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cited By 4; International audience; The influence of microstructure and temperature on the initiation of yield stress and strain of high-density polyethylene are examined using a set of linear and branched polyethylenes. The polymers were crystallized in different ways in order to get samples covering the range of crystallinity 0.5 ≤ Xc ≤ 0.8 and crystallite thickness 8 nm ≤ Lc ≤ 29 nm. In contrast to the conventional macroscopic yield strain and stress, the initiation strain εyi and stress σyi were estimated from the macroscopic stress-strain curves at the onset of local plasticity as judged from in situ WAXS experiments upon tensile deformation. Phenomenological linear relation was observed between σyi crystallinity at each draw temperature Td. The dislocation model was applied to check the correlation between σyi and crystal thickness. In order to also account for the chain topology, namely the concentration of stress transmitters ST, a modified Eyring's approach was proposed. This modelling provides a good prediction the σyi dependence on Lc and ST in the context of thermally activated rate processes. Finally, anelastic stress gauge, σccr, was determined from local strain measurements in crystals at the same local strain as for σyi. This critical elastic stress at initiation of crystal plasticity displayed a good correlation with σyi at high crystallinity. However, σccr was found to deviate from σyi with increasing Td particularly at low Xc values. This finding was attributed to the activation of the crystalline mechanical relation that involves a significant drop of σyi with increasing Td in the crystalline lamellae under shear yielding whereas it does not affect the theoretical σccr elastic stress. © 2017 Elsevier Ltd

cited By 0; International audience; Mechanical properties, especially rupture properties, of unfilled EPDM submitted to different radiation doses (with a dose rate of 1 kGy/h) are presented and discussed. The elastomer structure modifications, via chain scission and crosslinking, lead to a small increase in the strain at break, that can be described by the Kuhn approach, which relates it to the extensibility limit of the elastically active chains of the network. The critical strain energy release rate, G0, evaluated from tests on notched samples can be roughly described by the Lake and Thomas model, if the presence of soluble and dangling chains is taken into account in the calculation of the mean values used in the model equation. G0 is not correlated to the ultimate properties indicating the main influence of the crack initiation step on the material rupture. © 2017 Elsevier Ltd

International audience; Ultrahigh-molecular-weight polyethylene (UHMWPE) has been processed by means of sintering of a nascent powder. Particular attention was paid to the precompaction of the powder just below the melting point (Tm) under vacuum. The particle welding was subsequently carried out under pressure at various temperatures above Tm for various durations. Tensile drawing experiments performed on sintered samples either at room temperature or above Tm were specifically aimed at discriminating the role of chain interdiffusion through the particle interfaces from that of cocrystallization in the mechanism of particle welding. It turned out that efficient welding occurred within a very short time. One of the novel results of the work is the much weaker influence of sintering time as compared with temperature, giving evidence that chain interdiffusion is not governed by a reptation process. The entropy-driven melting explosion over distances much larger than the chain length between entanglements is suggested to be the main mechanism of the fast chain re-entanglement and particle welding in the present case of a nascent powder consisting of nonequilibrium chain-disentangled crystals. Another major aspect of this study is the demonstration of the huge cocrystallization efficiency in the interface consolidation in the solid state that significantly hides the kinetics of chain intertwining occurring in the melt.

International audience; We study the slow dynamics of water evaporation out of hydro-phobic cavities by using model porous silica materials grafted with octylsilanes. The cylindrical pores are monodisperse, with a radius in the range of 1–2 nm. Liquid water penetrates in the nanopores at high pressure and empties the pores when the pressure is lowered. The drying pressure exhibits a logarithmic growth as a function of the driving rate over more than three decades, showing the ther-mally activated nucleation of vapor bubbles. We find that the slow dynamics and the critical volume of the vapor nucleus are quantita-tively described by the classical theory of capillarity without adjust-able parameter. However, classical capillarity utterly overestimates the critical bubble energy. We discuss the possible influence of surface heterogeneities, long-range interactions, and high-curvature effects, and we show that a classical theory can describe vapor nucleation provided that a negative line tension is taken into account. The drying pressure then provides a determination of this line tension with much higher precision than currently available methods. We find consistent values of the order of −30 pN in a variety of hydrophobic materials. drying transition | hydrophobicity | kinetics | nanobubbles

Stack lamella deformation depends on their orientation with respect to the loading axis, the intrinsic properties of the lamellae, and the mechanical coupling between crystalline and amorphous phases. The aim of this work is to investigate the influence of the stress transmitter (ST) density and the crystallinity Xc on the local deformation. A wide experimental campaign has been undertaken on several polyethylenes with controlled molecular parameters and subjected to different thermal treatments. The ST density has been evaluated by the natural draw ratio and calculated by the Brown's model. The local deformation was measured by SAXS along a tensile test by using the long period stretching of the equatorial lamella stacks. The ratio elocal/emacro was found to be a constant close to 0.5. This surprising low value has highlighted that the equatorial regions could be either the stiffest zone of the spherulite or submitted to a lower stress. It is proposed that the stability of the ratio elocal/emacro is the result of two opposite phenomena: On one hand, the increase of Xc leads to unload the equatorial regions due to partial percolation of the crystalline phase and so decreases the stresses. On the other hand, when increasing Xc, the ST density decreases which causes the decrease in the local equatorial modulus. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1535–1542, 2010

A new technique, high-velocity compaction (HVC), allows the processing of nascent semicrystalline polymer powders via sintering. For the two polymers studied, polyoxymethylene (POM) and ultra-high molecular weight polyethylene (UHMWPE), it has been observed that during HVC a fraction of the nascent polymer undergoes melting. After cooling, the material thus comprises both nascent polymer that has not undergone melting and melted–recrystallized polymer. First, the localization of the melted–recrystallized polymer is investigated by microscopic techniques and nanoindentation. This reveals that melting occurs first at particle interfaces. Then, it is demonstrated that welding of the particles occurs via chain diffusion and recrystallization during the cooling. The efficiency of these welding mechanisms is discussed in order to gain an insight into the possibilities of using HVC to process different types of polymers. In addition it is shown that for a given material the overall sintering quality is governed by the fraction of recrystallized material.

A new processing method, high velocity compaction (HVC), is particularly adapted to process ultra high molecular weight polyethylene (UHMWPE). In a previous study, it has been shown that UHMWPE processed by HVC exhibits superior Young's modulus and significant ductility, owing to the preservation of nascent high crystallinity and high-quality sintering (Jauffres et al., Polymer 2007, 48, 6374). In this article, a comparative study of yield, creep, and wear properties of HVC-UHMWPE and compression-molded UHMWPE is reported. HVC-UHMWPE has an enhanced resistance to plastic deformation, likely due to its particular microstructure that improves its wear resistance. Concerning creep resistance, HVC-UHMWPE also exhibits enhanced performances thanks to its higher crystallinity. Consequently, HVC could be a new mean to improve UHMWPE performance for a wide range of applications, requiring high wear resistance, both in industrial and biomedical fields.

An extensive mechanical spectroscopy study in the low-frequency range is proposed to evidence and characterize the effect of a deformation below the glass transition temperature (Tg) on the mobility in an amorphous bisphenol-A polycarbonate. The mechanical treatment induces the appearance of a peak more than 100 K below Tα, which presents the characteristics of a relaxation associated to an unstable nature. This phenomenon is related to the creation of shear-induced high-mobility zones. The characteristics of these zones appear to be independent of the deformation level applied to the system. The use of a specific procedure allows the determination of the values of the apparent activation energy of the mechanism. The apparent activation energy increases continuously from the values of the β-relaxation to the main α-relaxation ones. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 497–505, 2008

International audience; Ultra High Molecular Weight Polyethylene (UHMWPE) is a semi-crystalline polymer with exceptional wear and impact properties, but also a very high melt viscosity, owing to its extremely long chains. Therefore, UHMWPE is non-melt processable and its processing is long and expensive. However, a new process, High Velocity Compaction (HVC), allows processing UHMWPE within short processing times via sintering. Several high velocity impacts are applied to a powder-filled die to provide self-heating. The sintering is then obtained by local fusion/recrystallization. In this study, the physical and mechanical properties of UHMWPE processed by HVC are investigated. Ductile UHMWPE with a high modulus was obtained. The particular microstructure of the material resulting from the sintering by fusion/recrystallization has then been characterized. It appears that mechanical properties of HVC-UHMWPE are governed by the microstructure induced by processing conditions, and hence can be adjusted for a given application.

Improvement of ball-bearing performance requires an optimization of desired properties of rubbery lip seals. Such an application necessitates specific material properties such as thermomechanical and tribological behaviours. These requirements led to investigate relationship between viscoelastic properties and tribological behaviour of filled elastomers. To achieve this objective, an analysis of two nitrile rubbers (NBR), which mainly differed in their glass transition temperature (Tg) values, was performed. This difference was obtained by a modification of the matrix composition. This work is focused on the difference of their tribological behaviour in relation to their viscoelastic, thermomechanic and physicochemical characteristics. It is pointed out that the sample with higher Tg shows the formation of an oxidised protective layer on the surface, which prevents its wear. The lower Tg sample, which shows high wear, does not have such a protective layer.

International audience; Conventional polymer processes, such as injection and extrusion, require the melting of the polymer. High-velocity compaction (HVC) allows the processing of polymer powders via sintering without the need of a melting stage. It opens up a new horizon for polymers that have processing issues linked to the melting stage. Because of chemical degradation above the melting point and significant shrinkage, the injection of semicrystalline polymer polyoxymethylene (POM) is often problematic. Nascent, highly crystalline POM powder has been successfully processed by HVC, and this process appears to be an interesting alternative to injection for certain applications. POM processed by HVC has a remarkably high stiffness but is brittle. A microstructural investigation, involving differential scanning calorimetry experiments and scanning electron microscopy, has been conducted to explain these unusual mechanical properties. It appears that in POM processed by HVC, the stiffness is due to particularly high crystallinity, and brittleness is intrinsic to nascent POM powder.

cited By 19; International audience; The small strain mechanical behavior of bulk polyethylene was investigated at the local scale of the lamella stackings by means of combined in situ SAXS and WAXS at different testing temperatures. Three different thermal treatments on four materials afforded studying a wide range of crystallinities (Xc) and microstructures. The local strain in tensile direction of the amorphous phase in equatorial region of the spherulites was determined via SAXS. The amorphous to macroscopic strain ratio proved to be fairly constant in the preyield strain domain for every materials. This ratio also proved to be strongly dependent on Xc. The local tensile stress on the amorphous phase in equatorial region was assessed from the strain on the crystals as measured by WAXS, using theoretical values of the elastic constants. The apparent tensile modulus of the amorphous phase, Ma, was shown to reach a maximum value of 300 MPa at RT for Xc = 50% and exhibited a monotonic drop with increasing both Xc and temperature. Evidence was given of the major role of the density of molecular stress transmitters on the amorphous phase stiffness over that of structural confinement. Comparison between Ma and macroscopic modulus revealed a significant modification of the mechanical coupling of the crystalline lamellae in relation to Xc that was assigned to an increasing lamella percolation throughout the spherulites with increasing Xc. © 2015 American Chemical Society.

International audience; The forced intrusion of water in hydrophobic nanoporous pulverulent material is of interest for quick storage of energy. With nanometric pores the energy storage capacity is controlled by interfacial phenomena. With subnanometric pores, we demonstrate that a breakdown occurs with the emergence of molecular exclusion as a leading contribution. This bulk exclusion effect leads to an osmotic contribution to the pressure that can reach levels never previously sustained. We illustrate on various electrolytes and different microporous materials, that a simple osmotic pressure law accounts quantitatively for the enhancement of the intrusion and extrusion pressures governing the forced wetting and spontaneous drying of the nanopores. Using electrolyte solutions, energy storage and power capacities can be widely enhanced.

International audience; Nascent powders of ultrahigh molecular weight polyethylene (UHMWPE) with different molecular weights in the range 0.6 < M-v < 10.5 million g/mol have been processed by means of sintering. The interface consolidation or particle welding was carried out under pressure at various temperatures above the melting point and for various durations. Tensile drawing experiments performed above the melting point enabled to study the role of chain interdiffusion through the particle interface. The melting explosion phenomenon was demonstrated to occur for each molecular weight. The higher is M-v, the greater is the efficiency of the phenomenon. But this does not allow a homogenization of the entanglement network of the sintered sample. Indeed, heterogeneity of deformation between the grain and the interface clearly appears during tensile tests above the melting temperature. The differences in mechanical behavior in the molten state between the molecular weights seem exacerbated by crystallization under tension. After melting explosion, end-chain diffusion as well as sideways motions are probably involved in the healing of very long chains polymer interfaces to reach a sufficiently high entanglement density. This latter phenomenon seems thermally activated.

cited By 17; International audience; A mapping of the initiation of the three major plastic deformation processes in polyethylene is established as a function of temperature and microstructure. In this aim, a collection of polyethylene samples covering large ranges of crystallinity 0.50 < Xc < 0.80 and crystal thickness, 8 nm < Lc c values at which a competition exists between two processes for various temperatures in the range 25-100 °C. These critical Lc values were reported on a temperature versus crystal thickness diagram where the various regions are assigning the order of occurrence that the three plastic processes should obey during tensile testing. Examples are given for illustrating the use of the map for predicting the order of occurrence of the three processes as a function of temperature for a given microstructure. Extrapolation is proposed for the behavior of materials having a crystal thickness out of the range of the present study. © 2015 American Chemical Society.

International audience; This work describes the influence of silica fillers on the thermomechanical properties of diglycidyl ether of bisphenol A/triethylenetetramine (DGEBA/TETA) epoxy resins during ageing under electron beam irradiation. Whatever be the silica filler (pure micrometric ground and spherical silicas, nanometric silicas and coupling agent treated silicas), the glass transition temperature of the epoxy resins decreases with increasing irradiation dose, meaning that the main effect of the irradiation is chain scission. No influence of the silica fillers has been detected from the changes in the glass transition temperature with the increase in the irradiation dose. The disappearance of the cooperativity of the ? relaxation, the decrease of the a relaxation and the decrease of the elastic modulus at the rubbery plateau observed by dynamic mechanical analyses involve a decrease in the crosslink density of the epoxy resins. The occurrence of chemical reactions between the epoxy resin and the silica surface at high irradiation doses has been shown. Moreover, we show evidence that chemical reactions between the epoxy resin and the silica surface occur at high irradiation dose.

Acrylonitrile-butadiene rubbers (NBR) are widely used in the automotive industry, especially for lip seals of ball-bearings. Such an application requires specific materials properties either in terms of thermo-mechanical or tribological behaviours in the presence of lubricants. NBR filled with carbon black properly fulfils these conditions. Specifically, the lip seal contact area has to be large and the contact pressure high to prevent leakage. Weak contact friction enables the heat dissipation in this area to be decreased and improves the durability of ball bearings. Both requirements for the optimization of these materials lead us to investigate the relationship between the viscoelastic and tribological behaviours of NBR. The present study focuses on the characterization of two NBR using thermo-mechanical, viscoelastic and tribological experiments. The tribological set up consists of a 52100 steel sphere sliding over a sheet of the rubber specimen under dry conditions, which is the critical issue of such a contact. Two rubbers are investigated, mainly differing by their T g values (Δ T g ≈ 15 °C). The tribological behaviour of the two materials is very different, while the difference in their mechanical properties is very limited. Rubber with a high T g value seems to be protected against wear, unlike the rubber with the lowest T g , on which long sliding induces a deep wear trace. Thermomechanical degradation occurs during the sliding and leads to surface modifications, which vary according to the nature of the rubber specimen. Experiments, results and comments address this difference of wear mechanism by relating it to the different characteristics of the sample surface: viscoelastic, thermo-mechanical and physicochemical (obtained by XPS analysis).

Understanding of the physics of confined fluids is of major theoretical and practical interest. In the field of materials science, the most popular techniques used to characterise the texture of mesoporous supports are based on the analysis of the properties of a confined fluid:thermoporometry, adsorption technique and mercury intrusion porosimetry. In particular, in the last two techniques the pressure parameter is related to the dimension of the pores. The present study is dedicated to an original analysis of the forced-intrusion of a non-wetting liquid (water) in hydrophobic mesoporous materials presenting different pore topologies. Among these supports, MCM-41 type materials allowed to point out distinct mechanisms for the intrusion and the extrusion of water. Whereas the intrusion process obeys to the Laplace law of capillarity, the extrusion is found to be preferentially governed by the formation of the vapor phase by a mechanism such as nucleation. Therefore, the conventional interpretation given for the hysteresis observed in mercury porosimetry, based on wetting hysteresis and pore-blocking effects is not directly transposable to the case of water intrusion experiments. Data obtained on other supports such as silica gels and Controlled Pore Glass supports are also reported and discussed taking into account the results obtained in MCM-41 type materials. Finally, the mechanisms leading to hysteresis in both phenomena (sorption of a wetting fluid and forced intrusion of a non-wetting liquid) are shown to present strong analogies in model materials as well as in disordered and interconnected porosities.

We present experimental and theoretical results on intrusion-extrusion cycles of water in hydrophobic mesoporous materials, characterized by independent cylindrical pores. The intrusion, which takes place above the bulk saturation pressure, can be well described using a macroscopic capillary model. Once the material is saturated with water, extrusion takes place upon reduction of the externally applied pressure; Our results for the extrusion pressure can only be understood by assuming that the limiting extrusion mechanism is the nucleation of a vapour bubble inside the pores. A comparison of calculated and experimental nucleation pressures shows that a proper inclusion of line tension effects is necessary to account for the observed values of nucleation barriers. Negative line tensions of order $10^{-11} \mathrm{J.m}^{-1}$ are found for our system, in reasonable agreement with other experimental estimates of this quantity.

Three types of low-density polyethylene materials were investigated with respect to the influence of the molecular architecture on the mechanical and use properties of blown films. The materials were a branched polyethylene synthesized by free-radical polymerization under high-pressure (HP-LDPE), a linear ethylene–hexene copolymer (ZN-LLDPE) produced by low-pressure Ziegler–Natta catalysis, and an ethylene–hexene copolymer (M-LLDPE) from metallocene catalysis. The extrusion and blowing conditions were identical for the three materials, with a take-up ratio of 12 and a blow-up ratio of 2.5. The blown films displayed a decreasing puncture resistance in the order M-LLDPE, ZN-LLDPE, and HP-LDPE. In parallel, the tear resistance of the films became increasingly unbalanced in the same order of the polymers. The morphological study showed an increased anisotropy of the films in the same polymer order, the crystalline lamellae being increasingly oriented normal to the take-up direction. This texturing caused a detrimental effect on the mechanical properties of the films, notably increasing the capacity for crack propagation. The phenomenon was ascribed to the kinetics of chain relaxation in the melt that governed the ability of the chains to recover an isotropic state from the flow-induced stretching before crystallization. The puncture resistance was examined in terms of both texture and strain-hardening capabilities. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 327–340, 2003

Molecular mobility is studied in poly(butyl methacrylate) isomers with molecular dynamics simulations in order to understand the role of the side chain steric hindrance on the β and δ relaxation mechanisms. The simulations suggest that in the temperature range studied the δ process is weakly influenced by the α process. Conversely, the β process seems to be significantly changed as compared with the low frequency regime. The results exposed also show evidence that the intramolecular cooperativity is higher in PnBMA than in PtBMA, which, could be related to the distance to the α-β crossover.

In the context of a European union-supported network on “Reactive Surfactants for Heterophase Polymerization,” different polymerizable surfactants (surfmers) have been synthesized and engaged in the emulsion polymerization of styrene, butyl acrylate, and acrylic acid. The thermomechanical properties of films cast from these different latices are reported in this article. The evolution of the mechanical properties with temperature and the effect of water molecules on these properties are studied. We observed that the studied surfactants do not influence the properties of the dry films. However, some differences due to grafting of reactive surfactants appeared when the films were wet. The amount of water uptake is drastically decreased when only reactive surfactants are present in the film. Concerning the mechanical behavior of the wet films, a decrease of the plastic flow stress is observed for all the samples whatever the nature of the surfactant (reactive or conventional). Hence, calorimetric measurements and dynamic mechanical analysis are used to identify the possible mechanisms that induce the change in the mechanical behavior of the latex films. In the case of reactive surfactant grafted to the polymer, the very low value of water uptake is accompanied by a plasticization of the polymer. In contrast, no plasticizing effect is observed in the case of nonreactive surfactant, even if the amount of water is very large. Finally, the tensile behavior of the styrene–butyl acrylate copolymer versus temperature is analyzed in the frame of the quasi point defects (qpd) model. Both rubber elasticity and chain orientation effects are taken into account to describe the behavior laws at large extensions (i.e., ϵ ≈ 1.2). © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1686–1700, 2002; DOI 10.1002/app.10548

Tensile drawing and short-term creep of ethylene/α-olefin copolymers having bimodal (BM) molar weight distribution are studied in comparison with unimodal (UM) copolymers of similar crystallinity. The natural draw ratio and viscoelastic recovery upon unloading strongly suggest that BM copolymers have more tie chains and chain entanglements than corresponding UM copolymers. The incorporation of co-units in the longest chains of BM copolymers is ascribed a major role on these topological changes. Creep of isotropic materials shows lower compliance for BM copolymers in parallel with higher-durability grades. This is attributed to a better “macromolecular network efficiency.” The creep behavior of strain-hardened samples, which is assumed to simulate the mechanical behavior of craze fibrils at the tip of a propagating crack, reveals similar trends. The better fibril strength in BM copolymers is again ascribed to a better network efficiency. Necked samples display an odd behavior of higher compliance at low stress and lower compliance at high stress for BM copolymers compared with the behavior of UM counterparts. This is associated with the exhaustion of viscoelastic capabilities with increasing draw ratio and stress. The phenomenon is discussed in relation to cavitation. A short-term creep test is proposed for comparative prediction of long-term behavior. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2308–2317, 2002

The deformation of polyethylene spherulites at the scale of the lamellar stackings is investigated by in-situ SAXS as a function of draw temperature in comparison with the macroscopic strain. Local elongation strain in equatorial region and local compressive transverse strain in polar region both increase in absolute value and follow linear relations versus macroscopic strain in the visco-elastic range. However, local strains turn out heterogeneous over the whole spherulite volume. This is attributed to the different type of mechanical coupling between the crystalline lamellae and the amorphous layers in the polar and equatorial regions. Increasing the temperature in the range 50–100 °C gradually promotes homogeneous distribution of strain at the local scale. The origin of this phenomenon is discussed in terms of activation of the chain mobility in the crystalline phase that modifies the mechanical coupling between the two phases. This ratio between local and macroscopic deformation increases with temperature up to the macroscopic value.

cited By 5; International audience; Different Ethylene Propylene Diene Monomers have been characterized prior and after aging under γ-irradiation. At small irradiation doses, the crosslinking process is the main mechanism occurring during irradiation, and is related to the residual ENB units remaining after the material processing. At higher doses, chain scissions (proportional to the radiation dose) are predominant and can be considered independent on the PE/PP ratio at a given ENB ratio, on the residual peroxide content, and on the initial EPDM cross-links density. © 2014 Elsevier Ltd. All rights reserved.

The thermomechanical properties of diglycidyl ether of bisphenol A (DGEBA)/4-4 diaminodiphenylmethane (DDM) resins are analysed as a function of the irradiation dose. Irradiation results in a decrease of the glass transition temperature and of the elastic modulus in the rubbery region, ascribed to a destruction of crosslinks. The Arrhenius diagrams show that the α relaxation becomes faster, that the β process is enhanced but that γ relaxation is less modified by the irradiation. The effects of irradiation on the stress–strain curves suggest a spatial heterogeneity of the crosslink density.

The β-crystalline form of isotactic poly(propylene) (PP) has been long recognized to have a greater mechanical absorption capacity than the α-crystalline form. This is of major importance for improving impact properties and crack resistance of injection-molding parts. Unfilled PP samples together with calcium carbonate-filled PP samples having various β/α-phase ratios, with nearly constant morphological parameters, have been investigated from the standpoint of ductile crack propagation and impact behavior. The presence of the β-crystalline phase turned out to improve both properties. The β spherulites are notably more prone to craze initiation than a spherulites that display a propensity for cracking. Subsequent crack propagation appears to be faster in the latter ones. The plastic zone ahead from the crack tip broadens, and the specific plastic energy increases with increasing β-phase content. The lower elastic limit of the β phase is likely to promote the early crazing. However, the suspected higher density of tie molecules in β spherulites provides more numerous and stiffer microfibrils. The impact strength of PP is also improved by the presence of β crystals as a result of greater energy-absorption capabilities. However, filled samples turned out insensitive to the β phase. A discussion is made about the origins of the β-phase-induced improvement of the mechanical properties. The possible role of the β → a transition is also explained.

Precursors of polyurethane acrylate based on hydroxy-terminated polybutadiene (HTPB) soft segments, different diisocyanate and hydroxy ethyl acrylate (HEA) as hard units, were synthesized in bulk or in solution in methyl methacrylate. During precursor synthesis (in bulk), microphase separation was observed by small-angle X-ray scattering (SAXS). Diffusing particles are around 50 A in size and are assumed to be assembling of hard segments. From these morphologies, it can be deduced that some isocyanate groups were trapped/or buried in hard domains. At a larger scale, around millimeters, hard segment crystallites were observed. Properties such as molar masses, melting and glass-transition temperatures, and viscosities were correlated with precursor structure and morphology. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 225–233, 2002

Effects of irradiation on the viscoelastic properties of peroxide-crosslinked PDMS elastomers (EVC) without filler (1), filled with 30% untreated nanoscopic SiO 2 (2) and with 30% surface modified silica (3) were investigated. Electron irradiation in air using a high dose rate allowing homogeneous irradiation at different doses ranging from 25 to 500 kGy have led to additional crosslinking of these three types of silicone rubbers making them become more rigid and brittle. The apparent crosslink density is higher for filled elastomers since links at the polymer–silica interfaces have been created as a result of irradiation. A further crosslink density enhancement is obtained for the surface-treated silica fillers. High resolution low frequency dynamic mechanical spectroscopy shows that the magnitude of the crystallisation peak located at approximately −40°C decreases with the irradiation dose. As a result, the amplitude of the relaxation process associated with the glass transition near −125°C increases, since a larger amount of amorphous phase is present due to the hindrance of the crystallisation behaviour. DSC measurements confirm that crystallisation is limited by the formation of a higher number of crosslinks, and that this effect is amplified for filled PDMS samples. A study of mechanical hysteresis at high strains (Mullins effect) shows an increase of the associated dissipative phenomena due to the induced polymer/silica bonds.

Ethylene/α-olefin copolymers having bimodal molar weight distribution are investigated in comparison with unimodal copolymers in order to understand the incidence of the molecular architecture on the stress cracking resistance. The preferred introduction of the co-units in the longest chains of bimodal copolymers is suggested to favour the occurrence of intercrystalline tie molecules during crystallisation. The more complex is the molecular architecture, the greater is the difficulty for crystallisation by regular chain folding. Intermolecular chemical heterogeneity resulting from preferred incorporation of the co-units in the long chains enhances the co-unit disturbing effect on crystallisation without reducing crystallinity. Intra-molecular heterogeneity of the co-unit distribution is also suggested to be an efficient means to generate tie molecules and random chain folding at the expense of regular chain folding. Isothermal crystallisation is used to probe the effect of molecular architecture on the crystallisation kinetics. It appeared that the correlations between kinetics, molecular architecture and molecular topology of unimodal copolymers no longer hold when considering bimodal copolymers. In contrast, the crystal surface free energy proved to be sensitive to topological changes resulting from molecular architecture modifications.

The influence of β crystals on the mechanical properties of isotactic polypropylene is studied on compression-moulded sheets without filler or filled with stearate-coated calcium carbonate particles. A crystallisation procedure is setup for producing sheets with various amounts of β crystals, trying to keep constant crystallinity, spherulite size and crystal thickness. An optimum of β crystals among α crystals is first produced by isothermal crystallisation at 110°C. Then gradual transformation of β into α crystals is carried out through thermal annealing at 152°C after an intermediate cooling down to room temperature. Due to the thermal inertia of the large sheet thickness, the amount of β crystals does exceed 10% for unfilled sheets and 50% for filled sheets. Dynamic mechanical behaviour shows that molecular mobility is higher in the β crystals than in the α crystals, as judged from the temperature of the crystalline relaxation. Loss modulus in the temperature range of the crystalline relaxation also indicates greater damping capacity for the β crystals. This is discussed in terms of activation of conformational defects moving along the chain stems in the crystal. Plane strain compressive testing reveals better ductility for β rich samples. Interpretation is provided using an approach of semi-crystalline polymer plasticity based on dislocation-governed crystallographic slip. Correlation is made with the viscoelastic behaviour through the concept of conformational defects.

The thermomechanical properties of DGEBA/TETA resins are analysed as a function of the irradiation dose. Irradiation by electrons results in a decrease of the glass transition temperature and the elastic modulus in the rubbery region. The Arrhenius diagrams obtained for the various doses show that the cooperative mobility associated with the α relaxation becomes faster after the irradiation. More local mobility corresponding to the β and γ relaxations is also modified, but in a lesser extent. These results can be interpreted invoking a destruction of the crosslinks within the resin. The effects of irradiation on the stress–strain curves are also discussed and are shown to be connected with both the physical aging kinetics and the spatial heterogeneity of the crosslink density in the resin.

Intercalated and exfoliated nanocomposites were prepared by the extrusion and injection of polyamide-6 and highly swollen or slightly swollen montmorillonite, respectively. The microstructure of pure compounds was characterized. The basal spacing was more homogeneous in swollen montmorillonite than in nontreated montmorillonite. This was attributed to the presence of a surfactant. Surfactant crystallites were observed in swollen montmorillonite and in the nanocomposites. Their distribution is discussed. The nanocomposites exhibited anisotropic properties attributed to the orientation of the montmorillonite sheets. The preferential orientation of the montmorillonite sheets was studied in detail by small-angle X-ray scattering (SAXS). It was found to be related to the injection direction. An average distance between two sheets was also measured from the SAXS spectra. The crystallographic state of the polyamide matrix was then analyzed. The orientation of the polymer lamellae relative to the montmorillonite sheets is discussed. The orientation of the montmorillonite sheets and the polyamide lamellae is expected to play a major role in the mechanical properties of nanocomposites.

Molecular mobility in two similar organic glass formers, namely sorbitol and maltitol, is studied in order to understand their difference in the cross-over between \(\) and \(\) relaxations, far above their respective glass transition temperatures. In this goal, the individual mobility of each carbon atom of the 6 carbon chain present both in sorbitol and maltitol is studied by means of ^13C nuclear magnetic resonance and molecular dynamics simulations. Both techniques imply that the mobility of carbons located at the end of the 6 carbon chain is greater than that of any other carbon of this chain and that the difference in carbon mobility is greater within the sorbitol moiety of maltitol than in sorbitol. The relaxation time obtained by magnetic resonance for carbons at the end of the 6 carbon chain is related to the \(\) relaxation time and the one of carbons in the middle of the chain is in relation with the value of the \(\) relaxation time. This result may suggest that the merging between the \(\) and \(\) relaxation processes in both sugars would be related to the decrease of the differences in mobility between the atoms of 6 carbon chain.

Ce travail s'inscrit dans la recherche de l'amelioration des proprietes a long terme du polyethylene (PE) pour applications aux tubes de distribution de gaz. Une etude du comportement thermique, microstructural et mecanique de trois PE de differentes architectures moleculaires et de differentes proprietes a long terme a ete realisee dans le but de progresser dans la connaissance des relations structure-proprietes, et de mettre au point un test a court terme permettant d'evaluer le comportement mecanique a long terme. La transformation fibrillaire qui intervient dans les mecanismes de fissuration constitue un element cle de cette etude. Le fluage d'echantillons etires jusqu'au stade de durcissement structural s'est avere etre un test approprie pour la prediction comparative du comportement a long terme.

Under special crystallization conditions from the melt, both α- and β-forms of isotactic polypropylene were produced simultaneously. The α- and β-spherulites of polypropylene were differentiated under optical microscope, allowing the nanoindentation of individual spherulites of each crystallographic form. Elastic modulus and hardness of β-spherulites were found to be 10 and 15% respectively lower than in α-spherulites. The higher stiffness of α may be related to the particular interlocked structure with cross-hatched lamellae, and to a lower molecular mobility in the crystallites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 195–200, 1999

In this paper, we investigate the final morphology of photocured polyurethane acrylates based on polypropylene oxide by means of Transmission Electron Microscopy (TEM), Small Angle X-ray Scattering (SAXS), and dynamic mechanical measurements. Two interrelated structural features on two different size scales can occur in these systems. TEM analysis demonstrates the presence of inhomogeneities on the length scale of 10–200 manometers, mostly constituted by clusters of small hard units (diacrylated diisocyanate) connected by polyacrylate chains. The bimodal shape of the dynamic mechanical relaxation spectra corroborates this two-phase structure. Besides, a suborganization of the reacted diisocyanate hard segments inside the polyurethane acrylate matrix is revealed by SAXS measurements, depending on the nature of the hydroxylacrylate used for the synthesis of the precursor. Finally, UV-exposure time is found to induce modifications on the viscoelastic properties of the final network, even at high double-bond conversion: this effect can be due to a postreaction and to an increase of the crosslinking density inside the hard segments domains. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 919–937, 1999

Hybrid systems based on 3-trimethoxysilyl propyl methacrylate, tetramethylorthosilicate, and methyl methacrylate were developed for moisture protection and strengthening of glass objects. The hydrophobic behavior of the hybrid was obtained by adding different fluorinated precursors to the hybrid solution. Experimental results show that among different fluorinated percursors, 1H,1H,2H,2H-perfluorooctyl trichlorosilane gives the best results, increasing the water contact angle up to 100° and decreasing the free surface energy. Coated glasses exhibit higher strength (more than 50%) than uncoated glasses. The strengthening was interpreted in terms of a healing mechanism.

The structures of in situ generated clusters and the level of physical interactions in two types of networks differing from the chemistry of crosslinking were studied by means of small-angle X-ray analysis and dynamic mechanical spectroscopy. For the first type of networks, the crosslinks result from the hydrolysis and condensation of ethoxysilane endgroups, thus generating silicon-rich dispersed phase. In the second case, the crosslinks result from the formation of urethane units by introducing a triol. In the two cases, different types of soft segment precursors having different polarities are considered. α, ω-hydroxyl-terminated oligomers of hydrogenated polybutadiene or polycaprolactone or a polyester from oleic acid are used. The miscibility of the soft-segment chains with the relatively polar crosslinks is the most important parameter for understanding the morphology and the mechanical behavior of such materials. The main difference obtained from the SAXS analysis and DMS experiments is that the silicon-rich clusters appear to be stiffer and well separated in comparison with the trimethylolpropane-urethane crosslinks. In addition, in the case of silica clusters generated in situ, the phase separation plays an important role. In the polycaprolactone-based systems, the formation of clusters is mainly governed by the nature and the reactivities of the functional groups. As a consequence, the clusters are more fractal-like. The mechanical behavior, i.e., the mechanical losses and the high-temperature behavior, is discussed as a function of the existing interactions and the concentration of elastically active network chains in the different types of networks considered. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2373–2386, 1997

This study focuses on the first occurrence of either cavitation or crystal shear in relation to temperature and microstructure during the tensile drawing of polyethylene. Four high density polyethylenes covering a range of crystallinity have been thermally treated to generate different microstructures displaying a large range of crystal thickness from 8 to 29 nm. The testing temperature spanned the domain 25–100 °C. In-situ SAXS measurements on synchrotron have been performed to capture the initiation of cavitation in parallel with stress-strain measurements. Depending on microstructure and temperature the strain onset of cavitation proved to be either before or after yielding associated with homogeneous or localized cavitation regimes respectively. The transition between the two regimes can be defined by a critical value of lamella thickness at each temperature. A physical modeling based on a thermally activated nucleation process has been developed for predicting the macroscopic stress for generation of cavities as well as the one for initiating crystal shearing. This modeling accounts for both temperature and microstructure effects on yielding. It allows describing successfully the delayed apparition of cavitation with increasing temperature and decreasing crystal thickness. The observation of complete disappearance of cavitation at high temperature is also predicted by the model in relation to crystal thickness. The more relevant aspects as well as the shortcomings of the model are discussed in the conclusion.

International audience; The sintering mechanism by high velocity compaction of UHMWPE nascent powders has been investigated with regard to the number of impacts, Nhia (otherwise the total impact energy), and the molecular weight, M. The mechanical energy provided to the powder produces a partial melting and subsequent welding of the powder particles. A Nryits threshold value is necessary to generate mechanical cohesion in the powder-compacted billets owing to the recrystallized interfacial material which provides wetting and welding of the powder particles. Mechanical strength increases with increasing Nryirs owing to the increasing fraction of recrystallized material. However, stiffness decreases in parallel due to the lower crystallinity of the recrystallized material compared to the original nascent powder. The M-w dependence of the mechanical properties of the various compacted powders is attributed to the interface strength of the welded particles in relation to the kinetics of interfacial chain diffusion. The mechanism Of particle welding is then discussed in terms of chain diffusion with regard to the time scale of the process. Wool's welding criterion based on the reptation of chains over their own radius of gyration through the interface only applies for M-w < 10(6) g/mol. It turned out that above this M-w value efficient welding is readily obtained after chain diffusion over a distance close to the stacking long period thanks to the occurrence of co-crystallization. (C) 2013 Elsevier Ltd. All rights reserved.

SUMMARY Emulsion polymerization of ethyl acrylate was carried out in the presence of different kinds of functionalized and non-functionalized silica particles. 3-(Trimethoxysily1)propyl methacrylate, 2-hydroxyethyl methacrylate and 3-(triethoxysily1)propanethiol functionalization agents were examined. It is demonstrated that use of the first two coupling agents results latexes able to give films which display remarkable mechanical properties similar to those of vulcanized elastomers reinforced with solid particles. A qualitative model is then set up to give account for these properties. It is proposed that a part of the polymer is covalently bound to silica as tight loops, giving rise to a set of strong entanglements with the non-grafted polymer during film formation. This model is supported by stress-strain measurements. Some validation experiments are described using the third grafting agent of the series. ZUS AMMENFAS S UNG : Die Emulsionspolymerisation von Ethylacrylat wurde in Gegenwart verschiedener funktionalisierter und nicht funktionalisierter Siliziumdioxid-Teilchen durchgefiihrt. Als Funktionalisierungsagens dienten 3-(Trimethoxysilyl)propylmethacrylat, 2-Hydroxyethylmethacrylat und 3-(Triethoxysi1yl)propanthiol. Mit den beiden erstgenannten Agentien wurden latexartige Materialien erhalten, aus denen Filme mit bemerkenswerten mechanischen Eigenschaften, vergleichbar denen von vulkanisierten und mit Feststoffen verstiirkten Elastomeren, hergestellt werden konnten. Ein qualitatives Modell zur Erklarung dieses Verhaltens wird vorgestellt, nach dem Polymermolekiile in Form von engen Ringen kovalent an Siliziumdioxid-Teilchen gebunden sind und wahrend der Filmbildung starke Verschlaufungen mit den ungepfropften Polymermolekiilen bewirken. Dieses Modell wird durch Ergebnisse von Zug-Span

Cellulose whiskers have been used as reinforcement in a copolymer matrix prepared from a latex phase. If a water suspension-mixing procedure is adopted, the fibril breakage that usually occurs during the mixing with a molten polymer can be avoided, and an enhanced filler dispersion can be expected. In this study, different processing methods have been used to prepare composite films, either by film casting (water evaporation) or by freeze drying, followed by classical compression or extrusion processes. The thermomechanical properties of these nanocomposites have been investigated, and the influence of processing conditions and the effect of whisker content have been considered. Processing conditions have a large influence on the mechanical behavior and can be classified in ascending order of their reinforcement efficiency : It can be attributed to a decrease of the apparent whisker aspect ratio, due to gradual breakage and/or orientation of the whiskers when hot pressing or extrusion is used. Below T g , good agreement is found between experimental moduli and the theoretical predictions of the Halpin-Kardos equation. On the other hand, above T g , a spectacular reinforcing effect is observed, which is widely underestimated by this short fiber composite model. This is related to the presence of a rigid cellulose network, linked by hydrogen bonds, when the whisker content is above its percolation threshold. The quality of this network (i.e., density and homogeneity) and thus, the magnitude of the reinforcing effect, depend on processing conditions.

Poly(butylene terephthalate) (PBT) was used as a semicrystalline modifier of epoxy-aromatic diamine formulations in concentrations ranging from about 3 wt% to 8 wt%. The epoxy monomer was based on diglycidylether of bisphenol A (DGEBA) and the diamines were either 4,4′-methylenebis [3-chloro 2,6-diethylaniline] (MCDEA) and 4,4′-diaminodiphenylsulfone (DDS). Using conversion-temperature transformation diagrams developed in part 1, thermal cycles were selected to generate different morphologies. In the case of PBT-DGEBA-DDS systems, phase separation in the course of reaction led to a random dispersion of spherical particles (sizes in the range of 1 μm), rich in PBT. Small and wide angle X-ray scattering, carried out in situ , during cure, revealed that the dispersion of spherical particles was produced by a nucleation-growth mechanism and that crystallization took place after phase separation. A completely different morphology, characterized by a distribution of large and irregular semicrystalline particles, was produced by crystallization before reaction. However, both types of morphologies introduced a small increase in the critical stress intensity factor. The main toughening mechanism was crack bridging produced by highly drawn thermoplastic particles. On the other hand, PBT-DGEBA-MCDEA formulations were cured at temperatures high enough to avoid crystallization of PBT during reaction. In this case, the PBT remaining dissolved in the matrix did not introduce any toughening effect.

The density fluctuations in poly(methyl methacrylate) are investigated by small-angle X-ray scattering (SAXS) as a function of temperature and microstructural state with time-resolved accuracy, by means of synchrotron radiation. We show for the first time the differences in electronic density fluctuations between quenched and annealed samples in relation to their dynamic behavior. At low temperature, the microstructural state slightly influences the electronic density fluctuations, whereas differences between both samples appear in the glass transition range. SAXS is thus essentially sensitive to fluctuations of dynamic nature. These features are understood and related to the differences observed directly in the relaxational behavior, as revealed by high resolution mechanical spectroscopy and differential scanning calorimetry measurements.

Crystallization of polytetrafluoroethylene (PTFE) from the melt state has been studied by wide angle X-ray scattering (WAXS), small angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) experiments, supported by scanning electron microscopy (SEM) observations. Isothermal crystallization of PTFE (between 315 °C and 325 °C) is analyzed and compared with PTFE cooled at 1 °C/min taken as the reference. We establish a TTC (time, temperature, crystallization) diagram and point out the time and temperature dependence of both crystallite thickness and crystallinity. It appears that the crystallization occurs in a very narrow temperature range. We assign the morphological changes to different crystal growth regimes in the range of study.

Lyophobic heterogeneous systems (LHS) are made of mesoporous materials immersed in a non-wetting liquid. One application of LHS is the nonlinear damping of high frequency vibrations. The behaviour of LHS is characterized by P - ΔV cycles, where P is the pressure applied to the system, and ΔV its volume change due to the intrusion of the liquid into the pores of the material, or its extrusion out of the pores. Very few dynamic studies of LHS have been performed until now. We describe here a new apparatus that allows us to carry out dynamic intrusion/extrusion cycles with various liquid/porous material systems, controlling the temperature from ambient to 120 °C and the frequency from 0.01 to 20 Hz. We show that for two LHS: water/MTS and Galinstan/CPG, the energy dissipated during one cycle depends very weakly on the cycle frequency, in strong contrast to conventional dampers.

cited By 7; International audience; The fibrillar structure that develops upon drawing of semi-crystalline polymers has been investigated at room temperature by means of in situ SAXS for a series of ethylene copolymers crystallized through various thermal treatments. The long period of the microfibrils for any given material proved to be only dependent on the experimental conditions of the drawing. This suggested the occurrence of a destruction and reconstruction of the semi-crystalline structure via fragmentation accompanied with melting-recrystallization. However, the diameter of the microfibrils proved to keep the memory of the lamellar microstructure via a correlation with the content of interphase at the crystal-amorphous boundary. A physical explanation is proposed for these apparently antagonist findings. The memory of the lamellar semi-crystalline microstructure is completely erased in the microfibrils owing to the melting-recrystallization process. In contrast, the diameter of the fibrils keeps the memory of the chain topology to which a major role is ascribed in the strain-induced fragmentation of the crystalline lamellae. © 2012 Elsevier Ltd. All rights reserved.

Isothermal agings have been carried out on poly(methyl methacrylate) at temperature far below the glass transition temperature region. Aging effects have been investigated by calorimetric and dynamic mechanical techniques; influence of quenching rate, time and temperature of aging have been studied. It is seen that the β-relaxation is not affected by physical aging and that the α-relaxation spreads out in the β-relaxation temperature domain. Comparison of the results obtained from the two techniques shows the occurrence of a specific mechanism for aging temperature far below the glass transition temperature.

The microstructure of polyurethanes and its evolution upon thermal treatment were investigated by means of small-angle X-ray scattering and low-frequency high-resolution mechanical spectroscopy. The segmented copolymer materials studied in this work were prepared in bulk and were completely amorphous as a consequence of the chemical composition of the soft and hard segments. This amorphous state results in model materials and, in principle, easier interpretation of the morphology from experimental results. It is shown in this work that thermal treatment induces a coarsening of hard-phase microdomains and an increase of the interfacial region, which remains of the order of a few angstroms. This morphology evolution leads, to some extent, to a weakening of the mechanical properties as observed above the glass transition temperature of the soft-phase matrix. The conclusion is that the synthesis conditions, as well as subsequent heat treatment, are to be carefully considered in order to obtain materials with well controlled physical properties.