Tribology of confined Fomblin-Z perfluoropolyalkylethers: molecular weight dependence and comparison between unfunctionalized and telechelic chains

The dynamic shear properties of molecularly-thin films of unfunctionalized and end-functionalized (telechelic) Fomblin-Z perfluoropolyalkylether (PFPAE) melts with number-average molecular weight M _n≈ 3000−4000 g,mol^-1 have been studied at shear rates of 10^-2−10⁵ s^-1 at normal pressures of 1 and 3 MPa. The shear responses are compared to measurements on end-functionalized polymers of the same chemical composition but lower molecular weight, M _n≈ 2000 g,mol^-1. The predominantly elastic response and high shear moduli of the confined film of unfunctionalized polymer, Fomblin Z03, suggest that it forms a structure likely to solidify already at low pressure. Its lubricating properties are less favorable than the ones found for hydroxyl- (DOL) and piperonyl-terminated Fomblin-Z (AM2001, AM3001), where associated molecules form a structure less prone to solidification under confinement. The thickness of the compressed films of the end-functionalized polymers increased more strongly with molecular weight than as M _n ^0.5 . The shear moduli were found to be larger, the higher the molecular weight, indicating slower relaxations. At a normal pressure of 3 MPa, these films solidified and displayed stick–slip as seen already at 1 MPa in the Z03 film. The limiting shear stress of the unfunctionalized Z03, σ > 3 MPa, exceeded by an order of magnitude the limiting shear stress of all of the end-functionalized polymers. The limiting shear stress of the hydroxyl-terminated polymer was larger than that of the piperonyl-terminated polymer. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mapping phases of poly(vinyl alcohol) and poly(vinyl acetate) blends by FTIR microspectroscopy and optical fluorescence microscopy

Fluorescence optical microscopy (FOM) of poly(vinyl alcohol) (PVA) and poly(vinyl acetate) (PVAc) blends in compositions 9/1, 1/1, and 1/9 (w/w) show that these blends present phase separation in the solid state. Each domain of the solid samples was identified by FOM as PVA-richer domains by green fluorescence of fluorescein and PVAc-richer domains by the blue fluorescence of anthracene. The dimensions, shapes, and distributions of these domains were dependent on the initial composition of the polymeric mixtures in the solution. Specific interactions between both homopolymers were studied using FTIR microspectroscopy, which allowed us to obtain spectra for both PVA-richer and PVAc-richer domains. These spectra demonstrated that partial miscibility could occur only for blends with a higher PVAc content and, in these cases, we observed interchain hydrogen-bonded carbonyl groups. Fluorescence microscopy of blends with this partial miscibility exhibited small interconnected domains produced by coalescence of droplets during the polymer phase separation process. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 645–655, 1998     

Microkinetics of high-temperature pyrolysis

High-temperature pyrolysis of various classes of polymers have been discussed. For the non-carbonizing polymers it was shown that polymer structure can affect the pyrolysis characteristics. Data on the high-temperature pyrolysis of some polypropylene samples with different crystallinity have been presented. Modification of the standard method allowing one to obtain kinetic data from the single experiment was discussed. A kinetic model for the degradation of char-forming polymers in conditions of high-temperature pyrolysis has been suggested. It has been supposed that pyrolysis of carbonizing polymers proceeds in some (two in present work) parallel stages with different activation energies, temperatures and pyrolysis rates. Questions of the applicability of the suggested model to describe the various types of charring systems have been discussed. High-temperature pyrolysis for a number of epoxy resin-based polymeric compositions have been investigated. Kinetic parameters of the pyrolysis have been computed. © 1998 John Wiley & Sons Ltd.     

Multinode finite element based on boundary integral equations

A finite element constructed on the basis of boundary integral equations is proposed. This element has a flexible shape and arbitrary number of nodes. It also has good approximation properties. A procedure of constructing an element stiffness matrix is demonstrated first for one-dimensional case and then for two-dimensional steady-state heat conduction problem. Numerical examples demonstrate applicability and advantages of the method. © 1998 John Wiley & Sons, Ltd.     

Structure of Mixed Reverse Microemulsions Based on Sodium Bis (2-Ethylhexyl) Sulfosuccinate and Sodium Cholate

The microenvironment of water droplets of sodium bis (2-ethylhexyl) sulfosuccinate (AOT) and sodium cholate mixed reverse microemulsions was studied. Structural changes of water pockets in mixed reverse micelles were investigated by IR spectroscopy. The O-H stretching vibrational absorption spectra in the region of 3000–3800 cm⁻¹ were fit to three subpeaks with the Monte Carlo method. It was revealed that additives of sodium cholate suppress free water fraction in the water droplets of reverse micelles from 31% to 20% and support rising of bound fraction from 53% to 65%. The binding of optical probe ortho-nitroaniline to the mixed reverse micelles was determined by UV–visible spectroscopy. It was found that introducing of additives of sodium cholate below its critical micelle concentration (CMC) causes increasing of values of binding constant K_b twice compared with reverse micelles modified with pure water. However, values of the binding constant were reduced 4-fold at concentrations of sodium cholate higher than its CMC. Electrical conductivity of the reverse mixed micellar solutions (AOT + sodium cholate) was measured. Water-induced percolation in conductance of mixed reverse microemulsions occurs at a lower value of water/surfactant molar ratio (W) under the influence of sodium cholate, viz. electrical percolation threshold decreases from W = 32 to W = 15. The size of water droplets was estimated with the dynamic light scattering method. It was found that additives of sodium cholate below and higher than the CMC results in increasing and decreasing of hydrodynamic diameters of the water droplets, respectively, but sizes of water droplets decrease at concentrations of sodium cholate higher than its CMC.     

Effect of Dietary Restriction on DNA-Adduct Formation of Benzo[a]pyrene and 2-Acetylaminofluorene in Mouse Liver

Dietary restriction (DR) alters hepatic drug metabolizing enzyme activities that affect the metabolic activation of xenobiotics. Previously, we have studied the effect of DR on the formation of benzo[a]pyrene (BaP)-DNA adducts in male Fischer 344 rats. In this study, the effects of DR on the formation of specific BaP-DNA adducts in mouse livers were investigated. In addition, 2-acetylaminofluorene (2-AAF) modified DNA adducts in livers of mice and rats were also examined. DR reduced the body and liver weights of male B6C3F₁ mice. Both the total [³H]BaP-DNA binding and the specific BaP-DNA adduct, N²-dG-BaP, detected by ³²P-postlabeling technique, were found to be greater in DR mice than in animals fed al libitum (AL). The formation of the 2-AAF-DNA adduct, AF-C8-dG, was not affected by DR. Similar results were obtained from the in vitro DNA adduct formation of these two carcinogens mediated by rat and mouse liver microsomes. The effect of DR on the formation of BaP- and 2-AAF-modified DNA adducts is discussed.     

New technique for effective dose estimation using Monte Carlo simulation for the patients undergoing radioiodine therapy

We presume that radioiodine capsule, used for therapy, stays in stomach for 15 min before the absorption starts when large amount of radioactivity needlessly expose a part of stomach and several surrounding organs. Investigations were performed at Institute of Pathophysiology and Nuclear Medicine, Faculty of Medicine, Ss. Cyril and Methodius University, Skopje, Macedonia. Eighty-seven patients were reviewed, age between 21 and 73. Patients were divided in 20 groups receiving average dose per group from 1813 MBq to 6105 MBq. We used Monte Carlo MCNP 4b code and gave the results only for administrated activity of 6105 MBq. Calculated results are given for imparted energy per transformation, dose equivalent and effective doses for different organs. The effective doses were between 9.17 nSv (for bone surface) to 122.4 mSv (for stomach). As the total dose is estimated to be 126.73 mSv it is obvious that the highest part is received by stomach.     

Assessment of Plastic Deformation Induced by Indentation on a Large Grain on Inconel 600 Using Synchrotron Polychromatic X-ray Microdiffraction

A synchrotron-based three-dimensional (3-D) Laue technique called Differential Aperture X-ray microscopy (DAXM) was used to investigate plastic deformation of Inconel 600 induced by indentation. The DAXM technique is capable of probing up to 60 to 100 ??m into the Ni alloy, with micron resolution. A conical indenter was used to generate an indent on a large surface grain on Inconel 600. The DAXM Laue images from the uppermost grain exhibited pronounced streaking and splitting of Laue spots, which increased as the grain boundary was approached. Splitting of the Laue patterns correlates with subgrain or dislocation cell formation. A significant amount of dislocation cell formation was found within the 1-??m voxels probed by DAXM. A change in total angle of 8.79 deg was determined for the uppermost grain from the diffraction data from depths 4 to 28 ??m, with an average misorientation angle of 2.15 deg between the dislocation cells. The next grain having a different crystallographic orientation continued to rotate in the same direction as the uppermost grain due to the large plastic deformation.     

An inter-comparison of three urban wind models using Oklahoma City Joint Urban 2003 wind field measurements

Three wind models are compared to near-surface time-averaged wind measurements obtained in downtown Oklahoma City during the Joint Urban 2003 Field Campaign. The models cover several levels of computational approximation and include in increasing order of computational demand: a mass-consistent empirical-diagnostic code, a Reynolds-averaged Navier-Stokes (RANS) computational fluid dynamics (CFD) model, and a Large Eddy Simulation (LES) CFD code. The models were run with identical inlet and boundary conditions using the same grid resolution; the choice of the specific computational set-up reflects demands for fast-response models, although it may be a sub-optimal choice for the more complex models. A qualitative comparison of the model-computed flow fields with the Joint Urban 2003 wind measurements shows that all three models compare favorably to the near-surface wind measurements in many locations, although there are often instances of winds being calculated poorly in specific locations. The CFD models, however, had clearly superior looking flow fields, whereas the empirical-diagnostic code produced fields that were less smoothly varying. The inter-comparison exercise was supported by point-by-point quantitative comparisons of the wind speed and wind direction and with statistical measures. The RANS-CFD code, for example, was within 50% of the measured wind speed 62% of the time as compared to 53% for the LES model and 49% for the empirical-diagnostic code. For wind direction, the RANS-CFD code was within 30° of the measured wind direction 58% of the time as compared to 50% for the LES code and 43% for the empirical diagnostic code. It is noticeable that throughout the various IOP cases examined, and under the specific computational set-up used in the simulations for fast-response needs, there is no clear superiority of one model over another. In addition, for the LES model, which in theory should provide the most realistic representation of the flow field, it appears that further to the sub-optimal computational set-up, as well as the uncertainty and natural variability persistent in the real world, has resulted in diminished performance.