Controllable fabrication of soap-bubble-like structured polyacrylic acid nano-nets via electro-netting

Abstract

Soap-bubble-like structured polyacrylic acid (PAA) nano-nets that comprise interlinked ultrathin nanowires with diameters of 10–35 nm are controllably prepared by a one-step electro-netting process.

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One-dimensional (1D) nanoscale materials have stimulated great interest in recent years because of their fundamental properties and potential applications in many different areas.¹ Nanofibers represent an important 1D nanostructure, standing at the leading edge of nanoscience and nanotechnology. Compared with other methods of fiber fabrication such as template synthesis, drawing, phase-separation, etc.,^2,3electrospinning has become a powerful fabrication method for producing micro- and nanoscaled fibers by applying a high voltage on a polymer solution from a micro-syringe pump.^4,5 Up to now, a wide variety of materials such as polymers, ceramics, and carbon have been processed into uniform fibers or fibrous assembliesviaelectrospinning,⁶ with applications in areas ranging from filtration, sensors, drug delivery platforms, tissue engineering, cosmetics, and so on.^7,8

Electrospun nanofibrous membranes typically have fiber diameters in the range 100–500 nm, but properties such as surface area and porosity become more significant when the fiber diameter falls below 50 nm.⁹ The large average diameter of common electrospun fibers limits their further applications in ultrafiltration, ultrasensitive sensors, catalysts, etc.¹⁰ Consequently, a current major challenge is to develop robust methods for manufacturing large-scale and extremely small nanofibers. Great efforts have been made to study the effects of basic processing parameters on the structure and morphology of electrospun fibers. The nanofiber diameter is commonly changed by adjusting the polymer concentration, with lower concentrations resulting in thinner but beaded fibers.¹¹ Doshi and Reneker¹² found that a higher net charge density of the polymer solution could yield thinner fibers with no beads. Deng et al.¹³ attempted to enhance the melting temperature in melt electrospinning to reduce the diameter of electrospun nanofibers. But the objective of reducing fiber diameter to 50 nm was rarely achieved.

Polyacrylic acid (PAA) is a highly water absorbing and protein resistive hydrogel material widely used in medical fields.¹⁴ Growing attempts were reported to regulate the morphology of PAA fibers to achieve desired functions.^9,15,16 Ding et al.⁹ first reported the formation of PAA nano-nets, and concluded that nano-nets can be obtained by adjusting the solution properties and several parameters in the process of electrospinning. Wang et al.¹⁵ have demonstrated that a humidity sensor could be prepared by electrospinning deposition of PAA nano-nets on the electrodes of quartz crystal microbalances. Despite previous publications which have demonstrated the formation feasibility of nano-nets, to date, current work has mainly focused on varying the processing parameters and the synthesis of well controlled nano-nets in large quantities remains a very important and challenging issue.

In the present study, we report a facile process to controllably prepare large-scale PAA nano-nets via the regulation of the intrinsic properties of PAA solutions such as conductivity, surface tension and viscosity. We refer to this novel process as “electro-netting”, which accompanies the electrospinning process and allows one-step fabrication of ultrathin nano-nets in large quantities and with uniform size. To achieve this objective, various additives, such as salts (NaCl, Cu(NO₃)₂, AgNO₃), dodecylbenzene sulfonic acid (DBSA), and silica, were incorporated into the PAA solutions and their effects on the internal morphology of the electrospun nanofibrous membranes were also investigated. To the best of our knowledge, there has been no research reported about the incorporation of various additives to control the structure of PAA nano-nets. The setup for electrospinning/electro-netting deposition of fibrous membranes on a grounded roller is depicted in Fig. 1a. The PAA solutions were loaded into a syringe that was attached to a high-voltage power supply (DW-P303-1ACD8) that was capable of generating voltages up to 30 kV. A syringe pump (LSP02-1B) was used to regulate the flow of the solutions at 3 mL h⁻¹. The fibrous membranes were deposited on the aluminium foil-covered grounded metallic rotating roller at a 15 cm tip-to-collector distance. Fig. 2 presents the field emission scanning electron microscopy (FE-SEM) images of pure PAA fibrous membranes formed from various kinds of solvents. As typical features of electrospun fibrous structures, the resultant nanofibers were randomly oriented as three-dimensional (3D) porous membranes with a wide fiber diameter distribution. The as-prepared PAA fibers spun using H₂O as the solvent have diameters ranging from 200 to 830 nm (Fig. 2a). Several discontinuous irregular defect films with maximum lengths of up to 20 μm were found among the electrospun PAA fibers that were spun using H₂O as the solvent. This could be due to the formation of strong hydrogen bonds between water and the large amount of carboxyl groups of PAA, which leads to incomplete solvent evaporation and thus the formation of defect films.⁹ The 3D porous fibrous membranes with irregular defect films were still observed in the membrane formed using ethanol and H₂O as cosolvents (Fig. 2b). The corresponding high-magnification FE-SEM image in Fig. 2b confirms that the defect films partly split into nano-nets, which may be attributable to the fast phase separation of the polymer and highly volatile solvent in the charged droplet. It is worth noting that the fibers formed from the cosolvents showed a slightly increased diameter (745 nm) which corresponds to the increased viscosity (585 cps) and decreased conductivity (129 μS m⁻¹).

Fig. 1 Schematic diagrams illustrating the possible mechanism of nano-net formation during the electrospinning/electro-netting process (a) and the forces acting on the charged droplet (b). (c) Typical FE-SEM image of PAA/DBSA nano-nets.

Fig. 2 FE-SEM images of pure PAA fibers spun using different solvents: (a) H₂O, (b) H₂O–ethanol (50/50 weight ratio), and (c) ethanol. (d) Optical image of soap bubbles. Histograms showing the wire diameter distribution (e) and pore-width distribution (f) of the nano-nets presented in (c).

Fig. 2c provides the morphology of fibrous PAA membranes formed from using ethanol alone as the solvent. In comparison with the fibers formed from H₂O or H₂O–ethanol cosolvents, the fibers showed an increased average diameter (841 nm) due to the sharply decreased conductivity.¹⁷ More interestingly, the formation of a few nano-nets with soap-bubble-like structure (Fig. 2d) was observed among the main fibers. The conventional electrospun PAA fibers acted as a support for the nano-nets comprising interlinked 1D nanowires. The histogram in Fig. 2e indicates that the major distribution region (over 70%) of PAA nanowire diameters was in the range of 10–22 nm with an average diameter of 20 nm, which was one order of magnitude less than that of common electrospun nanofibers. The region of pore-width distribution of PAA nano-nets ranged from 20 to 550 nm (Fig. 2f), which was much less than that of pores among electrospun nanofibers. Therefore, nano-nets possess great potential for application in filtration systems for the removal of particles or viruses with sizes in the nanometre range. Moreover, the nano-nets display clear geometric characteristics with ideal and weighted Steiner-tree structure,¹⁸ in which three neighboring nanowires form a three-way junction with angular symmetry and topological invariability (indicated by dotted circle). Fig. 1a presents the possible formation mechanism of nano-nets. The instability of the Taylor cone induced by the high electric field leads to the formation of the electrospray droplets.¹⁹ During the flight of charged droplets from the capillary tip to the collector, the microsized droplet is distorted and expanded into a thin film due to the various forces it endures (Fig. 1b) such as electrostatic force, air resistance, gravity, coulombic repulsion, surface tension and viscoelastic force.²⁰ The occurrence of rapid phase separation on the splitting-film and its obeyed minimal energy principle may result in the formation of such unexpected soap-bubble-like structured nano-nets (Fig. 1c). These versatile nano-nets create enhanced interconnectivity and additional surface area, which may significantly boost the properties for practical applications.

The FE-SEM images in Fig. 3 show the variation in morphology as the PAA fibers are loaded with three kinds of salts. All of the three samples possessed more or less net structures and the PAA/NaCl composite membranes exhibited a highly interconnected soap-bubble-like structure. The larger area density of the nano-nets may result from the greatly increased formation probability of microsized droplets due to the increased instability of the Taylor cone after NaCl is added into the polymer solutions. It is noteworthy that a small quantity of ribbon-like defect film with a uniform width of 2 μm was found among the PAA/AgNO₃ fibers (Fig. 3b), while the defect films of larger width distributed among the PAA/Cu(NO₃)₂ fibers were clearly revealed (Fig. 3c). Moreover, the enlarged FE-SEM images (insets of Fig. 3a and b) confirmed that the nano-nets were stacked layer-by-layer. The results from Brunauer-Emmett-Teller (BET) surface area measurements also support these observations (Table 1). Compared with the PAA/AgNO₃ and PAA/Cu(NO₃)₂ composite membranes, the PAA/NaCl composite membranes show the largest surface areas (6.57 m² g⁻¹) due to the smallest average fiber diameter (330 nm) and the existence of dense nano-nets with smaller nanowire diameters of 14 nm (Fig. 3a and Table 1). Additionally, ions with smaller atomic radii have a higher charge density and thus a higher mobility under an external electric field.²¹ Thus the elongation forces imposed on the jet with NaCl should be higher than that with AgNO₃ and Cu(NO₃)₂, since Na⁺ and Cl⁻ have smaller radii than Ag⁺, Cu²⁺ and NO₃⁻. Therefore, the PAA/NaCl composite membranes possessed the smallest average fiber and nanowire diameters.

Fig. 3 FE-SEM images of PAA fibers containing 0.1 wt% salts: (a) NaCl, (b) AgNO₃, and (c) Cu(NO₃)₂.

Table 1 Solution properties and characteristics of the resulting nanofibers

Solvents	Additives	Viscosity/cps	Conductivity/μS m^−1	Average fiber diameter/nm	Average nanowire diameter/nm	BET surface area/m^2 g^−1
H2O	—	170	1401	510	—	—
H2O–ethanol (50/50 wt/wt)	—	585	129	745	—	—
Ethanol	—	336	6.4	841	20	3.30
	0.1 wt% Cu(NO3)2	350	200	1360	20	6.35
	0.1 wt% AgNO3	340	145	364	20	6.42
	0.1 wt% NaCl	337	240	330	14	6.57
	0.1 wt% DBSA	345	75	818	23	5.84
	0.3 wt% DBSA	338	196	802	23	2.84
	0.5 wt% DBSA	340	314	646	22	5.00
	8 wt% silica	335	7.3	683	18	3.76
	10 wt% silica	350	7.2	1150	20	4.08
	12 wt% silica	360	7.3	1160	35	2.36

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Inspired by soap bubbles, which exhibit highly stable Steiner networks, here we considered the addition of surfactant into the electro-netting PAA solutions to decrease the surface tension and mimic the fascinating structure of soap bubbles (Fig. 2d), and the results conform to the expectation. Fig. 4 displays FE-SEM images of PAA fibers formed from the PAA solutions containing various concentrations of DBSA. The surfactant was so effective that a concentration as low as 0.1 wt% was enough to produce nano-nets of large area and uniform size. High magnification images (insets of Fig. 4) reveal that the nano-nets are more regular due to a more stable jet that encounters fewer perturbations from the surface tension effect of reducing the surface area. Additionally, the average fiber and nanowire diameters were both slightly decreased with increasing concentration of DBSA in the solutions (Table 1). This effect can be understood as follows. With a lower surface tension, the whipping jet is more easily elongated by the electrostatic forces, resulting in smaller fiber diameters.¹⁰ Moreover, the surfactant was also found to improve the solution conductivity, but had no obvious effect on the viscosity. The increased solution conductivity favored the formation of thinner fibers. Lin et al.²² have demonstrated that a small amount of cationic surfactant effectively prevents the formation of beaded fibers during electrospinning. They attribute this effect to the reduced surface tension caused by the surfactant. These results from Fig. 4 indicate that the surface tension is likely the primary factor that contributed to regulating the morphology of the fibers and nano-nets.

Fig. 4 FE-SEM images of PAA fibers formed from PAA solutions containing various concentrations of DBSA: (a) 0.1, (b) 0.3, and (c) 0.5 wt%.

Fig. 5 shows FE-SEM images of a series of PAA/silica composite membranes loaded with different amounts of silica nanoparticles at low and high magnifications. Relatively smooth fibers were observed when the silica content was 8 wt% (Fig. 5a), and the fiber surfaces became rough when the silica content was above 10 wt% (Fig. 5b and c) due to the presence of silica nanoparticles on the surfaces. Concerning the formation of nano-nets within the electrospun fibrous membranes, very visible nano-nets fastened to the main fibers are observed when the silica content is below 10 wt% (Fig. 5a and b). However, no nano-nets except several tiny nanowires (35 nm) were found in the PAA/silica composite membranes loaded with 12 wt% silica. Moreover, the diameters of PAA/silica fibers increased with increasing silica content, which may be mainly attributable to the increased viscosity of the solutions. At high silica content (12 wt%), the high viscosity of the solution limited the extent of jet stretching during whipping, thus leading to larger fiber diameter. This observation is the opposite to that reported in the literature²³ for PMMA/silica composite fibers that the average fiber diameter is decreased with increasing silica content. In general, the increase in solution viscosity increases the fiber diameter and the increase in solution conductivity shows the opposite effect. The conductivity has the predominant effect on fiber morphology in the PMMA/silica system, but the conductivity is basically unchanged with varying the contents of silica in the PAA/silica system (Table 1).

Fig. 5 FE-SEM images of PAA fibers formed from PAA solutions loaded with various contents of silica: (a) 8, (b) 10, and (c) 12 wt%.

In conclusion, we have developed a one-step method to controllably produce a large area density of nano-nets via the introduction of various additives into electro-netting solutions. Nano-nets that comprise interlinked 1D ultrathin nanowires (∼35 nm) are stacked layer-by-layer and widely distributed in the 3D structured membranes. The final morphology of the nano-nets, including nanowire diameter, area density and pore-width of the nano-nets, is highly dependent on the solvent, and the type and concentration of additives. Additionally, the versatile nano-nets with small pore-widths create additional surface area, which would be particularly useful for applications such as ultrafiltration, ultrasensitive sensors, catalyst supports, etc. We believe that the electro-netting process could be easily extended to the production of soap-bubble-like structured nano-nets for various polymeric materials.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 50803009 and 10872048), the “111 Project” (No. 111-2-04 and B07024), the Shanghai Committee of Science and Technology (No. 10JC1400600), the National Basic Research Program of China (973 Program, 2011CB606100), and the Fundamental Research Funds for the Central Universities.

References

1. J. Q. Hu, Y. Bando and Z. W. Liu, Adv. Mater., 2003, 15, 1000–1003.
2. S. Maheshwari and H. C. Chang, Adv. Mater., 2009, 21, 349–354.
3. Y. D. Xia, Z. X. Yang and R. Mokaya, Nanoscale, 2010, 2, 639–659.
4. B. Ding, M. R. Wang, X. F. Wang, J. Y. Yu and G. Sun, Mater. Today, 2010, 13, 16–27.
5. F. Yao, L. Q. Xu, B. P. Lin and G. D. Fu, Nanoscale, 2010, 2, 1348–1357.
6. Y. Dzenis, Science, 2004, 304, 1917–1919.
7. J. W. Xie, M. R. MacEwan, A. G. Schwartz and Y. N. Xia, Nanoscale, 2010, 2, 35–44.
8. X. F. Wang, B. Ding, M. Sun, J. Y. Yu and G. Sun, Sens. Actuators, B, 2010, 144, 11–17.
9. B. Ding, C. R. Li, Y. Miyauchi, O. Kuwaki and S. Shiratori, Nanotechnology, 2006, 17, 3685–3691.
10. K. Lin, K. N. Chua, G. T. Christopherson, S. Lim and H. Q. Mao, Polymer, 2007, 48, 6384–6394.
11. J. M. Deitzel, J. Kleinmeyer, D. Harris and N. C. B. Tan, Polymer, 2001, 42, 261–272.
12. J. Doshi and D. H. Reneker, J. Electrost., 1995, 35, 151–160.
13. R. J. Deng, Y. Liu, Y. M. Ding, P. C. Xie, L. Luo and W. M. Yang, J. Appl. Polym. Sci., 2009, 114, 166–175.
14. Y. Lu, D. F. Wang, T. Li, X. Q. Zhao, Y. L. Cao, H. X. Yang and Y. Y. Duan, Biomaterials, 2009, 30, 4143–4151.
15. X. F. Wang, B. Ding, J. Y. Yu, M. R. Wang and F. Pan, Nanotechnology, 2010, 21, 055502.
16. D. C. Parajuli, M. P. Bajgai, J. A. Ko, H. K. Kang, M. S. Khil and H. Y. Kim, ACS Appl. Mater. Interfaces, 2009, 1, 750–757.
17. L. W. Ji, A. J. Medford and X. W. Zhang, Polymer, 2009, 50, 605–612.
18. G. S. Li, Y. J. Yin, Y. Li and Z. Zhong, Chin. Sci. Bull., 2009, 54, 3220–3224.
19. G. Larsen, R. Spretz and R. Velarde-Ortiz, Adv. Mater., 2004, 16, 166–169.
20. R. L. Grimm and J. L. Beauchamp, J. Phys. Chem. B, 2005, 109, 8244–8250.
21. X. H. Zong, K. Kim, D. F. Fang, S. F. Ran, B. S. Hsiao and B. Chu, Polymer, 2002, 43, 4403–4412.
22. T. Lin, H. X. Wang, H. M. Wang and X. G. Wang, Nanotechnology, 2004, 15, 1375–1381.
23. Y. Z. Chen, Z. P. Zhang, J. Yu and Z. X. Guo, J. Polym. Sci., Part B: Polym. Phys., 2009, 47, 1211–1218.

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Footnotes

† Electronic supplementary information (ESI) available: Preparation procedure and characterization of PAA nano-nets. See DOI: 10.1039/c0nr00730g

‡ Co-first authors with the same contribution to this work.

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