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Hypothesis: Interpenetrating networks consisting of a polymer network with dynamic cross-links and a supramolecular network allow obtaining hydrogels with significantly enhanced mechanical properties. Experiments: Binary hydrogels composed of a dynamically cross-linked poly(vinyl alcohol) (PVA) network and a transient network of entangled highly charged mixed wormlike micelles (WLMs) of surfactants (potassium oleate and n-octyltrimethylammonium bromide) were prepared and studied by rheometry, SANS, USANS, cryo-TEM, and NMR spectroscopy. Findings: Binary hydrogels show significantly enhanced rheological properties (a 3400-fold higher viscosity and 27-fold higher plateau modulus) as compared to their components taken separately. This is due to the microphase separation leading to local concentrating of PVA and WLMs providing larger number of polymer–polymer contacts for cross-linking and longer WLMs with more entanglements. Such materials are very promising for the application in many areas, ranging from enhanced oil recovery to biomedical uses.

Transitional compositions or phase-changing structures in specific layers can respond to environmental changes differently and show intelligent behaviors.

Efficient recycling of crosslinked polyethylene has been challenging due to manufacturing difficulties caused by chemical crosslinking. This study focuses on simple processing via solid waste powder generation and particle fining for the subsequent crosslinked polyethylene inclusion and dispersion in rigid polyurethane foam. In addition, the concentration effects of crosslinked polyethylene in polyurethane were studied, showing a well-controlled foam microstructure with uniform pores, retained strength, better thermal degradation resistance, and, more importantly, increased thermal capabilities. Thus, the simple mechanical processing of crosslinked polyethylene and chemical urethane foaming showed the massive potential of recycling large amounts of crosslinked polyethylene in foams for broad applications in food packaging, house insulation, and sound reduction. This research was funded by Salt River Project.

International audience; Measuring the viscoelastic behavior of polymers in the vicinity of a surface or under confinement is an experimental challenge. Simple rheological tests of nanolayered films of polyethylene/polyamide 6 compatibilized in situ during the coextrusion process enabled the probing of these interfacial properties. Taking advantage of the different melting points and of the multiplication of the number of interfaces, a drastic increase of dynamic moduli was reported when increasing the interphase volume fraction in the films. A solid-like behavior for the interphase was identified. The complex viscosity of nanolayered films as a function of angular frequency was quantitatively captured for all samples using a weighted mixing law of bulk and interphase viscosities, without additional adjusting parameters, highlighting the interfacial synergy developed in nanolayered polymer films.

An experimental investigation is reported on the effect of shear on the bursting of molten ultrathin polymer films embedded in an immiscible matrix. By use of an optical microscope coupled with a shearing hot stage, the dewetting dynamics, i.e., the growth of dewetting holes, is monitored over time at various shear rates. It is observed that their circularity is modified by shear and that for all temperatures and thicknesses studied the growth speed of the formed holes rapidly increases with increasing shear rate. A model balancing capillary forces and viscous dissipation while taking into account shear thinning is then proposed and captures the main features of the experimental data, such as the ellipsoid shape of the holes and the faster dynamics in the direction parallel to the shear. This research will help to understand the instabilities occurring during processing of layered polymeric structures, such as multilayer coextrusion.

In this work, we investigated the self-assembly of a lamellar block copolymer (BCP) under different wetting conditions. We explored the influence of the chemical composition of under-layers and top-coats on the thin film stability, self-assembly kinetics and BCP domain orientation. Three different chemistries were chosen for these surface affinity modifiers and their composition was tuned in order to provide either neutral wetting (i.e. an out-of-plane lamellar structure), or affine wetting conditions (i.e. an in-plane lamellar structure) with respect to a sub-10 nm PS-b-PDMSB lamellar system. Using such controlled wetting configurations, the competition between the dewetting of the BCP layer and the self-organization kinetics was explored. We also evaluated the spreading parameter of the BCP films with respect to the configurations of surface-energy modifiers and demonstrated that BCP layers are intrinsically unstable to dewetting in a neutral configuration. Finally, the dewetting mechanisms were evaluated with respect to the different wetting configurations and we clearly observed that the rigidity of the top-coat is a key factor to delay BCP film instability.

International audience; Dual transient networks were prepared by mixing highly charged long wormlike micelles of surfactants with polysaccharide chains of hydroxypropyl guar above the entanglement concentration for each of the components. The wormlike micelles were composed of two oppositely charged surfactants potassium oleate and n-octyltrimethylammonium bromide with a large excess of anionic surfactant. The system is macroscopically homogeneous over a wide range of polymer and surfactant concentrations, which is attributed to a stabilizing effect of surfactants counterions that try to occupy as much volume as possible in order to gain in translational entropy. At the same time, by small-angle neutron scattering (SANS) combined with ultrasmall-angle neutron scattering (USANS), a microphase separation with the formation of polymer-rich and surfactant-rich domains was detected. Rheological studies in the linear viscoelastic regime revealed a synergistic 180-fold enhancement of viscosity and 65-fold increase of the longest relaxation time in comparison with the individual components. This effect was attributed to the local increase in concentration of both components trying to avoid contact with each other, which makes the micelles longer and increases the number of intermicellar and interpolymer entanglements. The enhanced rheological properties of this novel system based on industrially important polymer hold great potential for applications in personal care products, oil recovery and many other fields.

International audience; The objective of this work is to give new insight into the stability of thin polymer films under shear, in order to pave the way to a better control of the nanolayer coextrusion process. To do so, a finite-difference numerical scheme for the resolution of the thin film equation was set up taking into account capillary and van der Waals (vdW) forces. This method was validated by comparing the dynamics obtained from an initial harmonic perturbation to established theoretical predictions. With the addition of shear, three regimes have then been evidenced as a function of the shear rate. In the case of low shear rates the rupture is delayed when compared to the no-shear problem, while at higher shear rates it is even suppressed: the perturbed interface goes back to its unperturbed state over time. In between these two limiting regimes, a transient one in which shear and vdW forces balance each other, leading to a non-monotonic temporal evolution of the perturbed interface, has been identified. While a linear analysis is sufficient to describe the rupture time in the absence of shear, the nonlinearities appear to be essential otherwise.

The barrier performance and structural lightening of organic materials are increasingly desired and constitute a major challenge for manufacturers, particularly for transport and packaging. A promising technique which tends to emerge in recent years is that of multinanolayer coextrusion. The advantage is that it can produce multilayers made of thousands of very thin layers, leading to new properties due to crystalline morphology changes induced by confinement. This paper is focusing on the study of multinanolayered films with alternated polyethylene (PE), compatibilizer (PEgMA) and polyamide 6 (PA6) layers and made by a forced assembly coextrusion process equipped with layer multiplying elements (LME). PE/PA6 multilayer films consisting of 5 to 2049 layers (respectively 0 to 9 LME) were successfully obtained with well-organized multilayered structure. The evolution of the morphology and the microstructure of these two semi-crystalline polymers, when the thickness of each polymer layer decreases from micro-scale to nano-scale, was correlated to the water and gas transport properties of the PE/PA multilayers. The expected improvement of barrier properties was limited due to the on-edge orientation of crystals in very thin PE and PA6 layers. Despite this change of crystalline morphology, a slight improvement of the gas barrier properties was shown by comparing experimental results with permeabilities predicted on the basis of a serial model developed by considering a PE/PA6 interphase. This interphase observed by TEM images and the on-edge crystal orientation in multilayers were evidenced from mechanical properties showing an increase of the stiffness and the strength.

The 3D printing method, alternatively known as additive manufacturing (AM), is promising for rapid tooling and layered micromanufacturing. However, significant fundamental research and applied study in the 3D printing area are still necessary to develop new manufacturing mechanisms for combining multi-materials for multiscale and multi-functionality behaviors. Among those materials, particles with unique mechanical, thermal, electrical, optical, and other functional properties can find broad applications in structural composites, thermal packaging, electrical devices, optoelectronics, biomedical implants, energy storage, filtration, and purification. This review will first briefly cover the 3D printing basics before presenting the critical factors in polymer/particle-based printing. We will then introduce a spectrum of different printing mechanisms, i.e., vat polymerization-based, jetting-based, material extrusion-based, powder bed fusion-based, and a few other less utilized 3D printing methods, with a summary of the processing parameters, advantages, disadvantages, and future challenges of each printing technique. During this discussion of 3D printing, we will also present generally used polymers and particles, namely, liquid monomers, viscous inks, compliant gels, stiff filaments, and loosely packed pellets containing micro and nanoscale particles. The emphasis of this review is on the general printing mechanisms applicable in particle- and polymer-relevant processing. To end, this review identifies provides future perspectives regarding some new application examples. Identifying challenges in materials science and manufacturing processes will give direction to the fabrication of multifunctional systems for diverse applications, especially when using multi-materials (e.g., polymers and particles) at multiple scales (e.g., nanoscale morphologies and macroscale structures) for multifunctional systems.

Being able to predict the lifetime of elastomers is fundamental for many industrial applications. The evolution of both tensile and compression behavior of unfilled and filled neoprene rubbers was studied over time for different ageing conditions (70 &deg, C, 80 &deg, C and 90 &deg, C). While Young&rsquo, s modulus increased with ageing, the bulk modulus remained almost constant, leading to a slight decrease in the Poisson&rsquo, s ratio with ageing, especially for the filled rubbers. This evolution of Poisson&rsquo, s ratio with ageing is often neglected in the literature where a constant value of 0.5 is almost always assumed. Moreover, the elongation at break decreased, all these phenomena having a similar activation energy (~80 kJ/mol) assuming an Arrhenius or pseudo-Arrhenius behavior. Using simple scaling arguments from rubber elasticity theory, it is possible to relate quantitatively Young&rsquo, s modulus and elongation at break for all ageing conditions, while an empirical relation can correlate Young&rsquo, s modulus and hardness shore A. This suggests the crosslink density evolution during ageing is the main factor that drives the mechanical properties. It is then possible to predict the lifetime of elastomers usually based on an elongation at break criterion with a simple hardness shore measurement.

International audience; The effect of initial fiber orientation and welding conditions on the mechanical properties and microstructure of 30 wt% glass-fiber-reinforced polyamide 66 was systematically evaluated. For all conditions studied no significant change in the polymer matrix was evidenced. However, fibers in the welded zone were reoriented toward the squeeze and vibration flows and this reorientation is related to the appearance of cavities, as evidenced by high-resolution synchrotron-based X-ray microtomography. It is shown that stress at break values of the welded samples increase with the thickness of the weld zone together with the minimum value of the fiber orientation tensor component in the tensile direction. A drop of strain at break is also related to an increase in the fibers’ concentration in the weld. The maximum void volume fraction being measured on samples which have the thickest welded zones, counterintuitively it does not induce lower stress at break.

International audience; Rheological behavior of nanolayered films of polyethylene/polyamide 6 (PE/PA6) compatibilized in situ during the coextrusion process has been studied at a temperature between the melting temperatures of PE and PA6. Thanks to the high number of interfaces, a drastic increase in dynamic moduli has been measured when increasing the interphase volume fraction in the films, and a solid-like behavior for the interphase was identified. Different models are compared to capture the complex viscosity of nanolayered films as a function of angular frequency. A model considering interphase and bulk viscosities and a single fitting parameter, namely, the thickness over which the viscosity decreases linearly from the interphase to the bulk one, captures the complex viscosity of all samples. This thickness is comparable to the PE layer thickness up to values about 1 μm before a significant bulk region has to be added to capture the behavior for thicker layers. This suggests that the melt rheology is impacted by the presence of a nanometric interphase over very large (micronic) length scales.

3D printing (additive manufacturing (AM)) has enormous potential for rapid tooling and mass production due to its design flexibility and significant reduction of the timeline from design to manufacturing. The current state-of-the-art in 3D printing focuses on material manufacturability and engineering applications. However, there still exists the bottleneck of low printing resolution and processing rates, especially when nanomaterials need tailorable orders at different scales. An interesting phenomenon is the preferential alignment of nanoparticles that enhance material properties. Therefore, this review emphasizes the landscape of nanoparticle alignment in the context of 3D printing. Herein, a brief overview of 3D printing is provided, followed by a comprehensive summary of the 3D printing-enabled nanoparticle alignment in well-established and in-house customized 3D printing mechanisms that can lead to selective deposition and preferential orientation of nanoparticles. Subsequently, it is listed that typical applications that utilized the properties of ordered nanoparticles (e.g., structural composites, heat conductors, chemo-resistive sensors, engineered surfaces, tissue scaffolds, and actuators based on structural and functional property improvement). This review's emphasis is on the particle alignment methodology and the performance of composites incorporating aligned nanoparticles. In the end, significant limitations of current 3D printing techniques are identified together with future perspectives.

The size of representative microstructural samples obtained from atomic force microscopy is addressed in this paper. The case of an archetypal one-dimensional nanolayered polymer blend is considered. Image analysis is performed on micrographs obtained through atomic force microscopy, yielding statistical data concerning morphological properties of the material. The variability in terms of microstructural morphology is due to the thermomechanical processing route. The statistical data is used in order to estimate sample size representativity, based on an asymptotic relationship relating the inherent point variance of the indicator function of one material phase to the statistical, size-dependent, ensemble variance of the same function. From the study of nanolayered material systems, the statistical approach was found to be an effective mean for discriminating and characterizing multiple scales of heterogeneity.

International audience; A multilayer film composed of alternating layers of polycarbonate (PC) and poly(m-xylene adipamide) (MXD6) was elaborated by using an innovative multilayer coextrusion process. Quasi-continuous thin MXD6 layers (nanolayers) alternating with PC layers were successfully obtained. The PC/MXD6 multilayer film showed a confining effect of MXD6 exerted by PC layers leading to an improvement of barrier properties despite a low degree of crystallinity (X c < 10 wt%). In order to further improve the barrier performances, crystallization treatments induced by water and by heating were then applied on the multilayer film and allowed reaching around 30 wt% of crystallinity in MXD6 layers. To decouple crystallization and geometrical constraint effects on the barrier properties in the multilayer films, the two treatments were also applied on MXD6 films. Surprisingly, despite an increase of the degree of crystallinity from 6 to 26%, water crystallization did not permit to improve gas barrier performances of the MXD6 film nor into the PC/MXD6 multilayer film. On the other hand, thermal crystallization of MXD6 in the multilayer film seems to be a more efficient route to strongly decrease the gas and moisture permeability, up to 75% for nitrogen, 58% for oxygen, 84% for carbon dioxide and 43% for water.

In capillary-driven fluid dynamics, simple departures from equilibrium offer the chance to quantitatively model the resulting relaxations. These dynamics in turn provide insight on both practical and fundamental aspects of thin-film hydrodynamics. In this work, we describe a model trilayer dewetting experiment elucidating the effect of solid, no-slip confining boundaries on the bursting of a liquid film in a viscous environment. This experiment was inspired by an industrial polymer processing technique, multilayer coextrusion, in which thousands of alternating layers are stacked atop one another. When pushed to the nanoscale limit, the individual layers are found to break up on time scales shorter than the processing time. To gain insight on this dynamic problem, we here directly observe the growth rate of holes in the middle layer of the trilayer films described above, wherein the distance between the inner film and solid boundary can be orders of magnitude larger than its thickness. In otherwise identical experimental conditions, thinner films break up faster than thicker ones. This observation is found to agree with a scaling model that balances capillary driving power and viscous dissipation with, crucially, a no-slip boundary condition at the solid substrate/viscous environment boundary. In particular, even for the thinnest middle-layers, no finite-size effect is needed to explain the data. The dynamics of hole growth is captured by a single master curve over four orders of magnitude in the dimensionless hole radius and time, and is found to agree well with predictions including analytic expressions for the dissipation., Comment: 21 pages, 4 figures

International audience; Multilayer films comprising polystyrene (PS)/polymethyl methalcrylate (PMMA) and PS/polycaprolatone (PCL) alternating nanolayers with varied layer thickness were fabricated by multilayer coextrusion. The nanolayers breakup phenomena of PMMA and PCL were characterized using atomic force microscopy (AFM), oxygen permeability, light transmission, wide-angle X-ray scattering (WAXS), and differential scanning calorimetry (DSC). The continuous layers started to break up into nanosheets and nanodroplets during the coextrusion process when the nominal layer thickness decreased to between 30 nm and 40 nm. Further decrease of the nominal layer thickness of PMMA and PCL resulted in less nanosheets and more nanodroplets. Oxygen permeability was effective for characterizing the onset thickness of layer breakup. The oxygen permeability for the PS/PCL system was modeled and demonstrated good correlation with estimated composition of continuous layers, nanosheets, and nanodroplets.

International audience; Multilayer coextrusion was used to obtain nanolayered films of self-assembled commercial triblock copolymer poly(methyl methacrylate-b-butyl acrylate-b-methyl methacrylate) (MAM) confined by poly (methyl methacrylate) (PMMA). A double level of organization is achieved: the obtained films are made of thousands of alternating continuous nanolayers of each component, while within the nanolayers, MAM is self-assembled with a cylindrical morphology. The mechanical properties of the resulting films were compared to those of virgin PMMA and to classically extruded PMMA/MAM blends with the same compositions where no control of the nanostructure can be achieved. It is shown that significant reinforcement can be obtained with the multilayer films compared to virgin PMMA (on the order of 20 times in terms of fracture toughness) but also compared to classical blends, by a factor from 2 to 4. Reinforcing glassy thermoplastics has been a major industrial challenge due to their usually brittle behavior in the temperature range they are used. This industrially scalable one-step process shows promises for the design of nano-laminated organic glasses with high impact resistance.

International audience; Multilayer coextrusion was used to obtain nanolayered films of self-assembled commercial triblock copolymer poly(methyl methacrylate-b-butyl acrylate-b-methyl methacrylate) (MAM) confined by poly(methyl methacrylate) (PMMA). The MAM layer thickness was varied from 30 nm to 500 nm (i.e. roughly 1 to 15–20 nanodomains) by changing either the number of multiplying elements or the chill roll draw ratio. The as-extruded triblock morphology within the layers was identified as cylindrical using transmission electronic microscopy (TEM) and small-angle X-ray scattering (SAXS). Surprisingly, this differs from the lamellar morphology identified at equilibrium in bulk and thin films for this triblock. Moreover, as the triblock layer thickness is decreased, the triblock morphology is constrained into a preferential orientation. Slightly different packings were observed on films with similar layer thicknesses but achieved with different processing routes. This one-step and industrially scalable method allowing long-range control of the self-assembly is of interest for engineering applications with large quantity of materials needed.

Films of PC and MXD6 were prepared via multilayer coextrusion. Films presenting stretched nodules/lamellas of MXD6 in the PC matrix, and films with quasi-continuous thin MXD6 layers alternating with PC layers were obtained. The transport properties were specifically investigated from water and gas permeation kinetics, and the corresponding permeation parameters were determined. The confinement effect of MXD6 in the multilayer structure was highlighted by comparing the permeability predicted from the series model equation with the experimental data. Although the MXD6 layers were not homogeneous and continuous for all membranes, water and gas permeabilities of the PC/MXD6 multilayer film were largely reduced. Such improvement of barrier properties was related to the change of the MXD6 structure, considering that the confinement of the MXD6 layers induced by PC layers allowed to bring chain orientation and to reduce the chain segment mobility in the MXD6 amorphous constrained zones. In addition, in the case of a PC/MXD6 multilayer structure well defined with continuous layers and relatively homogeneous thicknesses, the degree of crystallinity was slightly increased.

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.; International audience; 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.

Poly(lactic acid) (PLA) is a biodegradable thermoplastic polyester derived from renewable resources which may replace conventional polymers for some applications. To overcome some of its limitations such as poor gas barrier properties and low elongation at break, one method is to blend PLA with small amounts of other bio-based polymers. In this study, two processes, eg classical twin screw extrusion and a multilayer co-extrusion process have been used to combine PLA and poly(3-hydroxybutyrate-co-3-valerate) PHBV to obtain films with different blend morphologies. The effect of the morphology on the crystallinity has been studied and has hightlightned new behavior of PHBV. The addition of a nucleating agent in the PHBV to modify its crystallinity, has also been studied.

International audience; This paper reports an investigation of the thermal degradation of a poly(styrene-b-butadiene-b-methyl methacrylate) triblock terpolymer compared with those of butadiene rubbers and poly(styrene-b-methyl methacrylate). According to sol-gel properties changes, it was proposed that polybutadiene blocks undergo mainly crosslinking. However, these latter coexist with chain scissions occurring in polybutadiene blocks, which generates poly(butadiene-b-methyl methacrylate) and poly(butadiene-b-styrene) diblock chains, and possibly chain scissions occurring in polystyrene and poly(methyl methacylate) blocks. The consequences of the degradation on the self-assembly of triblock terpolymer were studied through the annealing kinetics monitored by Atomic Force Microscopy. It appears that at low conversion degrees, chain scissions induce a faster self-assembly kinetics. At higher oxidation degrees, crosslinking of butadiene phase combined with scissions induces a “frozen” state at high annealing times characterized by a macrophase separation.

International audience; An experimental study was carried out to investigate the existence of a critical layer thickness in nanolayer coextrusion, under which no continuous layer is observed. Polymer films containing thousands of layers of alternating polymers with individual layer thicknesses below 100 nm have been prepared by coextrusion through a series of layer multiplying elements. Different films composed of alternating layers of poly(methyl methacrylate) (PMMA) and polystyrene (PS) were fabricated with the aim to reach individual layer thicknesses as small as possible, varying the number of layers, the mass composition of both components, and the final total thickness of the film. Films were characterized by atomic force microscopy (AFM), and a statistical analysis was used to determine the distribution in layer thicknesses and the continuity of layers. For the PS/PMMA nanolayered systems, results point out the existence of a critical layer thickness around 10 nm, below which the layers break up. This critical layer thickness is reached regardless of the processing route, suggesting it might be dependent only on material characteristics but not on process parameters. We propose this breakup phenomenon is due to small interfacial perturbations that are amplified by (van der Waals) disjoining forces

Blending poly(lactic acid) (PLA) with a small amount of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV; 10 wt%) using a custom multilayer co-extrusion process increases both ductility and gas barrier properties of extruded films compared with neat PLA and classical blending methods. The co-extrusion process allows multiplication of the number of alternate layers of PLA and PHBV within a film. It was observed that for a critical number of theoretical layers, PHBV layers are broken into lamellas. A well-developed lamellar morphology, with thin and long lamellas of highly crystalline PHBV in PLA matrix was obtained. A balance between aspect ratio and crystallinity of the lamellas, and their dispersion within the PLA matrix was needed to obtain films with improved permeability and mechanical properties.

Poly(lactic acid) (PLA) is a biodegradable, thermoplastic, aliphatic polyester derived from renewable resources, which is a potential candidate to replace conventional polymers in food packaging applications.. Other bio-based materials like polyhydroxyalkanoates (in our case PHBV) could be associated with PLA in order to improve its gas barrier properties and/or its mechanical properties. Two processes, classical 3 layer coextrusion and original polymer blending extrusion, have been used in this study to combine PLA and PHBV to obtain films with different PLA-PHBV structures. The aim of this paper is to compare the thermoformability of the different films and to investigate the final structure in relationship with their gas barrier properties. The effect of the thermoforming steep has been studied by separating heating and stretching effects. We show that the blend morphology offers better mechanical and barrier properties than single PLA or 3 layers PLA/PHBV films.

The mechanical properties of nanostructured polyelectrolyte hydrogels have been investigated targeting specifically the nonlinear deformation regime at large strains and the fracture properties. The hydrogels were synthesized using a thiol-ene cross-linking chemistry of polyacrylic acid precursor chains previously functionalized with double bonds. Some polymers were also grafted with C 12 side chains able to form hydrophobic clusters in water. The large strain deformation of these nanostructured gels was tested in uniaxial compression by performing loading/unloading cycles at different strain rates and up to different maximum strains. All hydrogels displayed a pronounced strain hardening before fracture and a strain dependent hysteresis but the magnitude of these effects depended on the details of the composition. The most pronounced strain hardening was observed for the unmodified hydrogels in pure water while the largest hysteresis was found for the gels containing the highest concentration of nanoclusters. These dissipative processes barely influence the mechanical behaviour at small strains where the gel remains very elastic but become important at large strains and improve the resistance to fracture of the gels.

International audience; is an open access repository that collects the work of Arts et Métiers ParisTech researchers and makes it freely available over the web where possible. This is an author-deposited version published in: https://sam.ensam.eu Handle ID: .http://hdl.handle.net/10985/16016 To cite this version : Yann ZHU, Adrien BIRONEAU, Frédéric RESTAGNO, Cyrille SOLLOGOUB, Guillaume MIQUELARD-GARNIER-Kinetics of thin polymer film rupture: Model experiments for a better understanding of layer breakups in the multilayer coextrusion process-Polymer a b s t r a c t We show that the kinetics of bursting of a thin film embedded in another polymer matrix can be well described by a simple equation balancing the viscous and capillary forces. The role of viscosity, interfacial tension and thickness of the layers under static conditions was investigated by adapting a simple model experiment initially designed to study the dewetting of a thin polymer film on another polymer sub-strate. Kinetics was correlated to instabilities occurring during multilayer coextrusion leading to the break-up of layers below a critical nanometric thickness. The results suggest that shear and elongation forces during the process actually stabilize the layers. Understanding these instabilities is of great interest since this innovative process showed potential to design at an industrial scale macromolecular materials displaying new macroscopic properties, due to interfacial and confinement effects. However, instabilities leading to the breaking of these nanometric layers will alter final properties (optical, gas barrier …).

International audience; We report the making of multilayered self-assembled films by coextrusion, composed of alternated layers of confining polycarbonate and confined poly(styrene-. b-butadiene-. b-methyl methacrylate), whose blocks are chemically different from the confining polymer, and presenting a self-assembled morphology directly after extrusion. The triblock copolymer layers thicknesses was varied from few hundreds to few tens of nanometers. As the triblock layer thickness is decreased and the draw ratio is increased, the triblock morphology is constrained into a preferential orientation and higher long-range order is observed by transmission electron microscopy and small angle X-ray scattering. This one-step and industrially scalable method allowing long-range ordering of the nanodomains is of interest for many engineering applications for which producing large quantity of materials is necessary.

Summary: We report the synthesis and characterization of a family of hydrophobically modified hydrogels designed to have an improved fracture resistance. A backbone of poly(acrylic acid) (PAA) was functionalized with double bonds and hydrophobic groups. The functionalized PAA was then crosslinked with a dithiol. The chemical structure of the gels was characterized with a combination of NMR, titration methods and rheological techniques. The crosslinked structure of the hydrogel was found to be dependent on the polymer concentration only, while the dissipative properties of the gel increased strongly at all frequencies, with the introduction of hydrophobic groups which formed reversible associations. We expect these viscoelastic hydrogels to display dramatically different properties from the unmodified hydrogels when stretched and fractured.

The influence of both viscoelastic and interfacial parameters on the surface properties of elastomers is difficult to study. Here, we describe a simple route to achieve surface modification of PDMS without impacting the viscoelastic properties of the bulk. PEG modified PDMS surfaces were synthesized by two step surface modification based on hydrosilylation. The covalent grafting of PEG on the surface has been evidenced by AFM and ATR-FTIR, and its effect on the hydrophilicity characterized by static and dynamic contact angle. The static water contact angle of the PEG-modified PDMS decreases from 110° (for unmodified PDMS) to 65°. Dynamic contact angles also show a significant decrease in both advancing and receding contact angles, along with a significant increase in the contact angle hysteresis, which can be related to an increase in the surface energy as estimated by JKR measurements. The viscoelastic properties of modified PDMS are found to be quantitatively comparable to those of the unmodified PDMS. This simple method is an efficient way to prepare model materials which can be used to get a better understanding of the exact contribution of the surface chemistry on surface properties of elastomers.

International audience; 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.

International audience; 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.

Mechanical interactions between individual cells and their substrate have been studied extensively over the past decade; however, understanding how these interactions change as cells interact with neighboring cells in the development of a cell sheet, or early stage tissue, is less developed. We use a recently developed experimental technique for quantifying the mechanics of confluent cell sheets. Living cells are cultured on a thin film of polystyrene [PS], which is attached to a patterned substrate of crosslinked poly(dimethyl siloxane) [PDMS] microwells. As cells attach to the substrate and begin to form a sheet, they apply sufficient contractile force to buckle the PS film over individual microwells to form a microlens array. The curvature for each microlens is measured by confocal microscopy and can be related to the strain and stress applied by the cell sheet using simple mechanical analysis for the buckling of thin films. We demonstrate that this technique can provide insight into the important materials properties and length scales that govern cell sheet responses, especially the role of stiffness of the substrate. We show that intercellular forces can lead to significantly different behaviors than the ones observed for individual cells, where focal adhesion is the relevant parameter.

Wrinkled surfaces are ubiquitous in Nature and can be used in a large range of applications such as improved adhesives, microfluidic patterns, or as metrology instruments. Despite wide-ranging applications, existing methods do not permit local pattern control since all existing methods impose extensive compressive strains. In this article, we describe a new process that exploits the local deformation of a soft substrate as it stretches to form an adhesive interface with a thin polymer film. The wrinkle pattern is effectively a measurement of the strain-field created during the adhesion process, which shows a strong dependence on the speed of attachment. We develop simple scaling arguments to describe this velocity dependence and a critical velocity above which wrinkles do not form. Notably, our approach allows us to define the surface pattern “wrinkle-by-wrinkle”, thus permitting the creation of single wrinkles. Intricate patterns on laterally extensive length scales can also be produced by exploiting the shape of the contact line between the film and the substrate. This level of control—the placement of single features of prescribed trajectory—which is not present in any other method of thin film wrinkling, is absolutely necessary for any realistic, scalable application.

Systematic large strain compression measurements have been performed on polyelectrolyte hydrogels based on modified PAA crosslinked by bifunctional thiols. For compressive strains larger than a critical value depending on polymer concentration, we observed a significant hysteresis, strain-hardening and a stress plateau during unloading. This was attributed to strain-induced ionic clustering due to electrostatic interactions that can become attractive if chains are close enough to each other. This phenomenon is dynamic and reversible but a long lifetime for the clusters has been identified. Although clustering between like-charge chains has been reported for hydrogels, it is the first time that this phenomenon is caused by deformation. This effect is potentially important as we strive to understand the behaviour of all polyelectrolyte hydrogels at large strains which are highly relevant for fracture properties.

The objective of this PhD work isto study the role of shear on the rupture of ultra-thin polymer films, by coupling a numerical study to an experimental one. First, a numerical method adapted to the resolution of the thin film equation was set up taking into account the capillary and van der Waals forces. This method was validated by comparing the dynamics obtained to theoretical predictions. With the addition of shear, two regimes have then been evidenced as a function of the shear rate: in the case of low shear rates the rupture is delayed, while at higher shear rates it is suppressed. The use of spatio-temporal diagrams allowed the characterization of the interface deformation. The transition from the delayed to the suppressed rupture regime is similar to a phase transition. The experimental study at low shear rate allowed to confirm this delayed rupture, but also showedafter the rupture an increased dewetting speed. This study will allow a better understanding of the rupture phenomena occurring during nanolayer coextrusion., L’objectif de ce travail de thèse est d’étudier le rôle du cisaillement sur la rupture de films ultra-minces de polymères, en couplant une étude numérique à une étude expérimentale. Tout d’abord, une méthode numérique adaptée à la résolution de l'équation des films minces a été mise en place en prenant en compte les forces capillaires et de van der Waals, et validée en comparant la dynamique obtenue à des prédictions théoriques. Un terme de cisaillement a ensuite été rajouté, et deux régimes ont pu être mis en évidence en fonction du taux de cisaillement : dans le cas de faibles taux de cisaillement la rupture est retardée, à plus haut taux de cisaillement elle est supprimée. L’utilisation de diagrammes spatio-temporels a permis de caractériser la déformation de l’interface. Le passage du régime de rupture retardé à celui de rupture supprimée présente une évolution analogue à une transition de phase. L’étude expérimentale à faible taux de cisaillement a permis de confirmer ce retard de rupture, mais également de montrer après la rupture une vitesse de démouillage augmentée. Cette étude permettra de mieux appréhender les phénomènes de rupture de couches survenant lors de la coextrusion multinanocouches.

Vibration welding is a common process used in automotive industry to assembly polymer parts. For pristine polymers, welding ratio, defined as the ratio between weld strength and tensile strength of non-welded material, is close to 1. However, for glass-fiber reinforced polymers, welding ratios are around 0.6 at best. In order to understand this discrepancy, several grades of polyamide (PA) 6 and 66 reinforced with 30% glass fibers as well as polypropylene (PP) with 0, 20, 35 and 50 % glass fibers have been injected in plates and vibration welded. A linear relationship was obtained between tensile strength of welded and non-welded PA specimens and their glass fiber orientation. Correlations were also found when plotting weld strength of samples regarding their welded zone thickness. In addition, voids are present only in the welded zone of glass fiber reinforced specimens, indicating that these voids are due to reorientation of fibers during the welding process. Finally, finite element modeling of mechanical behavior up to failure has been applied on a meshed specimen with a welded zone. Results show an amplification and distribution of stresses in the three directions inside the welded zone due to geometrical confinement. This generated triaxiality promotes growth and coalescence of cavities in the welded zone, explaining the weakening of the macroscopic uniaxial stress at failure.; Le soudage par vibration est un procédé largement utilisé dans l’industrie automobile pour assembler des pièces en polymère. Lorsque la matière à souder ne contient pas de renforts, le coefficient de soudage, défini comme le rapport entre la contrainte à la rupture du matériau soudé et celle du matériau non soudé est proche de 1. En revanche, il devient difficile d’obtenir des coefficients de soudage dépassant 0,6 lorsque le matériau contient des fibres de verre. Afin de mieux comprendre pourquoi, différents grades de polyamide (PA) 6 et 66 renforcés à 30% de fibres ainsi que du polypropylène (PP) à 0, 20, 35 et 50% de fibres ont été injectés en plaques et soudées par vibration. Pour les échantillons en PA, des corrélations sont obtenues entre la contrainte à la rupture des assemblages soudés et l’orientation des fibres mais aussi avec l’épaisseur de la zone soudée déterminée par tomographie RX. L’étude sur les échantillons en PP indique que des cavités, probablement nocives, sont présentes uniquement dans la zone soudée des polymères renforcés. Leur présence est alors principalement attribuée à la réorientation des fibres pendant l’opération de soudage. Enfin, une simulation par éléments finis du comportement mécanique jusqu’à la rupture a été réalisée sur une éprouvette maillée comprenant une zone soudée. Les résultats obtenus mettent en exergue un phénomène d’amplification et de redistribution des contraintes dans la zone soudée en raison du confinement. Cette triaxialité générée favorise la croissance et la coalescence des cavités dans la zone soudée expliquant ainsi l’affaiblissement de la contrainte macroscopique uniaxiale à la rupture.

Vibration welding is a common process used in automotive industry to assembly polymer parts. For pristine polymers, welding ratio, defined as the ratio between weld strength and tensile strength of non-welded material, is close to 1. However, for glass-fiber reinforced polymers, welding ratios are around 0.6 at best. In order to understand this discrepancy, several grades of polyamide (PA) 6 and 66 reinforced with 30% glass fibers as well as polypropylene (PP) with 0, 20, 35 and 50 % glass fibers have been injected in plates and vibration welded. A linear relationship was obtained between tensile strength of welded and non-welded PA specimens and their glass fiber orientation. Correlations were also found when plotting weld strength of samples regarding their welded zone thickness. In addition, voids are present only in the welded zone of glass fiber reinforced specimens, indicating that these voids are due to reorientation of fibers during the welding process. Finally, finite element modeling of mechanical behavior up to failure has been applied on a meshed specimen with a welded zone. Results show an amplification and distribution of stresses in the three directions inside the welded zone due to geometrical confinement. This generated triaxiality promotes growth and coalescence of cavities in the welded zone, explaining the weakening of the macroscopic uniaxial stress at failure.; Le soudage par vibration est un procédé largement utilisé dans l’industrie automobile pour assembler des pièces en polymère. Lorsque la matière à souder ne contient pas de renforts, le coefficient de soudage, défini comme le rapport entre la contrainte à la rupture du matériau soudé et celle du matériau non soudé est proche de 1. En revanche, il devient difficile d’obtenir des coefficients de soudage dépassant 0,6 lorsque le matériau contient des fibres de verre. Afin de mieux comprendre pourquoi, différents grades de polyamide (PA) 6 et 66 renforcés à 30% de fibres ainsi que du polypropylène (PP) à 0, 20, 35 et 50% de fibres ont été injectés en plaques et soudées par vibration. Pour les échantillons en PA, des corrélations sont obtenues entre la contrainte à la rupture des assemblages soudés et l’orientation des fibres mais aussi avec l’épaisseur de la zone soudée déterminée par tomographie RX. L’étude sur les échantillons en PP indique que des cavités, probablement nocives, sont présentes uniquement dans la zone soudée des polymères renforcés. Leur présence est alors principalement attribuée à la réorientation des fibres pendant l’opération de soudage. Enfin, une simulation par éléments finis du comportement mécanique jusqu’à la rupture a été réalisée sur une éprouvette maillée comprenant une zone soudée. Les résultats obtenus mettent en exergue un phénomène d’amplification et de redistribution des contraintes dans la zone soudée en raison du confinement. Cette triaxialité générée favorise la croissance et la coalescence des cavités dans la zone soudée expliquant ainsi l’affaiblissement de la contrainte macroscopique uniaxiale à la rupture.

The objective of this PhD work was to study the role of shear on the rupture of ultra-thin polymer films, by coupling a numerical study to an experimental one. First, a numerical method adapted to the resolution of the thin film equation was set up without shear but taking into account the capillary and van der Waals forces. This method was validated by comparing the dynamics obtained to theoretical predictions. With the addition of shear, two regimes have then been evidenced as a function of the shear rate: in the case of low shear rates the rupture is delayed, while at higher shear rates it is suppressed. The use of spatio-temporal diagrams allowed the characterization of the interface deformation. The transition from the delayed to the suppressed rupture regime is similar to a phase transition. The experimental study at low shear rate allowed to confirm this delayed rupture, but also showed after the rupture an increased dewetting speed. This study will allow a better understanding of the rupture phenomena occurring during nanolayer coextrusion., L’objectif de ce travail de thèse était d’étudier le rôle du cisaillement sur la rupture de films ultra-minces de polymères, en couplant une étude numérique à une étude expérimentale. Tout d’abord, une méthode numérique adaptée à la résolution de l'équation du film mincea été mise en place sans cisaillement mais en prenant en compte les forces capillaires et de van der Waals,et validée en comparant la dynamique obtenue à des prédictions théoriques.Deux régimes ont ensuite pu être mis en évidence en fonction du taux de cisaillement : dans le cas de faibles taux de cisaillement la rupture est retardée, à plus haut taux de cisaillement elle est supprimée. L’utilisation de diagrammes spatio-temporels a permis de caractériser la déformation de l’interface. Le passage du régime de rupture retardé à celui de rupture supprimée présente une évolution analogue à une transition de phase. L’étude expérimentale à faible taux de cisaillement a permis de confirmer ce retard de rupture, mais aussi de montrer après la rupture une vitesse de démouillage augmentée. Cette étude permettra de mieux appréhender les phénomènes de rupture survenant lors de la coextrusion multinanocouches.

Block copolymers (BCP) have proven to be of great interest, especially for their ability to spontaneously self-assemble in ordered and well-defined nanostructures. Some of their macroscopic properties can be altered by physical effects such as confinement (constraining domains at molecular scale) or shear (predominant orientation of domains with anisotropic mechanical response). Here, the study of these effects over the BCP structuration is achieved by using an industrially scalable technique, nanolayer coextrusion, which allows fabrication of macroscopic films made of thousands of alternating layers with individual thickness tuned down to a few tens of nanometers. Films are then coextruded using the triblock : poly(methyl methacrylate-b-butyl acrylate-b-methyl methacrylate) and the polymers : polymethyl methacrylate, polystyrene and polycarbonate (having different interfaces) by varying weight proportions and process parameters to target different layer thicknesses. A multi-scale characterization by coupling different technics as AFM, TEM (after samples staining) and SAXS allowed the local identification of nano-domains and the better understanding of process-structures-properties relation as a result of tensile tests showing improved mechanical behavior for these films. A low thermic stability for comparable process times showed a clear influence over structuration of this kind of BCP. We have observed a change in the triblock morphology from lamellar structures, when it is a close to thermodynamic equilibrium state, to cylindrical structures within the multilayer system which is maintained over long distances indistinct of layer thickness. The well understanding of the forming process parameters to create polymer-based materials at micro- or nanoscale scale is therefore an important factor in order to control nano-structures during a large-scale production of hierarchized materials consisting of BCP, which could strongly influence their macroscopic properties.; Les copolymères à blocs (BCP) sont des macromolécules capables de s’auto-assembler produisant des morphologies bien définies à l’échelle nanométrique. Un certain nombre de leurs propriétés macroscopiques peuvent être largement modifiées par des effets de confinement à l’échelle moléculaire, mais aussi par des effets de cisaillement (l’orientation des structures et donc un comportement mécanique fortement anisotrope). L’une des technologies permettant d’étudier ces effets sur la structuration des BCP est la coextrusion multinanocouches, qui permet de produire à grande échelle des matériaux sous forme de films possédant deux ou plusieurs constituants organisées en milliers de couches alternées ayant chacune une épaisseur nanométrique. À l’aide de ce procédé, des films constitués du tribloc poly(méthacrylate de méthyle-b-butyle acrylate-b-méthacrylate de méthyle) et des homopolymères polyméthacrylate de méthyle, polystyrène et polycarbonate (ayant donc différentes interfaces) ont été fabriqués en variant la composition du mélange et les conditions de coextrusion afin d’obtenir différentes épaisseurs du film (et donc différentes épaisseurs de couche). Une caractérisation multi-échelle a été effectuée en couplant différentes techniques, notamment AFM, MET (après une étape préalable de marquage) et SAXS. Elle a permis d’identifier les structures locales au sein des couches et de mieux comprendre la relation procédé-structure-propriétés suite à des essais en traction uni-axiale montrant de meilleures propriétés dans le cas de structures multicouches. Une faible stabilité thermique, à de temps comparables à ceux du procédé, a montré une influence sur la structuration de ce type de BCP. Nous avons mis en évidence un changement dans la morphologie du tribloc à partir d’une structure lamellaire, lorsque le matériau se trouve dans un état proche de l’équilibre thermodynamique, vers une structure cylindrique au sein du système multicouche et maintenue, quelle que soit l’épaisseur de couche, à grande distance. La maitrîse des procédés de transformation de matériaux à base de polymère à l’échelle micro ou nanométrique prend ainsi toute son importance afin de mieux contrôler, dans le cas des BCP, la structuration lors d’une production à grande échelle de matériaux hiérarchisés constitués de ces matériaux, ce qui influence fortement leurs propriétés macroscopiques.

Block copolymers (BCP) have proven to be of great interest, especially for their ability to spontaneously self-assemble in ordered and well-defined nanostructures. Some of their macroscopic properties can be altered by physical effects such as confinement (constraining domains at molecular scale) or shear (predominant orientation of domains with anisotropic mechanical response). Here, the study of these effects over the BCP structuration is achieved by using an industrially scalable technique, nanolayer coextrusion, which allows fabrication of macroscopic films made of thousands of alternating layers with individual thickness tuned down to a few tens of nanometers. Films are then coextruded using the triblock : poly(methyl methacrylate-b-butyl acrylate-b-methyl methacrylate) and the polymers : polymethyl methacrylate, polystyrene and polycarbonate (having different interfaces) by varying weight proportions and process parameters to target different layer thicknesses. A multi-scale characterization by coupling different technics as AFM, TEM (after samples staining) and SAXS allowed the local identification of nano-domains and the better understanding of process-structures-properties relation as a result of tensile tests showing improved mechanical behavior for these films. A low thermic stability for comparable process times showed a clear influence over structuration of this kind of BCP. We have observed a change in the triblock morphology from lamellar structures, when it is a close to thermodynamic equilibrium state, to cylindrical structures within the multilayer system which is maintained over long distances indistinct of layer thickness. The well understanding of the forming process parameters to create polymer-based materials at micro- or nanoscale scale is therefore an important factor in order to control nano-structures during a large-scale production of hierarchized materials consisting of BCP, which could strongly influence their macroscopic properties., Les copolymères à blocs (BCP) sont des macromolécules capables de s’auto-assembler produisant des morphologies bien définies à l’échelle nanométrique. Un certain nombre de leurs propriétés macroscopiques peuvent être largement modifiées par des effets de confinement à l’échelle moléculaire, mais aussi par des effets de cisaillement (l’orientation des structures et donc un comportement mécanique fortement anisotrope). L’une des technologies permettant d’étudier ces effets sur la structuration des BCP est la coextrusion multinanocouches, qui permet de produire à grande échelle des matériaux sous forme de films possédant deux ou plusieurs constituants organisées en milliers de couches alternées ayant chacune une épaisseur nanométrique. À l’aide de ce procédé, des films constitués du tribloc poly(méthacrylate de méthyle-b-butyle acrylate-b-méthacrylate de méthyle) et des homopolymères polyméthacrylate de méthyle, polystyrène et polycarbonate (ayant donc différentes interfaces) ont été fabriqués en variant la composition du mélange et les conditions de coextrusion afin d’obtenir différentes épaisseurs du film (et donc différentes épaisseurs de couche). Une caractérisation multi-échelle a été effectuée en couplant différentes techniques, notamment AFM, MET (après une étape préalable de marquage) et SAXS. Elle a permis d’identifier les structures locales au sein des couches et de mieux comprendre la relation procédé-structure-propriétés suite à des essais en traction uni-axiale montrant de meilleures propriétés dans le cas de structures multicouches. Une faible stabilité thermique, à de temps comparables à ceux du procédé, a montré une influence sur la structuration de ce type de BCP. Nous avons mis en évidence un changement dans la morphologie du tribloc à partir d’une structure lamellaire, lorsque le matériau se trouve dans un état proche de l’équilibre thermodynamique, vers une structure cylindrique au sein du système multicouche et maintenue, quelle que soit l’épaisseur de couche, à grande distance. La maitrîse des procédés de transformation de matériaux à base de polymère à l’échelle micro ou nanométrique prend ainsi toute son importance afin de mieux contrôler, dans le cas des BCP, la structuration lors d’une production à grande échelle de matériaux hiérarchisés constitués de ces matériaux, ce qui influence fortement leurs propriétés macroscopiques.

Nanolayer coextrusion enables the production of polymeric films composed of up to thousands of alternating layers. The thickness of each layer can in theory be controlled, by monitoring the number of layers, the mass ratio of the polymers, and the draw ratio of the film at the exit die, and can decrease down to several nanometers. It has been shown that such films can display drastically improved macroscopic properties, such as optical, gas barrier, or mechanical, due to confinement and interfacial effects. However, layer beak-up phenomenon occurring at such thicknesses, impacting the resulting properties, has also been reported for many polymer pairs. The goal of this thesis is to investigate the causes for these break-ups and for the instabilities leading to them. Most of this work deals with multilayer films of polymethyl methacrylate (PMMA) and polystyrene (PS), two amorphous polymers which blends were widely studied in the literature. Films with 65 to more than 8000 layers were fabricated by modifying processing and molecular parameters, to determine their impact on the homogeneity of the samples. These films were characterized mainly by using microscopy techniques, and especially atomic force microscopy (AFM), to extract not only the mean layer thickness, but distribution of thicknesses and the ratio of broken layers within the sample. A first necessary step was to develop a reliable statistical and quantitative analysis to obtain such information. Then, a first study focused on the effects of some process and material parameters on the homogeneity of multilayer films with micronic thicknesses. Choosing favourable experimental conditions at these scales, nanolayered films were then fabricated. We showed the existence of a critical layer thickness, below which layer breakup, estimated at around 10 nm for PS/PMMA films. When the targeted thickness is around or below 10 nm, the amount of broken layers increases significantly. We make the hypothesis that the layer breakup phenomenon is due to interfacial instabilities driven by van der Waals forces. The thicknesses of the layers we can reach with this process are so small that dispersive forces between two layers composed of the same polymer cannot be neglected (typically below 100 nm). Model experiments are proposed to quantitatively study the critical conditions of appearance of these layer breakups., La coextrusion multinanocouche est un procédé innovant qui permet de combiner deux polymères afin de produire des films composés de couches alternées dont le nombre peut être contrôlé et atteindre plusieurs milliers. Ainsi, les épaisseurs des couches individuelles dans le film peuvent en théorie atteindre quelques nanomètres. Les effets de confinement des chaînes macromoléculaires ainsi que la multiplication des interfaces peuvent alors conduire à des propriétés macroscopiques améliorées, pertinentes dans un contexte industriel (optiques, mécaniques, barrière aux gaz, …). Néanmoins, à ces échelles, des défauts dans la continuité des couches peuvent apparaître pendant la mise en forme et affecter ces propriétés. L’objectif de cette thèse est d’identifier les paramètres clés, procédés et matériaux, et de mieux comprendre les mécanismes à l’origine des instabilités conduisant à ces inhomogénéités de la nanostructure. Dans ce cadre, deux polymères amorphes ont été principalement étudiés, le polyméthacrylate de méthyle (PMMA) et le polystyrène (PS). Des films composés de 65 à plus de 8000 couches alternées, à différents taux d’étirage et compositions massiques, ont été fabriqués dans le but d’étudier la stabilité du procédé à différentes échelles et principalement à l’échelle nanométrique. Les films obtenus ont été caractérisés par microscopie, en particulier la microscopie à force atomique (AFM). Un premier travail a consisté à mettre en place une démarche statistique et quantitative pour caractériser l’épaisseur moyenne des couches obtenues, mais aussi la distribution d’épaisseur et la stabilité des couches. Puis, nous avons cherché à sonder l’effet de différents paramètres procédés et matériaux sur l’homogénéité des structures à l’échelle micronique. En se plaçant ensuite dans des conditions stables à ces échelles, nous avons cherché à faire varier de manière systématique les paramètres procédés pour étudier la stabilité des couches à l’échelle nanométrique. Nous avons mis en évidence l’existence d’une épaisseur critique en dessous de laquelle les couches rompaient, située autour de 10 nm pour le couple PS/PMMA. Lorsque l’épaisseur visée est de l’ordre de la dizaine de nanomètres ou inférieures, le taux de rupture de couches augmente également fortement. Des hypothèses sont faites quant aux causes de ces ruptures et de l’existence de cette épaisseur critique. Nous suggérons que ces ruptures peuvent être provoquées par des perturbations interfaciales (liées à des impuretés et/ou aux fluctuations thermiques) amplifiées par les forces de van der Waals qui deviennent non négligeables pour de faibles épaisseurs de couches (typiquement inférieures à 40 nm) et sont attractives entre deux couches de même nature. Des expériences modèles sont proposées dans la perspective d'une approche quantitative des conditions critiques d'apparition de ces défauts.