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bacterial cellulose nanocomposite membranes
Composites Science and Technology 159 (2018) 70e76
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Facile and simple fabrication of strong, transparent and flexible aramid nanofibers/bacterial cellulose nanocomposite membranes Yadong Wu 1, Fang Wang 1, Yudong Huang* MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2018 Received in revised form 18 February 2018 Accepted 24 February 2018 Available online 27 February 2018
Mechanical strength, transparency and flexibility are the leading bottlenecks for the application of a membrane. Thus, the development of co-effectively strong, transparent and flexible membranes is significant for various industries. Here, we fabricated the ANFs (aramid nanofibers)/BC (bacterial cellulose) nanocomposite membranes with different ANFs loadings (up to 8.0 wt%) via a facile and simple vacuumassisted flocculation route. FT-IR, XRD and SEM were applied to characterize the pure BC membrane and ANFs/BC nanocomposite membranes. The resultant membranes maintained excellent transparency and flexibility at relatively low ANFs concentrations (4.0 wt%). The mechanical properties of ANFs/BC nanocomposite membranes could be altered by changing the ANFs content, in which the ANFs served as an enforcement agent, and the nanocomposite membrane exhibited the highest tensile strength at ANFs content of 4.0 wt%. Besides the excellent tensile strength, transparency and flexibility, the surface wettability of the ANFs/BC decreased compared to that of pristine BC, indicating a relative stability in humidity. These results showed that the ANFs/BC nanocomposite membrane is strong, transparent and flexible, thus making it an excellent candidate for electronic substrates and optical materials. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Bacterial cellulose Aramid nanofibers Nanocomposite membrane Tensile strength Transparency and flexibility
1. Introduction The development patterns of composite materials science have been dramatically affected by nanomaterials and nanotechnology. The ever-growing library of nanoscale building blocks with variable shapes, sizes and compositions has provided great opportunities for designing the materials based on multi-features fusion [1,2]. Nanomaterials scientists predict that composite made with nanoscale building blocks such as nanotubes, platelets, and nanofibers will combine two or more desirable properties, as exhibiting exceptional mechanical performance [3], or provide thermal stability for otherwise highly labile materials [4]. In particularly, the effective use of nanoscale building blocks as reinforcement may not only result in superior properties to other systems [1,5,6] but also retain the original performance such as excellent transparency [7] and good flexibility [8]. In addition, the significance of mechanical reinforcement for optically functional materials has been witnessed by the rapid development of electronic industries which, in turn,
* Corresponding author. E-mail address: [email protected] (Y. Huang). 1 Note: These authors contributed equally to this work. https://doi.org/10.1016/j.compscitech.2018.02.036 0266-3538/© 2018 Elsevier Ltd. All rights reserved.
have created a great demand for transparent and strong materials [9]. Meanwhile, flexibility is necessary for high-strength materials to be employed in various applications [10]. Encouragingly, the availability of nanofibers offers a potential way to avoid this limitation of opaqueness [7]. One of the most versatile polymer matrixes may be BC, which is a homopolymer made up of b-D-glucopyranose units linked by (1e4)-glycosidic bonds [11], regarding as promising biodegradable fiber-reinforcement for polymeric composites [12e14]. Additionally, as the efficiency of reinforcement depends on the especial characteristics of nanofillers such as aspect ratios and intrinsic mechanical properties, nanoscale building blocks including graphene [15] and CNTs [16,17], and metal nanoparticles [18] have been widely used in the preparation of BC nanocomposites. BC nanocomposites with high mechanical strength have potential applications in a variety of industries. Addition of some nanomaterials to BC producing media or direct blend with BC could result in such nanocomposites, which subsequently leads to enhancement of the mechanical properties and the widen applications [19]. However, the incorporation of nanomaterials into BC for imparting increased properties or special characteristics sometimes deteriorates its intrinsic performance such as
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transparency [18,20,21], which may restrict its application to a certain extent. Therefore, it is important to open to the wide vision on the exploration of strong, transparent and flexible BC composite, which may serve as a basis for further application in various fields. To address the challenge, it is quite useful to have a suitable nanoscale building block which in nature can realize these aspects. Nanoscale fibers have the extremely high surface-to-volume ratio and is expected to exhibit superior mechanical performance, unique optical properties and other amazing characteristics [22,23]. Recently, the nanoscale form of aramid fiber has emerged with maintaining the excellent mechanical performance [1,23,24] similar its original macro-fiber (Kevlar™) [25], and the ANFs can be easily obtained via deprotonation in KOH/DMSO [23,26,27]. ANFs have been incorporated into some polymers as nanoscale fillers to achieve the attractive properties [1,6], and its hybrids with CNTs and graphene have been synthesized and exhibited high performance [5,28,29], in spite of its unsatisfactory biocompatibility compared to other biologically derived nanofibers such as phage nanofibers and bacterial flagella [30e32]. These previous studies show that ANFs are promising nanoscale building block in nanocomposites with multiple intriguing properties. Here we describe the ANFs/BC nanocomposite membranes based on BC and ANFs by a vacuum-assisted filtration method. The obtained membranes exhibit excellent mechanical properties at suitable ANFs concentration, which is required for highperformance nanocomposite membranes. Beyond that, the use of ANFs also could retain the good transparency and flexibility, which are essential for some special applications.
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2.4. Fabrication of ANFs/BC nanocomposites membranes BC suspension and ANFs dispersion were mechanically mixed in various proportions, giving a mixture ready for filtration. Specifically, ANFs dispersion of different volumes (0.837, 1.709, 2.618 and 3.566 mL) were added dropwise into 25 mL BC suspension with vigorously stirring at room temperature, which were corresponding to ANFs/BC mixtures containing 2.0, 4.0, 6.0 and 8.0 wt % of ANFs, respectively. Subsequently, the mixtures were further vigorously stirred for an additional 1.0 h to achieve a uniform dispersion, and after then the obtained dispersions were vacuum filtrated using the nylon membranes. Finally, the ANFs/BC nanocomposite membranes were dried in N2 atmosphere and peeled off from the nylon membranes. 2.5. Characterization and testing
2. Experimental section
FT-IR spectra were collected on a Perkin Elmer Spectrum One spectrometer, on which the membrane was immobilized for measurement directly. SEM observations were obtained in a Hitachi S4700 SEM. XPS measurements were performed on a Probe ESCA with the Al Ka radiation. XRD measurement was carried out on a D/ man-rBX X-ray generator operated at 30 mA and 40 kV. Tensile strength of the materials was measured by a Cmt8102 Electric Universal Testing Machine with a test speed of 10 mm/min. The wettability of the membranes was examined by static water contact angle SL200KB measurement at room temperature. Static contact angle measurements were performed by depositing 5 mL droplets of deionized water onto the surfaces of the membranes, and images were collected using an 87-340FPS Cam.
2.1. Materials
3. Results and discussion
Bacterial cellulose was purchased from Hainan Yeguo Foods Co., Ltd., Hainan, China. Kevlar-29 brand yarns (136 dtex) were obtained from DuPont, USA. DMSO, NaOH and KOH were supplied by Shuang Shuang Chemical Co., Ltd, Yantai, China. Nylon filtration membranes (0.1 mm pore size) were obtained Filtration Equipment Factory Co., Ltd., Haining, China.
3.1. Chemical structure
2.2. Pretreatment of BC and preparation of BC/water suspension
The FTIR spectra of pristine BC, pure ANFs and ANFs/BC nanocomposites containing varying amounts of ANFs are shown in Fig. 1. The spectrum for BC (Fig. 1a) reveals the presence of all fundamental peaks for various chemical groups, and Fig. 1b presents the typical characteristic absorptions of aramid [5,23,28]. For the ANFs/
BC sheets were rinsed with deionized water and then immersed in a solution of 0.1 mol/L NaOH at 80 C for 3 h to remove the impurities such as medium, microbial cells at al, followed by washing with deionized water to pH ¼ 7. In order to prepare a uniform suspension of BC nanofibers, the resulting BC sheets were cut into small pieces and pulped with a mechanical homogenizer at a speed of 20000 rpm. Then the slurry of BC was diluted with deionized water to obtain a suspension of BC at a concentration of 3.281 mg/ mL. 2.3. Preparation of ANFs/DMSO dispersion ANFs/DMSO dispersions were synthesized by following the method described previously with some small modifications [23,26]. Firstly, the extraction of Kevlar-29 yarns was realized by using acetone as solvent for 72 h followed with oven-drying at 60 C overnight. Next, accurately weighting of 0.5 g above Kevlar-29 yarns was dispersed into a 500 mL round-bottom flask which containing 250 mL of DMSO in the presence of 1.0 g of KOH. Then the flask was sealed, and the Kevlar/KOH/DMSO mixture was vigorously stirred for 7 days at room temperature to generate a homogenous, viscous and crimson solution of ANFs at a concentration of 2 mg/mL.
Fig. 1. FTIR spectra of (a) BC, (b) ANFs and ANFs/BC nanocomposite membranes with different amounts of ANFs: (c) 2.0 wt%, (d) 4.0 wt%, (e) 6.0 wt%, and (f) 8.0 wt%.
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BC membranes (Fig. 1c to f), the N-H characteristic peak of ANFs overlap the broad absorption occurring from 3600 to 3000 cm1, which is attributable to the stretching vibration of hydroxyl group. The twin peaks at around 2910 and 2852 cm1 are assigned to the asymmetrical and symmetric stretching vibration of C-H. Most importantly, three new peaks appear at 1652, 1540, 1515 cm1 which are representative of stretching vibration of C¼O in amide, bending vibration of N-H and stretching vibration of C¼C in benzene ring, respectively, and their intensity increase dramatically upon the addition of ANFs into BC. Overall, the results firmly verify the presence of ANFs in the nanocomposite membrane and are in good agreement with the ratio of ANFs to BC. 3.2. Morphology of BC, ANFs and the ANFs/BC nanocomposites The surface morphology of the original BC membranes as examined by SEM is shown in Fig. 2a. BC produced by some nonpathogenic bacteria is a highly pure and crystalline form of cellulose with cross-linked fibrillar structure. It can be found that the BC fibers have a high aspect ratio, 20e100 nm in width and 1e9 mm length. They are interconnected and overlapped, forming a threedimensional network without any preferential orientation. Commercial Kevlar is an organic fiber in the aromatic polyamide family. It is a golden yarn, consisting of amount of microfibers which have a unique combination of high strength, high modulus, toughness and thermal stability [25]. The diameter of the asreceived microfiber is around 9 mm (Fig. 2b), and the paradigm of aramid microfiber is relatively inert and possessing a low affinity for surface bonding has long been recognized. For increasing the affinity and specific surface area, we employ a deprotonation
method to synthesize ANFs (Fig. 2c). The diameter of the asprepared nanoscale fiber is about 20e50 nm and its length is up to10 mm (Fig. 2d). Through blending of ANFs with BC (the content of ANFs is not more than 4.0 wt%), the structure becomes more densely packed while the fibrillar network is visible (Fig. 3a and b). Upon the addition of ANFs, no severe morphology destruction is seen at relatively low loadings. This phenomenon may be explained that the ANFs have been uniformly dispersed and entangled with BC, but the two kinds of nanofibers could not be distinguished in SEM due to the similar structures and sizes of ANFs and BC. However, at higher concentrations (6.0, 8.0 wt%), big clusters and peel-off are visible (Fig. 3c and d) that could be due to the severe gel effect when ANFs were added into BC suspension in DI water during the preparation of ANFs/BC nanocomposite. This could be inferred from the re-protonation of the negatively charged nitrogen atoms on the surface of ANFs contributed to the interactions with Hþ in water [5,8]. As the concentration of ANFs goes higher, the gel phenomenon becomes more serious so that ANFs are agglomerated and failed to form a uniform dispersion in BC. 3.3. X-ray diffraction analysis Crystallinity is an important property for polymer membranes, as a change in crystallinity of the polymer could affect the physical performance. Fig. 4 illustrates XRD patterns of the membranes in the absence and in the presence of ANFs. The pattern of pure BC membrane (Fig. 4a) exhibits two peaks at 2q ¼ 15.8 and 21.5 corresponded to the (110) and (200) plane of cellulose Ⅰ, respectively. Diffraction peak at 2q ¼ 13.5 could be the
Fig. 2. SEM of BC and aramid at various states, from large to small: (a) pure BC membrane. (b) Kevlar-29 monofilament. (c) Aramid nanofibers networks obtained by freeze-drying method and observed in an SEM. (d) TEM image of aramid nanofibers.
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Fig. 3. SEM images of ANFs/BC nanocomposite membranes with different amounts of ANFs: (a) 2.0 wt%, (b) 4.0 wt%, (c) 6.0 wt%, and (d) 8.0 wt%.
beneficial to pack closely in order to form compact network structure. However, further increase in ANFs proportion causes smaller diffraction 2q, bigger d-spacing and lower peak intensity (Fig. 4d and e) that represents loose fibrillar stack and decrease in the crystallinity. This changing d-spacing and crystallinity trend is in agreement with the above SEM results. Thus, it could be concluded that high loadings (6.0e8.0 wt%) would induce the agglomeration and poor dispersion of ANFs in membranes, which ultimately leads to the disruption of the original well-organized BC crystal structure. 3.4. Transparency and flexibility analysis
Fig. 4. X-ray diffractograms of (a) pristine BC membrane, and membranes with various ANFs loadings: (b) 2.0 wt%, (c) 4.0 wt%, (d) 6.0 wt% and (e) 8.0 wt%.
result of the transformation of the crystalline structure from type Ⅰ to type Ⅱ. While the diffractograms of the ANFs/BC with low ANFs contents (Fig. 4b and c) show similarity to that of BC, neither do any new peak appear, nor do any of the existing peaks disappear. It is noticed that the reflective angle increases as the loading of ANFs increases (0e4.0 wt%), indicating the d-spacing has decreased. This could be concluded that the well-dispersion of ANFs in BC is
Transparency and flexibility are essential for high-strength membranes to be employed in optical applications, electronic industries and other special fields [33]. Fig. 5 shows the effect of the presence of ANFs in the BC on the appearance and transparency of the nanocomposite membranes. With increasing the weight fraction of ANFs, the color of the nanocomposite membranes are turning gradually from white to yellow (Fig. 5a), induced by the intrinsic gold color of aramid fiber. At low ANFs concentrations (0e4.0 wt%), the ANFs/BC membranes are considered as transparent (Fig. 5b and c) owing to the diameters of the both two kinds of nanofibers are much smaller than the wavelengths of visible light [7,27], and the well dispersion of ANFs in the nanocomposites also helps to retain the good transparency and makes the surfaces smooth. However, some wrinkles occur on the surface of nanocomposite membranes, and it is also obvious to see the transparency declines (Fig. 5d and e) with higher ANFs loadings, which is caused by the size of the agglomeration of ANFs
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Fig. 5. Optical images of (a) appearance of BC membranes and ANFs/BC membranes [12 o'clock position is pure BC, and ANFs loading increases in the clockwise direction], and transparency of membranes containing (b) 0, (c) 2.0 wt%, (d) 4.0 wt%, (e) 6.0 wt% and (f) 8.0 wt% of ANF.
approaching to the wavelengths of visible light and the further increase in the thickness of ANFs/BC membranes. The ANFs/BC membranes could be bent easily which has been illustrated in Fig. 6, indicating that the ANFs/BC membrane is flexible. Thus, the small weight fractions of ANFs (4.0 wt%) could be very suitable for retaining the good transparency and flexibility of nanocomposite membranes, which outperforms the BC nanocomposites reported in other references [18,22].
Fig. 6. Photographs of ANFs/BC membranes showing flexibility, in which the weight fractions of ANFs are (a) 0, (b) 2.0 wt%, (c) 4.0 wt%, (d) 6.0 wt% and (e) 8.0 wt%, respectively.
3.5. Mechanical properties of ANFs/BC nanocomposite membranes Membranes have to be durable for handling and resistant to the load applied, so the mechanical properties of composite membranes are critical for the design of devices. The tensile strength of pristine BC membrane and ANFs/BC membranes have been tested, in order to explore the effect of ANFs on the strength and establish the optimal loading ratio. Fig. 7 illustrates the consequence of the weight fraction of ANFs on the tensile strength. Statistical comparisons are performed using SPSS 16.0 software, and difference are considered significant for p < 0.05. The pure BC membrane shows a tensile strength of 77.13 MPa. The 4.0 wt% additions of ANFs
Fig. 7. Tensile strength of pure BC membrane and ANFs/BC with different ANFs concentrations.
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significantly improves the tensile strength (100.36 MPa) (p < 0.01), while a further increase of the concentration of ANFs (6.0e8.0 wt%) causes the decrease of the tensile strength (from 60.94 MPa to 50.80 MPa) (p < 0.01), even weaker than original BC membrane (p < 0.01). A conspicuous advancement of the incorporation of ANFs in BC matrix on tensile strength is recorded despite the low-down loadings of ANFs (2.0e4.0 wt%). This is very exciting because high mechanical performance, flexible and transparent membranes represent the bottleneck for many applications [5,26]. The FTIR spectra (Fig. 1) have indicated that the incorporation of ANFs in the nanocomposite membranes does not create or remove new adsorptions. Therefore, it is suggestible that no chemical interaction occurs between BC and ANFs except for the dominant hydrogen bonding. The noticeable trend of mechanical performance observed for the ANFs/BC membranes with low ANFs concentrations could be explained in terms of the suitability of ANFs as the filler for composite membranes, which is due to its excellent mechanical properties and high aspect ratio [26], leading to the effective internal stress transfer. Beyond that, SEM and XRD results have showed that ANFs are well dispersed and the nanofibers possess a higher packing density with low ANFs additions. This is helpful for the ANFs/BC membranes to enhance their basic mechanical properties based on the intermolecular hydrogen bonds. What's more, BC and ANFs are interconnected with each other through physically entanglement. Among the fabricated membranes, ANFs/BC with 4.0 wt% ANFs seems to provide the optimal potentiation. However, lower tensile strength (ANFs/BC containing 6.0 and 8.0 wt% of ANFs) is a result of agglomeration of ANFs and low crystallinity which could be observed from SEM and XRD results, subsequent with the bad interfacial adhesion between ANFs and BC. Besides, the agglomeration of ANFs maybe induce the stress concentration. 3.6. Wettability analysis Static water contact angle could be regarded as a representative parameter to evaluated the surface hydrophilic or hydrophobic characteristics of nanocomposite membranes. Fig. 8 shows the static water contact angle measurements used to compare the surface hydrophilicity of original BC membrane and modified nanocomposite membranes.
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It can be observed that the pristine BC membrane exhibits good surface hydrophilicity, and the water droplet spread out rapidly on its surface, showing a low contact angle (10.91 ), which could be explained by the intrinsic hydrophilic molecular structures in BC so that it is liable to absorb water and swell [34]. In contrast, an observable change of the surface wettability occurs upon the addition of ANFs. The hydrophilicity of ANFs/BC membranes decreases as the ANFs loading increases from 2.0 wt% to 4.0 wt%, and the contact angle of the nanocomposite membranes changes from 21.80 to 26.68 . This result could be explained with FTIR spectra, which show that the inherent hydrophobic structures have been introduced into the nanocomposites by the well-dispersed ANFs and their number increases gradually. Interestingly, contact angle does not follow a trend based on the concentration of ANFs in the nanocomposite membranes. Instead, the high ANFs additions yield small contact angles of 14.80 and 18.18 , respectively, corresponding to 6.0 wt% and 8.0 wt% of ANFs. The phenomenon may be attributed to the agglomeration of ANFs in the nanocomposite membranes. As a result, the ANFs/BC membrane with 4.0 wt% ANFs has the largest contact angle. Above all, the addition of ANFs into BC could improve the stability of nanocomposites in humidity and strengthen the mechanical property, which promises the potential applications of ANFs/BC nanocomposites, such as electronic substrates and optical materials. 4. Conclusions In the present study, we have reported a facile and simple method to fabricate the ANFs/BC nanocomposite membranes with different weight fractions of ANFs. Since the addition of ANFs, the mechanical properties of the nanocomposite membranes could be tuned, the membranes with a relatively low ANFs loadings exhibited an excellent tensile strength. It was also important to note that the achievement of such high mechanical performance at low concentration of ANFs was very desirable for retaining the good transparency and flexibility of ANFs/BC membranes. Besides, we further demonstrated that the hydrophilicity of the nanocomposite membranes decreased compared to pristine BC membrane. Therefore, the ANFs/BC nanocomposite membranes might be the promising candidates for potential application as strong, transparent and flexible nanocomposite in many fields. Acknowledgements This work was financially supported by China Postdoctoral Science Foundation (2013M541372). We would like to thank the financial support from Heilongjiang Postdoctoral Fund (LBHZ13086). References
Fig. 8. Contact angle measurements of original BC and ANFs/BC membranes (the diagrams above each column correspond to the optical images obtained from contact angle measurements).
[1] Q.X. Kuang, D. Zhang, J.C. Yu, Y.W. Chang, M.L. Yue, Y. Hou, M. Yang, Toward record-high stiffness in polyurethane nanocomposites using aramid nanofibers, J. Phys. Chem. C 119 (2015) 27467e27477. [2] G. Kickelbick, Concepts for the incorporation of inorganic building blocks into organic polymers on a nanoscale, Prog. Polym. Sci. 28 (2003) 83e114. [3] Y. Dzenis, Structural nanocomposites, Science 319 (2008) 419e420. [4] G. Decher, Fuzzy nanoassemblies: toward layered polymeric multicomposites, Science 277 (1997) 1232e1237. [5] J.Q. Zhu, W.X. Cao, M.L. Yue, Y. Hou, J.C. Han, M. Yang, Strong and stiff aramid nanofiber/carbon nanotube nanocomposites, ACS Nano 9 (2015) 2489e2501. [6] S.O. Tung, S. Ho, M. Yang, R.L. Zhang, N.A. Kotov, A dendrite-suppressing composite ion conductor from aramid nanofibers, Nat. Commun. 6 (2015), https://doi.org/10.1038/ncomms7152. [7] M. Yang, K.Q. Cao, B. Yeom, M.D. Thouless, A. Wass, E.M. Arruda, N.A. Kotov, Aramid nanofiber-reinforced transparent nanocomposite, J. Compos. Mater. 49 (2015) 1873e1879. [8] J.C. Fan, Z.X. Shi, M. Tian, J. Yin, Graphene-aramid nanofiber nanocomposite
76
[9]
[10]
[11]
[12] [13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
Y. Wu et al. / Composites Science and Technology 159 (2018) 70e76 paper with high mechanical and electrical performance, RSC Adv. 3 (2013) 17664e17667. J. Zhu, B.S. Shim, M.D. Prima, N.A. Kotov, Transparent conductors from carbon nanotubes LBL-assembled with polymer dopant with pi-pi electron transfer, J. Am. Chem. Soc. 133 (2011) 7450e7460. L. Jinhwan, L. Phillip, L. Hyungman, L. Dongjin, L.S. Seob, H.K. Seung, Very long Ag nanowire synthesis and its application in a highly transparent, conductive and flexible metal electrode touch panel, Nanoscale 4 (2012) 6408e6414. S. Gea, E. Bilotti, C.T. Reynolds, N. Soykeabkeaw, T. Peijs, Bacterial cellulosepoly(vinyl alcohol) nanocomposites prepared by an in-situ process, Mater. Lett. 64 (2010) 901e904. M.M. Rahman, A.N. Netravali, Oriented bacterial cellulose-soy protein based fully ‘green’ nanocomposites, Compos. Sci. Technol. 136 (2016) 85e93. H.Y. Yu, J.M. Yao, Reinforcing properties of bacterial polyester with different cellulose nanocrystals via modulating hydrogen bonds, Compos. Sci. Technol. 136 (2016) 53e60. M.M. Rahman, A.N. Netravali, High-performance green nanocomposites using aligned bacterial cellulose and soy protein, Compos. Sci. Technol. 146 (2017) 183e190. Y. Liu, J. Zhou, J. Tang, W.H. Tang, Three-dimensional, chemically bonded polypyrrole/bacterial cellulose/graphene composites for high-performance supercapacitors, Chem. Mater. 27 (2015) 7034e7041. W.I. Park, H.S. Kim, S.M. Kwon, Y.H. Hong, H.J. Jin, Synthesis of bacterial cellulose in multiwalled carbon nanotube-dispersed medium, Carbohyd. Polym. 77 (2009) 457e463. A. Jasim, M.W. Ullah, Z.J. Shi, X. Lin, G. Yang, Fabrication of bacterial cellulose/ polyanilin/single-walled carbon nanotubes membrane for potential application as biosensor, Carbohyd. Polym. 163 (2017) 62e69. H.R. Wei, K. Rodriguez, S. Renneckar, W.N. Leng, P.J. Vikesland, Preparation and evaluation of nanocellulose-gold nanoparticle nanocomposite for SERS applications, Analyst 140 (2015) 5640e5649. N. Shah, M.U. Islam, W.A. Khattak, J.K. Park, Overview of bacterial cellulose composites: a multipurpose advanced material, Carbohyd. Polym. 98 (2013) 1585e1598. W. Shao, S.X. Wang, H. Liu, J.M. Wu, R. Zhang, H.H. Min, M. Huang, Preparation of bacterial cellulose/graphene nanosheets composite films with enhanced mechanical performances, Carbohyd. Polym. 138 (2016) 166e171. S.H. Yoon, H.J. Jin, M.C. Koot, Y.R. Pyun, Electrically conductive bacterial cellulose by incorporation of carbon nanotubes, Biomacromolecules 7 (2006) 1280e1284. S. Ifuku, H. Maeta, H. Izawa, M. Morimoto, H. Saimoto, Facile preparation of
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33] [34]
aramid nanofibers from Twaron fibers by a downsizing process, RSC Adv. 4 (2014) 40377e40380. M. Yang, K.Q. Cao, L. Sui, Y. Qi, J. Zhu, A. Waas, E.M. Arruda, J. Kieffer, M.D. Thouless, N.A. Kotov, Dispersions of aramid nanofibers: a new nanoscale building block, ACS Nano 5 (2011) 6945e6954. J.U. Lee, B. Park, B.S. Kim, D.R. Bae, W. Lee, Electrophoretic deposition of aramid nanofibers on carbon fibers for highly enhanced interfacial adhesion at low content, Compos. Part A. Appl. S. 84 (2016) 482e489. I. O'Connor, H. Hayden, J.N. Coleman, Y.K. Gun’ko, High-strength, hightoughness composite fibers by swelling Kevlar in nanotube suspensions, Small 5 (2009) 466e469. K.Q. Cao, C.P. Siepermann, M. Yang, A.M. Waas, N.A. Kotov, M.D. Thouless, E.M. Arruda, Reactive aramid nanostructures as high-performance polymeric building blocks for advanced composites, Adv. Funct. Mater. 23 (2013) 2072e2080. J. Lyu, L. Liu, X. Zhao, Y.D. Shang, T.K. Zhao, T.H. Li, Facile fabrication of multifunctional aramid nanofiber films by spin coating, J. Mater. Eng. Perform. 25 (2016) 4757e4763. J.C. Fan, J.L. Wang, Z.X. Shi, S. Yu, J. Yin, Kevlar nanofiber-functionalized multiwalled carbon nanotubes for polymer reinforcement, Mater. Chem. Phys. 141 (2013) 861e868. J.C. Fan, Z.X. Shi, L. Zhang, J.L. Wang, J. Yin, Aramid nanofiber-functionalized graphene nanosheets for polymer reinforcement, Nanoscale 4 (2012) 7046e7055. F.K. Wang, D. Li, C.B. Mao, Genetically modifiable flagella as templates for silica fibers: from hybrid nanotubes to 1D periodic nanohole arrays, Adv. Funct. Mater. 18 (2008) 4007e4013. Y.C. Wang, Z.G. Ju, B.R. Cao, X. Gao, Y. Zhu, P.H. Qiu, H. Xu, P.T. Pan, H.Z. Bao, L. Wang, C.B. Mao, Ultrasensitive rapid detection of human serum antibody biomarkers by biomarker-capturing viral nanofibers, ACS Nano 9 (2015) 4475e4483. N. Gandra, G. Abbineni, X.W. Qu, Y.Y. Huai, L. Wang, C.B. Mao, Bacteriophage bionanowire as a carrier for both cancer-targeting peptides and photosensitizers and its use in selective cancer cell killing by photodynamic therapy, Small 9 (2013) 215e221. M.M. Bergshoef, G.J. Vancso, Transparent nanocomposites with ultrathin, electrospun nylon-4, 6 fiber reinforcement, Adv. Mater. 11 (1999) 1362e1365. Z.Y. Wu, C. Li, H.W. Liang, J.F. Chen, S.H. Yu, Ultralight, flexible, and fireresistant carbon nanofiber aerogels from bacterial cellulose, Angew. Chem. 52 (2013) 2925e2929.
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Facile and simple fabrication of strong, transparent and flexible aramid nanofibers/bacterial cellulose nanocomposite membranes Yadong Wu 1, Fang Wang 1, Yudong Huang* MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 January 2018 Received in revised form 18 February 2018 Accepted 24 February 2018 Available online 27 February 2018
Mechanical strength, transparency and flexibility are the leading bottlenecks for the application of a membrane. Thus, the development of co-effectively strong, transparent and flexible membranes is significant for various industries. Here, we fabricated the ANFs (aramid nanofibers)/BC (bacterial cellulose) nanocomposite membranes with different ANFs loadings (up to 8.0 wt%) via a facile and simple vacuumassisted flocculation route. FT-IR, XRD and SEM were applied to characterize the pure BC membrane and ANFs/BC nanocomposite membranes. The resultant membranes maintained excellent transparency and flexibility at relatively low ANFs concentrations (4.0 wt%). The mechanical properties of ANFs/BC nanocomposite membranes could be altered by changing the ANFs content, in which the ANFs served as an enforcement agent, and the nanocomposite membrane exhibited the highest tensile strength at ANFs content of 4.0 wt%. Besides the excellent tensile strength, transparency and flexibility, the surface wettability of the ANFs/BC decreased compared to that of pristine BC, indicating a relative stability in humidity. These results showed that the ANFs/BC nanocomposite membrane is strong, transparent and flexible, thus making it an excellent candidate for electronic substrates and optical materials. © 2018 Elsevier Ltd. All rights reserved.
Keywords: Bacterial cellulose Aramid nanofibers Nanocomposite membrane Tensile strength Transparency and flexibility
1. Introduction The development patterns of composite materials science have been dramatically affected by nanomaterials and nanotechnology. The ever-growing library of nanoscale building blocks with variable shapes, sizes and compositions has provided great opportunities for designing the materials based on multi-features fusion [1,2]. Nanomaterials scientists predict that composite made with nanoscale building blocks such as nanotubes, platelets, and nanofibers will combine two or more desirable properties, as exhibiting exceptional mechanical performance [3], or provide thermal stability for otherwise highly labile materials [4]. In particularly, the effective use of nanoscale building blocks as reinforcement may not only result in superior properties to other systems [1,5,6] but also retain the original performance such as excellent transparency [7] and good flexibility [8]. In addition, the significance of mechanical reinforcement for optically functional materials has been witnessed by the rapid development of electronic industries which, in turn,
* Corresponding author. E-mail address: [email protected] (Y. Huang). 1 Note: These authors contributed equally to this work. https://doi.org/10.1016/j.compscitech.2018.02.036 0266-3538/© 2018 Elsevier Ltd. All rights reserved.
have created a great demand for transparent and strong materials [9]. Meanwhile, flexibility is necessary for high-strength materials to be employed in various applications [10]. Encouragingly, the availability of nanofibers offers a potential way to avoid this limitation of opaqueness [7]. One of the most versatile polymer matrixes may be BC, which is a homopolymer made up of b-D-glucopyranose units linked by (1e4)-glycosidic bonds [11], regarding as promising biodegradable fiber-reinforcement for polymeric composites [12e14]. Additionally, as the efficiency of reinforcement depends on the especial characteristics of nanofillers such as aspect ratios and intrinsic mechanical properties, nanoscale building blocks including graphene [15] and CNTs [16,17], and metal nanoparticles [18] have been widely used in the preparation of BC nanocomposites. BC nanocomposites with high mechanical strength have potential applications in a variety of industries. Addition of some nanomaterials to BC producing media or direct blend with BC could result in such nanocomposites, which subsequently leads to enhancement of the mechanical properties and the widen applications [19]. However, the incorporation of nanomaterials into BC for imparting increased properties or special characteristics sometimes deteriorates its intrinsic performance such as
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transparency [18,20,21], which may restrict its application to a certain extent. Therefore, it is important to open to the wide vision on the exploration of strong, transparent and flexible BC composite, which may serve as a basis for further application in various fields. To address the challenge, it is quite useful to have a suitable nanoscale building block which in nature can realize these aspects. Nanoscale fibers have the extremely high surface-to-volume ratio and is expected to exhibit superior mechanical performance, unique optical properties and other amazing characteristics [22,23]. Recently, the nanoscale form of aramid fiber has emerged with maintaining the excellent mechanical performance [1,23,24] similar its original macro-fiber (Kevlar™) [25], and the ANFs can be easily obtained via deprotonation in KOH/DMSO [23,26,27]. ANFs have been incorporated into some polymers as nanoscale fillers to achieve the attractive properties [1,6], and its hybrids with CNTs and graphene have been synthesized and exhibited high performance [5,28,29], in spite of its unsatisfactory biocompatibility compared to other biologically derived nanofibers such as phage nanofibers and bacterial flagella [30e32]. These previous studies show that ANFs are promising nanoscale building block in nanocomposites with multiple intriguing properties. Here we describe the ANFs/BC nanocomposite membranes based on BC and ANFs by a vacuum-assisted filtration method. The obtained membranes exhibit excellent mechanical properties at suitable ANFs concentration, which is required for highperformance nanocomposite membranes. Beyond that, the use of ANFs also could retain the good transparency and flexibility, which are essential for some special applications.
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2.4. Fabrication of ANFs/BC nanocomposites membranes BC suspension and ANFs dispersion were mechanically mixed in various proportions, giving a mixture ready for filtration. Specifically, ANFs dispersion of different volumes (0.837, 1.709, 2.618 and 3.566 mL) were added dropwise into 25 mL BC suspension with vigorously stirring at room temperature, which were corresponding to ANFs/BC mixtures containing 2.0, 4.0, 6.0 and 8.0 wt % of ANFs, respectively. Subsequently, the mixtures were further vigorously stirred for an additional 1.0 h to achieve a uniform dispersion, and after then the obtained dispersions were vacuum filtrated using the nylon membranes. Finally, the ANFs/BC nanocomposite membranes were dried in N2 atmosphere and peeled off from the nylon membranes. 2.5. Characterization and testing
2. Experimental section
FT-IR spectra were collected on a Perkin Elmer Spectrum One spectrometer, on which the membrane was immobilized for measurement directly. SEM observations were obtained in a Hitachi S4700 SEM. XPS measurements were performed on a Probe ESCA with the Al Ka radiation. XRD measurement was carried out on a D/ man-rBX X-ray generator operated at 30 mA and 40 kV. Tensile strength of the materials was measured by a Cmt8102 Electric Universal Testing Machine with a test speed of 10 mm/min. The wettability of the membranes was examined by static water contact angle SL200KB measurement at room temperature. Static contact angle measurements were performed by depositing 5 mL droplets of deionized water onto the surfaces of the membranes, and images were collected using an 87-340FPS Cam.
2.1. Materials
3. Results and discussion
Bacterial cellulose was purchased from Hainan Yeguo Foods Co., Ltd., Hainan, China. Kevlar-29 brand yarns (136 dtex) were obtained from DuPont, USA. DMSO, NaOH and KOH were supplied by Shuang Shuang Chemical Co., Ltd, Yantai, China. Nylon filtration membranes (0.1 mm pore size) were obtained Filtration Equipment Factory Co., Ltd., Haining, China.
3.1. Chemical structure
2.2. Pretreatment of BC and preparation of BC/water suspension
The FTIR spectra of pristine BC, pure ANFs and ANFs/BC nanocomposites containing varying amounts of ANFs are shown in Fig. 1. The spectrum for BC (Fig. 1a) reveals the presence of all fundamental peaks for various chemical groups, and Fig. 1b presents the typical characteristic absorptions of aramid [5,23,28]. For the ANFs/
BC sheets were rinsed with deionized water and then immersed in a solution of 0.1 mol/L NaOH at 80 C for 3 h to remove the impurities such as medium, microbial cells at al, followed by washing with deionized water to pH ¼ 7. In order to prepare a uniform suspension of BC nanofibers, the resulting BC sheets were cut into small pieces and pulped with a mechanical homogenizer at a speed of 20000 rpm. Then the slurry of BC was diluted with deionized water to obtain a suspension of BC at a concentration of 3.281 mg/ mL. 2.3. Preparation of ANFs/DMSO dispersion ANFs/DMSO dispersions were synthesized by following the method described previously with some small modifications [23,26]. Firstly, the extraction of Kevlar-29 yarns was realized by using acetone as solvent for 72 h followed with oven-drying at 60 C overnight. Next, accurately weighting of 0.5 g above Kevlar-29 yarns was dispersed into a 500 mL round-bottom flask which containing 250 mL of DMSO in the presence of 1.0 g of KOH. Then the flask was sealed, and the Kevlar/KOH/DMSO mixture was vigorously stirred for 7 days at room temperature to generate a homogenous, viscous and crimson solution of ANFs at a concentration of 2 mg/mL.
Fig. 1. FTIR spectra of (a) BC, (b) ANFs and ANFs/BC nanocomposite membranes with different amounts of ANFs: (c) 2.0 wt%, (d) 4.0 wt%, (e) 6.0 wt%, and (f) 8.0 wt%.
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BC membranes (Fig. 1c to f), the N-H characteristic peak of ANFs overlap the broad absorption occurring from 3600 to 3000 cm1, which is attributable to the stretching vibration of hydroxyl group. The twin peaks at around 2910 and 2852 cm1 are assigned to the asymmetrical and symmetric stretching vibration of C-H. Most importantly, three new peaks appear at 1652, 1540, 1515 cm1 which are representative of stretching vibration of C¼O in amide, bending vibration of N-H and stretching vibration of C¼C in benzene ring, respectively, and their intensity increase dramatically upon the addition of ANFs into BC. Overall, the results firmly verify the presence of ANFs in the nanocomposite membrane and are in good agreement with the ratio of ANFs to BC. 3.2. Morphology of BC, ANFs and the ANFs/BC nanocomposites The surface morphology of the original BC membranes as examined by SEM is shown in Fig. 2a. BC produced by some nonpathogenic bacteria is a highly pure and crystalline form of cellulose with cross-linked fibrillar structure. It can be found that the BC fibers have a high aspect ratio, 20e100 nm in width and 1e9 mm length. They are interconnected and overlapped, forming a threedimensional network without any preferential orientation. Commercial Kevlar is an organic fiber in the aromatic polyamide family. It is a golden yarn, consisting of amount of microfibers which have a unique combination of high strength, high modulus, toughness and thermal stability [25]. The diameter of the asreceived microfiber is around 9 mm (Fig. 2b), and the paradigm of aramid microfiber is relatively inert and possessing a low affinity for surface bonding has long been recognized. For increasing the affinity and specific surface area, we employ a deprotonation
method to synthesize ANFs (Fig. 2c). The diameter of the asprepared nanoscale fiber is about 20e50 nm and its length is up to10 mm (Fig. 2d). Through blending of ANFs with BC (the content of ANFs is not more than 4.0 wt%), the structure becomes more densely packed while the fibrillar network is visible (Fig. 3a and b). Upon the addition of ANFs, no severe morphology destruction is seen at relatively low loadings. This phenomenon may be explained that the ANFs have been uniformly dispersed and entangled with BC, but the two kinds of nanofibers could not be distinguished in SEM due to the similar structures and sizes of ANFs and BC. However, at higher concentrations (6.0, 8.0 wt%), big clusters and peel-off are visible (Fig. 3c and d) that could be due to the severe gel effect when ANFs were added into BC suspension in DI water during the preparation of ANFs/BC nanocomposite. This could be inferred from the re-protonation of the negatively charged nitrogen atoms on the surface of ANFs contributed to the interactions with Hþ in water [5,8]. As the concentration of ANFs goes higher, the gel phenomenon becomes more serious so that ANFs are agglomerated and failed to form a uniform dispersion in BC. 3.3. X-ray diffraction analysis Crystallinity is an important property for polymer membranes, as a change in crystallinity of the polymer could affect the physical performance. Fig. 4 illustrates XRD patterns of the membranes in the absence and in the presence of ANFs. The pattern of pure BC membrane (Fig. 4a) exhibits two peaks at 2q ¼ 15.8 and 21.5 corresponded to the (110) and (200) plane of cellulose Ⅰ, respectively. Diffraction peak at 2q ¼ 13.5 could be the
Fig. 2. SEM of BC and aramid at various states, from large to small: (a) pure BC membrane. (b) Kevlar-29 monofilament. (c) Aramid nanofibers networks obtained by freeze-drying method and observed in an SEM. (d) TEM image of aramid nanofibers.
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Fig. 3. SEM images of ANFs/BC nanocomposite membranes with different amounts of ANFs: (a) 2.0 wt%, (b) 4.0 wt%, (c) 6.0 wt%, and (d) 8.0 wt%.
beneficial to pack closely in order to form compact network structure. However, further increase in ANFs proportion causes smaller diffraction 2q, bigger d-spacing and lower peak intensity (Fig. 4d and e) that represents loose fibrillar stack and decrease in the crystallinity. This changing d-spacing and crystallinity trend is in agreement with the above SEM results. Thus, it could be concluded that high loadings (6.0e8.0 wt%) would induce the agglomeration and poor dispersion of ANFs in membranes, which ultimately leads to the disruption of the original well-organized BC crystal structure. 3.4. Transparency and flexibility analysis
Fig. 4. X-ray diffractograms of (a) pristine BC membrane, and membranes with various ANFs loadings: (b) 2.0 wt%, (c) 4.0 wt%, (d) 6.0 wt% and (e) 8.0 wt%.
result of the transformation of the crystalline structure from type Ⅰ to type Ⅱ. While the diffractograms of the ANFs/BC with low ANFs contents (Fig. 4b and c) show similarity to that of BC, neither do any new peak appear, nor do any of the existing peaks disappear. It is noticed that the reflective angle increases as the loading of ANFs increases (0e4.0 wt%), indicating the d-spacing has decreased. This could be concluded that the well-dispersion of ANFs in BC is
Transparency and flexibility are essential for high-strength membranes to be employed in optical applications, electronic industries and other special fields [33]. Fig. 5 shows the effect of the presence of ANFs in the BC on the appearance and transparency of the nanocomposite membranes. With increasing the weight fraction of ANFs, the color of the nanocomposite membranes are turning gradually from white to yellow (Fig. 5a), induced by the intrinsic gold color of aramid fiber. At low ANFs concentrations (0e4.0 wt%), the ANFs/BC membranes are considered as transparent (Fig. 5b and c) owing to the diameters of the both two kinds of nanofibers are much smaller than the wavelengths of visible light [7,27], and the well dispersion of ANFs in the nanocomposites also helps to retain the good transparency and makes the surfaces smooth. However, some wrinkles occur on the surface of nanocomposite membranes, and it is also obvious to see the transparency declines (Fig. 5d and e) with higher ANFs loadings, which is caused by the size of the agglomeration of ANFs
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Fig. 5. Optical images of (a) appearance of BC membranes and ANFs/BC membranes [12 o'clock position is pure BC, and ANFs loading increases in the clockwise direction], and transparency of membranes containing (b) 0, (c) 2.0 wt%, (d) 4.0 wt%, (e) 6.0 wt% and (f) 8.0 wt% of ANF.
approaching to the wavelengths of visible light and the further increase in the thickness of ANFs/BC membranes. The ANFs/BC membranes could be bent easily which has been illustrated in Fig. 6, indicating that the ANFs/BC membrane is flexible. Thus, the small weight fractions of ANFs (4.0 wt%) could be very suitable for retaining the good transparency and flexibility of nanocomposite membranes, which outperforms the BC nanocomposites reported in other references [18,22].
Fig. 6. Photographs of ANFs/BC membranes showing flexibility, in which the weight fractions of ANFs are (a) 0, (b) 2.0 wt%, (c) 4.0 wt%, (d) 6.0 wt% and (e) 8.0 wt%, respectively.
3.5. Mechanical properties of ANFs/BC nanocomposite membranes Membranes have to be durable for handling and resistant to the load applied, so the mechanical properties of composite membranes are critical for the design of devices. The tensile strength of pristine BC membrane and ANFs/BC membranes have been tested, in order to explore the effect of ANFs on the strength and establish the optimal loading ratio. Fig. 7 illustrates the consequence of the weight fraction of ANFs on the tensile strength. Statistical comparisons are performed using SPSS 16.0 software, and difference are considered significant for p < 0.05. The pure BC membrane shows a tensile strength of 77.13 MPa. The 4.0 wt% additions of ANFs
Fig. 7. Tensile strength of pure BC membrane and ANFs/BC with different ANFs concentrations.
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significantly improves the tensile strength (100.36 MPa) (p < 0.01), while a further increase of the concentration of ANFs (6.0e8.0 wt%) causes the decrease of the tensile strength (from 60.94 MPa to 50.80 MPa) (p < 0.01), even weaker than original BC membrane (p < 0.01). A conspicuous advancement of the incorporation of ANFs in BC matrix on tensile strength is recorded despite the low-down loadings of ANFs (2.0e4.0 wt%). This is very exciting because high mechanical performance, flexible and transparent membranes represent the bottleneck for many applications [5,26]. The FTIR spectra (Fig. 1) have indicated that the incorporation of ANFs in the nanocomposite membranes does not create or remove new adsorptions. Therefore, it is suggestible that no chemical interaction occurs between BC and ANFs except for the dominant hydrogen bonding. The noticeable trend of mechanical performance observed for the ANFs/BC membranes with low ANFs concentrations could be explained in terms of the suitability of ANFs as the filler for composite membranes, which is due to its excellent mechanical properties and high aspect ratio [26], leading to the effective internal stress transfer. Beyond that, SEM and XRD results have showed that ANFs are well dispersed and the nanofibers possess a higher packing density with low ANFs additions. This is helpful for the ANFs/BC membranes to enhance their basic mechanical properties based on the intermolecular hydrogen bonds. What's more, BC and ANFs are interconnected with each other through physically entanglement. Among the fabricated membranes, ANFs/BC with 4.0 wt% ANFs seems to provide the optimal potentiation. However, lower tensile strength (ANFs/BC containing 6.0 and 8.0 wt% of ANFs) is a result of agglomeration of ANFs and low crystallinity which could be observed from SEM and XRD results, subsequent with the bad interfacial adhesion between ANFs and BC. Besides, the agglomeration of ANFs maybe induce the stress concentration. 3.6. Wettability analysis Static water contact angle could be regarded as a representative parameter to evaluated the surface hydrophilic or hydrophobic characteristics of nanocomposite membranes. Fig. 8 shows the static water contact angle measurements used to compare the surface hydrophilicity of original BC membrane and modified nanocomposite membranes.
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It can be observed that the pristine BC membrane exhibits good surface hydrophilicity, and the water droplet spread out rapidly on its surface, showing a low contact angle (10.91 ), which could be explained by the intrinsic hydrophilic molecular structures in BC so that it is liable to absorb water and swell [34]. In contrast, an observable change of the surface wettability occurs upon the addition of ANFs. The hydrophilicity of ANFs/BC membranes decreases as the ANFs loading increases from 2.0 wt% to 4.0 wt%, and the contact angle of the nanocomposite membranes changes from 21.80 to 26.68 . This result could be explained with FTIR spectra, which show that the inherent hydrophobic structures have been introduced into the nanocomposites by the well-dispersed ANFs and their number increases gradually. Interestingly, contact angle does not follow a trend based on the concentration of ANFs in the nanocomposite membranes. Instead, the high ANFs additions yield small contact angles of 14.80 and 18.18 , respectively, corresponding to 6.0 wt% and 8.0 wt% of ANFs. The phenomenon may be attributed to the agglomeration of ANFs in the nanocomposite membranes. As a result, the ANFs/BC membrane with 4.0 wt% ANFs has the largest contact angle. Above all, the addition of ANFs into BC could improve the stability of nanocomposites in humidity and strengthen the mechanical property, which promises the potential applications of ANFs/BC nanocomposites, such as electronic substrates and optical materials. 4. Conclusions In the present study, we have reported a facile and simple method to fabricate the ANFs/BC nanocomposite membranes with different weight fractions of ANFs. Since the addition of ANFs, the mechanical properties of the nanocomposite membranes could be tuned, the membranes with a relatively low ANFs loadings exhibited an excellent tensile strength. It was also important to note that the achievement of such high mechanical performance at low concentration of ANFs was very desirable for retaining the good transparency and flexibility of ANFs/BC membranes. Besides, we further demonstrated that the hydrophilicity of the nanocomposite membranes decreased compared to pristine BC membrane. Therefore, the ANFs/BC nanocomposite membranes might be the promising candidates for potential application as strong, transparent and flexible nanocomposite in many fields. Acknowledgements This work was financially supported by China Postdoctoral Science Foundation (2013M541372). We would like to thank the financial support from Heilongjiang Postdoctoral Fund (LBHZ13086). References
Fig. 8. Contact angle measurements of original BC and ANFs/BC membranes (the diagrams above each column correspond to the optical images obtained from contact angle measurements).
[1] Q.X. Kuang, D. Zhang, J.C. Yu, Y.W. Chang, M.L. Yue, Y. Hou, M. Yang, Toward record-high stiffness in polyurethane nanocomposites using aramid nanofibers, J. Phys. Chem. C 119 (2015) 27467e27477. [2] G. Kickelbick, Concepts for the incorporation of inorganic building blocks into organic polymers on a nanoscale, Prog. Polym. Sci. 28 (2003) 83e114. [3] Y. Dzenis, Structural nanocomposites, Science 319 (2008) 419e420. [4] G. Decher, Fuzzy nanoassemblies: toward layered polymeric multicomposites, Science 277 (1997) 1232e1237. [5] J.Q. Zhu, W.X. Cao, M.L. Yue, Y. Hou, J.C. Han, M. Yang, Strong and stiff aramid nanofiber/carbon nanotube nanocomposites, ACS Nano 9 (2015) 2489e2501. [6] S.O. Tung, S. Ho, M. Yang, R.L. Zhang, N.A. Kotov, A dendrite-suppressing composite ion conductor from aramid nanofibers, Nat. Commun. 6 (2015), https://doi.org/10.1038/ncomms7152. [7] M. Yang, K.Q. Cao, B. Yeom, M.D. Thouless, A. Wass, E.M. Arruda, N.A. Kotov, Aramid nanofiber-reinforced transparent nanocomposite, J. Compos. Mater. 49 (2015) 1873e1879. [8] J.C. Fan, Z.X. Shi, M. Tian, J. Yin, Graphene-aramid nanofiber nanocomposite
76
[9]
[10]
[11]
[12] [13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
Y. Wu et al. / Composites Science and Technology 159 (2018) 70e76 paper with high mechanical and electrical performance, RSC Adv. 3 (2013) 17664e17667. J. Zhu, B.S. Shim, M.D. Prima, N.A. Kotov, Transparent conductors from carbon nanotubes LBL-assembled with polymer dopant with pi-pi electron transfer, J. Am. Chem. Soc. 133 (2011) 7450e7460. L. Jinhwan, L. Phillip, L. Hyungman, L. Dongjin, L.S. Seob, H.K. Seung, Very long Ag nanowire synthesis and its application in a highly transparent, conductive and flexible metal electrode touch panel, Nanoscale 4 (2012) 6408e6414. S. Gea, E. Bilotti, C.T. Reynolds, N. Soykeabkeaw, T. Peijs, Bacterial cellulosepoly(vinyl alcohol) nanocomposites prepared by an in-situ process, Mater. Lett. 64 (2010) 901e904. M.M. Rahman, A.N. Netravali, Oriented bacterial cellulose-soy protein based fully ‘green’ nanocomposites, Compos. Sci. Technol. 136 (2016) 85e93. H.Y. Yu, J.M. Yao, Reinforcing properties of bacterial polyester with different cellulose nanocrystals via modulating hydrogen bonds, Compos. Sci. Technol. 136 (2016) 53e60. M.M. Rahman, A.N. Netravali, High-performance green nanocomposites using aligned bacterial cellulose and soy protein, Compos. Sci. Technol. 146 (2017) 183e190. Y. Liu, J. Zhou, J. Tang, W.H. Tang, Three-dimensional, chemically bonded polypyrrole/bacterial cellulose/graphene composites for high-performance supercapacitors, Chem. Mater. 27 (2015) 7034e7041. W.I. Park, H.S. Kim, S.M. Kwon, Y.H. Hong, H.J. Jin, Synthesis of bacterial cellulose in multiwalled carbon nanotube-dispersed medium, Carbohyd. Polym. 77 (2009) 457e463. A. Jasim, M.W. Ullah, Z.J. Shi, X. Lin, G. Yang, Fabrication of bacterial cellulose/ polyanilin/single-walled carbon nanotubes membrane for potential application as biosensor, Carbohyd. Polym. 163 (2017) 62e69. H.R. Wei, K. Rodriguez, S. Renneckar, W.N. Leng, P.J. Vikesland, Preparation and evaluation of nanocellulose-gold nanoparticle nanocomposite for SERS applications, Analyst 140 (2015) 5640e5649. N. Shah, M.U. Islam, W.A. Khattak, J.K. Park, Overview of bacterial cellulose composites: a multipurpose advanced material, Carbohyd. Polym. 98 (2013) 1585e1598. W. Shao, S.X. Wang, H. Liu, J.M. Wu, R. Zhang, H.H. Min, M. Huang, Preparation of bacterial cellulose/graphene nanosheets composite films with enhanced mechanical performances, Carbohyd. Polym. 138 (2016) 166e171. S.H. Yoon, H.J. Jin, M.C. Koot, Y.R. Pyun, Electrically conductive bacterial cellulose by incorporation of carbon nanotubes, Biomacromolecules 7 (2006) 1280e1284. S. Ifuku, H. Maeta, H. Izawa, M. Morimoto, H. Saimoto, Facile preparation of
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33] [34]
aramid nanofibers from Twaron fibers by a downsizing process, RSC Adv. 4 (2014) 40377e40380. M. Yang, K.Q. Cao, L. Sui, Y. Qi, J. Zhu, A. Waas, E.M. Arruda, J. Kieffer, M.D. Thouless, N.A. Kotov, Dispersions of aramid nanofibers: a new nanoscale building block, ACS Nano 5 (2011) 6945e6954. J.U. Lee, B. Park, B.S. Kim, D.R. Bae, W. Lee, Electrophoretic deposition of aramid nanofibers on carbon fibers for highly enhanced interfacial adhesion at low content, Compos. Part A. Appl. S. 84 (2016) 482e489. I. O'Connor, H. Hayden, J.N. Coleman, Y.K. Gun’ko, High-strength, hightoughness composite fibers by swelling Kevlar in nanotube suspensions, Small 5 (2009) 466e469. K.Q. Cao, C.P. Siepermann, M. Yang, A.M. Waas, N.A. Kotov, M.D. Thouless, E.M. Arruda, Reactive aramid nanostructures as high-performance polymeric building blocks for advanced composites, Adv. Funct. Mater. 23 (2013) 2072e2080. J. Lyu, L. Liu, X. Zhao, Y.D. Shang, T.K. Zhao, T.H. Li, Facile fabrication of multifunctional aramid nanofiber films by spin coating, J. Mater. Eng. Perform. 25 (2016) 4757e4763. J.C. Fan, J.L. Wang, Z.X. Shi, S. Yu, J. Yin, Kevlar nanofiber-functionalized multiwalled carbon nanotubes for polymer reinforcement, Mater. Chem. Phys. 141 (2013) 861e868. J.C. Fan, Z.X. Shi, L. Zhang, J.L. Wang, J. Yin, Aramid nanofiber-functionalized graphene nanosheets for polymer reinforcement, Nanoscale 4 (2012) 7046e7055. F.K. Wang, D. Li, C.B. Mao, Genetically modifiable flagella as templates for silica fibers: from hybrid nanotubes to 1D periodic nanohole arrays, Adv. Funct. Mater. 18 (2008) 4007e4013. Y.C. Wang, Z.G. Ju, B.R. Cao, X. Gao, Y. Zhu, P.H. Qiu, H. Xu, P.T. Pan, H.Z. Bao, L. Wang, C.B. Mao, Ultrasensitive rapid detection of human serum antibody biomarkers by biomarker-capturing viral nanofibers, ACS Nano 9 (2015) 4475e4483. N. Gandra, G. Abbineni, X.W. Qu, Y.Y. Huai, L. Wang, C.B. Mao, Bacteriophage bionanowire as a carrier for both cancer-targeting peptides and photosensitizers and its use in selective cancer cell killing by photodynamic therapy, Small 9 (2013) 215e221. M.M. Bergshoef, G.J. Vancso, Transparent nanocomposites with ultrathin, electrospun nylon-4, 6 fiber reinforcement, Adv. Mater. 11 (1999) 1362e1365. Z.Y. Wu, C. Li, H.W. Liang, J.F. Chen, S.H. Yu, Ultralight, flexible, and fireresistant carbon nanofiber aerogels from bacterial cellulose, Angew. Chem. 52 (2013) 2925e2929.