Studies on polyethylene crystallized at unusually high supercoolings: Fold length,habit, growth rate,epitaxy

The newly arisen possibility of crystallizing polyethylene at supercoolings much higher than were achievable previously has enabled the study of crystallization to be extended in several directions. Thus, fold length can be followed down to previously inaccessibly low crystallization temperatures, in the present case with sharp fractions, demonstrating the essential independence of the fold length of molecular weight. In this context the thinnest isolated crystal reported so far was obtained (ca. 6 nm). The faceted nature of crystals grown at such low temperatures and high rates has been noted, and is in line with new conceptions of polymer crystal growth. A previous observation of exceptionally high crystal growth rate (ca. 2 m/s) has been supplemented by measurements over a range of crystallization temperatures and the results found to be in good agreement with the predicated regime III behavior in the least theory of Hoffman. Observations of epitaxy on mica, while broadly in line with those by Lovinger, were revealing in several respects. Among these the observation that the substrate can influence the fold length when the chains are parallel to the substrate plane remains unexplained and puzzling.     

A Steel Pricing Case Study

Britain's entry into the European Community required fundamental changes to be made to the methods of charging U.K. home trade steel customers. The paper describes O.R. involvement in the design and implementation of the new charging methods for wide strip products sold by British Steel Corporation. The methodology employed was simple but none the less proved to be valuable in allowing management to make sense of what appeared at the outset to be a highly unstructured problem.     

Adsorption-entanglement layers in flowing high molecular weight polymer solutions II. The effect of the nature of the surface

Adsorption-entanglement layers are thick, i.e. multimolecular, layers formed at solid surfaces from solutions of high molecular weight polymers in a state of flow. This publication, the second part of a series of four, is concerned with the effect that the chemical nature of the surface has on formation of these layers. It is shown that for the solutions of polyethylene (PE) and polymethylmethacrylate (PMMA) the layers form most readily on carbon surfaces. Further it is shown that in the case of PE, addition of aluminium stearate can prevent formation of the layers along steel surfaces.These and other effects reported in the paper all bear on actual situations encountered in rheological practice and in particular on the measurement of viscosities. It is concluded that the marked dependence of layer formation on the chemical nature and treatment of the surface is responsible for the differences in viscosity measurements featuring in earlier works by different authors.     

Self-hardening of highly oriented polyethylene

Ultra-oriented polyethylene fibers obtained by drawing to approximately 30 times their original length have a Young's modulus of approximately 800 kbar. Such fibers, if unconstrained, contract on heating to a length near the original. We have studied the forces causing this contractile behavior by monitoring the stress in the fiber while maintaining it at constant length. In the course of this we observed a complex sequence of both reversible and irreversible behavior. In the reversible case we observed first energy and then entropy elastic behavior. The most significant feature observed is that at sufficiently high temperature the fiber stress relaxes to an unmeasurably low value. A fiber allowed to relax in this way possesses a much lower room temperature tensile modulus (ca. 80 kbar) immediately after relaxation but, remarkably, this modulus increases to approach the initial high value over a period of a few hours when the fiber is stored either clamped or unclamped at room temperature. High x-ray orientation is preserved throughout the storage period but the density which dropped during the stress decay rose again in the course of the spontaneous stiffening. None of the stress relaxed fibers displays large-scale contractile behavior on subsequent heating. A phenomenological composite model is proposed which involves stiff microfibrils of short length—surrounded by a matrix present as a minority component. The softening of this matrix on heating and its subsequent stiffening on storage, involving a certain amount of melting and recrystallization, respectively, could then be responsible for the observed variations in the macroscopic tensile properties using simple fiber composite theories. The fibers are likely to be of extended-chain type produced by the initial drawing while the matrix may consist of a combination of oriented amorphous material (tie chains), randomly oriented chains, and transverse lamellar overgrowth present in varying proportions in the different stages of sample treatment. The wider implications, fundamental and practical, of this remarkable self-hardening process are indicated.     

A fiber composite model of highly oriented polyethylene

A fiber composite model of highly drawn polyethylene is presented. Quantitative predictions and calculations are made using shear-lag theory. The drawing process is shown to occur in two stages, a neck and a postneck taper. It is shown that there is an empirical linear relationship, with a high correlation, between the parameter x in shear-lag theory (which involves the aspect ratio of the reinforcing elements and the square root of the ratio of matrix shear modulus to the Young's modulus of the reinforcing elements) and the 3/2 power of the taper draw ratio. It is concluded that crystalline fibrils (the reinforcing elements) deform homogeneously during the secondary, taper drawing process. The increase in aspect ratio resulting from this homogeneous deformation is held to be responsible for the increase in tensile modulus owing to the increased efficiency of the fibrils as reinforcing elements. The model is also used to explain the self-hardening process exhibited by these fibers and, using measurements of density of hardened fibers, to predict that immediately after the neck the aspect (length to diameter) ratio of the crystalline reinforcing elements is ca. 2 and that the shear modulus of the matrix material in as-drawn fibers is ～10³N/m² and does not change significantly during the taper-drawing process.     

Effect of donor-acceptor defect pairs on the crystal structure of In and Ga rich ternary compounds of Cu-In(Ga)-Se(Te) systems

A comparative study of the effect of donor-acceptor defect pairs on the unit cell parameters a, c and V of the ordered defect compounds that are intermediate phases of the pseudo-binary [Cu₂(Se,Te)]_1−X[(In₂,Ga₂)(Se₃,Te₃)]_X system has been carried out. It is found that a, c and V decrease linearly with the increase in the fraction of cation vacancies to the total number of cation positions, m, or the fraction of the interacting donor-acceptor defect l per unit, respectively, in the chemical formula. The reduction in the unit cell dimensions is explained as due to the decrease in the effective cation radius r_eff caused by the increase in m or l or decrease in n. The linear dependence of r_eff on a, c, and V has important consequences. This behavior can be used to predict the unit cell parameters of other ODCs that may have chalcopyrite-related structure and have not been reported so far.     

Determination of valproic acid by high-performance liquid chromatography with photodiode-array and fluorescence detection.

The derivatization of valproic acid and undecylenic acid with 4-bromomethyl-7-methoxycoumarin is described. The derivatives were detected by photodiode-array and fluorescence detectors. The optimum monitoring conditions and the stability of the suspension solution and derivatives were investigated. A high-performance liquid chromatographic (HPLC) method with isocratic or gradient elution has been established for the analysis. This method has been used for the determination of total and free valproic acid in serum. There is a satisfactory correlation between the results obtained by this HPLC method and those measured by the enzyme immunoassay. Some other common anti-epileptic drugs did not interfere with the analysis. The method is simple and fast and has better sensitivity and linearity than enzyme immunoassay. It is suited for routine therapeutic drug monitoring.     

A multi-orbital iterated perturbation theory for model Hamiltonians and real material-specific calculations of correlated systems

The dynamical mean field theory (DMFT) has emerged as one of the most importantframeworks for theoretical investigations of strongly correlated lattice models and realmaterial systems. Within DMFT, a lattice model can be mapped onto the problem of amagnetic impurity embedded in a self-consistently determined bath. The solution of thisimpurity problem is the most challenging step in this framework. The available numericallyexact methods such as quantum Monte Carlo, numerical renormalization group or exactdiagonalization are naturally unbiased and accurate, but are computationally expensive.Thus, approximate methods, based e.g. on diagrammatic perturbation theory have gainedsubstantial importance. Although such methods are not always reliable in various parameterregimes such as in the proximity of phase transitions or for strong coupling, theadvantages they offer, in terms of being computationally inexpensive, with real frequencyoutput at zero and finite temperatures, compensate for their deficiencies and offer aquick, qualitative analysis of the system behavior. In this work, we have developed such amethod, that can be classified as a multi-orbital iterated perturbation theory (MO-IPT) tostudy N-folddegenerate and non degenerate Anderson impurity models. As applications of the solver, wehave embedded the MO-IPT within DMFT and explored lattice models like the single orbitalHubbard model, covalent band insulator and the multi-orbital Hubbard model fordensity-density type interactions in different parameter regimes. The Hund’s couplingeffects in case of multiple orbitals is also studied. The limitations and quality ofresults are gauged through extensive comparison with data from the numerically exactcontinuous time quantum Monte Carlo method (CTQMC). In the case of the single orbitalHubbard model, covalent band insulators and non degenerate multi-orbital Hubbard models,we obtained an excellent agreement between the Matsubara self-energies of MO-IPT andCTQMC. But for the degenerate multi-orbital Hubbard model, we observe that the agreementwith CTQMC results gets better as we move away from particle-hole symmetry. We have alsointegrated MO-IPT+DMFT with density functional theory based electronic structure methodsto study real material systems. As a test case, we have studied the classic, stronglycorrelated electronic material, SrVO₃. A comparison of density of states and photo emissionspectrum (PES) with results obtained from different impurity solvers and experimentsyields good agreement.     

Entropy analysis of Powell–Eyring hybrid nanofluid including effect of linear thermal radiation and viscous dissipation

Journal of Thermal Analysis and Calorimetry - Hybrid nanofluids are introduced as heat transfer fluids with greater surface stability, diffusion and dispersion capabilities compared to traditional...