Substrate effect and application of the elastic foundation model to evaluate atomic force microscope nanoindentations of thin polymeric films

The analysis of the mechanical properties of thin films by nanoindentation has been recently subject of numerous theoretical and phenomenological studies as well as numerical simulations. In this work, we report on the application of the Winkler elastic foundation theory to analyze atomic force microscope nanoindentations of poly(n‐butyl methacrylate) films with thicknesses of 90, 196, and 485 nm. The elastic moduli of the samples were found to be 1.13 ± 0.43 GPa, 1.34 ± 0.32 GPa and 1.23 ± 0.24 GPa, respectively, after indentations of at least 50% of the film thickness. These data rely on the independent determination of the mechanical properties of 485‐nm thick films, using Sneddon's model at low penetration depth (yielding 1.27 ± 0.37 GPa). Our data show that the substrate effects begins to be noticeable only after indenting to a depth of more than 40% of the film thickness. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

原子力显微镜在聚氨酯材料性能分析中的应用

简述了原子力显微镜（AFM）探测物体表面形貌的工作原理和操作模式，介绍了AFM在观察聚氨酯（PU）材料微相分离、复合树脂的相容性的应用情况，综述了AFM应用于PU材料研究的新进展。     

Advanced techniques for nanocharacterization of polymeric coating surfaces

Surface properties of a polymeric coating system have a strong influence on its performance and service life. However, the surface of a polymer coating may have different chemical, physical, and mechanical properties from the bulk. In order to monitor the coating property changes with environmental exposures from the early stages of degradation, nondestructive techniques with the ability to characterize surface properties with micro- to nanoscale spatial resolution are required. In this article, atomic force microscopy has been applied to study surface microstructure and morphological changes during degradation in polymer coatings. Additionally, the use of AFM with a controlled tip-sample environment to study nanochemical heterogeneity and the application of nanoindentation to characterize mechanical properties of coatings surfaces are demonstrated. The results obtained from these nanometer characterization techniques will provide a better understanding of the degradation mechanisms and a fundamental basis for predicting the service life of polymer coatings. Presented at the 81st Annual Meeting of the Federation of Societies for Coatings Technology on November 12–14, 2003, in Philadelphia, PA.     

Multiscale physical characterization of an outdoor-exposed polymeric coating system

Surface topography and gloss are two related properties affecting the appearance of a polymeric coating system. Upon exposure to ultraviolet (UV) radiation, the surface topography of a coating becomes more pronounced and, correspondingly, its gloss generally decreases. However, the surface factors affecting gloss and appearance are difficult to ascertain. In this article, atomic force microscopy (AFM) and laser scanning confocal microscopy (LSCM) measurements have been performed on an amine-cured epoxy coating system exposed to outdoor environments in Gaithersburg, Maryland. The formation of the protuberances is observed at the early degradation stages, followed by the appearance of circular pits as exposure continues. At long exposure times, the circular features enlarge and deepen, resulting in a rough surface topography and crack formation. Fourier Transform Infrared Spectroscopy (FTIR) study indicates that the oxidation and chain scission reactions are likely the origins of the surface morphological changes. The relationship between changes in surface roughness and gloss has been analyzed. The root mean square (RMS) roughness of the coating is related to nanoscale and microscale morphological changes in the surface of the coating as well as to the gloss retention. A near-linear dependence of RMS roughness with the measurement length scale (L) is found on a double logarithmic scale, i.e., RMS ∼ L ^f. The scaling factor, f, decreases with exposure time. The relationship between surface topography, on nano- to microscales, and the macroscale optical properties such as gloss retention is discussed. Moreover, a recent development in using an angle-resolved light scattering technique for the measurement of the specular and off-specular reflectance of the UV-exposed specimens is also demonstrated, and the optical scattering data are compared to the gloss and the roughness results.

Exploring forces between individual colloidal particles with the atomic force microscope

Forces between individual colloidal particles can be measured with the atomic force microscope (AFM), and this technique permits the study of interactions between surfaces across aqueous solutions in great detail. The most relevant forces are described by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, and they include electrostatic double-layer and van der Waals forces. In symmetric systems, the electrostatic forces are repulsive and depend strongly on the type and concentration of the salts present, while van der Waals forces are always attractive. In asymmetric systems, the electrostatic force can become attractive as well, even when involving neutral surfaces, while in rare situations van der Waals forces can become repulsive too. The enormous sensitivity of the double layer forces on additives present is illustrated with oppositely charged polyelectrolytes, which may induce attractions or repulsions depending on their concentrations.     

Analysis of polymeric composite interphase regions with thermal atomic force microscopy

Previous work on the characterization of interphase regions in thermosetting composite systems has focused on the inference of an interphase layer from effects noticed through macroscale mechanical and thermal testing. With the development of atomic force microscopy and active thermal probes for this technique, it is now possible to examine material thermal properties on a much smaller scale. Variations in microscale thermal properties of an aerospace‐grade thermosetting resin system were evaluated for carbon and glass fiber reinforcement, using the modulated local thermal analysis mode of a TA Instruments 2990 μTA. The variations observed clearly demonstrate the presence of a soft interphase layer in the glass material and underline the importance of fiber–matrix interactions during the formation of the interphase. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1643–1649, 2001     

On the activation and physical degradation of boron-doped diamond surfaces brought on by cathodic pretreatments

The electrochemical activation and physical degradation of boron-doped diamond (BDD) electrodes with different boron doping levels after repeated cathodic pretreatments are reported. Galvanostatic cathodic pretreatment passing up to −14000 C cm⁻² in steps of −600 C cm⁻² using −1 A cm⁻² caused significant physical degradation of the BDD surface, with film detachment in some areas. Because of this degradation, a great increase in the electrochemically active area was observed in Tafel plots for the hydrogen evolution reaction (HER) in acid media. The minimum cathodic pretreatment needed for the electrochemical activation of the BDD electrodes without producing any observable physical degradation on the BDD surfaces was determined using electrochemical impedance spectroscopy (EIS) measurements and cyclic voltammetry: −9 C cm⁻², passed at −1 A cm⁻². This optimized cathodic pretreatment can be safely used when electrochemical experiments are carried out on BDD electrodes with doping levels in the range between 800 and 8000 ppm.     

Surface morphology of Nafion 117 membrane by tapping mode atomic force microscope

Surface morphology of Nafion 117 membrane was studied by tapping mode atomic force microscopy. Three different samples were analyzed and correspond respectively to dry membrane and wet membrane equilibrated either with water or with tributylphosphate. These studies show the supermolecular structure of the membrane, which is made of nodules or spherical grains of a mean diameter of 11 nm, and are surrounded by interstitial regions of a mean thickness of 50 Å. Roughness analysis of the samples shows the influence of the swelling properties of the membrane on its surface morphology. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 503–508, 1998     

Dynamic response of a cracked atomic force microscope cantilever used for nanomachining

ABSTRACT: The vibration behavior of an atomic force microscope [AFM] cantilever with a crack during the nanomachining process is studied. The cantilever is divided into two segments by the crack, and a rotational spring is used to simulate the crack. The two individual governing equations of transverse vibration for the cracked cantilever can be expressed. However, the corresponding boundary conditions are coupled because of the crack interaction. Analytical expressions for the vibration displacement and natural frequency of the cracked cantilever are obtained. In addition, the effects of crack flexibility, crack location, and tip length on the vibration displacement of the cantilever are analyzed. Results show that the crack occurs in the AFM cantilever that can significantly affect its vibration response.PACS: 07.79.Lh; 62.20.mt; 62.25.Jk.     

Sample rotation angle dependence of graphene thickness measured using atomic force microscope

A precise measurement of graphene thickness is required for the design and development of nano-devices based on the material. Many factors affect this measurement when using scanning tunneling microscope (STM) and atomic force microscope (AFM), including the interaction between the scanning tip and ripples on graphene; such effects have not previously been explored. To investigate this, we measure the sample rotation angle dependence of graphene thickness as determined by contact mode and tapping mode AFM. The graphene thickness as determined by contact mode AFM follows a cosine modulus function of sample rotation angle, while tapping mode AFM reveals a constant graphene thickness, independent of sample rotation angle. For comparison, the AFM torsion signal is measured and follows a sine function of the sample rotation angle. All the measured sample rotation angle dependences can be explained by the interaction between linearly aligned ripples on graphene and the AFM tip in contact with the graphene.     

原子力显微镜技术研究重组人内皮抑素对内皮细胞的作用

目的应用原子力显微镜(atomic force microscope,AFM)技术研究重组人内皮抑素(recombinant human endostatin,rhES)对内皮细胞的作用。方法采用MTT法检测不同浓度的rhES(0.05~2.4μg/ml)对人脐静脉内皮细胞ECV304增殖活力的影响;分别用0.8和2μg/ml的rhES处理ECV304细胞,应用AFM观察内皮细胞整体形貌的变化,SPI 3800 New DFM动力显微镜观察ECV304细胞表面局部形貌的变化。结果 rhES可明显抑制ECV304细胞增殖,且呈剂量效应(P0.001);rhES可降低贴壁的ECV304细胞的厚度,且呈剂量依赖效应,使较光滑的细胞表面变粗糙,产生了一些微小的突起;经rhES处理的ECV304细胞表面结构呈现不规则的变化。结论 AFM技术具有样品制备简便和分辨率较高等优点,适合贴壁培养细胞的原位观察。     

Quantifying the mechanical properties of polymeric tubing and scaffold using atomic force microscopy and nanoindentation

Measurement of mechanical parameters of polymeric scaffolds presents a significant challenge due to their intricate shape and small characteristics dimensions of their elements—around 100 μm. In this study, mechanical properties of polymeric tubing and scaffold, made of biodegradable poly(l ‐lactic) acid (PLLA), were characterized using atomic force microscopy (AFM) and nanoindentation, complemented with tensile testing. AFM was employed to assess the properties of the tube and scaffold locally, while nanoindentation produced results with a dependency on the depth of indentation. As a result, the AFM‐measured elastic modulus differs from the nanoindentation data due to a substantial difference in indentation depth between the two methods. With AFM, a modulus between 2 and 2.5 GPa was measured, while a wide range was obtained from nanoindentation on both the tube and scaffold, depending on the indentation scale. Changes in the elastic modulus with in‐vitro degradation and aging were observed over the 1‐year period. To complement the indentation measurements, tensile testing was used to study the structural behavior of the tube, demonstrating the yielding, hardening and fracture properties of the material. POLYM. ENG. SCI., 59:1084–1091, 2019. © 2019 Society of Plastics Engineers     

Some comments on the use of the EIS phase angle to evaluate organic coating degradation

The impedance of a disc electrode protected by an organic coating, with a thickness profile along its radius, was considered by EIS. The local and global impedances, the ohmic resistance corrected phase angle, and raw phase angle were calculated. The thickness profile leads to a well-defined minimum in the calculated phase angle curves, which can be observed in the high frequency domain. This effect is enhanced with higher permittivity, thinner coatings and low conductivity immersion baths. From these results, it appears that the graphical treatment of experimental phase angle curves to evaluate the coating degradation may lead to erroneous conclusions.     

Continuous electrospinning of polymer nanofibers of Nylon-6 using an atomic force microscope tip

An atomic force microscopy (AFM) probe is successfully utilized as an electrospinning tip for fabricating Nylon-6 nanofibers. The nanometre-size tip enabled controlled deposition of uniform polymeric nanofibers within a 1 cm diameter area. Nylon-6 nanofibers were continuously electrospun at a solution concentration as low as 1 wt% Nylon-6 in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). Wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) results of the AFM electrospun fibers indicated that the nanofibers predominantly display the meta-stable γ crystalline form suggesting rapid crystallization rate during the process. In addition to precise control over fiber deposition and diameter, some of the drawbacks of conventional electrospinning such as large volume of solutions and clogging of needles can be overcome using this AFM based electrospinning technique. Lastly, a comparison of electrospun fibers from syringe-needle based electrospinning and AFM probe-tip based electrospinning indicated significant morphological and microstructural differences in the case of AFM based electrospinning.     

A local field emission study of partially aligned carbon-nanotubes by atomic force microscope probe

Atomic force microscopy (AFM) was used to study the field emission (FE) properties of a dense array of long and vertically quasi-aligned multi-walled carbon nanotubes grown by catalytic chemical vapor deposition on a silicon substrate. The use of nanometric probes enables local field emission measurements to be made allowing the investigation of effects that are not detectable with a conventional parallel plate setup, where the emission current is averaged over a large sample area. The micrometric inter-electrode distance allows one to achieve high electric fields with a modest voltage. These features made us able to characterize field emission for macroscopic electric fields up to 250 V/μm and attain current densities larger than 10⁵ A/cm². FE behaviour is analyzed in the framework of the Fowler–Nordheim theory. A field enhancement factor γ ≈ 40–50 and a turn-on field E_turn-on 15 V/μm at an inter-electrode distance of 1 μm are estimated. Current saturation observed at high voltages in the I-V characteristics is explained in terms of a series resistance of the order of MΩ. Additional effects, such as electrical conditioning, CNT degradation, response to laser irradiation and time stability are investigated and discussed.     

Apparent contrast reversal in tapping mode atomic force microscope images on films of polystyrene-b-polyisoprene-b-polystyrene

Summary Thin films of phase separated polystyrene-b-polyisoprene-b-polystyrene block copolymers were studied by tapping mode atomic force microscopy. The relative contrast in height and phase mode images of the phase separated regions was found to be very sensitive to changes in the operating conditions of the microscope. Contrast variations and reversals were observed for height and phase mode images as a function of the set-point amplitude ratio and drive frequency. No unique height or phase contrast was observed for the the tri-block copolymer system examined in this study. Received: 30 December 1997/Revised version: 14 January 1998/Accepted: 15 January 1998     

Mechanical properties of porous methyl silsesquioxane and nanoclustering silica films using atomic force microscope

Mechanical properties of porous methyl silsesquioxane samples with dielectric constant 2.4 and 2.0 and a recently developed nanoclustering silica film samples with dielectric constants 2.3 and 2.0 were evaluated using an atomic force microscope based nanoindentation. It was found that the Young’s modulus and the hardness decrease while the fracture toughness increases with a decrease in the dielectric constant in the same type of material. Moreover, the Young’s modulus and the hardness of the nanoclustering silica films were observed to be at least twice and fracture toughness values ~1.3–1.5 higher than those for methyl silsesquioxane films with similar dielectric constants. The high resolution topographic imaging capability of atomic force microscope was shown to be particularly useful in the measurement of cracks generated by the ultra-low indentation loads, and the evaluation of the fracture toughness of the nanoscale volumes of materials.     

Nanopatterning of an organic monolayer covered Si (1 1 1) surfaces by atomic force microscope scratching

In the present work, an atomic force microscope (AFM) mounted with a diamond-coated tip was used to scratch through organic monolayer covered Si surfaces to create nanostructures by electrodeposition. The organic layer (1-octadecene) was covalently attached to a hydrogen-terminated Si (1 1 1) surface. Copper was deposited into the nanostructures either by immersion plating or electrodeposition. The morphology of copper deposits was studied using scanning electron microscope (SEM). The influence of the different types of semiconductor substrates (1-octadecene covered n-type Si and p-type Si) and different parameters such as immersion time on the copper deposition morphology is presented. Auger electron spectroscopy (AES) scans were performed to obtain information of the selectivity and the copper deposition. The results show clearly that under optimized conditions the organic layer can be used as an effective mask for the site selective patterning of copper nanostructures on Si.     

An atomic force microscope study of calcium carbonate adhesion to desalination process equipment: effect of anti-scale agent

Whilst carbon dioxide is water soluble the system is somewhat complex and results in the presence of carbonate anions which interact with cations such as Ca²⁺ and Mg²⁺ present in seawater to form insoluble carbonates, especially at high temperatures. In multistage flash (MSF) desalination plants CO₂ gas becomes less soluble in the brine as a result of the brines high temperature and high salinity which causes the pH to be in the range of 8–9. The presence of these conditions causes the release of CO₂, simultaneous to the formation of scale deposits since its solubility is a function of the solution pH.

The formation of scale deposits, such as CaCO₃ causes fouling in the MSF distillers which has previously been studied by many researchers. A great amount of work has been carried out and more is yet to come in order to fully understand the role of various components and their interaction including the effectiveness of scale control techniques. The deposits may serve as an adsorbing film raising the speed of the loss of crystals or promoting the formation of scale deposits and therefore further adhesion on the wall surfaces of the MSF distillers and other process plant equipment leading to deterioration in the performance and efficiency of the whole desalination plant.

This paper shows direct quantification of the adhesion forces between CaCO₃ crystals and different process equipment surfaces under different conditions. This was carried out using an atomic force microscope (AFM) with an attached CaCO₃ crystal as a colloid probe to bring the CaCO₃ directly into and out of contact with the surfaces and measuring the resultant adhesion. This involved using surfaces different grades of roughness and carrying out measurements in synthetic sea water solutions of differing ionic strengths as well as with real seawater samples. Furthermore, the effect on measured adhesion of adding anti-scalant to the solutions was examined.     

The structure and morphology of the skin of polyethersulfone ultrafiltration membranes: A comparative atomic force microscope and scanning electron microscope study

Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were used to investigate the surface structure and morphology of 10,000, 30,000, and 100,000 dalton molecular weight cutoff (MWCO) polyethersulfone (PES) ultrafiltration membranes, and the results are compared. Although both approaches reveal the pore structure in the 30,000 and 100,000 MWCO membranes, the pore diameters derived from SEM are smaller than those measured by AFM. This discrepancy is a result of the diminution in pore dimensions during the sample preparation for SEM, that is, the solvent exchange procedure needed to remove the water from the membrane prior to the high vacuum gold coating deposition step. In contrast to SEM, which requires a high vacuum both during heavy metal coating and during examination, AFM can be performed on wet ultrafiltration membranes. Consequently, the potential of altering the membranes' pore structures during sample preparation is eliminated. Therefore, the pore diameters obtained from AFM are more accurate than those derived from SEM.