Polyimide foams from powder: Experimental analysis of competitive diffusion phenomena

In the present study, various diffusive processes have been investigated during foaming of powdered precursors of polyimide. A detailed analysis of the powdered precursor's characteristics allows for an enhanced morphological understanding of the resulting microstructures and foam unit cell. Parameters that are central to the foaming process such as particle morphology, volatile concentration and sorption-desorption processes are evaluated. Isothermal and non-isothermal desorption experiments have been carried out by thermogravimetric analysis (TGA), and specific diffusive processes have been correlated to thermodynamic and kinetic transitions by means of modulated differential scanning calorimetry (MDSC) of the corresponding materials. It was found that two primary fluxes of volatiles, one out of the external surface of the particles (responsible for volatile desorption) and the other into the growing bubble (responsible for vapor supersaturation inside the bubble) compete against each other creating a competitive scenario that becomes the controlling factor for potential inflation within the precursor particles.     

Effect of porosity and hydrostatic pressure on water absorption in a semicrystalline fluoropolymer

This paper discusses the effect of porosity and hydrostatic pressure on diffusion kinetics and equilibrium water uptake in a semicrystalline fluoropolymer. Water sorption experiments at atmospheric pressure and under water pressures up to 250 MPa were carried out during 18 months at 40 °C on reference and porous samples. Porosity of samples was induced due to a cavitation process occurring at the highest triaxiality area of waisted and notched specimens during tensile tests. Water uptake was found to be very sensitive to porosity, showing an increase in samples with a high void fraction. On the other hand, water content decreased with increasing pressure suggesting a compaction of the porous space in which water can be stored. Two models describing this water uptake behaviour were considered. The first is a classical model which assumes that sorption occurs only by diffusion following Fick’s law. Fick’s model was found to be in agreement with the experimental results. A “Langmuir-type” sorption model was also proposed to describe water uptake in porous samples, considering a two-phase water transport mechanism: one portion of the absorbed water diffuses through the polymer matrix and the other portion is stored in voids. This model was implemented in a user subroutine using ABAQUS? software and simulations were confronted to experimental sorption curves showing satisfactory agreements. The potential of the Langmuir-type sorption model resides on its availability to be coupled to a poro-mechanical model, in order to improve the understanding of coupling between the mechanical behaviour and water sorption mechanism in a porous polymer.     

Cognitive control by brain-injured car drivers: an exploratory study

Cognitive control is a key tool for adaptation in dynamic situations. The aim of the study is to assess the relevance of a theoretical framework for cognitive control in dynamic situations, in order to understand brain-injured (BI) car drivers' cognitive impairment. The framework supports a cognitive control multimodality based on the crossing of two orthogonal dimensions: symbolic/subsymbolic; anticipative/reactive control. BI car drivers' behaviour was compared with that of a control group (CTRL) during driving simulator scenarios. Eye movement analysis, among other variables, revealed that BI car drivers made use of a more symbolic and reactive control than did CTRL drivers. CTRL drivers showed a more stable cognitive compromise than BI drivers. The latter became less symbolic and more reactive in the case of difficult scenarios. In addition, BI drivers focused on the main task of trajectory management, with fewer resources devoted to traffic interaction management. STATEMENT OF RELEVANCE: An explanation of differences between BI and CTRL drivers in terms of cognitive control requirements, attention and processing speed is put forward. From this, it is possible to derive some implications in terms of driver assistance (e.g. lane keeping or a warning assistance device) and rehabilitation.     

Do bugs foreshadow vulnerabilities? An in-depth study of the chromium project

As developers face an ever-increasing pressure to engineer secure software, researchers are building an understanding of security-sensitive bugs (i.e. vulnerabilities). Research into mining software repositories has greatly increased our understanding of software quality via empirical study of bugs. Conceptually, however, vulnerabilities differ from bugs: they represent an abuse of functionality as opposed to insufficient functionality commonly associated with traditional, non-security bugs. We performed an in-depth analysis of the Chromium project to empirically examine the relationship between bugs and vulnerabilities. We mined 374,686 bugs and 703 post-release vulnerabilities over five Chromium releases that span six years of development. We used logistic regression analysis, ranking analysis, bug type classifications, developer experience, and vulnerability severity metrics to examine the overarching question: are bugs and vulnerabilities in the same files? While we found statistically significant correlations between pre-release bugs and post-release vulnerabilities, we found the association to be weak. Number of features, source lines of code, and pre-release security bugs are, in general, more closely associated with post-release vulnerabilities than any of our non-security bug categories. In further analysis, we examined sub-types of bugs, such as stability-related bugs, and the associations did not improve. Even the files with the most severe vulnerabilities (by measure of CVSS or bounty payouts) did not show strong correlations with number of bugs. These results indicate that bugs and vulnerabilities are empirically dissimilar groups, motivating the need for security engineering research to target vulnerabilities specifically.     

Self-foaming diphenolic acid benzoxazine

This work concerns the investigation of porous polybenzoxazines based on the non-toxic renewable diphenolic acid. The approach described relies on the in situ generation of foaming agent (CO₂) during the thermal curing. For this purpose, the previously synthesized benzoxazine monomer from diphenolic acid was thermally polymerized at different temperatures. As the beginning of decarboxylation is about 200 °C, we selected five foaming temperatures (T_f) ranging from 190 to 230 °C. The influence of the foaming temperature on the cellular structure and its dependency on final properties is discussed.     

Diagnostic Tools for Wireless Sensor Networks: A Comparative Survey

The availability of tools to diagnose Wireless Sensor Network (WSN) failures is a key success factor for this type of networks as already demonstrated by several long-running deployments. By nature, WSNs are resource-constrained, fragile, complex to analyse, and failure-prone. Naturally, with the growing number of installations, it is becoming fundamental to efficiently diagnose failures as soon as possible, in order to deal with the underlying causes. In accordance with this, from 2005 onwards, the offer of diagnostic tools has been increasing, as the other base technologies (e.g. networking, operating system, localisation, synchronisation) become reasonably stable. The purpose of this survey is to provide an overview of existing post-deployment WSN diagnostic tools, by briefly presenting their functionality, architecture and constraints, in order to enable a basic understating of each tool. The survey also includes a multi-dimensional comparative analysis of the various tools, based on a proposed classification scheme and evaluation criteria, as well as an identification of the main open research issues. Although the number of diagnostic tools is high and considerable work has been done in this area, we conclude that there are still several challenges concerning post-deployment WSN diagnostic tools, regarding scope, flexibility, generality, mobility and security. Moreover, there is a need for mature, native diagnostic-oriented functionality in WSN platforms and operating systems.     

Photon absorption in a hybrid slurry photocatalytic reactor: Assessment of differential approximations

Radiative transfer inside a slurry photocatalytic reactor with hybrid illumination from both solar radiation and lamps is examined. The local volumetric rate of photon absorption is evaluated. For this purpose, the P1 and the modified differential approximations (MDAs) are used, and results compared to a solution by the Monte Carlo method. It is found that significant differences may arise between the predictions of the above approximations and the exact results provided by the Monte Carlo simulations. The P1 approximation is very inaccurate near to the radiation entrance for the partially collimated solar radiation, although it improves, as optical depth increases. As expected, the MDA improves the results near to the boundary. Surprisingly, it turns out to be much worse than the P1 approximation at medium and large optical depths. In the case of lamp irradiation, the behavior of the MDA is the opposite; it works better at small optical depths. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3256–3265, 2012     

Effect of cavity surface coating and molding parameters on pressure drop: Developing a new software for rapid calculations

Nowadays, many of the plastic components are aesthetic pieces, and they are fabric, thermoplastic vulcanizates (TPVs), or film coated. In the case of upholstered pieces, they are injected onto a textile tissue introduced into the mold, and they are finished with an edging process. Simulation programs used to optimize the injection process are not able to simulate the injection over fabrics, and therefore, it is difficult to select an injection machine without knowing the pressure drop inside the mold and without knowing how the parameters affect to the injection cycle or to the aesthetic aspect of this kind of pieces. This article presents a software for rapid calculations during the filling phase without the need for complex modeling. The program will be able to calculate pressure drop and other flow parameters that are fundamental for a good design of an industrial mold used in in‐mold decoration (IMD) technique. POLYM. ENG. SCI., 2012. © 2011 Society of Plastics Engineers     

Renewable polybenzoxazines based in diphenolic acid

The reaction of mixtures of renewable diphenolic acid (DPA) and its methylesterbenzoxazine derivative (MDP-Bz) has been studied. The DPA was introduced to lower the high temperature needed to complete the curing of the pure benzoxazine. In this way, samples with different DPA/MDP-Bz ratio (0, 2, 5, 10 and 25% of DPA) were investigated. Moreover, high performance flame retardant thermosetting resins with phosphorus were prepared through the mixture of MDP-Bz and a DPA-phosphazene derivative (DPA-PPZ). The curing behavior of these materials was studied by differential scanning calorimetry (DSC). Finally, the properties of the materials were evaluated by termogravimetric analysis (TGA), dynamic mechanical analysis (DMTA), tensile measurements, limiting oxygen index (LOI) and UL-94 Burn Test.