Entanglement network and relaxation temperature dependence of single‐site catalyzed ethylene/1‐hexene copolymers

In this work, we investigate the linear viscoelastic response of high molecular weight ethylene/1‐hexene copolymers, characterized by a narrow molecular weight distribution and comonomer content in the range from 0 to 10 mol %. A variation in the entanglement plateau modulus has been found in agreement with the recently developed packing length model. The packing model applied to viscoelastic data suggests decreased values of the characteristic ratio, accordingly with recent computer simulation results. The flow activation energy increases as the side chain content increases. This feature is thought to be related to the mobility of the molecules. The presence of side branches due to the comonomer hinders the mobility of the molecules, and increases the thermal barrier for the segmental motion. Then in the comonomer content range studied, the increase of the flow activation energy goes parallel with a decrease in the characteristic ratio. This result suggests that more parameters than only the stiffness of the chain modulate the thermal dependence of viscoelastic properties. A more refined study is necessary combining experiments with computer simulations in order to elucidate these aspects. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Influence of the amount of ferromagnetic addition on the rheological properties of spinning solutions,the structure,the strength,and thermal properties of polyacrylonitrile fibres

The rheological properties of spinning solutions of polyacrylonitrile in dimethyl formamide (DMF) containing various amounts of a ferromagnetic nanoaddition were investigated. The porous structure, the strength and thermal properties of fibers obtained from these solutions were assessed, as well as the uniformity of the nanoaddition distribution on the fiber surface was estimated. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Nonisothermal melt-crystallization kinetics of isotactic polypropylene synthesized with a metallocene catalyst and compounded with different quantities of an α nucleator

The nonisothermal crystallization kinetics of a metallocene-made isotactic polypropylene (m-iPP) and its compounds with 0.1 wt % and 0.3 wt % of a sorbitol derivative [1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS); an α nucleator] were investigated by differential scanning calorimetry at different cooling rates from the melt. The nucleation efficiency was proved by a significant increase in the crystallization temperatures (accompanied by a slight augmentation of the degree of crystallinity and a decrease in the crystal sizes). This increase in the crystallization temperatures led to higher amounts of fractional content in the γ polymorph, even though DMDBS was supposed to be a nucleator for the α form. The Avrami and Ozawa methods effectively described only the early stage of crystallization, whereas a combined Avrami–Ozawa method was valid for the whole crystallization process. The values of the exponent for this method decreased for nucleated samples in the later stage of crystallization, especially in the case of m-iPP with 0.3 wt % DMDBS added (m-iPP03). The activation energy of the process and the surface free energy were also estimated. The production of considerable proportions of the γ polymorph in m-iPP03 corresponded to higher values of the activation energy and lower values of the surface free energy. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Potential applications of poly(vinyl alcohol)‐congo red aqueous solutions and hydrogels as liquids for hydraulic fracturing

The authors report on the viscoelastic characterization of guar hydrogels obtained through complexation reactions with borax ions. These gels are compared with hydrogels obtained from poly(vinyl alcohol) of different degree of hydrolysis through complexation reactions with congo red. The effect of the degree of hydrolysis and both, the concentration of PVA and the concentration of congo red, on the viscoelastic properties of the hydrogels is analyzed. The potential use of the PVA‐based hydrogels as hydraulic fracturing liquids is discussed in relation to the commonly used fracturing liquid based on the guar–borax system. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Extended Hyperbolic Model for Sand-to-Concrete Interfaces

A relatively simple, four-parameter extended hyperbolic model for interfaces was developed for use in soil-structure interaction analyses. The model accommodates arbitrary stress path directions and includes three important elements: (1) development of a yield surface during interface shear; (2) a formulation for yield-inducing shear stiffness that is applicable to any stress path orientation; and (3) a formulation for unloading-reloading shear stiffness. The model was evaluated against the results of shear tests performed at the interface between three different types of sand and a concrete surface under a variety of stress paths. Comparisons between measured and calculated interface response indicate that the model provides accurate estimates of the response of sand-to-concrete interfaces.     

Balance-dendrogram. A new routine of CoDaPack

A statistical analysis of compositional data based on the Aitchison geometry of the simplex requires an appropriate basis for representing the data. A simple and intuitive way of building such a basis employs a sequential binary partition of the compositional vector. The partition, together with some statistical summaries of the coordinates, or balances, can be represented in a dendrogram-type graph. In this paper we introduce an implementation of this methodology inside CoDaPack, which is freeware. An example with real data illustrates the use of the Balance-Dendrogram routine.     

Cross sections of proton induced nuclear reactions on enriched 111Cd and 112Cd for the production of 111In for use in nuclear medicine

Proton induced nuclear reactions on enriched 111Cd and 112Cd have been studied up to 30 MeV in the context of routine production of the medically used isotope 111In with low and medium energy cyclotrons. The excitation functions of 111Cd(p,n)111m,gIn and 112Cd(p,2n)111m,gIn as production reactions and 111Cd(p,2n)110mIn, 111Cd(p,2n)110In, 111Cd(p,3n)109ml,m2,In, 112Cd(p,3n)110mIn, 112Cd(p,3n)110gIn as competing processes have been measured using the activation method involving the stacked-foil technique. The deduced thick target yields are compared with those obtained experimentally.     

Degradation of the light-stress protein is mediated by an ATP-independent, serine-type protease under low-light conditions

Green plants respond to light stress by induction of the light-stress proteins (ELIPs). These proteins are stable as long as the light stress persists but are very rapidly degraded during subsequent low light conditions. Here we report that the degradation of ELIPs is mediated by an extrinsic, thylakoid-associated protease which is already present in the membranes during light stress conditions. Partial purification of the protease by perfusion chromatography indicates that this proteolytic activity may be represented by a protein with an apparent molecular mass of 65 kDa. The ELIP-directed protease is localized in the stroma lamellae of the thylakoid membranes and does not require ATP or additional stromal factors for proteolysis. The protease has an optimum activity at pH 7.5-9.5 and requires Mg2+ for its activity. The ELIP-degrading protease show an unusual temperature sensitivity and becomes reversibly inactivated at temperatures below 20 degree C and above 30 degree C. Studies with protease inhibitors indicate that this enzyme belongs to the serine class of proteases. The enhanced degradation of ELIP in isolated thylakoid membranes after addition of the ionophore nigericin suggests that a trans-thylakoid delta pH or changes in ionic strength may be involved in the mechanism of protease activation.     

Biochemical changes in cerebrospinal fluid associated with the neurotoxic action of HIV-1]

Different intravascular ultrasound (IVUS) systems vary in their image presentation. The purpose of this study was to compare four IVUS systems in vitro to determine the accuracy of tissue characterization of atherosclerotic plaque compared with histology. Ninety-eight plaque segments from 23 formalin-fixed human iliac arteries were imaged in saline at room temperature with four different IVUS systems. To assess the accuracy of IVUS in describing plaque, three types of analysis were performed: (1) the ability to identify the presence and extent of lumen or plaque boundary; (2) sensitivity, specificity, and interobserver variability of IVUS in qualitatively identifying plaque components compared with histology; and (3) quantification of calcification. The synthetic aperture device had a lower sensitivity in identifying lumen and plaque boundaries (87%, 38% respectively) compared with other machines (96%-100%, 95%-100%). All three mechanically rotating systems had fair to good sensitivities for identifying calcification (57%-73%) or lipid filled areas (50%-83%). The sensitivity of discriminating fibrous tissue from fatty areas was low (39%-52%). The synthetic aperture system had a significantly lower sensitivity for identifying all three tissue types (4%-21%). There was significant interobserver variability (kappa value = 0.47-0.68) as well as machine to machine variability (kappa value = 0.52) for tissue characterization. Calcified areas were underestimated by System 1 (p < .05) and System 4 (p < .01) because of weaker echo reflections or poor image quality. There are significant differences in image representation among these four IVUS systems in the diagnosis of tissue components of complex atherosclerotic plaque. These variabilities should be considered when interpreting studies performed with different machines.