A Flexible Polymer Ultra-Fine Fiber with Extreme Toughness - ACS

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A Flexible Polymer Ultra-Fine Fiber with Extreme Toughness Guangliang Zhou, Guang Yang, Xilin Li, Baiyi Chen, Jing Fan, Xu Hou, and Shaobing Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Materials & Interfaces

A Flexible Polymer Ultra-Fine Fiber with Extreme Toughness Guangliang Zhou,‡,a Guang Yang,‡,a Xilin Li,a Baiyi Chen,b Jing Fan,c Xu Hou *,b and Shaobing Zhou*,a

a

Key Laboratory of Advanced Technologies of Material, Minister of Education, School of Materials Science

and Engineering & School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, China b

College of Chemistry and Chemical Engineering & School of Physical Science and Technology, Xianmen

University, Xiamen 361005, China c

Department of Mechanical Engineering, City College of New York, New York, NY 10031, USA

KEYWORDS: bioinspired; nano-shell; flexibility; microfluidic; toughness

ABSTRACT Fiber materials with multilevel interior structures have myriad applications in many fields due to their unique properties. In this study, we develop a bio-inspired flexible ultra-fine polymer fiber via an integrated microfluidic-electrospinning technology. The fiber possesses periodic hollow and tubular chambers with a shell layer of approximately 150 nm in thickness extremely like natural bamboo. The single fiber with a diameter of ~1.5 µm exhibits the Young’s modulus

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ranging from 2-7 MPa measured with atomic force microscopy (AFM). The fiber with periodic hollow chambers and extreme toughness can find many applications in medicine, industry, and agriculture.

The design and fabrication of materials with multilevel interior structures have spurred the interest of numerous scientists in recent years because the hierarchically complex interior architectures can result in superior mechanical, chemical, or anisotropic properties.1-5 For example, trees can be blown off by heavy storm while bamboos with internodal hollow stems only bend. Previous studies report that the elastic modulus of bamboo is 5-7 times lower than that of trees,6 which is mainly ascribed to the unique hierarchical structure of bamboo with a hollow, tubular culm with periodic nodes. The multi-compartmental material is also of importance because of its myriad applications in the fields of drug delivery, tissue engineering, sensors, water and hydrogen storage.4,5, 7,8 This unique structure could bring fantastic properties, such as anisotropic responses to external fields with a high degree of functional disparity on the same materials.7, 8,9 However, how to precisely control this complex structure and further achieve multilevel interior artificial materials is still a great challenge. In these years some approaches have been developed to fabricate the fibers with multilevel interior structures. Among these, electrospinning as an economical and effective approach is frequently applied for producing continuous micro/nano scaled fibers.10 By changing the process parameters, the multilevel structures can be achieved including core-shell structure,11 multicompartmental structure 12 and Janus structure.13 However, to directly fabricate the ultrathin fiber with a periodic hollow interior structure remains a great challenge. Another approach for producing the fibers with the similar structures is via microfluidic fabrication based on a


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microfluidic device.14-16 However, the size of the fibers obtained with this approach is generally over 50 μm or more. Additionally, the internal structure of the fibers appears to be like natural bamboo or peapod-like structures, however, there aren’t hollow and tubular chambers in the interior of the fibers.14-16 The structures can limit the fibers’ applications. The microfluidicelectrospinning technology derived from a combination of the above two approaches is prospective to produce small diameter fibers with multilevel internal structures. For example, Thorsen et al. reported that the microfluidic-electrospinning approach based on polydimethylsiloxane (PDMS)based microfluidic device was employed to produce a new bicomponent Janus nanofibers. 17 In this study we present a flexible bioinspired ultra-fine polymer fiber extremely like natural bamboo (Figure 1a) via the microfluidic-electrospinning technology. As shown in Figure 1b, firstly, monodispersed water-in-oil (W/O) emulsion droplets are generated through a glass capillary microfluidic device, which is very simple and low-cost.18 The oil phase is 20 wt.% biodegradable poly(d, l-lactic acid) (PLA, molecular weight: 102 kDa) in dimethyl carbonate (DMC); the water phase is glycerol water solution with various concentrations. Later, the droplets at the end of outlet of metal capillary are electrospun into bamboo-like polymer fibers by a conventional electrospinning device. By the combination of the two conventional and simple devices, we can obtain the well-defined bamboo-like fiber easily. The tubular fibers were fabricated under the glycerol concentration of 62% with the microfluidic-electrospinning fabrication; and the solid fibers with a same diameter were fabricated as control by the electrospinning device alone.


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Figure 1. Fabrication of the bamboo-like ultra-fine fibers. (a) Natural bamboo; (b) The combination of glass capillary microfluidic device and electrospinning setup; (c) TEM image of the fiber (the inserted is the SEM image of the cross-section of the fiber); (d) CLSM image of the fiber; (e) The schematic process and the actual process of generating the bamboo-like fibers.

From the transmission electron microscope (TEM) image of the fiber in Figure 1c, we can clearly see the hollow interior cavity structures with a shell layer of approximately 150 nm in thickness (arrow direction) connected with period nodes. In the inserted scanning electron microscope (SEM) image of the cross-section of the fiber, the interior hollow tubular structure can be also clearly seen. The internal structure is completely different from the solid interior of the fibers reported previously.14-16 From the confocal laser scanning microscopy (CLSM, Leica)


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images in Figure 1d, we can find that the polymer ultra-fine fibers with an average diameter of approximately 1.5 μm have a structure extremely like natural bamboo, which possess a hollow, tubular culm and periodic nodes. The empty chamber (black) can also be clearly seen, which was formed due to the evaporation of water; the shell layer and nodes (green) were marked with oilsoluble Coumarin-6, which is added in outer oil phase. Moreover, the multilevel interior structure of the bamboo-like fibers including the diameter and density of empty chambers can be precisely controlled by simply changing the concentration of inner water phase. The formation of the bamboo-shaped fiber is a combined process of electrospinning and electrospraying. We can thus estimate the size of the elliptical chamber in theory by the scaling law for electrified drops. For fluids with relatively small conductivity, e.g., the mixture of water and glycerol used in our work, the following scaling law has been identified as equation (1):19 𝑉∝

𝜀𝜌𝛾 2 𝜎𝜇3

（1）

where V is the drop volume corresponding to the chamber volume in fiber, ε, ρ, σ, μ are the permittivity, density, conductivity, and viscosity of the fluid, and γ is the interfacial tension between the drop phase and the continuous phase. We calculate the permittivity and density of the water-glycerol mixture by assuming that these two properties vary linearly with the water fraction. We measure the conductivity, viscosity, and the interfacial tension of the mixture at different glycerol concentrations. The values of these parameters and Support Information (SI). The variation trend of

𝜀𝜌𝛾2 𝜎𝜇3

𝜀𝜌𝛾2 𝜎𝜇3

are listed in Table S1 in the

agrees very well with our measurement on

the chamber volume, as shown in Figure 2.


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Figure 2. The precise control of the bamboo-like structure by changing glycerol concentration from 62% to 91%. (a) CLSM images of the fiber with various diameters of the empty chambers (D) along the long axis, Scale bar: 1 μm; (b) The aspect ratios of the chamber (D/L); (c) The average volume (V) of the chamber.

We also observe that the aspect ratio of the chamber (D/L) increases with the water concentration, while the width of the chamber along the short axis (L) does not change much, as shown in Figure 2a. We accredit the invariable chamber width to the restraint from the fiber diameter. In electrospinning process, the fiber diameter can be influenced by many factors including the polymer concentration, electric voltage, surface tension, and flow rate.20 In our work the former three parameters remain the same, the flow rate of oil phase varies within a small range


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between 1.8 mL/h and 2.2 mL/h and the flow rate of water phase is fixed at 0.5 mL/h. The fiber diameter is thus almost the same for different experiments; so does the chamber width. Therefore, larger chamber volume in Figure 2c results in larger aspect ratio (Figure 2b). At certain chamber size and aspect ratio, the distance between two chambers depends on the ratio of the flow rate of the two phases. The effect of flow rate of the inner water phase on the fiber structure was also investigated, where the concentration of glycerol is fixed at 62 wt%; however, the result displays that this influence almost can be ignored (Figure 2b, c and Figure S1 in SI). Therefore, we are able to manipulate the ultrathin-structure of the bamboo-shaped fibers by simply controlling the glycerol concentration in the drop phase. The modulus on a single bamboo-like fiber was analyzed with finite element (FEM) analysis. As shown in the upper images in Figure 3a, given the same force on these fibers including a bamboo-like fiber, a solid fiber and a tubular fiber, the deformation of the bamboo-like fiber is the biggest among the three kinds of samples. The stress distribution maps on both the cavities and nodes are also displayed in the bottom images of Figure 3a. We can see that when the force acts on the cavity, the stress generated is dispersed and focused on the adjacent nodes; therefore, the bamboo-like fiber exhibits the best stiffness in contrast to other two samples without the multilevel structure.


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Figure 3. The Young's modulus of a single fiber. (a) The strain distribution (Up) and the stress distribution (Down) of a bamboo-like fiber, a solid fiber, a tubular fiber under the same force calculated by finite element analysis; (b) The modulus map with fiber topography (Left) and the modulus distribution (Right) of the three classes of fiber measured with AFM.


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The mechanical property of the single bamboo-like fiber was also investigated with Atomic force microscopy (AFM, FlexAFM 5) measurements with C3000 controller and Tap190 cantilever (k = 21 N/m, ds = 68.3 nm/V) as shown in Figure S2 in SI. AFM imaging is in dynamic mode. Force spectroscopy map (32 × 32 pixels) is obtained with a maximum contact force of 800 nN. A Hertz model fit was applied to the forward force curve. For each sample, the topography was overlaid with the modulus map. The black and pink colors in the modulus map correspond to pixels for which no modulus could be calculated (e.g. the surface was not reached in the gap of the substrate) or which were clipped to emphasize the differences of the modulus on the fiber. In Figure 3b, the left images are the overlaid images of the fiber topography and the modulus map of a bamboo-like fiber, a solid fiber and a tubular fiber, respectively; the right images are the Young’s modulus distributions for the three classes of samples. From the modulus distribution images, it can be seen that the modulus on the bamboo-like fiber is ranging from 2-7 MPa, which is much lower than that (60-150 MPa) of tubular fiber, also smaller than that (15-50 MPa) of solid fiber. Accoding to the modulus distribution, the average young’s modulus of different fibers can be calculated as shown in Figure S3 in supporting information. The average young’s modulus of bamboo-like fibers is 4.95 ± 10.55 MPa, which is significant lower that of tubular fiber (73.82 ± 60.54 MPa) and solid fiber (43.78 ±54.94 MPa). This result indicates that the polymer fiber has a superior flexibility, endowing it a large shape deformation. The result is in good agreement with the FEM analysis in Figure 3a. The significant decrease of the Young’s modulus can be ascribed to the multilevel interior structure of combined empty chambers and periodic nodes similarly with natural structure of bamboo. The multilevel hollow interior structure of the fiber with extreme toughness makes it possess wide applications in biomedical field, industry and agriculture fields.


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In summary, we successfully develop a flexible bamboo-like ultra-fine polymer fiber via a combined microfluidic-electrospinning technology for the first time. The fiber possesses periodic hollow and tubular chambers with a shell layer of approximately 150 nm in thickness. Both the diameter and density of the chambers can be readily tuned by simply changing the concentration of inner water phase. A single bamboo-like fiber with a diameter of approximately 1.5 µm exhibits the Young’s modulus ranging from 2-7 MPa measured by Atomic force microscopy (AFM), which is much lower than that (60-150 MPa) of tubular fiber, also smaller than that (15-50 MPa) of solid fiber. The modulus on a single bamboo-like fiber analyzed with finite element (FEM) analysis is also in good agreement with the AFM test. The periodic hollow interior structure and resultant extreme toughness have a great potential for broadening the polymer fiber applications in medicine, industry, and agriculture.

ASSOCIATED CONTENT Supporting Information The properties of the glycerol solutions in water at different concen-trations, the CLSM images showing the effect of flow rate of the inner water phase on the fiber structure and the measurement of modus of a single fiber with AFM. The Supporting Information is available free of charge on the ACS Publications website. AUTHOR INFORMATION

* S. Zhou, Email: [email protected]; X. Hou, Email: [email protected]


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‡These authors contributed equally.

The authors declare no competing financial interests.

This work was partially supported by the China National Funds for Distinguished Young Scientists (51725303), National Natural Science Foundation of China (Nos. 21574105, 51703189) and the Sichuan Province Youth Science and Technology Innovation Team (Grant No.2016TD0026). We greatly thank Dr. F. Liu at Nanosurf China Co. at Shanghai for AFM measurements.

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A Flexible Polymer Ultra-Fine Fiber with Extreme Toughness - ACS

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