Conformational transitions of a semiflexible polymer in nematic solvents

Conformations of a single semiflexible polymer chain dissolved in a low molecular weight liquid crystalline solvents (nematogens) are examined by using a mean field theory. We takes into account a stiffness and partial orientational ordering of the polymer. As a result of an anisotropic coupling between the polymer and nematogen, we predict a discontinuous (or continuous) phase transition from a condensed-rodlike conformation to a swollen-one of the polymer chain, depending on the stiffness of the polymer. We also discuss the effects of the nematic interaction between polymer segments.Comment: 4 pages, 4 figure

Anisotropy of a flexible polymeric chain in a nematic field : neutron scattering versus magnetic resonance

Flexible polymeric chains are dissolved in a nematic liquid crystal; the effect of the nematic field is probed through s.a.n.s. and 2H n.m.r. The orientational order for the segments of perdeuterated polystyrene (Mw = 2 100), dissolved in p-ethoxybenzilidene p-n butylaniline, is obtained from 2H n.m.r. spectra : this parameter has the same temperature dependence as the order parameter of the solvent. Yet s.a.n.s. experiments do not show an anisotropy of the overall dimensions of the same macromolecules dissolved in p-azoxy-anisole. This apparent conflict is resolved when the anisotropy of the radius of gyration is calculated as a function of the spherical harmonics which describe the averaged orientation of the segments.La diffusion des neutrons aux petits angles et la résonance magnétique du deutérium sont utilisées pour étudier l'effet du champ nématique sur des polymères flexibles en solvant mésomorphe. A partir des spectres de 2H r.m.n., on obtient l'ordre orientationnel des chainons de polystyrène perdeutéré (Mw = 2 100) en solution dans du p-éthoxybenzilidène p-n butylaniline : ce paramètre présente la même dépendance en température que le paramètre d'ordre du solvant. Toutefois, les expériences neutroniques ne révèlent pas une anisotropie de forme des mêmes chaines solubilisées dans du p-azoxy-anisole. Cette contradiction apparente s'explique si on calcule l'anisotropie du rayon de giration à l'aide des harmoniques sphériques définissant l'orientation moyenne des chainons

Flexible polymers in a nematic medium : ultrasonic measurements

Using a shear wave reflectance technique, two viscosity coefficients in solutions of flexible polymers in a nematic solvent were measured. A large increase in the viscosities was observed when the chains were solubilized in the nematic solvent even at very low concentration. The variation of this effect as a function of the temperature shows the existence of two different situations for the polymer chains. The results are consistent with the theoretical expectations of Brochard.Nous présentons une étude de deux coefficients de friction d'un mélange nématique-polymère, effectuée au moyen de la technique d'absorption des ondes ultrasonores transversales. Les résultats obtenus montrent que la présence du polymère augmente de façon considérable les deux coefficients de friction du nématique pur, bien que les chaînes soient courtes et en faible concentration. Les mesures effectuées en fonction de la température mettent en évidence deux régimes correspondant à des conformations différentes des chaînes polymériques. Ces résultats sont compatibles avec les prévisions théoriques de Brochard

Détermination des Caractéristiques Viscoélastiques du polystyrène au sein de billes expansibles

No abstract provide

Molecular Analysis of the Mechanical Behavior of Plasticized Amorphous Polymer

Plasticization effects on the mechanical behavior were investigated on two families of materials based on poly(methyl methacrylate) (PMMA) and poly(vinyl chloride) (PVC), respectively. For this purpose, PMMA was blended with poly(vinylidene fluoride) (PVDF) by co-precipitation from solution, all over the PVDF range 0-40 wt% where the samples remain amorphous. Di-octylphtalate (DOP) was mechanically dispersed in PVC over the DOP range 0-20 wt%. The relaxation behavior of the samples was studied by differential scanning calorimetry at heating rate of 10 °C$\cdot$min-1 and by dynamic mechanical analysis at the frequency 1 Hz over the temperature range $-100~^{\circ}$C/150 °C. Stress strain curves were recorded during compression testing at a deformation rate of 2.10-3 s-1. Data analysis was carried out on the molecular scale; it permitted to highlight the influence of the β elaxation motions on the plastic behavior. Consideration of the non elastic part of the energy to yield was clearly related to the contribution of $\alpha$ and $\beta$ motions