Students modelling environmental issues

This work is based on a Brazilian project that aims to promote the use of a semi‐quantitative tool with students (11 ‐ 18 years old) in topics about local environmental problems. The paper presents the VISQ program, a network to analyse models, a set of tasks and three examples of models made by students. Results suggest that the VISQ program is suitable for use in Environmental Education as while working with the proposed tasks students thought at a system level becoming aware of some local environmental issues.     

Reactive processing of LLDPES in corotating intermeshing twin-screw extruder. II. Effect of peroxide treatment on processability

Commercial ethylene–octene linear low-density polyethylenes (LLDPEs) were reactively extruded with low levels of a peroxide [2,5-dimethyl-2,5-di(t-butylperoxy)hexane] to modify polymer molecular structure and processing properties. Peroxide levels were kept low to avoid crosslinking. This article reports the effects of this reactive extrusion on viscoelastic properties. Rheological properties are more sensitive than are molecular structure characteristics to the changes produced by reactions of very low peroxide concentrations. Complex viscosity increases are seen, especially at low frequencies. Shear-thining behavior is also accentuated. The crossover between G′ and G″ moves to lower frequencies. A modified Cole–Cole presentation of these data shows that the elastic component is more predominant for extrusion-reacted materials. Peroxide-modified materials all have higher Bagley end correction values than those of barefoot resins. The former exhibit lower power indices (more shear thinning). All these properties indicate more long-chain branching and higher melt elasticity. However, die swell decreased as a function of peroxide concentration. Peroxide treatment results in an enhancement of elongational viscosity, both under isothermal and cooling conditions, along with a decrease in drawdown ability. The rheological changes parallel those reported earlier in molecular characteristics but are more sensitive and suitable to evaluate the effects of reactive processing. The effectiveness of the reactive extrusion process for improving processability of LLDPEs depends critically on the extrusion conditions. © 1995 John Wiley & Sons, Inc.     

Atomic force microscopy on tree-like crystals in polyvinylidene fluoride blends

Polyvinylidene fluoride (PVDF), in homogeneous blends with an amorphous polymer under specific conditions, forms crystals with a tree-like architecture. Tree-like crystals have a loose fractal structure of crystalline branches which are impregnated with the amorphous polymer. In a previous study, this spectacular morphology was investigated in blends of PVDF and a random copolymer of styrene and methylmethacrylate (SMMA, 13% wt/wt styrene) by light microscopy. The crystal growth was interpreted in terms of a rate competition of crystallization and interdiffusion. The submicroscopic structure of tree-like PVDF spherulites has now been characterized by atomic force microscopy. Surface profiles were prepared by etching PVDF/SMMA blend films with toluene. The tree-like spherulites proved to be less disordered than expected. Their core consists of a compact array of lamellae, the planes of which are periodically twisted, producing a ring pattern. Near the crystallizing front, branches grow, separated from each other, into the amorphous phase. They are internally as compact as the core, but are covered by a loose fur of disordered lamellae. The tree-like appearance of these crystals comes from frequent splitting of the branches into twigs. Crystallographic bifurcation as in dendritic crystals was not observed. At interfaces between two tree-like crystals, the lamellae form patterns of backfolding. © 1998 Chapman & Hall     

A new CITEL fosters telecommunications in the Americas

The Inter-American Telecommunication Commission (CITEL) is an entity of the Organization of American States (OAS). Prior to 1993, CITEL was a series of conferences that existed in various forms over the years since 1889. The CITEL Assembly in 1993 organized CITEL into four permanent committees: a Permanent Executive Committee (COM/CITEL) and three Permanent Consultative Committees (PCCs) to carry out the work program of the Commission. There are a number of factors regarding the organization and operation of CITEL that are rather obvious. Probably the most important of these is the fact that CITEL must confront the future in partnership with the private sector in order to position itself to be of assistance to member states. With the increased participation of the private sector, it is anticipated that the scope of activities CITEL will be addressing in the future will also broaden. This is already taking place, especially in support of the establishment of a free trade zone in the Americas. One obvious step in this direction would be the expansion of the standards coordination activities into the area of information technology standards, which not only supports the trade negotiations but also the implementation of the global information infrastructure     

Sizing design sensitivity analysis of non-linear structural systems. Part II: Numerical method

A numerical method is presented to carry out sizing design sensitivity calculations outside established finite element analysis codes, using postprocessing data only. Geometric as well as material non-linearities are treated. To demonstrate the accuracy of the proposed method, numerical results are presented for structural systems with linear elastic material, large displacements, large rotations and small strains. A distributed parameter approach to structural design sensitivity analysis is used to retain the continuum elasticity formulation throughout the derivation of design sensitivity results. Using this approach and an adjoint variable method, design sensitivity computations are carried out. For structural performance functionals stress and displacement are considered. It is shown that computations can be performed with the same computational effort as for sizing design sensitivity analysis of linear structural systems. Accurate design sensitivity results are obtained for both linear and non-linear structural systems without the uncertainty of numerical accuracy and high cost associated with the selection of finite difference perturbations. Also, the method does not require differentiation of element stiffness and mass matrices in conventional finite element models.