Nematic ordering in dilute solutions of rodlike polyelectrolytes

Quantitative theory of orientational behavior of rodlike polyelectrolytes in dilute solution is developed. We find that in salt-free solutions many-body Coulomb interactions between macro- and counterions favor nematic ordering. It is shown that the orientationally isotropic phase of the solution becomes unstable toward nematic ordering at polymer concentration smaller than the overlap concentration. Our predictions are consistent with experimental observations for synthetic polyelectrolytes poly(p-phenylene)sulfonates in aqueous solutions.     

HPLC Characterization of the Association Mechanism of Organic Compounds with β Cyclodextrin Using a Novel Boron Nitride Nanotube Particulate Stationary Phase

In this paper, a simple homogeneous coating of silica spherical particles with pristine boron nitride nanotubes (BNNTs) was described. BNNTs dissolved in dimethylacetamide (DMAc) were mixed with amino-functionalized silica particles having a 5 μm diameter. Favorable interaction between the amino group and the BNNT surfaces induces the absorption of the BNNTs on the silica. The BNNT-coated silica particles were used as stationary phase for HPLC. For the first time, it was demonstrated that this new particulate BNNT stationary phase can be used for the study of the complexation of solute molecules (terpene molecules used as test drugs in this work) with β cyclodextrin (βCD). The apparent formation constants Kf of terpene derivative/βCD were in the same magnitude as those reported in the literature. The plot of Kf versus the water fraction in the methanol/water mobile phase showed that the BNNT surface played an active role in the complex formation due to terpene/BNNT-specific polar interactions. This work demonstrated that our novel particulate BNNT HPLC stationary phase was an efficient tool to study molecular recognition mechanism and more specifically the association between a drug substance and a target molecule with the aim of reaching biopharmaceutic and clinical applications.     

Perfectly dissolved boron nitride nanotubes due to polymer wrapping

We report for the first time that boron nitride nanotubes (BNNTs) may be dissolved in organic solvents by wrapping them with a polymer. Transmission electron microscopy and cathodoluminescence studies indicate the strong pi-pi interactions between BNNTs and the polymer. A band gap ranging from 5.2 to 5.5 eV was documented for the BNNTs independent of their geometrical characteristics by using ultraviolet-visible absorption experiments on composite films and thin BNNT films prepared from solutions.     

Synthesis of boron nitride nanotube films with a nanoparticle catalyst

Boron nitride nanotube (BNNT) films were synthesized by combining ball milling and thermal chemical vapor deposition (CVD) using nano-Fe₃O₄ as a catalyst. The as-produced BNNTs have a bamboo-like structure and have a diameter in the range of 50~200 nm with an average length of more than 40 mm. Moreover, BNNT nanojunction structures were synthesized. The structure and morphology of the BNNTs were characterized by XRD, SEM, TEM and HRTEM. The possible growth mechanism of BNNTs and BNNT nanojunction structures were proposed. Though the BNNT films were observed, out of our expectation, BNNTs with thin tube wall and small average diameter have not been achieved, and this could be mainly ascribed to the aggregation of the nanoparticle catalyst, resulting in greater catalyst particles during the process of BNNT growth. This result will provide a promising approach to obtain the desired shape of BNNTs and produce branched junctions of BNNTs.     

In situ real-time study buckling behavior of boron nitride nanotubes with axial compression by TEM

The real time analysis structure evolution of BNNT with compression showed that the formation of V-shape in the post-buckling before BNNT fracture was reversible.     

Theoretical simulation of thermally induced phase separation in a main‐chain liquid‐crystalline polymer solution

Phase diagrams of main‐chain liquid‐crystalline polymer (MCLCP) solutions have been calculated self‐consistently on the basis of a simple addition of the Flory–Huggins free energy for isotropic mixing, the Maier–Saupe free energy for nematic ordering, and the Flory free energy for chain rigidity of the MCLCP backbone. The calculated phase diagram is an upper critical solution type overlapping with the nematic–isotropic transition. The phase diagram consists of liquid–liquid, liquid–nematic, and pure nematic regions. Subsequently, the dynamics of thermally induced phase separation and morphology development have been investigated by the incorporation of the combined free energy density into the coupled time‐dependent Ginzburg–Landau (model C) equations, which involve conserved compositional and nonconserved orientational order parameters. The numerical calculations reveal a variety of the morphological patterns arising from the competition between liquid–liquid phase separation and nematic ordering of the liquid‐crystalline polymer. Of particular interest is the observation of an inflection in the growth dynamic curve, which may be attributed to the nematic ordering of the MCLCP component, which leads to the breakdown of the interconnected domains. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 913–926, 2003     

Water phase transition induced by a Stone-Wales defect in a boron nitride nanotube

Boron nitride nanotubes (BNNTs) have been reported to possess superior water permeation properties. In this work, using molecular dynamics simulations with partial charges, capturing BNNT polarization effects obtained from quantum calculations, we found that Stone-Wales (SW) defects in a (5,5) BNNT result in phase transition of water, i.e., a transition between liquid-like phase and vapor-like phase was observed. The 90 degree rotation of the B-N bond, SW transformation, in an SW-defective (5,5) BNNT results in breaking of hydrogen bonding with neighboring water molecules and leads to the existence of a vapor-like phase near the SW defect. Water transport rate was evaluated by measuring translocation time. Water in an SW-defective (5,5) BNNT has fewer translocation events, longer translocation time, and a higher axial diffusion coefficient compared to water in a nondefective (5,5) BNNT.     

铂掺杂氮化硼纳米管及CO在其表面的吸附行为的理论研究

采用基于密度泛函理论的PBEPBE方法对铂(Pt)掺杂的氮化硼(BN)纳米管进行了理论研究. 计算结果表明, Pt原子突出BN纳米管表面, Pt的d轨道暴露到外面, 使它更容易和外来分子发生相互作用, 提高了纳米管的反应活性. Pt取代掺杂缩小了纳米管的能隙, 从而提高BN纳米管的导电性. 一氧化碳(CO)在Pt掺杂BN纳米管上的吸附行为表明, 2个CO能化学吸附到纳米管表面, 更多的CO分子吸附是物理吸附.     

Nitrogen monoxide storage and sensing applications of transition metal–doped boron nitride nanotubes: a DFT investigation

The structural properties, electronic properties, and adsorption abilities for nitrogen monoxide (NO) molecule adsorption on pristine and transition metal (TM = V, Cr, Mn, Nb, Mo, Tc, Ta, W, and Re) doping on B or N site of armchair (5,5) single-walled boron nitride nanotube (BNNT) were investigated using the density functional theory method. The binding energies of TM-doped BNNTs reveal that the Mo atom doping exhibits the strongest binding ability with BNNT. In addition, the NO molecule weakly interacts with the pristine BNNT, whereas it has a strong adsorption ability on TM-doped BNNTs. The increase in the adsorption ability of NO molecule onto the TM-doped BNNTs is due to the geometrical deformation on TM doping site and the charge transfer between TM-doped BNNTs and NO molecule. Moreover, a significant decrease in energy gap of the BNNT after TM doping is expected to be an available strategy for improving its electrical conductivity. These observations suggest that NO adsorption and sensing ability of BNNT could be greatly improved by introducing appropriate TM dopant. Therefore, TM-doped BNNTs may be a useful guidance to be storage and sensing materials for the detection of NO molecule.

     

SnO2 nanoparticle-functionalized boron nitride nanotubes

Boron nitride nanotubes (BNNTs) were synthesized by a carbon-free chemical vapor deposition method using boron and metal oxide as reactants. Then SnO(2) nanoparticles were functionalized on them via a simple wet chemistry method. Detailed transmission electron microscopy (TEM) observations reveal that SnO(2) nanoparticles may cover the tube surface or be encapsulated in tube channels. The lattice distances of both BNNT and SnO(2) have been changed due to the strong interactions between them. The band gap energy of SnO(2) particles is found enlarged due to the size effect and interaction with BNNTs.     

Dually-functionalized boron nitride nanotubes to target glioblastoma multiforme

Boron nitride nanotubes (BNNT) were functionalized under mild conditions, using a difunctional amine, such as glycine, with one of three targeting ligands, folic acid, a nerve growth factor, or an antibody against nerve growth factor. In addition, non-fouling BNNTs were obtained by a facile and versatile, non-destructive method via controlled surface-initiated grafting of polyzwitterions. The BNNTs were loaded with a fluorescent probe for convenient imaging of glioblastoma multiforme cells treated with BNNTs. BNNTs bearing targeting factors on their outer surface demonstrated an increased efficiency of internalization in glioblastoma multiforme cells, compared to non-modified BNNTs. The degree of internalization was affected both by the nature of the ligand/agonist linked to the BNNTs surface and by the presence of serum proteins. The polyzwitterion grafts prevented the spontaneous adsorption of serum proteins on the BNNTs.     

NBO,AIM, and TD-DFT assisted screening of BNNT optimum diameter on ethyl phosphorodimethylamidocyanidate sensor design

A computational study based on DFT calculations was performed to investigate the effect of phosphorodimethylamidocyanidate (PDMAC) molecule adsorption on the surface of pure and Ga-doped (4,0), (5,0), (6,0), (7,0), and (8,0) zigzag boron-nitride nanotubes (BNNTs). Our results reveal that the interactions between PDMAC molecule and (5,0), (6,0), (7,0), and (8,0) BNNTs are weak. However, according to the AIM and NBO analysis the PDMAC exhibits strong affinity towards the (4,0) BNNT with appreciable adsorption energy (?111.03 kJ/mol). The adsorption of PDMAC molecule onto the (4,0) BNNT affect the electronic conductance, hypsochromic, and hyperchromic shifts in the calculated UV-Visible spectrum. Based on the obtained results, it is expected that the pristine and Ga-doped (4,0) BNNT could be promising candidates in gas sensor devices for detecting the PDMAC molecule.     

Electronic structure of octagonal boron nitride nanotubes

The effect of an octagonal lattice configuration on a boron nitride nanotube is explored using first principle calculations. Calculations show that the formational energy of an octagonal boron nitride nanotube (o‐BNNT) is an exothermic reaction. Boron and nitrogen atoms within an o‐BNNT have an average of 2.88 electrons and 9.09 electrons, respectively, indicating ionic‐like bonding. In addition, the electronic structure of the octagonal boron nitride nanotube shows semiconductive properties, while h‐BNNT is reported to be an insulator. Additional o‐BNNTs with varying diameters are calculated where the results suggest that the diameter has an effect on the binding energy and bandgap of the o‐BNNT. The defect sites of the o‐BNNT are reactive against hydrogen where a boron defect is particularly reactive. Thus, this work suggests that physical and chemical properties of a boron nitride nanotube can be tailored and tuned by controlling the lattice configuration of the nanotube.     

Structural characterizations and electronic properties of boron nitride nanotube crystalline bundles

The structural characterizations and electronic properties of aligned armchair single-walled boron nitride nanotube (BNNT) bundles are theoretically investigated. In the spontaneous bundling process, the cylindrical shapes of bundled BNNTs are preserved all along, whereas their diameters expand, then shrink, and return back to the initial dimensions. Owing to the nonuniform distribution of positive and negative charges among BNNTs, the multipole interaction in bundles is completely dependent upon the chirality of each BNNT and the arrangement of bundled BNNTs. The effect of intertube coupling on the dispersions of BNNT bundles is demonstrated. Our systematical simulations might be helpful for the understanding of potential applications of BNNT bundles in the nanometer manufacturing techniques such as doping, adsorption, and derivative synthesis.     

Single‐wall Boron Nitride Nanotube with Various Diameters under the Influence of Electric Field,A Computational Investigation

Structural and electrical response of the (4, 0), (5, 0) and (6, 0) zigzag model of single‐walled boron nitride nanotube (BNNT) with H‐terminated at the open ended, have been investigated under the external electric field (EF) with intensities 0–1.6 × 10^?2 a.u. using the DFT B3LYP/6‐31G* level of theory. Results of this study show that with increasing BNNTs diameter, the HOMO‐LUMO gap (HLG) values increase, and with increasing the EF intensity, the HLG values decrease. In both cases with increasing EF intensity and the BNNT diameters, the electric dipole moment is increased significantly. Also the calculated natural bond orbital (NBO) atomic charges on the atoms of the BNNT show that the separation of the center of the positive and the center of the negative electric charges of the boron nitride nanotubes are increases in both case. We have found that the properties of the BNNTs are dependent on their diameters and can be tuned by applied electric fields intensity.     

Effect of polydispersity and soft interactions on the nematic versus smectic phase stability in platelet suspensions

We theoretically discuss, using density-functional theory, the phase stability of nematic and smectic ordering in a suspension of platelets of the same thickness but with a high polydispersity in diameter, and study the influence of polydispersity on this stability. The platelets are assumed to interact like hard objects, but additional soft attractive and repulsive interactions, meant to represent the effect of depletion interactions due to the addition of nonabsorbing polymer, or of screened Coulomb interactions between charged platelets in an aqueous solvent, respectively, are also considered. The aspect (diameter-to-thickness) ratio is taken to be very high, in order to model solutions of mineral platelets recently explored experimentally. In this regime a high degree of orientational ordering occurs; therefore, the model platelets can be taken as completely parallel and are amenable to analysis via a fundamental-measure theory. Our focus is on the nematic versus smectic phase interplay, since a high degree of polydispersity in diameter suppresses the formation of the columnar phase. When interactions are purely hard, the theory predicts a continuous nematic-to-smectic transition, regardless of the degree of diameter polydispersity. However, polydispersity enhances the stability of the smectic phase against the nematic phase. Predictions for the case where an additional soft interaction is added are obtained using mean-field perturbation theory. In the case of the one-component fluid, the transition remains continuous for repulsive forces, and the smectic phase becomes more stable as the range of the interaction is decreased. The opposite behavior with respect to the range is observed for attractive forces, and in fact the transition becomes of first order below a tricritical point. Also, for attractive interactions, nematic demixing appears, with an associated critical point. When platelet polydispersity is introduced the tricritical temperature shifts to very high values.     

Morphology development of main‐chain liquid crystalline polymer fibers during solvent evaporation

A nonequilibrium thermodynamic approach has been developed for describing the emergence of fiber morphologies from a liquid crystalline polymer solution undergoing solvent evaporation, including fibrillar structures, concentric rings, and spiral structures. We utilized Matsuyama–Kato free energy for main‐chain liquid crystalline polymer (MCLCP) solutions, which is an extension of Maier–Saupe theory for nematic ordering and incorporates a chain‐stiffening, combined with Flory‐Huggins free energy of mixing. Temporal evolution of the concentration and nematic order parameters pertaining to the above free energy density of liquid crystalline polymer solution was simulated in the context of time‐dependent Ginzburg–Landau theory coupled with the solvent evaporation rate equation under the quasi‐steady state assumption. The emerged morphological patterns are discussed in relation to the phase diagram of the MCLCP solution and the rate of solvent evaporation. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 429–435, 2007     

Structure of single and double stranded dna solutions as studied by ultracentrifugation

The osmotic pressure of dense solutions of DNA fragments in aqueous 0.2-2 M salt (NaCl, etc.) solutions were easily obtained from equilibrium ultracentrifugation data. Short, helical double stranded DNA fragment solutions showed a well defined ordering transition. The osmotic pressure of these helical solutions could be explained by the scaled particle theory of rigid sphero-cylinders. On the other hand, dense solutions of (single stranded) etheno-DNA derivatives (of about 100 nucleotides) showed no ordering phase transition. The osmotic pressure as a function of the ε-DNA concentration verifies a scaling prediction for random coils in the semi-dilute regime. Good fits to the reduced osmotic pressure are obtained by using the Kleintjens and Koningsveld modified mean field lattice gas equations of state. As inferred from the Zimm clustering function, essentially all intermolecular interactions at high ionic strength are purely repulsive.     

Texture formation under phase ordering and phase separation in polymer-liquid crystal mixtures

Computational modeling of texture formation in coupled phase separation-phase ordering processes in polymer/liquid crystal mixtures is performed using a unified model based on the nematic tensor order parameter and gradient orientation elasticity. The computational methods are able to resolve defect nucleation, defect-defect interactions, and defect-particle interactions, as well as global and local morphological features in the concentration and order parameter spatiotemporal behavior. Biphasic structures corresponding to polymer dispersed liquid crystals (PDLCs), crystalline filled nematic (CFNs), and random filled nematics (RFNs) are captured and analyzed using liquid crystal defect physics and structure factors. Under spinodal decomposition due to concentration fluctuations, the PDLC structure emerges, and the nucleation and repulsive interaction of defects within nematic droplets leads to bipolar nematic droplets. Under spinodal decomposition due to ordering fluctuations, the CFNs structure emerges, and the stable polymer droplet crystal is pinned by a lattice of topological defects. For intermediate cases, where the mixture is unstable to both concentration and nematic order fluctuations, the RFN structure emerges, and polymer droplets and fibrils are pinned by a defect network, whose density increases with the curvature of the polymer-liquid crystal interface. The simulations provide an information of the role of topological defects on phase separation-phase ordering processes in polymer-liquid crystal mixtures.     

Origins of thermodynamically stable superhydrophobicity of boron nitride nanotubes coatings

Superhydrophobic surfaces are attractive as self-cleaning protective coatings in harsh environments with extreme temperatures and pH levels. Hexagonal phase boron nitride (h-BN) films are promising protective coatings due to their extraordinary chemical and thermal stability. However, their high surface energy makes them hydrophilic and thus not applicable as water repelling coatings. Our recent discovery on the superhydrophobicity of boron nitride nanotubes (BNNTs) is thus contradicting with the fact that BN materials would not be hydrophobic. To resolve this contradiction, we have investigated BNNT coatings by time-dependent contact angle measurement, thermogravimetry, IR spectroscopy, and electron microscopy. We found that the wettability of BNNTs is determined by the packing density, orientation, length of nanotubes, and the environmental condition. The origins of superhydrophobicity of these BNNT coatings are identified as (1) surface morphology and (2) hydrocarbon adsorbates on BNNTs. Hydrocarbon molecules adsorb spontaneously on the curved surfaces of nanotubes more intensively than on flat surfaces of BN films. This means the surface energy of BNNTs was enhanced by their large curvatures and thus increased the affinity of BNNTs to adsorb airborne molecules, which in turn would reduce the surface energy of BNNTs and make them hydrophobic. Our study revealed that both high-temperature and UV-ozone treatments can remove these adsorbates and lead to restitution of hydrophilic BN surface. However, nanotubes have a unique capability in building a hydrophobic layer of adsorbates after a few hours of exposure to ambient air.