L. David

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.

Influence of SiO2 fillers on the irradiation ageing of silicone rubbers

Abstract Effects of irradiation on the viscoelastic properties of peroxide-crosslinked PDMS elastomers (EVC) without filler (1), filled with 30% untreated nanoscopic SiO 2 (2) and with 30% surface modified silica (3) were investigated. Electron irradiation in air using a high dose rate allowing homogeneous irradiation at different doses ranging from 25 to 500xa0kGy have led to additional crosslinking of these three types of silicone rubbers making them become more rigid and brittle. The apparent crosslink density is higher for filled elastomers since links at the polymer–silica interfaces have been created as a result of irradiation. A further crosslink density enhancement is obtained for the surface-treated silica fillers. High resolution low frequency dynamic mechanical spectroscopy shows that the magnitude of the crystallisation peak located at approximately −40°C decreases with the irradiation dose. As a result, the amplitude of the relaxation process associated with the glass transition near −125°C increases, since a larger amount of amorphous phase is present due to the hindrance of the crystallisation behaviour. DSC measurements confirm that crystallisation is limited by the formation of a higher number of crosslinks, and that this effect is amplified for filled PDMS samples. A study of mechanical hysteresis at high strains (Mullins effect) shows an increase of the associated dissipative phenomena due to the induced polymer/silica bonds.

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.

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.

Influence of electron irradiation on the mobility and on the mechanical properties of DGEBA/TETA epoxy resins

Abstract The thermomechanical properties of DGEBA/TETA resins are analysed as a function of the irradiation dose. Irradiation by electrons results in a decrease of the glass transition temperature and the elastic modulus in the rubbery region. The Arrhenius diagrams obtained for the various doses show that the cooperative mobility associated with the α relaxation becomes faster after the irradiation. More local mobility corresponding to the β and γ relaxations is also modified, but in a lesser extent. These results can be interpreted invoking a destruction of the crosslinks within the resin. The effects of irradiation on the stress–strain curves are also discussed and are shown to be connected with both the physical aging kinetics and the spatial heterogeneity of the crosslink density in the resin.

Influence of the molecular modifications on the properties of EPDM elastomers under irradiation

The degradation of the mechanical behaviour of EPDM elastomers used as cable insulation materials has been investigated by mechanical spectroscopy and tensile tests for different formulations: unvulcanised EPDM, vulcanised and stabilised elastomer with an antioxidant. In all cases, γ-irradiation of EPDM under oxygen leads to a reduction of the molecular mobility indicated by the shift of the glass transition relaxation temperature towards higher temperatures. Moreover, the molecular flow occurring above Tg is suppressed after irradiation for the unvulcanised EPDM providing evidence of cross-linking. The competition between cross-linking and chain scissions is shown by the decrease of the storage modulus above the crystallites melting temperature (∼40 °C) at doses larger than 100 kGy. A strong increase of the Young modulus and reduction of the elongation at break of the non-vulcanised EPDM becoming more brittle are shown by stress/strain characterisations performed at 80 °C. At the opposite vulcanised EPDM exhibits higher elongation at break after crystallites melting. This evolution is interpreted by the competition between cross-linking and chain scissions, being hindered by the crystallites at room temperature. The intrinsic irradiation effects can be isolated after crystallite melting. The reduction of the molecular mobility can be explained by a chemi-crystallisation process assisted by chain scissions, leading to a more rigid phase upon irradiation.

Relaxation behaviour of radiochemically aged EPDM elastomers

Abstract EPDM-type elastomers (70% ethylene; 28% propylene; diene monomer: 2% norbornene) were characterised before and after irradiation using differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Three formulations were studied: pure elastomer, cross-linked elastomer with dicumyl peroxide and protected with a phenolic-type antioxidant agent. Two irradiation methods were used: γ radiation, under oxygen, at a fixed dose rate of 1 kGy/h, for total integrated doses ranging from 50 to 450 kGy and electron irradiation, under inert atmosphere, at a dose rate of 500 kGy/h, for doses of 100 and 500 kGy. For both types of irradiation, for the three EPDM formulations and for any total integrated dose, DSC and DMA show that a cross-linking phenomenon predominates. Cross-linking increases with the total integrated dose. It is also observed that cross-linking is stronger with the γ irradiation than with the electron irradiation, which confirms the influence of the oxidising atmosphere.

Poly(imide-amide)-poly(ethylene adipate) hybrid networks. I.Nanostructure and segmental dynamics

Abstract Segmental dynamics at 200–500xa0K and nanometer-scale structure were studied by SAXS, DSC and creep rate spectroscopy (CRS) techniques in two series of poly(ethylene adipate)-crosslinked poly(imide–amide)s with regularly varied composition and different lengths of PEA crosslinks (Mn=1.300 and 2.700xa0gxa0mol−1). The crystallization ability was observed in networks with longer PEA crosslinks. Shorter PEA crosslinks induced non-crystalline, nanostructured PIA–PEA networks. Combined DSC/CRS analysis indicated the pronounced heterogeneity of segmental dynamics in the latter networks. The wide dispersions of motional activation energies within the glass transitions were found by DSC, and manifestation of a few constituent segmental motions was shown in the creep rate spectra. The observed effects could be explained in terms of (i) the different constraining influence of PIA rigid constituents on dynamics in PEA crosslinks, and (ii) the breakdown of motional cooperativity as a result of loosening of PEA chain packing in presence of the PIA rigid nanodomains.

Intermolecular and intramolecular contributions to the relaxation process in sorbitol and maltitol

Molecular mobility in sorbitol and maltitol is studied with spectroscopic techniques and molecular dynamics simulations in order to evaluate the relative contributions of the intermolecular and intramolecular interactions involved in the relaxation processes. The results of the molecular dynamics simulations performed on the polyols in the bulk or in vacuum compares well with the results of the analysis of the relaxation diagrams in the framework of the Perez et al. model. They both imply that the difference in the relative contributions of the intermolecular and intramolecular interactions associated with the different chemical architectures of the two polyols must be taken into account. The intermolecular interactions cannot be neglected and they are stronger in sorbitol than in maltitol in relation with the linear structure of this polyol. The intramolecular barrier, higher in the maltitol molecule with a more complex structure, could be at the origin of the higher junction temperature between the α and β relaxation processes.

Recent developments in Small-Angle X-ray Scattering for the study of metals and polymers

New applications of Small-Angle X-ray Scattering in Materials Science of metallic alloys and polymers are presental. The use of powerful X-ray ources like synchrotrous is shown to enable in-situ study of dynamic system changes. Examples are given m welding of precipitation hardening aluminium alloys, where precipitate microstructures in welded structures are studied with high resolution and simulated through non-isothermal heat treatments. The interest of in situ measurements is alse shown for polymeric systems. Demixion phenomena occuring in the DGEBA/MCDEA/PEI modified thermoset system far from the critical concentration is studied during the curing of the epoxy amide polymerization. A methodology showing evidence of a nucleation/grewth mechanism is presented. In addition, scattering measurements during deformation of polyethylenes is shown to yield the density of the molecules that are in direct relation to their long term properties.