H. Sautereau

Surface modifications of montmorillonite for tailored interfaces in nanocomposites

Different kinds of clays based on sodium montmorillonite have been modified by (1) cationic exchange of alkylammonium ions and (2) covalent grafting of organosilane. We have prepared organophilic clays with different gallery heights (due to alkylammonium ions with alkyl chain length varying from 8 to 18 carbon atoms) and with a specific functionalization (due to the organosilane). The experimental parameters of the cationic exchange process (exchange temperature, initial amine/clay ratio, washing of the clay) govern the quantity of the intercalated alkylammonium ions as well as their organization in the montmorillonite galleries. We have generated organoclays with different kinds of organic layer properties, namely ionically bonded or physically adsorbed alkylammonium ions and covalently grafted organosilane. These different surface treatments will be useful for different applications, varying as a function of the coupling with the matrix and as a function of filler dispersion and resulting properties (thermal stability, mechanical and barrier properties) of the final nanocomposite.

Structural heterogeneities and mechanical properties of vinyl/dimethacrylate networks synthesized by thermal free radical polymerisation

Dimethacrylate based networks usually have a poor impact resistance. In this work, we have tried to understand the origin of this brittleness. Two systems based on a same dimethacrylate monomer polymerised with styrene and divinylbenzene comonomers, respectively, have been chosen to correlate structural parameters with mechanical properties. The increase of structural heterogeneity, characterized by the width of relaxation time distribution, was measured as a function of double bonds conversion using dynamic mechanical tests. Atomic force microscopy observations of network structure after laser ablation show that the heterogeneity of networks is spacially organised due to the formation of microgels and their agglomeration. The presence of microgels strongly affects the polymerisation kinetics and controls the mechanical behaviour. Results show that the more densely crosslinked the network, the more heterogeneous is its structure. The impact resistance is shown to be related to the level of networks heterogeneities.

Intractable high-Tg thermoplastics processed with epoxy resin: Interfacial adhesion and mechanical properties of the cured blends

The intractable, high-temperature-resistant thermoplastics (TPs) polyphenylenether (PPE) and polyetherimide (PEI) were processed by dissolution into epoxy–amine precursors and a subsequent reaction of the precursors. Because the TP concentration was higher than the critical concentration, the phase separation produced a dispersion of crosslinked thermoset (TS) particles into a TP matrix. The morphology of the blends was examined with transmission electron microscopy and dynamic mechanical thermal spectroscopy, which showed completion of the phase separation. The interfacial adhesion at the TP-matrix/TS-particle interface was estimated on TP/TS bilayers to be 10 J/m2 in PEI blends, whereas it was 70 J/m2 in PPE blends, where there is strong evidence for in situ grafting between PPE phenolic chain ends and glycidyl functions of the reactive TS. Yielding in the compressive mode occurred at an intermediate yield stress between the components values, and the anelastic deformation was separated from the plastic deformation. Fractures in the tensile mode occurred through debonding at the matrix/particle interfaces and coalescence of these defects, which led to microcrack formation and brittle failure. Mode I fracture toughness was, therefore, higher for PPE blends than for PEI blends, a result of the higher interfacial adhesion. However, a decrease from pure TP was observed.

Epoxy– diamine thermoset/thermoplastic blends: thermoset reactions and rheological evolutions during curing

Isothermal rates of reaction during the cure of epoxy-amine/thermoplastic blends were studied. Epoxy-amine reaction induces a phase separation. Experimental results show that when TP concentration is higher than 30 wt% an increase of reaction rate is observed after phase separation. A modelling of the kinetics of each phase before and after phase separation, shows that in the epoxy-amine rich phase, gelation occurs for a conversion close to 0.6. Rheological behaviour was studied during the cure. The viscosity was found greatly dependent of the morphology, the epoxy amine conversion and of the evolution of the phase composition. Modelling of the viscosity using simple relations gives a good fit of the experimental results during the cure.

Use of in-situ dielectric sensing for intelligent processing and health monitoring

Frequency dependent dielectric measurements (FDEMS) provide a sensitive, automated in situ sensor for intelligent processing and an in situ means for monitoring durability, that is degradation of polymers during use. FDEMS in situ sensors can be designed and calibrated to monitor changes in processing properties during fabrication and changes in mechanical or electrical service life properties of polymer materials during use. With a proper understanding of the type of polymer and the use environment, the sensor output can be related to changes in modulus, maximum load and elongation at break. The FDEMS technique has advantages over other monitoring techniques: nondestructive; accuracy/reproductivity; sensitivity; in situ capability; remote sensing; automated.