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1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol (DMDBS) is highly effective in nucleation of the &alpha, form of isotactic polypropylene (iPP). However, its role in high-pressure crystallization of iPP, facilitating the formation of the &gamma, polymorph, has not been explored. The present paper focuses on the influence of DMDBS on nucleation of high-pressure crystallization of iPP. iPP with 0.2&ndash, 1.0 wt.% of the DMDBS was crystallized under elevated pressure, up to 300 MPa, in various thermal conditions, and then analyzed by PLM, WAXD, SEM, and DSC. During cooling, crystallization temperatures (Tc) were determined. It was found that under high-pressure DMDBS nucleated crystallization of iPP in the orthorhombic &gamma, form. As a consequence, Tc and the &gamma, form content increased for the nucleated iPP, while the size of polycrystalline aggregates decreased, although the effects depended on DMDBS content. The significant increase of Tc and the decrease of grain size under high pressure of 200&ndash, 300 MPa required higher content of DMDBS than the nucleation of the &alpha, form under lower pressure, possibly due to the effect of pressure on crystallization of DMDBS itself, which is a prerequisite for its nucleating activity.

It is well known in practice that the shape and dimensions of injected parts are highly dependent on the packing-holding stage. A major problem in semi-crystalline polymers is the prediction of the solidified layer, whose thickness has an important effect on shrinkage and warpage. We propose a pragmatic approach based on the concept of no-flow temperature. This temperature should be related to crystallization temperature, but the choice is not easy because it depends on cooling rate and pressure which are functions of time and position. The objective of the work is to evaluate the sensitivity of an injection molding computation to the no-flow temperature and to evaluate the relevance of its choice. The crystallization temperature of an isotactic polypropylene is determined as a function of cooling rate and pressure in laboratory experiments. The pressure dependence is measured using the original Cristapress cell. As a case study, we simulate the filling and post-filling of a plate mold using Rem3D, a 3D code for injection molding. Three no-flow temperatures and two sets of parameters for temperature dependence of viscosity are tested. Their respective influences on the pressure evolution are shown, and the crystallization temperature calculated a posteriori using the experimental material data is compared to the “arbitrary” no-flow temperature.

International audience; Poly(ethylene 2,5-furandicarboxylate), referred to as PEF, was uni-axially stretched for temperatures above glass transition temperature. This bio-based polymer is considered as a serious competitor for the petroleum analogous poly(ethylene terephthalate), named PET. To replace PET in bottle forming, PEF has to be deformed to large strains which are only reachable when it is in its rubbery state. In the present work, the stretching conditions have been chosen by determining precisely the range of temperature and strain rate where PEF exhibits a rubbery-like state. This was feasible through the building of a master curve at a reference temperature. Local strain field measurements allow the description of PEF intrinsic mechanical behaviour. Above a draw ratio of around 6 to 8, the mechanical response presents an impressive strain hardening whereas a well-defined crystalline phase with a high orientation is formed. Diffraction peaks were indexed and compared to previous papers. Only one crystalline phase was observed either under mechanical loading or during static crystallization. Mechanical loading significantly speeds up crystallization.

International audience; The crystallization of 3 polyamides 66 has been studied by optical microscopy and differential scanning calorimetry (DSC). Nonisothermal crystallization kinetics has been analyzed using Oza-wa's equation. Two polymers have close behaviors with a crystallization mainly spherulitic. The third one, which has the highest molar mass, is subject to intense transcrystallinity, which disturbs the crystallization kinetics (shoulder on the DSC curves and low Avrami exponent). For this polymer, original methods based on a detailed analysis of transcrystallinity have been applied to measure crystal growth rate and to determine the "intrinsic" crystallization kinetics, that is, not polluted by transcrystallinity

International audience; Interest in graphite fillers has grown since the separation of graphene from graphite by micromechanical cleavage. The object of the paper is to understand the influence of graphene nanoplatelets (GNPs) with different sizes on the crystallization behavior of a polyolefin matrix such as polypropylene (PP), after elaboration by melt mixing and compression molding. Composites with volume fractions of graphene nanoplatelets ranging from 0.3 to 2 vol% were prepared. The particle dispersion states in the composites were characterized at different scales using Scanning and Transmission Electron Microscopies (SEM and TEM). Polypropylene crystallization and orientation were investigated using optical microscopy, Differential Scanning Calorimetry (DSC) and X-ray diffraction. This paper discusses the strong acceleration of crystallization kinetics due to the presence of GNPs. The micrometric flake-shaped GNPs act as nucleating agent and induce an epitaxial growth of alpha (α) crystalline phase of PP. The nucleating effect is related to the surface of the particles available for heterogeneous nucleation. Radial spherulitic growths are observed from the smallest micrometric particles. The coarsest GNPs, easily oriented by flow, favor PP transcrystallinity, in such a way that (010) plane of PP is parallel to (001) plane of graphene nanoplatelets.

Nucleating agents used for the monoclinic α-form of isotactic polypropylene (PP), commonly occurring under atmospheric pressure, are able to nucleate its crystallization in the orthorhombic γ-form under high pressure. Nucleation of γ-lamellae is possible through epitaxy involving (001)γ crystallographic plane on substrates nucleating the α-form through epitaxy engaging (010)α plane because of equivalency of both planes. Another possibility is nucleation of α-lamellae through epitaxy involving either (010)α or (110)α plane, and formation of the γ-phase through γ/α epitaxy. The study focuses on the structure and mechanism of nucleation of high pressure crystallization of PP with selected nucleating agents: poly(tetrafluoroethylene) particles, commercial Hyperform HPN-20E and ADK Stab NA11UH. The nucleated PP was isothermally crystallized in the γ-form under pressure of 200 and 300 MPa and studied ex-situ by WAXD, PLM and SEM. Analysis of lamellar structure provided insight into the nucleation mechanism of high pressure crystallization of PP. In all the materials the α-lamellae were nucleated first and served as seeds for the γ-form.

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International audience; In this paper, a general numerical method to simulate polymer crystallization under various conditions is proposed. This method is first validated comparing its predictions with well-validated analytical models in infinite volumes. Then, it is compared to Billon et al. validated model for thin films, without or in presence of transcrystallinity on the films surfaces. It is also compared with Chenot et al. model for thin films, proposed in a conference in 2005 and never yet compared with other methods. Finally, it is also compared with an extension of this model for the transcrystalline case. These models are valid for general nucleation cases (not only sporadic or instantaneous), and can be used for any thermal conditions. All the numerical and analytical results are consistent, except in a case which is shown to be out of the validity domain of the transcrystalline case extension of Chenot et al. model.

In polymer processing, crystallization generally occurs in complex, inhomogeneous and coupled mechanical (flow, pressure), thermal (cooling rate, temperature gradient) and geometrical (surface of processing tools) conditions. A first route to understand crystallization in processing conditions is to design model experiments to isolate the specific influence of a given parameter. The emphasis will be laid here on the influence of: (i) shear flow through rheo-optical measurements using the commercial RheoScope module, (ii) high cooling rates obtained with the modified hot stage Cristaspeed (up to 2 000 °C min−1) and (iii) high pressures in the original Cristapress cell (up to 200 MPa). Numerical simulation is also a useful tool to understand and predict the coupled phenomena involved in crystallization. Based on Avrami's ideas and equations, a general differential formulation of overall crystallization kinetics has been proposed by Haudin and Chenot (2004). It is able to treat both isothermal and non-isothermal cases, and has been extended to crystallization in a limited volume without and with surface nucleation inducing transcrystallinity.

International audience; Natural fillers composed of cellulose, lignin and fillers with a varying lignin/cellulose ratio (flax, jute, curauà fibers and miscanthus stem fragments) were used to prepare composites with polypropylene using the same procedure, with or without a maleic anhydride–grafted polypropylene (MA-g-PP) coupling agent. A clear acceleration of the crystallization kinetics was observed in the presence of miscanthus stem fragments. For non-coupled composites, the size and aspect ratio of fragments had no significant influence onto the crystallization kinetics of polypropylene. The presence of the MA-g-PP coupling agent increased even more the kinetics. A clear effect of the nature of the polymers present in the filler was observed. There is a direct relationship between the values of the Avrami kinetic constant k for the six fillers and their lignin content, the more effective to enhance crystallization kinetics being pure cellulose. Lignin has no effect onto the crystallization kinetics of polypropylene.

International audience; The aim of this work is to assess the potential of graphene nanoplatelets (GNP) according to their production method to improve properties of composites based on polypropylene and elaborated by melt mixing. The effects of GNP size on structural, thermal, rheological and mechanical properties of composites were investigated. Three grades of GNPs, obtained by a thermomechanical process and commercialized by Knano under KNG-180, KNG-150 and KNG-G5 references, were considered in this work. KNG-180 and KNG-150 GNPs were obtained from graphite through a three step process: i) graphite was intercalated by sulfuric acid under stirring and then washed and dried, ii) then, it underwent a thermal treatment and finally iii) the obtained expanded graphite was ultrasonicated in hydroalcoholic solution. From this step, particles were considered as graphene nanoplatelets [1]. An additional ball milling process in good solvent, followed by centrifugation, enabled to obtain thinner KNG-G5 from KNG-180 [2]. Composites were elaborated by melt mixing in an internal mixer using different conditions and thus mixing energies. The dispersion state of the composites was evaluated at different scales using SEM, TEM and X-ray diffraction. Thermal, rheological and mechanical properties of the composites were investigated. Although mechanical delamination followed by centrifugation enables to improve dispersion state of GNP in thermoplastics, PP/GNP mixtures have to be considered as non-exfoliated microcomposites. Even if, different mixing conditions were investigated, they did not affect the dispersion state. However, thermal, rheological and mechanical properties of PP/KNG-G5 composites were outstandingly close to those of thermoplastic/montmorillonite or reduced graphene nanocomposites and show positive results regarding the use of this graphene manufacturing process without chemical treatment. Ackowledgements GNPs were kindly provided by the CNRS research group project n°3661 "GDR PolyNano". References 1 G. Chen, C. Wu, W. Weng, D. Wu, W. Yan, Preparation of polystyrene/graphite nanosheet composite, Polymer. 44, 1781 (2003) 2 W. Zhao, M. Fang, F. Wu, H. Wu, L. Wang, G. Chen, Preparation of graphene by exfoliation of graphite using wet ball milling, J. Mater. Chem. 20, 5817 (2010)

A comprehensive understanding of the inherent link between in-situ growth kinetics of a polymer spherulite and high-pressure constraints under controlled temperature is concerned. As a matter of fact, while the link with temperature is well illustrated, little comprehensive study has been conducted to quantify the effect of pressure. This is yet required to model ‘extreme’ polymer processing conditions.Mainly, the experimental set-ups developed to reproduce the pressure effect can be classified into four families: “simple” cells, dilatometric set-ups, differential thermal analysis and diamond anvil plus in-situ measurement. In this context, an original model experiment, named CRISTAPRESS, has been constructed. The cell design exploits the optical properties of semi-crystalline spherulites. Time-resolved light depolarizing microscopic observations are conducted concomitantly with a fine PVT control, for high pressure up to 200 MPa and temperature up to 300 °C. The physical analysis of isothermal and isobaric holding of a model polymer shows the influence of temperature and pressure on the key kinetic parameters of crystallization, i.e., the growth rate and the number of activated nuclei, as well as on the subsequent morphologies. Simple modeling dealing with the Avrami equation and the Hoffman & Lauritzen theory is established.

International audience; The crystallization of isotactic polypropylene (PP) in γ-modification, which forms under high pressure but also at high temperature, can be nucleated by nucleating agents used for PP α-modification crystallizing under atmospheric pressure, like poly(tetrafluoroethylene) (PTFE) particles and commercial Hyperform HPN-20E. The study focuses on answering the question whether these nucleants are sufficiently active under high pressure to nucleate crystallization of PP during cooling in the temperature range of crystallization of the γ-form. To this aim, neat PP and PP with 0.2 wt% of nucleating agents were annealed at 250 °C for 5 min and subsequently cooled under a pressure ranging from 1.3 to 300 MPa. After releasing pressure, the specimens were analyzed by DSC, WAXD and PLM to have an insight into morphology, crystallinity and content of crystallographic forms. Both, Hyperform HPN-20E and PTFE, nucleated the crystallization of PP during cooling under high pressure in the γ-form causing an increase of crystallization temperature, the γ-form content and a decrease of grain size, although the first nucleant was more efficient. Calcium pimelate, a known nucleant of the β-form, studied for comparison, was ineffective.

The growing interest for continuous fiber-reinforced polymer composites leads to the development of new processes such as resin transfer molding for thermoplastics (RTM-TP) or tape placement. In the aim of optimization, their simulations are required and have to include all involved physical phenomena and the associated couplings. During the consolidation step, the crystallization of the semicrystalline matrix occurs between the fibers of the multiscale reinforcement. A tricky task is to provide a realistic model able to describe the crystallization kinetics, which includes the effect of fibers on the polymer phase change and avoiding large computation time. In 2004, Haudin and Chenot proposed a generalization of the Avrami model, written in a differential form to compute the evolution of the crystallization of a neat thermoplastic in an infinite volume. In the present article, new extensions are proposed to predict the crystallization in long-fiber thermoplastic composites, without or in the presence of transcrystallinity on fiber surfaces. In both cases, they are compared to three-dimensional numerical simulations using a previously validated numerical method. All the numerical and analytical results are consistent. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 44508.

International audience; Miscanthus Giganteus – reinforced polypropylene composites have been prepared by internal mixing. The effect of the size of the miscanthus fragments used as fillers onto the thermal properties of composites has been evaluated by differential scanning calorimetry (DSC) and polarized light optical microscopy (POM). The kinetics of the crystallization processes have been evaluated by isothermal methods. The influence of the fiber-matrix interface onto the melting/crystallization processes has been evaluated by adding a maleic anhydride–grafted polypropylene (MA-g-PP) coupling agent. A clear acceleration of the crystallization kinetics was observed in the presence of miscanthus stem fragments. However, the results showed that, for non-coupled composites, the size and aspect ratio of those fragments had no significant influence onto the crystallization kinetics of polypropylene. The presence of the MA-g-PP coupling agent increased the kinetics and reduced the crystallization activation energy. Although relatively small, if a good fiber-matrix interface is achieved, the size of the filler plays a significant role onto the crystallization kinetics of the matrix, with smaller fragment leading to faster kinetics and reduced activation energy

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International audience; A prototype hot-stage termed "Polymer High Cooling - Optics" was especially designed to follow on-line the nucleation and growth of crystallizing entities at constant cooling rates. The optically transparent hot-stage receives a gaseous flow at a constant temperature, which passes over the sample. Quiescent crystallization is monitored in real-time at relatively moderate (from 30 to 500 degrees C/min) and even at high (from 500 to 2000 degrees C/min) constant cooling rates. As a first application of this new set-up, two relevant parameters of the cc-modification of isotactic polypropylene, i.e., the crystallization temperature and the spherulite growth rate, were successfully captured for different constant cooling rates in a reproducible way. The superposition of these newly-collected growth rate data with those compiled by Janeschitz-Kriegl (2006) demonstrates the reliability of our experimental determination.

International audience; In polyamide 12 (PA12) tube extrusion, calibration is the key step of the process that affects the subsequent mechanical properties. In previous work it has been shown that according to the calibration conditions, a very oriented skin layer may be created, which has been correlated to an important decrease of elongation at break. In this paper, we present new results showing a good correlation between molecular orientation and fracture toughness, as evaluated by the EWF (Essential Work of Fracture) approach. They concern notched specimens and confirm the results obtained in classical tensile testing. EWF is very sensitive to processing conditions, and especially to induced orientation: it decreases from the external to the inner regions of the tube, and increases with skin orientation.

International audience; Shear-induced isothermal crystallization in iPP based nanocomposites with organo-modified montmorillonite was followed by light depolarization technique. Prior to the crystallization, samples were sheared at 1 or 2 s(-1) for 10s in a plate-plate system at crystallization temperature of 136 degrees C. Structure of the solidified specimens was investigated by light microscopy and electron microscopy, X-ray techniques and IR spectroscopy. Strong enhancement of the nucleation and crystallization after shearing was observed in the compatibilized nanocomposites with the clay. Clay exfoliation was found to accelerate strongly the shear-induced nucleation and overall crystallization. However, the sheared samples exhibited only weak orientation of alpha crystals with (040) crystallographic planes parallel to shearing direction that resulted probably from a small population of oriented crystals that formed due to shear-induced orientation of iPP chains and served as nuclei for further nearly isotropic growth.

In polyamide 12 tube extrusion, calibration is the key step of the process that affects the subsequent mechanical properties. In previous work it has been shown that according to the calibration conditions, a very oriented skin layer may be created, which has been correlated to an important decrease of elongation at break. In the present paper we develop a thermomechanical model which consists of two parts: a viscoelastic mechanical model for the calculation of stresses and a thermal model for the determination of the temperature field. Three types of inputs are necessary: processing parameters, material data (e. g., crystallization kinetics and rheology in the melt, the solid state and the transition zone) and heat transfer coefficients to describe the heat exchanges along the extrusion line. The model allows us to propose a physical interpretation of the oriented layer. During cooling high axial stresses are frozen in the first solidified layers. They induce a plastic deformation of the polymer leading to a high level of orientation in the outer zones.

Mechanical properties of flax-fibre reinforced composites (FFRC) are strongly affected by water ageing compared to glass-fibre reinforced composites (GFRC). This study highlights the influence of water absorption during immersion at 30 °C on mechanical properties of unsaturated polyester reinforced composites . Flax-fibre composites showed a Fickian absorption behaviour and a water uptake 15 times higher than that of glass-fibre composites. GFRC’s tensile modulus and maximum stress were slightly affected by water uptake while FFRC’s tensile modulus decreased by 37 %, and ultimate stress increased by 34 %. A silane-based chemical treatment (1 % compared to flax fibres) was applied onto flax fibres. Water uptake was slightly reduced by 9 % while tensile modulus at saturation was enhanced by 22 % on treated FFRC compared to untreated ones. Moreover, the complete recovery of the tensile modulus after desiccation suggests that ageing was mainly reversible: fibre and matrix plasticizing phenomena occurred during immersion at 30 °C. No damage was noticed but composites’ initial properties changed with the action of water: further crosslinking of matrix and release of fibre’s small cell-walls components into water were observed. Finally, the drying conditions influenced the return to the initial state before ageing insofar the flax fibres partially lost their initial humidity.

In packaging industry, many structures are produced by extrusion coating. In this process, a polymer film is extruded through a slit die, then stretched in air, coated on a substrate (paper, aluminium or steel foil) between a chill roll and a flexible pressure roll, and finally cooled on successive chill rolls. Due to their non-polar character, polypropylenes are not suitable for extrusion coating on metallic surfaces. Adhesive properties can be improved by grafting on the polymer chain a polar group like maleic anhydride which may react with the aluminium surface. Our purpose was to develop a general model in order to predict the temperature field in the thickness of the multilayered structure along the stretching, laminating and cooling steps, and especially near the polymer/metal interface. This model includes crystallization kinetics and accounts carefully for the heat transfer coefficient with the successive rolls and surrounding air. Its predictions have been successfully compared to experimental temperature measurements along the coating line for various processing conditions (velocity, roll temperature, etc). Moreover, thermal history has a real impact on structure and morphology in the film. All these aspects are revealed by microscopic observations of thin microtomed sections of the film and X-ray diffraction experiments. Finally, adhesion properties of the laminate have been tested for the same process conditions. A good correlation has been established between adhesion properties and the thermal history experienced by the grafted polypropylene near the interface with aluminium before crystallization.

Apres quelques rappels sur la cristallisation des polymeres, les effets des parametres de mise en forme (mecaniques, thermiques, geometriques) sur les phenomenes de cristallisation (germination, croissance, cinetique globale, developpement des morphologies) sont d’abord discutes de facon generale. Les principales methodes de caracterisation de l’orientation due a la mise en forme sont ensuite decrites. Les principes de la modelisation du developpement de structures sont enfin exposes, avec les lois utilisees pour decrire la cinetique globale de cristallisation.

Dans cet article, les concepts generaux du developpement de structures sont illustres par quatre exemples, qui correspondent a des influences differentes des parametres de mise en forme sur la microstructure. Dans l'extrusion de film a plat, la cristallisation se produit apres cessation de l'ecoulement. Au contraire, dans le filage ou le soufflage de gaine, elle se produit pendant l'ecoulement. Dans le procede d'injection, des situations tres differentes sont rencontrees en fonction de la localisation a l'interieur de la piece.

The widely applied approaches to the description of the conversion of melt into spherulites are reviewed and critically assessed. The attention is focused on the polymer crystallization in quiescent conditions. Both isothermal and nonisothermal crystallization processes are considered, including also the crystallization in a temperature gradient. In addition, the approaches to the description of the influence of spatial confinement and additional nucleation processes on surfaces confining the material on the conversion of melt into spherulites are discussed. Since similar phenomena occur also in fiber-reinforced systems, the theoretical predictions of the overall crystallization kinetics in these systems are also included in the present review. The possible reasons of discrepancies between the theoretical predictions and the experimental data are analyzed.

In polyamide 12 (PA12) tube extrusion, calibration is crucial for the major final properties such as elongation at break or burst pressure. In external calibration, the extruded tube is pulled through a cylindrical calibrator located in a water tank under vacuum. A water flow rate is applied at the calibrator inner side, creating a lubricating water layer at the polymer outer surface. A combination of a quenching and a mechanical drawing was highlighted during calibration by on-line measurements. A subsequent high molecular orientation in the outer tube layers was also featured by X-Ray diffraction and birefringence evaluation in light microscopy. Besides, influence of lubrication level in the sizing-sleeve was investigated by an estimate of the lubricating water layer and a characterization of the tube final surface state in different calibration conditions. Firstly, we quantitatively showed that rising the water layer thickness leads to a diminution of the draw ratio in the calibration tank, DrCAL (ratio between line velocity and velocity at the calibrator entrance). Excellent correlation has been found between tube superficial orientation and DrCAL: a reduced elongation of the polymer in the calibrator leads to a lower level of superficial orientation. Secondly, two kinds of surface defects were detected on the tubes by light microscopy and microtopography. At last, several correlations were featured with mechanical properties. Elongation at break was found to strongly depend on the molecular orientation resulting from calibration. Moreover, origin of rupture was investigated during tensile testing in light microscopy. We emphasized that surface defects alignments perpendicular to the extrusion initiate the rupture by creating a significant crack via coalescence of initial defects. Thus, microstructure, surface state and tensile properties can be controlled by fitting the calibration parameters to improve lubrication and reduce the draw ratio in calibrator.

Direct link: http://www.polymer-process.com/directlink.asp?IPP1892; International audience; Elongation at break is one of the major end-use properties of polyamide 12 extruded tubes. It is strongly affected by the tube microstructure and the molecular orientation resulting from extrusion conditions. Molecular orientation was characterized by X-ray diffraction and birefringence evaluation in light microscopy. Measurements were carried out on (r z) sections obtained by polishing and microtoming. On the other hand, polymer drawing was measured on line by tracer techniques. Calibration stage was determined as the key step of the process that generates orientation in tubes: as the tube is drawn through a cylindrical calibrator under vacuum and cooled from its outside surface, calibration leads to a highly oriented zone in the twenty external microns. Calibration conditions and elongation at break have been connected through orientation level in this region. Molecular orientation was found to strongly depend on the draw ratio in the calibration tank. Finally, birefringence of the tube external layers and elongation at break were successfully correlated. Elongation at break can be enhanced by reducing orientation resulting from calibration conditions.

The purpose of the present work is to introduce a crystallization law into Rem3D, a 3D code written in C++ and dedicated to the injection molding of polymers. We kept the basic hypotheses of Avrami's model and cast the kinetic equations into a differential system that is solved numerically. The variation of the density of potential nuclei with temperature is taken into account. Furthermore, the distribution of mean spherulite sizes can be deduced from the calculations. The second part of the paper is an experimental study of crystallization in well-controlled conditions (2D, isothermal or constant cooling-rate). It establishes a procedure for the determination of the nucleation and growth parameters used in the theoretical model, and gives a first validation of this model. Finally, the crystallization equations are introduced into Rem3D, in order to assess the feasibility of our new approach. Some typical results concerning the evolution of the transformed volume fraction in injection-molded parts are presented.

Crystallization of thin polypropylene films was performed in isothermal, constant cooling-rate and mixed conditions. The experiments were first analyzed using the classical procedures based on simplified forms (Avrami, Ozawa) of the general Kolmogoroff-Avrami-Evans (KAE) theory. These analyses, which can be applied over an unusually wide transformation range, show that the crystallizations are actually 2 D. Then, a procedure has been established for the determination of the nucleation and growth parameters involved in the theoretical model presented in the first paper of this series. These parameters have been introduced into the model in order to predict the crystallization behavior in isothermal, constant-cooling-rate and mixed-conditions: transformed fraction, number of activated nuclei, final size distribution of semi-crystalline entities. A very good agreement is generally found between predictions and experimental results.

The crystallization of nanocomposites of isotactic polypropylenes with organo modified montmorillonite compatibilized by maleic anhydride grafted polypropylene was studied by light microscopy in isothermal conditions in quiescent state and in shear. The isothermal and nonisothermal crystallization of the composites was also investigated by DSC method. Only weak nucleation activity of montmorillonite was observed during crystallization in static conditions. The clay nucleation activity was greatly enhanced in shear-induced crystallization and resulted in a drastic decrease of spherulite sizes. In nanocomposite films crystallized isothermally the intense nucleation of isotactic polypropylene spherulites was observed when the polymer was forced to flow to compensate the volume shrinkage due to crystallization. It was also found that the presence of a glass support enhances spherulite nucleation in nanocomposites which is caused possibly by shear due to a difference in thermal shrinkage of a polymer matrix and a glass support.

Ce chapitre introductif a pour but de presenter les surfaces on interfaces de polymeres et leurs proprietes, dans une approche pluridisciplinaire combinant chimie, physico-chimie, science des materiaux, mecanique, etc. L'accent est mis sur les proprietes optiques et surtout mecaniques et tribologiques, qui seront peu reprises par la suite. Les traitements de surface sont introduits en conclusion.

Interrelations between processing, orientation and shrinkage in low density polyethylene blown films. IV. Study of shrinkage. In this last paper, the shrinkage properties of the blown films are investigated. It appears that they are governed by the orientation of the crystalline phase. A good correlation between the cumulated strains during processing and the amounts of shrinkage is found.

International audience; Polymer crystallization often occurs in the presence of foreign bodies, such as walls of processing tools. In such cases, there is a competition between nucleation in the bulk polymer and nucleation on well-identified surfaces. If many nuclei are activated at the surfaces, their proximity imposes that entities emanating from these nuclei grow preferentially normal to the surfaces, leading to transcrystalline zones. The competition between surface and bulk nucleation can be studied through crystallizations of thin polymer films in contact with pan surfaces in a DSC apparatus. These experiments show that in thin samples transcrystallinity is limited by sample thickness. When thickness increases, the transcrystalline zones can grow, but up to a limiting value, because at a certain stage their development is stopped by the growth of bulk spherulites. A specific analysis of these DSC experiments gives access to crystallization parameters such as the number of nuclei per unit surface or the growth rate, and makes it possible to determine the crystallization kinetics of the polymer not disturbed by transcrystallinity.

International audience; Crystallization under shear of many different polypropylenes has been studied using a fiber pull-out device. It appears that growth can be considerably enhanced by flow. The best correlation is obtained with weight average molecular weight. Modeling the flow pattern gives access to the mechanical parameters at the growth front (shear rate and shear stress) as well as to the total strain applied to the polymer. The residual strain can be calculated taking into account relaxation processes.

International audience; Nucleation of the γ-form in isotactic polypropylene (PP) under high pressure was investigated. Three nucleating agents were used to nucleate crystallization of PP under atmospheric pressure: commercial Hyperform HPN-20E from Milliken Chemical, poly(tetrafluoroethylene) particles nucleating the α-form, and calcium pimelate nucleating the β-form. Crystallization of neat PP and PP with addition of 0.2 wt% of the nucleating agents was studied. Specimens were either kept at 200 °C under pressure of 200 MPa for time ranging from 2 min to 4 h or for 15 min under pressure ranging from 1.3 to 300 MPa. After cooling to ambient temperature and releasing the pressure, the specimens were analyzed by DSC, WAXD, and PLM to have an insight into the structure and to determine a crystallinity level and contents of crystallographic forms. Both α-nucleating agents strongly nucleated crystallization of PP under high pressure in the γ-form, whereas the β-nucleating agent had only a slight effect. The results show the possibility to use nucleating agents to nucleate the γ-form of PP under high pressure.

International audience; Shear refinement has been studied in four LDPE, which have been characterized molecularly and rheologically. They have been subjected to a pre-shearing in a cone-and-plate rheometer, and the decrease of viscosity, associated with the disentanglement of macromolecules, has been analyzed as a function of different mechanical parameters: shear rate, strain, stress, specific energy. It appears that disentanglement mechanisms are complex and cannot be explained by a unique molecular parameter, e.g., the molar mass between entanglements.; Le phénomène de « shear refinement » a été étudié sur quatre polyéthylènes basse densité (PEBD), caractérisés sur le plan moléculaire et rhéologique. Ils ont été soumis à un pré-cisaillement dans un rhéomètre cône-plan et la diminution de viscosité, associée au désenchevêtrement des macromolécules, a été analysée en fonction de différents paramètres mécaniques : taux de cisaillement, déformation, contrainte, énergie spécifique. Il apparaît que les mécanismes de désenchevêtrement sont complexes et ne sont pas explicables par un seul paramètre moléculaire, comme par exemple la masse molaire entre enchevêtrements. Mots-clé : Polyéthylène basse densité, structure moléculaire, enchevêtrements, rhéologie, « shear refinement » [Abridged English version on last page]

Flow marks in injection moulding is quite often encountered on different type of polymers. This defects is known as successive and periodic bright and dull bands oriented perpendicular to the flow direction. Their appearances meanly depend on processing and observation conditions. This study was lead on two industrial polypropylene polymers with a simplified polished rectangular mould geometry. We show that this defect appears during the filling stage and comes from a local change of morphological and molecular orientation along the filling direction on the surface. An instability of the flow front due to a limited sliding at the contact combined to the strong changed of molecular orientation before and after the stagnation point are invoked to explain this instability.

In this second paper, the orientation of the crystalline phase is characterized by three methods : Debye-Scherrer diagrams, pole figures and orientation functions. We try on the one hand to establish correlations between orientation and processing conditions, and on the other hand to interpret our experimental results using morphological models.

Cette etude experimentale porte sur la caracterisation et la propagation des defauts de vagues sur des plaques injectees en polypropylene. Nous avons mis en evidence deux sortes de bandes brillantes (tres orientees et non orientees). Le front de matiere des pieces incompletes est desequilibre alternativement d'un cote et de l'autre du moule. La periodicite correspond aux differentes bandes orientees ou non. Nous avons montre que les conditions de contact entre le moule et le polymere fondu influence fortement l'initiation ou non de ce defaut. Nous comparons nos resultats avec quelques modeles theoriques proposes dans la litterature.

Three series of isotactic polypropylene characterized by different molecular weights and the same isotactic index have been studied during crystallization under static and shearing conditions. The shear is induced by the displacement of a glass fiber in the molten polymer. The monoclinic α-phase is here formed under shear with a columnar organization at the surface of the glass fiber, and does not appear under static condition. The growth-rate, constant during the shear-induced crystallization experiment, is compared with the result obtained from static crystallization. An important increase of the growth-rate due to the shear flow is observed. This increase depends on the molecular structure. The average molecular weights ―Mw and ―Mz seem to be the most important molecular parameters, for which an excellent correlation is obtained. The increase of these parameters ―Mw and ―Mz leads to a significant enhancement of the growth-rate, which can be multiplied by a factor of 10 in the present conditions.

Cette etude porte sur la microstructure et les proprietes de tubes PVC biorientes. Ces tubes sont compares a un tube monooriente et a un tube extrude de facon classique. La biorientation se traduit par une augmentation du module de traction, de la limite d’elasticite, de la contrainte a rupture et de la tenue a l’impact. Cette amelioration des proprietes est clairement reliee aux evolutions de la microstructure induites par le procede.

In this second paper, the orientation of the crystalline phase is characterized by three methods : Debye-Scherrer diagrams, pole figures and orientation functions. We try on the one hand to establish correlations between orientation and processing conditions, and on the other hand to interpret our experimental results using morphological models.

The isothermal crystallization of isotactic polypropylenes has been studied under static conditions and under shear flow. The shear was induced by displacement of a glass fibre in the molten polymer. An α-monoclinic crystalline phase was formed with a columnar organization at the surface of the glass fibre, which did not appear under static conditions. The growth rate, constant during shear-induced crystallization experiments, is compared with the results obtained from static crystallizations. An important increase of the growth rate due to the shear flow is observed. This increase depends on the molecular weight and the tacticity of the polypropylene. Actually, the molecular weight and the isotactic index seem to be the most important molecular parameters. Their augmentation leads to a significant increase of the growth rate, which can be increased by a factor of seven, under the present conditions.

A way for the recovery of plastic waste is the reprocessing. As they cannot be easily separated, the reprocessing of polyolefins appears as a special case of the processing of polymer blends. In order to understand the basic phenomena involved in these systems, binary and ternary blends are prepared from a high density polyethylene, a linear low density polyethylene, and a random ethylene-propylene copolymer. In a first step, the interactions between the different polyolefins are characterized in laboratory experiments (differential scanning calorimetry, optical microscopy, small-angle light scattering, wide-angle X-ray diffraction, mechanical properties). Then, it is shown that the original poor mechanical properties of the blends can be largely improved by optimizing the processing conditions. Finally, an extrapolation of these results to real plastic waste is presented.

An erratum has been published for this content. Erratum: "CRISTAPRESS: An optical cell for structure development in high-pressure crystallization" Review of Scientific Instruments, 85 (2) art. no. 029902; International audience; An original optical high-pressure cell, named CRISTAPRESS, has been especially designed to investigate phase transitions of complex liquids, i.e., polymers, polymer blends, nano-composites, etc. The design of the cell is based on the optical properties of morphological entities through in situ light depolarizing microscopic observations. Pressure up to 200 MPa with a fine temperature control up to 300 °C can be applied. A striking advantage of this cell is the possibility to select the pressure transmitting medium that can be water, silicone oil, a fluid in the supercritical state, etc. The potential of the novel technique was demonstrated by carrying out time-resolved measurements during polymer crystallization induced by water pressure. These preliminary experimental investigations permit to discriminate the role of the barometric and thermal histories on the kinetics of polymer growth, as well as on the subsequent morphologies. It should lead to new reliable crystallization kinetics models.

The plane-strain compression test provides quantitative data on the polymer rheology in homogeneous large deformation at constant strain-rate, and on the friction shear stress between the film and the dies. Thin HDPE films with a homogeneous morphology were formed by quenching or isothermal crystallization. The structure was modified by varying the crystallization temperature over a wide range: 118°C ≤ T C ≤ 130°C. A mechanical model with a pressure-dependent rheology and two friction laws was tested. The rheological parameters are different in tension and in compression. The occurrence of brittle fracture under compression depends on the crystallization temperature, but the rheological parameters are almost independent of the crystallization conditions.

Low-energy impact tests were performed on a polyamide 6 using a dart test technique with a 20-mm-diameter hemispherical striker. Partial penetrations at controlled and increasing energies allowed to visualize the different steps of the deformation. The evolution of the deformation is described, taking into account the shape and the thickness of the samples, the crystalline orientation, the microstructure and the resulting mechanical properties of the polymer. The deformation results from several steps, which are revealed by the evolution of the force measured on the striker. The deformation induces important and inhomogeneous changes in the microstructure and the orientation of the polymer. Variations may be abrupt. The resulting mechanical behavior is strongly dependent on this evolution and, as a consequence, depends on the location in the sample. These observations confirm that it is impossible to predict such a complex behavior using data from simple tensile tests.

International audience; A combined effect of organo-modified montmorillonite (o-MMT) and shear flow on nonisothermal crystallization and emerging structure of isotactic polypropylene (iPP) was studied. Composite materials of iPP with 3 wt.% of o-MMT, nanocomposite compatibilized with maleic anhydride grafted iPP and non-compatibilized composite, blend of iPP with the compatibilizer and neat iPP were sheared for 10 s at a rate ranging from 1 to 40 s−1 during cooling at 10 °C min−1, beginning at different temperatures: 143, 153 and 163 °C. The crystallization behavior of all the materials studied did not differ under quiescent conditions. However, the effects of flow, that is an increase of crystallization temperature, enhancement of nucleation and decrease of grain size, and also orientation of crystals, although dependent on shear temperature and rate, were amplified by the presence of o-MMT. The strongest effects were observed for the compatibilized nanocomposite with exfoliated clay. In the nanocomposite sheared at 143 °C crystallization temperature increased by 16 °C, more than 12 °C achieved for the blend and the non-compatibilized composite. However, even in the latter case the increase was larger than 7 °C measured for neat iPP. The sheared nanocomposite exhibited the orientation of α-phase crystals with (0 4 0) crystallographic planes parallel to the shearing direction and (1 1 0) planes bimodally oriented, parallel and normal to the shearing direction.