Tailored Polymer-Based Nanofibers and ... - ACS Publications

Langmuir 2009, 25, 1181-1187

1181

Tailored Polymer-Based Nanofibers and Nanotubes by Means of Different Infiltration Methods into Alumina Nanopores Jaime Martıń and Carmen Mijangos* Instituto de Ciencia y Tecnologıá de Polı´meros, CSIC, Juan de la CierVa 3, 28006 Madrid, Spain ReceiVed September 23, 2008. ReVised Manuscript ReceiVed NoVember 6, 2008 Template synthesis is one of the most effective methods for the preparation of one-dimensional polymer-based nanostructures (1DPNs). Both hollow nanotubes and solid nanorods or nanofibers with tailored dimensions can be obtained by simply templating a porous material with suitable pore size. The mechanism of polymer infiltration into the pores is also very important in order to obtain the desired one-dimensional nanostructures and to control their final morphology. In this study, several infiltration methods were explored with the aim to obtain different 1DPNs. It was shown that, with these infiltration methods, it is possible to obtain nanofibers and nanotubes of any diameter and length composed of polymers with a wide chemical nature (poly(methyl methacrylate), poly(vinyl chloride), poly(vinyl alcohol), poly(vinylidene fluoride), etc.), or even composed of nanoparticulate composites. Finally, the selection of infiltration method for desired nanostructure is discussed.

1. Introduction Over the past decade one-dimensional (1D) nanostructures have attracted increasing interest due to their new or improved properties.1 Of these, one-dimensional polymeric nanostructures (1DPN) have been the least studied; however, in recent years the number of publications related to 1DPNs has significantly increased. 1D nanostructure-based polymeric materials can be generally classified in two large families: high aspect ratio (length/diameter) free 1D nanostructures, such as nanofibers, nanotubes, and so forth, and ordered arrays of lower aspect ratio nanorods or nanotubes supported on a film. Ordered arrays of nanorods or nanotubes exhibit interesting behavior for applications in photonics, electronics, sensors, coatings, catalysis, or biomedicine.2-5 In addition, these kinds of nanostructures are widely used as templates for nanostructuration of nonpolymeric materials.6-8 On the other hand, high aspect ratio 1D nanostructures can be used as fillers for nanocomposites, as precursor materials for fabrication of other 1D nanostructures,4,9-11 or as scaffolds for tissue engineering.12 Moreover, they are appropriate for * Corresponding author. E-mail: [email protected]. Tel: +34 91 562 29 00. Fax: +34 91 528 74 97. (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 354. (2) Richter, S.; Steinhart, M.; Hofmeister, H.; Zacharias, M.; Go¨sele, U.; Gaponik, N.; Eychmu¨ller, A.; Rogach, A. L.; Wendorff, J. H.; Schweizer, S. L.; von Rhein, A.; Wehrspohn, R. B. Appl. Phys. Lett. 2005, 87, 142107. (3) Pinto, N. J.; Johnson, A. T.; MacDiarmid, A. G.; Mueller, C. H.; Theofylaktos, N.; Robinson, D. C.; Miranda, F. A. Appl. Phys. Lett. 2003, 83, 4244. (4) Martıń, J.; Va´zquez, M.; Hernańdez-Ve´lez, M.; Mijangos, C. J. Nanosci. Nanotechnol., in press. (5) Dersch, R.; Steinhart, M.; Boudriot, U.; Greiner, A.; Wendorff, J. H. Polym. AdV. Technol. 2005, 16, 276. (6) Navas, D.; He´rnandez-Ve´lez, M.; Lee, W.; Nielsch, K.; Va´zquez, M. Appl. Phys. Lett. 2007, 90, 192501. (7) Tanaka, T.; Morigami, M.; Atoda, N. Jpn. J. Appl. Phys. 1993, 32, 6059. (8) Zschech, D.; Kim, D. H.; Milenin, A. P.; Scholz, R.; Hillebrand, R.; Hawker, C. J.; Russell, T. P.; Steinhart, M.; Go¨sele, U. Nano Lett. 2007, 7, 1516. (9) Parthasarathy, R. V.; Phamik, K. L. N.; Martin, C. R. AdV. Mater. 1995, 7, 896. (10) Ginzburg-Margau, M.; Fournier-Bidoz, S.; Coombs, N.; Ozin, G. A.; Manners, I. Chem. Commun. 2002, 3022. (11) Chen, J.-T.; Shin, K.; Leiston-Belanger, J. M.; Zhang, M.; Russell, T. P. AdV. Funct. Mater. 2006, 16, 1476. (12) Yoshimoto, H.; Shin, Y. M.; Terai, H.; Vacanti, J. P. Biomaterials 2003, 24, 20077.

studying size-dependent properties, such as optical or transport properties,13,14 or other processes with length scales comparable to some of the sizes of the nanostructure, such as phase separation in block copolymers,15,16 crystalline textures,17,18 or even molecular dynamics. Several strategies have been developed in order to prepare 1D nanostructures,19-23 and among them one of the most successful and even industrially promising24 fabrication method is the use of templates or “template synthesis”. In this work, porous anodic aluminum oxide (AAO) has been chosen as the template for the preparation of the 1D polymer nanostructures. It is highly versatile with respect to the diameter and length of the obtained 1DPN, of relatively low cost, has a high long-range ordered architecture, and has uniform tailored pores with hexagonal symmetry. These two aspects (dimensional flexibility and ordering) allow the preparation of both types of 1DPN previously mentioned: high aspect ratio free 1DPN, and ordered arrays of low aspect ratio 1DPN. The “template synthesis” method, introduced first by Martin et al.25,26 and widely developed and studied for polymers mainly by Martin, Steinhart, and Russell’s groups,27,28 is a nanomolding (13) Aleshin, A. N.; Lee, H. J.; Park, Y. W.; Akagi, K. Phys. ReV. Lett. 2004, 93, 196601. (14) Kong, F.; Wu, X. L.; Huang, G. S.; Yang, Y. M.; Yuan, R. K.; Yang, C. Z.; Chu, P. K.; Siu, G. G. J. Appl. Phys. 2005, 98, 074304. (15) Xiang, H.; Shin, K.; Kim, T.; Moon, S. I.; McCarthy, T. J.; Russell, T. P. Macromolecules 2004, 37, 5660. (16) Sun, Y.; Steinhart, M.; Zschech, D.; Aldhikari, R.; Michlar, G. H.; Go¨sele, U. Macromol. Rapid Commun. 2005, 26, 369. (17) Steinhart, M.; Go¨ring, P.; Dernaika, H.; Prabhukaran, M.; Go¨sele, U. Phys. ReV. Lett. 2006, 97, 027801. (18) Steinhart, M.; Senz, S.; Wherspohn, R. B.; Go¨sele, U.; Wendorff, J. H. Macromolecules 2003, 36, 3646. (19) Li, D.; Xia, Y. AdV. Mater. 2004, 16, 1151. (20) Luchnikov, V.; Sydorenko, A.; Stamm, M. AdV. Mater. 2005, 17, 1177. (21) Pisignano, D.; Maruccio, G.; Mele, E.; Persano, L.; Di Benedetto, F.; Cingolani, R. Appl. Phys. Lett. 2005, 87, 123109. (22) Chen, X.; Steinhart, M.; Hess, C.; Go¨sele, U. AdV. Mater. 2006, 18, 2153. (23) Song, T.; Zhang, Y.; Zhou, T.; T. Lim, C.; Ramakrishna, S.; Liu, B. Chem. Phys. Lett. 2005, 415, 317. (24) Grimm, S.; Schwirn, K.; Go¨ring, P.; Knoll, H.; Miclea, P. T.; Greiner, A.; Wendorff, J. H.; Wherspohn, R. B.; Go¨sele, U.; Steinhart, M. Small 2007, 3, 993. (25) Martin, C. R. Science 1994, 266, 1961. (26) Martin, C. R. Acc. Chem. Res. 1995, 28, 61. (27) Steinhart, M.; Wendorff, J. H.; Greiner, A.; Wherspohn, R. B.; Nielsch, K.; Schilling, J.; Choi, J.; Go¨sele, U. Science 2002, 296, 1997.

10.1021/la803127w CCC: $40.75  2009 American Chemical Society Published on Web 12/23/2008

1182 Langmuir, Vol. 25, No. 2, 2009

process. As any molding process, it consists first of the infiltration of a polymeric fluid (melt or solution) into a nanocavity with a well-defined shape, then the polymer is solidified within the cavity, and finally the molded polymeric material is removed (when a free polymeric nanostructure is required). The determining step in the process is usually the infiltration, a common step in the template-synthesis of all types of 1DPN. In addition, it will be shown that it is an important process for the control of the final shape of the nanostructure. The aim of the present work is to report the study carried out on infiltration methods of polymers into porous aluminum oxide (AAO) templates, to clasify them and to establish a relationship between the infiltration methods and the obtained morphologies of the 1DPN, providing rules for the selection of the infiltration method as a function of the required final 1DPN. Moreover, the efficiency of some of the infiltration methods is demonstrated for the obtention of nanoparticulate polymer-based composite nanostructures. Finally, a new class of polymer-based magnetic nanotubes is presented.

2. Materials and Methods 2.1. Polymers and Nanoparticles. Polymers were infiltrated into AAO pores both as melts and as solutions. Poly(methyl methacrylate) (PMMA, Mw ) 120 000 g/mol, supplied by Aldrich, Ltd.), poly(vinylidene fluoride) (PVDF, Mw ) 180 000 g/mol, supplied by Aldrich, Ltd.), polyethylene oxide (PEO, Mw ) 100 000 g/mol, supplied by Scientific Polymer Products) and polystyrene (PS, Mw ) 192 000 g/mol, supplied by Aldrich, Ltd.) were used in the molten state, and poly(vinyl alcohol) (PVA, Mw ) 94 000 g/mol, supplied by Aldrich, Ltd.) and poly(vinyl chloride) (PVC, Mw ) 112 000 g/mol, supplied by Rio Rodano) were used as solutions. On the other hand, CoFe2O4 and Fe50Pt50 nanoparticles 2029 and 3-430 nm in diameter, respectively, were used for the preparation of 1D composites. Both nanoparticles present magnetic properties potentially useful for information storage. 2.2. AAO Templates. Different AAO templates have been prepared by two-step electrochemical anodization of aluminum as described elsewhere.31-34 AAO templates were used both as closedend-pore templates or as membranes after removal of the aluminum substrate with a mixture of HCl, CuCl2, and H2O, and the alumina barrier layer (grown between the bottom of the pores and the aluminum substrate) with 10 wt % H3PO4 at 30 °C for 20 min. The AAO templates used had an average diameter of 70, 160, 170, and 360 nm. A birds-eye view of one of these templates is shown in Figure 1. Pore lengths of the templates used varied between 2 and 150 µm. 2.3. Infiltration Methods. Tailored polymer nanotubes and nanofibers can be prepared by choosing the suitable AAO template, and the appropiate infiltration method and conditions. Depending on the process involved, polymer infiltration methods can be classified in three groups: methods based on wetting phenomena, methods based on vacuum, and methods based on rotation. 2.3.1. Wetting-Based Methods. This group of infiltration methods is based on the wetting properties of liquids onto solid surfaces and they have the peculiarity of being spontaneous. Using these methods, PEO, PVDF, PMMA, PS, and PS together with magnetic Fe50Pt50 nanoparticles were infiltrated into AAO templates. However, for the sake of brevity, not all the prepared polymer nanostructures are (28) Zhang, M.; Dobriyal, P.; Chen, J.-T.; Russell, T. P.; Olmo, J.; Merry, A. Nano Lett. 2006, 6, 1075. (29) Lo´pez, D.; Cendoya, I.; Torres, F.; Tejada, J.; Mijangos, C. J. Appl. Polym. Sci. 2001, 82, 3215. (30) Va´zquez, M.; Luna, C.; Morales, M. P.; Sanz, R.; Serna, C. J.; Mijangos, C. Physica B 2004, 354, 71. (31) Masuda, H.; Fukuda, K. Science 1995, 268, 1466. (32) Masuda, H.; Yada, K.; Osaka, A. Jpn. J. Appl. Phys. 1998, 37, L1349. (33) Hernandez-Velez, M. Thin Sol. Films 2005, 495, 51. (34) Va´zquez, M.; Pirota, K.; Hernandez-Velez, M.; Prida, V.; Navas, D.; Sanz, R.; Batallań, F. J. Appl. Phys. 2004, 95, 6642.

Martıń and Mijangos

Figure 1. SEM micrographs of surfaces of prepared AAO templates: (a) 70 nm, (b) 160 nm, (c) 170 and (d) 360 nm in pore diameter.

described in this work. Depending on the process involved in the infiltration, three different methods can be considered: Precursor Film Infiltration. This method was used for infiltrating PVDF, PEO, and PS. The polymer is in the molten state at a temperature well above the glass transition temperature (Tg) or melting point (Tm) during the infiltration. PVDF infiltration into two alumina templates with 360 and 70 nm diameter pores was carried out by placing some pellets onto the AAO at 240 °C for 30 min. Both AAO templates were 100 µm in length. PS and PS/Fe50Pt50 infiltrations into a template with 360 nm diameter and 50 µm length pores were performed by placing the solid materials on the AAO surface at 200 °C for 30 min. The infiltration of PEO was carried out into different sized nanopores at 105 °C for 60 min. Melt Capillary Infiltration. For using capillarity, the polymer must be at a temperature slightly above Tg. This method was used for preparing PMMA nanocolumns (or short nanorods) using a 170 nm diameter and 5 µm length pore AAO template, and it was performed placing a PMMA film on the AAO at 120 °C for 40 h. Solution Wetting. PS nanotubes were fabricated by submerging an AAO template with 360 nm diameter and 50 µm length pores, into a 0.5 wt % solution of PS in toluene for 2 days. Solid nanofibers composed of PS and Fe50Pt50 nanoparticles were also prepared by submerging an identical AAO template for 2 days into a 10 wt % solution of PS in toluene which contained the magnetic nanoparticles. 2.3.2. Vacuum-Based Methods. Vacuum-based infiltration and spin-based infiltration (described below) differ from the previous methods mainly in the nonspontaneity of the infiltrations. In both methods, external forces induce the filling of the pores. Thus, these methods do not depend on the wetting properties of the materials but on the viscosity of the fluid and the size of the nanoparticles. For these reasons, polymer solutions are preferred to polymer melts. In this work, a 5 wt % aqueous solution of PVA containing 2.5 wt % of CoFe2O4 nanoparticles was infiltrated into 70 and 170 nm diameter pore templates. The first AAO template was a 150 µm pore length membrane (open pores at both surfaces), while the second was a 2 µm pore length closed-end-pore template. Thus, the method for creating a vacuum into the pores is different: for closed-end-pore templates, the vacuum and the infiltration must be carried out from the same surface of the template,4 whereas for through-hole membranes, it is posible to infiltrate the solution by suction from the opposite side to that at which the vaccum is generated.35,36 2.3.3. Spin-Based Methods. An easy way to perform spin infiltration of a polymeric solution is to use a spin coater directing the pore axis along the radial direction of the rotating plate. In our (35) Cepak, V. M.; Martin, C. R. Chem. Mater. 1999, 11, 1363. (36) Martıń, J.; Va´zquez, M.; Hernańdez-Ve´lez, M.; Mijangos, C. Nanotechnology 2008, 19, 175304.

Tailored 1D Polymer-Based Nanostructures

Langmuir, Vol. 25, No. 2, 2009 1183

Table 1. Prepared Samples and Their Main Fabrication Conditions AAO infiltration infiltration diameter length temperature (°C) time (nm) (µm)

infiltration method

polymer

fluid

wetting-based methods precursor film wetting precursor film wetting precursor film wetting precursor film wetting precursor film wetting precursor film wetting melt capillarity solution wetting solution wetting vacuum infiltration into closed end pore template

PS PVDF PS/FePt PEO PEO PVDF PMMA PS PS/FePt PVA/CoFe2O4

melt melt melt melt melt melt melt 0.5 wt % in toluene 10 wt % in toluene 5 wt % in water

into through hole membrane PVA/CoFe2O4 5 wt % in water spin infiltration

PVC

3. Results and Discussion The fabrication of 1DPNs (nanofibers and nanotubes) is presented below as a function of the infiltration method. As mentioned above, Table 1 summarizes the different infiltration methods, the conditions employed, and the nanostructure obtained. 3.1. By Wetting-Based Methods. Wetting-based infiltrations of polymer liquids (melts or solutions) into porous templates allow one to obtain both nanotubes and nanofibers. This is due to the fact that low surface energy liquids, such as polymer fluids and organic fluids in general, tend to wet high surface energy solids.41-43 When a small drop of a certain organic liquid settles on a high-energy solid surface, two wetting regimes are possible in equilibrium depending on the mobility of the molecules: (i) a partial wetting regime, in which the drop spreads until an equilibrium position is attained, characterized by a certain contact angle between the solid and the liquid; (ii) a complete wetting regime, which leads to the spreading of a liquid precursor film over the whole solid surface. High molecular mobility conditions (high temperature, low molecular weight, etc.) lead to the complete wetting regime, whereas low mobility conditions (low temperature, high molecular weight, etc.) lead to the partial wetting regime. Depending on the conditions under which the infiltration is carried out, three different infiltration methods can be considered. (37) Guo, C.; Feng, L.; Zhai, J.; Wang, G.; Song, Y.; Jiang, L.; Zhu, D. ChemPhysChem 2004, 5, 750. (38) Zheng, R. K.; Chen, H. L.; Choy, C. L. Nanotechnology 2005, 16, 1928. (39) Li, H.; Ke, Y.; Hu, Y. J. Appl. Polym. Sci. 2006, 99, 1018. (40) Martin, C. R. Chem. Mater. 1996, 8, 1739. (41) De Gennes, P.-G.; Brochard-Wyart, F.; Que´re´, D. Capillarity and Wetting Phenomena. Drops, Bubbles, Pearls, WaVes; Springer: New York, 2004. (42) De Gennes, P.-G. ReV. Mod. Phys. 1985, 57, 827. (43) Auserre´, D.; Picard, A. M.; Le´ger, L. Phys. ReV. Lett. 1986, 57, 2671.

30 min 30 min 30 min 60 min 60 min 30 min 40 h 2 days 2 days

room temp

10 wt % in THF

case, a P6700 Series spin coater was used for infiltrating a 10 wt % solution of PVC in tetrahydrofuran (THF) into a template with 2 µm length and 160 nm diameter pores. 2.3.4. Other Methods. Other infiltration methods can also be found in the literature. These methods are mainly related to hot press processes and are usually used for the preparation of low aspect ratio nanorods or their arrays.37-40 All the samples fabricated are listed in Table 1, together with their preparation conditions. 2.4. Characterization. All the prepared samples were morphologically characterized by scanning electron microscopy (SEM) (Philips XL-30 ESEM). In order to perform this analysis, the AAO templates were dissolved in 5 M NaOH when the 1DPNs were not hydrosoluble or subject to swelling in water. In the latter cases, the AAOs were fractured, or the polymeric nanostructure was mechanically removed from the AAO template before performing the SEM analysis.

200 240 200 105 105 240 120 room temp room temp room temp

room temp

60 s

360 360 360 360 40 360 170 360 360 170

50 100 50 100 100 100 5 50 50 2

70

150

160

2

obtained nanostructure PS nanotubes PVDF nanotubes PS/FePt nanotubes PEO nanotubes PEO nanofibers PVDF nanofibers PMMA nanocolumns PS nanotubes PS/FePt nanofibers PVA/CoFe2O4 nanocolumns PVA/CoFe2O4 nanofibers PVC nanocolumns

3.1.1. Melt Precursor Film Wetting. In complete wetting, a mesoscopic melt precursor film spreads, coating the whole surface of a flat substrate. For porous substrates, the melt precursor film covers the pore walls together with the flat surface of the substrate. This phenomenon was first used by Steinhart et al. for the preparation of polymer nanotubes.27 The precursor film is governed by long-range forces, i.e., van der Waals forces, and the explanation of the 1D nanostructure formation and several examples of nanotubes and nanofibers composed of different polymers can be found.18,27,44 Both polymer nanofibers and nanotubes can be prepared, depending mainly on the pore diameter of the template. If the precursor film thickness is larger than the nanopore radius, solid nanofibers are obtained, whereas if the thickness of the precursor film is shorter than the pore radius, nanotubes are formed. (i). Polymer-Based Nanotubes. PS nanotubes and PVDF nanotubes were achieved by carrying out the wetting process at 200 °C in the case of PS (well above the glass transition temperature, Tg ≈ 100 °C) and at 240 °C for PVDF (Tm ≈ 170 °C). The pores of the AAO template used for fabricating PS nanotubes were 50 µm in length and 360 nm in diameter, whereas the template used for the preparation of PVDF nanotubes had 100 µm length and 360 nm diameter pores. It is important to mention that nanotubes with the same dimensions were obtained after nanomolding. In Figure 2, SEM micrographs of the (a) PS and (b) PVDF nanotubes prepared can be observed. Taking into account the previous consideration about the wetting method and especially the formation of the precursor film, the obtention of these nanostructures can be explained by the fact that the polymer melt coats only the pore walls and not the center of pore as a result of the complete regime conditions and the “large” diameter of the template (360 nm). The thickness of the nanotube wall was aproximately 40 nm for PVDF and 70 nm for PS, as can be observed in the inset. These values are in the range of those reported in the literature by Steinhart et al.27,44 In the case of PVDF nanotubes, closed nanotubes are shown in Figure 2b due to the use of a closed-end-pore template. Taking into account that the advance of the precursor film is caused by the advance of polymer monolayers,42 one could imagine that the thickness of the nanotube wall could only measure an integer value of the size of the monolayer. PEO nanotubes have been also produced by precursor wetting infiltration; however, for the sake of brevity, the fabrication and study of these nanomaterials will be reported elsewhere. Precursor film wetting infiltration is also an efficient method for preparing polymer-based composite nanotubes. Nanotubes (44) Steinhart, M.; Wehrspohn, R. B.; Go¨sele, U.; Wendorff, J. H. Angew. Chem. 2004, 43, 1334.

1184 Langmuir, Vol. 25, No. 2, 2009

Figure 2. (a) Birds-eye view of PS and (b) PVDF nanotubes. The inset micrographs show broken nanotubes in which the tubular shape of the nanostructure can be observed.

composed of PS and magnetic Fe50Pt50 nanoparticles were prepared by melting a bulk PS/Fe50Pt50 composite on the 360 nm pore diameter AAO template at 200 °C. The nanotubes formed were 50 µm in length and 360 nm in diameter. Figure 3a shows standing PS nanotubes containing embedded Fe50Pt50 nanoparticles. The small size of the nanoparticles (3-4 nm) compared to the thickness of the nanotube wall (aprox. 70 nm; see inset image) allowed the infiltration of nanoparticles together with the PS. The presence of nanoparticles embedded in the nanotube wall was confirmed by testing the magnetic character of the nanotube dispersion in the presence of a magnet, as can be observed in Figure 3b, and also by Rutherford backscattering spectroscopy (RBS) (not shown here). RBS measurements also showed the homogeneous distribution of the nanoparticles along the whole nanotube, as will be reported elsewhere. It is worth mentioning that this is the first time that polymer nanotubes with embedded nanopartcles have been prepared, and, in addition, it is also the first reported 1D nanocomposite prepared by template synthesis from the melt. (ii). Polymer-Based Nanofibers. When melt precursor wetting infiltration is used, and the thickness of this precursor film is larger than the pore radius, solid nanofibers are obtained instead of hollow nanotubes. The infiltration was carried out under the same conditions as for the previous PVDF nanotubes: 240 °C for 30 min. An AAO closed-end-pore template with pores of 60 nm diameter and 50 µm length was used. In Figure 4 it can be observed that nanofibers with the same dimensions were obtained. Using this method, it is possible to obtain solid polymer nanofibers with diameters less than twice the thickness of the


Figure 3. (a) PS-Fe50Pt50 composite nanotubes prepared by precursor wetting. The tubular nature of the nanostructure is shown in the inset. (b) The magnetic character of the nanotubes can be observed in the photograph.

Figure 4. PVDF nanofibers prepared by precursor wetting.

precursor film (typically up to 100 nm in diameter). For the preparation of nanofibers with larger diameters, other infiltration methods are required. 3.1.2. Melt Capillarity. Polymer solid nanorods or nanofibers can be obtained by capillary infiltration. This can be carried out under partial wetting regime conditions, where the polymer chains have low mobility within the melt and are unable to form a melt precursor film. However, the polymer melt is still a low surface energy liquid that tends to wet the pore walls. In this way the


Langmuir, Vol. 25, No. 2, 2009 1185

Figure 5. Solid PMMA nanorods prepared by melt capillary wetting. In the inset, the truncated tips of the nanocolumns indicate the presence of menisci of the capillar infiltration.

Figure 6. PS nanotubes prepared by solution wetting. The inset micrograph shows an enlarged view of a nanotube.

polymer melt infiltrates into the pore, not only by advancing over the alumina pore walls, but also through the whole pore section. Capillary processes of polymer liquids into templates have been studied by Whitesides et al. and later by Russell and co-workers.15,28,45,46 Generally, capillary infiltration is orders of magnitude slower than precursor wetting infiltration, and is characterized by a meniscus in the advance extreme of the melt. Thus, PMMA short nanorods were obtained by melt capillary infiltration. After 40 h of infiltration at 120 °C, 1.5 µm length and 170 nm diameter nanocolumns were obtained (Figure 5). Despite the low infiltration rate compared to precursor film infiltrations, capillar infiltration allows the preparation of solid large-diameter nanofibers (above 100-150 nm). The agglomeration of the nanorods observed in Figure 5 is probably caused by the capillary forces of the rinsing liquid after dissolving the AAO template. Another characteristic of capillary infiltration are the menisci at the tips of nanorods, which are related to the contact angle between the polymer melt and the AAO pore walls (Figure 5, inset). This inset image corresponds to the same PMMA nanocolums prepared after 24 h of infiltration. Similar examples of solid polymer nanorods prepared by the “melt capillary” method heve been reported recently for homopolymers and block copolymers.12,15,28,46 3.1.3. Solution Wetting. Polymer infiltrations by submersion of templates into a polymer solution (“solution wetting infiltration”) are probably the most traditional methods used for the preparation of 1DPN.47,48 Polymer solutions usually infiltrate into pores of AAO filling their complete volume;47 however, depending on the concentration of the solution, solid or hollow 1DPNs can be obtained. (i). Polymer-Based Nanotubes. PS nanotubes nanotubes were also obtained by infiltration and subsequent drying of a dilute solution. First, the polymer solution completely fills the nanopores, and, as the solvent evaporates, the polymer settles on the pore walls. Nanotubes were fabricated by submerging an AAO template into a 0.5 wt % PS solution in toluene. Nanotubes obtained in this way were 50 µm in length and 360 nm in diameter with a wall thickness of around 100 nm. Figure 6 shows laid PS nanotubes prepared by solution wetting after the removal of the

AAO template. It is important to note that this procedure can be an alternative for the preparation of polymeric nanotubes for polymers that are not molten at the temperatures at which the precursor wetting infiltration takes place. Examples of PMMA, PS and poly(lactide) (PLLA) nanotubes prepared by solution wetting infiltration can be found in the literature.48-51 In these works, polymer nanotubes similar to those reported here can be observed. (ii). Polymer-Based Nanofibers. Melt capillarity allows the formation of high-diameter solid nanofibers or nanorods, but the process is extremely slow, and not all polymers can be used in the molten state. Solid polymer and composite nanofibers can also be obtained by solution wetting. As an example, composite nanofibers composed of PS and Fe50Pt50 were achieved by this method after submerging the template into a 10 wt % solution of PS in toluene for 2 days. Nanofibers were 50 µm in length and 360 nm in diameter, the same dimensions of the original AAO template (see Figure 7a). It is possible to confirm their solid nature in the broken nanofibers. However, it should be noted that the nanofibers often present lack of material, which means that imcomplete nanofibers are obtained, as can be observed in Figure 7a. The detection of the nanoparticles was carried out by observing the magnetic attraction when a magnet is approached to a PS-Fe50Pt50 nanofiber dispersion (Figure 7b), and also by RBS (not shown). 3.2. By Vacuum-Based Methods. Under a pressure gradient, fluids flow toward lower pressure values. This has been used for infiltrating polymers into nanopores by generating a vacuum within the latter. Depending on the required length of the nanofibers, closed-end-pore templates or through hole membranes must be used: closed-end-pore templates for short or medium nanofibers (40 µm). The way to create a vacuum in the pores of these two types of AAOs is different, but the principle for the infiltration is the same: a vacuum is created within the pores, and then a polymer solution is placed in contact with the pore, avoiding the infiltration of anything but the solution. This way, the solution is forced to enter and fill the nanopores. (i). Into Closed-End-Pore Templates. The solution wetting method allows for the preparation of high-diameter and high

(45) Kim, E.; Xia, Y.; Whitesides, G. M. Nature 1995, 376, 581. (46) Shin, K.; Obukhov, S.; Chen, J.-T.; Huh, J.; Hwang, Y.; Mok, S.; Dobriyal, P.; Thiyagarajan, P.; Russell, T. P. Nat. Mater. 2007, 6, 961. (47) Bernardiner, M. G. Transp. Porous Media 1998, 30, 251. (48) Steinhart, M.; Jia, Z.; Scheper, A. K.; Wehrspohn, R. B.; Go¨sele, U.; Wendorff, J. H. AdV. Mater. 2003, 15, 706.

(49) Chen, J.-T.; Shin, K.; Leiston-Belanger, J. M.; Yhang, M.; Russell, T. P. AdV. Func. Mater. 2006, 16, 1476. (50) Chen, J.-T.; Zhang, M.; Russell, T. P. Nano Lett. 2007, 7, 183. (51) Nielsch, K.; Castanõ, F. J.; Ross, C. A.; Krishnan, R. J. Appl. Phys. 2005, 98, 034318.

1186 Langmuir, Vol. 25, No. 2, 2009


Figure 8. SEM image of the PVA/SPS-CoFe2O4 short nanorod array.

Figure 7. (a) Solid nanofibers composed of PS and Fe50Pt50 nanoparticles. (b) The magnetic character of the PS-Fe50Pt50 nanowires can be observed when a magnet is approached.

aspect ratio polymer and composite nanofibers. However, nonspontaneus infiltration methods, such as vacuum infiltrations, present the advantage of allowing the infiltration of large nanoparticles. In the previous case, the AAO pores were 360 nm in diameter, whereas the nanoparticles were 3-4 nm, and, because of this large difference, the nanoparticles do not have problems infiltrating into the pores. Difficulties arise when there is a smaller difference between nanoparticle size and pore diameter. For this reason, forced infiltration methods were developed.4,36 In this work, a hexagonal array of nanorods composed of PVA and CoFe2O4 nanoparticles (20 nm in diameter) was obtained by vacuum infiltration and using a closed-end-pore template. The nanorods formed are 170 nm in diameter and ∼500 nm in length (Figure 8), and the fact that a relatively strong force induced such a flow was important to ensure the presence of nanoparticles inside the nanopores. The detection of the nanoparticles has already been reported.4 A noncomplete uniformity in the length of the nanorod is appreciable because the nanorods were manually extracted from the template, and some were broken in the process. (ii). Into Through-Hole Membranes. When larger nanofibers are required, it is possible to perform a vacuum infiltration of polymers through a permeable membrane in which pores are open at both surfaces. This infiltration method needs a certain mechanical stability of the membrane, which requires the membrane to be thick enough to support the vacuum conditions (typically 40-50 µm). The SEM image in Figure 9, recorded after the fracture of the AAO membrane, shows a row of ultrahigh aspect ratio composite PVA/CoFe2O4 nanofibers. The dimensions of the PVA/CoFe2O4

Figure 9. Aligned PVA nanowires with embebed CoFe2O4 magnetic nanoparticles.

nanofibers are determined by those of the AAO: 70 nm in diameter and 150 µm in length. Thus, the aspect ratio is above 2000. The presence of the nanoparticles in the nanofibers is confirmed by the magnetic response of the material, as reported elsewhere.36 A similar result was obtained by Cepak et al. using a 2.5wt. % PS solution and a membrane with 30 nm in diameter pores.30 3.3. By Spin-Based Methods. This method is based on the centrifugal force exerted by an object when it is rotating. The force is directed away from the axis of rotation, and the resulting displacement can be used for infiltrating a polymer solution into the nanopores. In this work, solid PVC nanorods were achieved. A 10 wt % PVC solution in THF was spin coated and directed to AAO pores. The used AAO closed-end-pore template was 2 µm in length and 160 nm in diameter. Figure 10 shows PVC nanorods obtained. The nanorods had the same dimensions than the pores, so a complete infiltration took place.

Discussion From our point of view, when possible, the use of melts instead of solutions is generally recommended to obtain both nanofibers or nanotubes. Spontaneous melt infiltrations are usually more reproducible, and solvent-related problems such as evaporation time, incomplete evaporation, lack of material in nanotubes or


Langmuir, Vol. 25, No. 2, 2009 1187

Because of the applied force, nanoparticles can be introduced. For low diameter composite nanofibers, precursor melt wetting can also be effective if the nanoparticles are small enough. For the fabrication of solid polymer nanorods with low or moderate aspect ratio (also referred to previously as nanocolumns; length

Tailored Polymer-Based Nanofibers and ... - ACS Publications

Recommend Documents