Control of Gelation and Network Properties of Cationically Copolymerized Mono- and Diglycidyl Ethers

International audienceThe development of low temperature curing systems has become a major objective in thermoset technologies for both environmental and economic reasons. The use of protic and chelating additives have recently been underlined for the control of the cationic ring opening polymerization of epoxies, a curing mode that is very efficient at temperatures close from the ambient but that can easily runaway. In this paper, we propose to use this strategy to control the kinetics of the cationic copolymerization of a diepoxy monomer(diglycidyl ether of bisphenol A, DGEBA) with a monoepoxy monomer(phenyl glycidyl ether, PGE). The purpose of the study is to tune the crosslink density (ν e) in order to control the mechanical properties of the materials. The sol-gel transition was first investigated in details at several frequencies by using the Fourier transform mechanical spectroscopy method (FTMS). We found that the gel time (t gel) and the critical conversion (α gel) can be controlled to a great extent by promoting transfers and complexing cationic species involved in the polymerization mechanism. The FTMS method also gives some insight into the structure of the polymer clusters at the sol-gel transition. The results indicate that the various additives used to control the transition have mostly no influence on the clusters' structure. The properties of the fully-cured networks were then investigated via swelling and dynamic mechanical measurements. Both methods indicate that ν e is strongly influenced by the crosslinker content (DGEBA) bu

Connection Between Microscopic Dynamics And Macroscopic Properties Of Soft Particle Glasses

The heterogeneous dynamics of jammed suspensions of soft particles is analyzed using three-dimensional particle dynamic simulations. Our results reveal how particle softness determines the dynamics at different length scales. The long-time shear-induced diffusion coefficient, the structural relaxation time, the shear stress and the normal stress differences are found to be universal functions of the same non-dimensional shear rate that depends on the solvent viscosity, particle elasticity and volume fraction. They are interconnected through simple relationships that predict the macroscopic rheology from microscopic dynamics. Please click Additional Files below to see the full abstract

Rheological Behavior and in Situ Confocal Imaging of Bijels Made by Mixing

International audienc

Particle-wall tribology of slippery hydrogel particle suspensions

Slip is an important phenomenon that occurs during the flow of yield stress fluids like soft materials and pastes. Densely packed suspensions of hydrogel microparticles are used to show that slip is governed by the tribological interactions occurring between the samples and shearing surfaces. Both attractive/repulsive interactions between the dispersed particles and surface, as well as the viscoelasticity of the suspension, are found to play key roles in slip occurring within rheometric flows. We specifically discover that for two completely different sets of microgels, the sliding stress at which slip occurs scales with both the modulus of the particles and the bulk suspension modulus. This suggests that hysteresis losses within the viscoelastic particles contribute to friction forces and thus slip at the particle-surface tribo-contact. It is also found that slip during large amplitude oscillatory shear and steady shear flows share the same generic features

Excess entropy scaling for soft particle glasses

The transport properties of soft particle glasses, such as dynamic viscosity, normal stress coefficients, and shear-induced diffusivity of its particles, are determined by the microstructure of the suspension under flow. A thermodynamic measure of the microstructure is the excess entropy, which we show here accurately correlates the transport properties of soft particle glasses onto master curves across a wide range of volume fractions, suspending fluid viscosities, particle moduli, and shear rates. The excess entropy for soft particle glasses is approximated with the two-body excess entropy computed from the pair distribution function extracted from dynamic simulations. The shear viscosity and normal stress functions diverge and the diffusivity vanishes at a critical excess entropy, corresponding to the yield stress of the suspension. An effective temperature is computed and is found to vary linearly with the shear stress and the elastic energy of the sheared soft particle glass. From this, an equation of state is derived relating the excess entropy to the shear stress. Consequently, three of the four transport properties are determined from the measurement of just one. Finally, a single master curve of particle diffusivity versus excess entropy is presented that unifies observations for both equilibrium and nonequilibrium suspensions.F.K. and R.T.B. gratefully acknowledge partial financial support from the National Science Foundation (NSF) (MRSEC under Award No. DMR-1720595)Center for Dynamics and Control of Material

Transient dynamics of soft particle glasses in startup shear flow. Part I: Microstructure and time scales

The rheology and microstructure of soft particle glasses during startup flow are studied using three-dimensional particle dynamics sim- ulations at different particle volume fractions and shear rates. The behavior of transient stress depends on the applied shear rate. At high shear rates, soft particle glasses exhibit a static yield stress signaled by a stress overshoot followed by a relaxation to a steady- state value. The buildup of the stress is driven by an interplay between structural anisotropy due to an accumulation of particles along the compression axis and a depletion along the extension axis and a compression of particles that are soft and deformable. At low shear rates, the stress increase is monotonic and without any stress overshoot. The time scale at which structural anisotropy and the stress are maximum is correlated to the nonaffine dynamics of SPGs through the persistence time of shear-induced particle collisions and to the residence time of particles inside their transient cages. The static yield strain γp and the reduced static yield stress σp/σy, where σy is the dynamic yield stress deduced from steady flow measurements, follow universal behaviors when correlated with the dimensional shear rate ηs γ_/G 0 , with ηs being the suspending fluid viscosity and G0 the storage modulus, which expresses the competition between elastic restoring forces and viscous dissipation. Dense suspensions of thermosensitive core–shell colloids, star-like micelles, and poly(ethylene oxide)-protected silica particles follow the same universal curves, suggesting the generality of our results.F.K. and R.T.B. gratefully acknowledge financial support from the National Science Foundation (MRSEC under Award No. DMR-1720595). B.F.D.D. and M.C. acknowledge financial support from the Innovative Training Network Dodynet (European Program H2020-MSCA-ITN-2017; Grant Agreement No. 765811).Center for Dynamics and Control of Material

Phenomenes d'agregation de particules sur une interface, induits par interaction capillaire, a deux dimensions

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Engineering Glasses And Gels Using Soft Colloids

At high concentration, dispersions of deformable particles such as emulsion droplets microgels, micelles and star polymers form entropic and jammed glasses that behave like weak elastic solids at rest but yield and flow at high stresses. In the jammed regime, the packed amorphous microstructure of the suspensions lies at the heart of their generic rheological behavior. Individual particles are trapped in cages and can only move past one another appreciably if the local stress exceeds the strength of the contact interactions. Chemistry offers a panel of strategies to tune the internal architecture of particles and the contact interactions, which in the general case involve elastic repulsion and attractions of different origins. We will review recent advances that bridge the gap between particle scale properties, local architecture, and macroscopic rheology, thereby opening new routes towards the rational design of glasses and gels using soft colloids. A special attention will be given to stimuli-responsive microgels which can be seen as the archetype of soft colloids. We will demonstrate that weakly crosslinked microgels belong to a new universality class of soft colloids whereas attractive interactions lead to a dual glass-gel paradigm

Adjuvants polymères pour l'extrusion de pâtes de ciment (conception, formulation, rhéologie)

PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF