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Self-Patterning Polyelectrolyte Multilayer Films: Influence of Deposition Steps and Drying in a Vacuum
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Self-Patterning Polyelectrolyte Multilayer Films: Influence of Deposition Steps and Drying in a Vacuum
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  • Amir Azinfar
    Amir Azinfar
    Institute of Physics, University of Greifswald, Felix-Hausdorff-Straße 6, D-17489 Greifswald, Germany
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  • Sven Neuber
    Sven Neuber
    Institute of Physics, University of Greifswald, Felix-Hausdorff-Straße 6, D-17489 Greifswald, Germany
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  • Marie Vancova
    Marie Vancova
    Institute of Parasitology, Biology Centre of the Czech Academy of Sciences, Branisovska 31, 37005Ceske Budejovice, Czech Republic
    Faculty of Science, University of South Bohemia, Branisovska 1760, 37005 Ceske Budejovice, Czech Republic
  • Jan Sterba
    Jan Sterba
    Faculty of Science, University of South Bohemia, Branisovska 1760, 37005 Ceske Budejovice, Czech Republic
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  • Vitezslav Stranak
    Vitezslav Stranak
    Faculty of Science, University of South Bohemia, Branisovska 1760, 37005 Ceske Budejovice, Czech Republic
  • Christiane A. Helm*
    Christiane A. Helm
    Institute of Physics, University of Greifswald, Felix-Hausdorff-Straße 6, D-17489 Greifswald, Germany
    *Email: [email protected]. Phone: +49 3834 420 4710. Fax: +49 3834 420 4712.
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Langmuir

Cite this: Langmuir 2021, 37, 35, 10490–10498
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https://doi.org/10.1021/acs.langmuir.1c01409
Published August 26, 2021

Copyright © 2021 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Typically, laterally patterned films are fabricated by lithographic techniques, external fields, or di-block copolymer self-assembly. We investigate the self-patterning of polyelectrolyte multilayers, poly(diallyldimethylammonium) (PDADMA)/poly(styrenesulfonate) (PSS)short. The low PSS molecular weight (Mw(PSSshort) = 10.7 kDa) is necessary because PSSshort is somewhat mobile within a PDADMA/PSSshort film, as demonstrated by the exponential growth regime at the beginning of the PDADMA/PSSshort multilayer build-up. No self-patterning was observed when the PDADMA/PSS film consisted of only immobile polyelectrolytes. Atomic force microscopy images show that self-patterning begins when the film consists of seven deposited PDADMA/PSSshort bilayers. When more bilayers are added, the surface ribbing evolved into bands, and circular domains were finally observed. The mean distance between the surface structures increased monotonously with the film thickness, from 70 to 250 nm. Scanning electron microscopy images showed that exposure to vacuum resulted in thinning of the film and an increase in the mean distance between domains. The effect is weaker for PSSshort-terminated films than for PDADMA-terminated films. The mechanism leading to domain formation during film build-up and the effect of post-preparation treatment are discussed.

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Introduction

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Layer-by-layer films (LbL films) are prepared by sequential deposition of alternating layers of oppositely charged polyelectrolytes (PEs) (synthetic PEs, proteins, DNA, or nanoparticles). (2−4) The targeted integration of suitable building blocks enables many technological and biomedical applications. (5,6) The vertical structures of LbL films have been studied in detail, especially the respective influence of the layer sequence and interdiffusion. (2,7−9) Their lateral structure is poorly understood; the focus has been on the film/air roughness because that limits their usefulness for applications. In some cases, scanning electron microscopy has been used to demonstrate the formation of surface domains. (10) Understanding self-assembly during layer-by-layer growth allows controlled fabrication of nano-patterned films.
Micro- or nano-patterned films are used in applications such as nanolithography, (11,12) nanotemplating, nanoporous membranes, (13) or surface functionalization to improve cell growth. (14,15) For surface patterning, one can modify the polymer films by external fields, (16) or one can use self-assembly. (17−19) Ten years after the discovery of LbL films, it was recognized that the film surface could be rough. (10) The increasing roughness could have two reasons: film destabilization or pattern formation. (10,20,21) In the first case, only films with limited thickness can be formed because increasing roughness limits the number of deposition steps which can be achieved. We investigate the second case, increased roughness due to pattern formation during film build-up.
Spontaneous pattern formation was also observed in PEMs before, (22,23) but it was never investigated systematically. More work on pattern formation has been done with gel films adhering to a surface. The freshly prepared gel film was homogeneous, and the pattern formed only during drying. The decrease in film volume due to drying caused asymmetric stresses, since the film could only shrink vertically, but not laterally. (24) This observation was explained by calculations considering the film thickness, and the elastic moduli of the film and the substrate. These calculations predict that the average distance between domains increases linearly with the film thickness. The formation of the surface pattern of drying gels was enabled by the movement of water molecules. (24) In LbL films, the asymmetric stress is dictated by the conformation of the adsorbed PEs. Just like a drying gel film, a LbL film can only expand in the direction perpendicular to the substrate during its fabrication. The PEs adsorb in a non-equilibrium conformation: If the PEs have a low diffusion coefficient, the original conformation is further immobilized as subsequent layers adsorb. (25−27) The water content is much lower than in gels, it stands to reason that the mobile species must also be a polyelectrolyte.
To obtain mobility within the film that is necessary for surface patterning, a mobile PE species is required. The diffusion coefficient increases as the polymer length (i.e., molecular weight) is reduced. (28,29) As the domains form, PE movement occurs not only within the film but also on the surface and the sides of the domains. As the average distance between domains increases when additional layers are deposited, not only mobile PEs in the bulk, but also PEs with an adsorption/desorption equilibrium at the surface may be involved. (27,30)
Additional stresses are applied to the LbL film post-preparation when the film is exposed to vacuum. Then, not only the weakly bound water leaves the film, but also the tightly bound water. (31,32) The absence of water molecules between the PEs and at the film/vacuum interface allows the chains to rearrange themselves to produce an energetically more favorable conformation. It was often observed that flat films deswell when moved from water to air as the weakly bound water evaporates. When the film is placed in a vacuum, tightly bound water leaves the film and changes the interaction between PE chains. (31,32) We will show that the additional stress caused by exposure to vacuum changes the surface pattern.
The polycation poly(diallyldimethylammonium) chloride (PDADMA), and polyanion poly(styrenesulfonate) sodium salt (PSS) were used to prepare the LbL films. This system was chosen because it is a well characterized system (1,3,4) with a wide range of biomedical applications. (33−35) The linear charge density of PSS is twice that of PDADMA, therefore, the system grows asymmetrically. (3) To reverse the surface charge, the PDADMA coverage must be larger than the PSS coverage at each deposition step. Therefore, the film growth is nonlinear, i.e., the film thickness increases nonlinearly with the number of PDADMA/PSS bilayers deposited. After PDADMA adsorption, the film is positively charged, and subsequent PSS adsorption leads to charge neutralization.
Finally, after the deposition of Ntrans bilayers, the film growth changes from nonlinear to linear, i.e., the film thickness increase per deposited PDADMA/PSS bilayer is constant. In the linear growth regime, PSS adsorption no longer leads to charge compensation within the film. As a result, there are more positive than negative monomers in the film (3,36) and electroneutrality within the film is achieved by the incorporation of monovalent anions from the deposition solution.
The exact nature of the nonlinear growth regime at the beginning of the LbL film build-up depends on the mobility of the PSS molecules. After Ntrans deposited PDADMA/PSS bilayers, a transition from a nonlinear towards a linear growth regime always occurs. However, for PSS with a molecular weight below Mw(PSSshort) < Mwthreshold(PSS), (1,27) film growth starts with an exponential growth regime, followed by a parabolic one. Neutron reflectivity measurements show that in the exponential growth regime, PSSshort is freely mobile within the film bulk, in a direction perpendicular to the film surface, (37) as expected for an exponential growth regime. (38) In situ monitoring of multilayer build-up indicated that fast adsorption of short PSS is followed by slow desorption of the PDADMA/PSS complexes. (27) In contrast to PSS, in all studies with low molecular weight PDADMA, the polycation was immobile within the film, even though the film thickness increase in the linear growth region was significantly decreased due to the low molecular weight of PDADMA. (37)
The picture is simpler for PSSlong with Mw(PSSlong) > Mwthreshold(PSS) ≈ 25 kDa. The smaller diffusion constant of PSSlong hinders the exponential growth. The remaining nonlinear growth regime is parabolic. Furthermore, no desorption of PSSlong/PDADMA complexes was observed. Although the mobility of PSSlong and PSSshort is dramatically different, the elastic modulus of the LbL film is assumed to be independent of the PSS molecular weight. (28)
We hypothesize that mobile PSS molecules are necessary for pattern formation. To evaluate this idea, PSSshort and PSSlong were used in the current study, with Mw(PSSshort) = 10.7 kDa and Mw(PSSlong) = 130 kDa. The other parameters characterizing film growth were kept constant: the molecular weight of the positively charged polyelectrolyte (Mw(PDADMA) = 322 kDa) and the ion concentration in the adsorption solution (0.1 M). To demonstrate that the chains must not only be short but also mobile, the surface structure of a film of PDADMAshort (23.6 kDa) and PSSlong was examined. PDADMAshort is found to be immobile by neutron reflectometry. (37) The film thickness was determined by X-ray reflectivity and ellipsometry. The evolution of surface patterning as a function of film thickness was followed by atomic force microscopy (AFM) images in air and in pure water. Surface domains in a vacuum were studied from different tilt angles using scanning electron microscopy (SEM).

Materials and Methods

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Materials

Branched poly(ethylenimine) (PEI; Mw = 750 kDa) was purchased from Sigma-Aldrich (Munich, Germany). Poly(styrenesulfonate) sodium salt (PSS) with two different molecular weights was used as the polyanion, PSSshort with the polymer weight Mw(PSSshort) = 10.7 kDa and PSSlong with Mw(PSSlong) = 130 kDa and 666 kDa (all PSS polymers with a PDI < 1.20). The polycation was poly(diallyldimethylammonium) chloride (PDADMA) with the polymer weight Mw(PDADMAlong) = 322 kDa and a PDI of 2.19 and Mw(PDADMAshort) = 23.6 kDa (PDI ≈ 2). PSS and PDADMA polymers were purchased from PSS (Polymer Standard Service, Mainz, Germany). Sodium chloride (NaCl) was obtained from Merck KGaK (Darmstadt, Germany). All solutions were prepared with ultrapure water, using a reverse osmosis system (Sartorius Arium Advance, Göttingen, Germany) which was followed by a millipore purification system (Millipore, Milli-Q synthesis, Molsheim, France, nominal conductivity 0.054 μS/cm). Each polyelectrolyte (PE) deposition solution was prepared at a concentration of 1 mmol/L with respect to the monomer unit. The NaCl concentration of the deposition solution was 100 mM. Single side polished silica wafers were used as substrates (⟨100⟩, Silicon Materials, Kaufering, Germany and Andrea Holm GmbH, Tann, Germany). Prior to sample preparation, silicon wafers were cleaned according to the RCA cleaning protocol. (39)

Preparation of Multilayer Films

Polyelectrolyte multilayers were fabricated via a dipping self-assembly technique (40) using a dipping robot (Riegler & Kirstein, Berlin, Germany). Each adsorption step lasted 30 min, followed by three washing steps in salt free ultrapure water for 1 min each. Branched PEI was always used as the first layer, which reverses the surface charge of the silicon wafers and serves as substrate anchoring layer. All adsorption solutions contained 0.1 M NaCl and 1 mM polyelectrolyte with respect to the monomer concentration. During the adsorption process, all solutions were maintained at 20 °C using a thermostat (Thermo Fisher Scientific, Haake A25, Haake AC200).

X-Ray Reflectometry

The X-ray measurements were performed with a Seifert XRD 3003 TT diffractometer (GE Sensing and Instrument Technology, Ahrensburg, Germany) using Cu Kα radiation (wavelength λ = 1.54 Å). This technique measures the interference between the light reflected from the nanometer-thick surface layer and the light reflected from the substrate/layer interface. With this small wavelength, the index of refraction n = 1 – δ depends linearly on the electron density of the constituting molecules. Since n deviates a little from 1 (δ ≤ 3 × 10–5), approximations are possible, and the measured reflectivity R can be normalized with respect to the Fresnel reflectivity RF of an infinitely sharp interface modulated by interference effects from the surface layer. Above about two critical angles of total external reflection () the reflectivity is given by the kinematic approximation (50)
where ρ is the electron density of the substrate, is the gradient of the electron density along the surface normal, and is the wave vector transfer normal to the surface (α is the incident angle that is equal to the exit angle in a specular scattering geometry), RF(Qz) is the Fresnel reflectivity of an ideally smooth surface. To quantify the molecular parameters, the exact, optical matrix formalism (dynamical approach) is used. The surface layer is parametrized as consisting of different slabs (each with a density and a thickness parameter, as well as a roughness parameter). In all cases, the simulated reflectivity is convoluted with the angular divergence of the respective spectrometer (0.012°).
The structure of the sample perpendicular to the surface is represented by a stack of three slabs, each with a constant refractive index (Si substrate with electron density 0.7 e3, PEM, and air with electron density 0 e3). The parameters to be determined are thickness dPEM and electron density ρe,PEM of the PEM. In a good approximation, dPEM can be determined from the separation of two interference minima of the X-ray reflectivity curve (cf. Figure S3) according to
The relative humidity varied slightly (3–5%) during one measurement. The low humidity in the chamber was achieved by inserting a small dish with P2O5 (Merck KGaG, Darmstadt, Germany).

Ellipsometry

For thicker films (≥130 nm), the thickness was determined by null-ellipsometry. The measurements are performed with an ellipsometer (Multiskop; Optrel GbR, Sinzing, Germany) in the PCSA configuration (polarizer-compensator-sample-analyzer). A He–Ne laser (power = 4 mW; wavelength λ = 632.8 nm) is used as the light source. The measured quantities were the ellipsometric angles Ψ and Δ, which correspond to the ratio of amplitude and phase shift of light due to reflection at the sample, respectively.
The ellipsometric angles are related to the ratio of the Fresnel reflection coefficients by rp/rs = tan(Ψ)e, where rp and rs are the reflection coefficients of the parallel and normal components of the electric vector (with respect to the plane of incidence). The relative humidity within the laboratory averaged around 18% during the measurements.
The structure of the sample perpendicular to the surface is represented by a stack of four slabs, each with a constant refractive index (Si substrate, SiO2 layer, PEM, and air). The roughnesses between different slabs are set to zero. In this model the refractive indices of the Si wafer and the SiO2 layer are fixed to 3.882 – 0.02i and 1.457. (41) The index of refraction of air is 1. Before the preparation of a PEM, the thickness of the native oxide layer was determined, it was usually about 1 nm. The only remaining unknown sample parameters are the PEM film thickness and refractive index (d, nPEM).
The angle of incidence traverses during the measurement, the range from 66 to 72° (with respect to the surface normal) in 1° steps. In each position the corresponding ellipsometric angles of the sample are measured. The resulting angles Ψ and Δ are particularly sensitive to changes caused by the PEM film. d and nPEM are determined by a least mean square algorithm.

Atomic Force Microscopy (AFM)

AFM imaging in air was performed using a Multimode microscope (Veeco/Digital Instruments, Santa Barbara, CA) equipped with a Nanoscope IIIa controller. For measurements in liquids, a Bioscope Resolve microscope (Bruker, Karlsruhe, Germany) was used. The images were recorded using AFM tapping-mode in air with standard cantilevers (OMCL-AC160TS, k ≈ 40 N/m, f ≈ 320 kHz, tip curvature radius < 10 nm as specified by the manufacturer; Olympus Inc., Hamburg, Germany), while for imaging in pure water, FESP-V2 rectangular cantilevers (k ≈ 1–5 N/m, f ≈ 50–100 kHz, tip curvature radius ≈ 8 nm as specified by the manufacturer; Bruker AFM Probes, Palaiseau, France) were used. AFM images were processed using Bruker NanoScope Analysis 1.9; with this program also the surface roughness σAFM ≙ σRMS, root-mean-square roughness was determined. To determine the average domain separation, two-dimensional fast Fourier transform (2D FFT) of the images was performed, leading to a 2D power spectrum. In the case of images that exhibit a coarse structure, the 2D power spectrum shows radial symmetry with a preferred spatial wavelength. The directional information on the 2D power spectrum was removed by radial averaging. (42)

Scanning Electron Microscopy (SEM)

SEM was performed using a field emission scanning electron microscope (FESEM, JSM-7401F, JEOL, Japan) incorporating a cold cathode field emission gun, ultra-high vacuum, and sophisticated digital technologies for high resolution, high-quality imaging of micro structures. Using SEM, high-quality imaging depends on the film conductivity. Therefore, all PEMs had to be coated (30 s) with a thin gold layer (∼6 nm) via plasma sputtering using a SCD 050 sputter coater (Bal-Tec, Los Angeles, CA). Measurements from three selected tilt angles (0, 45, 90°) were taken to acquire a highly and detailed demonstration of the PEM surfaces. The PEM micrographs were taken at selected options: Voltage 1–8 kV, working distance (WD) 8.0 mm. To determine the mean domain distance, a 2D FFT was performed using the Gwyddion 2.56 program.

Results and Discussion

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The theories about pattern formation predict that the separation of the surface domains increases with film thickness. (24,43) Experimentally, large patterns are easier to resolve than small ones. Therefore, we started the investigation with thick films. PSS-terminated films consisting of 30 polycation/polyanion bilayers (bl) were imaged with atomic force microscopy (cf. Figure 1). The film consisting of PSSlong was flat. The same surface roughness (σAFM = 1.5 nm) was obtained as that with X-ray and neutron reflectometry. (27,44) In contrast, the film consisting of PSSshort shows domains with an average distance exceeding 100 nm. The surface roughness was almost an order of magnitude larger (σAFM = 21 nm). The domains were also observed in pure water (cf. Figure S1). However, in pure water, the surface roughness was smaller. These results indicate that the domains of PDADMA/PSSshort films were formed during the LbL film self-assembly.

Figure 1

Figure 1. AFM images (5 μm × 5 μm) of three different PEI/PSS/(PDADMA/PSS)29 films in air (in water cf. Figure S1). The polyelectrolyte molecular weight is varied as indicated. (a): PSSshort (10.7 kDa) and PDADMAlong (322 kDa); (b): PSSlong (666 kDa) and PDADMAshort (23.6 kDa); (c): PSSlong (130 kDa) and PDADMAlong (322 kDa). All AFM measurements were performed under ambient conditions (r.h. = 40%). Note the different height scales of the images.

To demonstrate that it is not sufficient to use a long and a short polyelectrolyte, films made of PDADMAshort/PSSlong were also investigated (cf. Figure 1b). As expected, (1) these films are only half as thick as the films from PDADMAlong and PSSlong. Furthermore, they are flat and show the same film/air roughness as the PDADMA/PSSlong films. It is not sufficient that the polyelectrolyte is short, it has also to be mobile.
The forces which led to the domain formation are mechanical stresses within the film, similar to those which led to wrinkling in gel films. (24,43) However, only the films consisting of PSSshort show a surface pattern. Therefore, while the stresses are probably similar, domain formation is made possible by the increased mobility of PDADMA/PSSshort complexes. (27)
To monitor the formation of domains during film growth, the surface topology of different LbL films was imaged in dependence on the number of deposited PDADMA/PSSshort bl. The obtained AFM images are shown in Figure 2. When the first polyelectrolyte layers are deposited, the film thickness increases exponentially with the number of deposited layers, i.e., the film is in the exponential growth regime. PDADMA/PSSshort films consisting of up to five bl appear smooth. However, films consisting of seven bl start to show corrugations on the surface. Eventually, when the film consists of 15 bl, the corrugations turn into ribbons. Concomitantly, the surface roughness increases from 1.1 nm up to ≈ 4 nm. Now, at 15 bl, the transition from the exponential to the parabolic growth regime occurs. In the beginning of the parabolic growth regime, a similar trend was observed as in the exponential growth regime: the ribbons get more distinct and their separation increases (from 72.5 to 209 nm according to FFT), simultaneously the roughness increases further. Eventually, the ribbons change into small circular domains. At the end of the parabolic growth regime the film consists of 25 bl, and the surface shows circular domains only.

Figure 2

Figure 2. AFM images (5 μm × 5 μm) in air of the topography development of PEI/PSSshort /(PDADMA/PSSshort)N−1 films from N = 5 up to N = 40 bl. Shown is the surface structure and roughness (σAFM) as the films progress through the three different growth regimes: (a) exponential, (b) parabolic and (c) linear. The height scales are also constantly increasing with increasing number of deposited bilayers. Therefore, only nine representative examples for the height scale are given: 8 nm for 5 bl, 20 nm for 9 bl, 27.7 nm for 13 bl, 41.6 nm for 16 bl, 63.4 nm for 19 bl, 99.0 nm for 23 bl, 152 nm for 27 bl, 175 nm for 33 b and 167 nm for 40 bl (at r.h. = 40%).

During the subsequent linear growth regime, the circular domains persist. Their separation increases further on the addition of PDADMA/PSSshort bilayers (from 209 to 226 nm when the number of bl is increased from 25 to 40). However, the roughness is basically independent of the number of deposited bilayers (σAFM = 23.6 ± 1.4 nm).
The average distance between the surface corrugations was determined by fast Fourier transform (FFT) in two-dimensions. Only one peak is observed, showing a well-defined average distance between domains (cf. Figure S2), but no crystalline order. As Figure 3 shows, the average distance increases monotonously with the number of deposited bilayers. First the average distance increases steeply, and then it gets flatter. The influence of different growth regimes on domain formation can be better discerned when the average distance is plotted against the film thickness (cf. Figure 3, right; X-ray reflectivity measurements of the film thickness are shown in Figure S3 and the film thickness vs the number of PDADMA/PSSshort bl in Figures S4). When the domains (or rather the corrugations) form, the initial very pronounced growth of the average domain distance can be discerned. With each deposited bilayer, the average distance increases by a smaller amount. Eventually, in the linear growth regime, the increase in domain separation is small. In other words, as predicted theoretically, (24) the domain separation increases linearly with the film thickness.

Figure 3

Figure 3. Average distance of PDADMA/PSSshort surface patterns as calculated by the FFT power spectrum from AFM images in Figure 2. The average distance of domains is shown in dependence of the number of PDADMA/PSSshort bilayers deposited (left) and of film thickness (right). Additionally, the film/air roughness (σAFM) is shown (right). Film thickness was measured with X-ray reflectivity up to thicknesses of 125 nm, then ellipsometry was used. Different growth regimes of the LbL film are indicated.

The surface area of the domains could be determined when the film consisted of 20 bl (i.e., is in the middle of the parabolic growth regime) or more. Both the diameter of domain area and the mean separation of the domains increase monotonically. However, the diameter of the domain area is always smaller (cf. Figure S7).
The predictive power of the theory is limited if one calculates in the linear growth regime the elastic modulus of the film from the domain separation, one gets elastic moduli on the order of MPa as was found experimentally. (45) However, at the beginning of the film build-up the calculated elastic modulus is on the order of GPa, which is unrealistic. These observations illustrate that the domains are not necessarily equilibrium structures but formed during the PDADMA adsorption process due to the lateral movement of the PDADMA/PSSshort complexes. The surface roughness σAFM as determined by AFM can be taken as a measure of the height of the surface structures. It increases linearly in the exponential and parabolic growth regimes with the film thickness, and it is constant in the linear growth regime, suggesting that the domain height is constant in this growth regime. Since the domains are so tunable and well-defined, we examine the domains in the linear growth regime in more detail.
The films were investigated by scanning electron microscopy (SEM). For investigation with an electron microscope, the films were placed in a vacuum chamber, and water within the LbL film was extracted using a vacuum pump system (all water molecules), not only the weakly bound ones which desorb when the film is brought from aqueous solution into air. (31) Scanning electron microscopy images of PDADMA/PSSshort films under different angles of PSSshort- and PDADMA-terminated films are shown in Figure 4, top views with different magnifications are shown in Figure 5. In the side view, the films do not look homogeneous, but grainy. The grainy structure is attributed to polyelectrolyte clustering during drying. PSSshort terminated films have corrugations which are separated by shallow trenches. For films consisting of 35 bl, these trenches are about 83 nm deep, with a film thickness of about 336 ± 27 nm. The SEM images of PDADMA-terminated films differ dramatically from those of PSSshort-terminated films. Instead of corrugations with shallow trenches, pillars surrounded by deep trenches are found. The side view shows that the pillars are homogeneous with similar heights (341 ± 22 nm). Some pillars are connected by thin and short filaments. These thin filaments are attributed to PDADMA/PSSshort complexes which shuttled between the domains during PDADMA adsorption and adjusted the domain separation. (27)

Figure 4

Figure 4. Scanning electron microscopy images of PEMs made from PSSshort (10.7 kDa) at different tilt angles and magnifications. PSS-terminated (top) and PDADMA-terminated (bottom) films are shown. The number of PDADMA/PSSshort bilayers (bl) and the tilt angle (0, 45, 90°) are indicated in each panel.

Figure 5

Figure 5. Top: Scanning electron microscopy images of PSSshort (left) and PDADMA (right)-terminated multilayers consisting of 40 or 40.5 bilayers (top view). Different magnifications are shown. Insets: FFT power spectrum of the images. For the 40 bl film, the angle-averaged peak position is 26.38 μm–1 (white ring) and for the 40.5 bl film 11.69 μm–1 (blue ring) and 19.2 μm–1 (white ring). Bottom: Radially averaged power spectral density spectra obtained from the PSSshort-terminated (blue symbols) and the PDADMA-terminated film (black symbols). The lines are fits to Gaussian functions.

The very different lateral structure of PDADMA and PSSshort terminated films is attributed to the different configuration of PSS and PDADMA on PDADMA/PSSfilms: PSS with its large linear charge density adsorbs flatly, it compensates positive charges at the film surface. One PSS chain can adsorb on adjacent domains. Earlier experiments showed that upon a decrease in the ion concentration of the surrounding solution, PSS remains flatly adsorbed. (36) This flat adsorption is due to many electrostatic bonds between the monomers of PSSshort and the film surface. (36)
PDADMA adsorbs in a coiled structure since PDADMA has a low linear charge density. To overcompensate or balance the charges at the film surface, a flat adsorption is sterically not possible. This leads to fewer electrostatic monomer/monomer bonds between the PDADMA chain and the substrate. If a PDADMA-terminated film is immersed in a solution containing a low salt concentration, one finds that PDADMA chains protrude into the solution and form pseudo-brushes, which scale with the ion concentration as is known for polyelectrolyte brushes. (36,46,47) The occurrence of pseudo-brushes shows that a substantial part of the PDADMA chain is not bound to the surface of the film. If a PDADMA-terminated film forms three-dimensional domains, then it is to be expected that not only the top but also the sides of the domains are covered by polyelectrolyte pseudo-brushes. Two opposing surfaces covered with polyelectrolyte brushes repel each other. (48) Therefore, a PDADMA-terminated film shrinks vertically and laterally on drying. We suggest that this is the reason why SEM images of PDADMA-terminated films are very different from those of PSSshort -terminated films.
To further analyze the SEM images, the mean distance between the domains is determined by a two-dimensional fast Fourier transform (FFT) just as that for the AFM images. As representative examples, the analysis of films consisting of 40 and 40.5 PDADMA/PSSshort bilayers are shown (cf. Figure 5). The average distance between the domains of a PSSshort terminated film (40 bl) was 238 nm. If an additional PDADMA layer is added (40.5 bl) the average distance between pillars increased to 326 nm. For the PDADMA-terminated film a second, weaker peak can be discerned at 537 nm, which is a factor of ∼√3 larger than the first peak. This factor is consistent with the pair correlation function of a fluid, provided each atom in the fluid has about 6 nearest neighbors. (49) The difference between PSSshort- and PDADMA-terminated films is substantial, and is attributed to polymer movement during drying, and exposure to vacuum.
To better describe the changes that the films undergo under exposure to vacuum, Figure 6 shows the film thickness obtained from SEM images (original data are shown in Figure 4 at a tilt angle of 90° and Figure S5) together with the results of ellipsometric measurements in air. As expected for the linear growth regime and confirmed with ellipsometry, the thickness of PSSshort-terminated films in a vacuum increases linearly with the number of deposited PDADMA/PSSshort bilayers. At 18% relative humidity (r.h.) the film is 32% thicker than in a vacuum. Shrinkage by ≈6% is expected when r.h. is changed from 18 to 0% r.h. Additional shrinking is not found when a non-structured film is brought from 0% r.h. to vacuum. (32)

Figure 6

Figure 6. Film thickness (in linear growth regime) depending on the number of deposited PDADMA/PSSshort bilayers. Values obtained by ellipsometry in air (18% r.h.) and from tilted SEM images are shown (original data in Figures S4–S6). The lines are least-square fits assuming a linear thickness increase (slopes are 25 and 17 nm/bl, respectively). The dotted line connects the SEM data obtained from PDADMA-terminated films and is a guide to the eye.

Removal of tightly bound water molecules (8 vol % as deduced from the change in the refractive index) leads to voids within the film. For PDADMA/PSSshort films with surface domains, lateral and vertical movement of polymer molecules lead to additional thinning when tightly bound water molecules are removed. The thickness of the PDADMA-terminated films as measured with SEM does not increase linearly but in steps (cf. Figure 6), a finding which is attributed to the inhomogeneous shrinking of the films when exposed to vacuum. Apparently, not only the trenches which separate the domains broaden and deepen, but also the pillars shrink. Note that even when the roughness is two times bigger for PSSshort -terminated films measured by AFM (∼24 nm), it is much less than the depth of trenches observed with SEM (∼ 150 nm). Note that PDADMA-terminated films swell more, (32) therefore it is not unexpected that these films change drastically.
To further quantify the changes in PSSshort-terminated during exposure to vacuum, the average distance between domains is compared for films in a vacuum, in air, and in pure water (cf. Figure 7). The data obtained in air and in pure water coincide and the inter-domain distance under these conditions is smaller than that in a vacuum. It is noteworthy that the average distance increases more in a vacuum than in air, 1.7 and 1.1 nm/bl, respectively. Dehydration in a vacuum increases the elastic modulus of the film which should lead according to theory to an increased distance. (24) Finally, the simultaneous extraction of tightly bound water molecules and the increase in elastic modulus allows for an increased polymer mobility leading not only to film shrinking but also to an increase in the mean distance between the domains which is in qualitative agreement with theoretical predictions.

Figure 7

Figure 7. Mean distance of domains of PSSshort-terminated films in the linear growth regime as a function of the number of PDADMA/PSSshort bilayers deposited. The average distance was determined from AFM images in air, in water, and from SEM experiments (original data in Figures 4, 5, and S1, respectively). A representative error bar for the AFM measurements in air is shown; it results from the imaging of various films at different locations.

Conclusions

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The morphology of PDADMA/PSS multilayers was studied by AFM in air and in pure water as a function of the number of PDADMA/PSS bilayers deposited. Films made from PSSshort showed a patterned surface, the pattern changed during film growth. As the number of deposited PDADMA/PSSshort bilayers increased, surface grooves, then stripes, and finally circular domains were observed. The morphology of the surface patterns correlated with the growth regime of the PDADMA/PSSshort film (exponential, parabolic, linear). The average distance between adjacent patterns increased with the number of deposited PDADMA/PSSshort bilayers. The pattern formation was attributed to vertical and lateral mechanical stress in the film (24) and the ability of PSSshort and/or PDADMA/PSSshort complexes to move laterally to release the stress. In the linear growth regime, a linear relationship was observed between the mean distance of circular domains and the film thickness.
PSSshort has a high diffusion constant, which resulted in an exponential growth regime at the beginning of the multilayer build-up. In this process, the adsorbing PSSshort molecules diffuse through the entire multilayer, and the thickness of the top PDADMA/PSSshort bilayer is proportional to the film thickness. Films made from PSSlong do not show an exponential growth regime which is attributed to the small diffusion constant of PSSlong. Furthermore, films made from PSSlong did not show any surface pattern and were always flat. Similarly, films built with immobile PDADMAshort do not show self-patterning. We conclude that self-patterning films with tunable inter-domain spacing require at least one mobile polyelectrolyte.
Scanning electron microscopy was used to image the films and to see whether and how the morphology changed when placed in a vacuum. The images were very different depending on whether the films were PSSshort- or PDADMA-terminated. PSSshort-terminated films were vertically shrunk and the average distance between domains was slightly increased, which was attributed to the adaptation to the increased elastic modulus caused by dehydration. PDADMA-terminated films showed tall pillars (the pillar height exceeded the roughness determined by AFM by a factor of 10) and the average distance between domains was increased by a factor 1.4.
In summary, the self-patterning of the films and the appearance of circular domains were enabled by polymer mobility. Their average spacing can be adjusted by the number of deposition steps. The self-patterning changes further when the films are exposed to vacuum. We assume that the changes of the film in vacuum depend on whether many (as in PSSshort-terminated films) or not so many (as in PDADMA-terminated films) electrostatic monomer–monomer bonds are present in the film. Furthermore, the conformation of the top polyelectrolyte layer may have an influence.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.1c01409.

  • AFM images of the self-patterned and flat films measured in water (Figure S1); power spectral-density profiles obtained by the FFT analysis of the images shown in Figure 2 (Figure S2); X-ray reflectivity curves and deduced electron density profiles of different films (Figure S3); the film thickness and surface roughness in dependence of the number of deposited polycation/polyanion bilayers (Figure S4); vertical profiles of various films imaged by electron microscopy, both PSSshort- and PDADMA-terminated (Figures S5 and S6, respectively); and the diameter of the cross-sectional area of the domains (Figure S7) (PDF)

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Author Information

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  • Corresponding Author
  • Authors
    • Amir Azinfar - Institute of Physics, University of Greifswald, Felix-Hausdorff-Straße 6, D-17489 Greifswald, Germany
    • Sven Neuber - Institute of Physics, University of Greifswald, Felix-Hausdorff-Straße 6, D-17489 Greifswald, Germany
    • Marie Vancova - Institute of Parasitology, Biology Centre of the Czech Academy of Sciences, Branisovska 31, 37005Ceske Budejovice, Czech RepublicFaculty of Science, University of South Bohemia, Branisovska 1760, 37005 Ceske Budejovice, Czech Republic
    • Jan Sterba - Faculty of Science, University of South Bohemia, Branisovska 1760, 37005 Ceske Budejovice, Czech Republic
    • Vitezslav Stranak - Faculty of Science, University of South Bohemia, Branisovska 1760, 37005 Ceske Budejovice, Czech Republic
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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We are grateful for the financial support from the German Research Foundation (DFG) Collaborative Research Centre (CRC) ELAINE 1270 (SFB 1270/1 - 299150580) and support from the Czech Science Foundation Agency through the project GACR 19-20168S. Furthermore, we acknowledge the Laboratory of Electron Microscopy (LEM) of the Biology Centre of the Czech Academy of Sciences (CAS) supported by the Ministry of Education, Youth and Sports of the Czech Republic (LM2018129, Czech-BioImaging). Thanks to Dr. Heiko Ahrens, Dr. Peter Nestler, Dr. Heba Mohamad, Dr. Oliver Otto, Prof. Dr. Mihaela Delcea and Prof. Dr. Thomas Ihle for fruitful discussions.

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  • Abstract

    Figure 1

    Figure 1. AFM images (5 μm × 5 μm) of three different PEI/PSS/(PDADMA/PSS)29 films in air (in water cf. Figure S1). The polyelectrolyte molecular weight is varied as indicated. (a): PSSshort (10.7 kDa) and PDADMAlong (322 kDa); (b): PSSlong (666 kDa) and PDADMAshort (23.6 kDa); (c): PSSlong (130 kDa) and PDADMAlong (322 kDa). All AFM measurements were performed under ambient conditions (r.h. = 40%). Note the different height scales of the images.

    Figure 2

    Figure 2. AFM images (5 μm × 5 μm) in air of the topography development of PEI/PSSshort /(PDADMA/PSSshort)N−1 films from N = 5 up to N = 40 bl. Shown is the surface structure and roughness (σAFM) as the films progress through the three different growth regimes: (a) exponential, (b) parabolic and (c) linear. The height scales are also constantly increasing with increasing number of deposited bilayers. Therefore, only nine representative examples for the height scale are given: 8 nm for 5 bl, 20 nm for 9 bl, 27.7 nm for 13 bl, 41.6 nm for 16 bl, 63.4 nm for 19 bl, 99.0 nm for 23 bl, 152 nm for 27 bl, 175 nm for 33 b and 167 nm for 40 bl (at r.h. = 40%).

    Figure 3

    Figure 3. Average distance of PDADMA/PSSshort surface patterns as calculated by the FFT power spectrum from AFM images in Figure 2. The average distance of domains is shown in dependence of the number of PDADMA/PSSshort bilayers deposited (left) and of film thickness (right). Additionally, the film/air roughness (σAFM) is shown (right). Film thickness was measured with X-ray reflectivity up to thicknesses of 125 nm, then ellipsometry was used. Different growth regimes of the LbL film are indicated.

    Figure 4

    Figure 4. Scanning electron microscopy images of PEMs made from PSSshort (10.7 kDa) at different tilt angles and magnifications. PSS-terminated (top) and PDADMA-terminated (bottom) films are shown. The number of PDADMA/PSSshort bilayers (bl) and the tilt angle (0, 45, 90°) are indicated in each panel.

    Figure 5

    Figure 5. Top: Scanning electron microscopy images of PSSshort (left) and PDADMA (right)-terminated multilayers consisting of 40 or 40.5 bilayers (top view). Different magnifications are shown. Insets: FFT power spectrum of the images. For the 40 bl film, the angle-averaged peak position is 26.38 μm–1 (white ring) and for the 40.5 bl film 11.69 μm–1 (blue ring) and 19.2 μm–1 (white ring). Bottom: Radially averaged power spectral density spectra obtained from the PSSshort-terminated (blue symbols) and the PDADMA-terminated film (black symbols). The lines are fits to Gaussian functions.

    Figure 6

    Figure 6. Film thickness (in linear growth regime) depending on the number of deposited PDADMA/PSSshort bilayers. Values obtained by ellipsometry in air (18% r.h.) and from tilted SEM images are shown (original data in Figures S4–S6). The lines are least-square fits assuming a linear thickness increase (slopes are 25 and 17 nm/bl, respectively). The dotted line connects the SEM data obtained from PDADMA-terminated films and is a guide to the eye.

    Figure 7

    Figure 7. Mean distance of domains of PSSshort-terminated films in the linear growth regime as a function of the number of PDADMA/PSSshort bilayers deposited. The average distance was determined from AFM images in air, in water, and from SEM experiments (original data in Figures 4, 5, and S1, respectively). A representative error bar for the AFM measurements in air is shown; it results from the imaging of various films at different locations.

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  • Supporting Information

    Supporting Information


    The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.1c01409.

    • AFM images of the self-patterned and flat films measured in water (Figure S1); power spectral-density profiles obtained by the FFT analysis of the images shown in Figure 2 (Figure S2); X-ray reflectivity curves and deduced electron density profiles of different films (Figure S3); the film thickness and surface roughness in dependence of the number of deposited polycation/polyanion bilayers (Figure S4); vertical profiles of various films imaged by electron microscopy, both PSSshort- and PDADMA-terminated (Figures S5 and S6, respectively); and the diameter of the cross-sectional area of the domains (Figure S7) (PDF)


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