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Langmuir 2008, 24, 5508-5513
Hydrothermal-Induced Structure Transformation of Polyelectrolyte Multilayers: From Nanotubes to Capsules Qiang He,†,‡ Weixing Song,†,‡ Helmuth Möhwald,‡ and Junbai Li*,† Beijing National Laboratory for Molecular Sciences (BNLMS), International Joint Lab, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Max Planck Institute of Colloids and Interfaces, Am Mühlenberg 1, Golm/Potsdam D-14476, Germany ReceiVed NoVember 29, 2007. ReVised Manuscript ReceiVed February 5, 2008 The assembled polyelectrolyte nanotubes composed of poly(styrenesulfonate) and poly(allylamine hydrochloride) multilayers by using the layer-by-layer assembly combined with the porous template method can be transformed into capsules by a high-temperature treatment. Scanning electron microscopy and confocal laser scanning microscopy images revealed the whole transition process. The structure transformation of polyelectrolyte multilayers after annealing can be initiated by the input of thermal energy which leads to a breakage of ion pairs between oppositely charged polyelectrolyte groups. The transition process from tubes to capsules is supposed to be driven by the Raleigh instability and leads to the generated polyelectrolyte capsules with different sizes.
Introduction Controlled self-assembly of natural or synthetic macromolecules into supramolecular structures with different morphologies is an interesting topic in a wide range of scientific fields. Formation of various well-defined nanostructures with specific sizes and shapes can be realized by precisely controlling molecular arrangement at the nanometer scale.1–5 A number of assembled systems containing biological molecules in forming deferent types of structures, like vesicles, tubules, or fibrils, mimic various biological features in nature. Especially, the structural transition of tubular to vesicular or vesicle-like systems is often considered for the delivery of drugs or genes as well as for the preparations of nonreactors or biochemical sensors.6–9 The layer-by-layer (LbL) assembly of charged species including synthetic polyelectrolytes, biomacromolecules, and nanoparticles offers a versatile means to prepare ultrathin organic or hybrid films with different properties at various substrates.10–14 Moreover, micro- and nanosized capsules also can be prepared by alternately depositing oppositely charged polyelectrolytes onto charged colloid templates followed by removal of the cores.15–18 * Corresponding author,
[email protected]. † BNLMS, International Joint Lab, Institute of Chemistry, Chinese Academy of Sciences. ‡ Max Planck Institute of Colloids and Interfaces. (1) Vriezema, D. M.; Hoogboom, J.; Velonia, K.; Takazawa, K.; Chirstianen, P. C. M.; Maan, J. C.; Rowan, A. E.; Nolte, R. J. M. Angew. Chem., Int. Ed. 2003, 42, 772. (2) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (3) Diaz, N.; Simon, F. X.; Rawiso, M.; Decher, G.; Jestin, J.; Mesini, P. Angew. Chem., Int. Ed. 2005, 44, 3260. (4) Zhang, S. G. Nat. Biotechnol. 2003, 21, 1171. (5) Li, J. B.; Cui, Y. J. Nanosci. Nanotechnol. 2006, 6, 1. (6) Martin, C. R.; Kohli, P. Nat. ReV. Drug DiscoVery 2003, 2, 29. (7) Guo, X.; Szoka, F. C. Acc. Chem. Res. 2003, 36, 335. (8) Vriezema, D. M.; Aragones, M. C.; Elemans, J. A. A. W.; Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M. Chem. ReV. 2005, 105, 1445. (9) Caruso, F.; Caruso, R. A.; Möhwald, H. Science 1998, 282, 1111. (10) Decher, G.; Hong, J. D. Makromol. Chem. Makromol. Symp. 1991, 46, 321. (11) Decher, G. Science 1997, 277, 1232. (12) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Film 1992, 210, 831. (13) Caruso, F. AdV. Mater. 2001, 13, 11. (14) Tang, Z. Y.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. AdV. Mater. 2006, 18, 3203. (15) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Möhwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202. (16) Peyratout, C. S.; Daehne, L. Angew. Chem., Int. Ed. 2004, 43, 3762.
The properties of these micro- and nanosized polyelectrolyte capsules with a given size and number of polyelectrolyte layers have been widely investigated.19–24 They are subsequently altered by changing temperature, pH value, solvent, and ionic strength of the systems. The corresponding change of the strength of the intermolecular interaction between the oppositely charged polyelectrolytes leads to a denser or looser arrangement of the assembled multilayers. It means that after the formation of polyelectrolyte multilayer structures in certain morphology, they can be still changed to another by varying the external conditions. For instance, it has been reported that polyelectrolyte microcapsules in an aqueous phase at high temperatures lead to their shrinking, and it is accompanying by an increase of wall thickness.25–30 This phenomenon is of particular interest for a fundamental understandings of the internal layer structure and the practical applications. Recently, many reports show that the LbL technique combined with the template method can be used to create nanotubes with a diameter of few hundred nanometers and a length of tens of micrometers through the electrostatic attraction, covalent binding, (17) Li, J. B.; Möhwald, H.; An, Z. H.; Lu, G. Soft Matter 2005, 1, 259. (18) Sukhorukov, G.; Fery, A.; Möhwald, H. Prog. Polym. Sci. 2005, 30, 885. (19) Sukhorukov, G. B.; Donath, E.; Davis, S.; Lichtenfeld, H.; Caruso, F.; Popov, V. I.; Möhwald, H. Polym. AdV. Technol. 1998, 9, 759. (20) Antipov, A. A.; Sukhorukov, G. B.; Fedutik, Y. A.; Hartmann, J.; Giersig, M.; Möhwald, H. Langmuir 2002, 18, 6687. (21) Gao, C.; Donath, E.; Moya, S.; Dudnik, V.; Möhwald, H. Eur. Phys. J. E 2001, 5, 21. (22) Georgieva, R.; Moya, S.; Leporatti, S.; Neu, B.; Baeumler, H.; Reichle, C.; Donath, E.; Möhwald, H. Langmuir 2000, 16, 7075. (23) Antipov, A. A.; Sukhorukov, G. B.; Leporatti, S.; Radtchenko, I. L.; Donath, E.; Möhwald, H. Colloids Surf., A 2002, 198, 535. (24) Ibarz, G.; Daehne, L.; Donath, E.; Möhwald, H. Chem. Mater. 2002, 14, 4059. (25) Leporatti, S.; Gao, C.; Voigt, A.; Donath, E.; Möhwald, H. Eur. Phys. J. E 2001, 5, 13. (26) Gao, C.; Leporatti, S.; Moya, S.; Donath, E.; Möhwald, H. Chem. Eur. J. 2003, 9, 915. (27) Köhler, K.; Shchukin, D. G.; Sukhorukov, G. B.; Möhwald, H. Macromolecules 2004, 37, 9546. (28) Köhler, K.; Shchukin, D. G.; Möhwald, H.; Sukhorukov, G. B. J. Phys. Chem. B 2005, 109, 18250. (29) Köhler, K.; Dejugnat, C.; Dubois, M.; Zemb, T.; Sukhorukov, G. B.; Guttmann, P.; Möhwald, H. J. Phys. Chem. B 2007, 111, 8388. (30) Köhler, K.; Biesheuvel, P. M.; Weinkamer, R.; Möhwald, H.; Sukhorukov, G. B. Phys. ReV. Lett. 2006, 97, 188301.
10.1021/la703738m CCC: $40.75 2008 American Chemical Society Published on Web 04/10/2008
Nanotubes to Capsules Scheme 1. Schematic Representation of Hollow LbL-Assembled (PSS/PAH)n Nanotube Transformation into Closed Capsules Induced by a High Temperature
the hydrogen interaction, and so on.31–41 The length and outer diameter of as-prepared composite nanotubes are mostly depending on the used template. The wall thickness (or inner diameter) of the assembled tubes can be controlled by changing the number of LbL circular layers. It also allows modifying the permeability and mechanical properties of tubes by the similar way. Meanwhile, the diversity of nanotube functions can be realized by assembling different components or surface modification of the functional groups on the inner and outer surfaces of the tubes. Transformation of the assembled nanostructures between different morphologies can also result in the formation of different functions.42–48 It is reported that the reversible transition of the self-organized peptide structure change between nanotube and vesicle was triggered by the change of the peptide concentration.49–51 In the present work, we find that the tubes assembled by using poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) as pairs to coat the inner wall of the template of polycarbonate membranes can be transformed into capsules at a high temperature above 100 °C. Such a transformation process of the tubes into capsules is also observed by controlling the assembled layers as the temperature increases as illustrated in Scheme 1.
Experimental Section Materials. Commercially available porous polycarbonate (PC) filters (Purchased from Whatman Corp.) were used as templates. These membranes have nominal pore diameters of 400 nm and 1.0 µm, respectively, and a membrane thickness of ca. 10 µm. Sodium poly(styrenesulfonate) (PSS, Mw ∼ 70 kDa), poly(allylamine hydrochloride) (PAH, Mw ∼ 70 kDa), and sodium chloride were purchased from Sigma-Aldrich. All compounds were used without (31) Ai, S. F.; Lu, G.; He, Q.; Li, J. B. J. Am. Chem. Soc. 2003, 125, 11140. (32) Liang, Z. J.; Susha, A. S.; Yu, A. M.; Caruso, F. AdV. Mater. 2003, 15, 1849. (33) Yang, Y.; He, Q.; Duan, L.; Cui, Y.; Li, J. B. Biomaterials 2007, 28, 3083. (34) Hou, S.; Harrell, C. C.; Trofin, L.; Kohli, P.; Martin, C. R. J. Am. Chem. Soc. 2004, 126, 5674. (35) Tian, Y.; He, Q.; Cui, Y.; Tao, C.; Li, J. B. Chem. Eur. J. 2006, 12, 4808. (36) Lee, D. Y.; Cohen, R. E.; Rubner, M. F. Langmuir 2007, 23, 123. (37) Tian, Y.; He, Q.; Cui, Y.; Li, J. B. Biomacromolecules 2006, 7, 2539. (38) Tian, Y.; He, Q.; Tao, C.; Li, J. B. Langmuir 2006, 22, 360. (39) Huang, J. G.; Ichinose, I.; Kunitake, T. Chem. Commun. 2005, 1717– 1719. (40) Huang, J. G.; Kunitake, T. J. Mater. Chem. 2006, 16, 4257–4264. (41) He, Q.; Tian, Y.; Cui, Y.; Möhwald, H.; Li, J. B. J. Mater. Chem. 2008, 18, 748. (42) Park, C. Y.; Lee, I. H.; Lee, S. H.; Song, Y.; Rhue, M.; Kim, C. H. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 1199. (43) Shimizu, T.; Masuda, M.; Minamikawa, H. Chem. ReV. 2005, 105, 1401. (44) Kim, B. S.; Hong, D. J.; Bae, J. Y.; Lee, M. J. Am. Chem. Soc. 2005, 127, 16333. (45) Tellini, V. H. S.; Jover, A.; Meijide, F.; Tato, J. V.; Galantini, L.; Pavel, N. V. AdV. Mater. 2007, 19, 1752. (46) Ryu, J. H.; Kim, H. J.; Huang, Z.; Lee, E.; Lee, M. Angew. Chem., Int. Ed. 2006, 45, 5304. (47) Cornelissen, J. J. L. M.; Rowan, A. E.; Nolte, R. J. M.; Sommerdijk, N. A. J. M. Chem. ReV. 2001, 101, 4039. (48) Gazit, E. Chem. Soc. ReV. 2007, 36, 1263. (49) Ajayaghosh, A.; Varghese, R.; Mahesh, S.; Praveen, V. K. Angew. Chem., Int. Ed. 2006, 45, 7729. (50) Yan, X. H.; He, Q.; Wang, K. W.; Duan, L.; Li, J. B. Angew. Chem., Int. Ed. 2007, 46, 2431. (51) Song, Y. J.; Challa, S. R.; Medforth, C. J.; Qiu, Y.; Watt, R. K.; Pena, D.; Miller, J. E.; van Swol, F.; Shelnutt, J. A. Chem. Commun. 2004, 6, 1044.
Langmuir, Vol. 24, No. 10, 2008 5509 further purification except for PSS, which was dialyzed against water (Mw cutoff 20 kDa) and lyophilized. The water used in all experiments was prepared in a Milli-Q purification system and had a resistivity higher than 18.2 MΩ · cm. Polyelectrolyte Tube Preparation. Hollow polyelectrolyte tubes were fabricated using the LbL technique combined with the porous template method as described previously.31,33,41 The alternating adsorption of PSS and PAH onto PC membranes was carried out from 1 mg/mL polyelectrolyte solutions containing 0.5 M NaCl. Each solution was freshly prepared and filtered using 5 µm filters before use. Positively charged PAH was always first absorbed on the inner pore of PC membranes by immersing the membranes into PAH solution for 30 min. The templates were then rinsed with water for 2 min. This process was repeated three times in different beakers before the next immersion into a polyelectrolyte solution. Next, negatively charged PSS and positively charged PAH were alternately adsorbed for 30 min in the pores of the membranes and then washed three times with water until the desired number of layers was obtained. Then, multilayers depositing on the top and bottom surfaces of the PC membranes were removed by plasma etching at a pressure of 10 mbar for 5 min. The templates with the inner pore composites were sonicated in water for 5 min. The PAH/PSS tubes were finally obtained by dissolving the PC membranes in dichloromethane and subsequently sonicating for 10 min again. After sonication, the tubes were completely washed in fresh dichloromethane three times and ethanol twice, followed by redispersion in water. In all experiments, the final concentration of the nanotubes is roughly 4 × 105 to 4 × 108 tubes per milliliter water or the tubes prepared by two pieces of PC membranes were dispersed in 5 mL of water. High-Temperature Treatment. The nanotube suspensions were incubated for 20 min at 121 or 138 °C using a Fedegari autoclave from Biomedis GmbH (Germany). This setup permits that temperature can be set between 60 and 138 °C. The vessel containing the aqueous capsule suspension was put together with the temperature controlling sensor into a water storage jar. The maximum pressure is 2800 kPa. The whole process takes around 2 h including heating and cooling. Confocal Laser Scanning Microscopy (CLSM). Optical images of polyelectrolyte nanotubes and capsules in water were obtained using a Leica TCS SP confocal system (Leica, Germany) equipped with a 100× oil immersion objective with a numerical aperture of 1.4. To visualize the polyelectrolyte shells, PAH was labeled by fluorescein isothiocyanate (FITC, Aldrich) in advance according to the method described in literature. Scanning Electron Microscopy (SEM). SEM measurements were conducted with a Gemini Leo 1550 instrument at an operation voltage of 3 keV. Some samples also were observed by using an environmental scanning electron microscope (ESEM, Quanta 600 FEG, FE-ESEM, FEI Europe). A drop of the sample solution was placed onto a glass wafer, dried at room temperature, and sputtered with gold.
Results and Discussion Hollow PSS/PAH tubes with different diameters and wall thickness were fabricated by using the LbL technique combined with the porous template method via electrostatic attraction according to the previously reported procedure.35–38 In order to obtain a temperature above 100 °C without change of the solution volume, an autoclave was used to incubate the capsules. At first we studied the temperature-dependent behavior of (PSS/ PAH)8PAH tubes assembled in 400 nm PC templates in aqueous suspension. Typical scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of as-assembled (PSS/PAH)8PAH tubes before hydrothermal treatment are shown in Figure 1. We can see that a well-defined tubular structure is formed. The maximum length of the tubes is around 11 µm corresponding to the thickness of the PC template used. It is noted that these assembled (PSS/PAH)8PAH tubes collapsed on the surface after dissolving the PC membranes and drying in
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Figure 1. (A) SEM and (B) TEM images of the assembled (PSS/ PAH)8PAH nanotubes with a diameter of 0.4 µm by the LbL assembly combined with the template technique.
vacuum. The thin shell is obviously not stable enough to keep the tubular shape. A similar phenomenon has also been observed for polyelectrolyte multilayer microcapsules fabricated based on colloid particle templates.15,19 Figure 2 displays confocal laser scanning microscopy (CLSM) images of FITC-labeled nanotubes in aqueous solution and of the subsequently obtained spherical capsules after heating. The length of the nanotubes is estimated to the order of the template thickness, ca. 11 µm (Figure 2A). Here, the tubular structure of the PAH/PSS complex film is hardly observed due to the limited resolution of CLSM. However, a magnification image in Figure 2B clearly shows the formation of the PAH/PSS tubular structure. The corresponding intensity distribution profile (Figure 2B, inset) further confirms that the fluorescent maximum intensity mainly arises from the wall of the nanotubes. It can be seen from Figure 2C that most of the tubes have been transformed into spherical capsules after heating and the obtained capsules have different shapes and sizes. Similarly, the magnification image in Figure 2D clearly shows the formation of the hollow PAH/PSS capsule structure. The corresponding intensity distribution profile (Figure 2D, inset) also confirms that the fluorescence maximum intensity stems from the wall of the obtained capsule. Compared to the original nanotubes (Figure 2B, inset), one can see that the obtained capsule has a much thicker wall. In order to study the structural transformation of the assembled nanotubes after heating and to measure the changes of their surface morphology more precisely, SEM was applied. The SEM images in Figure 3 reveal that most tubes had disappeared after heating and compact particles or capsules were formed accordingly. It can be seen that the resulting capsules keep spherical or ovoid shapes and do not collapse any more during drying like the initial tubes. These capsules also indicate a more rigid wall. This means that the mechanical strength of these transformed capsule shells after heating increases distinctly in contrast to the initial tubes before hydrothermal treatment. Compared to the rough surface of the initial assembled tubes (Figure 1A), the surface of the capsules (Figure 3B) seems more smooth, which should be ascribed to a healing of the wall defects. From the SEM images in Figure 3 it is seen that the transformed capsules have not only different shapes but also different sizes. From the obtained capsules with several incompletely transformed tubes marked with arrows in Figure 3A, one can see that many transformed capsules are obviously swollen. From the SEM images one can roughly estimate the diameter of the capsules in a range of 500 nm to 1.00 µm (by measuring 40 capsules), which is obviously larger than that of the initial tubes (∼400 nm). Furthermore, some tubes were not transformed into capsules after heating (Figure 3), but their shapes still changed in contrast to those before annealing (Figure 1). They display a pearlnecklace-like structure, which can be considered as an intermediate state of the transformation process from tubes to capsules. A similar morphological transition of cylindrical surfactant tubular
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membranes, polymersomes, and metal wires has been reported.52–58 The so-called pearling instability, observed with lipid and polymer membranes, is analogous to the well-known Rayleigh instability in which a fluid cylinder with a circular cross-section breaks up into small spherical droplet with the same volume but less surface area if its length exceeds its circumference.56,57 The Rayleigh instability is driven by the surface tension of liquids to decrease surface tension or to minimize their surface area. According to the Rayleigh instability, average spacing (λ) and diameter (d) of the resultant spherical droplet are described as follows
λ ) 2π√2r = 8.89r d ) 3.78r
(1) (2)
where r stands for the radius of the initial fluid cylinder. Obviously, the transformation phenomenon of the assembled polyelectrolyte multilayer structures can also be explained by the above well-known Rayleigh instability. However, in our system, a significant part of surface tension arises from electrostatic interactions. In other words, the surface tension can be written as a sum of two contributions:
γ ) γe + γo
(3)
Here, γe is the contribution from the electrostatic interaction between the charged polyelectrolytes and γo is the contribution from other interactions such as the van der Waals force or hydrophobic action between polyelectrolyte molecules.52 Briefly, electrostatic surface tension (γe) is ascribed to the electrostatic repulsion of the osmotic brush constituting the outer layer.30 The nonelectrostatic surface tension (γo) is mainly due to the interaction of PSS and PAH hydrocarbon chains. Previous studies have demonstrated that the polyelectrolyte chains adsorb irreversibly onto the solid substrate retaining their solution conformation during the formation of the polyelectrolyte multilayer.11,19 The alternating assembly of oppositely charged polyelectrolytes results in a kinetic entrapment of the polyelectrolyte coils. Furthermore, the polyelectrolyte multilayer is here in a glasslike state and the polyelectrolyte chains are immobile at room temperature; thus the as-assembled PAH/PSS tubes are stable. Annealing of the (PAH/PSS)8PAH tubes provides thermal energy to overcome the electrostatic attraction between PAH and PSS, leading to an increased degree of mobility of polyelectrolyte chains in the multilayer films.27,61 The polyelectrolyte chains can thus rearrange into an energetically more favored coiled conformation. Hence, the polyelectrolyte multilayer tubes enables the reassembly of their shape according to the above-mentioned Rayleigh instability mechanism when these tubes were incubated for 20 min at 121 °C. According to eq 2, the diameter of the obtained capsules is calculated to about 1.5 µm. As we stated previously, the diameter of most of capsules is ranging from 500 nm to 1.00 µm, which is lower than that of the theoretical calculation. This should be ascribed to the fact (52) Bar-Ziv, R.; Moses, E.; Nelson, P. Biophys. J. 1998, 75, 294. (53) Reinecke, A. A.; Dobereiner, H. G. Langmuir 2003, 19, 605. (54) Chen, J. T.; Zhang, M. F.; Russell, T. P. Nano Lett. 2007, 7, 183. (55) Nguyen, T. T.; Gopal, A.; Lee, K. Y. C.; Witten, T. A. Phys. ReV. E 2005, 72, 051930. (56) Müller, R.; Dähne, L.; Fery, A. J. Phys. Chem. B 2007, 111, 8547. (57) Molares, M. E. T.; Balogh, A. G.; Cornelius, T. W.; Neumann, R.; Trautmann, C. Appl. Phys. Lett. 2004, 85, 5337. (58) Bendejacq, D.; Joanicot, M.; Ponsinet, V. Eur. Phys. J. E 2005, 17, 83. (59) Plateau, J. Statistique Experimentale et Theorique des LiquidesSoumis aux Seules Forces Moleculaires; Gautier-Villars: Paris, 1873. (60) Rayleigh, L. Philos. Mag. 1892, 34, 145. (61) Müller, R.; Köhler, K.; Weinkamer, R.; Sukhorukov, G.; Fery, A. Macromolecules 2005, 38, 9766.
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Figure 2. CLSM images of the obtained tubes or capsules of (PSS/PAH)8PAH before (A, B) the hydrothermal treatment at 121 °C and after (C, D).
Figure 3. SEM images of the obtained capsules after hollow (PSS/ PAH)8PAH nanotubes with a diameter of 0.4 µm heated for 20 min at 121 °C. (B) An enlarged image of (A).
that our studied polyelectrolyte multilayer system exists in a clear difference with a fluid cylinder (Raleigh instability model). Actually, several studies have shown that PSS/PAH capsules can shrink after heating due to a temporal breakage of ion pairs and subsequently an increased mobility of polyelectrolyte chains.25,27 In this case, the charged groups can recombine with other oppositely charged polyelectrolyte groups in the neighborhood. This recombination results in new local configurations of the multilayer film. Here, the shrinkage of capsules should be accompanied by the decay of nanotubes or the generation of capsules after heating. Similarly, defects like small pores and voids heal and the surface became smoother after this large transformation of nanotube to capsule (Figure 3). Moreover, the decrease in capsule diameter after heating is accompanied by a thickening of the wall, which leads to an increased mechanical stability of the shells preventing them from collapsing in the dried state (Figure 3). Corresponding to eq 3, the driving force for the rearrangement is thought to be the increased entropy of
the more coiled and interlapped polymer arrangement after annealing. A second driving force is hydrophobic interactions of PAH and PSS consisting of a hydrophobic hydrocarbon chain as backbone which also corresponds to an entropy increase.62 The shrinkage of the transformed capsules from tubes during heating should be accompanied by an enormous decrease of the surface area of the capsules (Figure 3) and the wall densification can be explained by the hydrophobic effect. It is also worthwhile to point to the fact that polyelectrolyte multilayers contain a large amount of water before heating. Recently, Köhler et al. revealed that the (PAH/PSS)7PAH shells contain about 12% less water in the initial as well as in the shrunk state after annealing by using transmission microscopy with soft X-rays.29 Thus, a larger interpenetration and coiling of polyelectrolyte molecules after annealing leads to a denser structure with fewer water-filled cavities. It means that water is expelled from the multilayers even though the capsules were heated in aqueous suspension, hence reducing the water/polyelectrolyte interface. Therefore, smaller capsules with thicker polyelectrolyte multilayers were formed. The more coiled state of the polyelectrolyte molecules and the hydrophobic effect may be due to the increase of entropy during the rearrangement process. It should be noted that the wide distribution of the obtained capsule diameters is partly due to the drawback of the used template and incomplete removal of the deposited polyelectrolyte multilayers on the top and bottom surface of the PC templates. As previously reported,35–37 some cylindrical pores in the commercial PC membranes are not totally single channels, which lead to the production of roughly 20% nanotubes with different lengths. The outer diameter of the nanotubes corresponds to the (62) Kotov, N. A. Nanostruct. Mater. 1999, 12, 789.
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Figure 4. Optical microscopy (A) and SEM (B) images of the produced capsules after hollow (PSS/PAH)8PAH nanotubes with a diameter of 1.0 µm heated for 20 min at 121 °C.
Figure 6. SEM images of the (PSS/PAH)19PAH nanotubes with a diameter of 0.4 µm: (A) before heating; (B) after hydrothermal treatment at 121 °C for 40 min; (C) after hydrothermal treatment at 138 °C for 20 min. (D) A magnification of (C). Images C and D were obtained by using ESEM.
Figure 5. SEM images of (PSS/PAH)12PAH (A), (PSS/PAH)16PAH (B), and (PSS/PAH)19PAH (C) nanotubes after heating at 121 °C for 20 min. (D) is the magnification of (C).
pore diameter of the PC membranes used. Since the diameter of each pore is not identical throughout the templates, the length of as-assembled tubes and diameter of tubes are not uniform. Thus the productions of the transformed tubes have also different shapes and sizes. According to Rayleigh instability (eq 2), polyelectrolyte nanotubes prepared by the templates with different diameters should produce capsules with different sizes after hydrothermal treatment. In order to prove it, a (PAH/PSS)8PAH nanotube prepared by the PC template with a diameter of 1.0 µm was treated for 20 min at 121 °C according to the above-mentioned condition. As shown in Figure 3, one can see that the (PAH/PSS)8PAH nanotube with a diameter of 1.0 µm also can transform into the capsules after hydrothermal treatment. The average diameter of the produced capsules is roughly ranging from 1.0 to 2.3 µm, which is obviously larger than that transformed by the nanotubes with a diameter of 0.4 µm. This further indicates that the tube-tocapsule transformation of polyelectrolyte tubes obeys the Rayleigh instability and the produced capsule size can conveniently be tuned by using nanotubes with different diameters. In order to investigate the effect of the number of assembled polyelectrolyte layers on the above-mentioned transformation process, we also studied the thermal behavior of the (PAH/ PSS)12PAH, (PAH/PSS)16PAH and (PAH/PSS)19PAH tubes at 121 °C for 20 min (Figures 4 and 5). The SEM image in Figure 5A shows that most of the as-assembled (PAH/PSS)12PAH nanotubes transformed into the capsules after hydrothermal treatment. However, no capsule-like structure can be observed in Figure 5B after as-assembled (PAH/PSS)16PAH nanotubes were heated under a same condition. Similarly, the structure
transformation of the (PAH/PSS)19PAH nanotubes after heating was not observed (Figure 5C), and the shape and size still remain unchanged (Figure 5D). These results suggest that the thickness of the assembled polyelectrolyte multilayer has significant influence on the structure transformation. In fact, we also found that the polyelectrolyte nanotubes with at least 12 bilayers can maintain the tubular structure in the dried and vacuum state, indicating that the amount of assembled polyelectrolyte layers has a great effect on the structure–stability of the PAH/PSS nanotubes. This finding is identical with our previously reported poly-L-lysine hydrochloride (PLL)/poly-L-glutamic acid (PGA) nanotubes.41 Furthermore, the (PSS/PAH)19PAH nanotubes were selected to explore the influence of temperature and time on the tubeto-capsule transformation of as-assembled polyelectrolyte nanotubes. It was found that as-assembled (PAH/PSS)19PAH tubes after heating at 121 °C for 40 min still preserved the tubular structure (Figure 6B) like that before heating (Figure 6A). However, when the heating temperature was increased to 138 °C for 20 min, most of the (PAH/PSS)19PAH tubes were transformed into the capsule-like structure as shown in panels C and D of Figure 6. This indicates that temperature also has large effect on the structure stability of the polyelectrolyte nanotubes. As previously discussed, the thicker wall strengthens the structure stability of as-assembled polyelectrolyte nanotubes and correspondingly more thermal energy has to be input to break the ion pairs between polycation and polyanion. Thus, only after most of the ion pairs in the multilayer were disconnected, the tube-to-capsule transformation of these nanotubes can occur.
Conclusions In summary, the assembled polyelectrolyte nanotubes composed of PSS/PAH multilayers can be transformed into capsule shape with high-temperature treatment in an aqueous phase. The amount of the assembled polyelectrolyte layers has significant influence on the transformation process. The structure transformation of polyelectrolyte multilayers from tubes to capsules after annealing can be explained by the input of thermal energy which leads to a breakage of ion pairs between oppositely charged
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polyelectrolyte groups. The transition of polyelectrolyte nanotubes is also supposed to be driven by the Raleigh instability and leads to the generated polyelectrolyte capsules with different sizes. The shrinkage of those capsules is accompanied by a strong increase of wall thickness and a smooth of the surface. The driving force for this rearrangement process can be explained by the entropy increase through the more coiled state of the polyelectrolyte molecules and the decreased interface.
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Acknowledgment. This work was financially supported by the National Nature Science Foundation of China (No. 20574077 and 20520130213), the Chinese Academy of Sciences, and the German Max-Plank Society. Q. He gratefully thanks R. Pitschke for the electron microscopy measurements and the Alexander von Humboldt Foundation for a research fellowship. LA703738M
