S. Etienne

Effect of physical aging on the low-frequency vibrational density of states of a glassy polymer

The effects of the physical aging on the vibrational density of states (VDOS) of a polymeric glass is studied. The VDOS of a poly(methyl methacrylate) glass at low energy ( < 15 meV), was determined from inelastic neutron scattering at low temperature for two different physical thermodynamical states. One sample was annealed during a long time at temperature lower than Tg, and another was quenched from a temperature higher than Tg. It was found that the VDOS around the boson peak, relatively to the one at higher energy, decreases with annealing at lower temperature than Tg, i.e., with the physical aging.

Effect of a plastic deformation on the nanostructure of polycarbonate: study by low-frequency Raman scattering

Abstract The effect of a plastic deformation, induced by a simple shear, on the nanostructure of amorphous polycarbonate (PC), is studied by low-frequency Raman scattering (LFRS). As previously observed for poly(methyl methacrylate) (PMMA), an excess appears in the low-frequency part of LFRS spectra after plastic deformation. The excess is stronger in the case of PC than in the case of PMMA, and its light polarization, relative to the molecular orientation, is different. In the frame of a non-continuous glassy polymer network, the experimental results are interpreted by an orientation of macromolecular strands in the less cohesive zones of the polymer matrix, between the more cohesive regions. When the deformation is carried out at a temperature lower than the glass transition temperature, the more cohesive regions are not affected by the plastic deformation. It is no more the case when the polymer is deformed at a temperature higher than T g .

Low frequency Raman scattering study of the nanostructure of plastically deformed polymer glasses

Abstract Plastic deformation of three amorphous polymers, such as poly(methyl methacrylate) (PMMA), poly(carbonate) (PC) and cross-linked PMMA, was investigated by low frequency Raman scattering. The induced structure modifications are found to differ in cross-linked and linear polymers. In the latter case, the deformation gives rise to a large scattering excess, maximum around 35 cm −1 , which is stronger with PC than with PMMA. The intensity of this excess depends on the inclination of the shear stress direction relative to the light polarization, thus indicating a significant anisotropy in the deformed amorphous structure. In the frame of a non-continuous glassy structure model, these changes are interpreted by a partial orientation of macromolecular strands in nanometric, less cohesive zones of the matrix. In the case of a three-dimensional polymeric network, the deformation occurs through a less localized process, affecting the more cohesive regions. Similar arguments are developed for specimens that were either plastically cycled or deformed at the glass transition temperature.

The effect of the moulding conditions on the semicrystalline structure of polypropylene

The semicrystalline structure of isotactic polypropylene (iPP) moulded by oscillating melt-flow (rheomoulding) has been studied using low-frequency Raman scattering (coupled with optical interference reflection microscopy and wide-angle X-ray scattering). It was compared to the structure of typically moulded iPP. We observed that only the relatively narrow central zone of the bar has a spherulitic organization in the rheomoulded specimen, while the external layers on each side have a perfectly oriented crystalline structure. The crystal structure, the orientation, the size of the crystallites and the effect of annealing were determined.

Molecular mobility and structural state relationship in amorphous polymers

The dynamics of slow relaxations, α and β, exhibited by an amorphous polymer is studied by means of low-frequency mechanical spectroscopy. The complex shear modulus spectra of the material were measured as a function of temperature in the liquid state and at temperature less than the glass transition temperature. The effect of structural relaxation in the glassy state was analysed. The α relaxation is analyzed in terms of the magnitude of cooperativity of molecular motions as a function of structural state, i.e. of disorder. It is shown that cooperative effects decrease as the disorder increases.

Physical aging and nanostructure of poly(methyl methacrylate): Effect of methanol

The effect of physical aging on the macroscopic thermophysical properties of amorphous polymers is well documented. The problem of physical aging of amorphous polymers at a nanoscopic scale is addressed in this work. To achieve this goal, we take advantage of low-frequency Raman scattering spectroscopy (LFRS) that makes it possible to investigate the structure at such a scale and its evolution by observing the LFRS intensity in the spectral range of the boson peak. The physical aging is usually observed according a given thermal history (i.e., quenching and isothermal aging below the liquid glass transition temperature). A particular procedure is considered out in this paper. Namely, the specimen of poly(methyl methacrylate) experiences a physical aging at room temperature (i.e., far below its conventional Tg) during a drying process following immersion in methanol until saturation. It is shown that this procedure increases the physical aging rate but in an inhomogeneous way. This observation together wit...

Effect of aging on the Boson peak and relaxation processes in a glassy polymer

The effect of aging following different treatments on the high frequency dynamics of a polymeric glass was measured by means of low frequency Raman scattering and inelastic neutron scattering. The spectra, in particular the Boson peak, are consistent with the discontinuous network model at the nanometric scale. The effects of similar aging procedures on slow dynamics are investigated by low frequency mechanical spectroscopy. The set of results is discussed within a multiscale description of the glassy state disorder.

Inelastic light, neutron, and X-ray scattering related to the heterogeneous elasticity of glasses

The effects of plasticization of poly(methyl methacrylate) glass on the Boson peaks observed by Raman and neutron scattering are compared. In plasticized glass the cohesion heterogeneities are responsible for the neutron Boson peak and partially for the Raman one, which is enhanced by the composition heterogeneities. Because the composition heterogeneities have a size similar to that of the cohesion ones and form quasiperiodic clusters, as observed by small angle X-ray scattering, it is inferred that the cohesion heterogeneities in a normal glass form nearly periodic arrangements too. Such structure at the nanometric scale explains the linear dispersion of the vibrational frequency versus the transfer momentum observed by inelastic X-ray scattering.