpm) could decrease the average fiber diameter (from 60 μm to 25 μm). In addition, PU fibers containing –COOH favored in situ polymerization of nanostructured PANi on fiber surface. Thus, the resultant PU/PANi composite fibers could reach an electrical conductivity of ∼10⁻² S cm⁻¹. The above results indicate that the solvent-free thermal assisted e-spinning of PU and PU/PANi microfibers may have interesting application in functional composite fibers because the recipe of PU is easily adjustable. The resulting fibers may be employed as special fabrics, devices and sensors.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51673103 and 51373082) and the Taishan Scholars Programme of Shandong Province, China (ts20120528).

Notes and references

1. C. Huang, S. J. Soenen, J. Rejman, B. Lucas, K. Braeckmans, J. Demeester and S. C. De Smedt, Chem. Soc. Rev., 2011, 40, 2417–2434.
2. L. Persano, A. Camposeo, C. Tekmen and D. Pisignano, Macromol. Mater. Eng., 2013, 298, 504–520.
3. J. W. Jung, C. L. Lee, S. Yu and I. D. Kim, J. Mater. Chem. A, 2016, 4, 703–750.
4. S. Cavaliere, S. Subianto, I. Savych, D. J. Jones and J. Rozière, Energy Environ. Sci., 2011, 4, 4761–4785.
5. Y. Z. Long, M. M. Li, C. Z. Gu, M. X. Wan, J. L. Duvail, Z. W. Liu and Z. Y. Fan, Prog. Polym. Sci., 2011, 36, 1415–1442.
6. S. Zhang, W. S. Shim and J. Kim, Mater. Des., 2009, 30, 3659–3666.
7. K. M. Yun, C. J. Hogan, Y. Matsubayashi, M. Kawabe, F. Iskandar and K. Okuyama, Chem. Eng. Sci., 2007, 62, 4751–4759.
8. S. Lee and S. K. Obendorf, Text. Res. J., 2007, 77, 696–702.
9. F. E. Ahmed, B. S. Lalia and R. Hashaikeh, Desalination, 2015, 356, 15–30.
10. D. P. Lin, H. W. He, Y. Y. Huang, W. P. Han, G. F. Yu, X. Yan, Y. Z. Long and L. H. Xia, J. Mater. Chem. C, 2014, 2, 8962–8966.
11. B. Ding, M. Wang, X. Wang, J. Yu and G. Sun, Mater. Today, 2010, 13, 16–27.
12. D. J. Yang, I. Kamienchick, D. Y. Youn, A. Rothschild and I. Kim, Adv. Funct. Mater., 2010, 20, 4258–4264.
13. S. C. Xu, C. C. Qin, M. Yu, R. H. Dong, X. Yan, H. Zhao, W. P. Han, H. D. Zhang and Y. Z. Long, Nanoscale, 2015, 7(29), 12351–12355.
14. T. J. Sill and H. A. von Recum, Biomaterials, 2008, 29, 1989–2006.
15. F. Yang, R. Murugan, S. Wang and S. Ramakrishna, Biomaterials, 2005, 26, 2603–2610.
16. S. L. Liu, Y. Z. Long, Y. Y. Huang, H. D. Zhang, H. W. He, B. Sun, Y. Q. Sui and L. H. Xia, Polym. Chem., 2013, 4, 5696–5700.
17. J. M. Deitzel, J. Kleinmeyer, D. Harris and N. C. Beck Tan, Polymer, 2001, 42, 261–272.
18. E. Zhmayev, D. Cho and Y. L. Joo, Polymer, 2010, 51, 4140–4144.
19. R. J. Deng, Y. Liu, Y. Ding, P. C. Xie, L. Luo and W. M. Yang, J. Appl. Polym. Sci., 2009, 114, 166–175.
20. P. D. Dalton, K. Klinkhammer, J. Salber, D. Klee and M. Möller, Biomacromolecules, 2006, 7, 686–690.
21. C. C. Qin, X. P. Duan, L. Wang, L. H. Zhang, M. Yu, R. H. Dong, X. Yan, H. W. He and Y. Z. Long, Nanoscale, 2015, 7, 16611–16615.
22. L. H. Zhang, X. P. Duan, X. Yan, M. Yu, X. Ning, Y. Zhao and Y. Z. Long, RSC Adv., 2016, 6, 53400–53414.
23. Y. Z. Long, H. W. He, L. Wang, L. H. Zhang, X. P. Duan, R. H. Dong, C. C. Qin, H. Zhao, and L. H. Xia, An UV-curing solventless electrospinning apparatus, China Patent, ZL201520643134.5, 2015.
24. N. Levit and G. Tepper, J. Supercrit. Fluids, 2004, 31, 329–333.
25. J. D. Kakisis, C. D. Liapis, C. Breuer and B. E. Sumpio, J. Vasc. Surg., 2005, 41, 349–354.
26. J. Y. Tan, C. K. Chua, K. F. Leong, K. S. Chian, W. S. Leong and L. P. Tan, Biotechnol. Bioeng., 2012, 109, 1–15.
27. W. Steuber, Elastic filaments of linear segmented polymers, US Pat., 2929804 (A), 1960.
28. H. Rinke, Angew. Chem., Int. Ed., 1962, 1, 419–424.
29. M. S. Khil, D. I. Cha, H. Y. Kim, I. S. Kim, I. S. Kim and N. Bhattarai, J. Biomed. Mater. Res., Part B, 2003, 67(2), 675–679.
30. Y. K. Kang, C. H. Park, J. Kim and T. J. Kang, Fibers Polym., 2007, 8, 564–570.
31. S. A. Guelcher, Tissue Eng., Part B, 2008, 14, 3–17.
32. M. M. Demir, I. Yilgor, E. Yilgor and B. Erman, Polymer, 2002, 43, 3303–3309.
33. S. Y. Lee, J. S. Lee and B. Kim, Polym. Int., 1997, 42, 67–76.
34. H. Du, Y. H. Zhao, Q. F. Li, J. W. Wang, M. Q. Kang, X. K. Wang and H. W. Xiang, J. Appl. Polym. Sci., 2008, 110, 1396–1402.
35. Y. L. Zhang, L. S. Shao, D. Y. Dong and Y. H. Wang, RSC Adv., 2016, 6, 17163–17171.
36. G. Anzuino, A. Pirro, O. Rossi and L. Polo Friz, J. Polym. Sci., Polym. Chem. Ed., 1975, 13, 1657–1666.
37. C. H. Chen, Y. T. Kan, C. F. Mao, W. T. Liao and C. D. Hsieh, Surf. Coat. Technol., 2013, 231, 71–76.
38. T. Jeevananda and Siddaramaiah, Eur. Polym. J., 2003, 39, 569–578.
39. K. Lakshmi, H. John, K. T. Mathew, R. Joseph and K. E. George, Acta Mater., 2009, 57, 371–375.
40. J. Njuguna and K. Pielichowski, J. Mater. Sci., 2004, 39, 4081–4094.
41. SDBS No.15340, http://www.sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi.

---

Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21882b

‡ These authors contributed to this work equally.

---