Sparing our natural resources is very important for tyre industry, on both the society side and the economical side. Optimal design - lighter tyres but even safer and with longer life – is a real challenge that can only be addressed with a thorough understanding of crack growth mechanisms. In the specific case of filled elastomers, representing most of the tyre composition, fatigue crack growth approach is very empirical and potential progress on material design is limited. We want to open new areas of innovation and optimization of filled elastomers, by developing a new understanding approach of the damaging phenomena in the crack tip area (from using new experimental and simulation techniques to predicting tools for guiding new material design). This way, we hope to understand the first order effects of micro-structural parameters on intrinsic crack growth resistance properties. Today, influencing mechanisms are rather poorly understood and much discussed. Among other difficulties is the key issue of coupling various phenomena at different scales: large strain constitutive law, including softening and self-heating, mechanical and thermal fields evolution through geometric modification of the crack tip, and so on … Our project consists in a multi-scale approach linking the physico-chemical scale (material structure, from a few nanometers to some micrometers), the crack tip scale (hundreds of micrometers) and the scale of the structure (a few centimeters). This approach must link physico-chemics, physical damage and continuum mechanics. It includes proposing a new constitutive law for filled elastomers, taking into account its fatigue evolution, and a damaging model at the crack tip based on the understanding of local mechanisms. These models shall be included in a finite element crack simulation. Digital image correlation technique will be used for comparing experimental displacement fields near the crack tip with simulated fields. Then simulation shall be upgraded, in several experimental/simulation loops. The first model difficulties lie in the fact that it is compulsory to have a good coherence between the small scale field and the far field at the scale of the whole structure. The project is also very ambitious in trying to put together several aspects which are individually poorly mastered in the case of filled elastomers. On one hand, physico-chemical damage origin at the crack tip is still unknown, partly because measurements are very tricky near this crack tip. On the other hand, constitutive laws for filled elastomers are difficult to measure and to simulate, due to high non linearities and to their strong sensitivity to loading history. Finally, existing damage simulation principles, developed for other materials, and displacement field measurements, by digital image correlation, have both never been applied to elastomers. Thus, the global project approach mixes several analysis scales (from micro-stuctural physico-chemics of the material to continuum mechanics). Each of these analysis hits strong difficulties and their coupling is in itself very tricky. Common work between so many specialists joining their expertises together on the same problem and on the same experimental setup has yet never been tried. We think that complementarity of gathered expertises is the only way to reach significant progress in understanding crack propagation in filled elastomers and in identifying innovative tracks for proposing more resistant materials.

Over the last two decades it has become well-accepted that incorporating nanoparticles (NPs) into a polymer matrix can lead to materials with significantly improved properties, e.g., mechanical, optical, electrical or gas barrier. As a result, polymers filled with NPs - polymer nanocomposites (PNCs) – have the potential to strategically impact many critical emerging industrial applications like energy storage, tires, paint and medical implants. These interesting mixtures, whose structure and dynamics are driven by nanoscale interactions, have also opened up a series of fundamental physics questions related to the role of polymer chain confinement, entropic elasticity and colloidal aggregation in this context. Up to now, the field of PNCs research has mainly focused on the optimization of the filler contribution – specifically, the controlling the NP spatial arrangement in the polymer matrix – to improve the macroscopic material properties. However, it’s not obvious how to extract general trends about the mechanisms governing the structure-property relationships. One has some evidence showing clear correlation between the filler morphology and the macroscopic properties, for example the concept of NP percolation seems to underpin mechanical reinforcement, electrical and thermal conductivity in these hybrid materials. However, it is unclear if this percolation effect results purely from the NPs or from the corresponding polymer-NP interfacial contribution involving the modification of polymer chain dynamics that can range from local monomer relaxations to the large-scale entanglement network diffusion. Significant breakthroughs can therefore be expected in the field of PNCs by overcoming the lack of the proper characterization of interfacial and chain dynamics. The proposal aims to develop new methodologies to design well-defined PNCs with modulated interfacial properties to optimize their application-relevant functionality. To reach this objective, we propose a self-consistent multi-disciplinary project incorporating polymer synthesis, PNC formulation and characterization (including dynamics and macroscopic properties) and transfer this knowledge to real-life, industrially critical tire systems (Michelin). It is critical to emphasize the novel aspects of the proposed work, especially in relation to the established state-of-the-art in this well-travelled field. First, we will synthesize a series of grafted spherical NPs presenting various degrees of chain-NP interactions, chain mobility, interfacial stiffness and polydispersity. While many previous works have also focused on such interfacial modification of the NP surface, we propose to introduce labile bonds so that we can systematically (and controllably) “degraft” chains from the NPs and examine the deterioration of properties with the “worsening” interface, while presumably not affecting NP percolation. Secondly, we will design a new class of PNCs by controlling the NP dispersion state focusing of a new assembly paradigm. While many previous works have used two strategies, namely NP surfactancy or flow-induced effects to achieve this goal, here we propose to control NP assembly by a new mechanism, i.e., the rate of crystallization of a semi-crystalline polymer host. Finally, we shall transfer methodologies and concepts to systems relevant to the industrial manufacture of tire. Hence the project presented is a research collaborative project PRCE between four partners including an industrial partner, experts in (i) innovative grafting polymerization, (ii) theoretical and experimental studies to assemble these functionalized NPs in a polymer matrix and (iii) characterizing and modeling the macroscopic functionalities of the composite materials in line with structure and dynamics. We believe this collaboration, which builds on a long-standing relationship between the different partners, is central to achieving the goal of disruptively impacting industrial practice in this field.

Among the four main families of natural compounds produced by plants and animals, terpene derivatives constitute the largest and most diverse group of functional hydrocarbon-based derivatives including molecules from C5 hemiterpenes to very high molar mass polyisoprenes such as Natural Rubber (NR). They are synthesized in vivo in presence of specific enzymes from a common C5 elementary precursor, isopentenyl pyrophosphate (IPP). Although in vitro biosynthesis of many terpenoids is described, this strategy of synthesis remains restricted to laboratory studies because of the low availability and stability of IPP. We have recently discovered that isoprene (IP) can be recognized by enzymes and used as initial substrate in place of IPP to produce isoprenoids. In particular, it was found that addition of IP to fresh Hevea brasiliensis latex containing active cis-prenyltransferase yields to an over production of very high molar mass 1,4-cis polyisoprene with a structure close to NR. In this project we plan to develop this approach for the production of various terpenoids. The elementary mechanisms leading to the formation of new NR from IP will be first investigated. Besides the possibility to significantly improve the NR biosynthesis, this strategy will be extended to the preparation of other terpenoid of economical interest. To that aim complementary experiments using labeled IP will be conducted on fresh rubber particles and on specific enzymes involved in terpenoid synthesis (IPP/DMAPP isomerase, prenyl transferase,…). Enzymatic systems will be obtained from plant extracts and through cloning protocols. It is anticipated that the use of IP, a substrate readily available from oil and the biomass and easy to handle, in place of IPP will open extremely interesting perspectives for the production and valorization of terpenoids and of related compounds. This new synthesis pathway should permit enhancing the productivity of NR producing plants as well as an industrial scale-up of the production of terpenoids of interest using microbial platform technology.

The industry of the tyre built itself since the end of the XIXth century thanks to the development and the adaptation of innovative technologies, like for example the radial tire or the introduction of silica in the energy-saving tires. The French Factory of Pneumatics Michelin is one of the world leaders of the tyre. But this business sector is strongly competitive, because new actors stemming from emerging countries appear and win consequent market shares on the historic leaders. While in 2000 the Chinese manufacturers represented only a marginal part of the market, in 2011 the volume of tires produced by these last ones amounts to the total volume of 3 world leaders. In this context, it is essential to innovate on products, in particular in the field of the materials for the development of the low energy-saving tires (grading A). Today, the main development limitation of new rubbers is the industrial processing. In particular, when the new silica-filled elastomers are extruded into profiles, volume defects appear on extrudates, preventing from the feasibility of such formulations. Michelin thus wishes to open new ways of innovation, at present forbidden by these industrial constraints. For that purpose, it is necessary to understand the causes of these extrusion instabilities and to develop knowledge on the nanostructure and the properties of these compounds. This kind of study already exists in the field of the thermoplastic polymers but in the field of nano filled elastomers. Our final goal is to obtain physical multi-scales criteria that describe the rheologic behaviour of these materials, connected with microstructural and molecular parameters, so as to determine the link of causality with the appearance of volume instabilities in extrusion.