Adsorption and desorption of polymer/surfactant mixtures at solid-liquid interfaces: substitution experiments

The adsorption of mixtures of aqueous solutions of cationic hydroxyethylcellulose polymer JR400 and anionic surfactant, sodium dodecyl sulfate, using atomic force microscopy (AFM) has been studied. Samples with various compositions from different regions of the ternary phase diagram presented in our previous work were imaged by atomic force microscopy on freshly cleaved mica, and hydrophobically modified mica and silica in soft-contact mode. A series of "washing" (subsequent injection of compositions with gradually decreasing polymer/surfactant ratio) and "scratching" (mechanical agitation of the surface material with an AFM tip) experiments were performed. It was revealed that the morphology of the adsorbed layer altered in a manner following the changes in morphology in the bulk solution. These changes were evidenced in cluster formation in the layer. The results suggest that the influence of the surface was limited to the formation of the adsorbed layer where the local concentrations of polymer and surfactant were higher than those in the bulk. All further modifications were driven by changes in the mixture composition in bulk. Force measurements upon retraction reveal the formation of network structures within the surface aggregates that will greatly slow structural reequilibration.     

Mechanical Properties of Poly(dimethylsiloxane)-block-poly(2-methyloxazoline) Polymersomes Probed by Atomic Force Microscopy

Poly(dimethylsiloxane)-block-poly(2-methyloxazoline) (PDMS-b-PMOXA) vesicles were characterized by a combination of dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), and atomic force microscopy imaging and force spectroscopy (AFM). From DLS data, a hydrodynamic radius of ～150 nm was determined, and cryo-TEM micrographs revealed a bilayer thickness of ～16 nm. In AFM experiments on a silicon wafer substrate, adsorption led to a stable spherical caplike conformation of the polymersomes, whereas on mica, adsorption resulted also in vesicle fusion and formation of bilayer patches or multilayer stacks. This indicates a delicate balance between the mechanical stability of PDMS-b-PMOXA polymersomes on one hand and the driving forces for spreading on the other. A Young's modulus of 17 ± 11 MPa and a bending modulus of 7 ± 5 × 10(-18) J were derived from AFM force spectroscopy measurements. Therefore, the elastic response of the PDMS-b-PMOXA polymersomes to external stimuli is much closer to that of lipid vesicles compared to other types of polymersomes, such as polystyrene-block-poly(acrylic acid) (PS-b-PAA).     

Deformation controlled assembly of binary microgel thin films

We describe the assembly of two-component, hydrogel microparticle (microgel) monolayer films onto solid substrates via passive Coulombic adsorption from solution. By using two different microgel types with nearly identical sizes but different degrees of softness, the influence of particle deformation on film composition was determined. Determination of the microgel properties using a variety of light scattering techniques allowed for predictions of the film composition as a function of solution composition using a random sequential adsorption (RSA) model. The films were then studied via atomic force microscopy (AFM), and surface coverage and population statistics were determined from the images and compared to the model predictions. Deviations from the predicted particle adsorption behavior can be directly traced to differences in particle softness, deformation, and particle footprint following adsorption, which biases the particle coverage to the more rigid (smaller footprint) particles. Furthermore, by using a mixture of degradable and nondegradable core/shell particles, the identity of the particles can be unambiguously determined by measuring AFM height changes following erosion of the core from the microgels. These results show that, regardless of the solution diffusion properties of soft particles, their competition for surface adsorption from a binary mixture is largely dictated by their interactions with the surface and their deformation at the surface.     

疏水缔合聚丙烯酰胺在盐水中的自组装行为

使用静态光散射、动态光散射以及原子力显微镜(AFM)研究了疏水缔合聚丙烯酰胺(HAPAM)在盐水溶液中的自组装行为.研究了聚合物分子在不同盐浓度中的表观重均分子量(Mw,a),均方根回转半径(),流体力学半径(),第二维里系数(A2)的变化,并根据/的比值得出聚合物分子的聚集形态.实验结果表明:随着盐浓度的增强,聚合物溶液的分子链由舒展变得卷曲.     

Direct AFM observation of an opening event of a DNA cuboid constructed via a prism structure

A cuboid structure was constructed using a DNA origami design based on a square prism structure. The structure was characterized by atomic force microscopy (AFM) and dynamic light scattering. The real-time opening event of the cuboid was directly observed by high-speed AFM.     

Analogy in adsorption behavior of homopolyelectrolyte mixtures as compared to analogous diblock polyampholytes

The adsorption behavior of aqueous mixtures of the homopolyelectrolytes poly(methacrylic acid) (PMAA) and poly[(dimethylamino)ethyl methacrylate] (PDMAEMA) was investigated in comparison with the adsorption of the ampholytic diblock copolymer PMAA‐b‐PDMAEMA on silicon substrates. Ellipsometry was used to determine the amount of adsorbed homopolyelectrolyte and diblock polyampholyte. Furthermore, the topography of the adsorbed polymers was investigated with atomic force microscopy (AFM) and compared with the structures observed in aqueous solutions by dynamic light scattering (DLS). For all types of investigated polyelectrolytic mixtures or the single polyampholyte, the adsorption was strongly influenced by the pH of the polymer solution. Although single homopolyelectrolytes showed only one maximum in adsorption according to their charge, the mixtures made from these homopolyelectrolytes showed two or three maxima. The third maximum near the isoelectric point of the mixture was assigned to a new species formed by aggregation of the two homopolyelectrolytes. Altogether, the adsorption behavior of the polyelectrolytic mixtures was in between the behavior of the pure homopolyelectrolytes and the analogous polyampholytes and therefore understandable from both of these polymer species. © 2002 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 338–345, 2002; DOI 10.1002/polb.10091     

Investigation of the role of the interplay between water and temperature on the growth of alkylsiloxane submonolayers on silicon

We have investigated the influence of the interplay of the temperature and the water concentration in the adsorption solution on the growth of self-assembled monolayers on silicon using octadecyltrichlorosilane as the precursor. Toluene has been used as the solvent. The morphology of the submonolayer films has been investigated by atomic force microscopy (AFM). The surface coverages have been determined both with ellipsometry and through quantitative evaluation of AFM images. The size distribution of species in the precursor solution has been studied with dynamic light scattering. The influence of water concentrations between 8 and 18 mmol/L has been investigated in the temperature range from 2 to 35 degrees C. Dynamic light scattering revealed a unimodal size distribution of ordered aggregates in solution with a hydrodynamic radius of 200 nm regardless of the temperature and water concentration. However, formation of these features was faster at higher water contents and lower temperatures. Moreover, a characteristic temperature, which was higher for higher water concentrations, was found, above which such aggregates could not be detected anymore. Below this temperature an increase of the aggregate concentration has been observed until a plateau had been reached within a temperature range of approximately 5 degrees C. AFM measurements and ellipsometry on the corresponding submonolayer films showed that this temperature range is also associated with a transition from fast growth via characteristic fractally shaped islands to comparatively slow homogeneous growth via adsorption of individual molecules. The results are discussed in terms of diffusion and adsorption limitations.     

Atomic force microscopy colloid-probe measurements with explicit measurement of particle-solid separation

We describe the use of evanescent wave scattering to measure the separation between the surface of a solid and a particle that is attached to an atomic force microscope (AFM) cantilever. Termed evanescent wave atomic force microscopy, our approach involves measuring the intensity of the light scattered from an evanescent field formed by the total internal reflection of a laser beam at a solid/fluid interface. In a conventional AFM "colloid probe" measurement, this separation must be inferred from an examination of the surface forces. Direct measurement of this separation with an evanescent wave atomic force microscope (EW-AFM) removes some ambiguity in the surface force measurement and, in addition, allows new types of measurements. For example, the force can be monitored at a constant separation. Our evanescent scattering apparatus is essentially identical to that used in total internal reflection microscopy (TIRM), except that we collect the light that scatters back into the incident medium, because the AFM partly obscures the forward scattered light (i.e., light scattered into the transmitted region). Compared to a conventional TIRM measurement, where the particle moves freely, attaching the particle to the cantilever in an EW-AFM gives much greater control of the particle position.     

Micellization of non-surface-active diblock copolymers in water. Special characteristics of poly(styrene)-block-poly(styrenesulfonate)

Strongly ionized amphiphilic diblock copolymers of poly(styrene)-b-poly(styrenesulfonate) with various hydrophilic and hydrophobic chain lengths were synthesized by living radical polymerization, and their properties and self-assembling behavior were systematically investigated by surface tension measurement, foam formation, hydrophobic dye solubilization, X-ray reflectivity, dynamic light scattering, small-angle neutron scattering, small-angle X-ray scattering, and atomic force microscope techniques. These copolymer solutions in pure water did not show a decrease of surface tension with increasing polymer concentration. The solutions also did not show foam formation, and no adsorption at the air/water interface was confirmed by reflectivity experiments. However, in 0.5 M NaCl aq solutions polymer adsorption and foam formation were observed. The critical micelle concentration (cmc) was observed by the dye solubilization experiment in both the solutions with and without added salt, and by dynamic light scattering we confirmed the existence of polymer micelles in solution, even though there was no adsorption of polymer molecules at the water surface in the solution without salt. By the small-angle scattering technique, we confirmed that the micelles have a well-defined core-shell structure and their sizes were 100-150 A depending on the hydrophobic and hydrophilic chain length ratio. The micelle size and shape were unaffected by addition of up to 0.5 M salt. The absence of polymer adsorption at the water surface with micelle formation in a bulk solution, which is now known as a universal characteristic for strongly ionized amphiphilic block copolymers, was attributed to the image charge effect at the air/water interface due to the many charges on the hydrophilic segment.     

Oxidation‐induced the Formation of Vesicles from Ferrocene Derivative Superamphiphiles

The stimuli‐responsive vesicles were prepared via electrochemical oxidation on the mixture solution of ferrocenylmethyl‐trimethylammonium iodide (FcMI) and sodium deoxycholate (NaDC). The vesicle structure and morphology are characterized respectively by transmission electron microscopy (TEM), dynamic light scattering (DLS) and atomic force microscopy (AFM). The vesicle shells can be clearly observed by TEM with the thickness ranging from several to several tens of nanometers and their outer diameters at the range of 50‐200 nm. The formation of vesicular structures is also supported via AFM measurements, and the ratio of the diameter and height of the nanospheres was estimated to be ca. 10, indicating the shell collapse. The obtained results are significant for the preparation of smart supramolecular aggregates.     

Study of polydimethylacrylamide- and polydiethylacrylamide-adsorbed coatings on fused silica capillaries and their application in genetic analysis

The adsorption of two polymers (polydimethylacrylamide and polydiethylacrylamide) on the inner surface of a fused silica of capillary (or wafer) was investigated by means of atomic force microscopy (AFM), multi-angle laser light scattering (MALLS) technique, and by measuring the electroosmotic flow (EOF) and contact angle. The AFM images showed that PDMA and PDEA tightly adsorbed on the fused silica surface and formed stable coatings. The contact angle data demonstrated that the polymer-adsorbed coatings have different hydrophobicities, which are related to the structures of the polymers. The adsorbing capability and stability of the adsorption coating, perhaps, were mainly dependent on the hydrogen bond force between oxygen atom on the carbonyl group of polymers and the hydroxyl group of the silica surface, and the hydrophobic nature of polymers. Our data also illustrated that the polymer-adsorbed coatings efficiently suppressed the EOF and the adsorption of DNA fragments on the capillary surface. These polymers were successfully used as sieving media in capillary electrophoresis of DNA fragments and detection of single point mutation of C677T from human methylenetetrahydrofolate reductase (MTHFR) gene.     

Surface morphology of nanostructured polymer-based activated carbons

Complementary techniques, including nitrogen adsorption, small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM), have been utilized to characterize the surface features of highly microporous carbon materials prepared from highly aromatic polymers. Nitrogen adsorption measurement interpreted by BET, DR, HK, and NLDFT methods reveals these nanostructured activated carbons exhibit a high surface area of up to 4000 m2/g, a micropore volume up to approximately 1.75 mL/g, and an average pore size of approximately 10-20 angstroms. A modified equation, based on Porod's law, the Debye-Bueche equation, and fractal dimension theories, has been proposed and successfully applied to analyze the SAXS spectra and to extract the porous texture of these unique activated carbons. AFM 3D imaging combined with the Fourier transform technique has been applied to statistically quantify pore sizes on the carbon surface.     

Grazing incidence small-angle X-ray scattering: an advanced scattering technique for the investigation of nanostructured polymer films

Hamburg workshop on the "application of synchrotron radiation in chemistry"With grazing incidence small-angle X-ray scattering (GISAXS) the limitations of conventional small-angle X-ray scattering with respect to extremely small sample volumes in the thin-film geometry are overcome. GISAXS turned out to be a powerful advanced scattering technique for the investigation of nanostructured polymer films. Similar to atomic force microscopy (AFM), a large interval of length between molecular and mesoscopic scales is detectable with a surface-sensitive scattering method. While with AFM only surface topographies are accessible, with GISAXS the buried structure is also probed. Because a larger surface area is probed, GISAXS also has a much larger statistical significance compared to AFM. Due to the high demand on collimation, GISAXS experiments are based on synchrotron radiation. Nanostructures parallel and perpendicular to the sample surface observable in thin poly(styrene- block-isoprene) diblock copolymer films are presented as an example of the possibilities of GISAXS.     

Interpolymer complexes based on the core/shell micelles. interaction of polystyrene-block-poly(methacrylic acid) micelles with linear poly(2-vinylpyridine) in 1,4-dioxane water mixtures and in aqueous media

The size and structural changes of nanoparticles formed after the addition of poly(2-vinylpyridine), PVP, to block copolymer micelles of polystyrene-block-poly(methacrylic acid), PS-PMA, were studied by light scattering and atomic force microscopy. Due to the strong hydrogen bonding between PVP and PMA segments, complex structures based on the core/shell micelles form in mixed selective solvents. As proven by a combination of light scattering and atomic force microscopy, individual PS-PMA micelles are "glued" together by PVP chains. The dialysis against solvents with a high content of water results in transient increase in polydispersity and turbidity of originally clear solutions. However, the precipitated polymer material dissolves in basic buffers and stable soluble nanoparticles reform in aqueous media. The behavior of their solutions was studied in a broad pH range by light scattering, atomic force microscopy and capillary zone electrophoresis.     

Adsorption and aggregation of cationic amphiphilic polyelectrolytes on silica

The adsorption of two cationic amphiphilic polyelectrolytes, which are copolymers of two charged monomers, triethyl(vinylbenzyl)ammonium chloride and dimethyldodecyl(vinylbenzyl)ammonium chloride (which is the amphiphilic one) with different contents of amphiphilic groups (40% (40DT) and 80% (80DT)), onto the hydrophilic silica-aqueous solution interface has been studied by in situ null ellipsometry and tapping mode atomic force microscopy (AFM). Adsorption isotherms for both polyelectrolytes were obtained at 25 degrees C and at different ionic strengths, and the adsorption kinetics was also investigated. At low ionic strength, thin adsorbed layers were observed for both polyelectrolytes. The adsorption increases with polymer concentration and reaches, in most cases, a plateau at a concentration below 50 ppm. For the 80DT polymer, at higher ionic strength, an association into aggregates occurs at concentrations at and above 50 ppm. The aggregates were observed directly by AFM at the surface, and by dynamic light scattering in the solution. The adsorption data for this case demonstrated multilayer formation, which correlates well with the increase in viscosity with the ionic strength observed for 80DT.     

Investigation of early stages of fibrin association

Interactions between fibrinogen molecules proteolytically cleaved with thrombin were investigated using atomic force microscopy (AFM) and dynamic light scattering (DLS). Gradually decreased fibrinogen concentrations were used to study the fibrin network, large separated fibrils, small fibrils in the initial association stages, and protofibrils. In addition, a new type of structure was found in AFM experiments at a low fibrinogen concentration (20 nM): the molecules in these single-stranded associates are arranged in a row, one after the other. The height, diameter, and distance between domains in these single-stranded associates were the same as those in the original fibrinogen molecules. DLS data assumed formation of extended associates in bulk solution at fibrinogen concentration as low as 20 nM.     

Characterization of DNA Layers Adsorbed on Glassy Carbon Electrodes

A DNA layer adsorbed at glassy carbon electrodes (GCE) was characterized by ellipsometry, atomic force microscopy (AFM) and scanning electron microscopy (SEM). The presence of the adsorbed DNA layer on polished glassy carbon electrodes was assessed indirectly by ellipsometric measurements. Ellipsometry was also useful to evaluate the influence of the oxide layer formed on glassy carbon electrodes, either spontaneously or after electrochemical pretreatments, on the DNA adsorption and further electrooxidation process. SEM and AFM images of the electrode surface covered by a thick layer of DNA reveal a nonuniform distribution, leaving channels and islands of the biological material.     

Synthesis and characterization of variable-architecture thermosensitive polymers for complexation with DNA

Copolymers of N-isopropylacrylamide with a fluorescent probe monomer were grafted to branched poly(ethyleneimine) to generate polycations that exhibited lower critical solution temperature (LCST) behavior. The structures of these polymers were confirmed by spectroscopy, and their phase transitions before and after complexation with DNA were followed using ultraviolet and fluorescence spectroscopy and light scattering. Interactions with DNA were investigated by ethidium bromide displacement assays, while temperature-induced changes in structure of both polymers and polymer-DNA complexes were evaluated by fluorescence spectroscopy, dynamic light scattering, laser Doppler anemometry, and atomic force microscopy (AFM) in water and buffer solutions. The results showed that changes in polymer architecture were mirrored by variations in the architectures of the complexes and that the overall effect of the temperature-mediated changes was dependent on the graft polymer architecture and content, as well as the solvent medium, concentrations, and stoichiometries of the complexes. Furthermore, AFM indicated subtle changes in polymer-DNA complexes at the microstructural level that could not be detected by light scattering techniques. Uniquely, variable-temperature aqueous-phase AFM was able to show that changes in the structures of these complexes were not uniform across a population of polymer-DNA condensates, with isolated complexes compacting above LCST even though the sample as a whole showed a tendency for aggregation of complexes above LCST over time. These results indicate that sample heterogeneities can be accentuated in responsive polymer--DNA complexes through LCST-mediated changes--a factor that is likely to be important in cellular uptake and nucleic acid transport.     

Aspects of the physical chemistry of polymers, biomaterials and mineralised tissues investigated with atomic force microscopy (AFM)

Beyond being merely a tool for measuring surface topography, atomic force microscopy (AFM) has made significant contributions to various scientific areas dealing with physical chemistry processes. This paper presents aspects of the physical chemistry at surfaces and interfaces of polymers, biomaterials and tissues investigated with AFM. Selected examples presented include surface induced self-assembly of polymer blends, copolymer interfacial reinforcement of immiscible homopolymers, protein adsorption on biomaterials and erosion of mineralised human tissues. In these areas, AFM is a useful and versatile tool to study structural or dynamic sample properties including thermodynamically driven surface evolution of polymer surfaces, lateral surface composition of interfaces, adsorption processes, and the metrology of demineralisation phenomena.     

Nanoscale conformational ordering in polyanilines investigated by SAXS and AFM

Understanding the adsorption mechanisms in nanostructured polymer films has become crucial for their use in technological applications, since film properties vary considerably with the experimental conditions utilized for film fabrication. In this paper, we employ small-angle X-ray scattering (SAXS) to investigate solutions of polyanilines and correlate the chain conformations with morphological features of the nanostructured films obtained with atomic force microscopy (AFM). It is shown that aggregates formed already in solution affect the film morphology; in particular, at early stages of adsorption film morphology appears entirely governed by the chain conformation in solution and adsorption of aggregates. We also use SAXS data for modeling poly(o-ethoxyaniline) (POEA) particle shape through an ab initio procedure based on simulated annealing using the dummy atom model (DAM), which is then compared to the morphological features of POEA films fabricated with distinct pHs and doping acids. Interestingly, when the derivative POEA is doped with p-toluene sulfonic acid (TSA), the resulting films exhibit a fibrillar morphology-seen with atomic force microscopy and transmission electron microscopy-that is consistent with the cylindrical shape inferred from the SAXS data. This is in contrast with the globular morphology observed for POEA films doped with other acids.