Analysis Of Low Temperature Impact Fracture Data Of Thermoplastic Polymers

Impact fracture toughness of polypropylene (PP) blends, high density polyethylene (HDPE) and rubber toughened polymethylmethacrylate (RTPMMA) has been studied by means of three-point bending falling weight impact testing at different temperatures ranging from -60 degrees C to room temperature using the cleavage fracture toughness, JC parameter [ASTM E1820-99a]. The latter Fracture Mechanics methodology was chosen due to its simplicity [Fasce et al., 2003]. Traces of the impact tests were analyzed using an inverse methodology just proposed by Pettarin et al. (2003). This methodology makes it possible to obtain from a three-point bending instrumented impact test the mechanical response of the material, discarding the dynamic effects associated with the test. The results show that the average JC values calculated with treated and untreated data are similar for a given material, while the standard deviations are larger when the calculations are made with the untreated data. It is clear that the inverse methodology used to correct the data reduces error propagation, giving place to more precise estimations, and therefore more reliable JC values

Uni- and biaxial impact behavior of double-gated nanoclay-reinforced polypropylene injection moldings

Polypopylene/nanoclay three-dimensional parts were produced without intermediate steps by direct injection molding to explore the influence of flow features and nanoclay incorporation in their impact performance. The nanocomposite was obtained by direct compounding of commercial PP with nanoclay masterbatch. The as-molded morphology was analyzed by X-ray and TEM analyses in terms of skin-core structure and nanoclay particle dispersion. The nanoclay particles induced the reduction of b-form spherulites, a known toughener. The impact behavior was assessed in tensile and biaxial modes. The PP nanocomposite molding toughness was practically unaffected by the processing melt temperature and flow rate. Conversely the nanoclay presence is influent in the impact performance. Under biaxial stress impact, the regions close to weld lines are tougher than the bulk and the fracture develops with main crack paths along the flow direction and the weld line. Cracking along the weld line results from less macromolecular interpenetration and chain entanglement, and unfavorable nanoparticle orientation. It seems that a failure mechanism which involves nanoclay delamination and multiple matrix crazing explains the toughening of PP in the directions where the nanoparticle orientation with respect to loading is adequate.Contract grant sponsors: CONICET, ANPCyT from Argentina, MINCyT (Argentina) - FCT (Portugal), Universities Nacional de Mar del Plata and Minho

Surface property effects of compounding a nanoclay masterbatch in PP injection moulding

Indicado para o prémio de melhor artigo mais inovador.The interest on the use of nanofillers in injection mouldings has been going on for more than a decade but a real breakthrough has not been achieved yet, especially in that mechanical properties are concerned. The nucleating effect of nanoclays in semicrystalline polymers suggests that surface effects may result interesting especially during processing. This paper includes some information on the surface properties of an injection moulding grade of polypropylene mixed with a commercial masterbatch of PP and 50% of organoclay. They were moulded as plates for testing in a prototype device for determining the coefficient of friction in as-moulding conditions. The surface was also characterised by depth sensing indentation tests. The through thickness microstructures of the mouldings were assessed by optical microscopy and differential scanning calorimetry, while surface morphology was assessed by X-ray diffraction. It was observed that independently of MB content, its addition caused a slight increase in elastic modulus and hardness in the skin layer.The friction properties directly associable to the product performance showed a slight improvement in terms of the dynamic friction coefficient. Conversely the static friction coefficient that is relevant in processing was no affected by the presence of the nanoclay

Synergistic effects of nanoclay and SGF on tribological and dynamic properties of polypropylene composites

In recent year’s polymer/layered silicate (PLS) nanocomposites have attracted great interest, both in industry and in academia, because they often exhibit remarkable improvement in materials properties when compared with virgin polymer or conventional micro and macrocomposites. These improvements can include high moduli, increased strength and heat resistance, decreased gas permeability and flammability, and increased biodegradability of biodegradable polymers. However these properties are strongly influenced by how the clay is dispersed in the polymer. In this study the synergistic effects in PP+short glass fiber+nanoclay systems in the tribogical and dynamic properties in injection mouldings were analysed. The materials used were a Polypropylene Homopolymer, Nanoclay (montmorillonite layer silicate) for Polyolefin Nanocomposites in percentages of 2%, 6% and 10% and a Polypropylene Homopolymer with content of 10% and 30% of glass fiber reinforced. The various materials systems were characterized in terms of dynamic properties and tribological properties. Several tests were conducted which includes the measurements of coefficient of friction in conditions similar to the ejection phase in injection moulding process. The microstructure of the mouldings was characterized by DSC. Polymer properties are determined by the incorporation of nanoclays, SGF and by processing. Moreover influencing the microstructure of the mouldings and a synergistic effect of the nano and micro reinforcements are also observed

Photoluminescence of Bridged Silsesquioxanes Containing Urea or Urethane Groups with Nanostructures Generated by the Competition between the Rates of Self-Assembly of Organic Domains and the Inorganic Polycondensation

The aim of this study was to investigate the changes produced in the nanostructures and the photoluminescence spectra of bridged silsesquioxanes containing urea or urethane groups, by varying the relative rates between the self-assembly of organic domains and the inorganic polycondensation. Precursors of the bridged silsesquioxanes were 4,4‘-[1,3-phenylenebis(1-methylethylidene)]bis(aniline) and 4,4‘-isopropylidenediphenol, end-capped with 3-isocyanatopropyltriethoxysilane. The inorganic polycondensation was produced using either high or low formic acid concentrations, leading to transparent films with different nanostructures as revealed by FTIR, SAXS, and ²⁹Si NMR spectra. For the bridged silsesquioxanes containing urea groups the self-assembly of organic domains was much faster than the inorganic polycondensation for both formic acid concentrations. However, the arrangement was more regular and the short-range order higher when the rate of inorganic polycondensation was lower. The photoluminescence spectra of the most ordered structures revealed the presence of two main processes:  radiative recombinations in inorganic clusters and photoinduced proton-transfer generating NH₂⁺ and N⁻ defects and their subsequent radiative recombination. In the less-ordered urea-bridged silsesquioxanes a third process was present assigned to a photoinduced proton transfer in H-bonds exhibiting a broad range of strengths. For urethane-bridged silsesquioxanes the driving force for the self-assembly of organic bridges was lower than for urea-bridged silsesquioxanes. When the synthesis was performed with a high formic acid concentration, self-assembled structures were not produced. Instead, large inorganic domains composed of small inorganic clusters were generated. Self-assembly of organic domains took place only when employing low polycondensation rates. For both materials the photoluminescence was mainly due to radiative processes within inorganic clusters and varied significantly with their state of aggregation.The financial support of the National Research Council (CONICET, Argentina), the National Agency for the Promotion of Science and Technology (ANPCyT, Argentina, PICT 14738-03), the University of Mar del Plata, the Grant Agency of the Czech Republic (Project 203/05/2252), and Project Nanoter (Project MAT2004/01347, MEC-DGI, Spain) is gratefully acknowledged. INTEMA and the Institute of Macromolecular Chemistry acknowledge the support of the European Network of Excellence Nanofun-Poly for the diffusion of their research results

Estructura factorial de la escala DREEM en estudiantes de medicina chilenos

Indexación: ScieloBackground: The entry to a University requires an adaptation process that not all students solve with the same kind of success. Even though students’ social adaptation and emotional skills are essential, the educational environmental that they perceive has a significant influence in their academic life. Aim: To describe the changes in the perception about academic environment that medical students experience during the first three years of undergraduate career. Material and Methods: The Dundee Ready Education Environment Measure (DREEM) scale was applied to 525 first to third year medical students and an exploratory factorial analysis was made. Results: Four factors were identified: Academic Perception: academic quality that students attribute to the process in which they take part, as well as to the assessment that they do of their learning outcomes (coefficient α = 0.85); Academic Experience: refers to positive emotions that students experience during the career such as confidence, pleasure and energy (coefficient α = 0.76); Atmosphere Perception, comfort and calm that students experiment during their academic activities (coefficient α = 0.79); Teachers Perception: the perception that students have of teachers about their interest and disposition towards students (coefficient α = 0.50). Conclusions: The assessment of academic environment quality is inversely associated with the lapse that the students have spent in their undergraduate careers. Key words: Education, Medical; Psychometrics; Students, medical; Undergraduate

Assessment of psychometric properties of the academic involvement questionnaire, expectations version

Indexación: Web of Science; Scielo.Background: Academic Involvement Questionnaire, Expectations version (CIA-A), assesses the expectations of involvement in studies. It is a relevant predictor of student success. However, the evidence of its validity and reliability in Chile is low, and in the case of Medical students, there is no evidence at all. Aim: To evaluate the factorial structure and internal consistency of the CIA-A in Chilean Medical school freshmen. Material and Methods: The survey was applied to 340 Medicine freshmen, chosen by non-probability quota sampling. They answered a back-translated version of CIA-A from Portuguese to Spanish, plus a sociodemographic questionnaire. For psychometric analysis of the CIA-A, an exploratory factor analysis was carried on, the reliability of the factors was calculated, a descriptive analysis was conducted and their correlation was assessed. Results: Five factors were identified: vocational, institutional and social involvement, use of resources and student participation. Their reliabilities ranged between Cronbach’s alpha values of 0.71 to 0.87. Factors also showed statistically significant correlations between each other. Conclusions: Identified factor structure is theoretically consistent with the structure of original version. It just disagrees in one factor. In addition, the factors’ internal consistency were adequate for using them in research. This supports the construct validity and reliability of the CIA-A to assess involvement expectations in medical school freshmen.http://ref.scielo.org/r2sn6

Treatment of hypertension in rural Cambodia: results of a 6-year programme

This study was aimed to describe the outcomes of a hypertension treatment programme in two outpatient clinics in Cambodia. We determined proportions of patients who met the optimal targets for blood pressure (BP) control and assessed the evolution of mean systolic and diastolic BP (SBP/DBP) over time. Multivariate analyses were used to identify predictors of BP decrease and risk factors for LTFU. A total of 2858 patients were enrolled between March 2002 and June 2008 of whom 69.2% were female, 30.5% were aged >/=64years and 32.6% were diabetic. The median follow-up time was 600 days. By the end of 2008, 1642 (57.4%) were alive-in-care, 8 (0.3%) had died and 1208 (42.3%) were lost to follow-up. On admission, mean SBP and DBP were 162 and 94 mm Hg, respectively. Among the patients treated, a significant SBP reduction of 26.8 mm Hg (95% CI: 28.4-25.3) was observed at 6 months. Overall, 36.5% of patients reached the BP targets at 24 months. The number of young adults, non-overweight patients and non-diabetics reaching the BP targets was more. Older age (>64 years), uncontrolled DBP (>/=90 mm Hg) on last consultation and coming late for the last consultation were associated with LTFU, whereas non-diabetic patients were 1.5 times more likely to default than diabetics (95% CI: 1.3-1.7). Although the definite magnitude of the BP decrease due to antihypertension medication over time cannot be assessed definitely without a control group, our results suggest that BP reduction can be obtained with essential hypertension treatment in a large-scale programme in a resource-limited setting

An instrument to measure psychosocial determinants of health care professionals' vaccination behavior: Validation of the Pro-VC-Be questionnaire

Objectives: The lack of validated instruments assessing vaccine hesitancy/confidence among health care professionals (HCPs) for themselves, and their patients led us to develop and validate the Pro-VC-Be instrument to measure vaccine confidence and other psychosocial determinants of HCPs' vaccination behavior among diverse HCPs in different countries.Methods: Cross-sectional survey in October-November 2020 among 1,249 GPs in France, 432 GPs in French-speaking parts of Belgium, and 1,055 nurses in Quebec (Canada), all participating in general population immunization. Exploratory and confirmatory factor analyses evaluated the instrument's construct validity. We used HCPs' self-reported vaccine recommendations to patients, general immunization activity, self-vaccination, and future COVID-19 vaccine acceptance to test criterion validity.Results: The final results indicated a 6-factor structure with good fit: vaccine confidence (combining complacency, perceived vaccine risks, perceived benefit-risk balance, perceived collective responsibility), trust in authorities, perceived constraints, proactive efficacy (combining commitment to vaccination and self-efficacy), reluctant trust, and openness to patients. The instrument showed good convergent and criterion validity and adequate discriminant validity.Conclusions: This study found that the Pro-VC-Be is a valid instrument for measuring psychosocial determinants of HCPs' vaccination behaviors in different settings. Its validation is currently underway in Europe among various HCPs in different languages.</p