Emmanuel Girard-Reydet

Reaction-induced phase separation mechanisms in modified thermosets

The reaction-induced phase separation in amorphous thermoplastic—modified epoxy systems was observed in situ using methods of different observation windows: small angle X-ray scattering (SAXS), light transmission (LT) and light scattering (LS). The transmission electron microscopy (TEM) technique was concurrently used to get direct representations of morphologies at different levels of the phase separation process. The selected systems were bisphenol-A diglycidylether cured with either 4,4′-diaminodiphenylsulfone or 4,4′-methylenebis[3-chloro,2,6-diethylaniline] in the presence of polyetherimide. The phase separation mechanisms involved were found to be greatly dependent on the initial modifier concentration and on the ratio of the phase separation rate with respect to the polymerization rate. Experimental results showed that, for modifier concentrations close to the critical fraction, the system was directly thrown into the unstable region, even at a low polymerization rate, and phase separation proceeded by spinodal demixing. On the other hand, for off-critical compositions the homogeneous solution demixed slowly via the nucleation and growth mechanism. In spite of the evolution of the phase diagram with reaction extent, the system remains in the metastable state whatever the cure temperature. The cure temperature has a strong effect on the extent of phase separation, since sooner or later vitrification of the thermoplastic-rich phase occurs and stops the evolution of morphologies. A post-cure allows the phase separation process to go further and sub-particles can be generated depending on the precure and post-cure temperatures.

Polyetherimide-modified epoxy networks : Influence of cure conditions on morphology and mechanical properties

The morphologies and mechanical properties of thermoplastic-modified epoxy networks generated through the reaction-induced phase separation procedure were studied as a function of isothermal cure conditions. The selected model system was diglycidyl ether of bisphenol A cured with 4,4′-methylenebis [3-chloro,2,6-diethylaniline] in the presence of a nonfunctionalized polyetherimide. Appropriate precuring and postcuring schedules were selected. The precure temperature had a strong effect on final morphologies because it affected the viscosity of the system at the cloud point and the extent of the separation process. The morphologies generated are discussed in connection with phase separation mechanisms. The ratio of the height of the loss peaks corresponding to each phase was an appropriate parameter to qualitatively predict the shape of morphology and to determine if the system was phase-inverted or not. The fracture toughness, KIc was significantly improved only when bicontinuous or inverted structures were generated, resulting from the plastic drawing of the thermoplastic-rich phase. Before phase inversion, KIc was hardly higher than that of the neat matrix due to poor interfacial adhesion. Nevertheless, the thermoplastic-rich particles constitute obstacles to the propagation of the crack and contribute to the toughening of the material, measured through impact resistance measurements. The observation of fracture surfaces revealed the occurrence of microcracking and crack-pinning. Strain recovery experiments showed that particle-induced shear yielding of the matrix was present as well.

Thermodynamic analysis of the phase separation in polyetherimide-modified epoxies

The miscibility of polyetherimides (PEIs) with epoxy monomers based on diglycidylether of bisphenol-A (DGEBA), and with reactive mixtures based on stoichiometric amounts of DGEBA and an aromatic diamine (DA) {either 4,4′-diaminodiphenylsulfone (DDS) or 4,4′-methylenebis[3-chloro 2,6-diethylaniline] (MCDEA)}; was experimentally studied. Cloud-point curves (temperature vs. composition) are reported for PEI-DGEBA and PEI-DGEBA-DA initial mixtures. Cloud-point conversions are reported for the reactive mixtures, for various PEI amounts and polycondensation temperatures. A thermodynamic model based on the Flory-Huggins-Staverman approach, taking polydispersity of both components into account, was used to analyze the experimental information. A single relationship between the interaction parameter and temperature, χ(T), could fit experimental results of mixtures of two commercial PEIs with DGEBA. The addition of DDS led to a decrease in miscibility whereas MCDEA improved the initial miscibility. In both cases, the interaction parameter decreased with conversion, meaning that PEI was more compatible with oligomeric species than with the mixture of starting monomers. The phase separation process in initially miscible rubber- or thermoplastic-modified thermosetting polymers is the result of two factors: increase in the average molar size of the thermosetting oligomer (main driving force favoring demixing), and variation of the interaction parameter with conversion, which may act to increase or decrease the cloud-point conversion determined by the first factor.

Use of block copolymers to control the morphologies and properties of thermoplastic/thermoset blends

Abstract A new method for increasing fracture toughness of brittle thermoplastic-modified thermosets by using triblock copolymers has been successfully investigated. The selected systems were polyphenylene ether (PPE)- and polyetherimide (PEI)-modified epoxy networks. Our choice was restricted to available commercial copolymers possibly with some chemical modifications. PPE presents the substantial advantage of having a negative enthalpy of mixing with polystyrene. The maleic anhydride-modified poly(styrene-b-ethylene-co-butene-b-styrene) triblock copolymer, containing an immiscible elastomer central block, was then selected. The reactivity of succinic anhydride functions towards primary amines was used to graft on ethylene-co-butene blocks, chains which are miscible or able to react with the growing epoxy network. The two problems encountered with PEI is that PEI is not miscible with any other polymer and that a commercial triblock with a PEI block does not actually exist. The only copolymer commercially available is a poly(etherimide-b-dimethylsiloxane) segmented copolymer, with elastomer segments which are known to be strongly immiscible with any components. In order to obtain the characteristics of the required compatibilizer, the poly(caprolactone-b-dimethylsiloxane-b-caprolactone) triblock copolymer was associated since (a) the polydimethylsiloxane elastomer central block is chemically identical to the elastomer segment of the previous copolymer, and (b) the polycaprolactone blocks are totally miscible with epoxy. For both thermoplastic-modified epoxy networks, spectacular mechanical reinforcements were measured with only 10%b.w. thermoplastic as a result of interfacial activities of selected compatiblizing systems, with a relative enhancement of fracture toughness close to 50% with around 1% of copolymer. The positive effects on mechanical properties always result from the same causes: large decrease of the particle size (submicron size) and formation of a copolymer-rich interphase characterized by a micromechanical transition in mechanical spectroscopy.

Reaction-induced phase separation in poly(butylene terephthalate)-epoxy systems: 1. Conversion-temperature transformation diagrams

Abstract Poly(butylene terephthalate) (PBT) was used as a semicrystalline modifier of epoxy-aromatic diamine formulations. The epoxy monomer was based on diglycidylether of bisphenol A (DGEBA) and the diamines were either 4,4′-methylenebis [3-chloro 2,6-diethylaniline] (MCDEA) or 4,4′-diaminodiphenyl-sulfone (DDS). PBT was more miscible in DGEBA-MCDEA than in DGEBA-DDS formulations, as revealed by the melting point depression observed in binary mixtures. Melting temperatures as a function of conversion were obtained for both systems using differential scanning calorimetry together with size exclusion chromatography. In the case of the PBT-DGEBA-DDS system, a cloud-point curve was also obtained, showing an upper-critical-solution-temperature behaviour. On the basis of melting, cloud-point, vitrification and gelation curves, conversion-temperature transformation diagrams were generated for both systems. These diagrams can be used to design particular cure cycles to generate different morphologies in the phase separation process. In the case of PBT-DGEBA-MCDEA systems, PBT could be either kept in solution in the matrix or separated by crystallization (initially or in the course of polymerization). For PBT-DGEBA-DDS systems, PBT was always segregated from the matrix, either initially through crystallization or by attainment of the cloud-point curve in the course of reaction. Morphologies generated and resulting mechanical properties will be discussed in the second part of the series.