Contribución de la ciencia a la cultura ciudadana : dificultades y perspectivas

La necesaria extensión de la cultura científica al conjunto de la ciudadanía se enfrenta a una serie de dificultades que se traducen en un alarmante descenso del interés de muchos jóvenes por los estudios científicos. El propósito de esta investigación es contribuir al estudio de dichas dificultades, así como de las estrategias para superarlas. Para ello, hemos analizado, en particular, las concepciones de los protagonistas del proceso de alfabetización científica, estudiantes y profesores, concediendo una especial importancia a las “buenas prácticas” susceptibles de mostrar vías de solución. En la comunicación se ofrecerán los primeros resultados obtenidos, tanto los correspondientes a las concepciones/obstáculo como a las pautas para promover el interés por la cultura científica propuestas por 17 profesores “innovadores/investigadores” consultados

Contribución de la ciencia a la cultura ciudadana : dificultades y perspectivas

La necesaria extensión de la cultura científica al conjunto de la ciudadanía se enfrenta a una serie de dificultades que se traducen en un alarmante descenso del interés de muchos jóvenes por los estudios científicos. El propósito de esta investigación es contribuir al estudio de dichas dificultades, así como de las estrategias para superarlas. Para ello, hemos analizado, en particular, las concepciones de los protagonistas del proceso de alfabetización científica, estudiantes y profesores, concediendo una especial importancia a las "buenas prácticas" susceptibles de mostrar vías de solución. En la comunicación se ofrecerán los primeros resultados obtenidos, tanto los correspondientes a las concepciones/obstáculo como a las pautas para promover el interés por la cultura científica propuestas por 17 profesores "innovadores/investigadores" consultados

Study of the molecular mobility in polymers with the thermally stimulated recovery technique : a review

Thermally stimulated recovery (TSR) is a non-conventional mechanical spectroscopy technique that allows to analyse in detail the relaxation processes of polymeric systems in the low frequency region. This work reviews the main aspects and potentialities of this technique. The different kinds of TSR experiments that can be performed, global and thermal sampling (TS) experiments, are described and illustrated with several examples. Also, the different methods for the determination of the thermokinetic parameters (activation energy and pre-exponential factor) of the thermal sampling (TS) procedure are explained and compared. In this context, the compensation phenomenon, which always appears in TSR results when the studies are performed in the glass transition region of a given system, is discussed. Examples of the application of this technique to different polymeric systems during the last 20 years are provided. An emphasis will be made on the analysis of the effect of crystallinity degree and crosslink density on the TSR response. A comparison between the results (characteristic times and activation energies) obtained by different techniques, namely TSR, dynamic mechanical analysis (DMA), and differential scanning calorimetry (DSC), is made

Study of the viscoelastic properties of PET by thermally stimulated recovery

The conformational chain mobility of semi-crystalline poly(ethylene terephthalate), PET, is studied in the glass transition region by thermally stimulated recovery (TSR). The thermal sampling (TS) experiments are analysed according to a simple Voigt- Kelvin model. Both the activation energy (Ea) and the pre-exponential factor follow the usual trends with temperature and present the compensation behaviour, that is discussed in this work. The influence of crystallinity and orientation on the glass transition dynamics is analysed. In particular, the profile of variation of Ea with temperature is found to be very sensitive to these two effects. TSR results are compared with the results obtained by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). In addition, this technique was successfully applied to the study of the structural relaxation process of semi-crystalline PET and it was found that the characteristic parameters of the TSR curves vary with the ageing temperature and ageing time. These variations are explained on the basis of the changes verified on the distribution of characteristic times with ageing

Enthalpy relaxation studies in polymethyl methacrylate networks with different crosslinking degrees

Structural relaxation of PMMA networks with distinct crosslink density has been studied by differential scanning calorimetry (DSC). The crosslinking agent used was ethylene glycol dimethacrylate (EGDMA). The experiments were carried out on heating after the samples have been subjected to distinct thermal histories, namely isothermal stages at different temperatures below the glass transition temperature for distinct times and cooling at different rates. These studies revealed a broadening of the glass transition with increasing crosslinking degree due to the constraints imposed by the crosslinks and suggested the presence of crosslink heterogeneity in the networks. A phenomenological model based on the configurational entropy concept was used to simulate the structural relaxation phenomenon and to evaluate the temperature dependence and distribution of the relaxation times of the conformational rearrangements for these networks. The agreement between the experimental results and the simulated thermograms was quite satisfactory. In addition, the kinetic fragility of the networks was evaluated from the results corresponding to the thermal treatments at distinct cooling rates. It was found an increase of the fragility index m with increasing crosslinking degree

Analysis of the thermal environment inside the furnace of a dynamic mechanical analyser

In this work, the thermal environment inside the furnace of a dynamic mechanical analyser was investigated by putting a standard sample in distinct positions inside the furnace. Penetration experiments were carried out in order to compare the measured melting temperature of the sample with the theoretical melting temperature. The thermal gradients were investigated for three distinct modes: compression, three-point bending and extension. In the compression mode a small variation of the measured melting point was found as a function of the radial distance. However, for both extension and three-point bending modes quite important variations were measured along the relevant directions. In the former case the bottom clamp was found to be warmer than the upper one and in the latter mode higher temperatures are found in the extremities of the samples (closer to the furnace wall). Other factors, such as the influence of the heating rate and the purge gas flow rate, were also investigated. In the particular case of the extension mode, it was found that the measured melting temperature decreased with increasing scanning rate. A simple model was used in order to investigate the influence of the temperature gradients on DMA measurements. For temperature gradients along the sample length below 10°C the differences in the viscoelastic parameters (tan d, D∗ and E∗) from the homogeneous case are small when compared with typical experimental errors

Influence of experimental variables on thermally stimulated recovery results: analysis of simulations and real data on a polymeric system

Thermally stimulated recovery (TSR) is a low frequency mechanical spectroscopy technique that allows investigation of conformational mobility in polymeric systems. In this study the effect of initial parameters chosen to perform experiments on the TSR response of a material in the thermal sampling mode is investigated. The studied experimental parameters are creep time (tσ) recovery time (tr) and window width (ΔTw); all are independently changed at one constant creep temperature. A simple model, able to describe global TSR and TS measurements, is used to evaluate the influence of each of the different parameters. The simulations are conducted for a system with a uniform distribution of activation energies and a fixed pre-exponential factor. These simulation results are qualitatively compared with some experimental data obtained for semicrystalline poly(ethylene terephthalate) under different conditions in the glass transition region. The tendencies resulting from the influence of the studied parameters on the intensity, the position of the TS peaks and the corresponding activation energies are found to be the same for the experimental and simulated results. Only the variation of the activation energy with tσ is opposite to that observed with the modelling results; this feature is explained on the basis of structural relaxation effects

Conformational mobility in a polymer with mesogenic side groups- dielectric and DSC studies

The relaxation times of the conformational rearrangements of the main-chain segments of a liquid crystalline side-chain polymer was determined from differential scanning calorimetry experiments in the temperature interval around and below its glass transition. Phenomenological models with fitting parameters were used to evaluate the temperature dependence of the relaxation times and the form parameter of the relaxation times distribution. These parameters were compared with its counterparts in the dielectric a relaxation process which appear in the temperature interval immediately above the calorimetric glass transition. For the temperature interval below the calorimetric glass transition the differential scanning calorimetry (DSC) results were compared with the dielectric results obtained by the thermally stimulated depolarisation technique

Aportación de la densitometría ósea en las artroplastias de rodilla

El presente trabajo consiste en una revisión clínica y radiológica de 84 artroplastias modulares de rodilla de las cuales, en 19 se ha estudiado la masa ósea periprotésica (7 cementadas y 12 sin cementar) antes, al mes y a los seis meses de la intervención mediante un densitómetro de doble fotón. Aparte, se ha probado la correlación directa entre la indicación peroperatoria de no cementar y una mayor masa ósea en todas las zonas periprotésicas; por otro lado, se ha observado una diferente distribución de la densidad mineral ósea en la tibia tras la intervención, según se haya utilizado o no cemento. Estos hallazgos confirman el estudio densitométrico previo a la intervención como un método complementario útil en la indicación de la cementación de la artroplastia total de rodilla.A clinical and radiological revision of 84 modular knee arthroplasties is presented. Periprosthesis bone mineral content was studied in 19 prostheses (7 cemented and 12 uncemented) before, surgery one and 6 months after the intervention using a densitometer of double energy. A direct correlation between the implantation of a non-cemented prosthesis and the growth of the bone mass in all the periprosthesis areas was found, existing a different distribution of the mineral bone density of the tibia after the intervention depending on the use of cement. These findings show that the densitometric study previous to an intervention as a useful complementary method in the indication of cemented total knee arthroplasty