Evolution of polymer blend morphologies during extrusion in a flat die

The control of blend morphologies during process is of prime importance in order to predict the final properties of polymer blends. A coextrusion technique combined with static mixers was developed in order to smartly blend polymeric melts and to optimize the blend morphologies during the flow in static mixers [1]. The aim of this paper is to study the evolution of those blend morphologies during extrusion in a flat die. The effect of the viscosity ratio and the interfacial tension are also investigated. The experimental observations are confronted with numerical simulation results

On-the-job learning and earnings

A simple model of informal learning on-the-job which combines learning by oneself and learning from others is proposed in this paper. It yields a closed-form solution that revises Mincer-Jovanovic's (1981) treatment of tenure in the human capital earnings function by relating earnings to the individual's learning potential from jobs and firms. We estimate the structural parameters of this non-linear model on a large French survey with matched employer-employee data. We find that workers on average can learn from others ten percent of their own human capital on entering the firm, and catch half of their learning potential in just two years. The measurement of worker's learning potential in their jobs and establishments provides a simple characterization of primary-type and secondary-type jobs and establishments. We find a strong relationship between the job-specific learning potential and tenure. Predictions of dual labor market theory regarding the positive match of primary-type firms (which offer high learning opportunities) with highly endowed workers (educated, high wages) are visible at the establishment level but seem to vanish at the job's level.Human capital, earnings functions, informal training, learning from others, learning by oneself, returns to tenure, dualism.

Crystallization behavior of poly(lactide)/poly(β-hydroxybutyrate)/talc composites.

The authors thank Dr. Patrice Lefrançois (CNAM) for SEM observations. They are also grateful to Pr. Jack R. Plimmer USDA, Agricultural Research Service, USA for his discussions during this work.The morphology and miscibility of commercial poly(lactide) (PLA)/poly(b-hydroxybutyrate) (PHB, from 5 to 20 wt %) blends prepared by melt extrusion method, were investigated using differential scanning calorimetry (DSC) and Fourier transform infra-red spectroscopy (FTIR) observations. The results show that for all the studied blend contents, PLA/PHB blends are immiscible. The effects of PHB and talc on the nonisothermal cold crystallization kinetics of PLA were examined using a differential scanning calorimetry (DSC) at different heating rates. PHB acted as a nucleating agent on PLA and the addition of talc to the blend yielded further improve-ment, since significant increase in the enthalpy peak was observed for samples containing 10 wt % PHB and talc (from 0.5 to 5 phr). The crystallization kinetics were then examined using the Avrami–Jeziorny and Liu–Mo approach. The simultaneous presence of PHB and talc induced a decrease of the crystallization half time. The evolution of activation energies determined with Kissinger’s equation suggests that blending with PHB and incorporating talc promote nonisothermal cold crystallization of PLA. The synergistic nucleating effect of PHB and talc was also observed on isothermal crystallization of PLA from the melt

Dispersion of carbon nanotubes in polypropylene via multilayer coextrusion: Influence on the mechanical properties

The authors would like to thank PSA for funding this research and providing some of the materials used in this study. We also would like to thank R. Glénat, P. Soria, E. Dandeu, A. Grand- montagne and A. Dubruc for their help in the preparation and the optical and mechanical characterizations of the samples presented in this study.Multilayer coextrusion was used to disperse Carbon Nanotubes (CNT) in polypropylene (PP). The dilution of commercially available masterbatches using a twin-screw extruder was first applied to produce several formulations, which were then mixed with PP using a multilayer coextrusion device to obtain films or pellets with CNT concentrations between 0.1 and 1%wt. The influence of the specific mechanical energy (SME) during the dilution step, of the addition of a compatibilizer, and of the multilayer tool on the CNT dispersion within the matrix was highlighted. The effect of the dispersion on the thermomechanical properties of the resulting materials was studied. We showed notably that films containing 0.2%wt CNT, 1%wt of PPgAm, prepared at high SME presented a Young’s modulus increase of 25e30% without significant decrease in the elongation at break. These results, using low amounts of CNT and industrially available devices, may show a new path for producing nanocomposites

Ion exchange membranes based upon crosslinked sulfonated polyethersulfone for electrochemical applications

Synthesis and characterization of new ion exchange membranes made from chlorosulfonated polyethersul- fone (SO2Cl-PES) crosslinked by polyaminated crosslinking reagents have been performed. Two examples are described: one crosslinked by hexane diamine, the other by amino-polyethersulfone (NH2-PES). Sulfonated polyether sulfone (S-PES) and NH2-PES have similar chemical structures that allow compatibility. Surpris- ingly enough, better results were obtained using amino-polyethersulfone. The best results have been obtained using SO2Cl-PES with 1.3 SO2Cl group per monomer unit crosslinked by 0.2 equivalent of NH2-PES. The membranes, less brittle than pristine SPES and insoluble in solvents such as DMAc, were characterized by TGA, DMA, DSC, ionic conductivity, transport numbers, and water swelling. The results showed that these membranes presented very promising performances for use in Proton Exchange Membrane Fuel Cells

Oxidative degradation of polylactide (PLA) and its effects on physical and mechanical properties

The thermo-oxidative degradation of polylactide (PLA) films was studied between 70 and 150 C. It was shown that the oxidative degradation of PLA leads to a random chain cission responsible for a reduction of the molar mass. These molar mass changes affect Tg and the degree of crystallinity, and it was found that Tg decreases according to the Fox–Flory theory whereas the degree of crystallinity increases due to a chemicrystallization process. A correlation between molar mass and strain at break during oxidation has been established: PLA displays a brittle behavior when Mn falls below 40 kg mol 1 in agreement with relationships linking the critical value for embrittlement with the molar mass between entanglements

Forced assembly by multilayer coextrusion to create oriented graphene reinforced polymer nanocomposites

A potential advantage of platelet-like nanofillers as nanocomposite reinforcements is the possibility of achieving two-dimensional stiffening through planar orientation of the platelets. The ability to achieve improved properties through in-plane orientation of the platelets is a challenge and, here, we present the first results of using forced assembly to orient graphene nanoplatelets in poly(methyl methacrylate)/ polystyrene (PMMA/PS) and PMMA/PMMA multilayer films produced through multilayer coextrusion. The films exhibited a multilayer structure made of alternating layers of polymer and polymer containing graphene as evidenced by electron microscopy. Significant single layer reinforcement of 118% at a concentration of 2 wt % graphene was achieveddhigher than previously reported reinforcement for randomly dispersed graphene. The large reinforcement is attributed to the planar orientation of the graphene in the individual polymer layers. Anisotropy of the stiffening was also observed and attributed to imperfect planar orientation of the graphene lateral to the extrusion flow

On-the-job learning and earnings

URL des Cahiers : https://halshs.archives-ouvertes.fr/CAHIERS-MSECahiers de la MSE 2005.22 - Série Blanche - ISSN : 1624-0340A simple model of informal learning on-the-job which combines learning by oneself and learning from others is proposed in this paper. It yields a closed-form solution that revises Mincer-Jovanovic's (1981) treatment of tenure in the human capital earnings function by relating earnings to the individual's learning potential from jobs and firms. We estimate the structural parameters of this non-linear model on a large French survey with matched employer-employee data. We find that workers on average can learn from others ten percent of their own human capital on entering the firm, and catch half of their learning potential in just two years. The measurement of worker's learning potential in their jobs and establishments provides a simple characterization of primary-type and secondary-type jobs and establishments. We find a strong relationship between the job-specific learning potential and tenure. Predictions of dual labor market theory regarding the positive match of primary-type firms (which offer high learning opportunities) with highly endowed workers (educated, high wages) are visible at the establishment level but seem to vanish at the job's level.Nous proposons un modèle d'apprentissage informel qui identifie deux des principales composantes de la formation en entreprise : l'apprentissage autonome et l'apprentissage par observation des autres. En reliant les gains au potentiel d'apprentissage dans l'entreprise, celui-ci révise la modélisation classique de l'ancienneté (Mincer et Jovanovic, 1981). L'estimation des paramètres structurels de ce modèle non-linéaire, à l'aide de données appariées françaises, fait apparaître, qu'en moyenne, les salariés peuvent apprendre l'équivalent de dix pour cent de leur capital humain initial et qu'il leur faut deux ans pour acquérir la moitié de ce potentiel d'apprentissage. Le potentiel d'apprentissage des individus dans leur établissement permet, à la fois au niveau des emplois et des établissements, de distinguer un secteur primaire (à forte accumulation en capital humain) d'un secteur secondaire (à faible accumulation en capital humain). Nous montrons qu'il existe une forte corrélation entre le potentiel d'apprentissage et l'ancienneté. Les prédictions de la théorie duale du marché du travail concernant l'appariement positif entre secteur primaire et salariés les plus éduqués, visibles au niveau des établissements, semble disparaître au niveau des emplois

Élaboration et propriétés des composites polypropylène recyclé/fibres de bambou

Cette étude présente des résultats obtenus sur l’élaboration de composites à base de polypropylène recyclé renforcé par des fibres végétales (fibres de bambou), fabriqués à l’aide d’une extrudeuse double vis modulaire pour produire un composite sous forme de granulés transformables. L’effet de la teneur en fibres de bambou, du compatibilisant et des méthodes de traitement chimique de la surface des fibres sur les propriétés mécaniques et rhéologiques du composite a été étudié. Nous avons montré que le traitement avec la soude enlève les impuretés et augmente la rugosité de surface des fibres, ce qui entraîne une meilleure adhésion des fibres dans la matrice polymère. Les composites fibres de bambou acétylées/polypropylène recyclé présentent de meilleures propriétés mécaniques, dues au greffage de groupes acétyles sur la surface des fibres et donc à l’amélioration de la compatibilité entre les fibres de bambou et la matrice polymère. Les propriétés rhéologiques des composites à différents taux de fibres de bambou et pour différentes méthodes de traitement de surface sont également analysées. Les facteurs prédominants qui influencent les propriétés des composites sont identifiés