pubs.acs.org/Langmuir © 2009 American Chemical Society
Growth Mechanism of Confined Polyelectrolyte Multilayers in Nanoporous Templates C. J. Roy, C. Dupont-Gillain, S. Demoustier-Champagne, A. M. Jonas,* and J. Landoulsi Institut de la Mati ere Condens ee et des Nanosciences, Universit e Catholique de Louvain, Place Croix du Sud 1, B-1348 Louvain-la-Neuve, Belgium Received August 21, 2009. Revised Manuscript Received October 15, 2009 We investigate the mechanism of polyelectrolyte multilayer (PEM) assembly in nanoporous templates with a view to synthesizing nanotubes or nanowires under optimal conditions. For this purpose, we focus on the effect of parameters related to the geometrical constraints (pore diameter), the size of the macromolecules (their molar mass and the ionic strength), and the interaction between the pore walls and the adsorbed chains (modulated by the ionic strength). Our results reveal the existence of two regimes in the mechanism of PEM growth: (i) the first regime is comparable to that observed on flat substrates, including the influence of ionic strength and (ii) the second regime, which is slower in terms of kinetics, results from the interconnection established between polyelectrolyte chains across the pores and leads to the formation of a dense gel. As a consequence, the diffusion of polyelectrolytes in nanopores becomes the controlling factor of PEM growth in this second regime. The dense gel, owing to its peculiar structure, enhances the formation of nanowires or of partially occluded nanotubes in some cases, depending on initial pore dimensions.
1. Introduction Nanoscience and nanotechnology constitute strategic and challenging fields of research that involve the engineering and fabrication of materials on the nanoscale with new features.1 Among the methodologies used to synthesize nano-objects, a promising technique is the template method combined with layerby-layer assembly (LbL). The template strategy, introduced by Martin et al.,2 consists of filling the void spaces of a nanoporous host material with the desired material (metals, polymers, inorganic oxides, semiconductors, or carbon).3,4 Compared to various deposition techniques (sol-gel chemistry, electrochemistry, chemical vapor, electroless),4,5 LbL assembly is a simple, versatile, low-cost strategy to replicate the open porosity of templates. It is based on the alternate adsorption of complementary species such as oppositely charged polyions and allows a wide range of materials to be assembled (synthetic polyelectrolytes,6 biological macromolecules,7 dyes,8 and nanoparticles9). On flat surfaces, many research groups have exhaustively studied the build up of polyelectrolyte multilayers (PEM) and the impact of different parameters (type of polyelectrolyte, ionic strength, pH, etc.) on their structure as well as on their kinetics of growth. Details can be found in a number of reviews.6,10-14 However, similar *Corresponding author. E-mail:
[email protected]. (1) Report of The Royal Society & The Royal Academy of Engineering, Plymouth, England, 2004; pp 1-111. (2) Martin, C. R. Science 1994, 266, 1961–1966. (3) Ozin, G. A. Adv. Mater. 1992, 4, 612–649. (4) Martin, C. R. Chem. Mater. 1996, 8, 1739–1746. (5) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N.; Nevanen, T. K.; Soderlund, H.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11864–11865. (6) Decher, G. Science 1997, 277, 1232–1237. (7) Lvov, Y. M.; Decher, G. Crystallogr. Rep. 1994, 39, 628–647. (8) Cooper, T. M.; Campbell, A. L.; Crane, R. L. Langmuir 2002, 11, 2713–2718. (9) Fendler, J. H. Chem. Mater. 1996, 8, 1616–1624. (10) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319–348. (11) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 1999, 4, 430–442. (12) Ariga, K.; Hill, J. P.; Ji, Q. Phys. Chem. Chem. Phys. 2007, 9, 2319–2340. (13) Wang, Y.; Angelatos, A. S.; Caruso, F. Chem. Mater. 2008, 20, 848–858. (14) Ariga, K.; Hill, J. P.; Lee, M. V.; Vinu, A.; Charvet, R.; Acharya, S. Sci. Technol. Adv. Mater. 2008, 9, 014109.
3350 DOI: 10.1021/la903121e
investigations are still scarce regarding deposition in nanoporous templates, even though polyelectrolyte nanotubes were successfully synthesized through the LbL templating technique.15-22 This lack of information about LbL assembly in nanopores is probably related to the difficulty in applying most common characterization techniques as used on flat substrates. Nevertheless, the confinement of macromolecular chains inside nanopores may significantly alter the mechanism of PEM growth. In previous studies, under identical deposition conditions, the PEM thickness was reported to be appreciably higher in nanopores than on flat substrates.15,17,19,23 Alem et al. have investigated this behavior by alternately filtering polyelectrolytes through nanopores of track-etched membranes using polyelectrolytes of varying molar mass.23 They showed that the maximum filling of the pores was reached after one to two cycles and suggested the rapid formation of a dense swollen gel in the whole cavity. In this study, we focus on the mechanism of PEM growth in nanopores by the alternate immersion of polycarbonate tracketched membranes in poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) solutions. This allows a comparison with results obtained on flat substrates to be made. Indeed, in contrast to filtration, the simple immersion procedure reproduces conditions comparable to those used in the case of flat substrates. The PAH/PSS system was selected because the effects of various (15) Ai, S.; Lu, G.; He, Q.; Li, J. J. Am. Chem. Soc. 2003, 125, 11140–11141. (16) Liang, Z.; Susha, A. S.; Yu, A.; Caruso, F. Adv. Mater. 2003, 15, 1849–1853. (17) Ai, S.; He, Q.; Tao, C.; Zheng, S.; Li, J. Macromol. Rapid Commun. 2005, 26, 1965–1969. (18) Tian, Y.; He, Q.; Tao, C.; Li, J. Langmuir 2006, 22, 360–362. (19) Lee, D.; Nolte, A. J.; Kunz, A. L.; Rubner, M. F.; Cohen, R. E. J. Am. Chem. Soc. 2006, 128, 8521–8529. (20) Yang, Y.; He, Q.; Duan, L.; Cui, Y.; Li, J. Biomaterials 2007, 28, 3083–3090. (21) Lee, D.; Cohen, R. E.; Rubner, M. F. Langmuir 2007, 23, 123–129. (22) He, Q.; Cui, Y.; Ai, S.; Tian, Y.; Li, J. Curr. Opin. Colloid Interface Sci. 2009, 14, 115–125. (23) Alem, H.; Blondeau, F.; Glinel, K.; Demoustier-Champagne, S.; Jonas, A. M. Macromolecules 2007, 40, 3366–3372. (24) Arys, X.; Jonas, A. M.; Laschewsky, A.; Legras, R.; Mallwitz, F. In Supramolecular Polymers; Ciferri, A., Ed.; Taylor & Francis: Boca Raton, FL, 2005; Vol. 2, pp 651-710.
Published on Web 11/09/2009
Langmuir 2010, 26(5), 3350–3355
Roy et al.
factors such as pH, ionic strength, polyelectrolyte concentration, and molar mass on the PEM growth are well known for this system as reviewed by Arys et al.24 The confinement of PEM in nanopores is investigated by tuning the size of the polyelectrolyte chains in solution as well as the membrane pore diameter. To this end, the polyelectrolyte molar mass (Mw) and the ionic strength of the solutions are varied using membranes with pore diameter from 100 to about 500 nm. The qualitative and quantitative evolutions of the PEM growth in nanopores are monitored stepby-step using electron microscopy and gas-flow porometry measurements.
2. Experimental Section 2.1. Assembly of Polyelectrolyte Multilayers. Sodium poly(styrene sulfonate) (PSS, low molar mass of Mw = 17 kDa or high molar mass of Mw = 150 kDa) and poly(allylamine hydrochloride) (PAH, low molar mass of Mw =15 kDa or high molar mass of Mw=70 kDa) were purchased from Sigma-Aldrich. Polyelectrolyte solutions at a concentration of 1 mg/mL were prepared in 100 mM acetate buffer (pH ∼4.7) with or without NaCl (0.5 M) under conditions of full protonation of the PAH. All solutions were freshly prepared before use. The templates used in this study were track-etched polycarbonate membranes, provided by It4ip, Seneffe, Belgium (http:// www.it4ip.be), with average pore diameters of ∼100, ∼200, and ∼500 nm and a thickness of 21 μm. These membranes will be designated as small pores, medium pores, and large pores, respectively. The buildup of PAH/PSS multilayers in nanopores was performed by alternately dipping the membranes in PAH and PSS solutions for 30 min each. An intermediate rinsing step was performed in two different baths (2 min each) with the buffer solution used for the LbL assembly. This process was repeated until the desired number of bilayers (n) was obtained. 2.2. Characterization of Polyelectrolyte Nanotubes. Microscopy observations of the obtained nanotubes were performed after air drying the samples. For scanning electron microscopy (SEM) analysis, the track-etched polycarbonate membranes were deposited on a porous silver membrane (average pore diameter of 0.45 μm, SPI Supplies) and then dissolved and rinsed with dichloromethane to entrap the liberated nanotubes. The samples were then imaged using a field-effect gun digital scanning electron microscope (FE-SEM, DSM 982 Gemini from LEO) operating at 1 kV. For transmission electron microscopy (TEM), the membranes were dissolved in dichloromethane with subsequent sonication. The extracted nanotubes were then collected on TEM carbon grids and imaged with a LEO 922 TEM operating at 200 kV. In both cases, the dissolution of the membrane in dichloromethane was carried out for less than 30 s and the extracted nanotubes were collected during the rapid solvent evaporation. 2.3. Gas-Flow Porometry Measurements. The PEM growth in membrane nanopores was monitored by measuring the evolution of the mean diameter of the pores as a function of the number of deposited bilayers. This was performed using gasflow porometry on air-dried samples at room temperature. The track-etched polycarbonate membrane was clamped in the sample holder, with an effective section area of 0.396 cm2, and nitrogen gas was delivered upstream from the sample at a pressure ranging from 104 to 4 105 Pa. The gas flow rate (mL/min) downstream from the sample was then measured using a flowmeter (Agilent), giving access to the mean diameter of the pores by using relationships based on the Knudsen diffusion and the viscous or HagenPoiseuille flow.19,25,26
(25) Bielza, J. M.; Kamusewitz, H.; Keller, M.; Paul, D. Langmuir 2002, 18, 8129–8133. (26) Albo, S. E.; Broadbelt, L. J.; Snurr, R. Q. Chem. Eng. Sci. 2007, 62, 6843– 6850.
Langmuir 2010, 26(5), 3350–3355
Article
Figure 1. SEM images of a track-etched polycarbonate membrane after the deposition of (PAH/PSS)26 (a) before and (b) after surface treatment to remove the PEM crust. The Knudsen diffusion flux and the viscous flux (mol/m2 s) are expressed respectively by 4 d ðPup -Pdown Þ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3l 2πMRT
ð1Þ
d 2 ðPup 2 -Pdown 2 Þ 64μRT l
ð2Þ
Jdiff ¼ and Jvisc ¼
where d is the pore diameter (m), l is the thickness of the membrane (m), Pup and Pdown are the pressures (Pa) upstream and downstream from the membrane respectively, M is the gas molar weight (kg/mol), μ is the dynamic viscosity of the gas (kg/m s), R is the ideal gas constant (J/K mol), and T is the gas temperature (K). The total volume flow rate Φ (m3/s) from eqs 1 and 2 can be expressed by Φ ¼ ðSPÞ
RT ðJdiff þ Jvisc Þ Patm
ð3Þ
where S is the effective section area of the membrane (m2), Patm is the atmospheric pressure (Pa), and P is the transparency of the membrane, defined as P¼ N
πd 2 4
ð4Þ
where N is the pore density (m-2). For each sample, the measured pore diameter was an average of at least 15 measurements. All measurements were found to be within 5% of the average value.
3. Results and Discussion 3.1. Formation and Characterization of PAH/PSS Multilayers in Nanopores. After the alternate adsorption of polyelectrolytes, the formation of a PEM film on the top and bottom surfaces of the membrane can be observed, leading to the encrusting of the pores (Figure 1a). This phenomenon, more pronounced with an increasing number of bilayers (data not shown), causes a major restriction in flow porometry measurements. To this end, prior to the characterization of the LbL-filled membranes, the film deposited on the top and bottom surfaces of the membrane was removed with a cotton swab immersed in a strongly basic aqueous solution (NaOH, pH ∼12) of high ionic strength (3 M NaCl). The membrane was then abundantly rinsed with Milli-Q water. This procedure, in addition to the mechanical polishing effect, causes the swelling and disturbing of the multilayers as a result of the combined effects of (i) pH, which drastically decreases the positive charge density of PAH, and (ii) salt, which reduces the electrostatic repulsion of charges along DOI: 10.1021/la903121e
3351
Article
Figure 2. (a) SEM and (b) TEM images of (PAH/PSS)26 nanotubes obtained in large pores.
the polyelectrolyte chains.27,28 Hence, it gives access to the nanopores without altering the membrane surface or the PEM inside the pores. This was carefully checked by comparing the length and the morphology of the extracted nanotubes using TEM images with and without the use of the cotton swab. SEM images revealed that, after the surface treatment, pores with welldefined contours can be observed in their entirety, suggesting that an appreciable part of the crust film was removed (Figure 1b). Accordingly, the flow porometry measurements can be performed under the correct conditions (i.e., the hindering effect of the film deposited on the top and bottom surfaces of the membrane can be neglected). Furthermore, the extraction from the membrane of individualized nanotubes for TEM or SEM characterization became easier after the dissolution of the membrane. The PAH/PSS nanotubes obtained after membrane dissolution exhibited smooth outer surfaces and outside diameters corresponding to that of the membrane nanopores with a narrow size distribution. These features can be seen in SEM images for all pore sizes under different deposition conditions. (See images of nanotubes obtained in large pores in Figure 2a.) One key issue is to know whether PEM growth occurs uniformly along the length of the nanopore walls. Independently of the ionic strength and Mw, TEM images revealed that the nanotube length corresponds to the thickness of the membrane (i.e., about 21 μm). (See images of nanotubes obtained in large pores in Figure 2b.) It should be noticed that this observation was valid only for a minimum number of bilayers, when nanotubes are mechanically stable after membrane dissolution (n=6 for small and medium pores, n=8 for large pores). For a smaller number of bilayers, the walls of the nanotubes are obviously too thin to preserve the mechanical integrity of the whole structure after membrane dissolution. More direct evidence regarding the hollow structure of nanotubes can be obtained by high-magnitude TEM images. The results given in Figure 3 show that the nanotube morphology evolves appreciably as a function of the number of bilayers. From the TEM images, it is possible to distinguish between hollow structures (nanotubes) and filled structures (nanowires): the former exhibit a higher optical density at their edges when seen in transmission mode (Figure 3a), whereas the latter appear to be more uniform in optical density (Figure 3b). Small pores rapidly lead to the formation of nanowires (Figure 3b), whereas larger pores lead to nanotubes whose walls were seen to thicken and become more rigid for a higher number of bilayers (Figure 3e,f). In pores of intermediate size, some portions of the nanotubes are clogged along the length of the nanopores, giving rise to partially occluded nanotubes (Figure 3d). (27) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 3736–3740. (28) Sukhorukov, G. B.; Schmitt, J.; Decher, G. Ber. Bunsen-Ges. 1996, 100, 948–953.
3352 DOI: 10.1021/la903121e
Roy et al.
It is worth noting that for a small number of bilayers, nanotubes were observed to flatten when deposited onto the TEM grids. This effect, attributed to the drying stage during solvent evaporation,16,23 did not allow us to provide reliable dimensions from the imaged nanotubes, in particular, for their inner diameter. Hence, flow porometry measurements were performed to monitor the growth of PEM in nanopores as a function of the number of bilayers (n=0-26) and to investigate the influence of (i) initial pore diameter, (ii) polyelectrolyte Mw, and (iii) the ionic strength of the solutions. Results, presented in Figure 4, show that the pore diameters decrease with an increasing number of bilayers, independently of the initial pore diameter, corresponding to an increase in the average multilayer thickness on the template walls. In some cases, for small and medium pores, no flow measurements can be performed (zero flow, Figure 4a,b,d). These findings are in agreement with TEM observations showing the formation of whole or partial nanowires. It was determined that the formation of nanowires as seen by TEM corresponds to zero flow measurements. The results also show that the evolution of the pore diameter is different whether the solution contains or does not contain NaCl (Figure 4, left vs right). The presence of NaCl in the polyelectrolyte solutions leads to a more pronounced decrease in the pore diameter with the number of bilayers (i.e., the deposition of thicker multilayers and even the formation of nanowires in the case of medium pores (Figure 4c,d)). In small pores, a dramatic drop in the diameter down to around 35 nm was observed in the presence of NaCl for two bilayers, whereas a more progressive decrease was noticed without NaCl (Figure 4a,b). These results suggest that the increase in the ionic strength leads to the deposition of thicker PEM inside the nanopores. Similar findings have been reported on flat substrates and were attributed to the more coiled conformation of polyelectrolytes in solutions of higher ionic strength.10,24 The effect of polyelectrolyte Mw on the PEM growth is also presented in Figure 4. The decrease in the pore diameter as a function of the number of bilayers was more pronounced for high polyelectrolyte Mw in medium pores (Figure 4c,d). This effect was smaller in large pores (Figure 3e,f) and not significant in small pores except that the formation of nanowires occurred for a smaller number of bilayers at low Mw (Figure 4a,b). On flat substrates, the influence of polyelectrolyte Mw on the PEM thickness is not as straightforward as the ionic strength and depends strongly on the polyelectrolytes used as well as the adsorption conditions.29,30 Under our deposition conditions, it appears that the increase in Mw leads to a slight increase in the PEM thickness depending on pore dimensions, which indicates a complex interplay between the size of the macromolecules and the pore diameter. 3.2. Mechanism of Multilayer Growth in Nanopores. In contrast to flat substrates, the diffusion of polyelectrolytes into the nanopores should be taken into account in the study of the growth mechanism of confined PEM. The parameters investigated in this study are related to the geometrical constraints (pore diameter), the size of the macromolecules (their molar mass and ionic strength), and the interaction between the pore wall and the adsorbed chains (modulated by ionic strength). The experimental parameters thus simultaneously influence (i) the thickness of each adsorbed layer, (ii) their interaction with the pore surface, and (iii) the rate of diffusion of the chains in the pores. The discrimination (29) Houska, M.; Brynda, E.; Bohata, K. J. Colloid Interface Sci. 2004, 273, 140– 147. (30) Tachaboonyakiat, W.; Serizawa, T.; Endo, T.; Akashi, M. Polym. J. 2000, 32, 481–485.
Langmuir 2010, 26(5), 3350–3355
Roy et al.
Article
Figure 3. Representative TEM images of (PAH/PSS)n nanotubes obtained in (a, b) small, (c, d) medium, and (e, f) large pores showing the evolution of nanotube morphology from (a, c) n = 6 or (e) n = 8 to (b, d, f) n = 26.
Figure 4. Evolution of the pore diameters estimated with gas-flow porometry as a function of the number of bilayers in (a, b) small, (c, d) medium, and (e, f) large pores. Polyelectrolyte multilayer (PEM) assembly was performed (a, c, e) without or (b, d, f) with the addition of 0.5 M NaCl. PAH and PSS of low (open symbols) or high (solid symbols) molar mass (Mw) were used.
between these three factors that are involved in the mechanism of PEM growth is challenging. We thus defined a heuristic parameter on the basis of porometry measurements, i.e., the relative radial PEM increment (Δr) defined as follows Δr ¼
dP ðiÞ -dP ðnÞ 100 dP ðiÞ
ð5Þ
where dP(i) is the initial pore diameter and dP(n) is the pore diameter measured after n bilayer deposition cycles. Langmuir 2010, 26(5), 3350–3355
This parameter corresponds to the relative gain in PEM thickness as a function of the number of bilayers. It may give information about the kinetics of the multilayer growth and allows the evolution obtained in small, medium, and large pores to be compared with each other (Figure 5). Despite a nonnegligible dispersion, data can clearly be fit according to linear regression (Figure 5). This procedure does not aim to establishing the best fit but allows a probable change in evolution to be noticed. Data corresponding to zero flow (i.e., corresponding to the formation of nanowires or part of the nanowires (4r=100%)) DOI: 10.1021/la903121e
3353
Article
Roy et al.
Figure 5. Evolution of the relative radial PEM increment (4r, eq 5) as a function of the number of bilayers in (triangles) small, (circles) medium, and (squares) large pores. Polyelectrolyte multilayers assembly was performed (a, c) without or (b, d) with the addition of 0.5 M NaCl. PAH and PSS of low (open symbols) or high (closed symbols) molar mass were used. Data corresponding to zero flow (i.e., corresponding to the formation of whole or partial nanowires (4r = 100%)) are not presented in the figure for the sake of clarity). In this case, the absence of data points above some number of cycles means that the sample enters the second regime (almost-filled pores). The shaded region indicates when the growth regime changes as justified by a change in slope of 4r (n).
are not presented in Figure 5 because clogging is sometimes a singular event that occurs in only a specific section of the pore. The results given in Figure 5 show clearly that a change appears in the evolution of the radial PEM increment when it reaches values ranging from 60 to 70%, independently of the ionic strength and Mw. This transition can be seen in small pores at four bilayers in the absence of NaCl (Figure 5a,c), whereas it occurred more rapidly at two bilayers in the presence of NaCl (Figure 5b,d). The slope change is also observed in large pores in the presence of NaCl for high Mw at 10 bilayers (Figure 5d). For all other cases, the relative radial PEM increment follows a constant regression and was still less than values near 70% for the higher number of deposition cycles probed in this study. These findings suggest a change in the PEM growth kinetics in nanopores and reveal the existence of two regimes. The first regime corresponds to relative radial PEM increment values lower than ∼70% where linear growth occurs from the first deposited layers. The second regime, highlighted by the shaded region in Figure 5, corresponds to the relative radial PEM increment larger than 70% and presents a noticeable decrease in the PEM growth kinetics compared to that in the first regime. To our knowledge, the existence of two regimes has never been seen in the buildup of PEM on flat substrates, which suggests that the diffusion of polyelectrolytes into the nanopores is a key parameter in that respect. Generally, the diffusion regime of polymer chains through cylindrical pores is related to the reduced variable rh/rp, where rh is the hydrodynamic radius of the polymer chains and rp is the pore radius.31-34 In this study, rh ranged from 2 to 11 nm (on the basis of dynamic light scattering measurements, (31) Adiga, S.; Curtiss, L.; Elam, J.; Pellin, M.; Shih, C.-C.; Shih, C.-M.; Lin, S.-J.; Su, Y.-Y.; Gittard, S.; Zhang, J.; Narayan, R. J. Mineral. Met. Mater. Soc. 2008, 60, 26–32. (32) Cannell, D. S.; Rondelez, F. Macromolecules 1980, 13, 1599–1602. (33) Guillot, G.; Leger, L.; Rondelez, F. Macromolecules 1985, 18, 2531–2537. (34) Prakash, S.; Piruska, A.; Gatimu, E. N.; Bohn, P. W.; Sweedler, J. V.; Shannon, M. A. IEEE Sens. J. 2008, 8, 441–450.
3354 DOI: 10.1021/la903121e
data not shown). In the first regime (i.e., the beginning of the formation of PEM), the value of rp is large enough compared to rh to assume that the diffusion of the macromolecules can be described by considering the polymer chains to be rigid spheres of much lower diameter than pore size.31-34 This suggests that the diffusion of polyelectrolytes into the pores is not limiting in the mechanism of PEM growth in the first regime and that the buildup takes place on the pore walls as on flat substrates. To explain the second regime, it must be kept in mind that porometry measurements were performed in the dried state. However, the buildup of PEM occurs in solution, leading to the formation of highly hydrated films. The thickness of hydrated PEMs is reported to be higher than that of dried PEMs. According to the literature,35-38 the swelling rate ranges from ∼20 to ∼40% depending on the deposition conditions. Although the swelling of PEM in the nanopores could be different from that in thin films, the highly stitched nature of PAH/PSS multilayers makes it improbable that the actual swelling value in nanopores would not fit within this broad 20-40% range. Note that Losche et al.38 have reported that dehydration leads to film shrinking and about 40% of the void volume created within the film, due to the removal of water molecules, is not filled by the polymer upon film shrinkage, presumably because of steric hindrance in the polymer chains. Thus, taking into account the PEM swelling, assembled polyelectrolytes should occupy the whole cavity in solution when the radial PEM increment obtained in the dried phase is near 70%. At the transition, the polyelectrolyte chains across the pore start to make interconnections between each other and this entanglement (35) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725–7727. (36) Harris, J. J.; Bruening, M. L. Langmuir 1999, 16, 2006–2013. (37) Miller, M. D.; Bruening, M. L. Chem. Mater. 2005, 17, 5375–5381. (38) Losche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893–8906.
Langmuir 2010, 26(5), 3350–3355
Roy et al.
Article
results did not allow the influence of ionic strength and Mw to be discriminated in a simple way, highlighting the complexity of the process.
4. Conclusions
Figure 6. Schematic representation of polyelectrolyte multilayer (PEM) structures in nanopores corresponding to regimes 1 and 2 before and after drying.
increases with the following deposited bilayers, leading to the formation of a denser gel. In this case, the “effective” rp is of the same order of magnitude as rh so that the rigid sphere model is no longer valid for the diffusion of polyelectrolytes in the second regime. The created gel, because of its peculiar structure, slows down diffusion and, as a consequence, PEM growth. Contrary to the first regime, the diffusion of polyelectrolytes in nanopores becomes the PEM growth controlling factor in the second regime (gray shaded areas in Figure 5). It should be noticed that the gel model has been previously proposed by Alem et al. to explain the rapid filling of the pores when filtering polyelectrolytes through track-etched membranes, although the formation of this filling gel seems to happen faster when the PEMs are grown by filtration rather than by simple diffusion. A representative structure of the confined PEM is depicted in Figure 6. The number of cycles required to form this spider-web-like structure depends mainly on the initial pore diameter. The size of the polyelectrolyte chains in solution, influenced both by the ionic strength and Mw, should also play an appreciable role. Indeed, the second regime was observed in medium and large pores in the presence of NaCl with high Mw, corresponding to chains of quite large rh. However, our
Langmuir 2010, 26(5), 3350–3355
In this article, the mechanism of PEM growth in confined media was evidenced, as were the main phenomena involved in this process. To this end, nanoporous templates with pore diameters ranging from ∼100 to ∼500 nm were used. Furthermore, the size of the polyelectrolyte chains in solution was tuned by varying the ionic strength of the medium and the molar mass (Mw). This strategy aimed to understand the mechanism of multilayer growth when the diffusion of polyelectrolytes into the pores may play a major role. Particular attention was given to the kinetics of PEM growth as a function of the number of bilayers. Our results showed the existence of two regimes in PEM growth. The first regime is comparable to that observed on flat substrates, including the influence of ionic strength: the increase in ionic strength leads to the formation of thicker PEM. The second regime, which is slower in terms of kinetics, corresponds to the development of a spider-web-like entangled structure inside the pores. Indeed, the buildup of PEM in confined media causes the interconnection between polyelectrolyte chains across the pores, leading to the formation of a dense gel. This happens when the swollen multilayers almost completely fill the pore. In this case, the role of polyelectrolyte diffusion into the pores is much more pronounced in the growth mechanism of PEM. Moreover, the peculiar structure of the gel leads to the formation of nanowires in small pores and some portions of the nanotubes that are clogged along the length for medium pores. The complexity of PEM growth in nanopores is inherent in the multiplicity of the parameters influencing this process. Although other parameters such as the dipping time and the polyelectrolyte concentration are also relevant, our results provide practical information and allow the syntheses of nanotubes and nanowires to be carried out under optimized conditions. The functionalization of nanoporous templates can be extended to sensitive polyelectrolytes or other macromolecules with biological interest, including proteins and DNA, and is particularly important in applications such as filtration and separation techniques involving suitable pore size, drug loading, and cell culture. Acknowledgment. We thank Etienne Ferain and the it4ip company for supplying polycarbonate membranes and for their contribution to gas-flow porometry measurements and J.-F. Gohy and C.-A. Fustin for DLS measurements. C.J.R. and J.L acknowledge financial support from FRIA-FNRS and BELSPO, respectively, in the framework of network IAP 6/27. S.D.-C. is an F.R.S.-FNRS Senior Research Associate.
DOI: 10.1021/la903121e
3355
