ARTICLE pubs.acs.org/Langmuir
Peculiarities of Polyelectrolyte Multilayer Assembly on Patterned Surfaces Maxim V. Kiryukhin,*,† Shu Mei Man,† Anton V. Sadovoy,† Hong Yee Low,† and Gleb B. Sukhorukov†,‡ †
Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), 3 Research link, 117602, Singapore ‡ School of Engineering and Material Science, Queen Mary University of London, Mile End Road, London E1 4NS, United Kingdom
bS Supporting Information ABSTRACT: The layer-by-layer assembly of poly(diallyldimethylammonium chloride) and poly(sodium 4-styrenesulfonate) is studied on templates with imprinted arrays of microwells ranging from 2 to 25 μm and different aspect ratios. The thickness and microstructure of polyelectrolyte multilayers (PEMs) are measured using scanning electron microscopy. At 0.2 M ionic strength, the PEM film evenly coats the template both inside and outside the microwells. If the film is thinner than the critical value of about 400 nm, PEM microstructures collapse upon dissolving the template. Euler’s model of critical stress is used to describe the collapse. At 2 M ionic strength, a substantially thinner PEM film is assembled inside the 25 μm wells than outside. If the well diameter is reduced to 7 and 2 μm, a much thicker PEM film is formed inside the microwells. These observations have been attributed to the changing of polyelectrolyte conformation in the solutions.
’ INTRODUCTION Nanostructured composite materials with defined spatial arrangements form a promising platform for a new generation of ultrasensitive sensors, biomaterials, and optoelectronic devices. Tailored modulation of nanocomposite functionalities is crucial for many technical applications, and still it is a significant and ongoing challenge within nanoscience and nanotechnology.13 The layer-by-layer (LbL) assembly of species containing complementary groups, as first introduced by Decher,4,5 is one of the promising approaches to combine different functions in ultrathin materials. It allows precise control on the nanometer length scale over the thickness and vertical (normal to the surface) composition of resulting films. Comprehensive reviews on the tunable and stimuliresponsive properties of LbL-assembled films have been reported.3,69 However, it remains challenging to achieve horizontal (along the surface) modulation of the properties of multilayered nanocomposites. Nowadays there is a growing interest in patterning of the LbL-assembled polyelectrolyte multilayer (PEM) films. A number of methodologies have been developed for direct patterning of prefabricated films by solvent-assisted room temperature imprinting,1012 contact printing,13,14 and lift-off technique.10,15,16 Uniform micro- and nanopatterns of PEMs were obtained upon strain-induced elastic buckling of PEM films preassembled on a flat compliant substrate.17,18 Other groups exploited assembly of PEMs on the templates with modulated surface topography or chemistry.3,1925 It has been reported that PEM films grew at different paces on top of a substrate and inside micro- or nanostructures, such as microtrenches imprinted on polystyrene,21 pores in silicon, or polycarbonate substrates.2224 Nonuniformity r 2011 American Chemical Society
on the film thickness could compromise the performance of patterned PEMs. Herein, the growth of PEM films on patterned templates with imprinted arrays of microwells ranging from 2 to 25 μm is studied. PEM films are made of poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC). For this pair of polyelectrolytes, two different regimes of PEM growth behavior have been reported depending on the ionic strength of the solutions; the structure and mechanical properties of PEM films built at these regimes were also very different.2628 In this paper, a dramatic change of the pace of multilayer assembly upon reducing the microwells depth and diameter is demonstrated and discussed in terms of the conformation and diffusion of polyelectrolytes in solutions. The micromechanics of the stress-induced deformation of PEM material upon dissolving the template is discussed here also. As a result, fabrication of the variety of microstructures is shown including hollow microchambers, micropools, and solid microstubs.
’ EXPERIMENTAL SECTION Materials. Poly(sodium 4-styrenesulfonate), Mw = 70 kDa (PSS); 20 wt % water solution of poly(diallyldimethylammonium chloride), Mw = 100200 kDa (PDADMAC); and branched poly(ethyleneimine), Mw = 25 kDa (PEI) were purchased from Sigma-Aldrich and used as Received: March 12, 2011 Revised: May 13, 2011 Published: June 06, 2011 8430
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Scheme 1. Master Silicon Molds of Different Patterns Used in This Work
received. Poly(methylmethacrylate) (PMMA) coil (0.05 mm thick) was purchased from Goodfellow. Deionized (DI) water from a Milli-Q (Millipore) water purification system was used to make all solutions.
Scheme 2. Schematic Illustration of the Route for Patterned Polyelectrolyte Multilayer Films
Fabrication of an Array of Microwells on PMMA Film. Scheme 1 shows five different patterns of master silicon molds used in this work. Pattern I is an array of blunted spherical cones 4 μm tall, with base and roof diameters of 10 and 7 μm, respectively, and 6 μm pitch size. Pattern II is an array of cylinders having 2 μm diameter, 4 μm height, and 2 μm pitch size. Patterns IIIV are arrays of 25 25 μm square posts with 25 μm pitch size, having 2, 4, and 12 μm height, respectively. The mold of pattern II was purchased from Kyodo (Japan). The rest of the molds were processed by traditional lithography and etching processes. All molds were cleaned in piranha solution (a 3:1 mixture of 96% sulfuric acid and 30% hydrogen peroxide) at 120 °C for 30 min, rinsed with deionized water, dried in a stream of dry nitrogen, and put in a clean oven at 100 °C for 1 h. Finally the molds were treated with a fluorosilane release agent through an overnight vapor deposition of 1H,1H,2H,2Hperfluorodecyltrichlorosilane. PMMA sheets were cut slightly larger than the mold size and placed between the mold and flat Si wafer. The microimprint process was performed with an Obducat imprinter: the assembly was heated to 140 °C and a pressure of 4 MPa was then applied for 5 min followed by cooling the temperature to 80 °C and releasing the pressure from the mold. As a result, the corresponding mold produced a negative replica of patterns IV on the PMMA surface, making arrays of microwells.
Layer-by-Layer Assembly of PEMs on the Patterned PMMA Substrates. LbL assembly was done by a standard dipping method using a dip-coating robot machine (Riegler & Kirstein GmbH, Germany). Prior to dip-coating, the PMMA sheet was sonicated in water for 5 min to remove air bubbles that may be trapped inside the wells. The PMMA film surface is negatively charged due to the presence of uniformly distributed carboxylate groups.29 So it was exposed for 15 min to 2 mg/mL branched PEI solution (which pH was adjusted to 5 with 1 M HCl) in order to generate the first anchoring layer with high density of positive charges.30 Further alternating layers of PSS and PDADMAC were introduced followed by three washings with DI water to remove all nonadsorbed macromolecules. Adsorption of polyelectrolytes was performed using NaCl solutions of chosen ionic strength, polymer concentration 2 mg/mL, pH ∼5.5, 15 min, and 30 s for each adsorption and washing step, correspondingly, except as otherwise specified in this paper. The terminal layer was always PSS. Then the PEM film was transferred onto the Si support precoated with PEI for better adhesion, and toluene was used to dissolve the sacrificial PMMA substrate in order to reveal the PEM structure. In some experiments, PEMs were treated with oxygen-CF4 plasma in a RIE I
Etcher, Sirus (Trion) apparatus operated at 300 mTorr gas pressure, a 12 sccm oxygen and 3 sccm CF4 flow rate, and a power of 40 W. Characterization. Scanning electron microscopy (SEM) was carried out in secondary electron imaging mode at 5 keV with JEOL JSM 5600 (for planar images) and high-resolution FE SEM JSM-6700F (for cross sections) instruments. The cross sections of samples were done by chilling the PEM film transferred onto a Si support in liquid nitrogen followed by dicing the Si wafer. The thickness of PEM films was determined by analyzing at least five different regions of the crosssection of corresponding sample.
’ RESULTS The fabrication of a patterned LbL film is shown in Scheme 2. In the first step, an array of microwells of selected size, shape, and arrangement is imprinted on the surface of a sacrificial poly(methylmethacrylate) (PMMA) film. PMMA is a typical thermal plastic resist and its thermal imprinting is a very well established surface patterning technique. In general, our imprinted PMMA has nearly 100% yield and high pattern fidelity. Second, it serves as a template for LbL assembly of oppositely charged polyelectrolytes. Finally, the assembled multilayer film is transferred onto the Si wafer (acting as a support for ease of characterization) followed by dissolving the template in toluene. The cross sections of various regions of the (PDADMACPSS)n films of pattern I built at 0.2 M NaCl have been analyzed by SEM after being transferred onto a PEI-coated Si wafers. We have found that films have uniform thickness through the pattern which increases with number of bilayers. The data are 8431
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Figure 1. Evolution of (PDADMAC-PSS)n thickness with number of bilayers for the films of pattern I built at 0.2 M NaCl (triangles) and at 2 M NaCl outside (squares) and inside (circles) the imprinted microwells. The inset shows the difference in thickness of multilayers formed inside and outside the imprinted wells developed with bilayer number, n.
accumulated in Figure 1 (triangles): all data fall well into a line with a fitted slope of 9 ( 0.3 nm, which represents the single bilayer thickness. This resembles the literature data on linear growth of (PDADMAC-PSS)n films on flat substrates if the ionic strength is less than 0.3 M.2628 Figure 2 shows typical SEM images of the films. The microstructures made of 20 and 40 bilayers collapse upon dissolution the PMMA template. Cross-sectional image of the individual collapsed microstructure reveals the walls bent like a jackknife and the collapsed roof contacting the underlying support. However, the PEM film made of 60 bilayers contains array of standing microstructures. A cross section of an individual microstructure demonstrates that it is a hollow microchamber made of an ∼540 nm thick shell and is shaped like a blunted cone with a sagging roof. The microchamber is roughly 3 μm tall, with base and roof diameters of 7 and 5 μm, respectively, thus being a slightly reduced replica of the mold of pattern I. The SEM analysis of the cross sections of PEMs of pattern I built at 2 M NaCl has demonstrated the different thickness of the film through the pattern. The data are accumulated in Figure 1, where squares and circles represent the thickness of PEM film formed outside and inside the imprinted wells, respectively. Both sets of data indicate dramatic nonlinear growth of the film thickness with number of bilayers. The same growth regime has been reported for PDADMAC-PSS films built at ionic strength > 0.3 M on the flat surfaces,2628 and attributed to the high diffusion of macromolecules inside the PEM film.3133 However, here we demonstrate that the PEM is growing faster inside imprinted microwells than outside; the difference in thickness increases rapidly until it reaches a plateau at ∼2.22.5 μm, as shown in the inset to Figure 1. Further PEM growth occurs at the same pace both inside and outside the imprinted microwells. Typical SEM images of a series are shown in Figure 3. Top views and cross sections clearly indicate that all films contain standing microstubs that are not hollow, but solid; their height corresponds to the difference in thickness of PEMs grown inside and outside the imprinted microwells, respectively. The array of microstubs having a 23 times thinner PEM film in-between
Figure 2. SEM images of (PDADMAC-PSS)n films of pattern I built at 0.2 M NaCl for different numbers of bilayers, n: 20 (a, d), 40 (b), and 60 (c, e). Scale bars for top views, 10 μm; for cross sections, 1 μm.
(like shown on Figure 2d) could be easily transformed into the array of isolated microstubs on bare Si wafer by reactive ion etching, a well-established technology to erase material in a precise manner. Figure 4 shows SEM images of (PDADMAC-PSS)8 film after 60 s of etching: the posts have a reduced height of ∼1.3 μm with no residual layer in-between. Figure 5 shows SEM images of the (PDADMAC-PSS)10 films of patterns IIV built at 2 M NaCl. For all patterns, the thickness of the PEM film formed outside the imprinted microwells is ∼1.5 μm, thus coinciding with the thickness of the (PDADMACPSS)10 film of pattern I (see Figure 3e). However, the PEM microstructure changes drastically for different patterns. PEMs of pattern II contain an array of standing microstructures that are neither hollow nor solid but look like snapshots of viscous material flowing down toward the underlying support. Microstructures of patterns IIIV are collapsed with folded walls and, contrary to all previous findings, much thinner roofs contacting the support. The deeper the microwells imprinted in the template were, the thinner were the collapsed roofs of PEMs microstructures: ∼1.4, ∼1.1, and ∼0.45 μm for 2, 4, and 12 μm deep wells of patterns IIIV, respectively. What is the reason behind such a drastic change?
’ DISCUSSION (PDADMAC-PSS)n films demonstrate linear growth regime on patterned PMMA template if built at 0.2 M ionic strength (see Figure 1, triangles). This resembles the literature data on linear growth of (PDADMAC-PSS)n films on the flat substrates if ionic strength is less than 0.3 M.2628 What is more important, the films of the same thickness were formed both inside and outside the imprinted wells (see Figure 2d,e). Earlier several research groups have reported the formation of thicker PEM films inside micro- and nanostructures, such as trenches imprinted on polystyrene21 or pores of polycarbonate membranes,22,23 if compared to PEM films formed outside, on top of the substrates. The phenomenon was 8432
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Figure 3. SEM images of (PDADMAC-PSS)n films of pattern I built at 2 M NaCl for different numbers of bilayers, n: 7 (a, c), 8 (b, d), and 10 (e). Scale bars for top views, 10 μm; for cross sections, 1 μm.
Figure 5. SEM images of (PDADMAC-PSS)10 film of patterns II (a, b), III (c, d), IV (e), and V (f) built at 2 M NaCl. All scale bars = 5 μm.
2πEh2 Pcr ¼ σcr A ¼ 2πahσcr ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3ð1 υ2 Þ
Figure 4. SEM images of isolated solid PEM microstubs on bare silicon prepared by reactive ion etching of (PDADMAC-PSS)8 film of pattern I built at 2 M NaCl. Scale bars for top view, 10 μm; for cross-section, 1 μm.
attributed to the incomplete drainage of solutions and uneven water access upon washing steps. Contrary to these findings, the PEM film formed inside the pores etched in Si wafer was thinner than that outside.24 Depletion of polyelectrolyte concentration within pores has been discussed as a possible explanation. Uniform thickness of the PEM film through the pattern in our case indicates that the time and number of adsorption/washing cycles during the LbL process chosen here is enough to ensure completion of a PE layer and removal of unadsorbed polyelectrolytes. Our data show that PDADMAC-PSS microchambers obtained upon dissolving the template collapse with the roofs contacting the underlying support if made of 40 bilayers and less (as shown in Figure 2a,b), but stand if made of 60 bilayers and more (Figure 2c). This gives us the range for the critical bilayer number or, using the linear fit of Figure 1, the range for the critical thickness hcr of PDADMAC-PSS film, 360540 nm. The theory of elasticity predicts the existence of a critical stress above which elastic shell collapses by buckling. The following equations predict critical stress, σcr, and load, Pcr, for cylindrical shells:34 Eh σ cr ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a 3ð1 υ2 Þ
ð1Þ
ð2Þ
where E is Young’s modulus, υ is Poisson’s ratio, A is the shell cross section, h is the shell thickness, and a is the radius of a cylinder. Microstructures usually collapse due to the adhesive contact with underlying support (roof collapse) or each other (lateral collapse).35,36 Following the model first reported by Navajas et al.,35 let us assume that collapse occurs if the adhesion strength of a chamber’s roof to the underlying Si wafer, σad, is larger than the critical stress chambers could withstand (such adhesion contact could be achieved upon deformation of microchambers by capillary force and solvent flow upon drying). Then using eq 2, the expression for the critical Young’s modulus is as follows: Ecr ¼
a2 σ ad
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3ð1 υ2 Þ 2h2
ð3Þ
The adhesion strength of a PDADMAC-PSS film to a silicon wafer was reported to be ∼6 MPa.37 Taking cylinder radius a = 3 μm, critical shell thickness in the range 360540 nm, and a Poisson’s ratio of 0.5 (Poisson’s ratio of incompressible elastic material), we calculate the range for critical elastic modulus, Ecr, 100300 MPa. The Young’s modulus of 102 MPa, reported for the wet (PDADMAC-PSS)8 shells of microcapsules dispersed in water,38 falls into this range. However, this range is far below the Young’s modulus of 673 ( 10 MPa, reported for the dried (PDADMACPSS)8 films.37 Thus, the collapse of (PDADMAC-PSS)n microchambers occurs upon dissolving the PMMA template while the multilayer film is still wet and highly plasticized by water. Only microchambers made of PEM film thicker than the critical thickness could withstand the template dissolving. Further drying makes these chambers even harder. PDADMAC-PSS films built at 2 M NaCl on top of patterned PMMA template are drastically thicker and exhibit a nonlinear growth regime (see Figure 1, squares). Again this resembles the literature data on the growth of PDADMAC-PSS films on the flat 8433
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substrates if ionic strength is higher than 0.3 M.2628 However, the PEM microstructure varies drastically depending on the size of the imprinted microwells in which they are assembled. The largest microstructures used in this work are 25 μm square wells of different depths (patterns IIIV). We discovered that the deeper the wells, the thinner the PEM film that is formed on their bottom (see Figure 5df). In our opinion, the reason for the effect is a depletion of polyelectrolyte concentration across the wells from the surface to the bottom, which becomes negligible upon reducing well depth from 12 to 2 μm. On one hand, electrostatic interaction between polyelectrolytes and charged surfaces confining the wells could cause such depletion, as it was discussed elsewhere.24 However, the electrostatic screening length at 2 M NaCl solution is ∼1 nm only (see the Appendix), thus making surface charge induced depletion inapplicable for our case. On the other hand, the reason could be physical exclusion of macromolecules, but the 25 25 μm wells are 3 orders of magnitude larger than the size of both PDADMAC and PSS coils (∼35 and 12 nm, respectively; see the Appendix). So macromolecules should freely pass into the wells even taking into account relatively high molecular mass distribution of PDADMAC used in the work. The only possible explanation we could suggest is the slow diffusion of macromolecules into the wells so that equilibrium concentration is not reached near the well bottom within dipping time (15 min) for deep enough wells. The diffusion of PDADMAC was measured by dynamic light scattering (see details in the Supporting Information); the normalized field autocorrelation function g(1)(t) and corresponding diffusion coefficient (D) distribution are shown in Supporting Information Figures S1 and S2, respectively. It appeared that there are two diffusion modes in 2 mg/mL PDADMAC solution at 2 M NaCl. The fast mode is characterized by the mean diffusion coefficient 8 108 cm2/s. The StokesEinstein equation gives 29 nm as the radius of the corresponding hydrodynamic sphere, Rh: Rh ¼
kB T 6πηD
ð4Þ
where kB is Boltzmann’s constant, 1.38 1023 J/K; T is temperature, 298 K; and η is the viscosity of NaCl aqueous solution,39 9.353 104 kg m1 s1. We attribute this mode to the diffusion of isolated polyelectrolyte coils. However, the slow mode has a much lower diffusion coefficient of 1.26 109 cm2/s, and the radius of the corresponding hydrodynamic sphere calculated by eq 4 is 1.85 μm. We attribute this mode to the diffusion of aggregates consisting of a large number of polyions. Really, at 2 M ionic strength, electrostatic screening length (∼1 nm) is just slightly larger than the average distance between the effective charges in the polymer (∼0.7 nm; see the Appendix), so electrostatic repulsion of ionized groups within the coil is negligible and association of hydrophobic PDADMAC backbones occurs. Yu et al. have shown that in dilute solutions (0.5 mg/mL) far below the overlap concentration intramolecular association occurs causing shrinkages of the extended PDADMAC coils to compact globules having hydrodynamic radius of 21 nm,39 that is close to the radius of fast particles measured in our case. Shear-force-induced stretching of polyelectrolytes brings the system into the semidilute regime above the overlap concentration, where intermolecular chain aggregations happen.39 Theoretical estimation gives 5.6 mg/mL as the overlap concentration for PDADMAC solution
at 2 M NaCl (see the Appendix); the 2 mg/mL PDADMAC solution used in this work is of the same order of magnitude, and hence intermolecular chain aggregations may happen. If PDADMAC adsorbs as aggregates of globules, it should lead to the exceeding of PDADMAC monomers amount over the PSS monomers in the assembled multilayer. In fact, it was reported that the number of PDADMAC monomers significantly (up to 7 times) exceeds the number of PSS monomers if a multilayer is built at that high ionic strength, and extra positive charges are compensated with Cl counterions occluded in multilayer.28 The characteristic diffusion time of aggregates tdiff across distance l could be estimated using the following expression: t diff ¼ l2 =D
ð5Þ
So it takes 20 min for polyelectrolyte aggregates with diffusion coefficient 1.26 109 cm2/s to travel through the 12 μm deep well. Retarded diffusion of aggregates leads to the lack of polymer near the bottom of imprinted wells after 15 min dipping, thus giving thinner PEM films. For smaller wells of pattern I the picture inverts: thicker (PDADMAC-PSS)n films are formed inside the wells. The difference increases rapidly until it reaches a plateau at ∼2.22.5 μm (see Figure 2cf). This thickness is determined analyzing the film cross sections by SEM. Wet PDADMAC-PSS films contain ∼40 vol % water,28 that should be inevitably lost upon sample preparation for SEM. So the difference in the wet PEM film thickness upon multilayer assembly should be ∼40% larger, giving ∼4 μm, which was the depth of the imprinted wells of pattern I. It means that PDADMAC-PSS film grows faster inside the wells until it completely fills them. Wells of 7 μm are still a few orders of magnitude larger than single polymer coils. However, large aggregates could stick inside the wells and be incompletely drained out upon washing steps. To check this, we have built the (PDADMAC-PSS)8 film of pattern I from dilute 0.5 mg/mL polyelectrolyte solutions at 2 M NaCl. No intermolecular aggregates are formed, but only intramolecular association of PDADMAC coils occurs at this concentration, as it is far below the overlap concentration of PDADMAC aqueous solution.39 SEM analysis demonstrates collapsed structures that have the same thickness of the PEM film, ∼30 nm, both inside and outside the imprinted wells (see Figure S4 in the Supporting Information). Hence, shrunken PDADMAC globules could pass freely in and out of the microwells, providing effective drainage of unadsorbed macromolecules and forming PEM film of even thickness through the pattern. However, the overall thickness of multilayers formed from isolated globules decreases dramatically if compared to the one formed from PDADMAC aggregates. It is worth noting that PEMs of pattern I have even thickness both inside and outside the microwells if built using 2 mg/mL polyallylamine hydrochloride (PAH) solution at 2 M NaCl instead of PDADMAC (see Figure S5 in the Supporting Information). PAH is a more hydrophilic polycation than PDADMAC and does not form aggregates at 2 M NaCl. Another interesting feature of Figure 3 is the presence of solid standing PEM posts. This is so even if the PEM film deposited inside the wells was much thinner than the well depth (as in Figure 3d). It was reported elsewhere that a wet PDADMACPSS film built at high ionic strength behaves like a gel (in contrast to PDADMAC-PSS films formed at ionic strength < 0.3 M which behave as elastic bodies).28 We assume that stresses that appear in the film upon template dissolution induce viscous flow of 8434
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Langmuir polymer toward the underlying support. While films are drying, they become harder and at some point relaxation stops. So the microstructures of pattern II having smaller diameter and higher aspect ratio dry faster, and their images look like snapshots of the viscous liquid flowing toward the support (Figure 5a,b). Relatively weak van der Waals forces between globules in the polyelectrolyte aggregates could be the reason for the high diffusion of macromolecules inside the PEM film3133 and gel-like behavior of PEMs under applied stress.
’ CONCLUSIONS In this work, we have reported a method to fabricate patterned arrays of PDADMAC-PSS multilayers using sacrificial PMMA templates. PMMA was dissolved with toluene in order to reveal the PEM structure. The post-treatment of multilayers by organic solvents could affect their properties. Depending on the solvent, significant softening (acetone)40 or stiffening (THF)41 of multilayers was reported. However, the mechanical properties of toluenetreated PDADMAC-PSS multilayers reported here, assembled at both low and high ionic strength, resemble the corresponding properties of untreated multilayers reported elsewhere.2628,38 We have demonstrated that the growth regime for PDADMAC-PSS polyelectrolyte multilayer films drastically changes depending on the size and aspect ratio of microwells imprinted on the templating surface. Conformation of polyelectrolytes in the solutions has been shown to be a crucial factor for the phenomenon. Polyelectrolytes exist as aggregated globules at 2 M ionic strength. If the template is patterned with an array of 25 μm square wells, the retarded diffusion of such aggregates leads to a substantially thinner PEM film assembled inside the microwells than outside. If the well diameter is reduced from 25 μm to 7 and 2 μm, incomplete drainage of polyelectrolyte aggregates occurs and, consequently, a much thicker PEM film is assembled inside the microwells than outside. Individual polyelectrolyte coils existing at 0.2 M ionic strength freely pass into microwells and evenly coat the template both inside and outside the wells. Critical thickness of the stable freestanding film as measured by SEM is about 400 nm. If the film is thinner than critical, PEM microstructures collapse upon dissolving the template. Adhesive contact of microstructures with the support is proposed as a guiding mechanism responsible for their collapse. Euler’s model of critical stress describing the collapse of elastic shells is used to estimate the Young’s modulus of the PDADMAC-PSS film. If the film is thick enough, it could withstand the template dissolution, giving free-standing empty chambers. The hollow interior of the chambers could be used for housing a variety of active components for envisaged location and time controlled activation. The highly ordered array of sealed microchambers containing substances of interest opens an avenue for programmed release-on-demand of cargo from individual chambers in a site-specific manner, for example, by remote opening with focused laser radiation by the way similar to shown before for PEM capsules.4244 These experiments are currently in progress. ’ APPENDIX An important parameter for consideration of polyelectrolyte adsorption on oppositely charged surfaces is the size of polymer coils in the solution. One means of characterizing coil dimensions is the chain end-to-end distance, R, which was estimated here using the scaling theory of flexible polyelectrolytes at high
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ionic strength, developed by Rubinstein et al.:45 R r B 2=5 L3=5
ð1Þ
where rB is the electrostatic screening length and L is the contour size of a chain of electrostatic blobs. For large enough salt concentrations, the electrostatic screening length is proportional to but larger than the Debye screening length: r B ¼ ðN=cLÞ1=2 ð1 þ 2cs A=cÞ1=2
ð2Þ
where N is the number of monomer units in polymer chain, c and cs are PE and salt concentration, respectively, and A is the number of monomers between uncondensed charges. In good solvent, contour size L is given by L NbðlB =bA2 Þ2=7
ð3Þ
where b is monomer size and lB is the Bjerrum length at which the energy of the Coulomb interaction between two elementary charges is equal to the thermal energy kT. In aqueous solution at 298 K, the Bjerrum length is 0.7 nm. For strong polyelectrolytes with high ionization degree, A = lB/b due to counterion condensation. Here we used PE concentration c = 2 mg/mL and salt concentration cs = 2M. For a PSS chain of molecular weight 70 000, N ≈ 340 and b ≈ 0.3 nm,45eqs 13 give rB ≈ 0.9 nm, L ≈ 67 nm, and R ≈ 12 nm. The same estimation for PDADMAC (Mw 100 000200 000) at cs = 2 M gives N ≈ 930, b ≈ 0.52 nm,28 rB ≈ 0.8 nm, L ≈ 444 nm, and R ≈ 35.5 nm. At lower salt concentration, cs = 0.2 M, rB ≈ 2.5 nm, and R ≈ 56 nm. The overlap concentration c* is determined as the concentration where the monomer density inside the coil is equal to the overall monomer density in the solution. At high salt limit (c , 2Acs), the overlap concentration is given by45 c ðN=LÞ6=5 ð2Acs Þ3=5 N 4=5
ð4Þ
So the overlap concentration of PDADMAC is 5.6 mg/mL at 2 M NaCl.
’ ASSOCIATED CONTENT
bS
Supporting Information. Detailed description of dynamic light scattering experiments, normalized field autocorrelation function g(1)(t), and corresponding diffusion coefficient distribution. SEM images of (PDADMAC-PSS)8 film of pattern I built from 0.5 mg/mL polyelectrolyte solutions at 2 M NaCl. SEM images of (PAH-PSS)40 film of pattern I built from 2 mg/mL polyelectrolyte solutions at 2 M NaCl. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Fax: (þ65) 6872 7528. Telephone: (þ65) 6874 8252. E-mail:
[email protected].
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