Yves Deslandes

Solid-state and solution conformation of scleroglucan

Abstract Scleroglucan is a neutral polysaccharide composed of a linear chain of (1→3)-linked β- d -glucopyranosyl residues with (1→6)-linked β- d -glucopyranosyl groups attached to every third residue. The conformational behaviour of scleroglucan has been investigated in solution and in the solid state. Order-disorder transitions in aqueous solution were studied by measurement of intrinsic viscosity. The results indicate the occurrence of such a transition at a pH /gt; 12, whereas gel formation under 10° is observed. X-Ray diffraction experiments performed on oriented fibers indicate that the backbone conformation is similar to that previously observed for curdlan, i.e. , a triple helix. The pendant (1→6)-linked β- d -glucopyranosyl residues protrude from the outside of the triplex, causing an expansion of the base plane parameters of the unit cell and further hampering lateral packing of the scleroglucan chains. The observed behaviour can be rationalized on the basis of a conformational analysis involving molecular modelling. As for the gentiobiose residue, extreme conformational flexibility about the (1→6)-β-linkage is disclosed. This conformational freedom is not significantly altered for the rotations about the (1→6)-β-linkage in the scleroglucan repeating-unit. Combination of solution and solid-state investigations provides insight into the aqueous gel-forming characteristics of scleroglucan.

Dispersion polymerization of styrene in polar solvents: II. Visualization of surface layers of steric stabilizer on dispersion-polymerized and precipitated polystyrene latex particles by transmission electron microscopy

Abstract Steric stabilizers employed in the dispersion polymerization of styrene in polar solvents include hydroxypropyl cellulose, poly(N-vinylpyrrolidone) and poly(acrylic acid). These steric stabilizers were found, for the first time, in 10–20-nm layers on the polystyrene particle surfaces by direct observation of stained thin sections of the particles using transmission electron microscopy. When the polystyrene latexes were dissolved in dioxane and precipitated with methanol, new latex particles with a similar surface stabilizer morphology were obtained. These results support the grafting mechanism of stabilization during dispersion polymerization of styrene in polar solvents and suggest that the growing or precipitating particles are able to rearrange to maintain the grafted steric stabilizer on the particle surface at all times.

High-resolution electron microscopy of poly(β-hydroxybutyrate)

Abstract High-resolution electron microscopy of poly(β-hydroxybutyrate) (PHB) single crystals has allowed direct visualization of the lattice planes of this highly beam-sensitive thermoplastic polyester from bacteria. From lattice images having a resolution of 0.35 nm, a simple Fourier averaging performed optically has generated an image of the crystal projected along its fibre ( c ) axis. The lattice image provides a molecular-level picture of the elliptical PHB cross-section with rows of alternating orientation clearly identifiable, as in the X-ray unit cell packing.

Crystalline conformation of homo- and regular heteroglucan chains

Abstract Crystalline polysaccharides with 1 → 3− β and 1 → 4− β glycosidic linkages are the most prevalent ones in nature. An interpretation of recent X-ray data on 1 → 3− α glucan shows that it has a ribbon-like crystalline conformation similar to cellulose. Comparison of the crystalline conformation of the four principal homoglucans shows that they fall either in the ‘ribbon-like’ or ‘large amplitude’ helix class. Heteroglucans with a regular sequence of glucosidic linkages show characteristics of the ‘extended conformation’ rather than the ‘coiled conformation’ even when there is 50% of a linkage which in a homoglucan leads to a large amplitude helix. It is concluded that X-ray diffraction analysis fully establishes the hypothesis that the glycosidic linkage type is the determinant of polysaccharide conformation. In this respect, polysaccharides are more like synthetic polymers than proteins or nucleotides; in the latter, it is variation in the substituents which are responsible for the conformational diversity.

CHARACTERIZATION OF CARBON BLACK FILLED POLYMERS USING SMALL ANGLE X-RAY SCATTERING AND TRANSMISSION ELECTRON MICROSCOPY: A COMPARATIVE STUDY

Abstract Carbon black (CB) is often incorporated in polymeric materials in order to modify their properties. The final properties of composite materials containing CB depend on, among other things, the nature of the carbon black, its concentration and its degree of dispersion. Therefore, there is a need to accurately characterize the degree of dispersion of carbon black in polymers. Although techniques based on optical microscopy can be used for detecting large agglomerates, they do not have the resolution to accurately quantify dispersions of particles having diameters less than 0.25 microns. Small angle X-ray scattering (SAXS) has the potential of being a useful and efficient tool for characterizing dispersions on length scales ranging from the order of 0.01 to 0.1 micron. The method is quick and provides an average value of the average agglomerate size through indirect measurements of the correlation length and the inner specific surface area. On the other hand. Transmission Electron Microscopy (TEM) a...

Bulk conductivity measurement of paper by a new insitu pressure cell

In this report a new bulk conductivity cell design is described which can be used to characterize both conductivized and nonconductive paper. By the use of optically flat stainless steel electrodes under a pressure of 115 bars during the conductivity measurement, it is shown that the contact resistance between the paper sample and the electrodes is reduced to negligible proportions. The effect of paper surface morphology on the contact resistance during the measurement of bulk conductivity was studied by a comparison between the polished stainless steel electrodes and electrodes composed of liquid Galium Indium, which make intimate contact with the paper surface. Bulk conductivity measurements were determined for paper samples having different basis weights and different degrees of calendering. The results obtained with the pressurized stainless steel electrodes for these paper samples indicate that the contact resistance is essentially eliminated for all the papers studied by using an applied pressure of...