New applications in studies of waterborne coatings by atomic force microscopy

Atomic force microscopy (AFM) is rapidly emerging as an important tool for coatings characterization. We report several new applications of AFM of particular value to the development of improved waterborne coatings systems. First, an AFM method was developed to quantitatively assess the extent of coalescence and film formation for latex films by measurement of particle number density of protruding (uncoalesced) particles in dried coatings. Second, the use of topographic imaging to evaluate environmental (temperature) effects on film formation was investigated for a waterborne latex system. Finally, specular gloss of waterborne epoxy coatings was studied by AFM and optical measurements, and topographic features analyzed using power spectral density calculations were found to correlate with optical gloss measurements. Mechanisms for gloss reduction over time (particularly in early pot life coatings) were elucidated in the studies. Further applications in coatings studies will be driven by the development of new modes of AFM (friction force, force modulation, and phase contrast) that can be used to map mechanical properties (friction, stiffness, and adhesion) while simultaneously imaging topography. Examples of the use of the phase contrast mode to identify chemically different domains in early pot life waterborne epoxy coatings are presented.     

Surface topography and surface chemistry of radiation‐patterned P(tBuMA)—analysis by atomic force microscopy

Poly‐(tert‐butyl methacrylate) (P(tBuMA)) thin‐film surfaces were patterned by UV radiation at doses in the range 10–100 mJ cm^?2, in order to induce laterally differentiated surface chemistry with µm resolution. The most likely pathway for the radiation chemistry predicts a transition from hydrophobicity to hydrophilicity. Outcomes of analysis by atomic force microscopy under air ambient conditions were consistent with that prediction. Topographic and lateral force imaging, in combination with friction loop analysis, revealed shrinkage and increased friction arising from exposure. Force versus distance analysis revealed greater adhesion in hydrophilic regions, due to greater meniscus force acting on the tip. The thickness of adsorbed moisture, increased by a factor of 2.5 from ca 0.8 nm for the unirradiated surface, as a result of greater hydrophilicity induced by radiation. The latter observation shows that the increased friction was due principally to the greater normal force on the tip from an additional meniscus force. Copyright © 2003 Society of Chemical Industry     

Self‐assembled multilayer films based on diazoresins studied by atomic force microscopy/friction force microscopy

The layer‐by‐layer self‐assembled NDR‐PSS (nitro‐containing diazoresin‐polysodium p‐styrenesulfonate) films were fabricated. The crosslinking structure formed from the conversion of ionic bond to covalent bond after UV irradiation, confirmed by small angle X‐ray diffraction. The roughness and microtribological properties of NDR‐PSS films were investigated by atomic force microscopy/friction force microscopy. The ordered multilayer films after photoreaction are better in microtribological performance than that of the monolayer film. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 631–638, 2000     

Surface topography and tribological properties of coatings prepared from microparticle size polyurethane dispersions studied by atomic force microscopy

The surface topography and mechanical properties of coatings prepared using large particle size polyurethane dispersions (PUD) are investigated using atomic force microscopy (AFM) imaging, AFM-based force measurements, and friction force microscopy. PUD coatings, which are prepared from dispersions containing particles of micron size, have surface roughness of 250–300 nm and waviness of 2.5–3 μm resulting from the particle size. The surface moduli of the PUD coatings are varied by tuning the ratio of hard-to-soft segmentation in the polyurethanes and are found to be between 40 and 100 MPa. The friction coefficient obtained in the study is found to be correlated with both the surface modulus of the coatings and the adhesion between the probe and the samples and is well in line with the perceived feel of an experienced human panel. The data are very well behaved and clearly show the utility of this technique in characterizing these types of surfaces.     

Fabrication of surface‐relief gratings on an azobenzene‐containing epoxy‐based polymer film using atomic force microscopy

Many studies have been reported on the photo‐fabrication of surface‐relief gratings (SRGs) in azo‐polymer films using the interference of two laser beams of appropriate polarization. However, there are few reports in the literature concerning the electro‐fabrication of SRGs on such types of polymer films. The goal of the work reported was the electro‐patterning of an azobenzene‐containing epoxy thermoplastic film. An epoxy‐based polymer functionalized with an azo‐chromophore was synthesized and characterized using thermal analysis. The reversible optical storage properties and photo‐induced dichroism were studied. SRGs were fabricated on a film of the synthesized azobenzene‐containing polymer using contact mode current‐sensing atomic force microscopy which locally applied an electric field that aligned the azobenzene moieties. The anisotropic mass transport of the azo‐polymer film was observed after applying an electric field. Additionally, the effect of the relief formation in the polymeric film surface was investigated by means of atomic force microscopy and electrostatic force microscopy. Copyright © 2010 Society of Chemical Industry     

Atomic force microscopy and scanning electron microscopy characterization of the controlling of surface morphology of epoxy-amine-cured spin-coated films

Atomic force microscopy (AFM) was successfully used to study spin-coated, amine-cured epoxy film microstructure and morphology. The air-epoxy and epoxy-substrate interfaces were examined using tapping-mode height and phase imaging AFM. The impact of relative humidity on the morphology and microstructure of the surfaces was determined. AFM was able to elucidate the changes on the surface as relative humidity during processing increased. It was observed that large nodular formations formed on the epoxy surface expose to the air but not epoxy surface formed on the substrate in addition to varying regions of more or less compliant structures, which was attributed to carbamate formation caused by the amine curing agent reaction with atmospheric CO₂. Scanning electron microscopy (SEM) was used to further elucidate interface and interphase morphology of spin-coated epoxies. Experimentation also demonstrated that post-curing above the glass transition did not change the morphology structure, suggesting surface structures are “locked-in.” SEM was used to further elucidate how the interface and interphase change with changing environmental conditions at both the air-epoxy and epoxy-substrate interface/interphases, including the impact of atmospheric CO₂ on Marangoni cell formation.     

Cellulosic nanocomposites. II. Studies by atomic force microscopy

Dissolving‐grade wood pulp fibers were partially esterified by mixed p‐toluene sulfonic/hexanoic acid anhydride in a nonswelling suspending agent. A biphasic morphology was revealed by atomic force microscopy (AFM) for the compression‐molded, partially modified pulp fibers. The AFM phase images indicated distinct periodicity on the scale of several 10's of nanometers. Surface etching with cellulolytic enzymes of the modified pulp fibers produced height images that had virtually the same periodicity. These results indicate that the modified pulp fibers are nanocomposites comprising unmodified cellulose and cellulose hexanoate. Regenerated lyocell fibers (from N,MMNO solvent) subjected to the same esterification system as applied to pulp fibers, by contrast, exhibited AFM phase images that indicated a high level of surface (skin) versus core reactivity. Modified lyocell fibers with an average diameter of about 12 μm and having an overall DS of 0.6 had surface layers that were approximately 1 μm thick. The latter represented a transitional phase in which the chemical composition and the physical properties were intermediate between a highly substituted surface (skin) and an unsubstituted core. When a compression‐molded sheet of the modified lyocell fibers was analyzed by microthermal analysis, the thermoplastic matrix on the lyocell fiber surface was revealed to have an apparent T_g of 75°C corresponding to cellulose hexanoate, whereas no significant thermal transition was determined for the (unmodified) fiber core. These results suggest that both partially modified lyocell fibers and partially modified pulp fibers are capable of producing composites with morphologies that have grossly different scales. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 2254–2261, 2000     

Effect of sidewall modification in the determination of friction coefficient of vertically aligned carbon nanotube films using friction force microscopy

The significance of the sidewall surface of vertically aligned carbon nanotubes (VACNTs) and the effect of humidity in the determination of VACNT film friction coefficient have been investigated. VACNT films of 2 μm thick were sidewall-modified by means of CF₄ and O₂ plasma treatments, and verified for the functionalization of the sidewalls. They were then characterized for wettability properties, as well as friction coefficient using friction force microscopy at different humidity levels. It was found that humidity had insignificant effect on the friction coefficient, and sidewall friction formed a major component of the friction force experienced by the tip. Sidewall modifications resulted in friction coefficient changes of up to 50%.     

Atomic force microscopy and contact angle studies of polymerizable gemini surfactant admicelles on mica

Atomic force microscopy (AFM) was used to directly observe and characterize a polymer‐modified mica surface prepared using a polymerizable gemini surfactant. Normal tapping mode and contact mode AFM were used to image the treated mica surface morphologies in air and liquid environments, respectively. The root mean square (RMS) roughness of mica surfaces before and after surface modification and polymerization was analyzed from these scans. To determine the effect of styrene adsolubilization on the surfactant‐modified mica, AFM measurements of the modified mica were made at various styrene concentrations. Contact angle measurements were also made to further characterize the nature of the surfactant‐modified mica surface. The surface morphology and surface hydrophilicity were observed to be different for the modified mica after polymerization. In addition, the polymerized surface maintained its morphology after washing/desorption studies demonstrating the stability of the polymerized surfactant film. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Latex characterization by atomic force microscopy

Ways, benefits and limitations of extracting the form and size of single latex particles or particle size distributions out of the surface topography of non-coalesced latex films obtained by atomic force microscopy (AFM) are discussed. The general ways to generate the desired information out of topographical information in terms of height, surface curvature and lateral extensions are shown for idealized particles and measurement conditions. The different sources of information are evaluated for practical use and the analysis of particle size distributions is demonstrated by practical examples. The information content in lateral particle dimensions is shown to be the most advantageous for practical use. Determination of latex particle size distributions by AFM is shown to be an interesting alternative to the arsenal of available methods with respect to exactness of results, time consummation and instrumentation costs.     

Anisotropic friction on lamellar crystals of polyethylene by lateral force microscopy

Summary Frictional forces are very sensitive to the interface structure and the chemical and atomic structure of surfaces. It has been shown in the late 80s (1) that lateral forces due to friction can be measured by atomic force microscopy (AFM) in the contact mode (2,3). This involves measurements which evoke vertical deflection of the AFM probe and observation of lateral forces which twist the AFM force sensor. A simultaneous detection of vertical deflection and twisting of the microcantilever can be done by using a four-sectored positional sensitive photodetector which measures the change in the deflection of a laser light reflected off the top of the microcantilever. The newest generation of SPM (Scanning Probe Microscope) equipment allows for simultaneous AFM and Lateral Force Microscopy (LFM) scans. There is ample evidence in the literature that contact-mode AFM can be used not only to investigate polymer morphology (4,5), but also to study polymer architecture from a true molecular perspective (6).     

Atomic force microscopy (AFM) studies of liquid crystalline polymer (LCP) surfaces

An atomic force microscope (AFM) operating in tapping or contact mode was used to study the surface topography and the molecular organization of Vectra‐A and Vectra‐B films. Large‐scale (15 × 15 μm) AFM images revealed that ribbonlike fibrils with a width/height ≫ 1.0 are the dominant surface features of these liquid crystalline polymers (LCPs). The region of local disorder, surface debris, and interfibrillar debris as well as possible amorphous regions were observed in both LCP samples. Large fibrils, 5.0–10.0 μm in width, can be thought of as formed by smaller microfibrils capable of forming ordered structures. Microfibrils can bend upward, forming raised surface features; bend inward, originating cracks 1–2 μm wide on the film surface; or divide and subdivide into smaller units. Longitudinal and lateral stresses are believed responsible for the variation in fibril size, shape, and orientation. AFM images containing molecular‐scale details showed that microfibrils consists of chains of molecules coiled around a central axis and that they can be only about 2.0 nm wide. These submicron surfaces consist of white spots (representing molecules) that can form ordered structures or that can cluster to form agglomerates distributed in a random manner. Submicron fibrils are believed to represent the LCP basic structural unit. AFM results indicate that the surface topography of Vecta‐B is more ordered and uniform than is the one observed for Vectra‐A. Seemingly, amorphous particles form debris on Vectra‐A surfaces. Short rods oriented crosswise on the fibril surface are instead what increases the Vectra‐B roughness. These LCPs can have a surface topography similar to the one observed in AFM images of a spiderweb. However, the spiderweb fibrils are formed by more uniform microfibrils that are oriented parallel to each other. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2243–2254, 1999     

Surface of poly(ethylene terephthalate/isophthalate) copolyesters studied by atomic force microscopy

An atomic force microscope (AFM) operating in Tapping™ and contact modes has been used to study the surface topography and the molecular organization of poly(ethylene terephthalate) (PET) films containing 2% (PET‐2I) and 10% (PET‐10I) isophthalate, and of injection/blow molded bottles containing 2.6% (PET‐2.6I) and 10% isophthalate. Large‐scale (15‐μm × 15‐μm) AFM images have shown that both surfaces are fairly flat and heterogeneous in nature, often containing inclusions. Whereas the PET surface appears to be formed mainly by microfibrils, isophthalic acid (IPA) incorporation at the 2–10 mol % level gives the surface a granular appearance. The IPA‐containing PET surfaces are frequently coated by a lacelike film consisting of submicron “beads” joined together by filaments. These “strings of beads” form bundles and can also connect bundles. AFM images of PET‐2I closely resemble those generated for PET films. By contrast, the lacelike structure becomes a dominant feature of the PET‐10I surface. The level of inclusions observed on film surfaces appears to correlate with the levels of extractable oligomers present in the polymers. Nanometer‐scale AFM images of PET‐10I exhibit surfaces composed of short stacks of plates or rods, with 30–50‐nm voids or pores between these stacks. Whereas surface deposits of what we suggest is most likely an oligomer correlates with isophthalate concentration, we see no gross structural features in PET‐2I and PET‐10I that explain the observed improvement in gas diffusion barrier in these polymers. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 750–762, 2001     

Photo‐oxidative degradation in polyisoprene: surface characterization and analysis by atomic force microscopy

Surface adhesion and surface mechanical properties of natural rubber cast films have been investigated by atomic force microscopy (AFM) before and after UV irradiation. Analysis in the force versus distance (F–d) mode probed changes in tip‐to‐surface adhesion. Adhesion was observed to increase as a result of exposure, in accord with the prediction that generation of polar groups in the surface should promote a hydrophilic transition. The hydrophilicity also gives rise to a stronger adhesive contribution from a meniscus force, as demonstrated by comparison of results obtained in air for irradiated and as‐received samples. Calculated values of work of adhesion, based on the Johnson–Kendall–Roberts model, reflected the changes in surface chemistry and the effects of the fluid environment. The outcomes of F–d analyses revealed both tip indentation and polymer extension; the former was fitted to expressions derived from the Snedden formalism, followed by calculation of an effective Young's modulus. Good agreement was obtained for as‐received surfaces with bulk measurements. The photochemistry predicts chain scission events which are likely to account for the observed softening of the polymer. © 2001 Society of Chemical Industry     

Friction and wear at nanometer scale: a comparative study of hard carbon films

Tribological properties of nanostructured carbon (ns-C) and tetrahedral amorphous carbon (ta-C) thin films were investigated by friction force microscopy. It was found that the ns-C films have a smaller friction coefficient than ta-C films for relative humidity greater than 30%. In particular, at 40% of humidity, ns-C films have lower friction coefficient (0.11±0.02) than the ta-C films (0.13±0.02), which can be attributed to both the presence of closed graphite nanoparticles and the passivation of the dangling bonds at the ns-C surface. The friction coefficient did not vary as a function of the tip scanning velocity for both films. The nanoscale wear was studied in a very low force regime, in the range of nanonewton, using an atomic force microscope (AFM) with a Si₃N₄ tip and with forces in the range of micronewton with the AFM equipped with a stainless still cantilever and a diamond tip. The ns-C provides better wear resistance compared to ta-C films in the range of forces studied. The sp²-rich ta-C surface layer was easily scratched during the wear test in contrast to the ns-C films. The wear in ta-C in the low forces regime is attributed to the presence of this low density layer at the surface of the film due to subplantation of energetic ions during deposition while the better resistance to wear of ns-C films is attributed to its highly elastic nature.     

Probing polymer interdiffusion in carboxylated latices with force modulation atomic force microscopy

Interdiffusion of polymer chains between latex particles is a prerequisite for the development of good mechanical strength and homogeneity in a latex film. This process may be retarded in carboxylated latices if the particles are surrounded by a hard cell wall consisting of ionic groups on the particle surface. The presence of an ionic cell wall can be indirectly detected by atomic force microscopy (AFM) because surfactant migration to the film/air interface is retarded compared with a non-ionic case. In this paper we have used force modulation atomic force microscopy to directly probe the relative polymer density across the film surface during annealing thereby qualitatively monitoring the interdiffusion process. The applicability of this method to study polymer interdiffusion will be discussed.     

Reactivity at the film/solution interface of ex situ prepared bismuth film electrodes: A scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM) investigation

Bismuth film electrodes (BiFEs) prepared ex situ with and without complexing bromide ions in the modification solution were investigated using scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM). A feedback mode of the SECM was employed to examine the conductivity and reactivity of a series of thin bismuth films deposited onto disk glassy carbon substrate electrodes (GCEs) of 3 mm in diameter. A platinum micro-electrode (? = 25 μm) was used as the SECM tip, and current against tip/substrate distance was recorded in solutions containing either Ru(NH₃)₆³⁺ or Fe(CN)₆⁴⁻ species as redox mediators. With both redox mediators positive feedback approach curves were recorded, which indicated that the bismuth film deposition protocol associated with the addition of bromide ions in the modification solution did not compromise the conductivity of the bismuth film in comparison with that prepared without bromide. However, at the former Bi film a slight kinetic hindering was observed in recycling Ru(NH₃)₆³⁺, suggesting a different surface potential. On the other hand, the approach curves recorded by using Fe(CN)₆⁴⁻ showed that both types of the aforementioned bismuth films exhibited local reactivity with the oxidised form of the redox mediator, and that bismuth film obtained with bromide ions exhibited slightly lower reactivity. The use of SECM in the scanning operation mode allowed us to ascertain that the bismuth deposits were uniformly distributed across the whole surface of the glassy carbon substrate electrode. Comparative AFM measurements corroborated the above findings and additionally revealed a denser growth of smaller bismuth crystals over the surface of the substrate electrode in the presence of bromide ions, while the crystals were bigger but sparser in the absence of bromide ions in the modification solution.     

Wang-Landau Monte Carlo simulation of capillary forces at low relative humidity in atomic force microscopy

A lattice gas model is used with Wang-Landau Monte Carlo sampling to predict the capillary force between a model of an atomic force microscopy (AFM) probe and a smooth surface as a function of separation, relative humidity (RH), and tip hydrophilicity. Completely wetting AFM tips exhibit a maximum in the capillary force as the RH increases, while the magnitude of the capillary force in the presence of partially wetting and partially drying tips is relatively independent of the RH. Capillary forces can also be significant in low RH environments and should not be discounted in AFM studies involving hydrophilic surfaces.     

Atomic force microscopy studies of poly(methyl methacrylate) doped with photoinitiator

Atomic force microscopy has been applied for measuring the nanomechanical characteristics of poly(methyl methacrylate) thin films containing 5% photoinitiator (Irgacure 651). The nanohardness, Young's modulus, and adhesion to AFM tip have been evaluated for the unexposed samples and after UV‐irradiation. Additionally, FTIR spectroscopy and differential scanning calorimetry (DSC) have been applied to explain the observed changes in nanomechanical properties. It was found that the exposure to ultraviolet changed the nanomechanical properties of polymer because of photo‐oxidative degradation and relaxation processes. These studies lead to the conclusion that the applied photoinitiator has no noticeable effect on nanohardness and Young modulus during PMMA irradiation, but efficiently participates in polymer photo‐oxidation increasing the surface hydrophilicity and adhesion to Si₃N₄. Moreover, the initiator hampers the relaxation of PMMA macromolecules, what was proved by DSC. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Prospect of nanoscale interphase evaluation to predict composite properties

For meeting the requirements of lightweight and improved mechanical properties, composites could be tailor-made for specific applications if the adhesion strength which plays a key role for improved properties can be predicted. The relationship between wettability and adhesion strength has been discussed. The microstructure of interphases and adhesion strength can be significantly altered by different surface modifications of the reinforcing fibers, since the specific properties of the interphase result from nucleation, thermal and/or intrinsic stresses, sizing used, interdiffusion, and roughness. The experimental results could not confirm a simple and direct correlation between wettability and adhesion strength for different model systems. The main objective of the work was to identify the interphases for different fiber/polymer matrix systems. By using phase imaging and nanoindentation tests based on atomic force microscopy (AFM), a comparative study of the local mechanical property variation in the interphase of glass fiber reinforced epoxy resin (EP) and glass fiber reinforced polypropylene matrix (PP) composites was conducted. As model sizings for PP composites, γ-aminopropyltriethoxysilane (APS) and either polyurethane (PU) or polypropylene (PP) film former on glass fibers were investigated. The EP-matrix was combined with either unsized glass fibers or glass fibers treated with APS/PU sizing. It was found that phase imaging AFM was a highly useful tool for probing the interphase with much detailed information. Nanoindentation with sufficiently small indentation force was found to be sufficient for measuring actual interphase properties within a 100-nm region close to the fiber surface. Subsequently, it also indicated a different gradient in the modulus across the interphase region due to different sizings. The possibilities of controlling bond strength between fiber surface and polymer matrix are discussed in terms of elastic moduli of the interphases compared with surface stiffness of sized glass fibers, micromechanical results, and the mechanical properties of real composites.