Complex dependence on the elastically active chains density of the strain induced crystallization of vulcanized natural rubbers, from low to high strain rate

Strain Induced Crystallization (SIC) of Natural Rubbers (NR) with different network chain densities (¿) is studied. For the weakly vulcanized rubber, the melting stretching ratio ¿m at room temperature is the lowest. This is correlated with larger crystallites in this material measured by in situ WAXS, suggesting their higher thermal stability. SIC kinetics is then studied via stretching at various strain rates (from 5.6 × 10-5 s-1 up to 2.8 × 101 s-1). For the slowest strain rates, SIC onset (¿c) is clearly the lowest in weakly vulcanized rubber. By increasing the strain rate, ¿c of the different materials increase and converge. For the highest strain rates, ¿c values still increase but less rapidly for the weakly vulcanized sample. This complex dependence on the elastically active chains (EAC) density of SIC has been confirmed by in situ WAXS during dynamic experiments and interpreted as a consequence of both the polymer chain network topology and of the entanglements dynamics.Peer ReviewedPostprint (author's final draft

Strain induced crystallization and melting of natural rubber during dynamic cycles

Strain-induced crystallization (SIC) of natural rubber (NR) is studied during dynamic cycles at high frequencies (with equivalent strain rates ranging from 7.2 s-1 to 290 s-1). The testing parameters are varied: the frequency, the temperature and the stretching ratio domain. It is found that an increase of the frequency leads to an unexpected form of the CI–¿ curve, with a decrease of the crystallinity during both loading and unloading steps of the cycle. Nevertheless, the interpretation of the curves needs to take into account several phenomena such as (i) instability of the crystallites generated during the loading step, which increases with the frequency, (ii) the memory of the previous alignment of the chains, which depends on the minimum stretching ratio of the cycle ¿min and the frequency, and (iii) self-heating which makes the crystallite nucleation more difficult and their melting easier. Thus, when the stretching ratio domain is above the expected stretching ratio at complete melting ¿melt, the combination of these phenomena, at high frequencies, leads to unexpected results such as complete melting at ¿min, and hysteresis in the CI–¿ curves.Peer ReviewedPostprint (author's final draft

Coarse-Grained Lattice Modeling and Monte Carlo Simulations of Stress Relaxation in Strain-Induced Crystallization of Rubbers

International audienc

De l' importance des conditions de synthèse sur la modification de l'interphase d'un système interpénétré de deux polymères réticulés

Un moyen relativement simple d'améliorer les propriétés thermo-mécaniques d'un matériau polymère est sa réticulation. Cependant, les matériaux modernes sont essentiellement constitués de deux ou plusieurs polymères et une réticulation de l'ensemble tend plutôt à faire diminuer leurs performances. La méthode couramment développée pour préserver les propriétés intrinsèques des constituants consiste en un mélange initial des monomères que l'on polymérise et réticule in situ par des mécanismes réactionnels différents. On obtient ainsi des réseaux polymères interpénétrés (IPN). L'enchevêtrement des phases qui résulte de cette technique de synthèse particulière permet une certaine stabilité morphologique, souvent associée à une synergie de propriétés. Notre étude consiste à déterminer les conditions et l'influence de l'introduction de liaisons covalentes inter-réseaux sur les propriétés d'un IPN polyuréthane / polystyrène (PUR / PS).Notre approche se décompose en trois parties. Tout d'abord nous avons synthétisé des échantillons modèles exempts de greffage inter-réseaux. Ceci a nécessité la mise au point d'un protocole expérimental adapté reposant sur la synthèse de PUR par "end-linking" et celle de PS par photopolymérisation radicalaire. Ensuite, en caractérisant ces échantillons, nous avons précisé l'importance du facteur cinétique sur la morphologie finale des matériaux suivant les conditions de synthèses. Nous proposons alors une relation semi-empirique, basée sur le gonflement des systèmes, permettant la mise en évidence de la présence d'enchevêtrements physiques entre les deux réseaux au sein des IPN. Finalement, en s'appuyant sur nos systèmes modèles nous abordons le greffage inter-réseau en présentant deux approches distinctes. Nous précisons les conditions de synthèse optimales pour lesquelles l'efficacité du greffage se traduit, notamment, par l'amélioration de la miscibilité du mélange.Crosslinking is relatively simple means to improve the thermo-mechanical properties of a polymeric based material. However, two or several polymers essentially form new materials, and crosslinking of the whole tends rather to decrease their performances. The method usually developed to maintain the intrinsic properties of constituents consists in an initial mixture of the mers which are then polymerised and crosslinked in situ by different mechanisms. Thus, interpenetrating polymers networks (IPN) are obtained. The entanglement of the phases that results from this particular synthesis technique allows a certain morphological stability, often associated with a synergy of properties. Our study consists in determining the conditions and the influence of the introduction of inter-network covalent grafts on the properties of a polyurethane / polystyrene IPN.Our approach decomposes into three parts. First, we have synthesized model samples without bonds between the networks. This requires the experimental settling of a fixed protocol based on the synthesis of PUR by end-linking and PS by free-radical photopolymerization. Then, by characterizing these samples, we have clarified the importance of the kinetic factor on the final morphology of the materials for various conditions of syntheses and we have proposed a semi-empirical relation, based on the swelling of the systems, which reveals the presence of physical entanglements between networks within the IPN. Finally, thanks to our model systems, we have considered the inter-network grafting considering two different types. We have clarified the optimal conditions of synthesis for which the efficiency of grafting is especially expressed by the improvement of miscibility of the mixture.STRASBOURG-Sc. et Techniques (674822102) / SudocSudocFranceF

Mechanical Properties of Rubber Nanocomposites: How, Why... and Then

International audienc

Functionally Graded Multilayer Composites Based on Poly(D,L lactide)/Bioactive Fillers Fabricated by a 3D Direct Pellet Printing Multi-Extrusion Process

International audienceThe aim of the present work was to investigate a 3D manufacturing process dealing with 3D Direct Pellet multi-Extrusion (DPPmE). Throughout this paper, various functionally graded materials (FGM) were obtained with tailored properties for bone regeneration. Therefore, an experimental investigation was performed to gain a better understanding of the structure-processing-properties relationships. A poly(D,l-lactic acid) (PDLLA) matrix and bioglass (BG) or hydroxyapatite (HA) bioactive fillers were chosen to obtain hybrid FGM in comparison to composite references. Different compositions and shape factors (from microspheres to fibers) of these bioactive fillers were used to control the PDLLA matrix degradation. Then, different strategies of DPPmE were used to tailor and tune the desired gradient of properties. Interestingly, good interfaces and adhesion properties were obtained. Subsequently, rheological, size-exclusion chromatography, and porosity measurements corroborated the present findings. Besides, tensile as well as thermomechanical properties showed the advantages of the DPPmE process to fabricate FGM composites with tailored architecture

Effect of ageing and annealing on the mechanical behaviour and biodegradability of a poly(3-hydroxybutyrate) and poly(ethylene-co-methyl-acrylate-co-glycidyl-methacrylate)blend

Poly(3-hydroxybutyrate) (PHB) undergoes an ageing process that contributes to its remarkable fragility. Blending it with an elastomer is a possibility to increase toughness. In this work, the mechanical properties of a 70/30 wt% blend of PHB and poly(ethylene-co-methyl-acrylate-co-glycidyl-methacrylate) were studied over time. The phenomenon of ageing affected the blend, which lost its ductility and became fragile days after its processing. Differential scanning calorimetry and small angle X-ray scattering analyses showed that this drop in mechanical properties was due to changes in the crystalline structure of the matrix. Annealing reduced fragility, increased toughness and prevented a re-ageing of the blend. Biodegradation in soil was also more intense for annealed samples.CAPESFAPESPcnp

Understanding the mechanical and biodegradation behaviour of poly(hydroxybutyrate)/rubber blends in relation to their morphology

In this work poly(hydroxybutyrate/poly(vinyl butyral)- co-(vinyl alcohol)-co(vinyl acetate) (or ethylene propylene diene monomer rubber) blends were prepared by conventional processing techniques (extrusion and injection moulding). A droplet type morphology was obtained for P(3HB)/PVB blends whereas P(3HB)/EPDM blends presented some extent of co-continuous morphology. In addition, rubbery domains were much smaller in the case of PVB. These differences in morphology are discussed taking into account solubility parameters and rheological behaviours of each component. For both blends, the increase of elastomer ratio led to a decrease of Young's modulus but an increase in elongation at break and impact strength. The latter increased more in the case of P(3HB)/EPDM blends although the rubbery domains were larger. These results are explained in the light of the glass transition of the rubber and the presence of plasticizer in the case of PVB. The addition of elastomer also resulted in an increase of P(3HB) biodegradation rate, especially in the case of EPDM. It is assumed that, in this case, the size and morphology of the rubbery domains induce a geometrical modification of the erosion front which leads to an increase of the interface between P(3HB) phase and the degradation medium and consequently to an apparently faster biodegradation kinetics of PHB/rubber blends. Copyright (C) 2011 Society of Chemical IndustryFAPESPFAPESPCAPESCAPESCNPqCNP

Influence of the rubbery phase on the crystallinity and thermomechanical properties of poly(3-hydroxybutyrate)/elastomer blends

Poly(3-hydroxybutyrate) (PHB) is a very promising biopolymer. In order to improve its processability and decrease its brittleness, PHB/elastomer blends can be prepared. In the work reported, the effect of the addition of a rubbery phase, i.e. ethylene - propylene-diene terpolymer (EPDM) or poly(vinyl butyral) (PVB), on the properties of PHB was studied. The effects of rubber type and of changing the PHB/elastomer blend processing method on the crystallinity and physical properties of the blends were also investigated. For blends based on PHB, the main role of EPDM is its nucleating effect evidenced by a decrease of crystallization temperature and an increase of crystallinity with increasing EPDM content regardless of the processing route. While EPDM has a weak effect on PHB glass transition temperature, PVB induces a marked decrease of this temperature thank to its plasticizer that swells the PHB amorphous phase. A promising solution to improve the mechanical properties of PHB seems to be the melt-processing of PHB with both plasticizer and EPDM. In fact, the plasticizer is more efficient than the elastomer in decreasing the PHB glass transition temperature and, because of the nucleating effect of EPDM, the decrease of the PHB modulus due to the plasticizer can be counterbalanced. (C) 2010 Society of Chemical Industr