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Characterisation of low temperature and semi-insulating GaAs lateral photo-Dember THz Emitters

Poster for IRMMW-THz conference in Mainz, Germany 2013

Photo España 2006 : fotografía y naturaleza.

Se celebra en Madrid la novena edición de PhotoEspaña con el tema de la naturaleza como marco de la exposición. En total son treinta y una exposiciones en diferentes museos, instituciones y grandes centros de exposiciones y el Festival Off, que cuenta con 39 exposiciones en galerías y otros espacios. Un tercer grupo de acontecimientos relacionados con esta iniciativa está constituido por clases magistrales de figuras como Sugimoto, Ouka Lele o Álex Hütte. Se celebran talleres, debates, premios, visionado de portafolios que permite descubrir nuevos talentos, o actividades en la calle. Además, la cineasta y fotógrafa belga Agnès Varda será objeto de una retrospectiva en la Filmoteca Española.

Time for a photo story!. 'Tiempo para una photo story!'.

La propuesta puede descargarse en la dirección: http://aula.grao.com

Linear viscoelasticity from molecular dynamics simulation of entangled polymers

The linear viscoelastic (LVE) spectrum is one of the primary fingerprints of polymer solutions and melts, carrying information about most relaxation processes in the system. Many single chain theories and models start with predicting the LVE spectrum to validate their assumptions. However, until now, no reliable linear stress relaxation data were available from simulations of multichain systems. In this work, we propose a new efficient way to calculate a wide variety of correlation functions and mean-square displacements during simulations without significant additional CPU cost. Using this method, we calculate stress−stress autocorrelation functions for a simple bead−spring model of polymer melt for a wide range of chain lengths, densities, temperatures, and chain stiffnesses. The obtained stress−stress autocorrelation functions were compared with the single chain slip−spring model in order to obtain entanglement related parameters, such as the plateau modulus or the molecular weight between entanglements. Then, the dependence of the plateau modulus on the packing length is discussed. We have also identified three different contributions to the stress relaxation:  bond length relaxation, colloidal and polymeric. Their dependence on the density and the temperature is demonstrated for short unentangled systems without inertia.

Hierarchical modeling of entangled polymers

We demonstrate that it is possible to link multi-chain molecular dynamics simulations with the tube model using a single chain slip-links model as a bridge. This hierarchical approach allows significant speed up of simulations, permitting us to span the time scales relevant for a comparison with the tube theory. Fitting the mean-square displacement of individual monomers in molecular dynamics simulations with the slip-spring model, we show that it is possible to predict the stress relaxation. Then, we analyze the stress relaxation from slip-spring simulations in the framework of the tube theory. In the absence of constraint release, we establish that the relaxation modulus can be decomposed as the sum of contributions from fast and longitudinal Rouse modes, and tube survival. Finally, we discuss some open questions regarding possible future directions that could be profitable in rendering the tube model quantitative, even for mildly entangled polymers

Microscopic Mechanisms of Strain Hardening in Glassy Polymers

The mechanisms underlying the increase in stress for large mechanical strains of a polymer glass, quantified by the strain-hardening modulus, are still poorly understood. In the present paper we aim to elucidate this matter and present new mechanisms. Molecular-dynamics simulations of two polymers with very different strain-hardening moduli (polycarbonate and polystyrene) have been carried out. Nonaffine displacements occur because of steric hindrances and connectivity constraints. We argue that it is not necessary to introduce the concept of entanglements to understand strain hardening, but that hardening is rather coupled with the increase in the rate of nonaffine particle displacements. This rate increases faster for polycarbonate, which has the higher strain-hardening modulus. Also more nonaffine chain stretching is present for polycarbonate. It is shown that the inner distances of such a nonaffinely deformed chain can be well described by the inner distances of the worm-like chain, but with an effective stiffness length (equal to the Kuhn length for an infinite worm-like chain) that increases during deformation. It originates from the finite extensibility of the chain. In this way the increase in nonaffine particle displacement can be understood as resulting from an increase in the effective stiffness length of the perturbed chain during deformation, so that at larger strains a higher rate of plastic events in terms of nonaffine displacement is necessary, causing in turn the observed strain hardening in polymer glasses.

Developing responsive preventative practices: key messages from children's and families' experiences of the Children's Fund

As part of the prevention and social inclusion agenda, the Children's Fund, set up in 2000, has developed preventative services for children at risk of social exclusion. Drawing on a large qualitative dataset of interviews conducted in 2004/05 with children, young people and their parents/carers who accessed Children Fund services, this article analyses key practices and approaches valued by children and parents. These included: specialist support tailored to individual support needs, family-oriented approaches, trusting relationships with service providers, multi-agency approaches and sustainability of services. Finally, the article draws out key lessons for the future development of preventative services.