Supramolecular Complexes of Multivalent Cholesterol-Containing Polymers to Solubilize Carbon Nanotubes in Apolar Organic Solvents

ArticleinChemistry - An Asian Journal 9(5) · May 2014with 113 Reads
Abstract
Copolymers of 2-ethylhexyl acrylate (EHA) and cholesteryloxycarbonyl-2-hydroxymethacrylate (CEM) were prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization. Supramolecular complexes of these copolymers with carbon nanotubes (CNTs) were soluble in THF, toluene, and isooctane. The colloidal solutions remained stable for months without aggregation. The rationale for the choice of CEM was based on the high adsorption energy of cholesterol on the CNT surface, as computed by DFT calculations. Adsorption isotherms were experimentally measured for copolymers of various architectures (statistical, diblock, and star copolymers), thereby demonstrating that 2-5 cholesterol groups were adsorbed per polymer chain. Once the supramolecular complex had dried, the CNTs could be easily resolubilized in isooctane without the need for high-power sonication and in the absence of added polymer. Analysis by atomic force microscopy (AFM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) indicated that the CNTs were devoid of bundles. The supramolecular complexes could also be employed in an inverse emulsion polymerization of 2-hydroxyethylmethacrylate (HEMA) in isooctane and dodecane, thereby leading to the formation of a continuous polymeric sheath around the CNTs. Thus, this technique leads to the formation of very stable dispersions in non-polar organic solvents, without altering the fundamental properties of the CNTs.

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  • ... 38 In order to stabilize the graphene sheets in isooctane, we exploited the fact that cholesterol forms supramolecular interactions with carbon-based materials in non-polar solvents. 39 Here, a diblock polymer containing 11 cholesterol units (CEM) and 7 ethyl-hexyl acrylate units (EHA) was used as graphene dispersant in isooctane ( Figure 1). Once stable dispersions of graphene were obtained, the printability of the resulting ink was evaluated. ...
    ... Cholesteryl chloroformate and hexamethyldisilazane (HMDZ) were purchased from Alfa Aesar. The diblock copolymer poly(CEM 11 -b-EHA 7 ) was prepared using the procedure outlined and described in reference 39 . ...
    ... The polymer is constituted of two blocks, a cholesterol containing block (CEM) and a 2-ethylhexyl acrylate block (EHA) (Figure 1). The former, which is insoluble in isooctane, is adsorbed at the graphene surface (the adsorption energy of cholesterol on graphene has been reported to be -63 kJ mol - 1 ) 39 while the latter, which is soluble in isooctane, forms a hairy layer responsible for steric stabilization. Please do not adjust margins ...
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    Reversible addition−fragmentation chain transfer (RAFT) controlled radical polymerization was employed, for the first time, to prepare well-defined (model) amphiphilic polymer co-networks based on n-butyl methacrylate (BuMA, hydrophobic monomer) and 2-(dimethylamino)ethyl methacrylate (DMAEMA, hydrophilic ionizable monomer) cross-linked with ethylene glycol dimethacrylate (EGDMA) and bearing elastic chains having the following comonomer distributions:  BuMA-b-DMAEMA-b-BuMA and DMAEMA-b-BuMA-b-DMAEMA triblock and BuMA-co-DMAEMA statistical copolymers. Two randomly cross-linked (not model) amphiphilic co-networks were also synthesized, the one by RAFT and the other by conventional free radical polymerization. The amphiphilic triblock copolymer-based model co-networks were obtained by stepwise synthesis in three stages:  (1) the synthesis of linear homopolymers bearing two active ends by using a bifunctional chain transfer agent (CTA), (2) the sequential addition of the second monomer on the macro-CTAs (homopolymers) to yield linear ABA or BAB triblock copolymers, and (3) the inter-linking of the linear copolymer chains at both ends using EGDMA cross-linker to form the final model co-networks. The homopolymer and copolymer precursors to the networks were characterized using gel permeation chromatography and 1H NMR spectroscopy. The swelling behavior of the networks was investigated in tetrahydrofuran and water. The co-networks swelled more in acidic than in neutral water due to the ionization of their DMAEMA units. The low pH aqueous swelling of the statistical co-network was higher than those of its triblock counterparts due to the lack of microphase separation with the statistical copolymer chains.
  • Article
    Azobenzene-derived photoactive polymers (P1−P3) containing pyrene pendants were designed and synthesized (Mw 30 000) for the noncovalent functionalization of single-walled carbon nanotubes (SWNTs). P1−P3 were found to be highly effective for the solubilization of SWNTs in common organic solvents, resulting in hybrid materials with enhanced thermal stability. The solubilization process was mostly driven by the π−π stacking interactions of pyrene with SWNTs. It also brings the azobenzene chromophores to the vicinity of nanotube surface, thereby allowing the electronic interactions between them. In addition to that, stacking of the pyrene and subsequent wrapping of the polymer around CNT surface provides more volume for the photoisomerization of azobenzene. These effects eventually accelerate the kinetics of photoisomerization of azobenzene in the polymer−SWNT composite. The photoalignment property of the composite was also increased when compared to that of the parent polymer which was studied by means of photoinduced birefringence.
  • Article
    Stable dispersions of both as-produced (raw soot) and purified laser-generated single-wall carbon nanotubes (SWNTs) have been demonstrated with several alkyl amide solvents. Optical absorption analysis over a range of concentrations has been utilized to estimate the dispersion limits for as-produced SWNTs in N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N,N-diethylacetamide (DEA), and N,N-dimethylpropanamide (DMP). In addition, extinction coefficients have been calculated using Beer's law for each solvent at energies of 1.27 and 1.77 eV, corresponding to the electronic transitions of semiconducting and metallic SWNTs, respectively. The results imply that high polarizability and optimal geometries (appropriate bond lengths and bond angles) may account for the favorable interaction between SWNTs and the alkyl amide solvents. The successful dispersion of purified SWNTs in DMA has enabled extinction coefficients of 43.4 and 39.0 mL·mg-1·cm-1 to be calculated at the selected energies, respectively. The magnitude of the dispersion limit and extinction coefficient values has been shown to be strongly dependent on the SWNT sample purity. These findings offer the potential for solution-phase analysis of SWNTs directed at purity assessment and electrophoretic separations in a simple organic solvent.
  • Article
    A systematic study has been performed in order to find an appropriate medium for solubilization/dispersion of pristine single-walled carbon nanotubes (SWCNTs). Five solvents, all featuring high electron pair donicity (β) and low hydrogen bond parameter (α) have demonstrated the ability to readily form stable dispersions. The best dispersions have been characterized by UV/visible-NIR spectra, ESR spectra, and atomic force microscopy (AFM).
  • Article
    We present a new hybrid meta exchange-correlation functional, called M05-2X, for thermochemistry, thermochemical kinetics, and noncovalent interactions. We also provide a full discussion of the new M05 functional, previously presented in a short communication. The M05 functional was parametrized including both metals and nonmetals, whereas M05-2X is a high-nonlocality functional with double the amount of nonlocal exchange (2X) that is parametrized only for nonmetals. In particular, M05 was parametrized against 35 data values, and M05-2X is parametrized against 34 data values. Both functionals, along with 28 other functionals, have been comparatively assessed against 234 data values:  the MGAE109/3 main-group atomization energy database, the IP13/3 ionization potential database, the EA13/3 electron affinity database, the HTBH38/4 database of barrier height for hydrogen-transfer reactions, five noncovalent databases, two databases involving metal−metal and metal−ligand bond energies, a dipole moment database, a database of four alkyl bond dissociation energies of alkanes and ethers, and three total energies of one-electron systems. We also tested the new functionals and 12 others for eight hydrogen-bonding and stacking interaction energies in nucleobase pairs, and we tested M05 and M05-2X and 19 other functionals for the geometry, dipole moment, and binding energy of HCN−BF3, which has recently been shown to be a very difficult case for density functional theory. We tested eight functionals for four more alkyl bond dissociation energies, and we tested 12 functionals for several additional bond energies with varying amounts of multireference character. On the basis of all the results for 256 data values in 18 databases in the present study, we recommend M05-2X, M05, PW6B95, PWB6K, and MPWB1K for general-purpose applications in thermochemistry, kinetics, and noncovalent interactions involving nonmetals and we recommend M05 for studies involving both metallic and nonmetallic elements. The M05 functional, essentially uniquely among the functionals with broad applicability to chemistry, also performs well not only for main-group thermochemistry and radical reaction barrier heights but also for transition-metal−transition-metal interactions. The M05-2X functional has the best performance for thermochemical kinetics, noncovalent interactions (especially weak interaction, hydrogen bonding, π···π stacking, and interactions energies of nucleobases), and alkyl bond dissociation energies and the best composite results for energetics, excluding metals.
  • Article
    Pyrene-containing polymers have been prepared for noncovalent sidewall functionalization of multiwalled carbon nanotubes (MWNTs). (1-Pyrene)methyl 2-methyl-2-propenoate (PyMMP) has been synthesized and copolymerized with methyl methacrylate (MMA). Poly(ethylene-co-butylene)-b-poly(MMA-co-PyMMP) diblocks have also been synthesized. The surface of MWNTs, produced by both the CCVD and arc discharge methods, has been modified by these copolymers for making them dispersible in a variety of organic solvents. The modified MWNTs have been characterized by thermogravimetric analysis, transmission electron microscopy, and atomic force microscopy.
  • Article
    Durch chemische Funktionalisierung einwandiger Kohlenstoffnanoröhren werden die Voraussetzungen für mögliche Anwendungen solcher Nanostrukturen geschaffen. Derivatisierte Röhren zeichnen sich im Unterschied zum Rohmaterial u. a. durch eine gute Löslichkeit aus und sind so einer umfangreichen Charakterisierung und Folgechemie zugänglich. Gängige Derivatisierungsmethoden umfassen die Defekt- und kovalente Seitenwandfunktionalisierung sowie die nichtkovalente exo- und endohedrale Funktionalisierung. Auf diese Weise wurde z. B. eine Reihe von seitenwandsubstituierten, polymerumwickelten oder gastmolekülhaltigen Nanoröhren erhalten. Der gegenwärtige Stand der Forschung wird in diesem Kurzaufsatz vorgestellt.
  • Article
    Microscale aggregate formation, resulting from high intrinsic filler attractions, is one of the major issues in nanocomposite preparation and processing. Herein, the dispersive effects achieved by a wide range of surface-active agents, as well as surface oxidation and functionalization, are investigated. The aim of our research is to form a uniform, multiwalled carbon nanotube (MWNT) distribution in water-soluble (poly(ethylene glycol)) and water-insoluble (polypropylene) polymers. In order to understand the surface-charge-related stability of the treated nanotubes solutions, zeta-potential measurements are applied. Quantification of the state of the MWNT dispersion is derived from particle-size analysis, while visual characterization is based on optical and electron microscopy. To estimate the nucleating ability of the surface-modified carbon nanotubes, the temperature of crystallization and the degree of crystallinity are calculated from differential scanning thermograms. Finally, we suggest general guidelines to produce uniform MWNT dispersions using a dispersive agent and/or surface treatment in water-soluble and water-insoluble polymers.
  • The effect of pyrene distribution within pyrene-functionalized random and block copolymers on noncovalent polymer/single-walled carbon nanotube (SWNT) interactions was investigated. The block copolymers served as superior solubilizing agents in comparison with the random copolymers. Also, increasing the pyrene content within a polymer, while a constant molecular weight was maintained, improved SWNT solubility and therefore had to result in stronger polymer–nanotube interactions. However, increasing the length of the pyrene-containing block diminished nanotube solubility, likely because of a lower number of polymer chains that were capable of binding to the nanotube surface. Atomic force microscopy and transmission electron microscopy indicated that the polymer–SWNT interactions were capable of partially debundling the nanotubes into individual solvated structures. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1941–1951, 2006
  • Article
    Stable dispersions of single-walled carbon nanotubes in deionized water were prepared using six common surfactants: sodium dodecylbenzene sulfonate (SDBS), sodium dodecyl sulfate (SDS), lithium dodecyl sulfate (LDS), tetradecyl trimethyl ammonium bromide (TTAB), sodium cholate (SC), and Fairy liquid (FL). For all nanotube dispersions (C-NT = 1 mg/mL), the optimum concentration of surfactant was found to be close to C-Surf = 10 mg/mL by measuring the fraction of nanotubes remaining after centrifugation for a range of surfactant concentrations. The aggregation state of each nanotube-surfactant dispersion was characterized as a function of nanotube concentration by AFM analysis of large numbers of nanotubes/bundles deposited onto substrates. The dispersion quality could then be quantified by four parameters: the saturation value (at low concentration) of the root-mean-square bundle diameter, the maximum value of the total number of dispersed objects (individuals and bundles) per unit volume of dispersion, the saturation value (at low concentration) of the number fraction of individual nanotubes, and the maximum value of the number of individual nanotubes per unit volume of dispersion. According to these metrics, the dispersion quality of the six surfactant-nanotube dispersions varied as SDS > LDS > SDBS > TTAB > SC > Fairy liquid. It was found that each of these dispersion-quality metrics scaled very well with the measured zeta-potential of the surfactant-nanotube dispersion. This confirms that dispersion quality is controlled by the magnitude of electrostatic repulsive forces between coated nanotubes.
  • Article
    A family of poly[(m-phenylenevinylene)-co-(p-phenylenevinylene)]s, functionalized in the synthetically accessible C-5 position of the meta-disubstituted phenylene rings have been designed and synthesized: they are essentially poly{(5-alkoxy-m-phenylenevinylene)-co-[(2,5-dioctyloxy-p-phenylene)-vinylene]} (PAmPV) derivatives. A range of these PAmPV polymers have been prepared both (1) by the polymerization of O-substituted 5-hydroxyisophthaldehydes and (2) by chemical modifications carried out on polymers bearing reactive groups at the C-5 positions. PAmPV polymers solubilize SWNT bundles in organic solvents by wrapping themselves around the nanotube bundles. PAmPV derivatives which bear tethers or rings form pseudorotaxanes with rings and threads, respectively. The formation of the polypseudorotaxanes has been investigated in solution by NMR and UV/vis spectroscopies, as well as on silicon oxide wafers in the presence of SWNTs by AFM and surface potential microscopy. Wrapping of these functionalized PAmPV polymers around SWNTs results in the grafting of pseudorotaxanes along the walls of the nanotubes in a periodic fashion. The results hold out the prospect of being able to construct arrays of molecular switches and actuators.
  • Article
    The incorporation of carbon nanotubes to a polymer generally improves the stiffness and strength of the polymer, but the ductility and toughness of the polymer are compromised in most cases. Here we report mechanical reinforcement of polyethlene (PE) using polyethlene grafted multiwalled carbon nanotubes (PE-g-MWNTs). The stiffness, strength, ductility and toughness of PE are all improved by the addition of PE-g-MWNTs. The grafting of PE onto MWNTs enables the well-dispersion of nanotubes in the PE matrix and improves MWNT/PE interfacial adhesion. The grafting was achieved by a reactive blending process through melt blending of PE containing 0.85 wt % of maleic anhydride and amine-functionalized MWNTs. The reaction between maleic anhydride and amine groups as evidenced by X-ray photoelectron spectroscopy and Raman spectroscopy, leads to the grafting of PE onto the nanotubes.
  • Article
    We have investigated a wide variety of surfactants for their efficiency in dissolving isolated single-walled carbon nanotubes (SWNTs) in water. In doing so, we have completely avoided the harsh chemical or mechanical conditions, such as acid or ultrasonic treatments, that are known to damage SWNTs. Bile salts in particular are found to be exceptionally effective in dissolving individual tubes, as evidenced by highly resolved optical absorption spectra, bright bandgap fluorescence, and the unprecedented resolution (similar to2.5 cm(-1)) of the radial breathing modes in Raman spectra. This is attributed to the formation of very regular and stable micelles around the nanotubes providing an unusually homogenous environment. Quantitative information concerning the degree of solubilization is obtained from absorption spectroscopy.
  • Article
    Intimate electrical contact occurs between a substituted poly(metaphenylenevinylene) (PmPv) and bundles of single-walled nanotubes (SWNT) as evidenced by atomic force microscopy, optical, and electronic measurements carried out on single, isolated SWNT/PmPv structures (see picture). PmPV may provide a useful route toward "functionalizing" the SWNT without destroying their electrical character.
  • Article
    We review experimental and theoretical work on electrical percolation of carbon nanotubes (CNT) in polymer composites. We give a comprehensive survey of published data together with an attempt of systematization. Parameters like CNT type, synthesis method, treatment and dimensionality as well as polymer type and dispersion method are evaluated with respect to their impact on percolation threshold, scaling law exponent and maximum conductivity of the composite. Validity as well as limitations of commonly used statistical percolation theories are discussed, in particular with respect to the recently reported existence of a lower kinetic (allowing for re-aggregation) and a higher statistical percolation threshold.
  • Article
    Suitable oxidative treatment of catalytically grown carbon nanotubes introduces oxygen containing surface groups. Infrared and titration studies indicate that they are predominately phenolic, carboxylic and lactonic groups. These groups stabilize dispersions of nanotubes at much higher concentrations than are possible with the raw material. Plots of the viscosity of dispersions as a function of their concentration show a dramatic increase in gradient above a critical concentration, leading to the formation of viscoelastic gels. During continued drying, the solvent mediates the formation of dense assemblies of nanotubes which then bond together through the surface groups. If the nanotubes are deposited from a dilute suspension by filtration they are able to maximize the number of intertube contacts by packing into a locally parallelized structure, reminiscent of liquid crystalline polymers.
  • Article
    The preparation of a new type of finite carbon structure consisting of needlelike tubes is reported. Produced using an arc-discharge evaporation method similar to that used for fullerene sythesis, the needles grow at the negative end of the electrode used for the arc discharge. Electron microscopy reveals that each needle comprises coaxial tubes of graphitic sheets ranging in number from two up to about 50. On each tube the carbon-atom hexagons are arranged in a helical fashion about the needle axis. The helical pitch varies from needle to needle and from tube to tube within a single needle. It appears that this helical structure may aid the growth process. The formation of these needles, ranging from a few to a few tens of nanometers in diameter, suggests that engineering of carbon structures should be possible on scales considerably greater than those relevant to the fullerenes.
  • Article
    The influence of a single walled carbon nanotube on the structure of a cholesterol cluster (domain) developed over the surface of the endothelial protein 1LQV has been investigated using the classical molecular dynamics (MD) simulation technique. We have observed a substantial impact of carbon nanotube on the arrangement of the cholesterol domain. The carbon nanotube can drag out cholesterol molecules, remarkable reducing the volume of the domain settled down on the protein.
  • Dispersions of multi-walled carbon nanotubes (MWNTs) assisted by surfactant adsorption were prepared for a number of ionic and non-ionic surfactants including sodium 4-dodecylbenzenesulfonate (NaDDBS), hexadecyl(trimethyl)azanium bromide (CTAB), sodium dodecane-1-sulfonate (SDS), Pluronic® F68, Pluronic® F127, and Triton® X-100 to examine the effects of nanotube diameter, surfactant concentration, and pH on nanotube dispersability. Nanotube diameter was found to be an important role in surfactant adsorption rendering single-walled carbon nanotube studies as unreliable in predicting MWNT dispersive behavior. Similar to other reports, increasing surfactant concentrations resulted in a solubility plateau. Quantification of nanotube solubility at these plateaus demonstrated that CTAB is the best surfactant for MWNTs at neutral pH conditions. Deviations from neutral pH demonstrated negligible influence on non-ionic surfactant adsorption. In contrast, both cationic and anionic surfactants were found to be poor dispersing aids for highly acidic solutions while, CTAB remained a good surfactant under strongly basic conditions. These pH dependent results were explained in the context of nanotube surface ionization and Debye length variation.
  • Article
    Full-text available
    Electromechanical actuators based on sheets of single-walled carbon nanotubes were shown to generate higher stresses than natural muscle and higher strains than high-modulus ferroelectrics. Like natural muscles, the macroscopic actuators are assemblies of billions of individual nanoscale actuators. The actuation mechanism (quantum chemical-based expansion due to electrochemical double-layer charging) does not require ion intercalation, which limits the rife and rate of faradaic conducting polymer actuators. Unlike conventional ferroelectric actuators, low operating voltages of a few volts generate large actuator strains. Predictions based on measurements suggest that actuators using optimized nanotube sheets may eventually provide substantially higher work densities per cycle than any previously known technology. Electromechanical actuators based on sheets of single-walled carbon nanotubes were shown to generate higher stresses than natural muscle and higher strains than high-modulus ferroelectrics. Like natural muscles, the macroscopic actuators are assemblies of billions of individual nanoscale actuators. The actuation mechanism (quantum chemical-based expansion due to electrochemical double-layer charging) does not require ion intercalation, which limits the life and rate of faradaic conducting polymer actuators. Unlike conventional ferroelectric actuators, low operating voltages of a few volts generate large actuator strains. Predictions based on measurements suggest that actuators using optimized nanotube sheets may eventually provide substantially higher work densities per cycle than any previously known technology.
  • Article
    A method is developed to enable emulsion polymerization to be performed under RAFT control to give living character without the problems that often affect such systems: formation of an oily layer, loss of colloidal stability, or loss of molecular weight control. Trithiocarbonate RAFT agents are used to form short stabilizing blocks from a water-soluble monomer, from which diblocks can be created by the subsequent polymerization of a hydrophobic monomer. These diblocks are designed to self-assemble to form micelles. Polymerization is initially performed under conditions that avoid the presence of monomer droplets during the particle formation stage and until the hydrophobic ends of the diblocks have become sufficiently long to prevent them from desorbing from the newly formed particles. Polymerization is then continued at any desired feed rate and composition of monomer. The polymer forming in the reaction remains under RAFT control throughout the polymerization; molecular weight polydispersities are generally low. The number of RAFT-ended chains within a particle is much larger than the aggregation number at which the original micelles would have self-assembled, implying that in the early stages of the polymerization, there is aggregation of the micelles and/or migration of the diblocks. The latexes resulting from this approach are stabilized by anchored blocks of the hydrophilic monomer, e.g., acrylic acid, with no labile surfactant present. Sequential polymerization of two hydrophobic monomers gives completely novel core-shell particles where most chains extend from the core of the particles through the shell layer to the surface.
  • Article
    A review on the controlled/living radical polymerization (CLRP) in dispersed systems is given so as to show its use from 2007 up to early 2008. This includes: the mechanisms of CLRP; CLRP in dispersed systems; the cross-linking of CLRP in dispersed systems; and lastly its particle morphology. Regarding the CLRP in dispersed systems, the following were also discussed. The colloidal stability, the theoretical aspects of CLRP in dispersed systems, the miniemulsion polymerization as well as its emulsion polymerization, the miscellaneous miniemulsion and emulsion CLRP, the microemulsion polymerization, and finally the dispersion and precipitation polymerizations.
  • Article
    Radical polymerization is one of the most widely used processes for the commercial production of high-molecular-weight polymers. The main factors responsible for the preeminent position of radical polymerization are the ability to polymerize a wide array of monomers, tolerance of unprotected functionality in monomer and solvent, and compatibility with a variety of reaction conditions. Radical polymerization is simple to implement and inexpensive in relation to competitive technologies. However, conventional radical polymerization severely limits the degree of control that researchers can assert over molecular-weight distribution, copolymer composition, and macromolecular architecture.