Macroscopically Oriented Research Articles

Anisotropic Thermal Interface Materials: Directional Heat Transfer in Uniaxially Oriented Liquid Crystal Networks.

For the development of anisotropic thermal interface materials (TIMs), a rod-shaped reactive monomer PNP-6MA is newly designed and successfully synthesized. PNP-6MA reveals a smectic A (SmA) mesophase between crystalline (K) and isotropic (I) phases. PNP-6MA can be oriented under a magnetic field ( B = 2 T), and its macroscopic orientation can be robustly stabilized by in situ polymerization. Even without macroscopic orientations, the fabricated thermal conducting liquid crystal (TCLC) films show the outstanding thermal conductivity of 1.21 W/m K, which is higher than conventional organic materials. The thermal conductivity of uniaxially and macroscopically oriented TCLC films can be 2.5 W/m K along the long axis of mesogenic core. The newly developed TCLC film can be used as a TIM between a high-power light-emitting diode and a heat sink.

Heat Transfer Organic Materials: Robust Polymer Films with the Outstanding Thermal Conductivity Fabricated by the Photopolymerization of Uniaxially Oriented Reactive Discogens.

For the development of advanced heat transfer organic materials (HTOMs) with excellent thermal conductivities, triphenylene-based reactive discogens, 2,3,6,7,10,11-hexakis(but-3-enyloxy)triphenylene (HABET) and 4,4',4″,4‴,4'''',4'''''-(triphenylene-2,3,6,7,10,11-hexaylhexakis(oxy))hexakis(butane-1-thiol) (THBT), were synthesized as discotic liquid crystal (DLC) monomers and cross-linkers, respectively. A temperature-composition phase diagram of HABET-THBT mixtures was first established based on their thermal and microscopic analyses. From the experimental results, it was realized that the thermal conductivity of DLC HTOM was strongly affected by the molecular organizations on a macroscopic length scale. Macroscopic orientation of self-assembled columns in DLC HTOMs was effectively achieved under the rotating magnetic fields and successfully stabilized by the photopolymerization. The DLC HTOM polymer-stabilized at the LC phase exhibited the remarkable thermal conductivity above 1 W/mK. When the DLC HTOM was macroscopically oriented, the thermal conductivity was estimated to be 3 W/mK along the in-plane direction of DLC molecule. The outstanding thermal conductivity of DLC HTOM should be originated not only from the high content of two-dimensional aromatic discogens but also from the macroscopically oriented and self-assembled DLC. The newly developed DLC HTOM with an outstanding thermal conductivity as well as with an excellent mechanical sustainability can be applied as directional heat dissipating materials in electronic and display devices.

Alignment under Magnetic Field of Mixed Fe2 O3 /SiO2 Colloidal Mesoporous Particles Induced by Shape Anisotropy.

When using the bottom-up approach with anisotropic building-blocks, an important goal is to find simple methods to elaborate nanocomposite materials with a truly macroscopic anisotropy. Here, micrometer size colloidal mesoporous particles with a highly anisotropic rod-like shape (aspect ratio ≈ 10) have been fabricated from silica (SiO2 ) and iron oxide (Fe2 O3 ). When dispersed in a solvent, these particles can be easily oriented using a magnetic field (≈200 mT). A macroscopic orientation of the particles is achieved, with their long axis parallel to the field, due to the shape anisotropy of the magnetic component of the particles. The iron oxide nanocrystals are confined inside the porosity and they form columns in the nanochannels. Two different polymorphs of Fe2 O3 iron oxide have been stabilized, the superparamagnetic γ-phase and the rarest multiferroic ε-phase. The phase transformation between these two polymorphs occurs around 900 °C. Because growth occurs under confinement, a preferred crystallographic orientation of iron oxide is obtained, and structural relationships between the two polymorphs are revealed. These findings open completely new possibilities for the design of macroscopically oriented mesoporous nanocomposites, using such strongly anisotropic Fe2 O3 /silica particles. Moreover, in the case of the ε-phase, nanocomposites with original anisotropic magnetic properties are in view.

Macroscopic ordering of helical pores for arraying guest molecules noncentrosymmetrically.

Helical nanostructures have attracted continuous attention, not only as media for chiral recognition and synthesis, but also as motifs for studying intriguing physical phenomena that never occur in centrosymmetric systems. To improve the quality of signals from these phenomena, which is a key issue for their further exploration, the most straightforward is the macroscopic orientation of helices. Here as a versatile scaffold to rationally construct this hardly accessible structure, we report a polymer framework with helical pores that unidirectionally orient over a large area (∼10 cm2). The framework, prepared by crosslinking a supramolecular liquid crystal preorganized in a magnetic field, is chemically robust, functionalized with carboxyl groups and capable of incorporating various basic or cationic guest molecules. When a nonlinear optical chromophore is incorporated in the framework, the resultant complex displays a markedly efficient nonlinear optical output, owing to the coherence of signals ensured by the macroscopically oriented helical structure.

Interaction of hydrophobic and amphipathic antimicrobial peptides with lipid bicelles.

Bicelles are model membrane systems that can be macroscopically oriented in a magnetic field at physiological temperature. The macroscopic orientation of bicelles allows to detect, by means of magnetic resonance spectroscopies, small changes in the order of the bilayer caused by solutes interacting with the membrane. These changes would be hardly detectable in isotropic systems such as vesicles or micelles. The aim of this work is to show that bicelles represent a convenient tool to investigate the behavior of antimicrobial peptides (AMPs) interacting with membranes, using electron paramagnetic resonance (EPR) spectroscopy. We performed the EPR experiments on spin-labeled bicelles using various AMPs of different length, charge, and amphipathicity: alamethicin, trichogin GA IV, magainin 2, HP(2-20), and HPA3. We evaluated the changes in the order parameter of the spin-labeled lipids as a function of the peptide-to-lipid ratio. We show that bicelles labeled at position 5 of the lipid chains are very sensitive to the perturbation induced by the AMPs even at low peptide concentrations. Our study indicates that peptides that are known to disrupt the membrane by different mechanisms (i.e., alamethicin vs magainin 2) show very distinct trends of the order parameter as a function of peptide concentration. Therefore, spin-labeled bicelles proved to be a good system to evaluate the membrane disruption mechanism of new AMPs.

Unidirectional self-assembly of soft templated mesoporous carbons by zone annealing

Surfactant or block copolymer-templated mesoporous films have been extensively explored, but achieving mesostructure coherence and unidirectional orientation over macroscopic dimensions has remained quite challenging for these self-assembled systems. Here, we extend the concepts associated with zone refinement of crystalline materials to soft templated mesoporous carbon films based on the cooperative assembly of commercial non-ionic surfactants (block copolymers) and phenolic resin oligomers (resol) to provide macroscopic alignment of both cubic (FDU-16) and hexagonal (FDU-15) mesostructures. The average orientation of these mesophases is determined from rotation grazing incidence small angle X-ray scattering (GISAXS) measurements. For FDU-15 templated by Pluronic P123, the orientation factor for the zone-annealed film is 0.98 based on the average of the second Legendre polynomial, but this orientation deteriorates significantly during carbonization. Notably, a thermal stabilization step following zone annealing preserves the orientation of the mesostructure during carbonization. The orientation factor for an isotropic cubic structure (FDU-16 templated by Pluronic F127) is only 0.48 (based on the 111 reflection with incident angle 0.15°) for the same zone annealing protocol, but this illustrates the versatility of zone annealing to different mesostructures. Unexpectedly, zone annealing of FDU-15 templated by Pluronic F127 leads to stabilization of the mesostructure through carbonization, whereas this structure collapses fully during carbonization even after extended oven annealing; despite no clear macroscopic orientation of the cylindrical mesostructure from zone annealing. Thermal zone annealing provides a simple methodology to produce highly ordered and macroscopically oriented stable mesoporous carbon films, but the efficacy is strongly tied to the mobility of the template during the zone annealing.

First evidences of macroscopic orientation of benzene in C 60.4C 6H 6 solvate at 293 K

We report the first spectroscopic (NMR) evidences of macroscopic orientation of benzene in C 60⋅4C 6H 6 solvate at room temperature. Unusually strong transcient oscillations have been observed in proton-carbon cross-polarization and cross-polarization inversion experiments for a static sample. This provides detailed geometrical information about macroscopically oriented benzene in C 60 solvate.

Effect of Polyacrylic Acid on Phase State of Lipids and Diffusion in Lipid-Water System

Lipid vesicles interacting with polyanions are promising for controlled drug delivery. However, different aspects of the interaction of these polymers with lipids are far from complete understanding. In this work we studied the influence of polyacrylic acid (PAA) with small concentrations (1–4 mol%) on the change of the phase state, lateral diffusion of these lipids in lamellar phase and transmembrane water diffusion in macroscopically oriented bilayers of lipid-water systems formed by dimiristoylphosphatidylcholine (DMPC) and dioleoylphosphatidylcholine. Measurements were performed by 31P nuclear magnetic resonance (NMR) spectroscopy and the 1H NMR technique with a pulsed field gradient. It was found that the presence of PAA does not change the lamellar structure of the system. However, a part of bilayers changes their originally flat geometry and forms vesicles with a higher surface curvature. Macroscopic orientation of bilayers disappeared. For DMPC the presence of PAA leads to a shift of the gel-to-liquid crystalline phase-transition temperature to higher temperatures. An increase of PAA concentration leads to a monotonous decrease in the lateral diffusion coefficient of lipids that is caused, probably, by the ordering of lipids in bilayers. The transbilayer diffusion coefficient of water increases in the presence of PAA, but it depends slightly on the PAA concentration. An increase of pH leads to a change of the lipid lateral and transbilayer diffusion coefficients to the values typical for a pure bilayer.

Morphology of a hydrogen-bonded LC polymer prepared by photopolymerization-induced phase separation under an isotropic phase

A hydrogen-bonded LC polymer was prepared by photopolymerization of an LC blend composed of 4-(6-acryloyloxyhexyloxy)benzoic acid (A6OBA) and 4-hexyloxy-4′-cyanobiphenyl (6OCB), containing small amounts of an inhibitor and photoinitiator, at two different temperatures in an isotropic phase. To elucidate the factors determining the morphology of the obtained polymer (poly(A6OBA)), we chose two irradiation temperatures: one in the LC temperature range of the polymer, the other in the isotropic range. We investigated structures of the polymers by optical microscopy and scanning electron microscopy. SEM images showed that the film obtained at the lower temperature consisted of randomly extended fibers having a diameter of ca. 1.0 μm and some branches, whereas the film prepared at the higher temperature was composed of polymer particles with a diameter ca. 1.5 μm. By comparing these results with those of an earlier experiment in which we obtained macroscopically oriented LC fibers by photopolymerization under the LC phase of the blend, we infer the following; (i) the presence of an LC phase in the resulting polymer itself during photopolymerization is necessary for it to form fibrous morphology and (ii) the LC ordering field present prior to photopolymerization is not indispensable for the fibrous morphology but it is for the macroscopic orientation and reduction of the branches in the fibers.

Effect of shear on cubic phases in gels of a diblock copolymer

The effect of shear on the orientation of cubic micellar phases formed by a poly(oxyethylene)–poly(oxybutylene) diblock copolymer in aqueous solution has been investigated using small-angle x-ray scattering (SAXS) and small-angle neutron scattering (SANS). SAXS was performed on samples oriented in a Couette cell using steady shear, and SANS was performed on samples subject to oscillatory shear in situ in a rheometer with a shear sandwich configuration. A body-centered-cubic (bcc) phase observed for gels with concentrations greater than 30 wt % copolymer was found to orient into a polydomain structure, with the close-packed {110} planes both parallel and perpendicular to the shear plane. For gels with 30 wt % copolymer or less, a face-centered-cubic (fcc) phase was observed, and this was also observed on heating the more concentrated gels that formed a bcc phase at room temperature. The fcc phase could be oriented to form a highly twinned structure, with a significant deviation from the ABCABC… stacking sequence of the ideal structure due to random sequences resulting from slip of {111} hexagonal-close-packed planes. For the lower concentration gels, a transition from hard to soft gel on increasing temperature was found to be characterized by a change in the susceptibility of the sample to macroscopic shear orientation, as probed using SAXS. The hard gel could be oriented by shear into a twinned fcc structure, whereas the soft gel comprised a fcc phase with a small grain size, which could not be sheared to form a macroscopically oriented domain. Shear only homogenized the sample, producing a powder SAXS pattern from a fcc structure.

The macroscopic organization of reconstituted M13 coat protein-phospholipid systems. An EPR spectroscopy and polarizing microscope study

The coat protein of the bacteriophage M13 in the α-helical state is reconstituted in macroscopically oriented systems of dioleoylphosphatidylcholine that are prepared by squeezing the reconstituted material between glass plates. The coat protein dramatically influences the macroscopic orientation of the multibilayers, as is investigated by polarizing microscopy and EPR spectroscopy of the cholestane spin label embedded in the bilayers. It is found that with increasing amounts of protein the spontaneous macroscopic orientation of the reconstituted system decreases. This effect is proposed to be due to an increase of the apparent viscosity of the lipid-protein systems with increasing amounts of protein. This is assumed to arise from a sticky effect of the C- and N-terminal protein parts that extend into the aqueous phase between the bilayers.

Structure and mobility within anisotropic networks obtained by photopolymerization of liquid crystal molecules

Liquid crystalline mixtures of mono- and diacrylates were polymerized after macroscopic orientation of the molecules. Refractive indices of the mixtures were investigated both in the LC phase and after polymerization in the macroscopically oriented state. In this way the effect of cross-link density on the optical anisotropy was measured and correlated with order parameter measurements using infrared dichroism. Dielectric measurements were also carried out to investigate the effect of cross-linking on the movement of the side groups around the main chain