Gilbert Weill

Osmotic swelling of unilamellar vesicles by the stopped-flow light scattering method. Influence of vesicle size, solute, temperature, cholesterol and three α,ω-dihydroxycarotenoids

Abstract α,ω-Dihydroxycarotenoids are postulated to play the role of membrane reinforcers in certain procaryotes, like cholesterol in eucaryotes. In order to evaluate the rigidifying effect of these polyterpenoids on lipid bilayers, osmotic swelling of unilamellar vesicles has been followed by measuring the light scattering intensity changes in a stopped-flow apparatus. A model based upon the Rayleigh-Gans theory was developped. It shows that the variation of light scattering intensity ( ΔI I 0 ) is proportional to that of vesicle radius ( ΔR R 0 ) for a given R0 (initial vesicle radius). An emperical relationship between −ΔI I 0 and Z (dissymmetry measured by light scattering) was established: −ΔI I 0 is proportional to (Z − 1). Therefore, the value −ΔI (I 0 · (Z − 1)) is independent of vesicle radius and can be used for the evaluation of bilayer rigidity. The water permeability was measured and it was shown that it is the limiting factor of the kinetics of swelling. When incorporated into dimyristoylphophatidylcholine vesicles, cholesterol and α,ω-dihydroxycarotenoids lower the water permeability and the value of −ΔI (I 0 · (Z − 1)) . So, at least on model systems, α,ω-dihydroxycarotenoids exert on a phospholipid bilayer a reinforcement effect similar to cholesterol.

Solution properties of poly(vinylidene fluoride): 1. Macromolecular characterization of soluble samples

Abstract Dissolution in a range of solvents reveals two kinds of behaviour of poly(vinylidene fluoride) (PVDF): readily soluble samples (mostly suspension-polymerized) and partly gel-forming samples (mostly emulsion-polymerized). From molecular characterization of a number of industrial and laboratory samples of the first kind in solution in different solvents by light scattering, intrinsic viscosity and size exclusion chromatography, we derive unperturbed dimensions and χ parameters. Our results differ from previous measurements where light-scattering experiments were carried out in solvents with very low d n d c and/or on less-soluble (second-kind) PVDF, where microgel formation cannot be avoided.

The structure and morphology of vinylidene fluoride and trifluoroethylene copolymers studied by wide-line nuclear magnetic resonance

Abstract Both hydrogen and fluorine nuclear magnetic resonance measurements have been performed on copolymers of vinylidene fluoride (VF 2 ) and trifluoroethylene (TrFE) containing from 30 to 100 mol% VF 2 . The temperature study of the 70 30 copolymer reveals that two crystalline and one amorphous phase must be considered to describe the ferroelectric transition. At low temperature, the structural change from α to β crystalline form with increasing TrFE content is clearly detected. At room temperature, the morphology of VF 2 -rich copolymers is well analysed with two components having the same 1 H to 19 F ratio. On the other hand, below 70 mol% VF 2 the two components have different 1 H to 19 F ratio, which implies segregation between TrFE-rich and VF 2 -rich sequences.

Solution properties of poly(vinylidene fluoride): 2. Relation between microgel formation and microstructure

Abstract In contrast to type I poly(vinylidene fluoride) (PVDF), solutions of type II PVDF contain, even in good solvents, a fraction of microgel that cannot be fully eliminated for characterization by light scattering. Intrinsic viscosity and size exclusion chromatography reveal that the soluble fraction has a molecular mass in the same range as type I samples, which are fully soluble in the same solvents. The shear moduli in the molten state reveal that the microgel fraction does not contain a high enough molecular mass to explain microgel formation by phase separation. From an analysis of the 19 F nuclear magnetic resonance spectra of both suspension- and emulsion-polymerized PVDF samples, with first-order Markov statistics, microgel formation is attributed to differences in microstructure, involving sequences with two reversed additions in a row.

Nuclear magnetic resonance lineshape studies of interpenetrating polymer networks

Abstract In the so-called in situ sequential interpenetrating polymer networks (IPNs), the two networks are formed after each other, and the network formed first is thought to impede gross phase separation in the final material. This is contrary to the other type of IPNs ( in situ simultaneous), in which the formation of both networks is initiated at once and proceeds to completion more or less simultaneously. In order to verify more accurately this assumption, which is not inconsistent with transmission electron microscopy findings, a solid-state nuclear magnetic resonance lineshape analysis technique has been used to investigate the degree of phase dispersion of IPNs of both types composed of an elastomeric polyurethane (PUR) (25 wt%) and a crosslinked poly(methyl methacrylate). The results confirm that such IPNs, when prepared sequentially, have a higher degree of phase dispersion than those obtained by the simultaneous synthesis method. Furthermore, in the corresponding neat PUR networks, built up from aromatic pluriisocyanate and poly(oxypropylene glycol) (POPG), the rigid crosslink points are not composed of isocyanate only, but include some oxypropylene mers; it appears that the amount of the rigidified part is the same, whatever the molecular weight of POPG.

Étude de l'action du chlorure de sodium sur l'oxyhémoglobine: I. Mesures de diffusion de la lumière

Resume On a utilise la methode de diffusion de la lumiere pour determiner la masse moleculaire de loxyhemoglobine humaine en solution, en presence ou en absence de NaCl, au voisinage de son pH isoelectrique. Le chlorure de sodium provoque une scission de la molecule, probablement en deux moities. Cette action nest jamais totale en solution, ni par augmentation du temps de contact dans le sel, ni par augmentation de la concentration saline. Apres elimination du sel, il y a reagregation des molecules. Il ne semble pas que luree ait une action comparable a celle du chlorure de sodium.

Morphologies of semi and full interpenetrating polymer networks by nuclear magnetic resonance relaxation times

Full and semi interpenetrating polymer networks, IPNs, formed with a network of polyurethane (PU), and either a crosslinked poly(methyl methacrylate), PAc, or a linear polystyrene, PS, were synthesized with the aim of obtaining homogeneous materials. As already shown in a previous paper, the degree of phase dispersion in such materials depends on the synthesis method. The purpose of the n.m.r. investigations is to estimate the degree of mixing of the polymer in such networks, together with the synthesis parameters. The n.m.r. lineshape evolution of the PU/PAc IPNs with temperature allowed us to conclude that the PAc and PU networks are more intimately mixed in the sequential mode. In this paper, the measurements of the spin-lattice relaxation times in the rotating frame and the Goldman—Shen experiment are used to confirm previous results, and to give an estimation of the degree of mixing of the two networks in the matrix and of the size of the inclusions of PS in the simultaneous IPN. Following the same protocol, different semi-1 IPNs of PU/PS are examined in order to relate the amount of PS added to the reaction medium with the degree of advancement of the phase separation. Nodules of pure PS are seen for some samples but they do not correspond to those observed by SEM.

A direct comparison of spinodal decomposition analysis by time and q resolved light scattering and n.m.r.

Abstract Time and q resolved light scattering and nuclear magnetic resonance spin-lattice relaxation time have been performed on polystyrene/poly(vinyl methyl ether) blends during the early stage of the spinodal decomposition. The two techniques are used to determine the apparent diffusion coefficient at two different temperatures. A satisfying agreement is found between the results of T 1 n.m.r. measurements and those from light scattering. Otherwise, it has been shown that n.m.r. spin-lattice relaxation measurements, during the decomposition time, permits discrimination between spinodal decomposition and nucleation and growth. T 1ρ experiments indicate that the homogeneous phase as well as the phase-separated quenched blend contain short scale inhomogeneities but it is not clear whether they compete with the larger scale spinodal decomposition process.

High pressure effect on molecular motions in the paraelectric phase of a vinylidene fluoride and trifluoroethylene copolymer (7030) studied by n.m.r.

Abstract The 19 F n.m.r. spin-lattice relaxation times in both the laboratory and the rotating frame have been measured in a 70 30 mol % random copolymer of vinylidene fluoride and trifluoroethylene, over a range of pressures from 0.1 to 200 MPa in the paraelectric phase. Molecular motions have been investigated in this condis phase, and correlation times were obtained as a function of pressure and temperature. Activation parameters have been determined and discussed in terms of a thermally activated model and compared with literature data.

Dynamics of single-stranded DNA in polyacrylamide gels during pulsed field gel electrophoresis. A birefringence study

The study of the orientation of single-stranded DNA in polyacrylamide gels in denaturing conditions has been undertaken by electric birefringence in order to determine the mechanism involved in the electrophoretic transport. The presence of an overshoot in the birefringence signal, when applying the electric field, and the study of the influences of the electric field and of the gel concentration on the dynamics show that a mechanism of reptation with elongation of the molecule occurs in polyacrylamide gels with low T values. Therefore it is suggested that the use of pulsed fields in sequencing electrophoresis is possible and can lead to a large increase of the length of the fragments that can be sequenced in one single run.