Journal of Colloid and Interface Science 421 (2014) 49–55
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Fluorinated hyperbranched polyurethane electrospun nanofibrous membrane: Fluorine-enriching surface and superhydrophobic state with high adhesion to water Fei Zheng a, Hongtao Deng a,⇑, Xinjun Zhao b, Xia Li a, Can Yang a, Yunyan Yang a, Aidong Zhang a,⇑ a b
College of Chemistry, Central China Normal University, Wuhan 430079, PR China College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 16 November 2013 Accepted 9 January 2014 Available online 25 January 2014 Keywords: Polyurethane Nanofibrous membrane Superhydrophobicity High adhesion
a b s t r a c t The fluorination of hyperbranched polyurethane (HPU) was achieved by atom transfer radical grafting polymerization (ATRgP) of dodecafluoroheptyl methacrylate that was initiated from 2-bromoisobutyryl bromide-modified end groups of HPU. The nanofibrous membrane of fluorinated HPU was prepared by electrospinning. The structure of fluorinated HPU was characterized by Fourier-transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance spectrum (1H NMR). The surface of nanofibrous membrane was investigated with scanning electron microscope (SEM), atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and water contact angle (WCA) analysis, respectively. The results suggested that compared with the reported linear fluorine-containing polyurethane materials, rather high fluorine content up to 29.14% was achieved on the surface of fluorinated HPU nanofibrous membrane. Meanwhile, a superhydrophobic surface (WCA 159.7°) with high adhesion to water was successfully fabricated via a convenient electrospinning process. The prepared material is promising for the application in microfluidic devices. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Polyurethane (PU) with excellent practical properties and tailorable molecular structure has been given much significance as one of the most versatile materials [1]. Recently, the superhydrophobic PU material has aroused great interest for its potential applications in antifouling coating, surface biocompatible biomaterial, separation membrane, etc. [2–4]. The fluorine-containing compounds with low surface tension and friction coefficient play an important role in fabricating superhydrophobic surface (water contact angle, WCA > 150°) [2–5]. Hence the introduction of fluoro moiety into PU materials is always regarded as one of the most effective ways to prepare superhydrophobic PU [2–4]. The surface and bulk fluorination are the common approaches to producing superhydrophobic PU materials. For surface fluorination, the fluoro moiety was introduced onto PU material surface to get a desirable superhydrophobicity. Although the favorable fluorine enrichment on surface can be achieved, the post-treatment on the surface of as-prepared materials was indeed cumbersome and sometimes ⇑ Corresponding authors. Fax: +86 27 88066253. E-mail addresses: [email protected] (H. Deng), [email protected]. edu.cn (A. Zhang). http://dx.doi.org/10.1016/j.jcis.2014.01.009 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
costly especially for massive production. Therefore, most of the relevant researches focused on the fluoro-modification of PU bulk. The bulk fluorination of PU was realized mainly via incorporating fluoro moiety into PU’s soft segments (e.g. perfluoro or semifluoro-polyether or polyester), hard segments (e.g. fluoro-polyisocyanate), chain extenders and end-capping structures. In these cases, the fluorine content on surface depended on the migration and surface enrichment of fluoro moieties in material. These processes were rather complicated and impacted by the molecular or aggregating structures of material as well as environments. In practice, the bulk-fluorinated PU materials often encountered the problems of low content or even depletion of fluorine on surface, which hindered fluoro moieties contributing to surface superhydrophobicity [5]. Therefore, it is necessary to design and fabricate specific molecular structure of PU to facilitate the fluorine enrichment on surface in favor of superhydrophobicity. On the other hand, the adhesive behavior of water droplet on surface is also a vital aspect of superhydrophobicity. Two distinct superhydrophobic phenomena in nature, namely ‘‘lotus effect’’ with low adhesion to water and ‘‘petal effect’’ with high adhesion to water, have been well-known [6]. The surface of lotus leaf composed of microscopic cells and nanoscopic wax crystals can prevent water from penetrating into the interstitial porosities. Thus,
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the water droplet easily rolls off the surface with low adhesion exhibiting the self-cleaning behavior. In contrast, for the surface of rose petal, the size of hierarchical micro and nanostructure are larger than that of lotus leaf. Water droplets preferably impregnate the larger-sized grooves. The capillary effect of sealed air pockets between water and substrate greatly contributes to the high adhesion [7–9]. Evidently, compared with the comprehensive and intensive studies of the artificial self-cleaning surfaces, the investigation of the artificial superhydrophobic surface with high adhesion was initiated just several years ago, but the amount of published researches is increasing rapidly [7–22]. In fact, the artificial surface bioinspired by rose petal has been verified to possess the capacity of manipulating the water droplet transfer without mass loss [23]. This kind of surface is promising for its applications in fluidic devices. At present, the reported fabricating methods for this sort of surface mainly included the replication of natural and artificial templates, surface-etching, surface deposition and self-assembly, plasma-processing, etc. Some of these methods were inconvenient and costly, especially for massive production [7–22]. Therefore, it is necessary to further explore more convenient and economical approaches to fabricate the superhydrophobic surface with high adhesion. In this study, a fluorinated hyperbranched polyurethane (HPU) was synthesized by initiating the atom transfer radical grafting polymerization (ATRgP) of dodecafluoroheptyl methacrylate from the initiator-modified end groups of HPU. It was anticipated that the nonlinear fluorinated HPU will facilitate the fluorine enrichment on material surface. Furthermore, electrospinning protocol was chosen to prepare a nanofibrous membrane of HPU. As reported in literatures [24–26], electrospinning is a convenient and economical method that is capable of fabricating the fibrous surface topography on nano- or micro-size and then regulating the surface wettability. So far, some nanofibrous interfaces with superhydrophobic, amphiphilic and stimuli-responsive properties have been fabricated successfully [25,26]. However, the electrospun nanofibrous superhydrophobic surface with high adhesion to water has been scarcely reported. In this study, we expected to explore a convenient and economical electrospinning method to achieve the superhydrophobic surface of HPU with high adhesion to water.
of DMF was dropped slowly into the mixture. The reaction was carried out at room temperature under nitrogen atmosphere for 48 h. The products were precipitated with 600 ml ethyl ether, washed with ethanol, deionized water and ethyl ether in turn and filtered. After desiccation under vacuum at 30 °C for 48 h, white powders were obtained. The fluorination of HPU was performed via ATRgP of DFHM initiated by as-prepared macroinitiator. In detail, 0.43 g of HPU-Br (0.30 mmol Br), 0.043 g (0.30 mmol) of CuBr and 0.094 g (0.60 mmol) of BPY were dissolved in 12 ml of DMF in a three-neck round flask. After deoxygenation by three freeze-pump-thaw cycles, 6.0 g of the monomer DFHM was added. The polymerization was performed under nitrogen atmosphere at 70 °C. The polymerization time was 1 h for HPU with longer grafted chain (HPU-L-F) and 0.5 h for HPU with shorter grafted chain (HPU-S-F), respectively. Afterward the mixture was cooled, diluted by 50 ml of DMF and passed through the neutral alumina column to remove the residual catalysts. The filtrate was concentrated under reduced pressure to remove most of solvents. The residue was poured into 100 ml of ethyl ether. The precipitates were filtered, washed with ethanol, deionized water and ethyl ether in turn, and dried under vacuum at 30 °C for 48 h. White powders were collected. 2.3. Preparation of nanofibrous membrane An electrospinning device constructed in our lab was applied to prepare the nanofibrous membrane of fluorinated HPU. In detail, 1.19 g of fluorinated HPU and 0.06 g of PEO were added into 5 ml of DMF, and stirred at 40 °C to complete dissolution. The homogenous polymer solution was drawn into a syringe fitted to a needle with a tip diameter of 0.7 mm. A high-voltage DC power generated a potential difference of 20 KV between the syringe tip and an aluminum collector plate. The polymer solution was pumped out from the syringe tip by a syringe pump at a feed rate of 1 ml/h with a tip-to-collector distance of 15 cm. The formed nanofibrous membrane on the collector plate was dried under vacuum at room temperature for 72 h. The nanofibrous membrane of HPU was prepared following the same procedures. The same polymer solution for electrospinning was cast directly onto a clean glass slide and dried under vacuum at 40 °C for 72 h to obtain a nonporous film for the comparative investigation with nanofibrous membrane.
2. Experimental 2.1. Materials 4,40 -Methylenebis(phenyl isocyanate) (MDI), 2-bromoisobutyryl bromide (BIBB) and 2,20 -bipyridyl (BPY) were purchased from Sigma–Aldrich. Copper (I) bromide (CuBr), diethanolamine (DEOA) and N, N0 -dimethyl formide (DMF) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Sinopharm). Dodecafluoroheptyl methacrylate (DFHM) and polyethylene oxide (PEO, Mn 500,000) was offered by Shenshi Chemical Reagent Co., Ltd. (China). DFHM was passed through a neutral alumina column to remove radical inhibitors. DMF was desiccated with anhydrous magnesium sulfate before use. All the other reagents were obtained from Sinopharm. 2.2. Preparation of fluorinated HPU The synthesis of HPU followed a modified synthetic procedure reported by Yan group [26] and described in Supplementary material. The macroinitiator (HPU-Br) was synthesized via the esterification of hydroxyl end groups of HPU with BIBB. In detail, 5 g of HPU (14.07 mmol hydroxyl) was dissolved in 100 ml of DMF followed by adding 0.35 g of triethylamine. The mixture was cooled to 0 °C in ice bath, and then 0.81 g (3.52 mmol) of BIBB in 10 ml
2.4. Characterization Fourier transform infrared spectroscopy (FTIR) (Avatar 360, Thermo Nicolet Co., USA) was used to investigate the differences of chemical structure among HPU, HPU-Br and fluorinated HPU with a standard KBr disk method. The 1H NMR analysis was performed on a 600 MHz nuclear magnetic resonance (NMR) spectrometer (Varian NMR System 600) with DMSO-d6 as solvent and tetramethylsilane (TMS) as internal standard. A field-emission scanning electron microscope (SEM) (Quanta 200, FEI Co., Holland) was applied to observe the surface morphologies of membranes. The membranes were sputter-coated with gold before observation. The membrane surface was also observed by atomic force microscope (AFM) (DI Nanoscope IV, Veeco, USA) in tapping mode. Xray photoelectron spectroscopy (XPS) (VG Multilab 2000, ThermoVG Scientific) analysis of membranes was performed using 300 W Al Ka as radiation source. A take-off angle of 90° was used. The static water contact angles (WCA) of membrane surface were measured by a sessile drop method at 25 °C with a contact angle goniometer (OCA-20, DataPhysics, Germany) equipped with video capture. 2 lL of deionized water droplet was used. Each WCA value was an average of at least five measurements on the different locations on surface.
F. Zheng et al. / Journal of Colloid and Interface Science 421 (2014) 49–55
3. Results and discussion 3.1. Preparation of fluorinated HPU The ATRgP protocol was used to fluorinate HPU. The synthetic route of fluorinated HPU was shown as Scheme 1. The hydroxyl end groups of HPU were esterified with ATRgP initiator BIBB to obtain macroinitiator HPU-Br, which subsequently initiated the graft polymerization of fluorine-containing monomers to form fluorinated HPU. Among the various synthetic methods of HPU reported, those based-on ‘‘AB2’’ type monomer with one isocyanate group (denoted as ‘‘A’’) and two hydroxyl groups (denoted as ‘‘B’’) are commonly used. However, ‘‘AB2’’ monomers are sometimes commercially unavailable and need to be synthesized through some complicated routes. Another synthetic protocol is based on ‘‘A2 + Bn’’ approach, but the combination of ‘‘A2’’ type monomer (e.g. toluene diisocyanate) and ‘‘Bn’’ type monomer (e.g. polyether polyol) often results in gelation during reaction [27]. Yan group has reported an ‘‘A2 + CBn’’ approach to synthesize hyperbranched
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polyurethane with commercially available monomers like MDI and DEOA and so on. This approach ensured that the reaction can be carried out smoothly with one step and avoid gelation [27]. Therefore, in this study, we adopted this approach to prepare HPU. For the preparation of macroinitiator, the esterification percentage of hydroxyl end groups of HPU can be controlled by changing the mole ratio of BIBB to hydroxyl. However, in our experiments, it was discovered that at the relatively higher esterification percentage, the subsequent ATRgP tended to form gelled mixture. This may be attributed to the fact that the densely grafted chains were prone to radical-radical coupling and then gelling [28,29]. So, according to our experience, an esterification percentage of 25% was relatively appropriate. Finally, the as-prepared macroinitiator initiated ATRgP of DFHM monomers. Through changing reaction time, we prepared HPU grafted by longer fluorine-containing chains (HPU-L-F) and shorter ones (HPU-S-F), respectively. The chemical structures of macroinitiator and fluorinated HPU were confirmed by FTIR and 1H-NMR, respectively (see Figs. S1 and S2 in Supplementary material).
Scheme 1. Reaction scheme of the synthesis of fluorinated HPU.
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3.2. Preparation of nanofibrous membranes Electrospinning method was applied to prepare the nanofibrous membranes. To improve the electrospinability of HPU and fluorinated HPU, 5 wt% of PEO was added in the electrospinning solution. As shown in SEM images (Fig. 1(a)–(c)), the morphology of the resultant membranes verified the successful fabrication of nanofibrous surface. 3.3. Superhydrophobicity The hydrophilicity/hydrophobicity on nanofibrous membrane surface was investigated via measuring WCA (h). The results were shown in Fig. 2. It can be seen that HPU exhibited moderate hydrophobicity (h 133.8°) and the fluorination significantly increased its hydrophobicity. WCA increased with increasing length of fluorinecontaining chain. Especially for HPU-L-F, a desirable superhydrophobicity (h 159.7°) was achieved. As known, the fluorinated segments tend to segregate to the polymer–air interface due to their lower surface free energy. The fluorine enrichment on surface will facilitate the enhancement of hydrophobicity [2–5]. To probe the surface chemistry on the as-prepared nanofibrous membranes, XPS analysis were performed. The results in Fig. 3 (a) indicated a strong peak of fluorine (F 1s) for HPU-S-F and HPU-L-F, which further verified the successful introduction of fluorinated segments onto HPU. Furthermore, the data of surface chemical composition listed in Table 1 suggested that the fluorinated HPU, especially HPU-L-F, exhibited rather high surface fluorine content up to 29.14%. The elemental composition herein was probed in the depth of around 10 nm away from surface with a take-off angle of 90°.
This result was superior to most of the reported data in the same depth [2,3,5,30–35]. In contrast to some reported linear fluorinated PU materials with relatively lower fluorine content, even fluorinedepleted surface, the obtained fluorine-enriching surface in our case was really encouraging. He group has also prepared a sort of electrospun nanofibrous membrane based on a linear PU that was modified by perfluoropolyether alcohol [5]. XPS analysis revealed the depletion of fluorine on surface. They explained this phenomenon as that the polymers aggregated in electrospinning solution with perfluoropolyether segments wrapped in hydrogen segments due to the relatively poorer solubility of fluorinated segments in solvent. During electrospinning, the volatilization of solvent in jet was too fast for perfluoropolyether segments to be unwrapped and move to the outer surface of fibers. Then the structure of aggregates in electrospinning solution was ‘‘frozen’’ in the formed fibers. However, in our case, we can presumably think that the fluorine-containing polymer chains, which were grafted from the end of HPU, existed on the outer layer of molecular structure wrapping the rest section. It was more convenient for them to stretch out and enrich on surface. In other words, the hyperbranched structure of HPU endowed fluoro-chains with the capability of surface enrichment. This may be the main reason for the high surface fluorine content of HPU-L-F. The fluorine-enriching state on surface may be depicted in Fig. 3 (b). In addition, it also can be noted that the length of grafted fluorine-containing chains has obvious impact on the hydrophobicity and fluorine content on surface. The longer grafted chains seem more prone to enrich on surface compared with the shorter ones, and thus gave rise to higher surface fluorine content and also higher WCA. Certainly, it is well known that superhydrophobicity lies on not only surface
(a)
(b)
200 nm
5 µm 4 3 2 1
(c)
(d)
Fig. 1. SEM images of HPU (a), HPU-S-F (b) and HPU-L-F (c) nanofibrous membranes and three-dimensional AFM image of HPU-L-F nanofibrous membranes (d) (SEM magnification: 3000, inset:10,000, scale bar: 1 lm).
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149.6
133.8
(a)
159.7
(b)
(c)
114.9
131.3
119.2
(d)
(e)
(f)
Fig. 2. Photographs of WCA on the surface of nanofibrous membrane of HPU(a), HPU-S-F(b) and HPU-L-F(c), respectively, as well as casting film of HPU(d), HPU-S-F(e) and HPU-L-F(f), respectively. The insets are the value of WCA corresponding to each photograph.
Counts/s
F 1s
a: HPU b: HPU-S-F c: HPU-L-F
c
O 1s C 1s N 1s
b a
1000
800
600
400
200
0
Binding Energy (ev)
(a)
Fluorine-containing grafted chain
Hyperbranched polyurethane
(b) Fluorine-enriching membrane surface Fig. 3. XPS of HPU, HPU-S-F and HPU-L-F nanofibrous membranes (a) and schematic representative for fluorine-enriching membrane surface (b).
chemistry but surface micro or nanostructure [8–10,14,16,18,20– 22]. To elucidate the effects of surface topography on the wettability, the casting films with relatively smooth surface were prepared from the solution with the same composition, and their WCA were also investigated for a comparative study. As shown in Fig. 2, the distinct decrease in WCA was observed for casting film in contrast to the corresponding nanofibrous membrane. The general interpre-
tation is that compared with smooth casting film surface, the roughness, exactly topography of nanofibrous surface (see Fig. 1) enhanced the original hydrophobic tendencies, namely a physical amplification of the chemistry of surface. It can increase contact angle well beyond that possible by chemistry alone [36]. Concretely, changing water contacting surfaces from smooth to rough actually promoted the transition of contact state from wetting
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Table 1 XPS analysis of chemical composition on the surface of HPU, HPU-S-F and HPU-L-F nanofibrous membranes. Nanofibrous membranes
HPU HPU-S-F HPU-L-F
Surface chemical composition (wt%) C 1s
O 1s
N 1s
F 1s
Br 3d
77.04 65.34 50.58
15.61 17.08 16.82
7.35 2.91 3.24
0 14.48 29.14
0 0.19 0.23
Wenzel mode to non-wetting Cassie mode. Many experiments have affirmed that for a droplet in Cassie mode, it is possible to obtain contact angle quite higher than droplet in Wenzel mode [2,8]. Therefore, it is conceivable that both of the high fluorine content and nanofibrous topography on surface lead to the resultant superhydrophobicity. 3.4. High adhesion to water It was interestingly discovered that the superhydrophobic surface of HPU-L-F nanofibrous membrane exhibited high adhesion to water. Generally, the property of water droplet adhesion on surface can be assessed by sliding angle or contact angle hysteresis as water droplet just starts to roll off a gradually tilting surface. Herein the water droplet did pin firmly on the surface as the substrate was tilted vertically and even turned upside down. The phenomenon was intuitively demonstrated in virtue of the photographs of water droplet pinning on the overturned membrane surface shown in Fig. 4. Furthermore, we also attempted to investigate the effects of droplet volume on its adhesion in order to assess the adhesive property in a quantitative way [15–17,19]. The volume of water droplet on overturned membrane was increased by an increment of 5 ll and the maximum value up to 20 ll was observed. The fact that the high adhesion to water occurred on such a surface with high fluorine content may be attributed to the surface micro and nanostructure. SEM images suggested that HPU exhibited a typical nanofibrous structure with relatively uniform fiber diameter (around 300–500 nm) (Fig. 1 (a)). However, as shown in SEM image of HPU-L-F (Fig. 1 (c)), the fibers highly adhered to each other and formed a surface composed of wider fibrous bands and pores among them. We assumed that the relatively poor solubility of HPU-L-F in DMF solvent gave rise to easier phase-separation
2 µL
5 µL
between polymer and solvent in electrospinning jet, which prompted the fast volatilization of solvent and subsequent ‘‘freezing’’ of material shape. Under this condition, polymers had no enough time to be stretched thoroughly by electric field force so that wider fibrous bands were observed. Further, the three-dimensional AFM image of HPU-L-F suggested that both the fibrous bands and pores among them were on microscale rather than nanoscale (Fig. 1 (d)). Therefore, it is conceivable that the wide fibrous bands offered a large contacting area between substrate and water droplet, which facilitated the interaction of van der Waals forces between them in favor of high adhesion to water. On the other hand, the water on the surface was allowed to impregnate the larger-size micro-pores among fibrous bands based on a Cassie impregnating wetting state [8]. This is similar to the water impregnating the large-size grooves on rose petal [7–9]. The strong capillary forces between water and micro-pores greatly contributed to the high adhesion. In addition, we also investigated the water adhesive property of HPU-S-F with shorter grafted chains, and no similar high adhesion phenomenon was observed on its surface. The SEM image of HPU-S-F (Fig. 1 (b)) suggested a sort of partly aggregating fibers, which lead to less uniform structure than HPU. However, compared with HUP-L-F, no marked fibrous band structure was observed in HUP-S-F membrane. In fact, HPU-S-F was more soluble in DMF solvent than HPU-L-F, thus only limited phase-separation between polymer and solvent existed in electrospinning jet. As a result, the nonuniform fiber rather than fibrous band structure was formed in HPU-S-F membrane. This phenomenon further implied that the fibrous band structure in larger-size played a key role in high adhesion to water.
4. Conclusion A fluorinated HPU nanofibrous membrane was prepared successfully. Compared with the reported linear fluorinated PU materials, the stretching-out fluorine-containing polymer chains of fluorinated HPU greatly contributed to the fluorine enrichment on surface. The high surface fluorine content up to 29.14%, coupled with the nanofibrous surface topography, contributed to a superhydrophobic surface with WCA of 159.7°. Meanwhile, the high adhesion to water was achieved via a convenient electrospinning method. The as-prepared PU nanofibrous membrane is promising for the potential application in microfluidic devices.
10 µL
15 µL
20 µL
Fig. 4. Photographs of water droplet adhered on the surface of HPU-L-F nanofibrous membrane. The insets are the volume of water droplet.
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Acknowledgment The authors gratefully acknowledge the financial support of the Natural Science Foundation of Hubei Province of China (No. 2011CDB158). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.01.009. References [1] E. Delebecq, J.P. Pascault, B. Boutevin, F. Ganachaud, Chem. Rev. 113 (2013) 80. [2] W.L. Wu, Q.Z. Zhu, F.L. Qing, C.C. Han, Langmuir 25 (2009) 17. [3] D.S. Tan, X.Q. Zhang, J.H. Li, H. Tan, Q. Fu, J. Biomed. Mater. Res. Part A 100A (2012) 380. [4] N. Wang, K. Burugapalli, W.H. Song, J. Halls, F. Moussy, Y.D. Zheng, Y.X. Ma, Z.T. Wu, K. Li, J. Membr. Sci. 427 (2013) 207. [5] W.L. Wu, G.C. Yuan, A.H. He, C.C. Han, Langmuir 25 (2009) 3178. [6] K. Uchida, N. Nishikawa, N. Izumi, S. Yamazoe, H. Mayama, Y. Kojima, S. Yokojima, S. Nakamura, K. Tsujii, M. Irie, Angew. Chem. Int. Ed. 49 (2010) 5942. [7] N.D. Boscher, D. Duday, S. Verdier, P. Choquet, ACS Appl. Mater. Interfaces 5 (2013) 1053. [8] L. Feng, Y.N. Zhang, J.M. Xi, Y. Zhu, N. Wang, F. Xia, L. Jiang, Langmuir 24 (2008) 4114. [9] J.M. Xi, L. Jiang, Ind. Eng. Chem. Res. 47 (2008) 6354. [10] M.H. Jin, X.J. Feng, L. Feng, T.L. Sun, J. Zhai, T.J. Li, L. Jiang, Adv. Mater. 17 (2005) 1977.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Fluorinated hyperbranched polyurethane electrospun nanofibrous membrane: Fluorine-enriching surface and superhydrophobic state with high adhesion to water Fei Zheng a, Hongtao Deng a,⇑, Xinjun Zhao b, Xia Li a, Can Yang a, Yunyan Yang a, Aidong Zhang a,⇑ a b
College of Chemistry, Central China Normal University, Wuhan 430079, PR China College of Chemistry and Materials Science, South-Central University for Nationalities, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 16 November 2013 Accepted 9 January 2014 Available online 25 January 2014 Keywords: Polyurethane Nanofibrous membrane Superhydrophobicity High adhesion
a b s t r a c t The fluorination of hyperbranched polyurethane (HPU) was achieved by atom transfer radical grafting polymerization (ATRgP) of dodecafluoroheptyl methacrylate that was initiated from 2-bromoisobutyryl bromide-modified end groups of HPU. The nanofibrous membrane of fluorinated HPU was prepared by electrospinning. The structure of fluorinated HPU was characterized by Fourier-transform infrared spectroscopy (FTIR) and 1H nuclear magnetic resonance spectrum (1H NMR). The surface of nanofibrous membrane was investigated with scanning electron microscope (SEM), atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS) and water contact angle (WCA) analysis, respectively. The results suggested that compared with the reported linear fluorine-containing polyurethane materials, rather high fluorine content up to 29.14% was achieved on the surface of fluorinated HPU nanofibrous membrane. Meanwhile, a superhydrophobic surface (WCA 159.7°) with high adhesion to water was successfully fabricated via a convenient electrospinning process. The prepared material is promising for the application in microfluidic devices. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Polyurethane (PU) with excellent practical properties and tailorable molecular structure has been given much significance as one of the most versatile materials [1]. Recently, the superhydrophobic PU material has aroused great interest for its potential applications in antifouling coating, surface biocompatible biomaterial, separation membrane, etc. [2–4]. The fluorine-containing compounds with low surface tension and friction coefficient play an important role in fabricating superhydrophobic surface (water contact angle, WCA > 150°) [2–5]. Hence the introduction of fluoro moiety into PU materials is always regarded as one of the most effective ways to prepare superhydrophobic PU [2–4]. The surface and bulk fluorination are the common approaches to producing superhydrophobic PU materials. For surface fluorination, the fluoro moiety was introduced onto PU material surface to get a desirable superhydrophobicity. Although the favorable fluorine enrichment on surface can be achieved, the post-treatment on the surface of as-prepared materials was indeed cumbersome and sometimes ⇑ Corresponding authors. Fax: +86 27 88066253. E-mail addresses: [email protected] (H. Deng), [email protected]. edu.cn (A. Zhang). http://dx.doi.org/10.1016/j.jcis.2014.01.009 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
costly especially for massive production. Therefore, most of the relevant researches focused on the fluoro-modification of PU bulk. The bulk fluorination of PU was realized mainly via incorporating fluoro moiety into PU’s soft segments (e.g. perfluoro or semifluoro-polyether or polyester), hard segments (e.g. fluoro-polyisocyanate), chain extenders and end-capping structures. In these cases, the fluorine content on surface depended on the migration and surface enrichment of fluoro moieties in material. These processes were rather complicated and impacted by the molecular or aggregating structures of material as well as environments. In practice, the bulk-fluorinated PU materials often encountered the problems of low content or even depletion of fluorine on surface, which hindered fluoro moieties contributing to surface superhydrophobicity [5]. Therefore, it is necessary to design and fabricate specific molecular structure of PU to facilitate the fluorine enrichment on surface in favor of superhydrophobicity. On the other hand, the adhesive behavior of water droplet on surface is also a vital aspect of superhydrophobicity. Two distinct superhydrophobic phenomena in nature, namely ‘‘lotus effect’’ with low adhesion to water and ‘‘petal effect’’ with high adhesion to water, have been well-known [6]. The surface of lotus leaf composed of microscopic cells and nanoscopic wax crystals can prevent water from penetrating into the interstitial porosities. Thus,
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the water droplet easily rolls off the surface with low adhesion exhibiting the self-cleaning behavior. In contrast, for the surface of rose petal, the size of hierarchical micro and nanostructure are larger than that of lotus leaf. Water droplets preferably impregnate the larger-sized grooves. The capillary effect of sealed air pockets between water and substrate greatly contributes to the high adhesion [7–9]. Evidently, compared with the comprehensive and intensive studies of the artificial self-cleaning surfaces, the investigation of the artificial superhydrophobic surface with high adhesion was initiated just several years ago, but the amount of published researches is increasing rapidly [7–22]. In fact, the artificial surface bioinspired by rose petal has been verified to possess the capacity of manipulating the water droplet transfer without mass loss [23]. This kind of surface is promising for its applications in fluidic devices. At present, the reported fabricating methods for this sort of surface mainly included the replication of natural and artificial templates, surface-etching, surface deposition and self-assembly, plasma-processing, etc. Some of these methods were inconvenient and costly, especially for massive production [7–22]. Therefore, it is necessary to further explore more convenient and economical approaches to fabricate the superhydrophobic surface with high adhesion. In this study, a fluorinated hyperbranched polyurethane (HPU) was synthesized by initiating the atom transfer radical grafting polymerization (ATRgP) of dodecafluoroheptyl methacrylate from the initiator-modified end groups of HPU. It was anticipated that the nonlinear fluorinated HPU will facilitate the fluorine enrichment on material surface. Furthermore, electrospinning protocol was chosen to prepare a nanofibrous membrane of HPU. As reported in literatures [24–26], electrospinning is a convenient and economical method that is capable of fabricating the fibrous surface topography on nano- or micro-size and then regulating the surface wettability. So far, some nanofibrous interfaces with superhydrophobic, amphiphilic and stimuli-responsive properties have been fabricated successfully [25,26]. However, the electrospun nanofibrous superhydrophobic surface with high adhesion to water has been scarcely reported. In this study, we expected to explore a convenient and economical electrospinning method to achieve the superhydrophobic surface of HPU with high adhesion to water.
of DMF was dropped slowly into the mixture. The reaction was carried out at room temperature under nitrogen atmosphere for 48 h. The products were precipitated with 600 ml ethyl ether, washed with ethanol, deionized water and ethyl ether in turn and filtered. After desiccation under vacuum at 30 °C for 48 h, white powders were obtained. The fluorination of HPU was performed via ATRgP of DFHM initiated by as-prepared macroinitiator. In detail, 0.43 g of HPU-Br (0.30 mmol Br), 0.043 g (0.30 mmol) of CuBr and 0.094 g (0.60 mmol) of BPY were dissolved in 12 ml of DMF in a three-neck round flask. After deoxygenation by three freeze-pump-thaw cycles, 6.0 g of the monomer DFHM was added. The polymerization was performed under nitrogen atmosphere at 70 °C. The polymerization time was 1 h for HPU with longer grafted chain (HPU-L-F) and 0.5 h for HPU with shorter grafted chain (HPU-S-F), respectively. Afterward the mixture was cooled, diluted by 50 ml of DMF and passed through the neutral alumina column to remove the residual catalysts. The filtrate was concentrated under reduced pressure to remove most of solvents. The residue was poured into 100 ml of ethyl ether. The precipitates were filtered, washed with ethanol, deionized water and ethyl ether in turn, and dried under vacuum at 30 °C for 48 h. White powders were collected. 2.3. Preparation of nanofibrous membrane An electrospinning device constructed in our lab was applied to prepare the nanofibrous membrane of fluorinated HPU. In detail, 1.19 g of fluorinated HPU and 0.06 g of PEO were added into 5 ml of DMF, and stirred at 40 °C to complete dissolution. The homogenous polymer solution was drawn into a syringe fitted to a needle with a tip diameter of 0.7 mm. A high-voltage DC power generated a potential difference of 20 KV between the syringe tip and an aluminum collector plate. The polymer solution was pumped out from the syringe tip by a syringe pump at a feed rate of 1 ml/h with a tip-to-collector distance of 15 cm. The formed nanofibrous membrane on the collector plate was dried under vacuum at room temperature for 72 h. The nanofibrous membrane of HPU was prepared following the same procedures. The same polymer solution for electrospinning was cast directly onto a clean glass slide and dried under vacuum at 40 °C for 72 h to obtain a nonporous film for the comparative investigation with nanofibrous membrane.
2. Experimental 2.1. Materials 4,40 -Methylenebis(phenyl isocyanate) (MDI), 2-bromoisobutyryl bromide (BIBB) and 2,20 -bipyridyl (BPY) were purchased from Sigma–Aldrich. Copper (I) bromide (CuBr), diethanolamine (DEOA) and N, N0 -dimethyl formide (DMF) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Sinopharm). Dodecafluoroheptyl methacrylate (DFHM) and polyethylene oxide (PEO, Mn 500,000) was offered by Shenshi Chemical Reagent Co., Ltd. (China). DFHM was passed through a neutral alumina column to remove radical inhibitors. DMF was desiccated with anhydrous magnesium sulfate before use. All the other reagents were obtained from Sinopharm. 2.2. Preparation of fluorinated HPU The synthesis of HPU followed a modified synthetic procedure reported by Yan group [26] and described in Supplementary material. The macroinitiator (HPU-Br) was synthesized via the esterification of hydroxyl end groups of HPU with BIBB. In detail, 5 g of HPU (14.07 mmol hydroxyl) was dissolved in 100 ml of DMF followed by adding 0.35 g of triethylamine. The mixture was cooled to 0 °C in ice bath, and then 0.81 g (3.52 mmol) of BIBB in 10 ml
2.4. Characterization Fourier transform infrared spectroscopy (FTIR) (Avatar 360, Thermo Nicolet Co., USA) was used to investigate the differences of chemical structure among HPU, HPU-Br and fluorinated HPU with a standard KBr disk method. The 1H NMR analysis was performed on a 600 MHz nuclear magnetic resonance (NMR) spectrometer (Varian NMR System 600) with DMSO-d6 as solvent and tetramethylsilane (TMS) as internal standard. A field-emission scanning electron microscope (SEM) (Quanta 200, FEI Co., Holland) was applied to observe the surface morphologies of membranes. The membranes were sputter-coated with gold before observation. The membrane surface was also observed by atomic force microscope (AFM) (DI Nanoscope IV, Veeco, USA) in tapping mode. Xray photoelectron spectroscopy (XPS) (VG Multilab 2000, ThermoVG Scientific) analysis of membranes was performed using 300 W Al Ka as radiation source. A take-off angle of 90° was used. The static water contact angles (WCA) of membrane surface were measured by a sessile drop method at 25 °C with a contact angle goniometer (OCA-20, DataPhysics, Germany) equipped with video capture. 2 lL of deionized water droplet was used. Each WCA value was an average of at least five measurements on the different locations on surface.
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3. Results and discussion 3.1. Preparation of fluorinated HPU The ATRgP protocol was used to fluorinate HPU. The synthetic route of fluorinated HPU was shown as Scheme 1. The hydroxyl end groups of HPU were esterified with ATRgP initiator BIBB to obtain macroinitiator HPU-Br, which subsequently initiated the graft polymerization of fluorine-containing monomers to form fluorinated HPU. Among the various synthetic methods of HPU reported, those based-on ‘‘AB2’’ type monomer with one isocyanate group (denoted as ‘‘A’’) and two hydroxyl groups (denoted as ‘‘B’’) are commonly used. However, ‘‘AB2’’ monomers are sometimes commercially unavailable and need to be synthesized through some complicated routes. Another synthetic protocol is based on ‘‘A2 + Bn’’ approach, but the combination of ‘‘A2’’ type monomer (e.g. toluene diisocyanate) and ‘‘Bn’’ type monomer (e.g. polyether polyol) often results in gelation during reaction [27]. Yan group has reported an ‘‘A2 + CBn’’ approach to synthesize hyperbranched
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polyurethane with commercially available monomers like MDI and DEOA and so on. This approach ensured that the reaction can be carried out smoothly with one step and avoid gelation [27]. Therefore, in this study, we adopted this approach to prepare HPU. For the preparation of macroinitiator, the esterification percentage of hydroxyl end groups of HPU can be controlled by changing the mole ratio of BIBB to hydroxyl. However, in our experiments, it was discovered that at the relatively higher esterification percentage, the subsequent ATRgP tended to form gelled mixture. This may be attributed to the fact that the densely grafted chains were prone to radical-radical coupling and then gelling [28,29]. So, according to our experience, an esterification percentage of 25% was relatively appropriate. Finally, the as-prepared macroinitiator initiated ATRgP of DFHM monomers. Through changing reaction time, we prepared HPU grafted by longer fluorine-containing chains (HPU-L-F) and shorter ones (HPU-S-F), respectively. The chemical structures of macroinitiator and fluorinated HPU were confirmed by FTIR and 1H-NMR, respectively (see Figs. S1 and S2 in Supplementary material).
Scheme 1. Reaction scheme of the synthesis of fluorinated HPU.
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3.2. Preparation of nanofibrous membranes Electrospinning method was applied to prepare the nanofibrous membranes. To improve the electrospinability of HPU and fluorinated HPU, 5 wt% of PEO was added in the electrospinning solution. As shown in SEM images (Fig. 1(a)–(c)), the morphology of the resultant membranes verified the successful fabrication of nanofibrous surface. 3.3. Superhydrophobicity The hydrophilicity/hydrophobicity on nanofibrous membrane surface was investigated via measuring WCA (h). The results were shown in Fig. 2. It can be seen that HPU exhibited moderate hydrophobicity (h 133.8°) and the fluorination significantly increased its hydrophobicity. WCA increased with increasing length of fluorinecontaining chain. Especially for HPU-L-F, a desirable superhydrophobicity (h 159.7°) was achieved. As known, the fluorinated segments tend to segregate to the polymer–air interface due to their lower surface free energy. The fluorine enrichment on surface will facilitate the enhancement of hydrophobicity [2–5]. To probe the surface chemistry on the as-prepared nanofibrous membranes, XPS analysis were performed. The results in Fig. 3 (a) indicated a strong peak of fluorine (F 1s) for HPU-S-F and HPU-L-F, which further verified the successful introduction of fluorinated segments onto HPU. Furthermore, the data of surface chemical composition listed in Table 1 suggested that the fluorinated HPU, especially HPU-L-F, exhibited rather high surface fluorine content up to 29.14%. The elemental composition herein was probed in the depth of around 10 nm away from surface with a take-off angle of 90°.
This result was superior to most of the reported data in the same depth [2,3,5,30–35]. In contrast to some reported linear fluorinated PU materials with relatively lower fluorine content, even fluorinedepleted surface, the obtained fluorine-enriching surface in our case was really encouraging. He group has also prepared a sort of electrospun nanofibrous membrane based on a linear PU that was modified by perfluoropolyether alcohol [5]. XPS analysis revealed the depletion of fluorine on surface. They explained this phenomenon as that the polymers aggregated in electrospinning solution with perfluoropolyether segments wrapped in hydrogen segments due to the relatively poorer solubility of fluorinated segments in solvent. During electrospinning, the volatilization of solvent in jet was too fast for perfluoropolyether segments to be unwrapped and move to the outer surface of fibers. Then the structure of aggregates in electrospinning solution was ‘‘frozen’’ in the formed fibers. However, in our case, we can presumably think that the fluorine-containing polymer chains, which were grafted from the end of HPU, existed on the outer layer of molecular structure wrapping the rest section. It was more convenient for them to stretch out and enrich on surface. In other words, the hyperbranched structure of HPU endowed fluoro-chains with the capability of surface enrichment. This may be the main reason for the high surface fluorine content of HPU-L-F. The fluorine-enriching state on surface may be depicted in Fig. 3 (b). In addition, it also can be noted that the length of grafted fluorine-containing chains has obvious impact on the hydrophobicity and fluorine content on surface. The longer grafted chains seem more prone to enrich on surface compared with the shorter ones, and thus gave rise to higher surface fluorine content and also higher WCA. Certainly, it is well known that superhydrophobicity lies on not only surface
(a)
(b)
200 nm
5 µm 4 3 2 1
(c)
(d)
Fig. 1. SEM images of HPU (a), HPU-S-F (b) and HPU-L-F (c) nanofibrous membranes and three-dimensional AFM image of HPU-L-F nanofibrous membranes (d) (SEM magnification: 3000, inset:10,000, scale bar: 1 lm).
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149.6
133.8
(a)
159.7
(b)
(c)
114.9
131.3
119.2
(d)
(e)
(f)
Fig. 2. Photographs of WCA on the surface of nanofibrous membrane of HPU(a), HPU-S-F(b) and HPU-L-F(c), respectively, as well as casting film of HPU(d), HPU-S-F(e) and HPU-L-F(f), respectively. The insets are the value of WCA corresponding to each photograph.
Counts/s
F 1s
a: HPU b: HPU-S-F c: HPU-L-F
c
O 1s C 1s N 1s
b a
1000
800
600
400
200
0
Binding Energy (ev)
(a)
Fluorine-containing grafted chain
Hyperbranched polyurethane
(b) Fluorine-enriching membrane surface Fig. 3. XPS of HPU, HPU-S-F and HPU-L-F nanofibrous membranes (a) and schematic representative for fluorine-enriching membrane surface (b).
chemistry but surface micro or nanostructure [8–10,14,16,18,20– 22]. To elucidate the effects of surface topography on the wettability, the casting films with relatively smooth surface were prepared from the solution with the same composition, and their WCA were also investigated for a comparative study. As shown in Fig. 2, the distinct decrease in WCA was observed for casting film in contrast to the corresponding nanofibrous membrane. The general interpre-
tation is that compared with smooth casting film surface, the roughness, exactly topography of nanofibrous surface (see Fig. 1) enhanced the original hydrophobic tendencies, namely a physical amplification of the chemistry of surface. It can increase contact angle well beyond that possible by chemistry alone [36]. Concretely, changing water contacting surfaces from smooth to rough actually promoted the transition of contact state from wetting
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Table 1 XPS analysis of chemical composition on the surface of HPU, HPU-S-F and HPU-L-F nanofibrous membranes. Nanofibrous membranes
HPU HPU-S-F HPU-L-F
Surface chemical composition (wt%) C 1s
O 1s
N 1s
F 1s
Br 3d
77.04 65.34 50.58
15.61 17.08 16.82
7.35 2.91 3.24
0 14.48 29.14
0 0.19 0.23
Wenzel mode to non-wetting Cassie mode. Many experiments have affirmed that for a droplet in Cassie mode, it is possible to obtain contact angle quite higher than droplet in Wenzel mode [2,8]. Therefore, it is conceivable that both of the high fluorine content and nanofibrous topography on surface lead to the resultant superhydrophobicity. 3.4. High adhesion to water It was interestingly discovered that the superhydrophobic surface of HPU-L-F nanofibrous membrane exhibited high adhesion to water. Generally, the property of water droplet adhesion on surface can be assessed by sliding angle or contact angle hysteresis as water droplet just starts to roll off a gradually tilting surface. Herein the water droplet did pin firmly on the surface as the substrate was tilted vertically and even turned upside down. The phenomenon was intuitively demonstrated in virtue of the photographs of water droplet pinning on the overturned membrane surface shown in Fig. 4. Furthermore, we also attempted to investigate the effects of droplet volume on its adhesion in order to assess the adhesive property in a quantitative way [15–17,19]. The volume of water droplet on overturned membrane was increased by an increment of 5 ll and the maximum value up to 20 ll was observed. The fact that the high adhesion to water occurred on such a surface with high fluorine content may be attributed to the surface micro and nanostructure. SEM images suggested that HPU exhibited a typical nanofibrous structure with relatively uniform fiber diameter (around 300–500 nm) (Fig. 1 (a)). However, as shown in SEM image of HPU-L-F (Fig. 1 (c)), the fibers highly adhered to each other and formed a surface composed of wider fibrous bands and pores among them. We assumed that the relatively poor solubility of HPU-L-F in DMF solvent gave rise to easier phase-separation
2 µL
5 µL
between polymer and solvent in electrospinning jet, which prompted the fast volatilization of solvent and subsequent ‘‘freezing’’ of material shape. Under this condition, polymers had no enough time to be stretched thoroughly by electric field force so that wider fibrous bands were observed. Further, the three-dimensional AFM image of HPU-L-F suggested that both the fibrous bands and pores among them were on microscale rather than nanoscale (Fig. 1 (d)). Therefore, it is conceivable that the wide fibrous bands offered a large contacting area between substrate and water droplet, which facilitated the interaction of van der Waals forces between them in favor of high adhesion to water. On the other hand, the water on the surface was allowed to impregnate the larger-size micro-pores among fibrous bands based on a Cassie impregnating wetting state [8]. This is similar to the water impregnating the large-size grooves on rose petal [7–9]. The strong capillary forces between water and micro-pores greatly contributed to the high adhesion. In addition, we also investigated the water adhesive property of HPU-S-F with shorter grafted chains, and no similar high adhesion phenomenon was observed on its surface. The SEM image of HPU-S-F (Fig. 1 (b)) suggested a sort of partly aggregating fibers, which lead to less uniform structure than HPU. However, compared with HUP-L-F, no marked fibrous band structure was observed in HUP-S-F membrane. In fact, HPU-S-F was more soluble in DMF solvent than HPU-L-F, thus only limited phase-separation between polymer and solvent existed in electrospinning jet. As a result, the nonuniform fiber rather than fibrous band structure was formed in HPU-S-F membrane. This phenomenon further implied that the fibrous band structure in larger-size played a key role in high adhesion to water.
4. Conclusion A fluorinated HPU nanofibrous membrane was prepared successfully. Compared with the reported linear fluorinated PU materials, the stretching-out fluorine-containing polymer chains of fluorinated HPU greatly contributed to the fluorine enrichment on surface. The high surface fluorine content up to 29.14%, coupled with the nanofibrous surface topography, contributed to a superhydrophobic surface with WCA of 159.7°. Meanwhile, the high adhesion to water was achieved via a convenient electrospinning method. The as-prepared PU nanofibrous membrane is promising for the potential application in microfluidic devices.
10 µL
15 µL
20 µL
Fig. 4. Photographs of water droplet adhered on the surface of HPU-L-F nanofibrous membrane. The insets are the volume of water droplet.
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