Hexagonal Lyotropic Liquid Crystal Research Articles

Liquid-crystal templating of conducting polymers.

Mimicking the texture and anisotropy of the template, birefringent conducting polymer films (see picture) have been obtained by electropolymerization of 3,4-ethyldioxythiophene (EDOT) within a hexagonal lyotropic liquid crystal. Using a self-assembled template to control molecular orientation in conducting polymers could enhance electronic and optical properties.

Aromatic Side Chain-Functionalized Long Chain Acid Salts: Structural Factors Influencing Their Lyotropic Liquid-Crystalline Behavior

The structural factors influencing the self-assembly of aromatic side chain functionalized long chain acid salts into lyotropic liquid crystal (LLC) phases were examined. Two sets of amphiphiles were synthesized containing two different ionic headgroups (i.e., sodium carboxylate and sodium sulfonate) and four different aromatic side chains (i.e., 4-styryl ether, 4-ethylphenyl ether, 1-naphthyl ether, and 4-phenylazophenyl ether) positioned at various locations along the amphiphile backbone. Systematic variation of the type of the aromatic side chain and its relative position on the amphiphile as a function of the two headgroups revealed that the position of the side chain on the amphiphile is the most important structural factor influencing the type and stability of the LLC phases formed. The nature of the aromatic side chain and the nature of the anionic headgroup play much more subtle roles in the LLC behavior of this particular amphiphile structural motif. Noncovalent aromatic−aromatic interactions are known to play important roles in the aggregation behavior of amphiphiles containing aromatic units along the alkyl chains; however, such interactions could not be detected spectroscopically in the LLC phases of our aromatic side chain amphiphiles. We speculate that the differences in alkyl chain interdigitation and packing efficiency arising from having the aromatic unit as a side chain rather than along the alkyl chains are responsible for this discrepancy. Control experiments with aliphatic ether side chain amphiphiles also corroborate the fact that aromatic interactions do not play a significant role in the aggregation behavior of these side chain amphiphiles. Side chain amphiphiles lacking an aromatic unit are still able to form the inverted hexagonal LLC phase, so long as the extent of branching and volume of the aliphatic side chain are sufficient.

Ordering effects on the photopolymerization of a lyotropic liquid crystal

The synthesis of polymers bearing the highly ordered nanostructure of lyotropic liquid crystal (LLC) phases has recently been of great interest. This work describes the polymerization behavior and structural evolution of a cationic amphiphile in various LLC phases. The type and degree of LLC phases formed from this monomer depend strongly on the composition and temperature. By adding a nonpolymerizable surfactant a variety of LLC phases are formed including hexagonal, bicontinuous cubic, and lamellar morphologies while maintaining a constant monomer concentration. The highly ordered lamellar LLC phase exhibits the fastest polymerization rate with the slowest occurring in the hexagonal phase. The polymerization rates of the bicontinuous morphology were intermediate to the lamellar and hexagonal phases. The faster polymerization kinetics is due to diffusional limitations imposed on the propagating polymer by the highly ordered lamellar LLC phase. Also, the order of this LLC system has a strong dependence on temperature. At higher temperatures, the degree of LLC order and correspondingly the polymerization rate decrease. The original LLC phase morphology appears to be retained to the greatest extent in the faster polymerizing lamellar phase. The original nanostructure is also retained in the hexagonal and cubic LLC phases but with some slight changes in structure. This LLC structure is preserved at temperatures well exceeding the thermal phase transitions of the unpolymerized LLC samples.

Probing matrix isolation effects in lyotropic liquid crystal nanocomposites using water-soluble PPv

The incorporation of water-soluble poly(sodium phenylenevinylenedicarboxylate) in the ordered nanochannels of a hexagonal lyotropic liquid crystal network is described. The absorption and emission behavior of the pristine, pre-formed PPV derivative in solution, in the bulk solid state, and in the nanocomposite are compared. The effect of matrix isolation in the nanostructured assembly on the conjugation length, conformation, stability, and alignment of the encapsulated PPV chains will be discussed.

A New Family of Polymerizable Lyotropic Liquid Crystals: Control of Feature Size in Cross-Linked Inverted Hexagonal Assemblies via Monomer Structure

An efficient and versatile synthesis of a series of polymerizable amphiphilic mesogens that affords control over tail length and position of the polymerizable group is described. The synthesis employs a novel and facile method of preparing styrene ethers. The monomers are sodium salts of styrene ether-modified fatty acids that can be used to form cross-linkable inverted hexagonal (HII) lyotropic liquid crystal (LLC) phases at ambient temperature with controllable nanometer-scale dimensions. Examination of a series of regioisomers with the same alkyl chain length but with the styrene ether group at different locations along the chain revealed that the position of the styrene ether has a profound effect on the dimensions of the resulting HII phase at a fixed temperature and composition. Increasing overall monomer tail length also has a significant, although smaller, effect on the unit cell dimensions of the LLC phase. By controlling the structure of the LLC monomer in this manner, cross-linked HII phases wi...

Self-Organization and Formation of Liquid Crystal Phases by Molecular Templates Related to Membrane Components of Archaebacteria.

Tubular supramolecular aggregates are formed from glycolipids 1, which are closely related to natural components of membranes of archaebacteria. The glycolipids exhibit disordered columnar thermotropic and hexagonal lyotropic liquid crystal phases. Whereas formation of the mesophases is insensitive to stereochemical factors, formation of the tubules is dependent on the configuration of the stereocenters that are shown in the picture but not specified.

Modification of the Microstructure in Poloxamer Block Copolymer−Water−“Oil” Systems by Varying the “Oil” Type

Amphiphilic block copolymers can self-assemble in the presence of selective solvents (“water” and “oil”) into a variety of different microstructures. The role that “oil” plays in the phase behavior and structure of copolymer−oil−water systems is the focus of the present investigation. We examined a poloxamer-type poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) triblock copolymer (Pluronic F127: E100P70E100) in ternary isothermal systems with water and each of three “oils” (of varying polarity), p-xylene, n-butyl acetate and butan-1-ol. A total of six phases with different microstructures have been identified in the ternary systems studied here: normal (oil-in-water) micellar cubic (I1), normal hexagonal (H1), lamellar (Lα), and reverse (water-in-oil) hexagonal (H2) lyotropic liquid crystals, and normal micellar (L1) and reverse micellar (L2) solutions. The number and types of phases, as well as the composition range of their stability, are different for each ternary system, reflecting the effects of oil. Systematic small-angle X-ray scattering (SAXS) measurements have been performed in order to establish the structure of the phases and determine their characteristic length-scales. The location of the “oils” in the different self-assembled microstructures has been deduced from the structural dimensions obtained from SAXS and the constraints imposed by the copolymer total volume fraction and interfacial area: all xylene is present in the interior (apolar core) of the oil-in-water microdomains and butanol is located at the interfacial region between poly(propylene oxide) and poly(ethylene oxide), while butyl acetate contributes to both the core and the interfacial domains.

Counterion Effects in Liquid Crystal Templating of Nanostructured CdS

We report the synthesis of cadmium sulfide containing a periodic array of nanometer-sized cylindrical pores. The nanostructured cadmium sulfide was synthesized in a hexagonal lyotropic liquid crystal by the reaction between H2S and various cadmium salts. Interestingly, the counterion of the cadmium salt had a pronounced effect on the regularity and periodicity of the pores. The dimension of the pores is commensurate with the hydrophobic cores of the liquid crystal, and one counterion even leads to hexagonally faceted nanostructured particles.

Solid-Like Director Reorientation in Sheared Hexagonal Lyotropic Liquid Crystals as Studied by Nuclear Maginetic Resonance

The shear-induced reorientation of the hexagonal surfactant mesophase is investigated by deuterium nuclear magnetic resonance spectroscopy. The NMR spectra of the sheared samples, measured in situ with two different shear geometries, either cone and plate or coaxial cylinders, prove that the hexagonal axis (the director) aligns parallel to the flow. Shear experiments starting from a monodomain sample, with the director initially aligned along the velocity gradient, exhibit an increasing tilt of the director as the shear strain γ grows. The tilt angle is given by arctan γ. This makes evident that the hexagonal lyomesophase behaves like a solid. The analysis of nuclear magnetic resonance spectra and textures observed by polarizing microscopy shows an undulation of the columns in both the quiescent and the sheared state. The amplitude of the undulations decreases with increasing shear strain.

Highly ordered polymer–inorganic nanocomposites via monomer self assembly: In situ condensation approach

A highly ordered organic inorganic nanocomposite can be synthesized by photopolymerizing an inverse hexagonal lyotropic liquid crystal phase consisting of an amphiphilic monomer and organic crosslinker as the organic matrix and a sol‐gel silica precursor solution as the hydrophilic component (see Figure). In situ silica condensation gives the ordered nanocomposite. A simple example is discussed.magnified image

Static H-2-H-2 Dipole Interactions in a Lyotropic Liquid-Crystal - A 2Dqe Study

The 2H NMR two-dimensional quadrupolar-echo spectra of an α-deuterated surfactant in an aligned hexagonal lyotropic liquid crystal were recorded. Whereas only indications of 2H-1H and 2H-2H dipole splittings were observed in the F2 spectra, these residual interactions were displayed in the F1, spectra as an apparently Lorentzian peak. When gated 1H decoupling was used, the 2H-2H dipole splittings became visible in the F1 spectra as three (apparently Lorentzian) peaks. From a fit to the experimental lineshape the frequencies, relative amplitudes, line-widths, and phases of these peaks were determined. The ratio between the 2H-2H dipole and 2H quadrupole splittings was found to be |q| = 0.0041. Since q values of about −0.0040 have been obtained in lamellar phases, these results indicate that the α-methylene angular distribution function with respect to the local surface normal of the cylindrical aggregates may not deviate significantly from threefold symmetry.

Anisotropic 23Na spin relaxation in liquid crystals. Determination of all nine spectral densities for a hexagonal lyotropic phase

The orientation dependence of three independent 23Na relaxation rates is determined from measurements on the counterions in a magnetically aligned hexagonal lyotropic liquid crystal. The data allow us to determine the nine director-frame spectral densities that comprise the full information content of the spin-relaxation behavior of a quadrupolar nucleus in a uniaxial phase. The model-independent director-frame spectral densities provide important information about the time scale and rotational symmetry of the anisotropic motions in the phase. More detailed information is obtained by invoking a specific dynamic model that separates dynamic and structural factors. The six high-frequency spectral densities are quantitatively accounted for in terms of fast local motions and counterion surface diffusion around the cylindrical surfactant aggregates, while the three zero-frequency spectral densities contain large contributions from diffusion-modulated director fluctuations.

Nuclear spin relaxation in a hexagonal lyotropic liquid crystal

The hexagonal (E) phase in the sodium dodecyl sulphate (SDS)/decanol/water system is investigated by 2H and 23Na nuclear magnetic resonance (NMR) of the selectively deuterated SDS and the sodium counterion. Using macroscopically oriented E phase samples, prepared from the magnetically aligned nematic (NC) phase, we measure the orientation-dependent relaxation rates R1Z and R1Q as well as the line shape of both nuclei. The orientation dependence of the lab-frame spectral densities, determined from the relaxation rates, allow us to separate contributions from different types of molecular motion. In particular, we find a dominant contribution from molecular diffusion around the cylindrical aggregate. From this contribution we determine the lateral diffusion coefficient of SDS to (1.4±0.2)×10−10 m2 s−1 at 25 °C (activation energy 26±2 kJ mol−1 ) and the counterion surface diffusion coefficient to (4.8±0.9)×10−10 m2 s−1 at 25 °C (a factor 2.8 smaller than in an infinitely dilute aqueous electrolyte solution). Furthermore, the flexibility of the cylindrical aggregates in the investigated E phase (aggregate volume fraction 0.27) is quantified in terms of an orientational order parameter SDC≊0.9.

Theory of intramolecular spin relaxation by translational diffusion in locally ordered fluids

The theory of spin relaxation induced by translational diffusion of small molecules or ions in locally ordered fluids, developed in parts I and II of this series, is extended to cylindrical geometry as, for example, in polyelectrolyte solutions or in hexagonal lyotropic liquid crystals. The theory is based on the Smoluchowski diffusion equation with nonuniform potential of mean force and translational diffusivity and on the cylindrical cell model. Formally exact closed-form expressions are derived for the zero-frequency spectral density associated with radial diffusion, while a general numerical algorithm is described for computing the full frequency-dependent spectral density. Several useful approximations, such as the dynamic cell approximation, the steady state approximation and the surface diffusion approximation, are formulated and their accuracy quantitatively assessed. Calculations are reported for a mean-field interaction model based on the nonlinear PoissonBoltzmann equation, with emphasis on applications of the theory to counterion spin relaxation in polyelectrolyte solutions. l. INTRODUCTION In locally ordered fluids, such as macromolecular solutions and lyotropic liquid crystals, nuclear spin relaxation can be induced by modulation, through translational motion of the fluid molecules, of the magnitude and orientation of the residual intramolecular spin-lattice coupling, which is only partially averaged by local molecular motions near an interface. In part I I-1] and part II I-2] of this series of papers, we developed a theoretical framework, based on the so-called continuous diffusion model (CDM), for treating spin relaxation in locally ordered fluids bounded by planar interfaces. In the CDM theory, the geometry of the fluid region and the potential of mean force acting on the spin-bearing species determine the equilibrium distribution of spins as well as their translational dynamics. The latter is characterized by a time-dependent distribution function, or propagator, which, in the CDM theory, is obtained from the Smoluchowski mean-field diffusion equation, containing also the translational diffusivity tensor of the spin-bearing species. These distribution functions are then used to construct the time correlation functions and spectral densities that determine the observable spin relaxation rates. We showed in parts I and II that the predictions of the CDM theory may differ qualitatively from those of the traditional discrete