AFM Phase Images Research Articles

Macromol. Biosci. 1/2009

Cover: The cover art illustrates the shape‐memory effect of a multiblock copolymer with poly(ε‐caprolactone) and poly(iso‐butylmorpholin dione) segments on two levels: the macroscopic shape recovery from the temporary shape (“stripe”) at 25°C (blue colour) to the permanent shape (“corkscrew”) at 60°C (red colour) as well as the changes of the morphology represented by AFM phase images (back image and small inset). Further details can be found in the article by Y. Feng, M. Behl, S. Kelch, A. Lendlein* on page 45.

Dynamic condensation of water at crack tips in fused silica glass

Water molecules play a fundamental role in the physics of slow crack propagation in glasses. It is commonly understood that, during stress-corrosion, water molecules that move in the crack cavity effectively reduce the bond strength at the strained crack tip and, thus, support crack propagation. Yet the details of the environmental condition at the crack tip in moist air are not well determined. In a previous work, we reported direct evidence of the presence of a 100nm long liquid condensate at the crack tip in fused silica glass during very slow crack propagation (10−9–10−10m/s). These observations are based on in situ AFM phase imaging techniques applied on DCDC glass specimens in controlled atmosphere. Here, we discuss the physical origin of the AFM phase contrast between the liquid condensate and the glass surface in relation to tip-sample adhesion induced by capillary bridges. We then report new experimental data on the water condensation length increase with relative humidity in the atmosphere. The measured condensation lengths were much larger than what predicted using the Kelvin equation and expected geometry of the crack tip.

Study of the morphology of poly(butylene succinate)/poly(ethylene oxide) blends using hot-stage atomic force microscopy

Blends of poly(butylene succinate) (PBS) and poly(ethylene oxide) (PEO) were cast into films, melted, and crystallized. A number of PBS/PEO blend compositions, ranging from 85/15 to 20/80 were used. The PBS, with a higher melting point, always crystallizes first, providing a scaffold on which the PEO would crystallize. AFM phase and height images were made at room temperature and at higher temperatures, as the PEO melted, allowing one to determine the morphology and location of the PEO. It was found that at low PEO concentrations (below 15 w/o) the PEO resides preferentially between PBS lamellae. This interlamellar PEO does not crystallize, except under extreme undercooling. At higher concentrations, larger amorphous domains exist within the PBS crystalline scaffold and PEO can crystallize in these domains. Two unexpected phenomena are observed: (1) the reversible exuding of PEO from interlamellar spaces to the surface for crystallization and (2) an unusual orientation of PEO lamellae within amorphous domains in the PBS scaffold.

Control of the Anodic Aluminum Oxide Barrier Layer Opening Process by Wet Chemical Etching

In this work, it has been shown that, through a highly controlled process, the chemical etching of the anodic aluminum oxide membrane barrier layer can be performed in such a way as to achieve nanometer-scale control of the pore opening. As the barrier layer is etched away, subtle differences revealed through AFM phase imaging in the alumina composition in the barrier layer give rise to a unique pattern of hexagonal walls surrounding each of the barrier layer domes. These nanostructures observed in both topography and phase images can be understood as differences in the oxalate anion contaminated alumina versus pure alumina. This information bears significant implication for catalysis, template synthesis, and chemical sensing applications. From the pore opening etching studies, the etching rate of the barrier layer (1.3 nm/min) is higher than that of the inner cell wall (0.93 nm/min), both of which are higher than the etching rate of pure alumina layer (0.5-0.17 nm/min). The established etching rates together with the etching temperature allow one to control the pore diameter systematically from 10 to 95 nm.

Waterborne Polyurethane/Clay Nanocomposites: Novel Effects of the Clay and Its Interlayer Ions on the Morphology and Physical and Electrical Properties

A modified clay was used to prepare waterborne polyurethane (WPU)/clay nanocomposite dispersions in this study. XRD and TEM examinations indicate that the clay platelets are exfoliated. Moreover, AFM phase image of the WPU/clay film shows that the isolated clay-rich phases are surrounded by a continuous WPU-rich phase. Clay does not influence the location and peak broadness of the Tg of soft segment domains in the WPU/clay films. However, clay raises the Tg of hard segment domains, which is probably due to the ionic interactions between the clay and hard segments of WPU. These ionic interactions could be evidenced by IR spectra of the WPU/clay nanocomposites. The WPU/clay dispersion with higher clay content exhibits a larger average particle size, a less negative zeta potential, and a higher viscosity, whereas the corresponding cast film possesses lower volumetric and surface electrical resistances. The volumetric and surface electrical resistances of the WPU/clay nanocomposite film are nearly identical. This indicates that clay platelets do not migrate to the surface regions of WPU/clay films because of the above-mentioned ionic interactions. In addition, the incorporation of clay is also capable of enhancing the thermal resistance and tensile properties of WPUs dramatically.

Adhesion properties of polyamide-6 fibres treated by dielectric barrier discharge

The adhesion properties of polyamide-6 fibres are improved by dielectric barrier discharge (DBD) treatment in helium at atmospheric pressure. These properties are usually expressed by means of the adhesion work, the surface energy and its polar and dispersive components, the polarity etc. Since the calculation of these quantities requires measurement of the contact angles between a liquid drop and the fibres, we use here a theoretical method that allows calculating the contact angle on cylindrical surfaces using measurable geometrical dimensions of the drop. Also, an experimental arrangement for contact angle measurement, based on the image-processing system, is described and the quantities related to the adhesion properties are estimated. The adhesion work, the surface energy and the surface polarity are increased after treatment, demonstrating the efficiency of the DBD technique in improving the adhesion properties of the polymer fibres. Moreover, AFM topography images show modifications at the micro and nanostructure level, relevant to enhanced adhesion. Microparticles and oligomers are removed from the treated fibres, leaving a cleaner and smoother surface at the microscale. Furthermore, a new structure is observed at the nanoscale, with an increased roughness and a larger surface area, favouring the adsorption. The AFM phase images confirm the cleaning of the fibre surface and also a better compositional homogeneity after DBD treatment.

Structure and thermal properties of titanium dioxide-polyacrylate nanocomposites

UV curable, transparent acrylic resin/titania organic–inorganic hybrid films were prepared by controlled hydrolysis of titanium tetrabutoxide (TTB) in Span85/Tweeen80 reverse micelles and subsequent in situ photopolymerization of acrylic monomers. The qualitative content of TiOTi group and TiOH was monitored by FTIR spectrum, and the thermal stability and film surface were characterized by TGA, DSC and AFM, respectively. TGA data show that TiO2-hybrid films can upgrade the decomposition onset temperature and the temperature at which there is a maximum mass loss rate (Tmax). DSC data show that prolonged exposure to UV light and post-thermal treatment can decrease the Tonset and Tpeak of the exothermal peak and the condensation temperature between TiOH, and increase ΔH of the exothermal peak. FTIR spectra show the presence of two νs(COO-) modes with the νa-νs splitting magnitude (Δν∼87 cm-1 and 148 cm-1, respectively), suggesting that acrylic acid acts as a bidentate coordination mode and therefore chemical linkages exist between inorganic and organic phases. AFM phase images showed the presence of inorganic domains, with mean size of 21.6 nm–28.8 nm, were uniformly dispersed in the polymeric networks.

Study of EVOH based single ion polymer electrolyte: Composition and microstructure effects on the proton conductivity

A comb-like EVOH based single ion polymer electrolyte (EVOH- g-SPEG) was synthesized by sulfonification of EVOH grafts 2-(2-chloroethoxy) ethanol (C 4H 9O 2Cl)/2-[2-(2-chloroethxy) ethoxy] ethanol (C 6H 13O 3Cl) with 1, 3-propane sultone. The main chain of the comb-like polymer is hydrophobic polyethylene segments; the side chain is hydrophilic poly (ethylene glycol) (PEG) segment, which can solubilize large amounts of inorganic salts. The sulfonic acid group was introduced onto the end of the PEG side chain. The acid form of SPE was successfully obtained by being dialyzed from the products with acid solution. The saturation water sorption of EVOH- g-SPEG membrane increased with the side chain length and the immersion time. The XRD results indicate that the water in SPE membrane region can destroy the membrane crystalline structure and the water absorption membranes are nearly amorphous. AFM phase images of the hydration membranes clearly show the hydrophilic domains, with sizes increasing from 10 to 35 nm as a function of the side chain length. A phase inversion could be observed when n ≥ 5, which was consistent with a rapid increase in water absorption. And the ion conductivity is also measured by AC impedance. The conductivity is greatly influenced by ion exchange capacity and water sorption. The comb-like EVOH- g-SPEG polymer electrolyte grafts with 2 PEG side chain provides the highest ionic conductivity (1.65 × 10 − 3 S cm − 1 ). The comb-like polymer could be a candidate as new polymeric electrolyte material for fuel cells and other electrochemical devices.

A comparative study of the structure–property behavior of highly branched segmented poly(urethane urea) copolymers and their linear analogs

The solid-state structure–property behavior of highly branched segmented poly(urethane urea) (PUU) copolymers and their linear analog was investigated. A limited study of their solution rheological behavior was also undertaken. The linear PUUs were synthesized by the two-step prepolymer method, whereas the oligomeric A 2+B 3 methodology was utilized to synthesize the highly branched materials. The soft segments (SS) were either poly(tetramethylene oxide) (PTMO) or poly(propylene oxide) (PPO). All copolymers utilized in this study, with one exception, contained 28 wt% hard segment (HS) content. DMA, SAXS, and AFM studies indicated that the linear as well as the highly branched PUUs were microphase separated. The SS T g of the highly branched PUUs was nearly identical to that of their respective linear analogs. However, the linear copolymers exhibited broader and less temperature sensitive rubbery plateaus, both attributed to one or both of two reasons. The first is better hydrogen bonding organization of the HS phase as well as greater HS lengths than in the highly branched analogs. The second parameter is that of a potentially higher chain entanglement for the linear systems relative to the branched analogs. Tapping-mode AFM phase images confirmed the microphase morphology indicated by SAXS and DMA. Ambient temperature strain-induced crystallization was observed in the PUU based on PTMO 2040 g/mol at a uniaxial strain of ca. 400%, irrespective of the chain architecture. Stress–strain, stress relaxation, and mechanical hysteresis of the highly branched copolymers were in general slightly poorer than that of their linear analogs. Ambient temperature solution viscosity of the highly branched materials in dimethyl formamide was substantially lower that that of the linear samples of nearly equal molecular weight.

Probing the Hard Segment Phase Connectivity and Percolation in Model Segmented Poly(urethane urea) Copolymers

Soluble model segmented poly(urethane urea)s (PUU) with or without hard segment (HS) branching were utilized to explore the importance of hydrogen bonding and chain architecture in mediating the long-range connectivity of the HS phase. The HS content of all the PUU copolymers was 22 wt %, and the soft segment (MW 970 g/mol) was a heterofed random copolymer of 50:50 ethylene oxide:propylene oxide, which possesses a single terminal hydroxyl group (monol). An 80:20 isomeric mixture of 2,4- and 2,6-toluene diisocyanate, 4,4',4''-triphenylmethane triisocyanate and water were utilized during the chain extension step of the synthesis to incorporate HS branching. DSC and SAXS results on the final plaques indicated that the samples were still able to establish a microphase morphology even in the presence of the highest extent of HS branching utilized in the study. The tapping-mode AFM phase image of the PUU sample without HS branching exhibited the presence of long ribbonlike hard domains that percolated through the soft matrix. The long-range connectivity of the HS was increasingly disrupted with higher levels of HS branching. Accompanying such disruption was a systematic mechanical softening of the PUU samples. FT-IR indicated that incorporation of HS branching disrupted the hydrogen-bonded network within the hard phase. These results demonstrate the importance of hydrogen bonding and chain architecture in mediating the long-range connectivity and percolation of the HS and achieving dimensional stability.

Curvature radius analysis for scanning probe microscopy

SFM images are the result of interactions between a sharp tip with a quasi-spherical apex and the surface of a sample. As a consequence, SFM images are highly influenced by the tip geometry especially when the surface features are sharper than the probe. For topographic images, this phenomenon is known as dilation. It can result in an image where the features reflect the tip apex characteristics rather than the sample’s surface. Mathematical algorithms are used to show the possibility of computing a curvature image from a topographic image to deduce the radius of the tip. In addition, the interpretation of a surface image obtained with modes other than topography can be achieved by comparing this image to a curvature image. As an example, the Tapping Mode AFM phase image contrast can show similarities or discrepancies with the curvature image contrast, leading to a direct relation between the phase and a topographic or a physical contrast.

Atomic Force Microscopy Studies of Mesoscopic Membranous Bubbles on Monolayers Derived from SiCl3-Terminated Carbosilane Dendrons on Mica

Monolayers of dendrimers were prepared on mica by spin-coating of the second generation carbosilane dendrons with 9 SiCl(3) periphery groups. AFM images of the films showed the presence of soft yet robust, dome-shaped features with a base diameter of 100-2000 nm. The apparent height of the features, ranging from 10 to 200 nm, rapidly reduced under increasing compression force, eventually to the same value ( approximately 2.5 nm) corresponding to a bilayer of the flattened dendrons. The change in shape of the features in response to the compression force from the AFM tip was fully reversible, indicating that the features were robust. The contrast of the features in the tapping mode AFM (TMAFM) phase images flipped at a setpoint ratio of approximately 0.55. In contrast to the reported amplitude vs displacement (A/z) curves for compliant materials, A/z curves of the features showed that the reduction of amplitude was larger than the tip displacement as if the cantilever tip were repelled by the soft features. This result cautions the use of amplitude/phase vs displacement (APD) curves for interpreting TMAFM images and for optimizing conditions for TMAFM imaging of very soft and "sticky" surfaces. On the basis of the AFM studies, we believe that the dome-shaped features are membranous air bubbles. The membranes of the bubbles were probably composed of a bilayer of the dendron molecules bound through the peripheral silanol groups. The bilayer could be formed by self-assembly of the molecules on top of the air bubbles entrapped at the monolayer/solution interface during spin-coating.

Fabrication of Metal‐Block‐Copolymer Composite Films by a Palladium‐Catalyzed Electroless Nickel‐Plating Process

AbstractSummary: Homogeneous films of PI‐b‐PDMAEMA are prepared on top of silicon (100) substrates. The free film surface shows microdomains of PDMAEMA within a PI matrix. These microdomains act as templates for the highly site‐selective synthesis of metal nanoparticles via palladium‐catalyzed electroless nickel plating. The particle formation is studied by atomic force microscopy in tapping mode and implications for a redox reaction and a nanoparticle growth mechanism on the surface of nanopatterned films are discussed.Chemical structure of the PI‐b‐PDMAEMA copolymer and AFM phase image of a PI‐b‐PDMAEMA film on Si (100) substrate.magnified imageChemical structure of the PI‐b‐PDMAEMA copolymer and AFM phase image of a PI‐b‐PDMAEMA film on Si (100) substrate.

Study of Langmuir and Langmuir−Blodgett Films of Odorant-Binding Protein/Amphiphile for Odorant Biosensors

To make ultrathin films for the fabrication of artificial olfactory systems, odorant biosensors, we have investigated mixed Langmuir and Langmuir-Blodgett films of odorant-binding protein/amphiphile. Under optimized experimental conditions (phosphate buffer solution, pH 7.5, OBP-1F concentration of 4 mg L(-1), target pressure 35 mN m(-1)), the mixed monolayer at the air/water interface is very stable and has been efficiently transferred onto gold supports, which were previously functionalized by self-assembled monolayers (SAMs) with 1-octadecanethiol (ODT). Atomic force microscopy and electrochemical impedance spectroscopy were used to characterize mixed Langmuir-Blodgett (LB) films before and after contact with a specific odorant molecule, isoamyl acetate. AFM phase images show a higher contrast after contact with the odorant molecule due to the new structure of the OBP-1F/ODA LB film. Non-Faradaic electrochemical spectroscopy (EIS) is used to quantify the effect of the odorant based on the electrical properties of the OBP-1F/ODA LB film, as its resistance strongly decreases from 1.18 MOmega (before contact) to 25 kOmega (after contact).

Surface morphology and chemistry of Prunus laurocerasus L. leaves: a study using X-ray photoelectron spectroscopy, time-of-flight secondary-ion mass spectrometry, atomic-force microscopy and scanning-electron microscopy

The surface properties of the plant cuticle play a crucial role in plant-pathogen interactions and the retention and penetration of agriculturally important chemicals. This paper describes the use of X-ray photoelectron spectroscopy (XPS), time-of-flight secondary-ion mass spectrometry (ToF-SIMS), tapping-mode atomic force microscopy (TM-AFM) and scanning electron microscopy (SEM) to determine surface-specific chemical and material properties of the adaxial surface of Prunus laurocerasus L. leaves. XPS data, derived from the uppermost few nanometres (< 10 nm) of the leaf surface, were consistent with the wax components and functionality known to be present within the waxes. ToF-SIMS provided molecular speciation from the outermost monolayer of the leaf surface, indicating the importance of a family of acetates with chain lengths ranging from C20 to C34. The presence of alkanes with C29 and C31 chain lengths was also confirmed. SEM and TM-AFM topography images revealed a textured granular surface, while simultaneously recorded AFM phase images revealed heterogeneous material properties at the nanoscale. The relevance of these data to plant cuticle development, allelochemistry and agrochemical delivery is discussed.

Synthesis and Characterization of Epoxidized Styrene‐Butadiene Block Copolymers as Templates for Nanostructured Thermosets

AbstractSummary: A wide range of epoxidized styrene‐block‐butadiene block copolymers were synthesized using hydrogen peroxide in the presence of an in situ prepared methyltrioctylammoniumtetrakis(diperoxotungstate)phosphate as the catalyst system in a water/dichloroethane biphasic system. 1H NMR and FT‐IR spectra revealed that the epoxidation procedure led mainly to 1,4‐epoxidized butadiene units. GPC analysis showed that epoxidation changed the copolymer architecture into stars with fewer arms. The copolymers showed two glass transition temperatures in accordance with the phase separation for block copolymers and, as revealed in AFM images, the self‐assembly takes place on a nanometer scale. Moreover, epoxidized styrene‐butadiene (SB) copolymers showed improved miscibility with epoxy monomers leading to self‐assembled nanostructures in the uncured state. This allows them to be used as templates for nanostructured resins.TM‐AFM phase images for: a) 40 mol‐% epoxidized copolymer (scale bar = 300 nm); b) uncured blend containing 50 wt.‐% DGEBA in 50 mol‐% epoxidized copolymer (scale bar = 1 μm), after annealing at 80 °C in vacuum for 3 h.imageTM‐AFM phase images for: a) 40 mol‐% epoxidized copolymer (scale bar = 300 nm); b) uncured blend containing 50 wt.‐% DGEBA in 50 mol‐% epoxidized copolymer (scale bar = 1 μm), after annealing at 80 °C in vacuum for 3 h.

Hard segment connectivity in low molecular weight model ‘trisegment’ polyurethanes based on monols

Low molecular weight model trisegmented polyurethanes based on monofunctional polyols, or ‘monols’, with water-extended toluene diisocyanate (TDI) based hard segments (HS) are investigated. The formulations of the materials generated are similar to those of flexible polyurethane foams with the exceptions that the conventional polyol is substituted by an oligomeric monofunctional polyether of ca. 1000 g/mol molecular weight; and no surfactant is utilized. Plaques formed from these model systems are shown to be solid materials at ambient even at their relatively low molecular weights of 3000 g/mol and less. SAXS, DSC, and AFM are utilized to investigate the microphase separated morphologies of the samples generated. WAXS results show that the local packing of the HS is of a similar nature as that in actual flexible polyurethane foams. AFM phase images, for the first time, reveal the ability of the HS to self-assemble through bidentate hydrogen bonding and form lath-like percolated structures, resulting in solid plaques, even though the overall volume of the system is well dominated by the two terminal liquid-like polyether segments.

Morphology Evolution in Nanoscale Light-Emitting Domains in MEH-PPV/PMMA Blends

The compositional evolution of the morphology of blends of a light-emitting conjugated polymer MEH-PPV [MEH-PPV = poly(2-methoxy-5-(2‘-ethylhexyloxy)-p-phenylenevinylene)] with poly(methyl methacrylate) (PMMA) on a PEDOT/PSS [PEDOT/PSS = poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)] coated glass substrate is reported. Results of AFM (topographic and phase images), optical microscopy, fluorescence microscopy, and photographs of the emission from light-emitting devices (LEDs) illustrate this morphological evolution. The 5 wt % compositional study demonstrates the stability of morphology for the blend films dried in air, vacuum-dried at room temperature, and vacuum-dried at 80 °C. Annealing above the Tg (106 °C) of PMMA caused significant dewetting between the blend and the PEDOT/PSS. At low MEH-PPV compositions, isolated light-emitting domains ranging in diameter from 300 to 900 nm and ∼100 nm in thickness were observed. At approximately 50 wt % MEH-PPV, a phase inversion was observed and a bicon...

Nanostructural features of demosponge biosilica

Recent interest in the optical and mechanical properties of silica structures made by living sponges, and the possibility of harnessing these mechanisms for the synthesis of advanced materials and devices, motivate our investigation of the nanoscale structure of these remarkable biomaterials. Scanning electron and atomic force microscopic (SEM and AFM) analyses of the annular substructure of demosponge biosilica spicules reveals that the deposited material is nanoparticulate, with a mean particle diameter of 74 ± 13 nm. The nanoparticles are deposited in alternating layers with characteristic etchant reactivities. Further analyses of longitudinally fractured spicules indicate that each deposited layer is approximately monoparticulate in thickness and exhibits extensive long range ordering, revealing an unanticipated level of nanoscale structural complexity. NMR data obtained from differentially heated spicule samples suggest that the etch sensitivity exhibited by these annular domains may be related to variation in the degree of silica condensation, rather than variability in the inclusion of organics. In addition, AFM phase imaging in conjunction with results obtained from HF and alkaline etching experiments suggest that at various stages in spicule biosynthesis, regions of unusually low silica condensation are deposited, indicating a possible interruption in normal spicule formation. While this discovery of nanoparticulate silica aggregation in demosponge skeletal elements is likely to reflect the intrinsic kinetic tendency of silica to form such particles during polycondensation, the heirarchical organization of these nanoparticles is biologically unique.

Investigation of surface changes of nanoparticles using TM-AFM phase imaging.

Tapping-mode AFM (TM-AFM) phase imaging was utilized to characterize the surface changes of nanosize particles, in regard to the effects of different amounts of condensed water and organic coatings on particle surfaces. Model nanoparticles were continuously examined under various relative humidity (RH) levels by concurrently obtaining both topographic and phase images. The condensed water appeared to soften particle surfaces and to increase tip-sample attractive interaction over relatively stiff surfaces, which were shown with dark phase contrasts and negative phase shift values in phase images. Under high RH, a massive amount of water gave the particles a droplet-like surface, which reversed the original negative phase shifts to positive values with bright contrasts. Glutaric-acid coatings provided a compliant surface with high viscosity resulting in a dark phase contrast, whereas water droplets containing relatively low viscosity gave a bright phase contrast and positive phase shift. Overall, our results show that it is essential to describe the physical properties of a sample surface as solid, soft, or droplet-like material in order to derive a meaningful understanding of the surface changes of nanosize particles based on TM-AFM phase images. In contrast to other phase imaging studies, this work clearly correlates continuous surface changes with phase images, demonstrating a promising approach to characterize environmental nanoparticles.