Polyelectrolyte blend multilayer films: surface morphology, wettability, and protein adsorption characteristics

We report the influence of polyelectrolyte (PE) multilayer films prepared from poly(styrene sulfonate)-poly(acrylic acid) (PSS-PAA) blends, deposited in alternation with poly(allylamine hydrochloride) (PAH), on film wettability and the adsorption behavior of the protein immunoglobulin G (IgG). Variations in the chemical composition of the PAH/(PSS-PAA) multilayer films, controlled by the PSS/PAA blend ratio in the dipping solutions, were used to systematically control film thickness, surface morphology, surface wettability, and IgG adsorption. Spectroscopic ellipsometry measurements indicate that increasing the PSS content in the blend solutions results in a systematic decrease in film thickness. Increasing the PSS content in the blend solutions also leads to a reduction in film surface roughness (as measured by atomic force microscopy), with a corresponding increase in surface hydrophobicity. Advancing contact angles (theta) range from 7 degrees for PAH/PAA films through to 53 degrees for PAH/PSS films. X-ray photoelectron spectroscopy measurements indicate that the increase in film hydrophobicity is due to an increase in PSS concentration at the film surface. In addition, the influence of added electrolyte in the PE solutions was investigated. Adsorption from PE solutions containing added salt favors PSS adsorption and results in more hydrophobic films. The amount of IgG adsorbed on the multilayer films systematically increased on films assembled from blends with increasing PSS content, suggesting strong interactions between PSS in the multilayer films and IgG. Hence, multilayer films prepared from blended PE solutions can be used to tune film thickness and composition, as well as wetting and protein adsorption characteristics.     

Effect of chain length and charge density on the construction of polyelectrolyte multilayers on colloidal particles

Two combinations of sodium poly(4-styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) of different chain length and charge density are employed to construct multilayer films. The polyelectrolytes are assembled layer-by-layer on colloidal particles in the absence of salt. We have investigated the formation and electrical characteristics of the films by using electric light scattering technique. The results show that the film thickness is independent of the chain length when fully charged PAH (at pH 4.6) is combined with fully charged PSS. When the films are prepared with less charged PAH (at pH 6.7) and fully charged PSS, lower thickness is found for the film with shorter polymer chains. In all cases, the thickness increment realized on addition of the polymer with lower molar concentration is partially lost on exposure to the solution with higher concentration of the oppositely charged partner. When the film growth is regular (at equal molar concentrations of the fully charged polyelectrolytes), the ratio of PSS to PAH charge, estimated from the electro-optical effect values, exceeds 1. The electro-optical effect is also higher for the films ending with PSS when fully charged PSS is combined with less charged PAH (at pH 6.7). This reveals the key role of the charge in the last-adsorbed layer for the electro-optical behavior of the whole film.     

Modulating the pattern quality of micropatterned multilayer films prepared by layer-by-layer self-assembly

Patterned multilayer films composed of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) were prepared using dip and spin self-assembly (SA) methods. A silicon substrate was patterned with a photoresist thin film using conventional photolithography, and PAH/PSS multilayers were then deposited onto the substrate surface using dip or spin SA. For spin SA, the photoresist on the substrate was retained, despite the high centrifugal forces involved in depositing the polyelectrolytes (PEs). The patterned multilayer films were formed by immersing the PE-coated substrates in acetone for 10 min. The effect of ionic strength on the pattern quality in dip and spin multilayer patterns (line-edge definition and surface roughness of the patterned region) was investigated by increasing the salt concentration in the PE solution (range 0-1 M). In dip multilayer patterns, the presence of salt increased the film surface roughness and pattern thickness without any deformation of pattern shape. The spin multilayer patterns formed without salt induced a height profile of about 130 nm at the pattern edge, whereas the patterns formed with high salt content (1 M) were extensively washed off the substrates. Well-defined pattern shapes of spin SA multilayers were obtained at an ionic strength of 0.4 M NaCl. Multilayer patterns prepared using spin SA and lift-off methods at the same ionic strength had a surface roughness of about 2 nm, and those prepared using the dip SA and lift-off method had a surface roughness of about 5 nm. The same process was used to prepare well-defined patterns of organic/metallic multilayer films consisting of PE and gold nanoparticles. The spin SA process yielded patterned multilayer films with various lengths and shapes.     

Assembly of poly(N-isopropylacrylamide)-co-acrylic acid microgel thin films on polyelectrolyte multilayers: Effects of polyelectrolyte layer thickness, surface charge, and microgel solution pH

Temperature- and pH-sensitive poly(N-isopropylacrylamide)?Cco-acrylic acid (pNIPAm-co-AAc) microgels were deposited on glass substrates coated with polyelectrolyte multilayers composed of the polycation poly(allylamine hydrochloride) (PAH) and the polyanion poly(sodium 4-styrenesulfonate) (PSS). The microgel density and structure of the resultant films were investigated as a function of: (1) the number of PAH/PSS layers (layer thickness); (2) the charge on the outer layer of the polyelectrolyte multilayer film; and (3) the pH of microgel deposition solution. The resultant films were studied by differential interference contrast optical microscopy, atomic force microscopy, and scanning electron microscopy. It was found that the coverage of the microgels on the surface was a complex function of the pH of the deposition solution, the charge on the outer layer of the polyelectrolyte thin film and the PAH/PSS layer thickness; although it appears that microgel charge plays the biggest role in determining the resultant surface coverage.     

Facile tailoring of film morphology and release properties using layer-by-layer assembly of thermoresponsive materials

Layer-by-layer self-assembly was used to prepare thermoresponsive thin films of poly(N-isopropylacrylamide) (PNIPAAm) and poly(acrylic acid) (PAA) based on hydrogen bonding. The temperature of PNIPAAm adsorption was shown to significantly affect both the mass proportion of PNIPAAm in the film and the film surface morphology. When the adsorption was conducted at temperatures close to the lower critical solubility temperature of PNIPAAm, the amount of PNIPAAm in the film increased significantly (from 51 to 59%), and the total film mass increased by 30-40%. The films prepared at 30 degrees C also exhibited a lower surface roughness (1-2 nm) compared with 5-8 nm when prepared at 10 or 21 degrees C. The resulting multilayer films ([PAA/PNIPAAm]10) were capable of being reversibly loaded and unloaded with dye (Rhodamine B) by exposure to solutions at elevated temperatures. The rate of loading and release was shown to depend on both the solution temperature and film preparation temperature, leading to tunable loading/release properties.     

Controlled loading and release of methylene blue from LbL polyurethane/poly(acrylic acid) film

The layer‐by‐layer (LbL) assembled multilayer films are widely used in the biomedical field for the controlled drug delivery. Here, multilayer films were assembled by LbL technique through alternating deposition of cationic polyurethane (PU) and poly(acrylic acid) (PAA) on glass slides. Methylene blue (MB) was used as a model drug to investigate the loading and release ability of the prepared multilayer film. The results showed that the loading rate and loading amount of MB were greatly influenced by pH value of the dye solution, and the release rate of MB was controlled both by ionic strength and pH value of immersing solution. The result also indicated that the film had a good reversibility of drug loading and release. It suggested that the PU/PAA multilayer film had potential applications in drug delivery and controlled release. Copyright © 2011 John Wiley & Sons, Ltd.     

Colloid surface engineering via deposition of multilayered thin films from polyelectrolyte blend solutions

Multilayer thin films were constructed on polystyrene colloidal particles by depositing alternating layers of poly(allylamine hydrochloride) (PAH) at pH 7.5 and varying composition blends of poly(acrylic acid) (PAA) and poly(styrenesulfonate) (PSS) at pH 3.5. Following the deposition of each layer, microelectrophoresis experiments showed alternating zeta-potentials, suggesting the formation of multilayered films on the particles. Scanning and transmission electron microscopy were used to examine the surface morphology of the colloidal particles, with homogeneous surface coatings apparent for films deposited from PAA/PSS blend solutions containing up to 90 wt % PAA. The colloidal stability of these particles is greater than those coated with individual PAH and PAA layers. In the case of the blend PAA/PSS = 25:75 wt %, up to 20 layers were assembled without compromising the colloidal stability of the dispersion. The results demonstrate that the deposition of layers from PE blend solutions containing a strong and weak PE can be used as a facile method for controlling the surface properties and hence the colloidal stability of core-shell particles, as well as the thickness and morphology of the coatings. Control of these parameters is important for subsequent processing and application of these particles in controlled delivery, photonics, catalytic, and separation applications.     

Synthesis, multilayer film assembly, and capsule formation of macromolecularly engineered acrylic acid and styrene sulfonate block copolymers

We report the use of copolymers synthesized with specific block ratios of weakly and strongly charged groups for the preparation of stable, pH-responsive multilayers. In this study, we utilized reversible addition-fragmentation chain transfer (RAFT) polymerization in the synthesis of novel pH-sensitive copolymers comprising block domains of acrylic acid (AA) and styrene sulfonate (SS) groups. The PAA x- b-SS y copolymers, containing 37%, 55%, and 73% of AA groups by mass (denoted as PAA 37- b-SS 63, PAA 55- b-SS 45, and PAA 73- b-SS 27, respectively), were utilized to perform stepwise multilayer assembly in alternation with poly(allylamine hydrochloride), PAH. The ratio of AA to SS groups, and the effect of the pH of both anionic and cationic adsorption solutions, on multilayer properties, were investigated using ellipsometry and atomic force microscopy. The presence of SS moieties in the PAA x- b-SS y copolymers, regardless of the precise composition, lead to films with a relatively consistent thickness. Exposure of these multilayers to acidic conditions postassembly revealed that these multilayers do not exhibit the characteristic large swelling that occurs with PAA/PAH films. The film stability was attributed to the presence of strongly charged SS groups. PAA x- b-SS y/PAH films were also formed on particle substrates under various adsorption conditions. Microelectrophoresis measurements revealed that the surface charge and isoelectric point of these core-shell particles are dependent on assembly pH and the proportion of AA groups in PAA x- b-SS y. These core-shell particles can be used as precursors to hollow capsules that incorporate weak polyelectrolyte functionality. The role of AA groups in determining the growth profile of these capsules was also examined. The multilayer films prepared may find applications in areas where pH-responsive films are required but large film swelling is unfavorable.     

Construction and deconstruction of multilayer films containing polycarboxybetaine: effect of pH and ionic strength

The influences of pH and NaCl concentration of dipping solutions and the pH and NaCl concentration of disintegration solutions on the disintegration behaviors of poly(4-vinylpyridiniomethanecarboxylate) (PVPMC)/poly(sodium 4-styrenesulfonate) (PSS) (PVPMC/PSS) multilayer films were investigated by ultraviolet-visible spectroscopy (UV-vis), Fourier transform infrared spectroscopy (FT-IR), quartz crystal microbalance (QCM) and atomic force microscopy (AFM). It was found that the disintegration rates and degrees of PVPMC/PSS multilayer films in neutral water could be well controlled by changing pH of dipping solutions and immersion time during the disintegration process. Furthermore, PVPMC/PSS multilayer films could be disintegrated completely and rapidly in pH 8 alkali solution or physiological condition (i.e., 0.15 M NaCl solution). The controllable disintegration of PVPMC/PSS multilayer films was then utilized to fabricate PEC/PSS free-standing multilayer films, in which PEC was a positively charged polyelectrolyte complex made from excessive poly(diallyldimethylammonium) (PDDA) and PSS. The experimental results indicated that the disintegration rates of PVPMC/PSS sacrificial sublayer strongly affected the integrity of the resultant PEC/PSS free-standing multilayer films. Only free-floating PEC/PSS was released from neutral water by disintegrating PVPMC/PSS multilayer sublayers. However, large size flat and tube-like PEC/PSS free-standing multilayer films with good mechanical properties were obtained facilely from pH 8 alkali solution and 0.15 M NaCl solution, respectively. The preparation of such free-standing films at physiological condition may be useful in the biological or medical application.     

Composite thin film by hydrogen-bonding assembly of polymer brush and poly(vinylpyrrolidone)

Based on hydrogen-bonding layer-by-layer (LBL) assembly in aqueous solution, poly(vinylpyrrolidone) (PVPON) and a spherical polymer brush with a poly(methylsilsesquioxane) (PSQ) core and poly(acrylic acid) (PAA) hair chains were used to fabricate composite multilayer thin films. Hydrogen bonding as the driving force was confirmed by FT-IR spectrometry. A simple method (Filmetric F20) was introduced to determine the thickness and refractive index of the films. The film thickness was found to be a linear function of the number of bilayers. The average increase in thickness per bilayer is 28.3 nm. The film morphology was characterized with scanning electron microscopy and atomic force microscopy. The images obtained from the two instruments show a great resemblance. The films were further calcined to get an inorganic film by removing the organic components, or treated with tetrabutylammonium fluoride (TBAF) to remove the PSQ core and get an organic film. The optical properties and morphological changes induced by these treatments were also studied.     

Amperometric uric acid sensors based on polyelectrolyte multilayer films

Uricase (UOx) and polyelectrolyte were used for preparation of a permselective multilayer film and enzyme multilayer films on a platinum (Pt) electrode, allowing the detection of uric acid amperometrically. The polyelectrolyte multilayer (PEM) film composed of poly(allylamine) (PAA) and poly(vinyl sulfate) (PVS) were prepared via layer-by-layer assembly on the electrode, functioning as H₂O₂-selective film. After deposition of the permselective film (PAA/PVS)₂PAA, UOx and PAA were deposited via layer-by-layer sequential deposition up to 10 UOx layers to prepare amperometric sensors for uric acid. Current response to uric acid was recorded at +0.6 V vs. Ag/AgCl to detect H₂O₂ produced from the enzyme reaction. The response current increased with increasing the number of UOx layers. Even in the presence of ascorbic acid, uric acid can be detected over the concentration range 10⁻⁶-10⁻³ M. The response current and deposited amount of UOx were affected by deposition bath pH and the addition of salt. The deposition of PAA/UOx film prepared in 2 mg ml⁻¹ solution (pH 11) of PAA with NaCl (8 mg ml⁻¹) and 0.1 mg ml⁻¹ solution (pH 8.5) of UOx with borate (100 mM) resulted in an electrode which shows the largest response to uric acid. The response of the sensor to uric acid was decreased by 40% from the original activity after 30 days.     

Silver nanocomposite layer-by-layer films based on assembled polyelectrolyte/dendrimer

Silver nanocomposite multilayer films were prepared through the in situ method. Multilayer thin films, prepared through the sequential electrostatic deposition of a positively charged third-generation poly(amidoamine) dendrimer (PAMAM) and negatively charged poly(styrenesulfonate) (PSS) and poly(acrylic acid) (PAA), were utilized as nanoreactors for the formation of silver nanoparticles. The silver ions were preorganized in layer-by-layer (LBL) films composed of PAMAM dendrimers and subsequently reduced with hydrogen to prepare the silver nanoparticles. The UV-vis spectrum and profilometer were used to characterize the regular growth of bilayers. UV-vis absorption from plasmon resonance at 435 nm and TEM images indicated the formation of the silver nanoparticles in the multilayer films. The silver nanocomposite LBL films were also constructed on the indium tin oxide-glass and investigated using cyclic voltammetry. The silver nanoparticles in the multilayer films have a stronger negative redox potential. The silver nanocomposite LBL films may have a potential application in the catalysis of reduction of 4-nitrophenol with sodium borohydride.     

pH-Induced antireflection coatings derived from hydrogen-bonding-directed multilayer films

Hydrogen-bonding-directed layer-by-layer assembled films, based on polystyrene-block-poly(acrylic acid) (PS-b-PAA) block copolymer micelles and poly(4-vinylpyridine) (P4VP), were successfully fabricated in methanol. Varying the PAA content in the PS-b-PAA micelles afforded control over the film growth properties, especially the multilayer film thickness. Interestingly, antireflection films with refractive indices that could be tuned between 1.58 and 1.28 were obtained by treatment with an aqueous HCl solution (pH 2.27), and the transmittance obtained was as high as 98.4%. In acid solution, the pyridine group was protonated, destroying the hydrogen bonding between P4VP and PAA. A concomitant pH-induced polymer reorganization in the multilayers resulted in a porous honeycomb-like texture on the substrate.     

Layered double hydroxides for ultrathin hybrid film preparation using layer-by-layer and spin coating methods

In our work, the synthesis and the structural properties of various layered double hydroxides (LDHs) were investigated. LDHs were prepared from metal salts with sodium hydroxide in aqueous medium at various molar ratios. Nitrate salts of, Mg²⁺, Ca²⁺, Co²⁺, Al³⁺ and acetate salt of Zn²⁺ were applied as precursors. The concentrations of LDH suspensions were in the range of 0.1–1 g/100 ml. The particle size and zeta-potential of LDH particles were measured by dynamic light scattering (DLS). The structure properties of LDHs were analyzed by several methods, namely XRD, UV-vis spectroscopy and AFM. We used the layer-by-layer dipping immersion technique and the spin coating method for the preparation of multilayer films from LDHs on glass surfaces. The cleaned glass surface has a negative surface charge, which is compensated by the positively charged LDH particles. Different negatively charged binders poly(sodium 4-styrenesulfonate), (PSS); poly(acryl-acid), (PAA) were utilized for the preparation of layer-by-layer films. Layer thickness was calculated from absorbance measurements by spectrophotometry and mass determination of the glass surface. Typical film thicknesses were in the range of 19–70 nm/layer and 6–40 μg/cm²/layer depending on the type of LDH and preparation technique. After n = 20 immersion cycles, maximal layer thickness was 1405 nm for 20 layers of 2:1 ZnAl–LDH/PSS film. The film preparation methods tested were compared and it was established that the films obtained by the layer-by-layer method are more stable but thicker than those prepared using the spin coating method.     

Dewetting Behavior of Hydrogen Bonded Polymer Complex Film under Hydrothermal Condition

Hydrogen-bonded polymer complex films with the thickness ranging from 50 nm to 2400 nm were prepared by layer-by-layer (LbL) assembly of poly(2-ethyl-2-oxazoline) (PEOX) and poly(acrylic acid) (PAA). The dewetting behavior of PEOX/PAA films under hydrothermal condition was investigated. It was found that the dewetting occurred at solid-liquid interface, and the typical morphologies such as holes, irregular cellular structure, and droplets were observed. Atomic force microscopy (AFM) revealed the initial rupture of the film. Microscopic Raman and infrared (IR) imaging demonstrated that the PEOX and PAA chains remained association during the dewetting process.     

PH effects in the complex formation and blending of poly(acrylic acid) with poly(ethylene oxide)

The effect of pH on the complex formation between poly(acrylic acid) (PAA) and poly(ethylene oxide) (PEO) has been studied in aqueous solutions by turbidimetric and fluorescent methods. It was shown that the formation of insoluble interpolymer complexes is observed below a certain critical pH of complexation (pH(crit1)). The formation of hydrophilic interpolymer associates is possible above pH(crit1) and below a certain pH(crit2). The effects of polymer concentrations in solution and PEO molecular weight as well as inorganic salt addition on these critical pH values were studied. The polymeric films based on blends of PAA and PEO were prepared by casting from aqueous solutions with different pHs. These films were characterized by light transmittance measurements and differential scanning calorimetry. The existence of the pH value above which the polymers form an immiscible blend was demonstrated. The transitions between the interpolymer complex, miscible blend, and immiscible blend caused by pH changes are discussed. The recommendations for preparation of homogeneous miscible films based on compositions of poly(carboxylic acids) and various nonionic water-soluble polymers are presented.     

Fabrication of free-standing multilayer films by using pH-responsive microgels as sacrificial layers

A facile way to prepare free-standing polyelectrolyte multilayer films of poly(sodium 4-styrenesulfonate)(PSS)/poly(diallyldimethylammonium)(PDDA) was developed by applying a new pH-dependent sacrificial system based on cross-linked poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) microgels. The tertiary amine groups of PDMAEMA microgels can be protonated in acidic environment, and the protonated microgels were deposited by layer-by-layer (LbL) technique with PSS. PSS/PDDA multilayer films were constructed on the top of the PSS/microgels sacrificial layers. The LbL assembly process was investigated by UV–vis spectroscopy. Further study shows that the free-standing PSS/PDDA multilayer films can be obtained within 3 min by treating the as-prepared films in alkali aqueous solution with a pH of 12.0. The pH-triggered exfoliation of PSS/PDDA multilayer films provides a simple and facile way to prepare LbL assembled free-standing multilayer films.     

Patterning Layered Polymeric Multilayer Films by Room‐Temperature Nanoimprint Lithography

Summary: Polyelectrolyte multilayer films of poly(acrylic acid) (PAA)/poly(allylamine hydrochloride) (PAH) and PAH/poly(sodium 4‐styrenesulfonate) (PSS) based on electrostatic interactions as a driving force are patterned by room‐temperature nanoimprint lithography (RT‐NIL). Under an imprinting pressure of 40 bar for 8 min, well‐defined pattern structures with a line width of ≈330 nm and a separation of ≈413 nm are achieved. Meanwhile, hydrogen‐bonding‐directed multilayer films of poly(vinyl pyrrolidone) (PVPON)/poly(methyl acrylic acid) (PMAA) and poly(4‐vinylpyridine)/PAA can also be patterned in a similar way by RT‐NIL. The successful imprinting of these films originates from the high compressibility and fluidity of the layered polymeric films under high pressure.

SEM image of an imprinted (PAH/PAA)*20 film on silicon wafer.     

Multilayered assemblies composed of brilliant yellow and poly(allylamine) for an optical pH sensor.

Multilayered thin films containing poly(allylamine) (PAA) and brilliant yellow (BY) were prepared on a quartz slide by a layer-by-layer (LBL) deposition technique. The UV-visible spectra of the PAA/BY films were sufficiently changed depending upon the pH value of the solution in which the film was immersed. The response of the PAA/BY films was very fast (within a second) upon pH change from 9.0 to 5.0, while the response time was ca. 100 s upon pH change from 5.0 to 9.0.     

Layer-by-layer electrostatic self-assembly of single-wall carbon nanotube polyelectrolytes

We have used anionic and cationic single-wall carbon nanotube polyelectrolytes (SWNT-PEs), prepared by the noncovalent adsorption of ionic naphthalene or pyrene derivatives on nanotube sidewalls, for the layer-by-layer self-assembly to prepare multilayers from carbon nanotubes with polycations, such as poly(diallyldimethylammonium) or poly(allylamine hydrochloride) (PDADMA or PAH, respectively), and polyanions (poly(styrenesulfonate), PSS). This is a general and powerful technique for the fabrication of thin carbon nanotube films of arbitrary composition and architecture and allows also an easy preparation of all-SWNT (SWNT/SWNT) multilayers. The multilayers were characterized with vis-near-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), surface plasmon resonance (SPR) measurements, atomic force microscopy (AFM), and imaging ellipsometry. The charge compensation in multilayers is mainly intrinsic, which shows the electrostatic nature of the self-assembly process. The multilayer growth is linear after the initial layers, and in SWNT/polyelectrolyte films it can be greatly accelerated by increasing the ionic strength in the SWNT solution. However, SWNT/SWNT multilayers are much more inert to the effect of added electrolyte. In SWNT/SWNT multilayers, the adsorption results in the deposition of 1-3 theoretical nanotube monolayers per adsorbed layer, whereas the nominal SWNT layer thickness is 2-3 times higher in SWNT/polyelectrolyte films prepared with added electrolyte. AFM images show that the multilayers contain a random network of nanotube bundles lying on the surface. Flexible polyelectrolytes (e.g., PDADMA, PSS) probably surround the nanotubes and bind them together. On macroscopic scale, the surface roughness of the multilayers depends on the components and increases with the film thickness.