Observation of an unprecedented body centered cubic micellar mesophase from rod-coil molecules

We have demonstrated that rod-coil molecules based on a tetra-p-phenylene rod and a poly(propylene oxide) coil self-assemble into an unprecedented body centered cubic micellar structure in the melt, through detailed morphological analysis by X-ray scattering and transmission electron microscopy experiments.     

Self-assembly of rod-coil molecules into molecular length-dependent organization

A series of rod-coil molecules (n-x, where n represents the number of repeating units in a PPO coil and x the number of phenyl groups in a rod segment) with variation in the molecular length, but an identical rod to coil volume ratio was synthesized, and their self-assembling behavior was investigated by using DSC and X-ray scatterings. The molecule with a short rod-coil molecule (16-4) shows a 3-D tetragonal structure based on a body-centered symmetry of the discrete bundles in addition to a lamellar structure. This 3-D lattice, on heating, collapses to generate a disordered micellar structure. Remarkably, the molecules based on longer molecular length (21-5 and 24-6) were observed to self-organize into, on heating, lamellar, tetagonally perforated lamellar, 2-D hexagonal columnar and finally disordered micellar structures. Further increase in the molecular length as in the case of 29-7 and 32-8 induces a 3-D hexagonally perforated lamellar structure as an intermediate structure between the lamellar and tetragonally perforated lamellar structures. Consequently, these systems demonstrate the ability to regulate the domain nanostructure, from 2-dimensionally continuous layers, long strips to discrete bundles via periodic perforated layers by small changes in the molecular length, at an identical rod-to-coil volume fraction.     

Incorporation of substituted acrylamides to the lamellar mesophase of Aerosol OT

The structure and stability of the lamellar liquid crystal formed by the surfactant sodium bis-2ethylhexyl sulfosuccinate (AOT) in water is perturbed by small amounts of the substituted acrylamides N-isopropyl, N,N-diethyl, N-acryloylmorpholine, and N,N-dimethyl methacrylamide, as revealed by small angle X-ray scattering (SAXS), deuterium NMR, and microscopy. These molecules are water soluble and stay mostly in the water layers between lamellae, but a small fraction of them (5-19%) are incorporated into the AOT bilayers, thereby producing dramatic changes. Both, the degree of anisotropy in the water molecules hydrating AOT (quadrupolar splitting in (2)H NMR) and the long period spacing between lamellae (SAXS), decrease with addition of this molecules at low concentrations, which is attributed to the lower average headgroup density at the AOT/water interface when the acrylamide is incorporated. The strength of these perturbations depends on the acrylamide, and goes in parallel with the hydrophobic character of the alkyl side groups in its molecule, which suggests that the acrylamides incorporated to the bilayer enter into contact with the lipophilic tails of the AOT molecule. An interaction with the hydrated heads of AOT is also suggested in the particular case of N-isopropylacrylamide. On increasing the molecule concentration an incipient melting of the lamellar phase towards an isotropic solution takes place, first at the microscopic level, then macroscopic. Near this phase transition, the ordered domains lose the random orientation prevailing at lower acrylamide concentrations, and adopt a preferred orientation, perpendicular to the magnetic field.     

Synthesis and mesophase behaviour of branched azobenzene-based supramolecular hydrogen-bonded liquid crystals

Five types of new pyridine-based azo compounds, namely, 4-(4′-pyridylazo)-4′‘-alkoxybenzoates or 4-(4′-pyridylazo)-4′’-substituted benzoates with straight or branch terminal were prepared as hydrogen bond (H-bond) acceptors to build mesogens. After self-assembled with different carboxylic acids, the resulted H-bond complexes were confirmed for the stability of H bond by fourier transform infrared spectroscopy (FTIR) spectroscopy, and investigated for the mesophase behaviour by differential scanning calorimetry and polarised optical microscopy. The influence of terminal chains in azo accepter or acid donor and the structure of acid donors on the mesogenic behaviour were discussed in detail.     

The role of polydispersity in the lamellar mesophase of model diblock copolymers

High-resolution small angle X-ray scattering (SAXS) measurements were performed on two series of poly(ethylene-alt-propylene)-b-poly(D ,L -lactide) (PEP-PLA) diblock copolymer materials exhibiting differences in the widths of the poly(D, L -lactide) block molecular mass distributions as measured by their polydispersity indices (PDI_PLA). At symmetric compositions of PEP-PLA (f_PLA ≈ 0.5), all SAXS data were successfully fit to an established model describing the small angle scattering from lamellar mesostructures. According to this model, the increase in the PDI_PLA negligibly affected the amount of lattice disorder. The apparent asymmetry of the poly(ethylene-alt-propylene)-block lamellae (ϕ), also determined by the fitting procedure, were more substantially affected; increasing the PDI_PLA resulted in a decrease in ϕ. At asymmetric compositions of PEP-PLA (f_PLA ≈ 0.67), only the data at the highest values of the PDI_PLA could be reasonably fit to this model. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3386–3393, 2007     

A study of anisotropic water self-diffusion and defects in the lamellar mesophase

The correlation of molecular diffusion coefficients obtained via a novel two-dimensional pulsed gradient spin-echo (PGSE) NMR method has been shown to reveal detailed structural information on the mesophases of lyotropic liquid crystals. A four-component system containing both nonionic (pentaethylene glycol monododecyl ether) and ionic (sodium dodecyl sulfate) surfactants, water, and decane was prepared and left to equilibrate. In the temperature region around 309 K, a lamellar mesophase forms. A two-dimensional Laplace inverse transformation was performed on the (gammadeltag)2(delta - delta/3) domain data to separate any multiexponential behavior that resulted from local anisotropy. The results of the double PGSE experiment with contiguous gradient pulse pairs, applied both collinearly and orthogonally, clearly show the presence of local anisotropic self-diffusion of the water molecules and suggest a preferred orientation of the lamellae. Information about defects/domain size was obtained by the insertion of a mixing time (t(m)') between the successive gradient pulse pairs. This work highlights the value of this new NMR correlation method in the study of surfactant systems.     

Simulation studies of branched polymer molecules

Monte Carlo simulation results are presented for lattice models of uniform stars (one branch point of functionality f = 3), combs (two branch points, ? = 3,3) and brushes (two branch points ? = 3,4 and ? = 3,5). We estimate the critical exponent γ(?), the ratio g(?) = 〈S(?)〉/〈S(1)〉 (where 〈S(?)〉 is the mean-square radius of gyration of a structure having ? branches and N monomers), and the meansquare end-to-end branch lengths, as a function of the number and arrangement of branches. Comparisons with theoretical predictions and experimental data are made where possible, leading to a test of some predictions, and a suggestion concerning future experiments.     

NMR spectra of molecules oriented in a lyotropic mesophase. III—The Spectrum of γ-Hydroxypyridine

Analysis of the ¹H NMR spectrum of γ-hydroxypyridine dissolved in the lyotropic mesophase formed by a 14 : 1 : 1 : 20 mixture (by weight) of sodium dodecyl sulphate, decanol, sodium sulphate and D₂O provides all the spectral parameters with reasonable precision. The chemical shift, the indirect and the direct proton–proton coupling constants are derived. The direct dipolar couplings thus determined are utilised to obtain the relative interproton distances. Information about the molecular orientation and the solvent solute interactions is deduced.     

Morphological control of synthesis and anomalous magnetic properties of 3-D branched Pt nanoparticles

Morphology-controllable platinum nanostructures could be obtained by modulating the growth kinetics in oleylamine. The nanostructures evolve from spherical particles to branched networks with decreasing reaction temperature, and the complexity of the branched-network nanostructures increases with the extended reaction period. Size-dependent magnetic properties and enhanced ferromagnetism in dodecanethiol-capped Pt branched nanostructures indicate that the permanent magnetic moments are probably introduced by broken symmetry and charge transfer because charge transfers more effectively from dodecanethiol than from oleylamine.     

Dynamics of branched polymer molecules in dilute solution

Theoretical formulas for the intrinsic viscosity and viscoelastic properties of some model branched molecules in dilute solution are calculated by means of the normal coordinate method of Rouse modified to include hydrodynamic interactions. The calculations are exact except for the usual approximation of the hydrodynamic interactions by the Kirkwood-Riseman formula. The ratio of the intrinsic viscosity of a branched molecule to that of a linear molecule of the same weight is found to vary almost as the square root of the ratio of the mean square radii, instead of as the latter ratio to three-halves power, as has been postulated before. It is proposed that this square root relation is applicable in general to branched molecules of all types. Several sets of experimental data in the literature are shown to agree well with this hypothesis.     

Spin transition at the mesophase transition temperature in a cobalt(II) compound with branched alkyl chains

A cobalt(II) compound, [Co(C5C12C10-terpy)2](BF4)2 [C5C12C10-terpy = 4',5' '-decyl-1' '-(heptadecyloxy)-2,2':6',2' '-terpyridine] with branched alkyl chains, based on a terpyridine frame, was synthesized. The cobalt(II) compound exhibits a spin transition between low-spin and high-spin with a thermal hysteresis loop (T(1/2) upward arrow = 288 K and T(1/2) downward arrow = 284 K) at the liquid-crystal transition temperature. It is the first example in the cobalt(II) compounds in which the spin transition occurs at the crystal-liquid crystal transition temperature.     

Preliminary communication Helical superstructures in a novel smectic mesophase formed by achiral banana-shaped molecules

A new smectic type mesophase without in-plane order is presented which is formed by nitro substituted banana-shaped compounds. The structure of this phase is not yet known in detail. The growth of germs and also the textures point to a helicoidal structure. The helical structure is obviously the result of the chirality of the smectic layers which is caused by the tilt of the molecules together with the polar arrangement of the angled molecules.     

Self-assembled lamellar nanostructures of wholly aromatic rod-rod-type block molecules

Wholly aromatic rod-rod type di- and triblock molecules, oligo(ether sulfone)-b-oligo(ether ketone)s (OES-OEK), were synthesized to study a solid-state self-assembled nanostructure. The OES and OEK segments in the block molecules form segregated crystalline domains. The energy-filtering transmission electron microscopy images revealed that the di- and triblock OES-OEK co-oligomers formed lamellar nanostructures with a periodicity of approximately 9 and 13 nm, respectively. [structure: see text].     

Assembling of amphiphilic highly branched molecules in supramolecular nanofibers

We found that the amplification of weak multiple interactions between numerous peripheral branches of irregular, flexible, polydisperse, and highly branched molecules can facilitate their self-assembly into nanofibrillar micellar structures at solid surfaces and the formation of perfect long microfibers in the course of crystallization from solution. The core-shell architecture of the amphiphilic dendritic molecules provides exceptional stability of one-dimensional nanofibrillar structures. The critical condition for the formation of the nanofibrillar structures is the presence of both alkyl tails in the outer shell and amine groups in the core/inner shell. The multiple intermolecular hydrogen bonding and polar interactions between flexible cores stabilize these nanofibers and make them robust albeit flexible. This example demonstrates that one-dimensional supramolecular assembling at different spatial scales (both nanofibers and microfibers) can be achieved without a tedious, multistep synthesis of shape-persistent molecules.     

Multi-morphological self-assembled structures of a rod-coil organometallic oligomer

A rod-coil amphiphilic organometallic oligomer based on o-ferrocenylcarbonyl benzoic acid (FcBA) as hydrophobic short rod and poly(ethylene glycol)₁₇ (PEG₁₇) as hydrophilic coil chain was synthesized via Friedel–Crafts acylation and esterification reaction and characterized by ¹H-NMR and FT-IR analyses. The rod-coil oligomer FcBA-PEG₁₇ with short-rod insoluble FcBA segment and long-coil PEG₁₇ segment could self-assemble into multi-morphological aggregates in water such as sphere, rod, strip and vesicle at different initial concentrations, which were characterized by transmission electron microscopy. Interestingly, two different kinds of vesicles were observed, where disfigured vesicles formed at low initial concentration and ideal vesicles at high initial concentration. And the disfigured vesicles can be transferred into ideal vesicles at lower initial concentration through the threading of rigid α-CDs onto the coil PEG₁₇ chains. A possible mechanism for the formation of multi-morphological self-assembled micelles was also proposed.     

Angular distribution of intensity of rayleigh scattering from comblike branched molecules

The angular distribution function P(θ) for intensity of light scattered by a dilute solution of comblike branched molecules has been determined for three situations of some interest for evaluation of experimental data: (1) the molecules are identical with branches of equal length attached equidistantly along linear backbone chains; (2) the molecules are homogeneous in mass, with the same number of branches on each molecule, but the branches are distributed at random along the chain; (3) branches and main chains are still uniform, but the molecules are heterogeneous in mass with the number of branches per molecule distributed according to the binomial distribution and the branches in any molecule spaced randomly along the backbone. Examination of numerical results shows that the scattering functions for models (1) and (2) are not very different. The function for case (3) is somewhat different from the others when the mean number of branches per molecule is small but they contain a large fraction of the mass of the molecule. Over a limited range of the pertinent variables (corresponding roughly to observations on typical vinyl polymers of molecular weights up to 10⁶) all three functions agree quite well with P(θ) for homogeneous linear chains with the same mean-square radius of gyration.     

Postsynthesis transformation of three-dimensional framework into a lamellar zeolite with modifiable architecture

Mild treatment of zeolite UTL results in degradation of its structure with preservation of the initially present dense layers connected by D4R "bridges". The lamellar product obtained through this 3D to 2D zeolite conversion has been structurally modified similar to methodologies applied to layered zeolite precursors, which show the opposite 2D to 3D zeolite transformation.     

Aggregates of rod-coil diblock copolymers adsorbed at a surface

The behavior of rod-coil diblock copolymers close to a surface is discussed by using extended scaling methods. The copolymers are immersed in selective solvent such that the rods are likely to aggregate to gain energy. The rods are assumed to align only parallel to each other, such that they gain a maximum energy by forming liquid crystalline structures. If an aggregate of these copolymers adsorbs with the rods parallel to the surface the rods shift with respect to each other to allow for the chains to gain entropy. It is shown that this shift decays with increasing distance from the surface. The profile of this decay away from the surface is calculated by minimization of the total free energy of the system. The stability of such an adsorbed aggregate and other possible configurations are discussed as well.