S. N. Magonov

Atomic force microscopy on epitaxially crystallized isotactic polypropylene

SummaryContact faces of thin films of α-phase isotactic polypropylene epitaxially crystallized on benzoic acid are examined by atomic force microscopy (AFM). The AFM images reveal the lamellar structure as well as the methyl side group pattern, thus enabling the discrimination between two structurally different contact surfaces in favour of the ‘four’ face type, with one methyl group per helix turn exposed. Furthermore it has been verified, that the observed helices are right-handed.

Atomic force microscopy on polymers and polymer related compounds

SummaryResults of Atomic Force Microscopy (AFM) on carbon fibers from polyacrylonitrile and pitch are presented in comparison with Scanning Electron Microscopy (SEM) and Scanning Tunneling Microscopy (STM) images. Single fiber surfaces and their crosssections have been imaged on scales from microns to nanometers. Morphological details beyond the resolution of SEM were revealed by AFM and STM. Grain-type structure was verified on surface of numerous nanofibrils orlented along the main fiber direction. Grains are bigger on pitch-based fibers generally, and on fibers of both types after treatment at higher temperatures. In the atomic scale AFM images traces of graphitic structure were recorded. AFM artefacts on rough surfaces are demonstrated. ac19920414

Characterization of surfaces of carbon fibres by scanning tunneling microscopy

SummaryThe experimental results obtained by scanning tunneling microscopy (STM) studies of different carbon fibres are presented and discussed. The comparative analysis of the STM images at scales from hundreds of nanometers down to atomic scale reveals the differences of surface features for carbon fibres processed from different precursors, polyacrylonitrile fibres and pitch. The high temperature treatment of carbon fibres — the so-called graphitization process — as used to improve the stress modulus induces drastically increased ordering phenomena at the atomic level. Structural information obtained by STM on the surface of the fibres as well as in their cross sectional areas is discussed in comparison with known results of diffraction studies. STM appears to be the new powerfull technique for the detailed structural studies of surfaces of carbon fibres. The perspectives of these studies are under discussion.

Structural and scanning microscopy studies of layered compounds MCl3 (M = Mo, Ru, Cr) and MOCl2 (M = V, Nb, Mo, Ru, Os)

Abstract Structural investigations on trichlorides (α- and β-MoCl3, solid solutions Ru1−5CrxCl3) and oxide dichlorides MOCl2 (M = V, Nb, Mo, Ru, Os) show that layer structures with stacking faults can be solved without evaluation of the diffuse parts of scattering. The combination of X-ray methods and atomic scale STM/AFM imaging yields information about the pattern of the surface atoms and the metal sublattice between the halide layers. Metal-metal bonds, structural distortions of coordination polyhedra and cation distribution in solid solutions have been determined. The correct layer symmetry, obtained by STM/AFM methods, was used in the X-ray structure analysis by single crystal and powder diffraction. Disorder problems due to stacking faults of the layers could be treated in that way.

Optimization of experiment in scanning force microscopy of polymers

Abstract A scanning force microscopy (SFM) study of an oriented polyethylene (PE) tape and a microporous isotactic polypropylene (iPP) film was conducted in the contact and tapping mode. In sub-water measurements on polymer surfaces with minimal tip-sample interactions, the images obtained in both modes correctly reproduce surface morphology and nanostructure. An advantage of the tapping mode in non-destructive imaging was shown in the observation of a weakly bound skin layer on the PE tape. This layer is destroyed during scanning with higher forces; however, its removal allows one to examine the core nanostructure of this material. Ambient-condition SFM measurements in both modes are accompanied by stronger tip-sample interactions. This leads to an overestimation of the dimensions of surface features and to a larger contribution of surface hardness to the image contrast.

Scanning force microscopy of oriented iso- and syndiotactic polypropylene films

SummaryDifferent scanning force microscopy (SFM) modes were applied for the examination of drawn films of isotactic and syndiotactic polypropylene, iPP and sPP. Uniaxially stretched films with a draw ratio λ=6 were studied at ambient conditions, and under water. In iPP films striated patterns of 40–50 nm in width, which are oriented along the stretching direction, exhibit shish-kebab morphology. The dominating nanoscale features are nanofibrils of 10–15 nm in width, and lamellar platelets, which are 40–50 nm in width and 30–35 nm in length. In many places lamellar platelets are closely packed, and they form periodical sequences with a repeat distance of 30–35 nm, which is consistent with thelong period. Similar and differing morphological features were found comparing the images of iPP and sPP films. Extended fibrillar patterns of sPP exhibit similar width in the 40–50 nm range, while the lamellar structure is more compact, and the platelets are less uniform in length than those in iPP. The length of lamellar platelets ranges from 30 to 60 nm, and they form local periodical patterns.

Scanning tunneling microscopy study of molecular order at liquid-solid interfaces

Adsorbates of normal alkane C36H74, cycloalkanes (CH2)48 and (CH2)72, decanol C10H21OH, 4-hexyl-4′-CyanoBiphenyl (6CB) and 4-octyl-4t′-CyanoBiphenyl (8CB) on graphite and β-Nb3I8 were studied by Scanning Tunneling Microscopy (STM), and the molecular arrangements at the liquid-solid interface were examined. Large-scale STM images show that the adsorbates possess complex multilayered structures, and that molecular ordering at the liquid-solid interfaces occurs primarily in the immediate vicinity of the substrate. Molecular-scale STM images are primarily determined by the electronic contributions of the most protruded atoms of the topmost overlayer. The underlying overlayers and the substrate affect the images indirectly by perturbing the topography of the topmost overlayer. The STM images of the adsorbates on graphite show that the atomically flat surface of graphite leads organic molecules to form lamella-like structures, while on the grooved surface of β-Nb3I8, long chain-like molecules are trapped in the grooves. We were unable to image the cycloalkanes on β-Nb3I8, which suggests that the cycloalkanes cannot assemble on the grooved surface due to a mismatch between the molecular shape and surface topography. The layers of 6CB and 8CB adsorbed on β-Nb3I8 exhibit two types of domains, which may be related to how the grooves of the β-Nb3I8 surface are occupied by the organic molecules. The STM images of decanol adsorbed on β-Nb3I8 show two domains of different brightness. The relative brightness of these domains switches reversibly as the gap resistance is changed in the region around −60 MΩ.

Atomic force microscopy on polymers and polymer related compounds: 3. Monocrystals of monomer and polymer 2,4-hexadiynylene bis(p-fluorobenzenesulfonate)

SummaryThe largest surfaces of lozenge-shaped monocrystals of polymer 2,4-hexadiynylene bis(p-fluorobenzenesulfonate), (pFBS), and its monomer have been studied by Atomic Force Microscopy (AFM). Tridimensional maps of interatomic forces between probe and surface have been recorded at areas from hundreds of nanometers down to the atomic scale. The examined surfaces, which generally correspond to the crystallographic [100] plane, are built from end groups of substituents, benzene rings, with main planes being perpendicular to the surface. Large scale AFM images reveal a fibrillar structure of the polymer surface in contrast to a not well-defined morphology of the monomer surface. The atomic scale maps of polymer and monomer [100] surfaces look quite similar. They can be characterized by alternating rows of ‘hills’, one of which shows zig-zag type arrangement. The repeat distances along the rows are .53±.04 nm for the polymer and .61±.06 for the monomer. In the close to perpendicular direction — α=89.5±3.0o for the polymer and the monomer — the repeat distances are 1.62±.16 nm for the polymer and 1.47±.08 nm for the monomer. These values are in general agreement with the crystallographic parameters, b=.4914 nm, c=1.4103 nm (polymer), and b=.5187 nm, c=1.4093 nm (monomer). The appearance of alternating rows in AFM images does not exactly correspond, however, to the surface atomic arrangement in the [100] plane, which has been reconstructed from the crystallographic data. Though the AFM patterns might be assigned to —CF-CH- groups of substituents, the differences found between images and diffraction data are discussed.

Surface structure of polymers and their model compounds observed by atomic force microscopy

Results of atomic force microscopy (AFM) of normal alkanes, polyethylene, isotactic polypropylene and of a diblock copolymer are presented. Various types of surfaces - naturally and epitaxially grown on different substrates - have been examined from hundreds of nanometers down to the atomic scale. Surface morphology and molecular arrangement have been visualized in AFM images. Atomic-scale AFM images with some defects have been observed on lamellar surfaces of normal alkanes C33H68. Arrays of oriented polyethylene molecules have been revealed at the surface of thin polyethylene films, epitaxially crystallized on the (001) face of an anthracene single crystal. Contact faces of thin films of isotactic polypropylene (iPP), epitaxially crystallized on various substrates (i.e. benzoic acid, potassium salt of 4-chloro-benzoic acid or polyamide 11), show the lamellar structure as well as the methyl side-group pattern in the exposed iPP (010) planes. AFM offers the way to distinguish the crystalline modification as well as the molecular conformation (left- and right-handed helices). Also the morphology of a microphase-separated block copolymer, polystyrene-b-poly-(2-vinylpyridine) (PS/PVP), has been observed by AFM.

Structure analysis of conductive polymer systems: Poly-4-vinylpyridine and poly(butadiene-b-4-vinylpyridine) with 7,7′,8,8′-tetracyanoquinodimethane

SummaryThe structure analysis of two conductive polymer systems-poly-4-vinylpyridine and poly(butadiene-b-4-vinylpyridine) with 7,7′,8,8′-tetracyanoquinodimethane (TCNQ)-was done by X-ray diffraction, scanning tunneling microscopy (STM) and FTIR. The charge transfer complex formed between the pyridine group and the known electron acceptor, TCNQ, is supposed to be the conductive element in these systems. In order to understand the structure of this complex, a model compound, the complex of 4-ethylpyridine (4EP) with TCNQ, 4EP/TCNQ2, was studied by the mentioned methods. It appears that there are two crystalline modifications of the model compound with different type of stacks of the TCNQ molecules. In polymer systems only one type of the complex is dominant as revealed by joint analysis of X-ray diffraction diagrams, STM and FTIR data. In the STM image of the polymer surface one can distinguish that molecular stacks with periodicities of 4.1 Å in a row are separated (12.5Å) from each other. Such organization is similiar to the one observed in conductive charge transfer complexes such as tetrathiofulvalene (TTF) with TCNQ. The ordered molecular domains are scattered on the polymer surface and take part in the formation of the conductive network.