H. J. Cantow

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.

Scanning force microscopy of nanofibrillar structure of drawn polyethylene tapes - 1. Different modes and tips

Different scanning force techniques and specially selected probes were employed to examine the nanofibrillar structure of gel-drawn ultra high molecular weight polyethylene. In order to avoid image artifacts and observe details of the surface structure with an improved resolution, the quality of probes was controlled with a new calibration gauge. Images recorded in the lateral force and tapping modes showed more structural features than those obtained in the normal force mode. Particularly a regular striped pattern was detected on the surface of nanofibrils. According to its repeating distance, ca. 25 nm, this pattern is consistent with the long period of UHMW PE observed earlier by SAXS and TEM

Measurement of the first and second normal stress differences: Correlation of four experiments on a polyisobutylene/decalin solution “D1”

Pressure distribution measurements for a polyisobutylene/decalin solution “D1” in the Truncated Cone-and-Plate (TCP) apparatus are combined with elastic hole pressures obtained for the same solution on the Lodge Stressmeter® in order to provide two independent estimates of the second normal-stress difference (N2). The values ofN2 from the TCP apparatus, obtained by numerical differentiation of a function of the center-hole pressure and the pressure gradient, are in good agreement with measurements made on the same sample by Tanner et al. with a direct method, namely the Tilted Trough Experiment, and by Christiansen et al. with a method that requires an extrapolation to the pressure at the free surface of coneand-plate rheogoniometer data obtained with flush-mounted pressure transducers. The viscosities from the modified Stressmeter for low shear rates extend over five decades of shear rate, including a zero-shear-rate region, and agree with the data of Christiansen on a torque-driven flow. The Higashitani-Pritchard-Baird-Lodge (HPBL) equation relatingN1–N2 to the hole pressure gives good agreement with the data over a certain range of shear stress. The Newtonian hole pressures for several liquids at 20 and 46 °C compare well with a finite-element calculation for a two-dimensional Poiseuille flow. When the elastic hole pressures from the Stressmeter are combined with the extrapolated rim pressures from the TCP Apparatus in order to extract the value ofN2, an agreement betweenN2 from the center-hole pressure andN2 from the rim pressure can only be obtained up to a shear rate of about 40 s−1, beyond which the value of −N2 from the rim pressure diverges abruptly to negative values. It is possible that this constitutes the first quantitative estimate of an edge effect in cone-and-plate rheometry. Alternatively, the elastic hole pressure in cone-and-plate flow is not equivalent to the elastic hole pressure in Poiseuille flow, at least at high shear rates. The data of Christiansen et al. with flush-mounted pressure transducers appear to confirm this second possibility. Finally, a single set of shift factors obeying the Williams, Landel and Ferry equation superposes the viscosity, the first and the second normal-stress difference within experimental scatter, which can be less than 1% for a certain combination of normal-stress differences. The data were recorded at 3, 20, 30, and 46 °C in the shear rate range 1–260 s−1.

Effect of chemical and configurational sequence distribution on the miscibility of polymer blends

SummaryA simple model is proposed to describe the miscibility of blends, which contain homo- and/or copolymers with tactic subunits. The directional-specific intermolecular interactions are assumed to be mediated by interactions caused by repulsive type intramolecular tacticity as well as chemical intersteps. Syndiotactic homopolymers are chosen as the reference state, for tactic as well as for mixed triads. The approximations developed in this paper include triad as well as diad treatment with respect to tacticity. Chemical intersteps are taken into account on a diad basis. Additive overlapping of tactic and chemical effects is assumed for mixed hetero- and isotactic triads.Complex copolymer blends, including multi-component copolymers, are treatable in this way. The modification of tactic and chemical sequences in polymer blends with the aim of developing compatible, as well as phase separated, systems may be facilitated.

ISOTHERMAL MElT CRYSTALLIZATION KINETICS OF POLY(L-LACTIC ACID)

A DSC study was carried out of the isothermal melt crystallization kinetics of poly(L-lactic acid), PLLA, at 110, 115, 120, 125 and 130‡C. The experimental data were evaluated within the framework of the well-known Avrami kinetic model and an extended model involving an additional third kinetic parameter [8]. In order to perform the necessary numerical calculations, a number of functions built into the Mathematica® software system were used. The results showed that the isothermal melt crystallization kinetics of PLLA can be described adequately by both these kinetic models. It should also be stressed that the kinetic model of Urbanovici and Segal offers a better description of the experimental melt crystallization data of PLLA than the classical Avrami model.

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.

Atomic scale surface studies of conductive organic compounds 3. Scanning tunneling microscopy studies of monocrystals of bis(ethylenedithio)tetrathiofulvalene with triiodide, ß-ET2I3

Abstract Scanning tunneling microscopy (STM) was applied to monocrystals of an organic superconductor, the charge transfer salt of bis(ethylenedithio)tetrathiofulvalene with triiodide, s-ET 2 I 3 , under ambient conditions. The obtained results reveal details of the charge distribution on the surfaces of differently shaped monocrystals. The periodicity of STM patterns obtained from the largest face of an elongated plate-like monocrystal is compatible with that of the crystallographic (001) plane. The STM patterns are therefore compared to drawings of corresponding computer-simulated models based on known crystallographic data. The imaged surface layer seems to consist of I 3 − anions. This interpretation agrees with the assumption that the central iodine atom of the anion has a higher charge density than the adjacent atoms. STM images of a surface of another (block-like) monocrystal display a pattern which is compared with the atomic arrangement in the (001) plane of the crystallographic structure. Well-separated linear patterns may be interpreted as a visualization of two-dimensional conductive layers parallel to the (001) plane.