Jean-François Gérard

Synthesis, structure, and morphology of polymer–silica hybrid nanocomposites based on hydroxyethyl methacrylate

Two types of polymer–silica nanocomposites have been prepared by undergoing free radical polymerization of 2-hydroxyethyl methacrylate (HEMA) either in the presence of HEMA-functionalized SiO2 nanoparticles (Type 1) or during the simultaneous in situ growing of the silica phase through the acid-catalyzed sol–gel polymerization of tetraethoxysilane (TEOS) (Type 2). Relationships between synthesis conditions, chemical structure, and resulting morphology have been studied. Type 1 systems exhibit a classical particle-matrix morphology, but where particles tend to form aggregates. Type 2 systems possess a finer morphology characterized by a very open mass-fractal silicate structure, which is believed to be bicontinuous with the organic phase at a molecular level.

A STRUCTURE-RELATED MODEL TO DESCRIBE ASPHALT LINEAR VISCOELASTICITY

Viscoelastic properties of paving grade asphalt cements (AC) were measured on a Rheometrics RDA II. Thermal properties were studied with a Mettler TA 2000 B differential scanning calorimeter. A review of the literature showed that most authors proposed that the time‐temperature superposition principle applies to asphalt over the whole temperature range. However, the results presented herein demonstrate that it indeed fails at high temperature, especially for high asphaltene content and high crystalline content materials. This paper focuses on the role of asphaltenes, which are, by definition, insoluble in n‐heptane. Asphaltenes are suspended in a colloidal manner within the AC matrix and are peptizided by the species called resins. Thus, a temperature dependent solid fraction exists within a liquid (or glassy) matrix. This paper shows that the viscoelastic properties of asphalt are bimodal: they are governed by both solid and continuous phases.Viscoelastic properties of paving grade asphalt cements (AC) were measured on a Rheometrics RDA II. Thermal properties were studied with a Mettler TA 2000 B differential scanning calorimeter. A review of the literature showed that most authors proposed that the time‐temperature superposition principle applies to asphalt over the whole temperature range. However, the results presented herein demonstrate that it indeed fails at high temperature, especially for high asphaltene content and high crystalline content materials. This paper focuses on the role of asphaltenes, which are, by definition, insoluble in n‐heptane. Asphaltenes are suspended in a colloidal manner within the AC matrix and are peptizided by the species called resins. Thus, a temperature dependent solid fraction exists within a liquid (or glassy) matrix. This paper shows that the viscoelastic properties of asphalt are bimodal: they are governed by both solid and continuous phases.

Processing and characterization of new thermoset nanocomposites based on cellulose whiskers

In the present study, the processing and the mechanical properties of new thermoset nanocomposites prepared from aqueous suspensions of microcrystalline cellulose fillers and epoxy are described. The nature of cellulose fibers, which display a large aspect ratio and the ability to associate by means of H-bonds implies that the processing method chosen in this study avoids the problem of a high level of viscosity of the epoxy reactive system-whiskers mixture. The reinforcing effect of this type of natural fiber in an epoxy matrix is mainly shown from the dynamic mechanical properties in the rubbery state. This unusual reinforcement is due to (i) the strong interactions existing between the cellulose whiskers and the epoxy network and, (ii) the creation of a percolating network linked by H-bonds between cellulose fibers. The existence of such a percolation effect is evidenced from the analysis of the rubbery shear modulus of nanocomposites based on various volume fractions of whiskers with mechanical modeling such as Halpin-Kardos and percolation approaches.

New waterborne epoxy coatings based on cellulose nanofillers

New nanocomposites based on a waterborne epoxy emulsion and a suspension of cellulose crystallites were prepared by film casting in order to obtain coatings displaying both flexibility and stiffness. Tunicin cellulose whiskers are attractive reinforcing fillers due to their high aspect ratio, large interface area, and, significant mechanical properties). Typical processing problems for this type of composite are related to high viscosity values of filled polymers in the in the molten state, the incompatibility between hydrophobic matrices and hydrophilic cellulose and the formation of hydrogen bonds between cellulose of this type of natural fibres in an epoxy matrix. The aim of this work is to solve some of the problems above mentioned by selecting the nature of the components used for coating synthesis film processing. The dynamic mechanical properties display an important improvement of the composite modulus in the rubbery state of the matrix at low concentrations of fillers, which can not be expected other cellulose based fibers.

A comparative study on different ionic liquids used as surfactants: Effect on thermal and mechanical properties of high-density polyethylene nanocomposites

Dialkyl imidazolium and alkyl phosphonium salts were synthesized to be used as new surfactants for cationic exchange of layered silicates, such as montmorillonite (MMT). The synthesized phosphonium (P-MMT) or imidazolium ion (I-MMT)-modified montmorillonites display a dramatically improved thermal degradation with respect to commonly used quaternary ammonium salts. This thermal degradation window can still be shifted toward higher temperatures after washing of modified clays. Two kinds of organic species can be identified onto clay: physically adsorbed species versus chemically adsorbed species. To evidence the impact of these thermally resistant ionic liquids, the modified montmorillonites were introduced in a great commodity polymer, i.e., high-density polyethylene. Thermoplastic nanocomposites with a very low amount of nanofillers were processed in melt by twin screw extrusion. If the thermal stability of polyethylene is slightly increased with only 2wt.% of thermostable made clays, the stiffness-toughness compromise is well improved since a strong increase in modulus is achieved with both thermostable clays without loss of fracture properties. But these mechanical performances are mainly obtained with unwashed thermostable clays because the physically adsorbed organic species onto clay surfaces behave like a compatibilizer that helps both the dispersion into the PE matrix and improves the clay/matrix interface quality.

The kinetics of α and β transcrystallization in fibre-reinforced polypropylene

Abstract The kinetics of α (monoclinic) and β (hexagonal) transcrystallization of isotactic polypropylene on aramid Kevlar 149 fibres, glass fibres and high modulus carbon fibres was investigated under isothermal and gradient cooling conditions. No difference was found between growth rates of bulk spherulites and transcrystalline layers, and Hoffmans theory led to the same results in both cases. Regarding α transcrystallization, a transition between regimes II and III occurred near 137°C and the ratio of the slopes of the two regimes was close to the theoretical value of 2. Regarding β transcrystallization, only regime II was exhibited in the temperature range studied. However, the induction time for transcrystallization was strongly influenced by the type of fibre, which in turn—based on Ishidas approach—resulted in variations in free energy differences at the fibre–crystallite interface for various fibres and bulk polypropylene. The respective values were 1.3, 1.5 and 2.1×10 −3 J m −2 for Kevlar 149 fibres, high modulus carbon fibres than in polypropylene, showing that α crystallization is more likely to occur in Kevlar 149 fibres and high modulus carbon fibres and bulk polypropylene. Gradient-thermal measurements were performed for α transcrystallinity which allowed estimation of the activation energy of transcrystallization for the different composites. Activation energies of transcrystallinity promoted on Kevlar 149 and high modulus carbon fibres were found higher than the activation energy for bulk crystallization.

Polymer modified asphalts as viscoelastic emulsions

Linear viscoelastic properties of polymer modified asphalts (PMAs) were studied at various temperatures and frequencies. The materials consisted of blends of paving grade asphalt cements (ACs) and diblock poly(styrene-b-butadiene) (SB) or triblock poly(styrene-b-butadiene-b-styrene) (SBS) copolymer up to 6 wt % concentrations, which yielded heterogeneous PMAs with an emulsionlike morphology: a polymer-rich phase dispersed within an asphalt phase. In addition, the 6% SB modified binder was studied before and after dynamic vulcanization (i.e., in situ crosslinking of the polymer-rich inclusions to increase the PMA stability). The rheological response of the blends was calculated using the Palierne emulsion model, knowing the mechanical properties of each phase, the volume fraction of dispersed phase and the capillary number of the dispersed droplets. The interfacial tension then acted as an adjustable parameter and was estimated to be of order of 10−5 N/m. As a consequence of the colloidal nature of ACs (a ...

Role of silane coupling agent and polymeric film former for tailoring glass fiber sizings from tensile strength measurements

Tensile strengths of differently sized E-glass fibres have been characterised using a bimodal Weibull two parameters cumulative distribution function. By comparing unsized fibres, pure silanes, different film formers, and silane/film former combinations, a comprehensive summary on the healing effect for surface flaws in relation to the type of sizing emulsion has been obtained. The great influence of the film former, which is the main component of the sizing by weight, was shown to affect both the healing of initially occuring flaws in the unsized fibre and the possibility of creating new defects. Besides the single influence of the film former, the synergetic effect of silane and film former polymer has been shown. In fact, the presence of sizing influences both the population of flaws on the fibre surface and the structure of the interphase, which will be created from the impregnation with a polymer matrix. Data from statistics of fracture as a function of the nature of sizings were discussed according to the literature on stress corrosion of E-glass filaments.

Mechanical characterisation of glass fibres as an indirect analysis of the effect of surface treatment

Tensile strength of E-glass fibres have been analysed using a bimodal Weibull two parameter cumulative distribution function. The bimodal character were associated to surface and internal flaws, respectively. Influence of silane coating as well as industrial sizing on the flaws distribution was quantitatively characterised. A qualitative interpretation of their effect based on a combination of probabilistic and deterministic approaches were proposed in terms of crack healing. It was demonstrated there that mechanical testing of fibres can be used as an indirect observation technique of the consequences of the surface treatment. An analogy is proposed between the sizing treatment of glass fibres and the strengthening of silica glass by means of hybrid organic-inorganic coatings.

Sepiolite-based epoxy nanocomposites: Relation between processing, rheology, and morphology

Rheology of sepiolite-based epoxy suspensions as well as morphology and dynamic mechanical properties of the corresponding nanocomposites are discussed in this paper. The influence of the type of sepiolite used, i.e. non-modified, aminosilane and glycidylsilane surface modified, and of the process developed to prepare the epoxy suspensions were investigated. Except for low amount of filler, a shear thinning behavior was observed in the others sepiolite-based epoxy suspensions. The interactions developed between the sepiolite and the epoxy matrix are responsible for the magnitude of the shear thinning effect and are related to the morphology of the nanocomposites. The best dispersion of sepiolite was achieved using either an emulsion process or a glycidyl functionalized sepiolite.