Showing papers by "Virgil Percec published in 1999"

Molecular Conformations of Monodendron-Jacketed Polymers by Scanning Force Microscopy

Abstract: Scanning force microscopy on monodendron-jacketed linear polystyrenes demonstrated unusual conformations and ordering depending on the branching density and interplay between intramolecular and surface interactions of the dendritic coat. Single molecules were visualized as wormlike cylinders, which could be twisted to plectonemic coils or aligned in nematic-like order on the substrate. The attained resolution enabled identification of chain-end stacking, hairpin folds and crossovers of individual macromolecules. The structure of the more densely branched 3,4,5-tris[3‘,4‘,5‘-tris(n-dodecan-1-yloxy)benzyloxy]benzyl was predominantly controlled by the intramolecular repulsion of the crowded dendron substituents which enforced an extended conformation of the molecular backbone. Formation of molecular monolayers was affected by wetting/dewetting events during casting, and the film structure did not depend on the substrate type. In contrast, the less crowded 3,4,5-tris[4-(n-tetradecan-1-yloxy)benzyloxy]benzoate...

Poly(p-phenylene)s with Mesogenic Side Groups: A Potential Class of NII Side Chain Liquid Crystalline Polymers?

Abstract: The synthesis and characterization of a series of n-[(4-cyano-4‘-biphenylyl)oxy]alkyl-2,5-dichlorobenzoate monomers containing from n = 2 to 12 methylenic groups in the alkyl spacer and their Ni(0)-catalyzed polymerization to produce poly(p-phenylene)s (PPP) containing mesogenic side groups are described. The resulting side chain liquid crystalline polymers (SCLCPs) have both a rigid rodlike mesogenic backbone and rigid rodlike mesogenic side groups. All polymers display only a broad enantiotropic nematic phase and no side-chain crystallization. The isotropization temperature of these SCLCPs exhibits a steeper increase with the degree of polymerization (DP) than that observed for polymers with flexible or semiflexible backbones. Therefore, above plateau DPs their isotropization temperature is higher than that of polymers with flexible or semiflexible backbones, although it displays a classic odd−even dependence on the spacer length. These unusual trends may indicate that the director of the nematic phase ...

Grain boundaries and stacking faults in a Pm3n cubic mesophase

Abstract: A monodendron that over a broad temperature range self-assembles in quasi-spherical supramolecular aggregates organized on a thermotropic cubic Pm3n lattice was examined by TEM. Grain boundaries were found parallel to the (1 0 0) and (3 2 0) planes. In well annealed specimens, in which grain boundaries are rare, two types of stacking fault were numerous, suggesting their relatively low energy. In the original Pm 3n lattice and in the two stacking fault arrangements, the supramolecular aggregates are distorted to an oblate shape, which is likely to be favoured by a crowded microscopic interface between core and tail moieties. Symmetry and phase selection for the ordered arrangement of amphiphilic molecules is discussed.

Phase identification in a series of liquid crystalline TPP polyethers and copolyethers having highly ordered mesophase structures. 7. Phase structures in a series of copolyethers containing odd and even numbers of methylene units of different compositions

Abstract: A series of liquid crystalline copolyethers have been synthesized from 1-(4-hydroxy-4'-biphenyl)-2-(4-hydroxyphenyl)propane with 1,7-dibromoheptane and 1,12-dibromododecene [coTPPs(7/12)], which represents copolyethers containing both odd and even numbers of methylene units. The molar ratio of odd to even methylene units in this series ranges from 1/9 to 9/1. The coTPPs(7/12) exhibit multiple phase transitions during cooling and heating in differential scanning calorimetry experiments. For all these thermal transitions, a small undercooling and superheating dependence is observed upon cooling and heating at different rates. Three types of phase behaviors can be classified in coTPPs(7/12) on the basis of the structural analyses by wide-angle X-ray diffraction on powder and fiber samples and by electron diffraction experiments in transmission electron microscopy. At room temperature, highly ordered smectic and smectic crystal (SC) phases are identified in coTPPs(7/12: 1/9 and 2/8), which is similar to the homopolymer TPP(m = 12). The coTPPs(7/12: 3/7, 4/6, and 5/5) possess a hexagonal columnar (Phi(H)) phase in which the molecular and columnar axes are parallel to the fiber direction and perpendicular to the hexagonal lateral packing. The coTPPs(7/12: 6/4, 7/3, and 8/2) possess a tilted hexagonal columnar (Phi(TH)) phase with a single tilt angle which increases with the increasing composition of the seven-numbered methylene units. However, in coTPP(7/12: 9/1), a Phi(TH) phase with multiple tilt angles is found. Upon heating, phase structures in most coTPPs(7/12) involving the columnar phases enter directly into the nematic (N) phase, while the coTPP(7/12: 1/9) exhibits a highly ordered smectic F (S-F) phase before it reaches the N phase. One exception is found in coTPP(7/12: 2/8), wherein the transformation from the S-F to Phi(H) occurs prior to the N phase. Combining the copolymer phase behaviors observed with the corresponding homopolymers TPP(n = 7) and TPP(m = 12), a phase diagram describing transition temperatures with respect to the composition can be constructed.

Phase identification in a series of liquid crystalline TPP polyethers and copolyethers having highly ordered mesophase structures. 6. Structure changes from smectic to columnar phases in a series of copolyethers containing odd and even numbers of methylene units in equal molar composition

Abstract: A series of liquid crystalline copolyethers has been synthesized from 1-(4-hydroxy-4'-biphenyl)-2-(4-hydroxyphenyl)propane and different alpha,omega-dibromoalkanes [coTPP(n/m)]. In this report, coTPPs having n = 5, 7, 9, 11 and m = 12 are studied, which represent copolyethers having both varying odd number and a fixed even number of methylene units. The compositions were fixed at an equal molar ratio (50/50). These coTPPs(nlm) show multiple phase transitions during cooling and heating in differential scanning calorimetry experiments. The undercooling dependence of these transitions is found to be small, indicating that these transitions are close to equilibrium, Although the coTPPs possess a high-temperature nematic (N) phase, the periodicity order along the chain direction is increasingly disturbed when the length of the odd-numbered methylene units decreases from n 11 to 5. in the coTPPs(5/12, 7/12, and 9/12), wide-angle X-ray diffraction experiments at different temperatures show that, shortly after the N phase formation during cooling, the lateral molecular packing improves toward a hexagonal lattice, as evidenced by a gradual narrowing of the scattering halo. This process represents the possible existence of an exotic N phase, which serves as a precursor to the columnar (Phi(H)) phase. A further decrease in temperature leads to a (PH phase having a long-range ordered, two-dimensional hexagonal lattice. In coTPP(11/12), the phase structures are categorized as highly ordered and tilted, smectic and smectic crystal phases, similar to homoTPPs, such as the smectic F (S-F) and smectic crystal G (SCG) phases. An interesting observation is found for coTPP(9/12), wherein a structural change from the high-temperature Phi(H) phase to the low-temperature S-F phase occurs. It can be proven that, upon heating, the well-defined layer structure disappears and the lateral packing remains hexagonal. The overall structural differences in this series of coTPPs between those of the columnar and highly ordered smectic phases are related to the disorders introduced into the layer structure by the dissimilarity of the methylene unit lengths in the comonomers.

A second columnar liquid crystalline phase formed by polymers with highly tapered side chains

Abstract: Previous work has shown that polymethacrylates with highly tapered side groups, such as poly{2-{2-[2-(2-methacryloyloxyethoxy )-ethoxy]ethoxy] ethyl-3,4,5-tris-(p-dodecyloxy-benzyloxy)benzoate (H12-ABG-4EO-PMA ), form columnar hexagonal (Col h , previously designated o h ) liquid crystalline structures. The small angle X-ray data for drawn fibers of H12-ABG-4EO-PMA, and also those of its hydroxy-tcrminated monomer precursor and decyloxy-homologue, contain several wcak equatorial Bragg reflections in addition to those characteristic of the Col h phase. The intensities of these extra reflections decline progressively on heating and disappear before the temperature of the Col h -to-isotropic transition is reached. It is proposed that the as-drawn fibers contain a second phase, which is probably a columnar liquid crystalline structure in view of the decreases in the observed d-spacings with increasing temperature. The additional reflections are indexed by a two dimensional monoclinic lattice, which probably contains columns with elliptical cross-section. The lower symmetry is consistent with the formation of a tilted columnar phase analogous to those formed by certain low molecular weight discotic materials.

Microstructure and Morphology of Thermotropic Amphiphilic Liquid Crystalline Materials

Abstract: Electron microscopy methods have been used to investigate the structure and morphology of a hexagonal columnar mesophase, formed by novel amphiphilic and dendrimeric liquid crystals. Alignment of the columns is examined by a surface condition that is suitable for the molecular architecture. For all the materials investigated, columns aligned perpendicular to an evaporated carbon surface. In the case of asymmetric amphiphilic compounds, planar alignment of asymmetric compounds was induced by a water surface. However, planar alignment on water was not possible for a symmetric dendrimer. Based on analysis of electron diffraction and images, the dimension and the stiffness of columnar assemblies is found to depend on molecular architecture.