Atomistic simulation of fluoropolymers : impact of regiodefects on characterization of polyvinylidene fluoride

L'alternance de deux groupes de polarités très différentes, CH2 et CF2, permet au poly fluorure de vinylidène (PVDF) d’être un polymère industriellement très intéressant. Cependant, cette spécificité mène aussi à d’importantes inversions du monomère lors de la polymérisation vinylique. Pendant la polymérisation, en complément de la propagation tête-queue, CH2CF2CH2CF2, les monomères inversés conduisent à l’addition en queue-queue, CF2CH2CH2CF2, et tête-tête, CH2CF2CF2CH2. Le taux de transformation de polymère se trouve expérimentalement entre 3 et 7%. Ce pourcentage élevé entraine sans aucun doute la modification de propriétés macroscopiques. En utilisant la dynamique moléculaire, cette thèse a pour but de montrer l'effet de ces défauts sur la température de transition vitreuse (Tg), la dynamique locale et sur la température de fusion (Tm) du PVDF. En phase amorphe, le PVDF avec différents pourcentages de régio-défauts a été étudié : 3.6, 4.1, 9.3 et 23%. Cette étude permet de prédire le comportement de polymères qui ne sont pas synthétisés. Étant donné que les Tg simulées et expérimentales concordent avec précision, les motifs moléculaires qui donnent lieu à l'effet plastifiant de l'inversion de monomères peuvent être envisagés. En plus d'accentuer leur effet de plastifiant, la conclusion significative est que la relaxation de la chaîne peut être révélée en abordant explicitement des mouvements locaux. Car cette procédure ne peut pas être déduite de la connaissance du Tg, nous avons basé notre analyse sur le fait cela : 1) Nous avons démontré que des relations linéaires directes entre Tg et l'énergie d'activation conformationnelle de transition (Ea) extraite à partir d'un graphe d'Arrhenius, existent. Ce diagramme correspond au logarithme naturel des taux de transition entre les états rotameriques contre l'inverse de la température. La pente de cette courbe rapporte directement à cet Ea efficace. Un tel lien a été seulement spéculé dans la littérature. 2) Nous avons calculé des relations d'Arrhenius pour différents genres de torsions le long de la chaîne d'épine dorsale. En conséquence, une barrière d'énergie potentielle, ea, est associée à la rotation d'un lien dans un environnement spécifique. L'addition de ces énergies pesées par le pourcentage de chaque lien le long de l'épine dorsale, donne un ea moyen qui est équivalent à l'ea efficace. À l'aide de cette procédure, nous avons maintenant accès au mouvement local de la chaîne entière. 3) Nous avons vérifié cette procédure pour calculer une valeur pour le Tg du copolymère alternatif du l'éthylène-tétrafluoroéthylène (E-TFE), qui possède les segments qui sont présents le PVDF changé. L'ambiguïté concernant la valeur de la Tg du copolymère E_TFE peut être résolue grâce à cette approche, puisque le PVDF avec 50% de défauts régio-isomériques conduit à l'E_TFE. D'ailleurs, nous avons étudié les temps de relaxation pour la fonction d'autocorrélation de torsion au-dessus d'un large éventail de température. La dynamique locale est alors spécifiquement étudiée. L'équation Vogel-Fulcher-Tammann (VFT) est utilisée pour décrire le processus de relaxation associée aux mouvements coopératifs des segments le long de la chaîne. Nous avons également étudié le possibilité d'utiliser le Kohlrausch-Williams-Watts (KWW), fonction exponentielle étirée, afin de décrire la dépendance temporelle du processus de relaxation, ce travail a été effectué à différentes températures. Les résultats concordent bien avec les données expérimentales. L'objectif principal de cette section est d'étudier conjointement la fréquence des transitions conformationnelles et le temps de relaxation obtenu par la fonction d’autocorrelation de torsion, sur une plage importante de température, afin d’établir un entre les fréquences des transitions conformationnelles et le comportement de type VFT. Nous montrons pour la première fois qu’une relation linéaire peut être établie entre la barrière de transition conformationnelle et l’énergie d’activation effective. Nous montrons pour la première fois qu'une relation linéaire peut être établie entre la barrière de transition conformationnelle, Ea et l'énergie d'activation effective, B, responsables de la dynamique locale. Parmi les cinq phases cristallines que présente le PVDF, les cristaux α et ß présentent des propriétés particulières intéressantes et ont fait l'objet d'une attention significative. Ces deux structures cristallines sont celles que l’on rencontre le plus souvent, la phase α est la plus thermodynamiquement stable le cristal β possède des propriétés ferroélectriques. Toutefois, le comportement lors de la fusion de ces deux phases cristallines n’est pas encore totalement compris. Certains chercheurs pensent que la température de fusion de la phase β est supérieure à la phase alpha . D'autres affirment que le pic endothermique vu sur le thermogramme obtenue par calorimétrie différentielle à balayage (DSC) a été attribué par erreur à la phase β, cela à cause d’une confusion dans les références . À cet égard, le comportement de la Tm des cristaux α et β par rapport à leur épaisseur est obtenu par la dynamique moléculaire. Différents types de nanocristaux composés de chaînes de PVDF, sans ou avec 10% de régio-défauts, ayant des longueurs différentes ont ainsi été simulées dans les phases α et β. On applique l'équation de Gibbs-Thomson (G-T) afin de déterminer l'énergie de surface et l’enthalpie de fusion des nanocristaux. Les valeurs déterminées sont en accord avec les données expérimentales. Nous avons montré que le PVDF en phase β pur a une température de fusion inférieure à celle du PVDF en phase α pur. Cependant, en insérant des défauts à l'intérieur du cristal, la phase α modifiée présente une température de fusion inférieure à celle de la phase β modifiée.Abstract : Alternating two groups, CH2 and CF2, of very different polarities along the backbone chain of polyvinylidene fluoride (PVDF) leads to very interesting properties, such as ferroelectricity. However, these properties are affected by the presence of regioisomerism defects (monomer inversion) that appear during the synthesis. During the polymerization, in addition to the Head-to-Tail (HT) sequences, CH2CF2CH2CF2, the reversed monomer units lead to formation of Tail-to-Tail (TT), CF2CH2CH2CF2, and Head-to-Head (HH), CH2CF2CF2CH2, links. The rate of this chain alteration experimentally lies between 3 and 7 %. This percentage undoubtedly brings changes in macroscopic properties. The aim of this thesis is to reveal the impact of these defects on the glass transition temperature (Tg), local dynamics and melting temperature (Tm) of PVDF by using Molecular Dynamics (MD) simulation. In amorphous phase, PVDF chains with different percentages of regiodefects were investigated: 0, 3.6, 4.1, 9.3, and 23 %. This study makes it possible to predict the experimental behavior of polymers which have not yet been synthesized. Once Tg is acquired, the relaxation of the chain can be investigated through the calculation of the activation energy (Ea) of the conformational transition. The significant conclusion is that the relaxation of the chain can be revealed by addressing the local motions. More specifically: a) We demonstrate a direct linear relationship between Tg and Ea extracted from an Arrhenius plot. This diagram corresponds to the natural logarithm of transition rates between rotameric states versus the inverse of the temperature. The slope of this curve yields directly Ea. Such a link was only speculated in the literature. b) A significant finding of this work is that the mobility of the chain can be associated with different types of bonds in PVDF with regiodefects. c) Based on the analysis of Ea for the different bond contributions, we proposed a value for the Tg of ethylene-tetrafluoroethylene (E-TFE), an isomeric polymer of PVDF with 50% regiodefects. Experimentally, the available data for the Tg of E-TFE are limited and highly variable. For example, it has been reported as varying from -108 °C to 145 °C. The ambiguity of Tg for this copolymer can be resolved with this approach. Furthermore, we studied the relaxation time associated with the torsional autocorrelation function (TACF) over a wide temperature range. The Vogel-Fulcher-Tammann (VFT) equation was used to describe the temperature dependence of the relaxation time. The Kohlrausch Williams Watts (KWW) stretched exponential function is then applied to fit the time dependence of the relaxation process at various temperatures. The results obtained from this work were found to be in good agreement with the experimental data. A particular interest in this study is the question of how the non-Arrhenius VFT of relaxation process is related to the Arrhenius behavior of conformational jump rates near the glass transition. In both cases, the energies (the conformational transition energy (Ea) and the effective activation energy (B) in VFT equation), were very close to the value of a single torsional barrier. However, in contrast to the relaxation time associated with TACF, the rates of conformational jumps show the activation energy higher than the single barrier value. We have shown that a linear relationship can be established between the conformational transition energy and the effective activation energy. In crystalline PVDF, among the five typical phases, the α and β crystals are of particular interest. The α phase is the most thermodynamically stable form and the β crystal possesses ferroelectric properties. The melting behaviour of these two crystal phases is not so clear. Some researchers believe that the melting temperature of the β phase is higher than that of the α phase. Others have claimed that the higher melting temperature of the peak in Differential Scanning Calorimetry (DSC) has been mistakenly attributed to β phase melting, due to confusion in the referencing of literature sources. In this regard, the melting temperatures of α and β crystals (with and without regiodefects) with respect to their thickness are captured by MD simulation. We then applied the Gibbs-Thomson (G-T) equation to determine the melting temperature, as well as the surface energy and enthalpy of fusion, for α and β nanocrystals. We have shown that pure β phase PVDF has a lower melting temperature than pure α phase PVDF. However, by inserting regiodefects randomly inside the crystal, the α phase with regiodefects shows a lower melting temperature than that of the β phase with regiodefects. We attributed this behaviour to the different structures of the two phases

PRIMM and Proper: Authentic Investigation in HE Introductory Programming with PeerWise and GitHub

We explore the use of the PRIMM methodology (Predict, Run, Investigate, Modify, Make) within a higher education introductory programming setting, particularly focusing on the three first three steps. Formative prediction questions on the effects of changes to HTML, CSS or JavaScript code are constructed by students using PeerWise system, based on their own investigation. Authenticity of the task is enhanced by presenting the peer prediction questions as pull requests to a GitHub repository, mirroring the code review process followed by professionals working within software development teams. We report on student engagement with the formative practical exercises and analyse the content of the questions they asked

A novel temporal trust-based recommender system

While Collaborative Filtering (CF) recommender systems, which focus on previous indicate preferences, are known for their traditional problems such as cold-start, sparsity and modest accuracy, trust-based CF has been previously proposed to solve such issues by focusing on trust values among the users. Nonetheless, most existing approaches use trust as an independent factor from time, in this paper we argue that trust value between users is dynamic; hence it change over time. For this reason we propose a novel temporal-trust based approach to calculate trust values aware of time of friendship. To validate the proposed approach in this paper, we used Delicious data set and compared our approach with traditional CF and trust-based approaches. Results showed that accuracy of proposed approach overcomes the traditional approaches

The Study of parents’ attitudes toward the status of execution of physical education course in Iranian elementary schools

The main objective of this research was to study the attitude of the elementary school students' parents in Tehran concerning the present status of physical education course on elementary schools. For this purpose, the personal characteristics of 300 individuals were randomly selected from among the parents whose children studying at public elementary schools in Tehran, Iran. Research method was a descriptive and the data were gathered through field method. In order to collect information, a self-made questionnaire including 15 closed questions was used. After doing data analysis, the results indicated that the parents’ attitudes toward the physical education lesson were positive. There was no significant difference between the attitude of students’ parents in case of execution. However, there was a significant difference between the attitude of the students’ parents about the importance of physical educations and sports

L’École polytechnique et l’international : un bilan historique

Cet article résume l’essentiel des hypothèses que nous avons développées dans une thèse de doctorat soutenue à l’Université Paris VII en décembre 1997 et intitulée « La formation des élites scientifiques et techniques à l’École polytechnique française aux 19e et 20e siècles ». Au cours de cette étude, nous avons tenté de préciser le rôle international de l’École pendant ses deux cents années d’existence, l’objectif étant d’effectuer un inventaire historique du « rayonnement » de l’établisseme..

Premature and Avoidable Mortality by Neighbourhood Income in Urban Canada

Background: Canadians are among the healthiest people in the world. Policy initiatives, such as implementing universal healthcare, have been employed to distribute this health equally among different groups of people in the country. Yet, health inequalities still exist and, in some cases, are growing across Canada. A key step to address health inequalities is to improve the measurement, monitoring, and reporting of social determinants of health, population health status, and the extent of health disparities. Purpose: The underlying purpose of this study is to provide evidence on health inequalities and how these inequalities are related to income disparities in urban Canada, at the municipal, provincial, and national levels, to empower policymakers in evidence-based decision-making to implement policies to tackle health disparity in Canada. Methods: This study was conducted in two phases. The first phase was a scoping review on application of the concept 'avoidable mortality' in socioeconomic-related health inequality research. The second phase of this study was a retrospective population-based analysis, conducted in two parts; first, the Canadian Vital Statistics - Death Database (CVSD) from 2011 to 2015 and Canadian Censuses 2011 and 2016 were used to calculate all-cause, premature, avoidable, preventable, and treatable mortality rates at municipal, provincial, and national levels; then, the National Household Survey (NHS) 2011 was used to measure income inequality in mortality by calculating concentration index, at municipal, provincial, and national levels. Results: Findings spanning the period of 2011 to 2015 suggest that overall, income inequality in mortality exists at every level of geography in Canada. This study found a higher inequality in preventable mortality as compared to the inequality in all-cause, premature, avoidable, and treatable mortality, at all geographic levels in Canada. Another finding of this study shows that, at the provincial level, Saskatchewan was the only province with an inequality higher than national level inequality in all the mortality indicators. At the Census Metropolitan Area (CMA) level, Winnipeg was the most unequal CMA in premature, avoidable, preventable, and treatable mortality indicators over the study period. Conclusion: Findings of this study highlight the need for an improvement in policies whose primary purpose is prevention, to curb the preventive mortality burden and reduce disparity in mortality. Additionally, the geographic variation in inequality found in this study points to the need for future research on identifying the factors leading to this difference in inequality across jurisdictions

The Study of parents’ attitudes toward the status of execution of physical education course in Iranian elementary schools

The main objective of this research was to study the attitude of the elementary school students' parents in Tehran concerning the present status of physical education course on elementary schools. For this purpose, the personal characteristics of 300 individuals were randomly selected from among the parents whose children studying at public elementary schools in Tehran, Iran. Research method was a descriptive and the data were gathered through field method. In order to collect information, a self-made questionnaire including 15 closed questions was used. After doing data analysis, the results indicated that the parents’ attitudes toward the physical education lesson were positive. There was no significant difference between the attitude of students’ parents in case of execution. However, there was a significant difference between the attitude of the students’ parents about the importance of physical educations and sports

Pour le Centenaire de 1894 : l’Annuaire de l’Ecole polytechnique

Première célébration de l’institution, le centenaire de l’Ecole polytechnique s’est déroulé en 1894 dans un contexte de remise en cause du rôle scientifique et de l’utilité publique de l’Ecole polytechnique. Il a porté la marque des questions vives de l’époque : consolidation des valeurs républicaines, réconciliation de l’Eglise et de l’Etat, expansion coloniale, front devant la crise boulangiste et rivalité franco-allemande. Au cours des cinq longues années de préparation et tout le long des..

Proizvodnja kruha s dodatkom mikroinkapsuliranih sinbiotika

Bread is a global staple food. Despite attempts to develop functional breads containing viable microorganisms, this has not been done yet because of the high temperature during baking. The aim of this study is to obtain synbiotic bread, hence hamburger bun and white pan bread were selected. Lactobacillus acidophilus LA-5 and L. casei 431 were encapsulated with calcium alginate and Hi-maize resistant starch via emulsion technique and coated with chitosan. The morphology and size of microcapsules were measured by scanning electron microscopy and particle size analyser. Inulin was added at 5 % wheat flour mass basis for prebiotic effect. The encapsulated probiotics were inoculated into the bread dough and bread loaves were baked. The survival of encapsulated probiotics was determined after baking; also sensory evaluation was performed. Both types of bread met the standard criteria for probiotic products. The probiotic survival was higher in hamburger bun. L. casei 431 was more resistant to high temperature than L. acidophilus LA-5. A significant increase in probiotic survival was observed when the protective coating of chitosan was used in addition to calcium alginate and Hi-maize resistant starch. Storage for 4 days did not have any effect on the viability of encapsulated bacteria. The addition of encapsulated bacteria did not have any effect on flavour and texture; however, 5 % inulin improved the texture of bread significantly. Results show that microencapsulation used in the production of synbiotic bread can enhance the viability and thermal resistance of the probiotic bacteria.Kruh je jedan od osnovnih prehrambenih proizvoda u svijetu. Dosad nije postignut uspjeh u proizvodnji funkcionalnog kruha jer mikroorganizmi ne preživljavaju visoku temperaturu pečenja. Stoga je svrha ovoga rada bila proizvesti kruh sa sinbioticima, i to pšenično bijelo pecivo za hamburgere i pšenični bijeli kruh. Sojevi bakterija Lactobacillus acidophilus LA-5 i L. casei 431 inkapsulirani su tehnikom geliranja emulzije s kalcijevim alginatom i rezistentnim kukuruznim škrobom Hi-maize kao nosačima, a dobivene su kapsule obložene hitozanom. Morfologija i veličina mikrokapsula ispitane su pomoću pretražnog elektronskog mikroskopa te s uređajem za mjerenje veličina čestica. Za postizanje prebiotičkog učinka pšeničnom je brašnu dodano 5 % inulina, a inkapsulirani probiotici inokulirani su u tijesto prije pečenja. Preživljavanje je inkapsuliranih probiotika određeno nakon pečenja, te je provedena senzorska analiza. Obje su vrste kruha zadovoljile zadane kriterije za probiotičke proizvode. Veći je broj stanica probiotika preživio postupak pečenja u pecivu za hamburgere. Soj L. casei 431 bio je otporniji na visoku temperaturu od soja L. acidophilus LA-5. Oblaganje kapsula zaštitnim slojem hitozana je uz primjenu kalcijevog alginata i rezistentnog škroba Hi-maize znatno povećalo preživljavanje probiotičkih sojeva. Broj živih stanica probiotika nije se smanjio nakon četiri dana skladištenja, a dodatak probiotika nije utjecao na okus i teksturu kruha. Međutim, dodatkom 5 % inulina bitno se poboljšala tekstura kruha. Rezultati pokazuju da postupak mikroinkapsulacije povećava postotak preživljavanja i toplinsku otpornost probiotičkih bakterija tijekom pečenja kruha