Self-Assembly of Electrospun Polymer Nanofibers: A General

Mar 22, 2011 - If the humidity is higher than the above limits, the self-assembled fiber clusters cannot grow high into the 3D walls although the self...
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LETTER pubs.acs.org/Langmuir

Self-Assembly of Electrospun Polymer Nanofibers: A General Phenomenon Generating Honeycomb-Patterned Nanofibrous Structures Guodong Yan,† Jie Yu,*,† Yejun Qiu,† Xiaohui Yi,† Jing Lu,† Xiaosong Zhou,† and Xuedong Bai‡ †

Department of Materials Science and Engineering, Shenzhen Graduate School, Harbin Institute of Technology, Xili, Shenzhen 518055, P. R. China. ‡ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P. R. China

bS Supporting Information ABSTRACT: A general phenomenon that electrospun polymer nanofibers self-assemble into honeycomb-patterned nanofibrous structures (HNFSs) is reported. We used electrospinning to produce charged polymer nanofibers, which were kept in liquid state (wet) on landing on the substrates by appropriately controlling the electrospinning conditions. Driven by the competitive actions of surface tension and electrostatic repulsion, these charged wet nanofibers self-assemble into the HNFSs. Fabrication of the well-defined three-dimensional HNFSs was successfully demonstrated for three different polymers, that is, polyacrylonitrile, polyvinyl alcohol, and polyethylene oxide. The pore diameter of the obtained honeycomb structures spans a wide range from micrometers to over 200 μm with depths as large as over 150 μm. The pore walls are composed of uniaxially aligned polymer nanofibers.

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elf-assembly allows arrangement of small components, especially nanoscale objects, into ordered systems or aggregates with ease, which otherwise is expensive, slow, and complex. By using self-assembling phenomena, small components from molecules1 3 through nanoscale particles4,5 to meso- and macroscopic objects6 9 can assemble into ordered aggregates with desired structures such as monolayers,3 superlattices,4,5 tubes,2,8 and honeycomb microporous films .1 So far the reported selfassembly occurred mainly for the equiaxial components (e.g., particles) and those with small aspect ratio (e.g., molecules and nanorods), while that of continuous or long nanofibers has rarely been reported because they are generally entangled together with no moving freedom. Here, we report on the well-controlled selfassembly of continuous polymer nanofibers into honeycombpatterned nanofibrous structures (HNFSs). We used electrospinning to produce charged polymer nanofibers, which were kept in liquid state (wet) on landing on the substrates by appropriately controlling the electrospinning conditions. Driven by the competitive actions of surface tension and electrostatic repulsion, these charged wet nanofibers self-assemble into the HNFSs. Electrospinning is a simple and versatile method to produce continuous polymer nanofibers by using electrostatic force.10,11 Many interesting applications have been demonstrated for the electrospun nanofibers, including functional filters and textiles,12,13 drug delivery,14 scaffolds for tissue engineering,15,16 sensors,17,18 catalysis,13,19,20 energy harvesting,21 electronic and r 2011 American Chemical Society

optical devices,22,23 and so forth. The electrospun nanofibers are highly charged and retain their charge even after deposition on substrates.23 26 Generally, the electrospun nanofibers are collected on the substrates with random orientation. Uniaxial alignment of the electrospun nanofibers has been achieved through the electrostatic repulsion induced self-assembly.23,26 The selfassembling phenomenon of the electrospun nanofibers into the honeycomb structures has ever been observed previously.27 However, up to now, the self-assembling mechanism remains a mystery and the HNFSs with well-controlled pore shape, size, depth, and wall structure have not been prepared yet. In this work, the factors influencing the self-assembly of the electrospun polymer nanofibers were disclosed and a possible mechanism to explain the self-assembling phenomenon of the electrospun nanofibers was established. The well-controlled self-assembly of the electrospun nanofibers was demonstrated for three different polymers, indicating that this is a general phenomenon applicable to many spinnable polymers. Self-assembly of the electrospun nanofibers was investigated for three different polymers, that is, polyacrylonitrile (PAN), polyvinyl alcohol (PVA), and polyethylene oxide (PEO). Both conducting and insulating substrates including Al foils, plastic films, glass sheets, and wooden boards were used to collect the Received: December 1, 2010 Revised: March 5, 2011 Published: March 22, 2011 4285

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Figure 1. SEM images showing the surface morphology and wall structure of the PAN HNFSs on Al substrates electrospun from PAN/ DMF solutions at a voltage of 22 kV and different concentrations: (A,B) 3% and (C,D) 2%.

electrospun nanofibers. During experiments, dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were used as the solvents to dissolve PAN and water was used to dissolve PVA and PEO. Figure 1 shows the scanning electron microscopy (SEM) images of the HNFSs electrospun from the PAN/DMF solutions with different concentration (w/v). All the SEM images shown in this paper were taken by rotating the sample holder for 45° (unless otherwise stated) in order to observe the threedimensional (3D) structures clearly and evaluate the pore depth. It is observed that the electrospun products self-assembled into the HNFSs instead of forming the uniformly distributed nanofibers as commonly observed for electrospinning. The pores exhibit the shape of polygonal channel running through the film thickness, and their walls tend to be three-branched with the intersecting angle approaching 120° generally (Figure 1A and C). Some pores are nearly hexagonal. The pore sizes of the HNFSs prepared at concentrations of 3% and 2% are 13 30 and 4 11 μm, respectively, suggesting a dependence of pore size on solution concentration. The pore walls are composed of many uniaxially aligned nanofibers (Figure 1B and D), which tend to conglutinate with each other. These nanofibers are not very uniform in diameter and exhibit the morphology of the so-called beaded nanofibers.28 The thinner parts and bead parts of the constituting nanofibers of the HNFSs electrospun at the concentration of 3% are about 20 40 and 80 160 nm, respectively, and that at the concentration of 2% are 15 40 nm and 70 150 nm, respectively. Appropriate solution concentrations were required for the self-assembly of the PAN nanofibers. The selfassembly can be observed in the concentration range of 2 6% for the PAN/DMF solution. However, the well-defined 3D HNFSs can only be obtained at concentrations of 2% and 3%. At concentrations of 4% 6%, the self-assembly can only occur for partial nanofibers that are thicker and wet (Support Information Figure S2). At the concentration of 7%, the self-assembly cannot be observed and the electrospun products are randomly distributed dry nanofibers without self-assembly. At too low concentration (1%), the electrospun products are solution droplets, which generally merge to form continuous films (Support Information Figure S3). We also found that the pore size increases with decreasing the electrospinning distance (Supporting

Figure 2. SEM images showing the surface morphology and wall structure of the HNFSs of PVA and PEO electrospun at different conditions: (A,B) PVA, concentration 6%, 22 kV, on plastic films; (C,D) PEO, concentration 16%, 22 kV, on Al substrates; (E,F) PEO, concentration 16%, 19 kV, on Al substrates.

Information Figure S4), which we believe to be related to the increase of the electrical field strength at shorter distance and similar voltage. It was observed that a portion of the nanofibers suspend between the pore walls sometimes rather than incorporating into the walls (Figure 1A), which generally occurs at higher solution concentration or longer electrospinning distance. This is because the nanofibers are drier and stronger under the circumstances. Electrospinning of PAN/DMSO solutions with concentrations of 1%, 2%, and 3% was also carried out under conditions similar to those for electrospinning PAN/DMF solutions (Figure 1 and Supporting Information Figure S3) in order to investigate the effects of solvent boiling point on the selfassembly of the electrospun products. However, only merged films were observed for all the obtained samples (Supporting Information Figure S5). This should be because the boiling point of DMSO (189 °C) is higher than that of DMF (153 °C), which makes the DMSO solvent evaporate slower and the electrospun products contain more solvent. However, further detailed work is needed to investigate the self-assembly of the electrospun products from the PAN/DMSO solutions. We believe that the self-assembled HNFSs can be obtained from the PAN/DMSO solutions by improving the electrospinning conditions, such as electrospinning at high temperature to accelerate solvent evaporation or increasing the electrospinning distance to increase the evaporation time. Figure 2 shows the SEM images of the electrospun products from the PVA and PEO solutions, which indicates that the electrospun products of both PVA and PEO can self-assemble into the well-defined 3D HNFSs. For PVA, the pore size is mostly in the range of 130 250 μm and the depth is over 150 μm (Figure 2A). The pore walls are also composed of many well 4286

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Langmuir aligned beaded nanofibers (Figure 2B). The diameters of these beaded nanofibers are 40 70 nm in the thinner parts and 260 500 nm in the bead parts. These nanofibers partially merged with each other, making the pore walls porous also. The electrospun PEO products can form even more regular HNFSs as shown in Figure 2C, the pores of which are mostly hexagonal. The decrease in pore size with decreasing the voltage was observed; for example, the pore size decreases to 65 95 μm (Figure 2E) from 90 to 130 μm (Figure 2C) when the voltage decreases to 19 kV from 22 kV. The pore walls of the HNFSs prepared at 22 kV are solid (Figure 2D), which is because the electrospun fibers are too wet and fully merged. At lower voltage (Figure 2F) or longer distance (Support Information, Figure S6), the pore walls tend to be nanofibrous. This is because the electrospun nanofibers are drier under the circumstances and cannot fully merge. We also found that the pore size decreases with increasing the collecting distance, which can be observed by comparing Figure 2C and Supporting Information Figure S6. This should be because the charge density may decrease for the nanofibers landing at longer distance due to the longer discharging time during flight. As reported,29 the diameter of the electrospun fibers may decrease at lower voltage due to the decrease of mass flow, which makes the fibers drier on reaching the substrates. At longer distance the electrical field strength decreases and the flight time increases, which makes the landing fibers drier. The self-assembly was observed in the concentration range of 5 7% for PVA and 13 20% for PEO. The pore size increases with increasing solution concentration was also observed. The substrates used in this work are connected with a 1  1 mm2 tip electrode. The electrostatic field between the charged polymer droplet (at the bottom of the needle tip) and the tip electrode can be approximated by the electrostatic field between two point charges. When introducing insulating substrates to the electrostatic field, in light of their few effects on the electrostatic field distribution,30 the approximation can still be effective. In such cases, the electrostatic field lines launch from the charged droplets and converge toward the point charge (tip electrode) just like that described in Theron’s paper.31 In that paper, a rotating disk with a tapered edge was used to catch electrospun fibers. They found that most electrospun fibers were attached to the sharp edge due to the higher field strength near the edge. A similar phenomenon was observed in our experiments: those places near the tip electrode attached more nanofibers due to their higher electrostatic field strength and consequently leading to deeper HNFSs on the substrate’s surface. As for using Al substrates, the electrostatic filed lines are not as converged as those field lines generated when using insulating substrates, leading to more uniform distribution of the HNFSs. However, the pore size is generally smaller for the HNFSs on Al substrates than that on insulating substrates (Supporting Information Figure S7). As indicated above, the self-assembling behavior of the electrospun nanofibers such as the effects of solution concentration, electrospinning voltage, substrate nature, and electrospinning distance is about similar for the three polymers, but the selfassembled structures are different more or less. In the optimal concentration ranges for self-assembly and at similar voltage and distance, the pore size of the obtained HNFSs decreases in the sequence of PVA, PEO, and PAN. The nanofibrous shape in the HNFSs is better kept for PAN, while the nanofibers are apt to merge for PEO and PVA. This is because the PEO and PVA nanofibers landing on the substrates contain more solvents due to their higher hydrophilicity. Simultaneously, the HNFSs tend

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Figure 3. Schematic showing the self-assembling mechanism of the wet electrospun nanofibers.

to be more regular for PEO and PVA (Figure 2). This is because the mobility of the electrospun nanofibers is higher for the PEO and PVA nanofibers containing more solvents, ensuring the full evolution of the self-assembled structures. We have found that ambient humidity plays a key role in the self-assembly of the electrospun nanofibers. The well-defined 3D HNFSs can only form when the humidity (RH) is decreased below 60%, 45%, and 40% for PAN, PVA, and PEO, respectively. If the humidity is higher than the above limits, the self-assembled fiber clusters cannot grow high into the 3D walls although the self-assembling phenomenon can still be observed (Supporting Information Figure S8). The self-assembling mechanism of the electrospun nanofibers is a problem that has puzzled us for a long time. We noticed that the self-assembly occurred at low solution concentrations and the nanofibers constituting the pore walls merged with each other, while the dry nanofibers cannot form the HNFSs. This indicates that the nanofibers must be in fluid state for the occurrence of self-assembly. Because the electrospun nanofibers are electrically charged, we consider that surface tension and electrostatic repulsion are the driving forces of the self-assembly of the wet electrospun nanofibers. The role of surface tension is to make the wet nanofibers stick and merge together when they contact each other, while the electrostatic repulsion may resist this mergence and try to push the fibers or the conglutinated fiber clusters away from each other when they approach. Based on the competitive actions of the above two forces, we propose a simplified model to explain the formation mechanism of the HNFSs. The cases occurring in the self-assembly of the electrospun nanofibers are very complicated and diversified. This model is established by analyzing the formation process of the three-branched walls. When two or more wet nanofibers cross or contact each other at certain point, the surface tension may drive the segments near the contacting point to merge together, while the electrostatic repulsion tries to push them away from each other, resulting in the formation of partially overlapped fiber clusters (Figure 3I). With the merging of the nanofibers, the charge density on the fiber clusters increases and therefore the electrostatic repulsion to the incoming nanofibers increases. When the nanofibers land 4287

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Langmuir and subsequently touch the fiber clusters, the surface tension may drive the part near the contacting position to incorporate into the clusters while the adjacent section farther away from the contacting point may be bent outward by the increased electrostatic repulsion. Simultaneously, the fiber clusters will be bent reversely, forming the branched structures (Figure 3II). Driven by the electrostatic repulsion, the depositing nanofibers try to land on or move toward the positions with less charge to make the merged clusters similar in size (Figure 3III and IV). Because the mechanical stability can be most easily achieved for the three-branched structure with an intersecting angle of 120°, the nanofibers tend to form the networks composed of three-branched clusters (Figure 3IV). Simultaneously, the landing nanofibers captured by the clusters are pushed upward by the electrostatic repulsion, making the clusters grow high into the walls of the 3D HNFSs (Figure 3V). The alignment of the nanofibers in the walls results from the electrostatic repulsion between adjacent fibers and that between adjacent sections of the individual fibers as occurred for preparing the aligned electrospun nanofibers.23,26 This model coincides with the experimental results. The shape of the hexagonal pores, the three-branched walls with intersecting angle approaching 120°, and the orientation of the nanofibers at the corners frequently observed from the SEM images (Supporting Information Figure S9) support the rationality of our model. For all three polymers, the self-assembly occurred mainly for the beaded nanofibers, which is because the bead parts are thick and can be kept wet on the substrates. It is believed that the humidity plays a role mainly by influencing the discharge process of the charged nanofibers. The charge amount dissipated by corona discharge increases with increasing humidity, resulting in the decrease of the charge density on the nanofibers.25,32 Consequently, at too high humidity, the electrostatic repulsion may be too small to support the self-assembly. The smaller pore size on conducting substrates than that on insulating substrates is because more charges were removed by the conducting substrates than the insulating substrates. We have found that with increasing concentration the electrical conductivity (σ) of the solutions increases and the surface tension (fs) changes not much for the three polymer solutions (Figure 4). It is known that the electrical conductivity of solutions is positively proportional to their ion concentration. So the charge density of the electrospun nanofibers increases with increasing the solution concentration and thus the electrostatic repulsion increases correspondingly, resulting in the increase of pore size at higher concentration. The differences in pore size for the three polymers can be ascribed to their differences in σ (σPVA > σPEO > σPAN) and fs (fsPVA > fsPEO > fsPAN) (Figure 4). Higher σ generates larger electrostatic repulsion and thus increases the pore size. The merging of the wet nanofibers is driven by their fs, so the length of the merged sections (corresponding to the side length of the polygonal pores) increases with fs, resulting in the corresponding increase of the pore size. The present work established a general method to prepare the HNFSs of polymers. By using the polymeric HNFSs as templates, various inorganic HNFSs such as oxides and composites can be prepared. Many applications as demonstrated for the electrospun nanofibers may be enhanced by this novel structure. The HNFSs of biocompatible polymers such as PVA prepared by the present method show great promise in applications such as scaffolds for tissue engineering due to the relatively regular porous structure, suitable pore size, nanofibrous walls, and the presence of the interpore channels.33,34 Other applications

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Figure 4. Electrical conductivity (A) and surface tension (B) versus concentration for the three polymer solutions.

include catalyst supports, filters, sensors, drug delivery, microcontainers, and so forth. In conclusion, self-assembly of the wet electrospun nanofibers into the well-defined 3D HNFSs was successfully demonstrated for three different polymers on various substrates by using both water and organic DMF as the solvents, indicating that this is a general phenomenon and applicable to many spinnable polymers. The factors influencing the self-assembly of the electrospun nanofibers were disclosed, and a possible mechanism to explain the self-assembling phenomenon of the electrospun nanofibers was established. The self-assembly of the wet electrospun nanofibers can be ascribed to the competitive actions of surface tension and electrostatic repulsion. Well-defined 3D HNFSs with controllable structure and dimension were successfully prepared for PAN, PVA, and PEO based on the systematic study on the self-assembling processes.

’ ASSOCIATED CONTENT

bS

Supporting Information. Experimental procedures, figures showing the schematic of the electrospinning setup, and additional SEM images of the electrospun products. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*Telephone: 0086-755-26033478. Fax: 0086-755-26033504. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the NSFC (Grant Nos. 50572019 and 50972033), new century excellent talents in university (NCET060343), and basic research foundation of Shenzhen Bureau of Science Technology & Information (JC200903120176A).

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