Terry L. Bluhm

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.

Crystal and Molecular Structures of Six Differently with Halogen Substituted Bis (benzylimido) perylene

The structures of bis (3,5-dichlorobenzylimido) perylene, bis (3-chlorobenzylimido) perylene, bis (3-fluorobenzylimido) perylene, bis (3,5-difluorobenzylimido) perylene, bis (4-chlorobenzylimido) perylene, and bis (2,6-difluorobenzylimido) perylene were solved by single crystal analysis. The compounds crystallize in four different space groups, that is, in sequence of the compounds listed P1, P1, P2 1 /a, C2/c, Pbcn. P2 1 /n, respectively, although their chemical constitutions are very similar. The packing in different arrangements causes the twist of the benzene ring to differ with respect to the perylene ring and amounts to 111.4°, 112.3°, 98.6°, 105.1°, 80.3 °, 72.0° for the sequence of compounds listed, respectively. The first two compounds crystallizing in space group P1 are isomorphous in structure and exhibit the same dark blue color. The C-F bond distances lie between 1.36 and 1.38 A, the C-Cl bond distances between 1.73 and 1.75 A. For all six structures investigated the planes of two adjacent perylene skeleton planes are 3.43 to 3.50 A apart and the tilt angles versus the shortest unit cell dimension lie between 20 and 45°.

Thermotropic Liquid Crystalline Polyesters Containing Naphthalenic Mesogenic Groups

SummaryRecently, the use of naphthalene in liquid crystalline compounds has become more commonplace primarily in the industrial research into LC polymers with wholly aromatic structures. A large number of compositions have appeared in the patent literature, and have been well summarized in several recent articles. Even though a large number of LC naphthalene-containing copolyesters have been prepared and reported, much of this work deals with wholly aromatic systems which include no flexible spacers. In order to investigate the effects that the naphthalene structure has on liquid crystalline behaviour, a series of low molecular weight model compounds and polymers with flexible spacers were synthesized and characterized.

Diffraction studies on metal free phthalocyanines (β-H2PC and X-H2Pc)

The crystal structure of the β-polymorph of metal free phthalocyanine (β-H2Pc) was redetermined from single crystals to obtain precise placements of the hydrogen atoms and a precise model for the X-polymorph (X-H2Pc) for which only limited data from different diffraction techniques are available. Partial occupancy for the hydrogens fixed to the nitrogens in the inner ring was found leading to a highly symmetric phthalocyanine molecule. Careful evaluation of X-ray, electron and neutron diffraction data for X-H2Pc results in an orthorhombic unit cell with a=2.10 nm, b=0.491 nm, c=2.31 nm, space group Pna21 containing four molecules in contrast to a recent determination of Oka and Okada who proposed a monoclinic unit cell with two molecules. As expected the phthalocyanine molecules pack in stacks. The overlap of molecules within one stack matches the current ideas for photosensitive phthalocyanine pigments.

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...

Application of Rietveld analysis to crystal structures of titanyl phthalocyanine

This paper discusses the use of Rietveld analysis to solve crystal structures of titanyl phthalocyanines. Rietveld analysis is based on whole pattern fitting in which every point in the x-ray powder diffraction pattern is used as a measure of one or more Bragg diffraction peaks. Thus the refinement of relatively complicated crystal structures from x-ray patterns containing a relatively small number of resolved diffraction peaks is enabled. Various profile-fitting functions used in Rietveld analysis were parameterized and tested against known crystal structures of type I and type II titanyl phthalocyanine. It was found that a split Pearson VII function was found to best correct for preferred orientation effects observed in the x-ray patterns. The final goodness-of-fit parameters were R(Bragg) equals 0.17 and 0.13 for type I and type II structures, respectively. A computer program was used to generate several unit cells for type IV titanyl phthalocyanine. These unit cells were tested by stereochemical packing analysis to first determine which unit cells allowed for good intermolecular packing arrangements. Energy minimized models were then used as phasing models for Rietveld refinement. A triclinic structure with space group P-1 having an x-ray goodness-of-fit parameter R(Bragg) equals 0.24 was proposed as the most probable crystal structure for type IV titanyl phthalocyanine. The unit cell parameters are a equals 1.083 nm, b equals 1.312 nm, c equals 0.996 nm, alpha equals 72.28 degrees, beta equals 77.25 degrees and gamma equals 104.48 degrees. There are two molecules in the unit cell related by a center of inversion.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Morphological studies on the blends of polyethyleneoxide and cellulose ethers

SummaryThe blends of the water soluble polymers, PEO/HPC and PEO/CMC show crystallization of PEO over a wide composition range. The crystallinity of the PEO fraction decreases drastically beyond an HPC concentration of 70%, whereas it persists throughout the composition range in the case of the blend with CMC. This leads to the conclusion that at low concentrations, PEO is more compatible with HPC than with CMC. The films of the blends exhibit a skin as seen in the cross-sections of the films. Calorimetric studies show that annealing above Tm of PEO or prolonged annealing below Tm causes disorder of the PEO crystalline domains.

Extended and threefold helical conformations in gels of isotactic polystyrene

SummaryGels of isotactic polystyrene (iPS) were prepared, using solvents of different molecular geometries, by rapid quenching of the solutions formed at high temperatures. The extended conformation was observed in gels prepared with bulky, cyclic hydrocarbon solvents, whereas the aromatic solvents promoted the threefold helical structure. The gels containing the threefold helix were more elastic compared to those with the extended conformation. The effect of the solvent geometry is explained qualitatively.