Generation of silicone poly-HIPES with controlled pore sizes via reactive emulsion stabilization

Macrocellular silicone polymers are obtained after solidification of the continuous phase of a PDMS (polydimethylsiloxane) emulsion, which contains PEG (polyethylene glycol) drops of sub-millimetric dimensions. Coalescence of the liquid template emulsion is prohibited by a reactive blending approach. We investigate in detail the relationship between the interfacial properties and the emulsion stability, and we use micro- and millifluidic techniques to generation macro-cellular polymers with controlled structural properties over a wider range of cell-sizes (0.2-2mm) and volume fractions of the continuous phase (0.1-40%). This approach could easily be transferred to a wide range of polymeric systems

Lien entre propriétés adhésives et structure de polyHIPEs de silicone stabilisées via des réactions chimiques

Macro-cellular polymers are highly searched-for materials thanks to their rich physical properties. These arise from the internal structuration of the material, in which discrete cells of gas or liquid are tightly packed within a continuous polymeric solid. The size and organization of these cells have an important influence on the overall material properties. The influence of the properties of spheres on their final packing morphology has led to numerous studies usually dealing with either hard frictional or soft frictionless grains, which are the two extremes of the spectrum of possible systems. An important question remains as to what happens for systems which are in-between these extremes, i.e. highly deformable grains presenting a frictional surface. To tackle this problem, we work with a model system of ultra-stable emulsions which consist of PEG (polyethyleneglycol) drops which are dispersed in a continuous phase of PDMS (polydimethylsiloxane). Coalescence of the drops is prohibited by a reactive blending approach which creates a solid-like skin around the PEG drops upon contact with the PDMS. This skin creates adhesion and friction between the drops. To study the influence of the skin properties on the sedimentation of the drops, we characterize the final drop packing under gravity using absorption contrast X-Ray. We show that the presence of friction and adhesion at the interface makes the liquid drops pack unconventionally regarding density and organization compared to classic surfactant stabilized emulsions. We then investigated the adhesive properties of the solid emulsions i.e. elastomers containing liquid drops in their substructure, using a probe-tack test. We studied the impact of the drop size and density on the increase of the bulk's dissipations of energy which enhance the adhesive properties of the material.Les matériaux cellulaires font l'objet de beaucoup de recherches du fait de leurs remarquables propriétés. Celles-ci proviennent de la structure interne du matériau, dans lequel des inclusions cellulaires sont compactées dans une matrice solide. Comprendre et contrôler l'organisation des cellules dans la phase continue est donc primordial pour pouvoir contrôler les propriétés finales du solide poreux. L'influence des propriétés d'objets sur leur organisation dans un volume a souvent été étudiée pour des systèmes granulaires durs monodisperses, où la friction entre deux grains implique que l'arrangement global sera désorganisé, ou pour des systèmes mous comme les bulles dans les mousses aqueuses, où la très faible friction aux interfaces conduit à un empilement organisé et compact de sphères. Une question importante est de comprendre comment s'empilent des objets déformables présentant de la friction à l'interface. Pour répondre à cela, nous nous intéressons ici à un système modèle de gouttes de PEG (polyéthylèneglycol) dispersées dans une phase continue de PDMS (polydiméthylsiloxane). La coalescence entre les gouttes est empêchée grâce à une réaction à l'interface qui crée un gel de polymère à la surface des gouttes au contact avec le PDMS. Cette peau de polymères induit de la friction et de l'adhésion entre les gouttes. Pour étudier l'influence des propriétés de la peau sur la sédimentation des gouttes, nous caractérisons la fraction volumique finale sous gravité grâce à la tomographie sous rayons X. Nous montrons que la présence de friction et d'adhésion à l'interface induit une organisation non-conventionnelle des gouttes en comparaison avec celle d'émulsions stabilisées par des tensioactifs. Nous examinons ensuite les propriétés mécaniques et adhésives des émulsions solides, composées de gouttes liquides dans une matrice solide, avec un test de probe-tack. Nous étudions l'impact de la taille ainsi que de la densité de gouttes sur l'augmentation des dissipations d'énergie dans le volume

Skinny emulsions take on granular matter

International audienceOur understanding of the structural features of foams and emulsions has advanced significantly over the last 20 years. However, with a search for "super-stable'' liquid dispersions, foam and emulsion science employs increasingly complex formulations which create solid-like visco-elastic layers at the bubble/drop surfaces. These lead to elastic, adhesive and frictional forces between bubbles/drops, impacting strongly how they pack and deform against each other, asking for an adaptation of the currently available structural description. The possibility to modify systematically the interfacial properties makes these dispersions ideal systems for the exploration of soft granular materials with complex interactions. We present here a first systematic analysis of the structural features of such a system using a model silicone emulsion containing millimetre-sized polyethylene glycol drops (PEG). Solid-like drop surfaces are obtained by polymeric cross-linking reactions at the PEG-silicone interface. Using a novel droplet-micromanipulator, we highlight the presence of elastic, adhesive and frictional interactions between two drops. We then provide for the first time a full tomographic analysis of the structural features of these emulsions. An in-depth analysis of the angle of repose, local volume fraction distributions, pair correlation functions and the drop deformations for different skin formulations allow us to put in evidence the striking difference with "ordinary'' emulsions having fluid-like drop surfaces. While strong analogies with frictional hard-sphere systems can be drawn, these systems display a set of unique features due to the high deformability of the drops which await systematic exploration