G. Vigier

Nanofillers in polymeric matrix: a study on silica reinforced PA6

Abstract The study was carried out on nanocomposites consisting of nanoscopic silica fillers embedded in polyamide 6. Various composite systems were prepared through in situ polymerisation, with different elementary particle diameters and filler contents as variables. The morphological investigation demonstrated the non influence of the particle presence on the crystalline phase of such composites. The introduction of filler leads to an obvious reinforcement of the matrix elastic modulus: the observed increase depends on the modulus difference between the various phases present, the filler content and its dispersion state. In the same way, the yield point, in both compressive and tensile tests, is found to be sensitive to the latter parameters. Complementary experiments enable to suggest possible local events leading to the rupture of these composite systems.

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.

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.

Influence of the β crystalline phase on the mechanical properties of unfilled and CaCO3-filled polypropylene. I. Structural and mechanical characterisation

Abstract The influence of β crystals on the mechanical properties of isotactic polypropylene is studied on compression-moulded sheets without filler or filled with stearate-coated calcium carbonate particles. A crystallisation procedure is setup for producing sheets with various amounts of β crystals, trying to keep constant crystallinity, spherulite size and crystal thickness. An optimum of β crystals among α crystals is first produced by isothermal crystallisation at 110°C. Then gradual transformation of β into α crystals is carried out through thermal annealing at 152°C after an intermediate cooling down to room temperature. Due to the thermal inertia of the large sheet thickness, the amount of β crystals does exceed 10% for unfilled sheets and 50% for filled sheets. Dynamic mechanical behaviour shows that molecular mobility is higher in the β crystals than in the α crystals, as judged from the temperature of the crystalline relaxation. Loss modulus in the temperature range of the crystalline relaxation also indicates greater damping capacity for the β crystals. This is discussed in terms of activation of conformational defects moving along the chain stems in the crystal. Plane strain compressive testing reveals better ductility for β rich samples. Interpretation is provided using an approach of semi-crystalline polymer plasticity based on dislocation-governed crystallographic slip. Correlation is made with the viscoelastic behaviour through the concept of conformational defects.

Physical and mechanical properties of polyethylene for pipes in relation to molecular architecture. I. Microstructure and crystallisation kinetics

Ethylene/α-olefin copolymers having bimodal molar weight distribution are investigated in comparison with unimodal copolymers in order to understand the incidence of the molecular architecture on the stress cracking resistance. The preferred introduction of the co-units in the longest chains of bimodal copolymers is suggested to favour the occurrence of intercrystalline tie molecules during crystallisation. The more complex is the molecular architecture, the greater is the difficulty for crystallisation by regular chain folding. Intermolecular chemical heterogeneity resulting from preferred incorporation of the co-units in the long chains enhances the co-unit disturbing effect on crystallisation without reducing crystallinity. Intra-molecular heterogeneity of the co-unit distribution is also suggested to be an efficient means to generate tie molecules and random chain folding at the expense of regular chain folding. Isothermal crystallisation is used to probe the effect of molecular architecture on the crystallisation kinetics. It appeared that the correlations between kinetics, molecular architecture and molecular topology of unimodal copolymers no longer hold when considering bimodal copolymers. In contrast, the crystal surface free energy proved to be sensitive to topological changes resulting from molecular architecture modifications.

Relaxation of non-crystalline solids under mechanical stress

Non-crystalline solids submitted to a thermomechanical stimulus have similar properties in materials as different as amorphous polymers, molecular, metallic or oxide glasses. The purpose of the present paper is to give some insights into the mechanical response of glassy or liquid media, in relation to their state of disorder. To describe thermomechanical response of non-crystalline solids, a molecular theory has been developed. We show how the concept of quasi-point defects (QPD) introduced in the theory is consistent with small angle X-ray scattering results. The calculations based on the QPD model are also compared to experimental data for various glass-forming systems. The magnitudes of the parameters, introduced in the model for these different systems, are discussed.

Reaction-induced phase separation in poly(butylene terephthalate)-epoxy systems. 2. Morphologies generated and resulting properties

Abstract Poly(butylene terephthalate) (PBT) was used as a semicrystalline modifier of epoxy-aromatic diamine formulations in concentrations ranging from about 3 wt% to 8 wt%. 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) and 4,4′-diaminodiphenylsulfone (DDS). Using conversion-temperature transformation diagrams developed in part 1, thermal cycles were selected to generate different morphologies. In the case of PBT-DGEBA-DDS systems, phase separation in the course of reaction led to a random dispersion of spherical particles (sizes in the range of 1 μm), rich in PBT. Small and wide angle X-ray scattering, carried out in situ , during cure, revealed that the dispersion of spherical particles was produced by a nucleation-growth mechanism and that crystallization took place after phase separation. A completely different morphology, characterized by a distribution of large and irregular semicrystalline particles, was produced by crystallization before reaction. However, both types of morphologies introduced a small increase in the critical stress intensity factor. The main toughening mechanism was crack bridging produced by highly drawn thermoplastic particles. On the other hand, PBT-DGEBA-MCDEA formulations were cured at temperatures high enough to avoid crystallization of PBT during reaction. In this case, the PBT remaining dissolved in the matrix did not introduce any toughening effect.

Precipitation effects on thermopower in Al-Cu alloys

Abstract The precipitation influence on the thermoelectric power (TEP) of Al-Cu alloys (with from 2 up to 5 wt% Cu) has been observed. It has been shown that incoherent θ precipitates have no influence on TEP, in agreement with previous observations in various metallic alloys where similar precipitates form. On the opposite metastable plate-like precipitates (GP zones, θ″, θ′) induce a contribution to TEP, which is the larger the higher the measuring temperature; furthermore a correlation between the sign of this contribution and the nature of the stresses induced by the particles is observed.

Poly(ethyl acrylate) latexes encapsulating nanoparticles of silica: 3. Morphology and mechanical properties of reinforced films

Abstract Films were prepared through coalescence of latexes of poly(ethyl acrylate) encapsulating nanoparticles of silica. Two kinds of silica have been used for the preparation of the latexes: one of them has been functionalized by treatment with methacryloylpropyltrimethoxysilane, and the other has not. The molecular and colloidal properties of the lateaxes were described previously. In both cases the resulting films are fully transparent up to a high silica content. Small-angle X-ray scattering gives evidence that a good dispersion of the filler is obtained when encapsulated functionalized silica is involved in film formation. On the other hand, small agglomerates are present in the elastomeric material when non-functionalized silica is employed; their size is estimated from scanning electron microscopy. The mechanical properties of the two series of films are strongly different, as shown by either stress-strain experiments or from dynamic mechanical measurements. Very high elongations at break are obtained when the silica is functionalized, so that a part of the polymer is covalently bound to it as tight loops. To account for these properties, a qualitative model is proposed and some validation experiments are described.

Activated drying in hydrophobic nanopores and the line tension of water

We study the slow dynamics of water evaporation out of hydrophobic cavities by using model porous silica materials grafted with octylsilanes. The cylindrical pores are monodisperse, with a radius in the range of 1–2 nm. Liquid water penetrates in the nanopores at high pressure and empties the pores when the pressure is lowered. The drying pressure exhibits a logarithmic growth as a function of the driving rate over more than three decades, showing the thermally activated nucleation of vapor bubbles. We find that the slow dynamics and the critical volume of the vapor nucleus are quantitatively described by the classical theory of capillarity without adjustable parameter. However, classical capillarity utterly overestimates the critical bubble energy. We discuss the possible influence of surface heterogeneities, long-range interactions, and high-curvature effects, and we show that a classical theory can describe vapor nucleation provided that a negative line tension is taken into account. The drying pressure then provides a determination of this line tension with much higher precision than currently available methods. We find consistent values of the order of −30 pN in a variety of hydrophobic materials.