Search

International audience; Thermosets are frequently toughened by a high-T g thermoplastic (TP). Blend morphologies, obtained by curing induced phase separation with scales of a few hundreds of nanometers are relevant for highperformance applications, but no quantitative description for obtaining these morphologies exist yet. We propose such a quantitative approach for predicting and controlling the final morphology. The key is the degree of curing and the corresponding T g of the blend and of both phases when phase separation takes place. It is controlled by the Flory interaction parameter  of the constituents and their respective T g 's. We show that if phase separation takes place too early during curing, the T g is too low and morphologies grow to reach sizes of a few micrometers, or more. Our study of different systems allows us to propose the relevant range of Flory interaction parameter  and temperature window T-T g for which the sizes of interest may be obtained. Our work opens the way for devising thermoplasticsthermosets couples with the appropriate affinity and T g s in order to make blends with tailored morphologies.

International audience; Thermoset (TS) epoxy resins can be toughened with a thermoplastic (TP) for high‐performance applications. The final structure morphology has to be controlled to achieve high mechanical properties and high impact resistance. Four polyethersulfone‐modified epoxy resins are considered. They consist of different epoxy monomer structure (TGAP, triglycidyl‐p‐aminophenol and TGDDM, tetraglycidyl diaminodiphenylmethane) and a fixed amount of thermoplastic, and they are cured with two different amounts of curing agent. A reaction‐induced phase separation occurs for all formulations generating morphologies, different in shapes and scales. The aim is to control the final morphology and in particular its dominant length scale. This morphology depends on the phase separation process, from the initiation to its final stage. The initiation relies on the relative miscibility of the components and on the stoichiometry between epoxy and curing agent. The kinetics depends on the viscosity of the systems. The different morphologies are characterized by electron microscopy or neutron scattering. Dynamic mechanical analysis allows confirming the presence of a phase separation even when it is not observable by electron microscopy. Vermicular morphologies with few hundreds nanometer width are obtained for the systems containing the TGAP as epoxy monomer. Systems formulated with TGDDM presents morphologies on much smaller scale of order a few tens of nanometers. We interpret the different sizes of the morphologies as a consequence of a larger viscosity for the TGDDM systems as compared to the TGAP ones rather than by a latter initiation of phase separation

The viscoelastic behaviour of cellulose acetate with a degree of substitution (DS) of 245 plasticized by triacetin was studied at short times by dynamic oscillatory measurements. Two distinct regimes and unexpected scaling behaviour according to plasticizer content were highlighted. The dynamics of chains and their structural organization are not modified up to 35 wt% of triacetin. The rheological behaviour is led by a constant correlation length corresponding to the distance between strong intermolecular interactions subsisting in the melt state at high temperature even in the presence of plasticizer. This particular structure involves the apparition of strain hardening effects during uniaxial extensional flow tests and an important elasticity corresponding to the apparition of a Weissenberg effect at really low shear rates during shear sweeps. Intramolecular hydrogen bonds are responsible of the high rigidity of cellulose acetate chains. Plasticized cellulose acetate in the melt state belongs to the class of associating polymers and its rheological behaviour is mainly led by stickers.

We consider the microscopic mechanisms of damaging in plasticized cellulose acetate under tensile stress. We show how they appear and develop during the course of deformation until failure. By usin...

The mechanical properties of injection molded plasticized cellulose acetate polymers processed with two different plasticizers (Triacetine and Diethyl phthalate) and various weight fractions comprised between 15 and 30 wt % have been investigated. Plasticized cellulose acetate exhibit a brittle-to-ductile transition from a low impact strength to a high impact strength of order 40 kJ/m². Obtaining a high impact resistance at room temperature requires plasticizer content larger than 25 wt.%. An important strain hardening is obtained for samples with both plasticizers during tensile experiment. At 15 wt.% plasticizer content, the measured strain hardening modulus is around 148 MPa at 60 °C. Different parameters influencing the strain hardening behavior have been identified: the tensile direction as compared to that of the injection flow, the temperature and the plasticizer, consistent with studies on pre-strained samples of synthetic amorphous polymers.

Atomistic simulations of cellulose acetates (CAs) differing in their degree of substitution have been performed and analyzed in terms of conformation and interaction schemes. The stabilization of the structure of these cellulose derivatives is understood as a subtle balance between hydrogen bonds and the dipolar acetate-acetate interactions that are associated with important changes in the macromolecular conformation. On the one hand, cellulose and cellulose triacetate (CTA) are characterized by a single stabilization process (H-bonds and dipolar interactions respectively), showing a similar structure in their melt phase together with similar radii of gyration. On the other hand partially acetylated CAs combine both the conformational properties of cellulose and CTA but present an unexpected conformational domain, named C2, which induces a local hydrophobic pocket. These CAs are also further stabilized by hydrogen bonds between the hydroxyl and acetyl groups. Although idealized, the proposed models are realistic since they are in good agreement with literature experimental results.

To understand, at a molecular scale, the effect of water on the structure of the amorphous region of polyamide 6,6 (PA6,6), atomistic molecular dynamics simulations have been carried out. Our results concerning the very local water organization relative to PA moieties agree qualitatively with a two-step sorption model. The first sorption mode seems to be saturated well below the lowest water concentration studied (5% relative to the amorphous phase). Moreover, above this saturation, the overall water organization displays at 300 K larger clusters than the 2−3 molecules usually assumed in the literature. The temperature dependence of free volume, hole size, and hydrogen bonding has also been investigated. It shows a transition between plasticized and antiplasticized behavior.

The interaction between cellulosic material and benzophenone was studied by molecular modeling. A model of the crystalline part of a native microfibril was built from previously published coordinates of the Iβ allomorph. This model presents three faces, namely (200), (110), and (110), of about the same surface area. The energetical and geometrical characteristics of the benzophenone adsorption onto this microfibril were studied with a Monte Carlo protocol. It was shown that the interaction does occur on the three faces and was stabilized by both van der Waals and electrostatic forces. On the hydrophobic (200) face, a large number of interacting sites without specific geometry were sampled by the adsorbing molecule. The hydrophilic surfaces, (110) and (110), also have many interaction sites, but in contrast, the orientation of the adsorbed molecules is more strict. These two hydrophilic surfaces display equivalent behavior. Three surfaces (crystalline (110) and (200) and amorphous) subjected to periodic...

The binary blend compatibility of polyamide 6 (PA6) with poly(vinyl alcohol) (PVOH), poly(vinyl acetate) (PVAC), and partially hydrolyzed PVAC was studied for a wide range of compositions, by atomistic and mesoscopic modeling. The Flory−Huggins interaction parameter χ, calculated for these mixtures by atomistic modeling, showed that favorable interactions develop for PVAC with a low hydrolysis degree for a specific composition and also for compositions rich in either component. The effect of the PVAC hydrolysis degree on the mixture compatibility was explained in terms of the reduced ability of the acetylated chains to form hydrogen bonds, which in turn, may result in weaker intramolecular interactions. Such an effect may also be due to the more extended conformations assumed by these chains because of the bulky side groups. Calculations at high temperature gave small negative χ parameters, in good agreement with results reported by others. The χ and other structure-dependent parameters, derived from the ...

Due to its biodegradability and renewability, a great interest has been devoted to investigating cellulose acetate in order to expand its potential applications. In addition, secondary cellulose acetate (CDA) could also be considered as a model system for strongly polar polymer system. The dynamical behavior of CDA is supposed to be governed by H-bonding and dipolar interaction network. Due to their high glass transition temperature, cellulose acetate-based systems are processed when blended with plasticizers. It is thus of utmost importance to study the miscibility and plasticizing effects of various molecules. We prepared CDA films via solvent casting method with diethyl phthalate as the plasticizer. Miscibility diagrams were established by calorimetry and thermo-mechanical (DMTA) experiments. Dynamical properties were analyzed by DMTA and broadband dielectric spectroscopy. We could identify the α-relaxation of these CDA-plasticizer systems in the frequency range from 0.06 Hz to 10 6 Hz, which allowed for describing the dynamics in the so-called Williams-Landel-Ferry/Vogel-Fulcher-Tammann regime.

Well-defined triblock copolymers comprising a poly(butylene terephthalate) (PBT) mid-block and three different hydrophobic blocks (poly(tert-butyl acrylate) P(t-BA), poly(n-butyl acrylate) P(n-BA) and poly(tert-butyl acrylamide) P(t-BAm)) were successfully prepared by the combination of step-growth and RAFT/MADIX polymerizations. Two different synthetic strategies were investigated for the preparation of O-ethylxanthate-terminated PBT RAFT/MADIX agent. Firstly α,ω-dihydroxy-PBT synthesized by a step-growth polymerization was transformed into the corresponding dixanthate-functionalized PBT according to a two-step procedure. An alternative approach was performed by using a hydroxyl-functional xanthate as chain stopper in a single step-growth polymerization process. In both cases, the presence of xanthate terminal groups was confirmed by NMR spectroscopy and MALDI-TOF mass spectrometry. The resulting PBT with xanthate end-groups was used as a macro-chain transfer agent for the RAFT/MADIX polymerization of n-BA, t-BA and t-BAm. The synthesis of well-defined PBT-based triblock copolymers was confirmed by SEC analysis.

An understanding of the molecular changes occurring during polymorphic transformations of amylose is sought through the use of energy calculations. Four stable families of low energy conformers are found; three of these correspond to conformations observed in linear and cyclic maltodextrins in the crystalline state. Upon change of solvent, significant variations have been shown to occur in the relative abundance of amylose conformers. This, in turn, influences properties such as chain extension. The relevance of the stable conformers with respect to helical structures of amylose is analyzed. Whereas one of the conformers would generate a V type shallow helix, another one would give rise to a 6 fold left-handed strand having a 3.6 A advance per residue. This would generate a double helical stranded structure.

A unified representation of helical parameters (n,h) based on a polar mapping is proposed. Its application to the case of polysaccharides allows a straightforward comparison of all secondary structures displayed by this class of biopolymers in the solid state. Based on the concept that a disordered state of a polymer may be described by a succession of local helical parameters, the application of this unified representation has been extended to the simulation of chain behavior in solution.

Determination of the steric features displayed by the bacterial polysaccharide xanthan is reported. Molecular modelling was conducted assuming an isolated, single polymer chain, having a well defined repeating unit. Potential energy was computed by taking into account van der Waals, torsional, exo-anomeric, and hydrogen bond contributions. Several families of stable conformers are found for the polymeric skeleton. Their occurences depend upon the conformations at the branching point, which are coupled to the orientations of the primary acetate group of the 6-O-acetate mannopyranosyl residue. The calculated stable conformations of the repeating unit would generate either hollow or extended helices, which, with the exception of one have left-handed chirality. One of these helices appears to correspond to the ordered conformation of xanthan in the solid state. The side chains exhibit a wide range of accessible conformations, but do not show any preferred tendency to fold back onto the polymeric backbone.

Solution and interfacial properties of water-soluble hybrid linear−dendritic polyether copolymers are investigated by static and dynamic surface tension measurements and adsorption experiments on polymeric substrates. The results obtained show that the block copolymers are able to form mono- and multimolecular aggregates in water. Contacting a solid polymeric substrate with an aqueous solution of hybrid block copolymer increases the hydrophilicity of the substrate. Adsorption on the hydrophobic surface of poly(ethylene terephthalate) proceeds only through the dendritic blocks of the hybrid macromolecule. For more hydrophilic substrates such as regenerated cellulose, both the poly(ethylene glycol) tail and the poly(benzyl ether) dendrons adsorb on the surface, increasing its hydrophilicity.

Cellulose acetate (CA) is a bio based polymer. Melt processing of cellulose based thermoplastic polymers is a real challenge. One problem is the existence of a narrow window between the melting point and the degradation temperatures for cellulose acetate with a substitution degree (DS) around 2.45 (which is developed and commercialized by Rhodia Acetow). As a consequence, its processing can only be considered with a sufficient amount of externalplasticizer (between 15 and 30% by weight). The corresponding polymer/plasticizer blends areamorphous and their mechanical properties are mainly governed by the presence of a high volume fraction of strong hydrogen bonds. The plasticization of cellulose acetate has been thesubject of many studies allowing us to focus on two plasticizers: triacetin (TA), an eco-friendlyplasticizer frequently used for cellulose acetate and diethyl phthalate (DEP) which is the historicplasticizer of cellulose acetate which constitutes a reference for this work as it is usually the case in the literature. Few studies have been published regarding the mechanical properties of bulk cellulose acetate (prepared via injection molding). It is described that they are comparable to those of PS or poly(methyl methacrylate) (PMMA) and have proven to be particularly interesting. Cellulose acetate based materials usually display a high Young modulus. But its small deformation at break limits its potential for new applications. The objectives of this thesis are to deeply understand the mechanical properties and damage mechanisms of bulk plasticized cellulose acetate polymers. For this purpose we first analyzed the tensile behavior and the influence of various parameters such as nature and content of the plasticizer, but also the influence of the injection process. We have thus been able to highlight the appearance of a strain hardening regime from 8% of deformation under certain conditions. It appears that the choice of the plasticizer, the temperature of the experiment and the macroscopic pre-orientation of the chains significantly influence this regime. Strain hardening has already been observed in other amorphous polymers such as polycarbonate (PC) or poly (methyl methacrylate) (PMMA) which are classified as amorphous polymers called "ductile". The origin of this regime is still undeveloped and much debated, however it appears that it stabilizes the deformation by avoiding the localization of damage and is therefore a key parameter for improving the ductility of these polymers. In order to better understand this ductility, we have made some analysis by Scanning Transmission Electron Microscopy (STEM) as well as Ultra Small Angles X-ray Scattering (USAXS). Thanks to these characterizations we have been able to describe the micromechanisms of damage from macro to nano-scales and thus precisely describe the micromechanisms related to initiation and propagation of damage. By these analyzes we highlight the simultaneous nucleation of nano crazes around pre-existing defects (related to the injection process). These crazes grow slowly until reaching the hundred microns. However, when the applied stress becomes sufficiently high, a small portion of these crazes starts to grow faster until the failure of the sample. With DEP the kinetics of growth is very fast, causing a brittle failure of the sample. With TA this growth is slower, which makes it possible to observe the evolution of the larger crazes. This work proposes a new mechanism of damage in plasticized cellulose acetate based on experimental results and physical interpretations; L'acétate de cellulose (CA) est un bio-polymère issu de la cellulose du bois. Sa température de dégradation (dont le degré de substitution 2,5 est développé et commercialisé par le Groupe Solvay) étant très proche de sa température de fusion, son procédé de mise en oeuvre par voie fondue ne peut être envisagé qu'avec l'ajout d'une quantité importante de plastifiant externe (entre 15 et 30% en poids). Le polymère plastifié obtenu est classé parmi les thermoplastiques amorphes et ses propriétés sont régies par un «réseau» de très fortes interactions polaires. La plastification de l'acétate de cellulose à fait l'objet de nombreux travaux nous permettant de nous concentrer in fine sur deux plastifiants: la triacétine (TA), un plastifiant biosourcé fréquemment utilisé dans l'acétate de cellulose et le Diethyl Phthalate (DEP) qui est le plastifiant historique de l'acétate de cellulose et constitue une référence. Les propriétés mécaniques de l'acétate de cellulose plastifié obtenu par voie fondue étant peu étudiées dans la littérature, nous avons dans un premier temps évalué le comportement en traction et l'influence de différents paramètres tels que le taux et le choix du plastifiant mais également l'influence du procédé d'injection sur ces propriétés. Nous avons ainsi pu mettre en évidence l'apparition d'un régime de durcissement plastique (strain hardening en anglais) dès 8% de déformation sous certaines conditions. Il apparaît que le choix du plastifiant, la température d'analyse et la pré-orientation macroscopique des chaînes influencent significativement ce régime. Le durcissement plastique a déjà été observé dans d'autre polymères amorphes tels que le polycarbonate (PC) ou le poly(méthyle methacrylate) (PMMA) qui sont classés parmi les polymères amorphes dit « ductiles ». L'origine de ce régime est encore peu connue et suscite de nombreux débats, cependant il semblerait qu'il stabilise la déformation en évitant la localisation de l'endommagement et serait donc un paramètre clé pour l'amélioration de la ductilité de ces polymères. Afin de mieux comprendre cette ductilité nous avons réalisé des observations par microscopie électronique à balayage en transmission (STEM) ainsi que par diffusion des rayons X aux très petits angles (USAXS). Grâce à ces caractérisations nous avons pu décrire les micro-mécanismes d'endommagement sous traction de nos polymères depuis l'échelle macroscopique jusqu'à l'échelle nanométrique et ainsi décrire précisément les micro-mécanismes liés à l'initiation et la propagation de l'endommagement. Par ces analyses nous mettons en évidence la nucléation simultanée de craquelures nanométriques autour des défauts préexistants (liés au processus de mise en oeuvre). Ces craquelures vont ensuite croitre de façon très limitée jusqu'à atteindre la centaine de micron. Cependant lorsque la contrainte appliquée devient suffisamment élevée, une petite portion de ces craquelures vont se mettre à croitre plus rapidement jusqu'à entrainer la rupture de l'échantillon. Avec le DEP la cinétique de croissance est très rapide, entrainant une rupture brutale de l'échantillon dès qu'une craquelure atteint une dimension critique. Avec la TA néanmoins cette vitesse est plus lente, ce qui permet d'observer l'évolution d'une deuxième famille de craquelures. Ces travaux proposent un nouveau mécanisme d'endommagement dans l'acétate de cellulose plastifié basé sur des résultats expérimentaux et un modèle physique permettant une meilleure compréhension de la ductilité dans ces polymères