SUMMARY When crystallisable polymers like isotactic poly(propy1ene) (iPP) or linear low density polyethylene (LLDPE) are finely dispersed in an incompatible matrix like atactic polystyrene (PS) a fractionated crystallisation process will develop if the number of dispersed droplets is greater than the number of active heterogeneities originally present in the bulk polymer. In this work, several PShPP and PSLLDPE blends were prepared in a composition range where PS was always the matrix component and in some cases compatibilizers were used to enhance dispersion. By applying a self-nucleation procedure we were able to corroborate that what causes the fractionated crystallisation is the lack of highly active heterogeneous nuclei (i.e., those normally active at low supercoolings in the bulk polymer) in every droplet. A detailed characterisation of the particle size distribution was carried out by SEM and the validity of using a Poisson distribution to calculate the concentration of heterogeneities present in one blend system was examined. The calculation of the concentration of heterogeneities can qualitatively explain the presence or absence of particular exotherms in the complex DSC cooling behaviour of some compositions of the PS/LLDPE/SEBS blends. However, the effect of the dispersity of the particle size distribution was found to greatly influence the results. When sufficient amount of a compatibilizer is used to obtain the minimum possible particle size the iPP crystallises exclusively at 45 "C. The origin of such crystallisation and the possibility that it may be interpreted as arising from homogeneous nucleation is discussed along with analogous data for dispersed LLDPE.

No abstract

No abstract

No abstract

ABSTRACT:In this work, the melt crystallization of immiscible blends of isotactic polypropylene (iPP) and branched polyethylenes (PE) was followed by oscillatory shear measurements during controlled cooling. All the blends contained 20% iPP finely dispersed in several ethylene/ a-olefin copolymer matrices (with and without a nucleating agent) with densities ranging from 0.88 to 0.92 g/cm 3 (linear low, very low, and ultra low density polyethylenes: LLDPE, VLDPE, and ULDPE). The rheological results were compared with parallel differential scanning calorimetry (DSC) measurements at the same cooling rate. During preliminary evaluations of the neat resins, no effect was found of the variation of the frequency of oscillation or the applied shear strain on their crystallization (at least in the range explored in this work). In the case of the blends, when the iPP crystallized in a fractionated fashion, only one sudden increase in the storage modulus ( G) was observed during cooling due to the partial coincident crystallization of both iPP and the PE matrix. In the presence of a nucleating agent, an almost complete separation between the crystallization of both components in the blend was achieved and two increases in G were clearly observed upon cooling. A close match between the dynamic crystallization kinetics obtained by DSC and torsion rheometry was demonstrated by a direct comparison between calorimetrically measured solid conversion and G during cooling from the melt.

The National System of Youth and Children's Choirs and Orchestras of Venezuela, now in its 41 st year, serves as a powerful tool for social inclusion by means of collective music education. This paper tries to illustrate the validity of its vision and mission and clarify the ways its particular didactic practices break traditional paradigms. Along these four decades and w i t h t h e s u p p o r t o f t h e Venezuelan state, El Sistema has adapted itself to changing times, deepening its social action as proposed by Maestro José Antonio Abreu. This has been accomplished through a number of educational principles and with the creation of new programs that have responded to the needs of thousands of children and youngsters, and with the practice of music making. Here there is a close look at t h e k e y p e d a g o g i c a l a n d didactical concepts, describing the individual and collective benefits that derive from their practice. Finally, we define the e d u c a t i o n a l p r i n c i p l e s underneath the musical and social work of the institution.

a b s t r a c tThe objective of this work was to obtain, through a simple experimental assembly, the real influence of the friction and process conditions in the assisted thermoforming process. The use of simulation tools to optimize this process requires the knowledge of the sheet-plug system friction coefficient [l], the used temperature and velocity conditions, and due to that, the software does not have a wide database. For this, the users suppose l with the previous known difference between the simulated and the experimental data. For this reason, the obtaining of l may allow to achieve more accurate simulations. In this work, a modified pendulous impact in Izod geometry was used to the measurements. The l increased with the sheet temperature. On the other hand, it was not possible to detect a significant l variations regarded with the plug material.

ABSTRACT:Two experiences based on the use of computer tools for teaching courses about the design and analysis of plastic pieces and molds are presented. The purpose is to offer students the opportunity to handle softwares by combining knowledge about plastics, metals, processing, mechanizing, marketing, stresses, costs, and design.