Calamitic bolaamphiphiles with (semi)perfluorinated lateral chains: polyphilic block molecules with new liquid crystalline phase structures

Novel bolaamphiphiles consisting of a rigid biphenyl unit, two terminal polar 1,2-diol units and laterally attached (semi)perfluorinated chains have been synthesized via palladium-catalyzed cross coupling reactions as the key step. The thermotropic liquid crystalline behavior of these compounds was investigated by polarized light optical microscopy, DSC, and X-ray scattering, and the influences of the length, number, structure, and position of the lateral chain on the mesomorphic properties were studied. A wide variety of unique liquid crystalline phases were found upon elongation of the lateral semiperfluorinated chains. For short- and medium-chain length a series of columnar phases were observed, and upon further elongation of the lateral chain a series of novel mesophases with layer structures were found. In the columnar phases, the nonpolar lateral chains segregate into columns, which are embedded in honeycomb-like networks of cylinders consisting of the biphenyl units. Strings of hydrogen-bonding networks of the diol groups provide cohesive forces, which maintain the overall structure. Changing the length of the lateral chains influences the diameter of the columns and thus determines the number of biphenyl units which are required to surround these columns. The number of these units [four (c2mm, p4mm), five (p2gg), six (p6mm), eight (c2mm) or 10 (p2gg)] defines the shape of the cylinders as well as the lattice type of the columnar phase. It is proposed that the columnar phases with a p2gg lattice result from the regular organization of pairs of cylinders which have a pentagonal cross sectional shape. In the mesophases with layer structure the aromatic rodlike cores are arranged parallel to the layer planes, and the onset of orientational and positional ordering of the biphenyl segments leads to a sequence of subtypes for these lamellar phases (Lam(Iso)-Lam(N)-Lam(X)).     

Properties of Star-Branched Polymer Chains Near a Surface

We considered two model systems of star-branched polymers near an impenetrable surface. The model chains were constructed on a simple cubic lattice. Each star polymer consisted of f = 3 arms of equal length and the total number of segments was up to 799. The excluded volume effect was included into these models only and therefore the system was studied at good solvent conditions. In the first model system polymer chain was terminally attached with one arm to the surface. The grafted arm could slide along the surface. In the second system the star-branched chain was adsorbed on the surface and the strength of adsorption was were varied. The simulations were performed using the dynamic Monte Carlo method with local changes of chain conformations. The internal and local structures of a polymer layer were determined. The lateral diffusion and internal mobility of star-branched chains were studied as a function of strength of adsorption and the chain length. The lateral diffusion and internal mobility of star-branched chains were studied as a function of strength of adsorption and the chain length. It was shown that the behavior of grafted and weakly adsorbed chains was similar to that of a free three-dimensional polymer, while the strongly adsorbed chains behave as a two-dimensional system.     

Synthesis and Crystal structure of Poly{[Co(4,4''-bpy)(H2O)4]SO4((4-abaH)2(3H2O}

The title complex has been obtained by the reaction of cobalt sulfate heptahydrate with 4,4'-bpy and 4-abaH (4,4'-bpy = 4,4'-bipyridine, 4-abaH = 4-aminobenzonic acid) in ethanol solution, and its structure was determined by X-ray crystallography with the following data: tetragonal, space group P42/n, Mr = 5692.46, Co8C192H288N32O120S8, a = b = 16.402(5), c = 22.750(5) (A), Z = 1, V = 6120(2) (A)3, F(000) = 2968, Dc = 1.544 g/cm3,μ = 0.707 mm- 1, the final R = 0.0786 and wR = 0.1935 for 2673 observed reflections (I ＞ 2σ(I)). The title complex consists of polymeric [Co(4,4'-bipy)(H2O)4]2+ cation chains, SO42- anions, lattice 4-abaH and water mole cules. The center CoⅡ ions are connected by bridging 4,4'-bpy ligands exhibiting one-dimensional chains, and coordinated by four water molecules into a distorted octahedral geometry. These chains are further extended by hydrogen bonds among SO42- anions, coordinated and lattice water molecules as well as lattice 4-abaH molecules into a three-dimensional network.     

Liquid crystalline bolaamphiphiles with semiperfluorinated lateral chains: competition between layerlike and honeycomb-like organization

Novel bolaamphiphilic triblockmolecules consisting of a rigid biphenyl unit, with a polar 2,3-dihydroxypropyloxy group and a phenolic OH group at opposite ends, as well as a semiperfluorinated chain in a lateral position have been synthesized via palladium catalyzed cross coupling reactions as the key steps. The thermotropic liquid crystalline behavior of these compounds was investigated by polarized light microscopy, DSC and X-ray scattering, and the influence of the length of the lateral chain on the mesomorphic properties was studied. The compound with the shortest chain as well as the long chain derivatives form lamellar mesophases composed of segregated layers of the bolaamphiphilic moieties and sublayers comprising the fluid lateral chains. The layers within the lamellar phases of the short chain compound adopt a positional correlation, leading to a 2D lattice (Col(r)/p2mm), whereas the layers of the lamellar phases of the long chain derivatives are noncorrelated (Lam). Compounds with a medium chain length organize into columnar phases, where the nonpolar lateral chains segregate into columns, which are embedded in networks of regular (Col(h)) or stretched (Col(r)/c2mm) hexagonal cylinder shells consisting of the bolaamphiphilic units. In total, an unusual phase sequence was found, where, with respect to the chain length, columnar mesophases occur between two mesophases with layer organization.     

Structure of polymer-stabilized magnetic fluids: small-angle neutron scattering and mean-field lattice modeling

Small-angle neutron scattering and mean-field lattice modeling were used to characterize a class of water-based magnetic fluids tailored specifically to extract soluble organic compounds from water. The fluids consist of a suspension of approximately 7 nm magnetite (Fe3O4) nanoparticles coated with a bifunctional polymer layer comprised of an outer hydrophilic poly(ethylene oxide) (PEO) region for colloidal stability and an inner hydrophobic poly(propylene oxide) (PPO) region for solubilization of organic compounds. The inner region of the polymer shell is increasingly depleted of water as the fraction of PPO side chains increases. The incorporation of PPO side chains also leads to a small increase in interparticle attraction. The lattice model predicted a shell structure similar to that of a PEO-PPO-PEO triblock copolymer (Pluronic) micelle, with equivalent levels of hydration but with more PEO present in the PPO-rich regions, as the side chains grafted to the surface are less able to segregate than when in free micellar systems.     

Molecular simulation studies of the structure of phosphorylcholine self-assembled monolayers

We report a study of the structure of phosphorylcholine self-assembled monolayers (PC-SAMs) on Au(111) surfaces using both molecular mechanics (MM) and molecular dynamics (MD) simulation techniques. The lattice structure (i.e., packing densities and patterns) of the PC chains was determined first, by examining the packing energies of different structures by MM simulations in an implicit solvent. The chain orientation (i.e., antiparallel and parallel arrangements of the PC head groups) was then evaluated. The initial azimuthal angles of the PC chains were also adjusted to ensure that the optimal lattice structure was found. Finally, the two most probable lattice structures were solvated with explicit water molecules and their energies were compared after 1.5 ns of MD simulations to verify the optimal structures obtained from MM. We found that the optimal lattice structure of the PC-SAM corresponds to a radical7 x radical7 R19degree lattice structure (i.e., surface coverage of 50.4 A(2)molecule) with a parallel arrangement of the head groups. The corresponding thickness of the optimal PC-SAM is 13.4 A which is in agreement with that from experiments. The head groups of the PC chains are aligned on the surface in such a way that their dipole components are minimized. The P-->N vector of the head groups forms an angle of 82 degrees with respect to the surface normal. The tilt direction of molecular chains was observed to be towards their next nearest neighbor.     

Two-dimensional ordered arrays of monodisperse silica particles grafted with concentrated polymer brushes

Hybrid particles which have a core of monodisperse silica particle and a shell of well-defined poly(methyl methacrylate) chains end-grafted on the core surface with a surface density as high as 0.8 chains/nm² were prepared by surface-initiated atom transfer radical polymerization of methyl methacrylate with an initiator-fixed silica particle. Monolayers of the hybrid particles were formed at the air-water interface by depositing a defined amount of the particle suspension onto water surface. Transmission electron microscopic and atomic force microscopic observations of these monolayers showed that the hybrid particles formed a two-dimensional hexagonally ordered lattice with a wide controllability of interparticle distance. This lattice structure was utilized as a template for the fabrication of a negatively patterned surface of poly(dimethylsiloxane) elastomer.     

Molecular dynamics simulation of cellulose-coated oil-in-water emulsions

The behaviors of cellulose chains and cellulose mini-crystal in oil-in-water emulsions were studied by molecular dynamics simulations to investigate the coating states and the structural features of cellulose in these emulsions. In oil-in-water emulsion, dispersed cellulose chains gradually assemble during the progress of the simulation, eventually surrounding the octane droplet. In case of a cellulose mini-crystal, the cellulose chain at the corner of the crystal first contacts with the octane droplet through its hydrophobic surface. The other cellulose chains along the crystal plane then gradually move toward the octane molecules. In both emulsions, the cellulose was found to interact with both water and octane surfaces with specific conformations that allow the CH groups of the glucose rings to contact with octane molecules, while the OH groups of these rings contact with water molecules to form hydrogen bonds. The cellulose chains on the octane droplet also contact with each other through lateral hydrogen bonding between chains. These interactions stabilize the emulsion formed by cellulose molecules as surfactants.     

Microfibril diameter in celery collenchyma cellulose: X-ray scattering and NMR evidence

Cellulose isolated from celery collenchyma is typical of the low-crystallinity celluloses that can be isolated from primary cell-walls of higher plants, except that it is oriented with high uniformity. The diameter of the microfibrils of celery collenchyma cellulose was estimated by three separate approaches: ¹³C NMR measurement of the ratio of surface to interior chains; estimation of the dimensions of the crystalline lattice from wide angle X-ray scattering (WAXS) measurements using the Scherrer equation; and the observation that microfibrils of this form of cellulose have the unusual property of packing into an irregular array from which small angle X-ray scattering (SAXS) shows features of both form and interference functions. The interference function contributing to the SAXS pattern implied a mean microfibril centre-to-centre distance of 3.6 nm, providing an upper limit for the diameter. However modelling of the scattering pattern from an irregular array of microfibrils showed that the observed scattering curve could be matched at a range of diameters down to 2.4 nm, with the intervening space more or less sparsely occupied by hemicellulose chains. The lateral extent of the crystalline lattice normal to the 200 plane was estimated as a minimum of 2.4 nm by WAXS through the Scherrer equation, and a diameter of 2.6 nm was implied by the surface: volume ratio determined by ¹³C NMR. The WAXS and NMR measurements both depended on the assumption that the surface chains were positioned within an extension of the crystalline lattice. The reliability of this assumption is uncertain. If the surface chains deviated from the lattice, both the WAXS and the NMR data would imply larger microfibril diameters within the range consistent with the SAXS pattern. The evidence presented is therefore all consistent with microfibril diameters from about 2.4 to 3.6 nm, larger than has previously been suggested for primary-wall cellulose. Some degree of aggregation may have occurred during the isolation of the cellulose, but the larger microfibril diameters within the range proposed are a consequence of the novel interpretation of the experimental data from WAXS and NMR and are consistent with previously published data if these are similarly interpreted.     

Negative thermal expansion of polymer crystals: Lattice model

The lateral thermal motion of atoms in an inextensible polymer chain would cause a shortening along the chain direction. This effect could explain the observed negative thermal expansivity α along the chain direction of a polymer crystal. Using this idea, the expansivity α is calculated for a lattice of parallel linear chains. A value of ?1.3 × 10^?5 K is obtained for chains with a carbon backbone, in good agreement with data for polyethylene.     

Conformation of Chains in Cores of Block Copolymer Micelles with Solubilized Homopolymer: a Monte Carlo Study

A system of compatible self‐avoiding polymer chains solubilized in spherical cores of block copolymer micelles was studied by lattice Monte Carlo simulations. The core is modeled as a spherical cavity on a simple cubic lattice, filled in partially by tethered (core‐forming) chains and partially by free (solubilized) chains. Molecular parameters (e.g., the ratio of the contour length of the model chains to the core radius) correspond to those in real micellar systems. The density (the fraction of lattice sites occupied) is 0.6 which corresponds to swollen micellar cores in real micellar systems. Simulations yield a constant segment density profile in the core. Both the tethered and solubilized chains acquire an ellipsoidal shape. The ellipsoids equivalent to both types of chains are more spherical than those in a melt and strongly oriented. The chains in the core show a Gaussian‐like behavior. Minor deviations from Gaussian behavior for tethered chains are due to surface effects.     

Synthesis of Amphiphilic Graft Copolymers with Poly(2‐methyl‐2‐oxazoline) Side Chains and a Backbone Containing Chloromethylstyrene and Methyl Methacrylate

New amphiphilic graft copolymers were synthesized by means of a cationic ring‐opening polymerization of 2‐methyl‐2‐oxazoline initiated with a statistical copolymer of methyl methacrylate and chloromethylstyrene. The synthesis was carried out in benzonitrile at 110°C. The grafting reaction was quantitative and the yield of grafting was more than 90%percent;. The graft copolymers were characterized by means of NMR, FT‐IR, and UV‐VIS spectroscopy, as well as elemental analysis and viscosity measurements of their aqueous solutions. The graft copolymers revealed amphiphilic properties and evidence for the formation of micelles and molecular aggregates in aqueous solution. Graft copolymers with relatively short lateral chains are soluble in methanol but insoluble in water, while the copolymers with longer lateral chains are soluble in both methanol and water.     

In‐Plane Structures of Synthetic Oligolactose Lipid Monolayers—Impact of Saccharide Chain Length

Grazing incidence X‐ray diffraction (GIXD) was used to investigate the in‐plane structure of monolayers of synthetic glycolipids with oligolactose head groups (Lac N, with N=number of lactose units) at the air/water interface. The Lac 1 monolayer exhibits three sharp diffraction peaks. One peak can be deconvoluted into two diffraction peaks, which suggests that alkyl chains in the monolayer form an orthorhombic lattice. On the other hand, the other two peaks are related to bulk crystalline materials residing at the interface. Another weak peak from the head group correlation, either in a monolayer or in bulk crystals, can also be observed in some experiments. The scattering patterns of the Lac 2 monolayer indicate that the alkyl chains order in an orthorhombic lattice, where a shift in the hydrophilic/hydrophobic balance seems to fluidize the film. Alkyl chains in the Lac 3 monolayer also seem to assume an orthorhombic lattice; however, a weak diffraction peak from the correlation between trilactose head groups can also be observed. The estimated lattice dimensions are compared systematically to those of bulk dispersions, as well as to the viscoelastic properties of the monolayer. The obtained results strongly suggest that the in‐plane structure of synthetic glycolipid monolayers can provide a well‐defined basis to understand the impact of the chemical structure on the cooperativity and function of the glycocalix of cellular surfaces.     

Properties of grafted amphiphilic chains. A computer simulation study

The model of a heteropolymer film formed by polypeptide chains was used for theoretical considerations. The linear chains consisting of amino acid residues were approximated by alpha carbon chains. Each chain was constructed on a very flexible [310] lattice. The inter- and intramolecular interactions consisted of the long-range contact potential between residues. The chains were built of hydrophilic and hydrophobic residues. Chains were terminally attached to an impenetrable surface with lateral motions possible. The Monte Carlo simulations of this model were carried out by using the Metropolis algorithm. The influence of the grafting density, the sequence of the amino acid residues, and the temperature on the static properties of the formed layer were studied and discussed. It was shown that homopolymer chains collapsed at higher temperature than the heteropolymers. The size of the polymers forming brush was smaller for homopolymers than for heteropolymers. The structure of the resulting polymer film and of its external surface was determined. The block copolymers formed well defined hydrophobic and hydrophilic layers, while for the amphiphilic case the composition of the brush layers changed continuously at high temperature. It was observed that the latter effect vanished at the collapsed amphiphilic copolymer.     

A simple model of stiff and flexible polymer chain adsorption: the influence of the internal chain architecture

In this study, we investigated the process of random sequential adsorption of stiff and flexible polymer chains on a two-dimensional square lattice. The polymer chains were represented by sequence of lattice points forming needles, T shapes, and crosses as well as flexible linear chains and star-branched chains consisted of three and four arms. The Monte Carlo method was employed to generate the model systems. The percolation threshold and the jamming threshold were determined for all systems under consideration. The influence of the chain length and the chain architecture on both thresholds was calculated and discussed. The changes in the ordering of the system were also studied.     

Raman spectroscopic studies of the solid-state polymerization of diacetylenes. I. Thermal polymerization of 1,6-di-p-toluenesulfonyloxy-2,4-hexadiyne

The resonance Raman spectra of polymer chains in partially polymerized crystals of 1,6-di-p-toluenesulfonyloxy-2,4-hexadiyne are reported. The polymer chain distortion is deduced using the results obtained previously for fully polymerized samples under tensile strain. Changes in crystal lattice dimensions both parallel and lateral to the polymer chains are found to be important in interpreting the variations in frequency of the Raman-active vibrational modes. Further evidence is found for the resonant interaction of backbone and side-group vibrations reported previously. This interaction is affected by the lateral dimensional changes and is also sensitive to residual strain fields in the monomer crystals. It is not necessary in the interpretation of the Raman spectra to take any account of changes in polymer chain length during polymerization.     

Structural characterization and properties of an azopolymer arranged in Langmuir and Langmuir-Blodgett films

The fabrication of Langmuir and Langmuir-Blodgett (LB) films of an acid-azopolymer (PAzCOOH) is reported. Several techniques were used in their characterization: surface pressure (pi) and surface potential (DeltaV) isotherms, UV-vis reflection spectroscopy, and Brewster angle microscopy (BAM) for the Langmuir films and contact angle measurements, UV-vis, fluorescence, IR and Raman spectroscopy and scanning electronic microscopy (SEM) for the LB films. Our study reveals that lateral chains of the polymer situate preferentially onto the water surface with the acid group in contact with the water, where aggregates are scarcely formed. Therefore, the lateral chains of PAzCOOH can be treated as individual monomers to determine structural properties of the fabricated Langmuir and LB films. Monomeric treatment has been used to interpret UV-vis reflection spectroscopy, and a monomer model has been performed to represent lateral chains using density functional theory at B3LYP 6-31G(d,p) level of theory to assign the observed vibrational spectra.     

Equilibrium states of self-assembly systems: Monte Carlo simulations

We investigated the equilibrium states of the self-assembly of amphiphilic molecules in water. The amphiphiles are represented by chains of the type H1T4, where H is the hydrophilic part of the molecule and T is its hydrophobic portion formed by four monomers. We have performed Monte Carlo simulations on a two-dimensional lattice, in which each water molecule occupies a single site, and the amphiphiles occupy five sites of the lattice. We have determined the aggregate distribution curves for the system at low concentration and fixed temperature. We have shown that the criterion to determine the equilibrium states of the system, based on the stabilization of energy curves as a function of the simulation time, is not reliable. The best way to ensure that the equilibrium state was reached was to follow the route to equilibrium of all aggregate sizes of the system.     

Correlated layer structures: a novel type of liquid crystalline phase with 2D-lattice

A novel liquid crystalline quaternary five-block molecule is reported which is composed of four incompatible molecular parts, a rigid biphenyl core, two polar 2,3-dihydroxypropoxy groups in the terminal 4- and 4'-positions, and a branched semiperfluorinated chain in the lateral 3-position, consisting of a perfluorinated and a lipophilic hydrocarbon wing. The self-organization of this compound was studied by polarized light optical microscopy, differential scanning calorimetry, and X-ray diffraction of aligned samples. These investigations confirm a novel liquid crystalline phase with two-dimensional (2D) lattice (columnar mesophase), which results from the positional correlation of smectic layers. The layer structure results from the segregation of the bolaamphiphilic parts from the side chains. Within the aromatic sublayers the biphenyl cores are arranged parallel to the layer planes, and the hydrogen-bonding networks of the terminal diol groups are segregated from the biphenyl cores, forming separate columns. The correlation between adjacent layers is due to the (partial) segregation of the fluorinated and hydrogenated parts of the lateral chains in the nonpolar sublayers.     

A computer simulation study of water drying at the interface of protein chains

This study investigated the water drying (cavitation) in the interfacial region of two chains of a dimeric protein by nanosecond molecular dynamics simulations using explicit water representation. Separation-induced cavity of water was directly observed in the region. We evaluated the separation length scale of two chains on which the drying transition occurs, and the average number of water molecules that are expelled from the interfacial region during the transition. The obtained values can be rationalized by Kelvin equation for finite lateral size of confinement [K. Lum and A. Luzar, Phys. Rev. E 56, R6283 (1997)]. Also, we found that the drying transition is accompanied by an exponential reduction in the average hydrogen-bond number per interfacial water molecule. The results of this study may deepen the understanding of how hydrophobic interaction drives the assembly of protein chains.