Surface crystallization of poly(ethylene terephtalate) studied by atomic force microscopy

Poly(ethylene terephtalate) (PET) crystallization was shown by atomic force microscopy (AFM) to occur at 85 °C in the first few nanometers near the polymer-air interface. The surface was fully transformed into spherulites after 30 min, while no signs of bulk crystallization were observed by FTIR. All the observed spherulites presented a nucleation centre, indicating that the crystallization process started at the surface of the film. Tapping mode AFM confirmed that the spherulites were not covered by an amorphous layer. The most probable explanation is a decrease of T_g near the surface. Due to the poor crystallization conditions, the constitutive units of the spherulites were small crystalline blocks. By changing the annealing time, it was possible to produce PET surfaces with different surface fractions consisting of semi-crystalline material (spherulites) and amorphous matrix. This provided a controlled surface heterogeneity on the submicrometer scale, with a contrast in terms of stiffness, roughness and swelling by organic solvents.     

Surface structure of Kevlar® fiber studied by atomic force microscopy and inverse gas chromatography

Kevlar® fiber surface structure was primarily and directly observed in the filament configuration by the tapping mode atomic force microscopy (AFM). The microfibrils feature was observed with an average width of 500 nm, composed of various types of periodical units of an average size 50 nm in a pleating appearance. At the less crystalline spot on the Kevlar® fiber surface, the periodical organizations exhibit the skin-core-like differentiation. In contrast, at the highly crystalline spot, the periodicity is more uniformly arranged by a rectangular network manner. Inverse gas chromatography (IGC) was used as a tool to investigate the surface structure heterogeneity by calculating the surface energy of different types of probes adsorbed on the Kevlar® fiber surface. The energy sites distribution plot of n-hexylamine adsorption at finite dilution exhibits a two-adsorbing-peaks curve. At the higher energy site, a possible hydrogen-bonding interaction was proposed between n-hexylamine and oxygen-containing groups formed at the less crystalline surface. According to the AFM and IGC results, a Kevlar® fiber surface organizations model at the nanometer scale was proposed.     

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.     

Introducing open films of nanosized cellulose—atomic force microscopy and quantification of morphology

A method for preparing open, sub-monolayer cellulose films on a silicon substrate is introduced, and the open films were quantified using the three-dimensional information from atomic force microscopy (AFM) height images. The preparation method is based on spin coating low concentrations of trimethylsilyl cellulose (TMSC) on silicon and hydrolysing the TMSC to cellulose using a vapour phase acid hydrolysis. AFM showed that the surfaces consist of nanosized cellulose patches which are roughly 50-100 nm long, 20 nm wide, and 1 nm high. The volume of the cellulose patches was quantified. Examination of the cross section of the cellulose patches revealed that the exaggeration of the lateral dimensions by the AFM tip is small enough to account for a mere ±2% error in the volume quantification. Pilot experiments showed that the volume of the cellulose was largely restored in a wetting/drying cycle but the morphology changed considerably. Because of their small size, the cellulose patches provide a novel approach for interpretation on the molecular architecture of cellulose.     

Single polymer chain rubber elasticity investigated by atomic force microscopy

A method, which is called as ‘nanofishing’, enabled us to stretch a single polymer chain adsorbed on an Au(111) substrate with picking it at its two thiol-modified termini using atomic force microscope equipped with a gold-coated probe. A force-extension curve obtained for a single polystyrene chain in a Θ solvent (cyclohexane) showed a good agreement with a so-called worm-like chain model, and thus gave microscopic information about entropic elasticity. Solvent effects on the statistical properties of single polymer chains were also investigated. For example, the second virial coefficient in cyclohexane was determined at a single polymer basis, which was almost comparable with a simple Flory's lattice model.     

Crystal growth of faujasite observed by atomic force microscopy

Atomic force microscopy (AFM) was employed to reveal the crystal growth mechanism of faujasite. The seeded growth of faujasite in dilute aluminosilicate solutions was observed. Two solutions were prepared: one was near equilibrium with the seed and the other was in growth mode for the seed. Morphological changes during the seeded growth were observed along with the growth period at the same position on the seed (ex situ observation). These observations showed the rough surface of the seed changing into a well-ordered (1 1 1) face in the solution that was near equilibrium with the seed. This surface ordering proceeded by thermodynamic stabilization of the top-surface structure via the mutual transfer of aluminosilicate species between the solution and solid phases, and/or by the dissolution of the amorphous matter on the seed. In growth mode, most of the top surfaces of the seeded crystals were terminated by double six-membered rings (D6Rs), while some were by complete or incomplete sodalite cage. These results showed that aluminosilicate species equal to or smaller than 6R contributed to the crystal growth.     

In situ atomic force microscopy of the melting of melt-crystallized polyethylene

High temperature AFM is used to observe the melting of polyethylene lamellae crystallized from the melt in situ in real-time. Both oriented and un-oriented samples are observed. The melting of shish-kebab structures, including revealing the bare oriented shish, is achieved. Lamellae are observed to melt from their edges, and this is proposed to be due to the inherent higher mobility at the crystal edges rather than differences in stability within the crystal due to different levels of post crystallization perfecting. Observation of the melting of structures that have been observed during growth confirms that material crystallized at lower temperatures melts first, followed by material crystallized in confined geometries, and finally the rest of the isothermally crystallized lamellae that melt back from the edges with an apparently random morphology that is not the reverse of the growth process. In situ observation during melting is confirmed as an alternative and complementary technique to etching to reveal behaviour during crystallization when crystallization is too rapid for in situ observation.     

The ultrastructure of spruce kraft pulps studied by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS)

Surface properties of spruce (Picea abies) kraft pulps cooked for different times and further OD₀E₁D₁E₂D₂-bleached were investigated with atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). A rough correlation between the increasing relative amount of the fibrillar surface structure in AFM images and increasing O/C atomic ratio in XPS-spectra was found with proceeding delignification. At the end of cooking (120 min) only about 1/3 of the fibre surface consisted of cellulose. The detailed analysis of the relative peak areas of the different C1s components in the XPS-spectra indicated that the granules at the beginning of cooking at 170 °C consisted mainly of lignin and extractives. The analysis also showed that different steps of the bleaching sequence were quite specific in removing structural components. Furthermore, the lignin removal was shown not to result automatically in increased fraction of exposed cellulose surface, but could also lead in increased relative amount of surface extractives. Evidence for the high surface content of hemicelluloses for the D2-stage sample was observed. Hemicelluloses with both fibrillar and amorphous morphology were found to be present.     

Observations and morphological analysis of supermolecular structure of natural bitumens by atomic force microscopy

The supermolecular structures of natural bitumens of the thermal consequent row asphaltites ⇒ lower kerites (albertites) ⇒ higher kerites (impsonites) ⇒ anthraxolites from the Timan-Pechora petroleum province and Karelian shungite rocks, Russia, were studied in details.The experimental technique used was atomic force microscopy (AFM), following fracture preparation. The element distribution of the sample surfaces was analyzed by an X-ray microanalyser “Link ISIS”, combined with a scanning electron microscope (SEM).In this work, we characterized the supermolecular evolution of natural solid bitumens in the carbonization sequence by quantitative parameters. We showed that supermolecular structure can be important in defining to which classification group solid bitumens belong.     

Morphology of extruded high-density polyethylene pipes studied by atomic force microscopy

Atomic force microscopy (AFM) was used to study the structure of extruded polyethylene (PE) pipe. During extrusion, the outer surface of the pipe was cooled with water. Two cross sections, parallel and transverse to the extrusion direction, were examined in order to spatially follow the structural development during extrusion. The morphology revealed was spherulitic, and the spherulites had a mostly banded appearance when viewed under the AFM. We were not able to distinguish an oriented skin layer at the surface of the pipe, either by AFM or polarizing microscopy. The changes in the pipe's structure resulting from the cooling conditions were found to be rather gradual, and no clearly defined zones were observed. A slight orientation towards the extrusion direction was detected only in the area of the pipe crystallized under the lowest degree of undercooling. Measured spherulitic size, band period, and lamellae thickness showed a gradual increase in their values from the cooled to the noncooled surface of the pipe. Transmission electron microscopy (TEM) was used to verify the band period and lamellae thickness measurements done by AFM. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 515–523, 1997     

Micromechanical properties of glassy and rubbery polymer brush layers as probed by atomic force microscopy

Carboxylic acid terminated polystyrene and polybutylacrylate were grafted from melt onto a silicon substrate modified with the epoxysilane monolayer. The tethered layers fabricated from polymers of different molecular weights are smooth, uniform, mechanically stable, and cover homogeneously the modified silicon surface. Micromechanical properties of the dry glassy and rubbery brush layers were measured with atomic force microscope. We observed that for the PS layers with the thickness higher than 7 nm, the average value of the elastic moduli reached 1.1 GPa, which is close, but still lower than the expected for bulk polymer. The elastic modulus of PS polymer brush layers dramatically depends upon molecular weight and follows the inverse law with segment molecular weight, M_c of 18,000 known for bulk PS. This result indicates that the process of the formation of the physical network within polymer melt of chains tethered to a solid substrate is similar to that occurring in unconstrained polymer melt. Under these conditions, three PS brush layers studied in this work represent different cases of chains without stable entanglements for M<M_c as well as chains with stable entanglements for brushes with M∼M_c. This transition shows itself in significant reduction of the compliance reflected in twofold increase in elastic modulus. Our estimation predicts that modest lowering of ‘limiting’ elastic modulus of 1.4 GPa can be expected for thicker polymer brushes.     

Temperature dependent loss tangent measurement of polymers with contact resonance atomic force microscopy

The loss tangent of individual components in a blend of polypropylene (PP) and polystyrene (PS) is measured as a function of temperature with contact resonance atomic force microscopy. The loss tangent is calculated directly from the experimentally obtained contact resonance frequency, cantilever quality factor and other operating parameters. The temperature dependent variation of the loss tangent, measured at the high frequency of AFM measurements, shows peaks at different temperatures for the different polymer materials. The loss tangent peak at approximately 53 °C for PP is identified as an alpha peak signifying crystal relaxation while the loss tangent peak at approximately 75 °C for PS is identified as a glass transition.     

Latex film formation: atomic force microscopy and theoretical results

We have critically examined the kinetics of latex film formation using an atomic force microscope to obtain corrugation height data as a function of time, temperature, molecular weight, particle size, etc. The results show that the film forming process obeys viscoelastic time/ temperature superposition principles, thus indicating a direct relationship between the kinetics of film formation and rheological properties. Film formation kinetics are examined under ‘wet’ and ‘dry’ conditions, with film formation occurring almost ten-times faster under wet conditions than dry. This proves for the latex system examined that capillary pressure from the water meniscus is the dominant driving stress for film formation. Past theories for latex film formation are reviewed, and problems and deficiencies are noted. A new theory for film formation from a dry latex system is presented, which is based on the use of the Boltzmann Superposition Principle to relate the changing stress and strain fields as the latex particles deform. The predictions of theory and the experimental data show excellent agreement over nearly four decades of time.     

Measuring local viscoelastic properties of complex materials with tapping mode atomic force microscopy

Tapping mode atomic force microscopy is a technique to measure the topography and properties of surfaces involving a micro-cantilever with a tip at one end that is excited into an oscillation near its resonance frequency. The phase lag between the excitation signal and the observed oscillation is sensitive to local mechanical properties under certain experimental conditions. We have found that by using silicon as an internal standard reference surface we can unambiguously relate the phase lag to local viscoelastic properties of a polymeric material. A model describing this relation has been built, validated with experimental data and finally inverted such that it can be used to determine local properties. This allows us to measure high frequency local viscoelastic properties on length scales as small as several nanometers. This technique works well for relatively compliant polymer surfaces with a shear modulus less than about 1 GPa.     

Atomic force microscopy study of comb-like vs. arborescent graft copolymers in thin films

This work deals with the study of comb-like vs. arborescent grafted copolymers made of poly(chloroethyl vinyl ether)-g-polystyrene (PCEVE-g-PS). We describe how the molecular architecture of the branched polymers affects their nanoscale organization in thin films, as observed using atomic force microscopy. The results indicate that modifying the molecular architecture from a ‘generation-zero’ comb-like (PCEVE-g-PS) to a ‘first-generation’ hyperbranched (PCEVE-g-(PS-b-PCEVE-g-PS)) architecture strongly modifies the observed geometrical parameters of the molecules, in good agreement with the expected evolution of the molecular dimensions and the corresponding data obtained in solution.The surface organization of the (PCEVE-g-PS) copolymer molecules is also strongly conditioned by the interplay between the molecule-substrate interactions and the molecule-molecule interactions, leading to different possible orientations of the lateral branches with respect to the surface and thus to different final morphologies.     

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 phase separations of PMMA/SAN blends investigated by atomic force microscopy

The thin films of poly(methyl methacrylate) (PMMA), poly(styrene-co-acrylonitrile) (SAN) and their blends were prepared by means of spin-coating their corresponding solutions onto silicon wafers, followed by being annealed at different temperatures. The surface phase separations of PMMA/SAN blends were characterized by virtue of atomic force microscopy (AFM). By comparing the tapping mode AFM (TM-AFM) phase images of the pure components and their blends, surface phase separation mechanisms of the blends could be identified as the nucleation and growth mechanism or the spinodal decomposition mechanism. Therefore, the phase diagram of the PMMA/SAN system could be obtained by means of TM-AFM. Contact mode AFM was also used to study the surface morphologies of all the samples and the phase separations of the blends occurred by the spinodal decomposition mechanism could be ascertained. Moreover, X-ray photoelectron spectroscopy was used to characterize the chemical compositions on the surfaces of the samples and the miscibility principle of the PMMA/SAN system was discussed.     

Growth process of homogeneously and heterogeneously nucleated spherulites as observed by atomic force microscopy

A poly(bisphenol A octane ether) (BA-C8) was synthesized. The isothermal spherulitic growth process was studied in situ using atomic force microscopy (AFM) at room temperature. For spherulites formed by homogeneous nucleation, the growth process includes the birth of a primary nucleus, the development of a founding lamella and the growth of the founding lamella into a spherulite. An embryo below a critical size is unstable. A stable embryo grows into a founding lamella. There is only one founding lamella in each spherulite. All other lamellae originate from this founding lamella. Two eyes can be seen at the center of a spherulite. For spherulites formed through heterogeneous nucleation, many lamellae grow at the nucleus surface and propagate outward radially. The spherulites acquire spherical symmetry at the early stage of crystallization. No eyes are found for this kind of spherulites.     

Study of the surface of the water treated cellulose acetate membrane by atomic force microscopy

Water treated cellulose acetate (CA) membrane's surface was studied by atomic force microscopy (AFM). It was observed that when CA membrane (water untouched) was treated with water, the morphology of the surface change was detected by AFM. The roughness parameter of the surface was increased. No significant change was observed on the surface on drying the water treated membrane at room temperature for four days. The results were discussed on the basis that CA membrane contains continuous channels (network pores), which were formed in water-swollen polymer matrix. These water channels are responsible for the rejection of salt in reverse osmosis (RO) phenomenon.     

An atomic force microscopy study of ozone etching of a polystyrene/polyisoprene block copolymer

The ozone etching of a commercial poly(styrene)/poly(isoprene) (PS/PI) block copolymer (Kraton D1117) was studied by atomic force microscopy. The copolymer contains 17% PS and forms a cylindrical phase in the melt. The copolymer dewetted when spin coated onto a silicon wafer but the film was stable on a grown silicon oxide layer. The structure of the stripe pattern formed was examined on substrates with different oxide layer thicknesses (surface energies). Finally etching by ozone was investigated. For low ozone doses, no degradation of polymer was observable. Extended ozone treatment resulted in more obvious degradation, but the etching was non-selective.