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Superamphiphilic Polyurethane Foams Synergized from Cellulose Nanowhiskers and Graphene Nanoplatelets Xiaotan Zhang, Dongyan Liu, and Guoxin Sui* oil are lower than 10°). In 1997, Fujishima and co-workers[14] first reported the photo generation of a superamphiphilic titanium dioxide surface, producing the microstructure composed of both hydrophilic and oleophilic phases by ultraviolet irradiation. Up to date, two approaches are used to construct superhydrophilic surfaces: one is that some special materials are induced to produce superhydrophilic properties under UV illumination conditions, i.e., photoinduced superhydropholic materials, such as TiO2, ZnO, and SnO2,[15,16] in which the slightly hydrophilic surface turned to highly hydrophilic surface; the other approach is to increase the surface roughness the hydrophilic material or to create a rough surface by assembling hydrophilic coatings, which is similar to the method of preparing superoleophilic surfaces.[17] Surface roughness, defined as the ratio of the actual surface area to the geometric surface area, has a critical effect on the wettability. It can adjust the wetting characteristics of materials,[18] based on the classical Wenzel[19] and Cassie and Baxter[20] models. Graphene,[21] a one-atom-thick planar sheet in a 3D honeycomb network arranged by carbon atoms, has drawn increasing attention due to high surface area, excellent thermal and chemical stability, and outstanding electrical conductivity since the famous 2004 “scotch tape” experiment by Novoselov et al.[22] 3D graphene-based materials[23–25] were extensively studied because of their superhydrophobicity, superior liquid absorption capacity, and excellent stability under harsh chemical and physical conditions. Most 3D graphene assemblies are acquired by reducing graphene oxide (GO), which features hydrophobic and hydrophilic domains, synthesized from graphite powder based on the well-known Hummers’ method.[26] Singh et al.[27] demonstrated that superhydrophobic graphene foams (water contact angle (WCA) approach 163°) were synthesized by using template-directed chemical vapor deposition, which contained pores with the size of several hundreds of micrometers, while the walls of the foam was comprised of few-layer graphene sheets coated with Teflon. Ren et al.[28] reported a robust graphene aerogel (water contact angle close to 150.5°) prepared by resorcinol–formaldehyde (RF) sol–gel chemistry for absorbing oils and organic solvents, and the absorption capacity of various oils and organic solvents was up to 19–26 times of its own weight. Oribayo et al.[29] employed polydopamine-reduced graphene oxide and octadecylamine to modify the surface of
Superamphiphilic materials are attracting significant attention because of their unique affinity to both water and oil. It is generally accepted that graphene-based assemblies are hydrophobic. Here, novel superamphiphilic polyurethane (PU) foams by the synergetic effect of cellulose nanowhiskers (CNWs) and graphene nanosheets (GNs) are reported. This process involves the creation of CNWs’ base coating on the surface of PU network prior to GN coatings to obtain superamphiphilic surfaces by the simple dip-coating method. Wetting behaviors of the foams start to turn from hydrophobicity into superamphiphilicity by increasing the concentration of the CNWs up to 0.5 wt%. The hydrophobicity recovers when the concentration of CNWs reaches 1.0 wt%. The superamphiphilic foam (CNWs’ concentration is 0.75 wt%) exhibits high storage capacity for water and various organic solvents up to 22–24 times of its own weight. Moreover, the coated foams can be reused for absorbing liquids for more than 20 cycles without losing their superamphiphilicity, exhibiting good reusability and durability. The discovery of this superamphiphilic foam is of great significance for the development of superwetting materials and finding their applications in oil emulsions purifying and catalyst anchoring fields.
1. Introduction The technology to tune the surface wetting characteristics of various underlying substrates has been of considerable interests in recent years.[1–3] Nowadays, many scientists focused on searching for the oil–water separation materials with superhydrophobic properties[4–8] or amphiphilic materials, by tailoring surface topographical features, self-assembling of molecular and block copolymers, and coating amphiphilic polymers.[9–13] Superamphiphilic materials become a novel topic of research owing to their superior affinity to both water and oil. Generally, superamphiphilic materials must have both superhydrophilicity and superoleophilicity (contact angles (CAs) for water and X. Zhang, Prof. D. Liu, Prof. G. Sui Institute of Metal Research Chinese Academy of Sciences Shenyang 110016, P. R. China E-mail:
[email protected] X. Zhang School of Materials Science and Engineering University of Science and Technology of China Shenyang 110016, P. R. China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.201701094.
DOI: 10.1002/admi.201701094
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lignin-based polyurethane (LPU) foam to form a superhydrophobic sorbent (water contact angle approach 152°) with a sorption capacity of 26–68 times its own weight. However, superamphiphilic graphene-based materials are rarely reported. Song et al. have fabricated a superamphiphilic multifunctional 3D graphene-based foam by the hydrothermal reaction of graphene with phytic acid, which makes a significant step forward in superamphiphilic property in graphene-based materials.[30] Superamphiphilic materials have a wide range of potential applications, such as the construction of implantable devices,[9] removal of low-density lipoprotein,[31] and improved cycle life of lithium–sulfur batteries.[32] So it is necessary to further explore the preparation methods and performance of superamphiphilic materials. In this work, a novel 3D superamphiphilic polyurethane (PU) foam has been successfully prepared via simple dip-coating 3D porous PU foam with cellulose nanowhiskers (CNWs) and graphene nanoplatelets (GNs) sequentially. The 3D porous PU foam is used as the template because it has mechanical and environmental stability. Commercial graphene powders are generally aggregated graphene nanoplatelets with less than ten layers of carbon atoms, which are difficult to disperse in water to form stable suspensions, making it impossible to further prepare graphene-based assemblies. CNWs,[33] extracted from natural fibers, are abundant biomass materials. It was found that minor addition of CNWs is conducive to graphene dispersion.[34–36] Furthermore, cellulose nanofibers are used to fabricate amphiphilic materials because of coexisted hydrophilic and oleophilic groups.[37,38] Therefore, we integrated 3D structure of PU foam, oleophilic property and rough surface of GNs, and hydrophilic property of CNWs to construct a superamphiphilic foam (graphene@polyurethane foams, CGPFs) through a facile and green dip-coating process.
2. Results and Discussion A higher proportion of researches on absorbing materials are confined to superhydrophobic materials (oil–water separation) or superhydrophilic materials, while we report a novel and facile fabrication method of a multifunctional material that integrates the advantages of both superamphiphilic properties and superior storage capacity. There is a strong binding force between coatings and PU substrate; the strong interactions between PU foam and CNWs are governed by various forces, but intermolecular hydrogen bonding is likely to play the vital role. CNWs are able to bond with PU foam well because of hydroxyl groups in CNWs and amidogen groups in PU foam. On the other hand, CNWs can be very well attached to the graphene sheets due to their high specific surface area. Furthermore, the CGPF inherits excellent properties of the three kinds of raw materials and 3D porous structure of PU foam.
2.1. Macroscopic and Microscopic Structures of PU Foams with Only CNW Coatings (cellulose nanowhiskers@polyurethane foams, CPFs) and CGPFs PU foams with only CNW coatings (CPFs) are tested by the scanning device. As shown in Figure 1, the amount of CNW
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Figure 1. FE-SEM photographs of polyurethane (PU) sponges with different concentrations of CNW coatings: a) CPF-1 (CNWs = 0.25 wt%); b) CPF-2 (CNWs = 0.5 wt%); c) CPF-3 (CNWs = 0.75 wt%); d) CPF-4 (CNWs = 1 wt%).
coatings gradually increases with the increase of CNWs’ concentration, which is different from the neat PU foam with bare surface (Figure 2a). CNW coatings are composed of randomly arranged whiskers at higher magnifications, as shown in the insets of Figure 1. As shown in Figure 2, the morphology of the foams, including the pristine PU foam and as-prepared CGPFs, was examined by field emission scanning electron microscopy (FE-SEM) observations. Compared with the surface of the CPFs (Figure 1), GN coatings cover the interconnected skeleton of the as-prepared foams as the concentration of CNWs is increased from 0.25 to 1.0 wt% (Figure 2b–e). The coexistence of CNW coatings and GN sheets enhances the hierarchical surface roughness, which is a necessary condition for the preparation of amphiphilic surface. After a series of dip-coating processes, the PU foam surface is covered with nanowhiskers and graphene sequentially. Moreover, the PU foam surface is completely covered with CNW coatings, while the GNs do not completely cover the CNW coatings. This means that although most of the surface of CNW coatings is covered by GN sheets, there is still a small amount of CNW coatings exposed to the air. It is also revealed that the morphologies of the as-prepared foams are greatly affected by the concentration of CNWs. The surface roughness of CGPF-4 (Figure 2e) is greater than other as-prepared foams, which indicates that CGPF-4 surface with a greater amount of CNWs adsorbs a greater amount of GNs due to the strong affinity of CNWs to GN (Table 1). A high-magnification SEM image of CGPF-3 (Figure 2h) can be a further proof that CNWs and GN sheets successively cover on PU foam surface. It also demonstrates the strong binding force between CNW coatings and GN sheets. Furthermore, high-resolution transmission X-ray tomography (HRTXRT) of CGPF-3 (Figure 2f,g) reveals the stereomorphology of the CGPF. The coating (CNWs and GN) favors the formation of interconnected network structure with a rough surface. In a word, CGPFs, fabricated via a simple dip-coating method, still maintain the 3D
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Figure 2. FE-SEM images of a) pristine PU foam, b) CGPF-1, c) CGPF-2, d,h) CGPF-3 at various magnification, and e) CGPF-4. f,g) High-resolution transmission X-ray tomography (HRTXRT) images of CGPF-3 at different magnifications.
network structure of PU foam and have synthetic properties of CNW coatings and GN sheets. It is also manifested that varying the concentration of CNWs leads to the different surface morphologies of the as-prepared foams.
2.2. The Interfacial Behavior of CGPFs at Air–Water Interface and Air–Oil Interface To characterize the wetting behavior of the foams, the WCA of the regular PU foams, CNW-coated PU foams, and both CNWs and graphene-coated PU foams were measured. Increasing the concentration of CNWs, the surface color of these foams becomes darker (Figure 3c). As shown in Figure 3a, the pristine PU foams give WCAs of 126.55 ± 2.22°, while the WCAs of different concentrations of CNW-coated foams are 92.75 ± 2.17° (0.25 wt%), 80.73 ± 1.93° (0.5 wt%), 74 ± 2.65° (0.75 wt%), and 49.9 ± 3.03° (1 wt%), respectively. This illustrates that PU foams coated with only CNW coatings (CPFs) change from hydrophobicity into hydrophilicity. The foams become more hydrophilic with the increase of CNWs’ concentration. In order to demonstrate the CNWs’ base coating on the wetting behavior of graphene-coated foams, CGPFs were prepared by dip-coating graphene sheets on the CPFs’ surface. As shown in Figure 3b, Table 1. Coatings density of CGPFs with different concentrations of CNWs. Sample
Coating density [mg cm−3]
CCNWs [wt%] CNWs
GNs
CGPF-1
0.25
1.449 ± 0.048
4.316 ± 0.075
CGPF-2
0.5
3.757 ± 0.101
4.502 ± 0.108
CGPF-3
0.75
5.485 ± 0.163
4.638 ± 0.070
CGPF-4
1.0
6.802 ± 0.202
6.002 ± 0.073
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WCA of CGPF-1 is 113 ± 2.94°, while WCAs of CGPF-2 and CGPF-3 change abruptly into 0° (Movies S1 and S2, Supporting Information). This phenomenon can be explained by Wenzel’ model[39,40] cosθr = rcosθ
(1)
first reported by Wenzel to describe the CA for a liquid droplet at a rough surface, θr and θ are the WCAs on a rough surface and a smooth surface of the same material, respectively, and r is the roughness factor. According to this equation, increasing surface roughness, the actual CA decreases for hydrophilic materials (θ < 90°) and increases for hydrophobic materials (θ > 90°). Therefore, the surfaces of the foams with CNW coatings were coated with GN sheets (rough surface); CGPF-1 possesses strong hydrophobicity, and both CGPF-2 and CGPF-3 show strong hydrophilicity. However, the WCA of CGPF-4 is 104 ± 2.35°, which is due to the fact that the surface has adsorbed a large amount of CNWs, resulting in the adsorption of more graphene than those of other CGPFs. Therefore, the foam surface is totally covered by graphene sheets. The foam mainly shows the hydrophobic properties of GNs, which is the reason why its contact angle changes differently from those of other foams. Figure 3d is a profile photograph of CGPFs; they show no difference in outer appearance. The highlight of our experiment is the fact that we can control the wetting behaviors of CGPFs by adjusting the amount of CNWs. Meanwhile, there are no limitations in substrate size so that we can fabricate any size of superamphiphilic CGPFs. The structure of CGPF with a monolithic 3D-interconnected and rough network resembles a 3D capillary imbibition phenomenon[41–44] of the spontaneous invasion of liquid into the material. Specific combination of wettability, such as superhydrophilicity and superoleophilicity, is the foundation for an absorbing mixture of oil and water, as CGPF-2 and CGPF-3,
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Figure 3. Water contact angles (WCAs) of a) the polyurethane foams only coated with different concentrations of CNWs (CPFs) and b) WCAs of CGPFs; and morphology of c) CPFs and d) CGPFs.
with the unique combination of hydrophilic CNWs and oleophilic GN on their surface. A droplet of water or oil (bean oil) was dropped on the surface of the CGPF-2 (Figure 4a,c) and CGPF-3 (Figure 4b,d), respectively, where the contact area first quickly absorbed the liquid, and then the liquid spontaneously penetrated into the foam interior. Furthermore, to examine the wettability of the CGPF-2 (Movies S3 and S5, Supporting Information) and CGPF-3 (Movies S4 and S6, Supporting Information), movies of a droplet of water on the surfaces were obtained at a capture speed of ten frames per second. By comparing their absorption speed, CGPF-3 can absorb oil and water in a short period time, which indicates that CGPF-3 possesses even better superhydrophilicity and superoleophilicity than CGPF-2 because its surface with a greater amount of CNWs can adsorb more GN sheets on the surface than that of CGPF-2. Besides, water is absorbed faster than oil, maybe, as a result of the different viscosities of water and oil. To further examine superamphiphilicity of the CGPFs, drops of water and oil were rapidly absorbed by a piece of CGPF-3 when parallel pushing the foam (Figure 4e; Movies S7, Supporting Information). For comparison, if CGPF-3 was vertically pushed over drops of water and oil, successively, the foam also completely absorbed water and oil (Figure 4f; Movie S8, Supporting Information). Compared with superamphiphilic multifunctional 3D graphene-based foam reported by Song,[30] the absorption speed of CGPF-3 for water and oil is faster.
2.3. The Absorption Capacity of CGPFs for Water and Various Organic Solvents The absorption capacity is an important parameter to evaluate absorption ability of the absorbant materials. Cho et al.[45] presented a new organic–inorganic hybrid particles prepared using tailor-made alkoxysilane-functionalized amphiphilic polymer precursors, which had the maximum sorption capacity of
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43.04 mg g−1 for water-soluble dye (azo-dye reactive orange-16). Jin et al.[46] synthesized a novel amphiphilic hybrid material ATP-P(S-b-DVB-g-AO) (ASDO), which was obtained after transforming acrylonitrile units into acrylamide oxime (AO) as the hydrophilic segment. The absorption capacity of ASDO for Pb(II) could achieve 131.6 mg g−1, and the maximum removal capacity of ASDO toward phenol was found to be 18.18 mg g−1 in the case of monolayer absorption at 30 °C. In our research, two organic solvents (ethanol and acetone) and three oils (soybean oil, lubricate oil, and olive oil) were used to evaluate the absorption capacity and versatility of modified foams (Figure 5a). The absorption capacity of CGPFs (Q) for liquids, including water, oils, and various organic solvents, was calculated according to the following equation Q=
Wa − Wb Wa
(2)
where Wa and Wb represented the weight of CGPFs before and after absorbing liquids, respectively.[47] The as-prepared foams exhibit weight gains more than 15 times of its own weight for a variety of liquids. The absorption capacity of CGPF-2 and CGPF-3 for water is, respectively, more than 17 and 23 times of their own weight, depending on their superhydrophilicity, while the absorption capacity of the others are zero due to their hydrophobicity. Moreover, the sorption capacities of CGPF-1 for soybean oil, lubricated oil, olive oil, ethanol, and acetone are 27, 24, 25, 26, and 29 g g−1, respectively, which are better than other foams because of excellent wettability for various oils and organic solvents. Interestingly, the CGPFs (CGPF-1 and CGPF-4) possessing hydrophobic performance get a better absorption capacity for organic liquids than other CGPFs (CGPF-2 and CGPF-3) with superhydrophilic performance. Hence, we can prepare CGPFs with different wetting performance by adjusting the amount of CNWs. CGPF-1 and CGPF-4 are hydrophobic and superoleophilic, while CGPF-2
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and CGPF-3 are superamphiphilic (both superhydrophilicity and superoleophilicity). More importantly, another issue regarding absorption performance of CGPFs is their recyclability. The superamphiphilic foams could be reused for absorbing water and organic solvents with high capacity and efficiency, which is an essential characteristic for practical application. For instance, the weight gain of the foams is 27 (CGPF-1), 17 (CGPF2), 22 (CGPF-3), and 24 (CGPF-4) g g−1 after 20 cyclic applications (Figure 4b), maintaining their original capability for soybean oil absorption.
3. Conclusion
Figure 4. Photographs of a) the spreading process of a droplet of water on the CGPF-2 (Movie S3, Supporting Information) and c) CGPF-3 (Movie S4, Supporting Information) surface. Photographs of the b) spreading process of a droplet of oil on the CGPF-2 (Movie S5, Supporting Information) and d) CGPF-3 (Movie S6, Supporting Information) surface. e) The side-by-side simultaneous absorption (Movie S7, Supporting Information) and f) the sequential absorption of water and oil (Movie S8, Supporting Information) by the CGPF-3 (water is dyed with methylene blue).
We demonstrated a new approach to synthesize superamphiphilic materials involving the formation of hydrophilic and oleophilic layers through a simple dip-coating process in the presence of porogens. The morphology of porous cellulose nanowhiskers and CGPFs is mainly affected by the concentration of CNWs, rendering them different wettability. The synergy arising from highly hydrophilic CNWs and highly oleophilic GN coexisted on PU foam surface results in a surprisingly large effect on surface wettability. Besides, the as-prepared foams exhibit the characteristics of superwettability, excellent absorption capacity, and superior recyclability. The superamphiphilic foams are capable of collecting liquids for more than 20 cycles. As a whole, the superamphiphilic 3D CGPFs represent a significant step forward in the design and application of absorbent materials. Therefore, the present study provides a
Figure 5. a) Absorption capacity of CGPFs for water and oils, as well as various organic solvents. b) Absorption capacity of the CGPFs for soybean oil at different cycle numbers.
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Figure 6. Fabrication schemes of CGPFs.
facile and inexpensive strategy for fabrication of robust superamphiphilic 3D porous materials.
4. Experimental Section Materials: Cellulose nanowhiskers were extracted from flax linens commercially provided by Xuyi Textile Material Co., Ltd., Ningbo, China. Graphene powders were purchased from Sichuan Jinlu Group Co., Ltd., Deyang, China. The polyurethane foams were supplied by sponge products Co., Ltd., Qingdao, China. Other reagents, including sodium hydroxide, acetone, and ethanol, were all purchased from Shenyang Xindongbao Scientific Instrument Co., Ltd., Shenyang, China. Measurements: The cross-sectional morphologies of the foams were characterized using FE-SEM, (JSM-6301F, JEOL, Japan). The wetting characteristics of the foams were measured by SL200KB apparatus using a droplet (6 µL) of water as an indicator at room temperature. The HRTXRT (VersaXRM-500, Xradia, USA) was used to analyze the 3D structure of superamphiphilic foam. Preparation of Superamphiphilic Polyurethane Foams: CNWs were extracted from linen fibers based on the method as the previous report.[48] The aqueous concentrations of CNWs were 0, 0.25, 0.5, 0.75, and 1.0 wt%, respectively. The graphene (GN; 1 g) was dispersed in distilled water (300 mL) with the assistance of ultrasonication for 36 h. Figure 6 illustrates the fabrication scheme of the foams. These foams were constructed through a simple and easy dip-coating process on a PU template, which consisted of the sequential dip-coating procedures of aqueous CNW solution and GN suspension. First, the PU foam with dimensions of 40 × 40 × 20 in mm was used as substrate with an average pore size of hundreds of micrometers. PU foams were successively cleaned in distilled water and ethanol for removal of impurities. Second, clean PU foam was completely immersed in CNWs solution for 10 min and dried at 40 °C in an oven. Finally, the prepared foams were immersed in GN (300 mL; 1 g) suspension for 20 min and dried in an oven. This dip-coating process was repeated twice in order to coat GNs uniformly on the foam surface. According to different weight percentages of CNWs, the as-prepared GN@PU foams were named CGPF-1 (0.25 wt%), CGPF-2 (0.5 wt%), CGPF-3 (0.75 wt%), and CGPF-4 (1.0 wt%), respectively. Density of CNW coatings and GN coatings are shown in Table 1.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
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Acknowledgements X.Z. and D.L. contributed equally to this work. The authors thank Fachun Cao for the help of the contact angle measurement, and would also like to extend their thanks to Shaogang Wang for his help in high-resolution transmission X-ray tomography test.
Conflict of Interest The authors declare no conflict of interest.
Keywords cellulose nanowhiskers, graphene, polyurethane foam, superamphiphili city Received: September 1, 2017 Revised: October 15, 2017 Published online:
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