Analysis of temperature effects near mode I cracks in glassy polymers

A previous isothermal study has shown that the toughness of glassy polymers is governed by the competition between shear yielding and crazing. The present work aims at investigating loading rates for which thermal effects need to be accounted for. The influence of the heat coming from the viscoplastic shear yielding and from crazing on their competition and on the toughness is examined. Crazing is shown to be the dominant heat source, and the dependence of the craze properties on temperature appears to be key in controlling the toughness of the material.

Influence of Eragrostis lehmanniana introduction on the grass community in the Ñacuñán Reserve (Mendoza, Argentina)

En 1976 se sembró E. lehmanniana Nees en la Reserva de Ñacuñán. Se evaluaron, después de 24 años de la siembra, la tasa lineal de dispersión, la abundancia y el impacto de esta especie exótica sobre la comunidad de gramíneas perennes nativas. Se compararon sitios dominados por la especie exótica con otros que incluían sólo vegetación nativa. Para comparar la cobertura de la canopia de las gramíneas, la cantidad de especies por parcela y la abundancia de la especie exótica se usó la prueba de Mann- Whitney. La especie exótica se dispersó 32 m año-1. Ella estaba presente en los caminos internos de la Reserva donde la vegetación nativa leñosa había sido eliminada y en el 45 % de los sitios ubicados en áreas no disturbadas adyacentes a los caminos. La cobertura total de las gramíneas nativas se redujo significativamente en los sitios donde la especie exótica estaba presente. La cantidad total de gramíneas por parcela no fue afectada por la presencia de E. lehmanniana. Pappophprum caespitosum, la gramínea dominante en Ñacuñán, y Sporobolus cryptandrus presentaron cobertura más baja en los sitios dominados por la especie exótica que en aquéllos no disturbados. La introducción de E. lehmanniana tuvo un impacto negativo en la composición de la comunidad de gramíneas perennes nativas en los sitios disturbados de la Reserva.E. lehmanniana Nees was seeded in Ñacuñán Reserve in 1976. The linear rate of spread, the abundance of this exotic grass and the impact of its introduction on the native perennial grass community were evaluated after 24 years seeding. Sites dominated by the exotic grass and those including only native vegetation were compared. The Mann- Whitney test was applied to compare grass canopy cover, number of species per plot and abundance of the exotic grass between the mentioned sites. The exotic grass spread 32 m year-1. It was present in tracks where the native woody vegetation had been removed, and in 45 % of the sites located in undisturbed areas adjacent to tracks. The total native grass canopy cover was significantly reduced in the sites where lovegrass was present. The number of total grass species per plot was not affected by the presence of the exotic grass. Pappophprum caespitosum, the dominant grass in Ñacuñán, and Sporobolus cryptandrus presented lower canopy cover in the sites dominated by the exotic species than that in the undisturbed ones. The introduction of E. lehmanniana has had a negative impact on the composition of the native perennial grass community in the disturbed sites of the Reserve.Fil: Guevara, Juan Carlos. Universidad Nacional de Cuyo. Facultad de Ciencias Agrarias. Departamento de Ciencias Económico-Jurídico y SocialesFil: Estevez, Oscar R..Fil: Stasi, Carlos R.

Biogenic silver nanoparticles: Understanding the antimicrobial mechanism using Confocal Raman Microscopy

The antimicrobial properties of silver nanoparticles (AgNPs) have made them ubiquitous in a number of real-world industrial applications; however, the antimicrobial mode of action of biogenic AgNPs is not entirely understood. The use of Raman spectroscopy can provide molecular fingerprint information on various chemical and biochemical components in complex systems like microbial cultures, without the need for any complex sample pre-treatment. Consequently, the antimicrobial mechanism of AgNPs can be inferred through morphological and compositional changes of microbial cells that are monitored via changes in Raman band profiles. Here we show the synthesis of biogenic AgNPs using the extracellular cell-free filtrates of Penicillium expansum. The antimicrobial activity of the Penicillium expansum synthesized silver nanoparticles (hereafter PeNPs) was evaluated and the interactions between the nanoparticles and Escherichia coli were studied using Transmission Electron Microscopy (TEM) and Environmental Scanning Electron Microscopy (ESEM), showing the attachment of PeNPs to the surface of the bacteria and rupture of the bacterial cell membrane. Importantly, we show how Confocal Raman Microscopy can be used as an innovative approach to study the antimicrobial mechanisms, the results of which confirm that the PeNPs induce damage to bacterial and fungal cells, resulting in critical changes to polysaccharides, lipids, proteins and nucleic acids

Computational Modeling of Failure Processes in Polymers

This paper deals with the modeling of the key mechanisms involved in the fracture of polymers: shear yielding and crazing. Along with the continuum model for shear yielding, we will discuss a recently proposed cohesive surface model for crazing. Applications to be presented include the study of the competition between the two mechanisms during growth of a mode I crack, and a numerical investigation of the role of localized deformations in failure of a polymer blend