Atomic Force Microscopy Phase Images Research Articles

Effect of latex on surface structure and wetting of pigment coatings

The amount and type of latex binder has a significant effect on both the structure and chemical properties of pigment coatings, which affects critical surface properties, e.g., gloss, ink setting rate via liquid absorption, and spreading properties. Increased knowledge is required for improved optimization in practical applications of pigment coating layers. To this end, the effect of binder amount for two different styrene–butadiene latices in kaolin pigment coatings on a base paper was studied, by measuring the surface topography, surface chemistry, and liquid absorption of these coatings. The topography was thoroughly analyzed from nanometer to millimeter length scales by atomic force microscopy (AFM), confocal optical microscopy (COM), and X-ray tomography. The area fractions of latex and pigments on the surface of the coating were determined from AFM phase contrast images, and the contact angles for pure latex and kaolin fractions were determined by using the Cassie equation. An explicit value for the effective surface area of the coatings was determined from the topographic data, which was used for determining the Wenzel correction for the contact angles. Work of adhesion and surface energy components were determined for the samples using three different methods (Owens–Wendt, Wu, vOCG). The latex was found to be evenly distributed over the surfaces, based on AFM phase imaging and ToF–SIMS mapping. The latex binder amount significantly affected the wetting behavior and interfacial properties of the coating layer. The results of this fundamental study can be used for optimization of functional properties of pigment coating systems by regulation of the amount and properties of their latex binder.

Improving the lifetime of white polymeric organic light-emitting diodes

We report on efficient polymeric white organic light-emitting diodes with unprecedented stability. The investigated devices are based on an electroluminescent copolymer of electron and hole-transporting units and red-, green-, and blue-emitting chromophores. We find that the glass transition of the polymer (Tg=182.5 °C) is the process determining the relation between thermal annealing during fabrication and device lifetime. For devices annealed below Tg, the device lifetime significantly increases with increasing annealing temperature. For annealing temperatures above Tg, however, the current density in the devices rapidly increases while their lifetime slightly decreases. Insight into the underlying processes is provided by atomic force microscopy phase imaging and by UV/visible and fluorescence spectroscopy. We also investigated the influence of the operating temperature of the device: besides the commonly known fact that elevated operating temperatures reduce the lifetime, we discovered that the acceleration coefficient, which determines the scaling of the device lifetime with applied current density, was reduced. At the glass transition, the device lifetime no longer depended on the current density. The device lifetime was improved even further by introducing an additional cross-linkable hole-transport layer. Optimized devices achieve a half-luminance lifetime of 1860 h when operated at room temperature and at an initial luminance of 500 cd m−2. As a result of the relatively balanced stability of the three chromophores, the emission spectrum remains virtually unchanged over the entire device lifetime. Finally, to reduce the time required for the lifetime measurements, we propose to analyze the voltage increase over the first 10–50 h of the lifetime test and find that this allows precisely estimating the lifetime of our devices.

Visualization of enzymatic hydrolysis of cellulose using AFM phase imaging

Complete cellulase, an endoglucanase (EGV) with cellulose-binding domain (CBD) and a mutant endoglucanase without CBD (EGI) were utilized for the hydrolysis of a fully bleached reed Kraft pulp sample. The changes of microfibrils on the fiber surface were examined with tapping mode atomic force microscopy (TM–AFM) phase imaging. The results indicated that complete cellulase could either peel the fibrillar bundles along the microfibrils (peeling) or cut microfibrils into short length across the length direction (cutting) during the process. After 24 h treatment, most orientated microfibrils on the cellulose fiber surface were degraded into fragments by the complete cellulase. Incubation with endoglucanase (EGV or EGI) also caused peeling action. But no significant size reduction of microfibrils length was observed, which was probably due to the absence of cellobiohydrolase. The AFM phase imaging clearly revealed that individual EGV particles were adsorbed onto the surface of a cellulose fiber and may be bound to several microfibrils.

Epoxy/acrylonitrile-butadiene-styrene copolymer/clay ternary nanocomposite as impact toughened epoxy

Epoxy resins have low impact strength and poor resistance to crack propagation, which limit their many end use applications. The main objective of this work is to incorporate both acrylonitrile-butadiene-styrene copolymer (ABS) and organically modified clay (Cloisite 30B) into epoxy matrix with the aim of obtaining improved material with the impact strength higher than neat epoxy, epoxy/clay and epoxy/ABS hybrids without compromising the other desired mechanical properties such as tensile strength and modulus. Impact and tensile properties of binary and ternary systems were investigated. Tensile strength, elongation at break and impact strength were increased significantly with incorporation of only 4 phr ABS to epoxy matrix. For epoxy/clay nanocomposite with 2.5% clay content, tensile modulus and strength, and impact strength were improved compared to neat epoxy. With incorporation of 2.5% clay and 4 phr ABS into epoxy matrix, 133% increase was observed for impact strength. Ternary nanocomposite had impact and tensile strengths greater than values of the binary systems. Morphological properties of epoxy/ABS, epoxy/clay and epoxy/ABS/clay ternary nanocomposite were studied using atomic force microscopy (AFM) phase imaging, scanning electron microscopy (SEM) and wide angle X-ray diffraction (WAXD). New morphologies were achieved for epoxy/ABS and epoxy/ABS/clay hybrid materials. Exfoliated clay structure was obtained for epoxy/clay and epoxy/ABS/clay nanocomposite.

Electropolymerized Tricopolymer Based on N-Pyrrole Derivatives as a Primer Coating for Improving the Performance of a Drug-Eluting Stent

The coating of medical implants by polymeric films aims at increasing their biocompatibility as well as providing a durable matrix for the controlled release of a drug. In many cases, the coating is divided into a primer layer, which bridges between the medical implant and the drug-eluting matrix. The primer coating must be very carefully designed in order to provide optimal interactions with the surface of the medical implant and the outer layer. Here we present a simple and versatile approach for designing the primer layer based on electropolymerization of a carefully chosen blend of three different pyrrole derivatives: N-methylpyrrole (N-me), N-(2-carboxyethyl)pyrrole (PPA), and the butyl ester of N-(2-carboxyethyl)pyrrole (BuOPy). The composition and physical properties of the primer layer were studied in detail by atomic force microscopy (AFM) and a nano scratch tester. The latter provides the in-depth analysis of the adhesion and viscoelasticity of the coating. AFM phase imaging reveals a uniform distribution of the three monomers forming rough morphology. This primer layer significantly improved the morphology, stability, and paclitaxel release profile of a paclitaxel-eluting matrix based on methyl and lauryl methacrylates.

AFM phase imaging of soft-hydrated samples: A versatile tool to complete the chemical-physical study of liposomes

Despite of the several approaches applied to the physicochemical characterization of liposomes, few techniques are really useful to obtain information about the surface properties of these colloidal drug-delivery systems. In this paper, we demonstrate a possible new application of tapping mode atomic force microscopy (AFM) to discriminate between conventional and pegylated liposomes. We showed that the differences on liposomal surface properties revealed by the phase images AFM approach well correlate with the data obtained using classical methods, such as light scattering, hydrodynamic, and nuclear magnetic resonance analysis.

Detecting sp 2 phase on diamond surfaces by atomic force microscopy phase imaging and its effects on surface conductivity

We show correlation of microscopic surface quality and morphology of nanocrystalline diamond films as a function of deposition temperature and post-growth acid treatment detected by atomic force microscopy in phase detection regime, X-ray photoelectron spectroscopy, X-ray induced Auger electron spectroscopy, Scanning Electron Microscopy, Raman spectroscopy, and the electrical conductivity of H-terminated diamond surfaces. The correlation reflects the decrease in sp 2 amount and enhanced surface conductivity of the diamond surface after the chemical treatment. These results indicate that the AFM phase can detect clearly and microscopically carbon sp 2 phase on facets and grain boundaries of nanocrystalline diamond films.

Honeycomb-patterned films of polystyrene/poly(ethylene glycol): preparation, surface aggregation and protein adsorption

Highly ordered honeycomb-patterned polystyrene (PS)/poly(ethylene glycol) (PEG) films were prepared by a water-assisted method using an improved setup, which facilitated the formation of films with higher regularity, better reproducibility, and larger area of honeycomb structures. Surface aggregation of hydrophilic PEG and adsorption of bovine serum albumin (BSA) on the honeycomb-patterned films were investigated. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) were used to observe the surface morphologies of the films before and after being rinsed with water. As confirmed by the FESEM images and the AFM phase images, PEG was enriched in the pores and could be gradually removed by water. The adsorption of fluorescence-labeled BSA on the films was studied in visual form using laser scanning confocal microscopy. Results clearly demonstrated that the protein-resistant PEG was selectively enriched in the pores. This water-assisted method may be a latent tool to prepare honeycomb-patterned biofunctional surfaces.

Effect of Molecular Structure of Cationic Surfactants on Biophysical Interactions of Surfactant-Modified Nanoparticles with a Model Membrane and Cellular Uptake

The aim of this study was to test the hypothesis that the molecular structure of cationic surfactants at the nanoparticle (NP) interface influences the biophysical interactions of NPs with a model membrane and cellular uptake of NPs. Polystyrene NPs (surfactant-free, 130 nm) were modified with cationic surfactants. These surfactants were of either dichained (didodecyldimethylammonium bromide [DMAB]) or single-chained (cetyltrimethylammonium bromide [CTAB] and dodecyltrimethylammonium bromide [DTAB]) forms, with the latter two having different hydrophobic chain lengths. Biophysical interactions of these surfactant-modified NPs with an endothelial cell model membrane (EMM) were studied using a Langmuir film balance. Changes in surface pressure (SP) of EMM as a function of time following interaction with NPs and in the compression isotherm (pi-A) of the lipid mixture of EMM in the presence of NPs were analyzed. Langmuir-Schaeffer (LS) films, which are EMMs that have been transferred onto a suitable substrate, were imaged by atomic force microscopy (AFM), and the images were analyzed to determine the mechanisms of the NP-EMM interaction. DMAB-modified NPs showed a greater increase in SP and a shift toward higher mean molecular area (mmA) than CTAB- and DTAB-modified NPs, indicating stronger interactions of DMAB-modified NPs with the EMM. However, analysis of the AFM phase and height images of the LS films revealed that both DMAB- and CTAB-modified NPs interacted with the EMM but via different mechanisms: DMAB-modified NPs penetrated the EMM, thus explaining the increase in SP, whereas CTAB-modified NPs anchored onto the EMM's condensed lipid domains and hence did not cause any significant change in SP. Human umbilical vein endothelial cells showed greater uptake of DMAB- and CTAB-modified NPs than of DTAB-modified or unmodified NPs. We conclude that (i) the dichained and single-chained cationic surfactants on NPs have different mechanisms of interaction with the model membrane and that (ii) NPs that demonstrate greater biophysical interactions with the membrane also show greater cellular uptake. Biophysical interactions of NPs with a model membrane thus could be effectively used for developing nanocarriers with optimized surface properties for drug delivery and imaging applications.

Microphase separation structure influences protein interactions with poly(urethane urea) surfaces

Microphase separation is an important characteristic of polyurethane copolymer biomaterials. An improved understanding of the effects of microphase structure on protein interactions with the polymeric biomaterial surface is essential for the development and application of new biomaterials intended for implantation into the body. In this study, an array of atomic force microscopy (AFM) techniques were used to visualize the phase separation structure in a hydrated poly(urethane urea) (PUU) material and to correlate that structure with molecular interactions at the molecular level. Sequential in situ AFM phase images showed that the hard domains present a dynamic environment and undergo rearrangement and enrichment at the surface when hydrated. Adhesion forces measured using a protein-modified AFM probe suggests that the PUU surface became less adhesive to protein with hydration time, consistent with other physical characterizations. Force measurements were used to quantify and correlate mechanical properties and local adhesion forces for bovine serum albumin, and results showed that low adhesion forces were primarily associated with polar hard domain regions. A nanogold-labeled protein conjugate was used to visualize individual protein adsorption to the separate microstructures on the PUU surface, with preferential protein adsorption seen on the more apolar hydrophobic soft segment regions. Together, the results suggest that the microphase separation structure mediates local surface microenvironments that influence biological interactions with the surface.

Fractal research on AFM images of polycrystalline aluminum surface with adsorption film of SDS inhibitor

We propose a new method of fractal analysis of the phase images obtained by the atomic-force microscopy (AFM) to study the properties of molecular adsorption films of corrosion inhibitors on the coarse polycrystalline surface, which can hardly be distinguished by using AFM height images due to the roughness of the surface. The system of polycrystalline aluminum and SDS (Sodium Dodecyl Sulfate) inhibitor is chosen as the research object. The electrochemical measurements of aluminum electrodes precoated with SDS films are carried out in HCl solutions, and the results show that the inhibition efficiency increases with the concentration of SDS in the prefilming solution and then remains nearly constant as the CMC (critical micelle concentration) of SDS is exceeded. The fractal analysis of the AFM images of prefilmed aluminum surfaces obtained in SDS solutions is performed and demonstrates that, as the SDS concentration increases, no obvious changes are observed for the fractal dimensions of height images, whereas the phase images present regular changes in the values of fractal dimensions, thus displaying the consistent alteration rule as revealed by electrochemical tests. The results prove the feasibility of the fractal analysis of AFM phase images in characterizing the adsorption behavior of the corrosion inhibitor on polycrystalline coarse surfaces.

Data fusion of XPS and AFM images for chemical phase identification in polymer blends

AbstractIn this study, multitechnique image fusion between imaging X‐ray photoelectron spectroscopy (XPS) and phase imaging atomic force microscopy (AFM) is discussed. It is beneficial for the purpose of facilitating correlation of different physical properties such as phase information from AFM images with chemical information from XPS images and enhancing spatial resolution by utilizing a nanometer spatial resolution of AFM images to enhance data from XPS images with spatial resolutions of microns. Small‐area‐imaging XPS is used to identify the phases observed through AFM for the data obtained from the same area on a sample. A patterned surface of polymer on indium tin oxide (ITO) is used to outline the procedure. The procedure is applied to poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) PVC blends of various compositions. The phases are clearly identifiable in AFM images based on imaging XPS results. Resolution merge was also applied to multispectral XPS and AFM images acquired from polystyrene/polybutadiene blend. This type of image fusion allows for a direct visualization of overlap between chemical and topographical heterogeneities, which is critical in understanding material properties. Copyright © 2008 John Wiley & Sons, Ltd.

Hydration behavior of perfluorinated and hydrocarbon-type proton exchange membranes: Relationship between morphology and proton conduction

The relationships between morphology and proton conduction for Nafion membranes and hydrocarbon-type proton exchange membranes, namely, sulfonated poly(arylene ether ether ketone) (S-PEEK) and sulfonated poly(arylene ether sulfone) (S-PES), were investigated by small-angle X-ray scattering (SAXS) and atomic force microscopy (AFM). The direct simultaneous observation of surface morphology and active regions of proton conduction on membranes by combined high-resolution AFM phase imaging and an electrochemical technique at controlled humidity provided significant morphological information, particularly for the hydrocarbon-type membranes that exhibit few or no features on SAXS profiles. For the Nafion membranes, the active proton paths became denser and congregated with each other at over 60% RH, resulting in the formation of well-connected networks. For the hydrocarbon-type membranes, however, only the relatively small and dispersed proton paths were observed, which showed no significant change even as water content increased. We have demonstrated that the differences in microscopic morphology between the Nafion and hydrocarbon-type membranes are associated with the differences between their macroscopic proton conductivities.

Origin of phase shift in atomic force microscopic investigation of the surface morphology of NR/NBR blend film

Atomic force microscopy (AFM) was used to study the morphology and surface properties of NR/NBR blend. Blends at 1/3, 1/1 and 3/1 weight ratios were prepared in benzene and formed film by casting. AFM phase images of these blends in tapping mode displayed islands in the sea morphology or matrix-dispersed structures. For blend 1/3, NR formed dispersed phase while in blends 1/1 and 3/1 phase inversion was observed. NR showed higher phase shift angle in AFM phase imaging for all blends. This circumstance was governed by adhesion energy hysteresis between the device tip and the rubber surface rather than surface stiffness of the materials, as proved by force distance measurements in the AFM contact mode.

Surface features on Sahara soil dust particles made visible by atomic force microscope (AFM) phase images

Abstract. We show that atomic force microscopy (AFM) phase images can reveal surface features of soil dust particles, which are not evident using other microscopic methods. The non-contact AFM method is able to resolve topographical structures in the nanometer range as well as to uncover repulsive atomic forces and attractive van der Waals' forces, and thus gives insight to surface properties. Though the method does not allow quantitative assignment in terms of chemical compound description, it clearly shows deposits of distinguishable material on the surface. We apply this technique to dust aerosol particles from the Sahara collected over the Atlantic Ocean and describe micro-features on the surfaces of such particles.

Application of atomic force microscopy to the study of natural and model soil particles

The structure and surface chemistry of soil particles has extensive impact on many bulk scale properties and processes of soil systems and consequently the environments that they support. There are a number of physiochemical mechanisms that operate at the nanoscale which affect the soil's capability to maintain native vegetation and crops; this includes soil hydrophobicity and the soil's capacity to hold water and nutrients. The present study used atomic force microscopy in a novel approach to provide unique insight into the nanoscale properties of natural soil particles that control the physiochemical interaction of material within the soil column. There have been few atomic force microscopy studies of soil, perhaps a reflection of the heterogeneous nature of the system. The present study adopted an imaging and force measurement research strategy that accounted for the heterogeneity and used model systems to aid interpretation. The surface roughness of natural soil particles increased with depth in the soil column a consequence of the attachment of organic material within the crevices of the soil particles. The roughness root mean square calculated from ten 25 microm(2) images for five different soil particles from a Netherlands soil was 53.0 nm, 68.0 nm, 92.2 nm and 106.4 nm for the respective soil depths of 0-10 cm, 10-20 cm, 20-30 cm and 30-40 cm. A novel analysis method of atomic force microscopy phase images based on phase angle distribution across a surface was used to interpret the nanoscale distribution of organic material attached to natural and model soil particles. Phase angle distributions obtained from phase images of model surfaces were found to be bimodal, indicating multiple layers of material, which changed with the concentration of adsorbed humic acid. Phase angle distributions obtained from phase images of natural soil particles indicated a trend of decreasing surface coverage with increasing depth in the soil column. This was consistent with previous macroscopic determination of the proportions of organic material chemically extracted from bulk samples of the soils from which specimen particles were drawn. Interaction forces were measured between atomic force microscopy cantilever tips (Si(3)N(4)) and natural soil and model surfaces. Adhesion forces at humic acid free specimen surfaces (Av. 20.0 nN), which are primarily hydrophilic and whose interactions are subject to a significant contribution from the capillary forces, were found to be larger than those of specimen surfaces with adsorbed humic acid (Av. 6.5 nN). This suggests that adsorbed humic acid increased surface hydrophobicity. The magnitude and distribution of adhesion forces between atomic force microscopy tips and the natural particle surfaces was affected by both local surface roughness and the presence of adsorbed organic material. The present study has correlated nanoscale measurements with established macroscale methods of soil study. Thus, the research demonstrates that atomic force microscopy is an important addition to soil science that permits a multiscale analysis of the multifactorial phenomena of soil hydrophobicity and wetting.

Use of Atomic Force Microscopy and Transmission Electron Microscopy for Correlative Studies of Bacterial Capsules

Bacteria can possess an outermost assembly of polysaccharide molecules, a capsule, which is attached to their cell wall. We have used two complementary, high-resolution microscopy techniques, atomic force microscopy (AFM) and transmission electron microscopy (TEM), to study bacterial capsules of four different gram-negative bacterial strains: Escherichia coli K30, Pseudomonas aeruginosa FRD1, Shewanella oneidensis MR-4, and Geobacter sulfurreducens PCA. TEM analysis of bacterial cells using different preparative techniques (whole-cell mounts, conventional embeddings, and freeze-substitution) revealed capsules for some but not all of the strains. In contrast, the use of AFM allowed the unambiguous identification of the presence of capsules on all strains used in the present study, including those that were shown by TEM to be not encapsulated. In addition, the use of AFM phase imaging allowed the visualization of the bacterial cell within the capsule, with a depth sensitivity that decreased with increasing tapping frequency.

Geometric Characterization of Carbon Nanotubes by Atomic Force Microscopy in Conjunction with a Tip Characterizer

An atomic force microscopy (AFM) probe tip characterizer with 14 line and space structures and two knife edges was fabricated by means of a superlattice technique. The shape of a probe tip both before and after AFM imaging was acquired by this tip characterizer with general variations <1.5 nm; depending on imaging conditions. The geometric structures of carbon nanotubes (CNTs) on a SiO2 substrate were studied by dynamic mode AFM in conjunction with this tip characterizer. Contact points between the tip and the CNTs were detected by observing changes in the AFM phase images. A modified CNT width correction model was established to calculate the estimated and upper-limit widths of two CNTs. The experimental results showed that imaging under a weak attractive force was suitable for obtaining accurate CNT height measurements, whereas a weak repulsive force provided the most accurate widths. Differing heights and widths between the two CNTs suggested that one CNT was double-walled, whereas the other had more than two walls; these results agree with transmission electron microscopy (TEM) measurements of the CNTs.

Two Novel Cyclopentadithiophene-Based Alternating Copolymers as Potential Donor Components for High-Efficiency Bulk-Heterojunction-Type Solar Cells

Polymer/fullerene bulk heterojunctions have recently generated a lot of scientific interest due to their potential in low-cost photovoltaic applications. In this paper we detail the synthesis and characterization of two new low-band-gap polythiophenes, poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-4,7-bis(2-thienyl)-2,1,3-benzothiadiazole-5′,5′′-diyl] (PCPDTTBTT) and poly[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-2,6-diyl-alt-2,3-dioctylquinoxaline-5,8-diyl] (PCPDTQ), for use in these applications. The PCPDTQ polymer did not produce efficient solar cells. A high power efficiency of 2.1% under one sun was found for a PCPDTTBTT/fullerene mixture. The high efficiency was achieved by alteration of the morphology using a solvent additive. Analysis of atomic force microscopy phase images shows that material phases with distinct mixing ratios are formed and altered with the addition of the solvent additive.

Self‐assembly and morphology of polydimethylsiloxane supramolecular thermoplastic elastomers

AbstractFunctionalization of polydimethylsiloxanes (PDMS) polymers with hydrogen‐bonding ureidopyrimidinone (UPy) groups leads to supramolecular thermoplastic elastomers. In previous studies, no lateral stacking of UPy dimers was observed in UPy‐functionalized polymers, unless additional urethane or urea groups were built into the hard block. However, we have shown that when PDMS is used as the soft block, this lateral aggregation of UPy dimers does take place, since long fibers could be observed in the atomic force microscopy (AFM) phase image. Also in bulk, the presence of these interactions was proven by oscillatory shear experiments. We attribute this aggregation to the incompatibility of soft block and hard block, leading to phase separation. Moreover, we have shown that additional urethane or urea groups in the hard block do lead to materials with more fibers and higher melting points. For the UPy‐urea functionalized PDMS even single fibers are observed with AFM when dropcasted from a very diluted solution. When the length of the soft block is increased, the morphology changes from fibrous to spherical. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3877–3885, 2008