Investigation of the fold-surface problem in polyethylene single crystals with the nitric acid oxidation technique

Single-crystal preparations of polyethylene were treated with the selective oxidizing agent, fuming nitric acid. The degraded products were examined as regards layer thickness (by low-angle x-ray studies), chemical and weight changes, recrystallization and annealing treatments, and by broadline NMR, as part of a systematic investigation aimed at clarifying the nature of disordered material in single crystals. It emerges that there is a disordered-mobile region along the fold surface of the crystals in agreement with other parallel works along similar lines. In a more detailed analysis we can now decompose the nitric acid attack into components affecting the basal and side surfaces, respectively. Taking into account the recrystallization–annealing observations, we infer that the fold surface is heterogeneous with folds of more than one kind. These results were combined with a preliminary molecular weight distribution study by gel permeation chromatography. Taking into account all the available evidence, we are led to suggest a composite structure where surface looseness, coresponding to long loops and hairs, is superimposed on the more regular folded surface. This model is in the process of being tested. The problems concerning the assignment of a value to the amount of surface looseness are being discussed. In addition, a discontinuity in the thermal behavior of the crystals between 75 and 80°C. has been detected.     

Electrochemical Synthesis and Characterization of Nickel(II) Complexes with N‐2‐Pyridyl‐sulfonamide Ligands

Heteroleptic nickel(II) complexes [NiL₂L′] of a series of monoanionic and potentially bidentate N‐2‐pyridyl‐sulfonamide ligands [HL] and 2,2′‐bipyridine or 1,10‐Phenanthroline (L′) have been prepared by electrochemical oxidation of a nickel anode in an acetonitrile solution of the ligands. The complexes have been characterized by microanalysis, IR and electronic spectroscopy, magnetic measurements and LSI mass spectrometry. The crystal structure of [Ni(Ms6mepy)₂(bipy)] has been determined by x‐ray diffraction and shows the metal in an octahedral NiN₆ environment. Octahedral structures are also proposed for the other complexes with the N‐2‐pyridyl‐sulfonamide ligands acting as N,N′ or N, O bidentate systems, depending on the position of the methyl substituent on the pyridine ring.     

Solution grown polymer crystals

Summary A survey is presented on some front line issues in the field of polymer crystallisation from dilute solution. The major portion of the review is devoted to the discussion of the problem of disorder in the simplest single crystal forms which can be considered as a preliminary for the understanding of the nature and behaviour of the amorphous content in a crystalline polymer. Available evidence for the existence of both disorder and crystallographic regularity along the fold surface is critically reviewed, and the compatibility of theoretical considerations with the various conceivable fold surface models is being analysed. The critical surveying of these issues suggests some obvious ways in which the apparently conflicting claims as regards the nature of the true fold surface could be reconciled, and along which future investigations could be fruitfully conducted. Some exploratory work aimed at the clarification of the disorder problem in single crystals pursued along some new lines in our laboratory is being outlined. In addition, single crystal growth from chemically inhomogeneous molecules (copolymers) is also touched upon.In the second part of the review a brief survey is presented of the research in a newly emerging field concerned with crystals with intrinsically fibrous habit resulting from crystallisation during flow, a topic which opens up new perspectives in the subject of polymer crystals.Finally some rather illuminating pictorial similarities between crystals from solution and the melt are placed in juxtaposition.     

Electrical resistivity, optical and magnetic properties of the layered oxyselenide SmCuOSe

The electrical and magnetic properties of the tetragonal phase SmCuOSe are reported as a function of the temperature. The optical properties were studied by means of diffuse reflectance spectrum in the UV-Vis range. The electrical resistivity measurements as well as diffuse reflectance spectrum show that SmCuOSe is a semiconductor with an optical band gap (E_g) of 2.6 eV. In this phase, Cu is at its monovalent oxidation state and, as such, it does not contribute to the total magnetic moment, whereas Sm is in its 3+ oxidation state, with a large VanVleck contribution due to the admixture of the fundamental state with higher energy levels.     

Ab initio SCF Calculation of the Fluoronium Ion: Geometry,electronic structure and vibrational constants

An ab initio SCF calculation of 42 points of the energy hypersurface of the fluoronium ion is presented using a contracted F(5s/3p), H(2s) gaussian basis set. In its equilibrium structure a bond length of 1.812 a.u. and a HFH bond angle of 127.2° are predicted. The calculated vibrational frequencies for H₂F⁺, HDF⁺, and D₂F⁺ are in good agreement with the experimental data.     

Profit maximization in an inventory system with time-varying demand,partial backordering and discrete inventory cycle

In this paper, an inventory problem where the inventory cycle must be an integer multiple of a known basic period is considered. Furthermore, the demand rate in each basic period is a power time-dependent function. Shortages are allowed but, taking necessities or interests of the customers into account, only a fixed proportion of the demand during the stock-out period is satisfied with the arrival of the next replenishment. The costs related to the management of the inventory system are the ordering cost, the purchasing cost, the holding cost, the backordering cost and the lost sale cost. The problem is to determine the best inventory policy that maximizes the profit per unit time, which is the difference between the income obtained from the sales of the product and the sum of the previous costs. The modeling of the inventory problem leads to an integer nonlinear mathematical programming problem. To solve this problem, a new and efficient algorithm to calculate the optimal inventory cycle and the economic order quantity is proposed. Numerical examples are presented to illustrate how the algorithm works to determine the best inventory policies. A sensitivity analysis of the optimal policy with respect to some parameters of the inventory system is developed. Finally, conclusions and suggestions for future research lines are given.

     

On the use of Helmholtz resonators as sound attenuators

Summary In the present paper a nonlinear acoustic theory is proposed, to accurately describe the properties of a generalized type of Helmholtz resonators. The theory may be used as a layout tool to design sound attenuators for combustion facilities. The present investigation shows that, in addition to the nonlinear effects, the effects due to thermoacoustic boundary layers and the first frequency correction to the Helmholtz approximation should be taken into account, in order to predict the frequency-dependent attenuation properties of Helmholtz attenuators with the accuracy required by typical technical applications. A series of experiments is presented which is used to validate the theoretical predictions.

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Zusammenfassung In der vorliegenden Abhandlung wird eine nichtlineare akustische Theorie vorgeschlagen, mit der das Verhalten einer verallgemeinerten Art von Helmholtzresonatoren beschrieben werden kann. Die Theorie kann als Werkzeug zur Auslegung von Schalldämpfern für Verbrennungsanlagen benutzt werden. Die vorliegende Arbeit zeigt, dass neben den nichtlinearen Effekten auch die Einflüsse der thermoakustischen Grenzschichten und der ersten Frequenzkorrektur zur Helmholtzapproximation berücksichtigt werden müssen, damit die frequenzabhängigen Eigenschaften von Helmholtzdämpfern mit technisch ausreichender Genauigkeit beschrieben werden können. Mit einer Reihe von Experimenten werden die theoretischen Ergebnisse untermauert.

     

Exponential weak Bernoulli mixing for collet-eckmann maps

We prove exponential weak Bernoulli mixing for invariant measures of certain piecewise monotone interval maps studied in [BK] and [KN]. In particular we prove this for unimodal maps with negative Schwarzian derivative satisfying lim , wherec is the unique critical point ofT.