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Biomedical subjects

Goran Ungar

Publications and source records attributed to Goran Ungar.

12 recordsLinked to original sources
PubMed2005

Liquid crystalline networks composed of pentagonal, square, and triangular cylinders.

T-shaped molecules are designed in such a way that they self-organize into nanoscale liquid crystalline honeycombs based on polygons with any chosen number of sides. One of the phases reported here is a periodic organization of identical pentagonal cylinders; the other one is a structure composed of square-shaped and triangular cylinders in the ratio 2:1. These two different packing motifs represent duals of the same topological class. The generalization of the concept applied here allows the prediction of a whole range of unusual complex liquid crystalline phases.

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PubMed2005

Organisation in two series of low-dimensional polymer electrolytes with high ambient lithium salt conductivity.

The amphiphilic polymers poly[2,5,8,11,14-pentaoxapentadecamethylene(5-hexadecyloxy-1,3-phenylene)] I (abbrev. C16O5), its homologues C16On, n = 1-4, and its copolymers with C16O1 have been synthesised by two Williamson procedures. Method X gives polyether-esters; method Y gives pure polyethers. DSC, optical microscopy, small-to-wide angle X-ray diffraction and complex impedance spectroscopy have been used to investigate organisation and ionic conductivities of the C16On and the C16O5-C16O1 copolymer series and their complexes with LiClO4 and LiBF4. Four-component mixtures of the latter systems (polymers I) with polyoxytetramethylene-decamethylene copolymer (II) and an octadecyl-terminated ABA block copolymer (III) (interfacial stabiliser) ('type D' systems) gave high ambient conductivities ( > or = 10(-4) S cm(-1)) with low temperature dependence. Highest conductivities (8 x 10(-4) S cm(-1) at 20 degrees C) were observed with a type D system based upon C12C18O5X.

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PubMed2004

Structure and inter-phase stability in solvent-free low-dimensional polymer electrolytes with high lithium conductivity.

Two Williamson procedures for the synthesis of the amphiphilic polymers poly[2,5,8,11,14-pentaoxapentadecamethylene(5-alkyloxy-1,3-phenylene)]I(abbrev. CmO5) are compared. Method X gives polyether-esters; method Y gives pure polyethers. In both, a dehydration reaction gives rise to CmO5-CmO1 copolymers. Two-phase systems of I with polyoxytetramethylene and polyoxytrimethylene copolymers (II) and LiBF(4) have been prepared with and without an interfacial stabiliser copolymer III. Highest and most stable conductivities (>5 x 10(-4) S cm(-1) at ambient) with low temperature dependence were observed with III, but I from method Y showed a tendency to phase separate at ambient.

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PubMed2004

A new type of square columnar liquid crystalline phases formed by facial amphiphilic triblock molecules.

A series of three novel liquid crystalline amphiphilic molecules is reported which are composed of three incompatible molecular parts, a rigid terphenyl core, two lipophilic decyloxy chains in the terminal 4- and 4' '-positions, and a polar group in the lateral 2'-position. The polar group comprises a polyether chain, an amide group, and a polyhydroxyalkyl end group (1-acylamino-1-deoxy-d-sorbitol derivatives). The self-organization of these compounds was studied by polarized light microscopy, differential scanning calorimetry, and different X-ray diffraction techniques. These investigations confirm a novel liquid crystalline phase with a square 2D-lattice (square columnar mesophase, plane group p4mm). This structure is built up by a set of three distinct columns, namely columns containing the polar lateral groups, columns incorporating the alkyl chains, and ribbons of the rodlike terphenyl units. The calamitic cores form walls bounding square-shaped channels occupied by the microsegregated polar lateral chains. The lipophilic columns containing alkyl chains are at the corners interconnecting the aromatic rods end-to-end.

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PubMed2004

Designing libraries of first generation AB3 and AB2 self-assembling dendrons via the primary structure generated from combinations of (AB)(y)-AB3 and (AB)(y)-AB2 building blocks.

Structural analysis of three libraries of up to five generations of self-assembling dendrons based on AB(3), AB(2), and combinations of AB(3) with AB(2) building blocks (Percec et al. J. Am. Chem. Soc. 2001, 123, 1302) facilitated the discovery of several nanoscale lattices previously unknown for organic compounds (3-D Pm3n cubic, 3-D P4(2)/mnm tetragonal, and a crystallographically forbidden 12-fold symmetry liquid quasicrystal) and provided fundamental correlations between the molecular structure of the dendron and the shape and the diameter of the supramolecular dendrimers which, in these experiments, were limited to less than 75 A. That study concluded that alternative design principles should be elaborated for the assembly of supramolecular dendrimers of larger dimensions. Here we report design principles, synthesis and analysis of first and higher generations AB(3) and AB(2) self-assembling dendrons, based on various primary structures, and combinations of (AB)(y)-AB(3) and (AB)(y)-AB(2) (i.e., from nondendritic AB where y = 1 to 11 and dendritic AB(3) and AB(2)) building blocks that produced the largest structural (including six new lattices) and dimensional (100 to 217 A diameter) diversity of supramolecular dendrimers.

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PubMed2004

Supramolecular dendritic liquid quasicrystals.

A large number of synthetic and natural compounds self-organize into bulk phases exhibiting periodicities on the 10(-8)-10(-6) metre scale as a consequence of their molecular shape, degree of amphiphilic character and, often, the presence of additional non-covalent interactions. Such phases are found in lyotropic systems (for example, lipid-water, soap-water), in a range of block copolymers and in thermotropic (solvent-free) liquid crystals. The resulting periodicity can be one-dimensional (lamellar phases), two-dimensional (columnar phases) or three dimensional ('micellar' or 'bicontinuous' phases). All such two- and three-dimensional structures identified to date obey the rules of crystallography and their symmetry can be described, respectively, by one of the 17 plane groups or 230 space groups. The 'micellar' phases have crystallographic counterparts in transition-metal alloys, where just one metal atom is equivalent to a 10(3)-10(4)-atom micelle. However, some metal alloys are known to defy the rules of crystallography and form so-called quasicrystals, which have rotational symmetry other than the allowed two-, three-, four- or six-fold symmetry. Here we show that such quasiperiodic structures can also exist in the scaled-up micellar phases, representing a new mode of organization in soft matter.

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PubMed2004

Solvent-free low-dimensional polymer electrolytes for lithium-polymer batteries.

The development of solvent-free low-dimensional polymer electrolytes intended for use in solvent-free lithium batteries operating at ambient or sub-ambient temperatures is described. The synthetic routes to the amphiphilic polymers I having 5-alkoxy-3,4-phenylene units connected with oligoethoxy segments via polyester-ether or pure polyether links (abbrev. CmOn, m = 12, 16, 18, n = 1-5) and to the copolymers CmO1-CmOn are described. The structures, thermal properties and SAXS long spacings of their complexes with lithium salts (type A) and with long chain n-alkane or alkyl side chain intercalation (type B) are discussed. However, high ambient conductivities (10(-4)-10(-3) S cm(-1)) are observed in type C systems when a second copolymer based on polytetramethylene oxide segments (II) is incorporated as a microphase between the lamellae of I and serving as an ion bridge or "glue". DC polarization between Li electrodes also gives ambient conductivities >/=ca.10(-3) S cm(-1). In type D systems the I/II interface is stabilized by including a copolymer III, promoting high reproducibility in performance. Copolymers I of CmO1-CmO5 having CmO1 in excess give optimum conductivities with low temperature-dependence. This, together with molecular modeling, suggests uncoupled ion mobilities by hopping between small aggregates in the interlamellar spaces.

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PubMed2003

Application of isomorphous replacement in the structure determination of a cubic liquid crystal phase and location of counterions.

A second generation monodendron with dodecyl end-groups based on the AB(3) monomer 3,4,5-trihydroxy benzoate has previously been shown to form a thermotropic cubic phase with Pmthremacr;n symmetry (Balagurusamy et al., J. Am. Chem. Soc. 1997, 119, 1539). A structure consisting of spherical "micelles" was proposed originally, but an alternative choice of structure factor phases, giving a structure of interlocked squashed columns, could not be ruled out by diffraction data on the original material alone. We have therefore synthesized two selectively fluorinated equivalent compounds, the carboxylic acid and its Rb salt, to be able to apply a variant of the isomorphous replacement crystallographic technique. On the basis of the electron density maps of the new labeled compounds, reconstructed using small-angle X-ray diffraction intensities, the interlocking columns model is unequivocally rejected and the spheres model is upheld. Furthermore, the location of the metal cation in the center of the "micelles" is directly confirmed. Micellar diameter was shown to decrease on fluorination of the dodecyl chain ends, and increase significantly on introduction of Rb. This is interpreted in terms of changes in the number of wedge-shaped dendrons fitting into a spherical micelle due to their changing taper angle. It was found that the Rb-rich regions at the centers of six out of eight "micelles" in the unit cell are elongated in the direction of their closest packing. This adds support to the suggestion of a partial "column-like" character of stacked rows of such micelles, consistent with the position of the Pmthremacr;n phase next to the columnar phase in the phase sequence of most taper-shape compounds. The results illustrate the potential of isomorphous replacement, used a great deal in protein crystallography, in structure investigation of liquid crystals and supramolecular soft matter.

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PubMed2003

Giant supramolecular liquid crystal lattice.

Self-organized supramolecular organic nanostructures have potential applications that include molecular electronics, photonics, and precursors for nanoporous catalysts. Accordingly, understanding how self-assembly is controlled by molecular architecture will enable the design of increasingly complex structures. We report a liquid crystal (LC) phase with a tetragonal three-dimensional unit cell containing 30 globular supramolecular dendrimers, each of which is self-assembled from 12 dendron (tree-like) molecules, for the compounds described here. The present structure is one of the most complex LC phases yet discovered. A model explaining how spatial arrangement of self-assembled dendritic aggregates depends on molecular architecture and temperature is proposed.

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PubMed2002

Synthesis and NaOTf mediated self-assembly of monodendritic crown ethers.

The synthesis of ten benzyl ether based self-assembling monodendrons containing benzo[15]crown-5 at their focal point is presented. These dendritic building blocks self-assemble either directly or via complexation with NaOTf in two-dimensional smectic B, smectic A, and p6mm hexagonal columnar (Phi(h)) and three-dimensional Pm3n cubic lattices. Retrostructural analysis of these lattices and of the lattices generated from the same monodendrons containing various other functional groups at their focal point by X-ray diffraction experiments provided for the first time a correlation between the molecular structure and the shape of the monodendron, the shape of the supramolecular dendrimer and the symmetry of the lattice. It has been shown that complexation with NaOTf provides the following five different trends: a) stabilization of the three-dimensional Pm3n cubic lattice self-organized from spherical dendrimers that are self-assembled from conic monodendrons; b) stabilization of the two-dimensional S(A) phase generated from parallel-piped monodendrons; c) no effect on the stability of the two-dimensional S(B) phase generated from parallel-piped monodendrons; d) stabilization of the two-dimensional p6mm hexagonal columnar phase self-organized from cylindrical supramolecular dendrimers that are self-assembled from tapered monodendrons; and e) destabilization of the two-dimensional p6mm hexagonal columnar phase self-organized from cylindrical supramolecular dendrimers self-assembled from half-disc monodendrons. Mechanisms of NaOTf mediated self-assembly processes were suggested. These monodendritic crown ethers and their NaOTf complexes provide the largest diversity of liquid crystalline phases encountered so far in any library of supramolecular dendrimers.

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PubMed2002

Exploring and expanding the three-dimensional structural diversity of supramolecular dendrimers with the aid of libraries of alkali metals of their AB(3) minidendritic carboxylates.

The synthesis of the alkali metal salts of 3,4,5-tris(n-alkan-1-yloxy)benzoic acid [(3,4,5)nG1-CO(2)M, where n is the number of methylenic units in the alkane group for n=12, 14, 16, 18 and M=Li, Na, K, Rb, Cs] is described. The structural analysis of these AB(3) molecules by a combination of methods which includes X-ray diffraction experiments was performed. These experiments have demonstrated that (3,4,5)nG1-CO(2)M self-assemble at low temperatures into supramolecular cylinders and at high temperatures into spheres which subsequently self-organize into two-dimensional c2mm rectangular columnar, p6mm hexagonal columnar, three-dimensional Pm(-)3n and Im(-)3m cubic lattices. In addition a novel unidentified liquid crystalline lattice was also discovered. The dependence between the symmetry of the lattice and the molecular structure of (3,4,5)nG1-CO(2)M was established. (3,4,5)nG1-CO(2)M represents one of the AB(3) minidendrons (i.e., first-generation monodendron attached to the periphery of larger generation dendrons) that is responsible for the control of the three-dimensional structures created from libraries of larger generations of dendrimers. Therefore, the molecular structure-lattice dependence elaborated here will aid the rational design of the three-dimensional shapes from larger generations of supramolecular dendrimers and of their lattices. In addition, the temperature responsive shape change of these supramolecular objects may generate new supramolecular concepts and technological applications.

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