Light‐driven atom transfer radical polymerization on supramolecular complexes of conjugated polymers and single‐walled carbon nanotubes

Previous approaches used to decorate latently reactive conjugated polymer‐coated carbon nanotube complexes have utilized “grafting‐to” strategies. Here, we coat the carbon nanotube surface with a conjugated polymer whose side chains contain the radical initiator, α‐bromoisobutyrate, which enables atom transfer radical polymerization (ATRP) from the polymer–nanotube surface. Using light to generate Cu(I) in situ, ATRP is used to grow narrow dispersity polymer chains from the polymer–nanotube surface. We confirm the successful polymerization of (meth)acrylates from the polymer–nanotube surface using a combination of gel permeation chromatography and infrared spectroscopy. Strikingly, we demonstrate that nanotube optoelectronic properties are preserved after radical‐mediated polymer grafting using Raman spectroscopy and photoluminescence mapping. Overall, this work elucidates a method to grow narrow dispersity polymer chains from the polymer–nanotube surface using light‐driven radical chemistry, with concurrent preservation of nanotube optoelectronic properties. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2015–2020     

Solubilizing single‐walled carbon nanotubes with pyrene‐functionalized block copolymers

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     

Adsorption properties of comb-like polymer on nanotube surface

The adsorption and wrapping process of a single flexible comb-like polymer to a single wall nanotube was studied by Molecular Dynamics simulation of a coarse-grained model. We varied the grafting density and length of the side chains, the radius of the nanotube and strength of interaction between the monomers of nanotube and side chains of polymer brush. We investigated the structural and dynamical characters of interactions of the nanotube-polymer composite, such as the effect of Lennard-Jones energy parameter ɛ_LJ and the nanotube radius on the adsorption behavior and how the wrapping conformation is affected by the structure of the polymer brush. The simulation results indicate that single comb-like polymer with flexible backbone tends to adsorb and wrap around the nanotube, when the interaction energy exceeds a critical value. The monomer adsorption ratio, interaction energy profiles and moment of inertia are obtained. The helical wrapping only occurs when the interaction energy is large enough. Also, the influence of the polymer structure on the conformational behavior is analyzed. This work underscores design elements important for engineering well-defined nanotube-polymer nanocomposite.     

Polymerization and characterization of self‐assembled β‐cyclodextrin‐threaded chiral monomers

A helical inclusion complex polymer was fabricated through the polymerization of β‐cyclodextrin‐threaded chiral monomers. The photo induced polymerization of inclusion complex clusters caused shrinkage of the polymer and decreased the pitches, leading to the disappearance of spring‐like construction under TEM. From the results of circular dichroism of the inclusion complex polymer, the helical construction was confirmed, and an entanglement of the polymer chains is proposed. After removal of the β‐cyclodextrins from the pendant groups of the inclusion complex polymer, the helical structure was found to be maintained. The highly ordered molecular arrangement of β‐cyclodextrins removed from the inclusion complex polymer was confirmed using POM. Here we demonstrate the fabrication of helical polymer fibers composed of entangled polymers through self‐assembled β‐cyclodextrin‐threaded chiral monomers. The helical polymer construction was maintained by the entwisted polymer chains even after the removal of β‐cyclodextrins from the pendant groups of the inclusion complex polymer. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2975–2981, 2010     

Circular Dichroism Study of the Inclusion-Dissociation Behavior of Complexes between a Molecular Nanotube and Azobenzene Substituted Linear Polymers

We have investigated the inclusion properties of molecular nanotubes composed ofcrosslinked -cyclodextrin. Induced circular dichroism was used to probe theformation and dissociation of complexes between the nanotubes and azobenzenemodified linear polymers. The polymer was poly(ethylene glycol) (PEG), either withor without a hydrophobic alkyl chain.It was found that the inclusion complex betweenthe nanotubes and polymers formed at room temperature, and that the polymers dissociated from the nanotubes with increasing temperature. Further, the polymer with hydrophobic alkyl chain was bound inside the nanotube more strongly and dissociated more abruptly with increasing temperature than its hydrophilic counterpart as expected theoretically.     

Threading-unthreading equilibrium in solution of molecular nanotubes and linear flexible polymer chains

Using the Flory-Huggins lattice model, we investigate the threading-unthreading equilibrium in a solution of linear flexible polymer chains and molecular nanotubes formed by covalently bonding the ringlike molecules, such as cyclodextrins (CDs). It is found that the threading-unthreading equilibrium depends on the temperature and the molar concentrations of the ringlike molecules and polymer chain segments but is independent of the polymer chain length, which agrees with the experimental observations. By fitting the experimental data of alpha-CD and poly(ethylene glycol) (PEG), the inclusion energy between alpha-CD and PEG, which includes the conformation energy loss of PEG resulted from the inclusion, is calculated to be approximately -20.45 kJ/mol per PEG unit.     

Preparation of stimulus-responsive,polyfluorene-wrapped carbon nanotubes via palladium cross coupling

Decoration of carbon nanotube surfaces without damaging nanotube optoelectronic properties is an ongoing challenge. Here, we utilize Sonogashira coupling chemistry to decorate the nanotube surface without perturbing optoelectronic properties. Reactive, noncovalently functionalized polymer–nanotube complexes were prepared using a polyfluorene with aryl iodide groups in its side chains. The aryl iodides enable Pd cross coupling between polymer–nanotube complexes and small molecules or polymers derivatized with an alkyne. Modestly efficient coupling was found to occur under dilute conditions at elevated temperatures. Successful coupling between aryl iodide and alkyne partners was observed using infrared spectroscopy via the appearance of carbonyl stretches that originate from covalently linked, carbonyl-containing alkynes, and thermogravimetric analysis was used to measure reaction conversion under various conditions. Grafting of the hydrophobic polymer–nanotube complex with poly(ethylene glycol) enabled the dispersion to be transferred from organic to aqueous solution. This chemistry resulted in no damage to the nanotube sidewall, as evidenced by Raman spectroscopy. The aryl iodide-containing polyfluorene–nanotube complex was also coupled to a photoswitchable alkyne-containing spiropyran moiety and it was found that the photoswitch retained its functionality after coupling to the polymer–nanotube complex. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2723–2729     

Architectural control of polystyrene physical properties using branched anionic polymerization initiated at ambient temperature

Ambient temperature-initiated anionic polymerization has generated branched polystyrenes of varying molecular weights and architectures by inclusion of a distyryl branching comonomer into a conventional sec-Butylithium-initiated polymerization of styrene. Primary chain length control within the branched polymers, and restriction of the branching points to varying segments of the primary chains, led to variations of glass transition temperature with no direct correlation to the branched polymer molecular weight but a strong relationship to the length of individual chains comprising the branched macromolecules.     

Conformational behavior of polymers adsorbed on nanotubes

The importance of hydrophobic interactions in determining polymer adsorption and wrapping of carbon nanotubes is still under debate. In this work, we concentrate on the effect of short-ranged weakly attractive hydrophobic interactions between polymers and nanotubes (modeled as an infinitely long and smooth cylindrical surface), neglecting all other interactions apart for chain flexibility. Using coarse-grained Monte Carlo simulation of such simplified systems, we find that uniform adsorption and wrapping of the nanotube occur for all degrees of chain flexibility for tubes with sufficiently large outer radii. However, the adsorbed conformations depend on chain stiffness, ranging from randomly adsorbed conformations of the flexible chain to perfect helical or multihelical conformations (in the case of more concentrated solutions) of the rigid chains. Adsorption appears to occur in a sequential manner, wrapping the nanotube nearly one monomer at a time from the point of contact. Once adsorbed, the chains travel on the surface of the cylinder, retaining their helical conformations for the semiflexible and rigid chains. Our findings may provide additional insight to experimentally observed ordered polymer wrapping of carbon nanotubes.     

Linear nano-templates of styrene and maleic anhydride alternating copolymers

Poly(styrene-alt-maleic anhydride) (SMA) self-assembles in aqueous solution to form nanotube structures. These can be used as templates to linearly guide the growth of a secondary polymeric or inorganic material. Templates are made starting from a basic SMA solution, followed by slow pH decrease by dialysis against deionized water, until a 50% degree of protonation is reached. The nanotube structure is composed of multiple polymer chains, associating sideways by π-stacking to form the nanotube walls. The SMA templates were used to grow linear composites, which shows the applicability of the template properties and also confirms the nanotube association mechanism. Linear polymer composites were formed using this SMA template: pyrrole was polymerized, silver nitrate was reduced to silver and silver cyanide nanowires were grown.     

Molecular dynamics simulations of capillary rise experiments in nanotubes coated with polymer brushes

The capillary filling of a nanotube coated with a polymer brush is studied by molecular dynamics simulations of a coarse-grained model, assuming various conditions for the fluid-wall and fluid-brush interactions. Whereas the fluid is modeled by simple point particles interacting with Lennard-Jones forces, the (end-grafted, fully flexible) polymers that form the brush coating are described by a standard bead-spring model. Our experiments reveal that capillary filling is observed even for walls that would not be wetted by the fluid, provided the polymer brush coating itself wets. Generally, it is found that the capillary rise always proceeds through a t1/2 law with time t while the underlying molecular mechanism differs for wettable and nonwettable walls. For wettable walls, fluid imbibition is compatible with the Lucas-Washburn mechanism whereby the total influx of matter drops steadily with growing chain length N and the meniscus speed goes through a minimum at intermediate chain lengths. Moreover, because of flow, the polymer brush reorganizes its structure by forming a dense plug of chain segments under the meniscus that follows the meniscus in its motion. When the tube wall does not wet, one observes no meniscus formation for short chains although the fluid seeps through the wet brush. For a brush coating with longer chains, axial segregation between the brush segments and the fluid occurs by a kind of diffusive spreading, reminiscent of invasion percolation transport in a random medium, leading to the formation of a moving meniscus. For even longer chains that reach the tube axis, the rise of a meniscus with vanishing curvature-like imbibition in a porous medium is observed to take place.     

The Nano-threading of Polymers

When guest polymers are threaded by host cyclodextrins (CDs) to form crystalline inclusion compounds (ICs), the included polymer chains are highly extended and separated from neighboring chains. This is a consequence of the stacking of the cyclic oligosaccharides, α-, β-, or γ-CD containing 6, 7, or 8 glucose units, respectively, which produces continuous narrow channels (~0.5–1.0 nm diameters), where the guest polymers are included and confined. Observations that illuminate several important aspects of the nano-threading of polymers to form polymer-CD-ICs are described. These include (i) the competitive CD threading of polymers with different chemical structures and molecular weights from their solutions containing suspended solid or dissolved CDs, (ii) the threading and insertion of undiluted liquid polymers into solid CDs, and (iii) suspension of polymer A or B-CD-IC crystals in a solution of polymer B or A and observation of the transfer of polymer B or A from solution to displace polymer A or B and form polymer B or A-CD-ICs, without dissolution of the CD-ICs. In addition, we report observations of polyolefins adsorbed on zeolites, where we believe the adsorbed polyolefin chains are actually threaded and absorbed into the interiors of the zeolite nano-pores, rather than adsorbed on the zeolite surfaces. All of the above observations were made to assist in answering the question “Why do randomly-coiling polymer chains in solution or the melt become threaded or thread into the nano-pores of dissolved or solid CDs and solid zeolites, where they are highly extended and segregated from other polymer chains?” Though still not fully able to answer this question, we are able to assess the importance of several factors that have been previously suggested to be important in the formation of CD-ICs with both polymer and small-molecule guests and to the nano-threading of polymers in general. In particular, the value in observations of the inclusion of guest polymers, as well as small-molecule guests, into solid CDs suspended in their solutions and in neat guest liquids were made apparent, because interactions between host CDs, between CDs and solvents, and between quests and solvents, which complicate and make understanding the formation of polymer-CD-ICs difficult, are either eliminated or can be independently varied in these experiments.     

Review of recent studies on interactions between polymers and nanotubes using molecular dynamic simulation

Nanotubes have extraordinary properties, which have attracted the attention of many researchers from diverse fields. The interaction between carbon nanotube (CNT) or boron nitride nanotube (BNNT) and polymer/copolymer/surfactant has shown potential improvement in properties and performance. This paper reviews the recent studies in this field obtained from molecular dynamics simulation calculations, focusing on the interaction energies between nanostructure and polymer, radial distribution function, diffusion and radius of gyration and some other physical chemistry properties. Recent studies show that the intermolecular interaction in mentioned systems is strongly influenced by the specific monomer structure of polymers. The high values of intermolecular interaction energy of such composites offer that an efficient load transfer exists at the interface between nanotube and polymer, which is of a key role in the composite reinforcement practical applications. Our study reviewed the possibility of wrapping CNT/BNNT/CNT bundles by polymers and also the effects of CNTs/CNT bundles’ length on the conformational behavior of polymer adsorbed on these nanostructures.     

Estimation of the molecular weights of noncovalent columnar polymers based on β-cyclodextrin by the measurement of the aggregation rate of their polymer inclusion complexes with polypropylene glycol

Noncovalent columnar polymers (NCPs) based on cyclodextrins (CD) are polymeric assemblies of molecules that have continuous hollow channels, the width of which is determined by the diameter of the cavity of the initial CDs. The repeating fragment in an NCP is the CD molecule. For NCPs that were obtained by the exclusion of polymer backbone macromolecule from the corresponding inclusion complexes (ICs) based on β-cyclodextrin (NCP_excl), the polymer length, expressed as the number of macrocycles in a single chain (n) is determined by the size of the included ligand, polypropylene glycol (PPG), and is the PPG polymerization degree divided by two. The determination of the molecular weight of an NCP obtained by the precipitation method (NCP_prec) is rather difficult, since they are present in the aggregated state rather than in the form of individual molecules in solution. To estimate the molecular weight of NCP_prec, an indirect method is used, which is based on the determination of the aggregation rate of the ICs formed as a result of the interaction between an NCP and polypropylene glycol with a fixed molecular weight (MW), in this case PPG 1000. The comparison of the aggregation rates of the inclusion between NCP_excb (which were synthesized using PPGs with different molecular weights) and PPG 1000 with the aggregation rate of the inclusion complex on the basis of NCP_prec provided the estimation for the MWs of single polymer chains. The fact that the samples of NCPprec contain ∼30% of the monomeric β-CD was taken into account when constructing the calibration curve. It was demonstrated that the MW of the polypropylene glycol corresponding to NCP_prec is 1320 Da. Consequently, ∼11–12 molecules of β-CD are included in the single chains of NCP_prec.     

Vibrational Spectroscopic and Mechanical Investigation of Carbon Nanotube-Reinforced Styrene-Butadiene Rubbers

This paper reports investigations carried out on elastomeric matrices based on a styrene-butadiene copolymer filled with multiwall carbon nanotubes. Stress-strain measurements of the composites demonstrate that carbon nanotubes bring significantly improvements in the mechanical properties with regard to the pure polymer. Raman spectroscopy, which is one of the most extensively employed methods for the characterization of carbon materials, has been applied for the analysis of the dependence of the Raman spectra on the nanotube content and on application of an uniaxial deformation. The results reveal a negligible stress transfer suggesting a weak interface between the tubes and the polymer chains.     

Synthesis of multiwalled carbon nanotube/fluorine‐containing poly(p‐phenylene benzoxazole) composites exhibiting greatly enhanced dielectric constants

A series of high‐performance polymer/carbon nanotube (CNT) composites with different nanotube contents have been prepared via condensation of N‐silylated diamino terminated precursor of the polymer with acid chloride‐functionalized CNTs and subsequent thermal cyclodehydration. The composites have been fully characterized by infrared and Raman spectroscopy, electron microscopy, and thermal analysis. Various interesting morphologic features including helical structures have been observed in the composites as a result of covalent attachment of the polymer. The composites exhibit excellent thermal stability and a significant improvement in the dielectric constant and mechanical strength with the inclusion of CNTs. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Exfoliation of a stack of platelets and intercalation of polymer chains: Effects of molecular weight,entanglement, and interaction with the polymer matrix

Exfoliation of a stack of sheets (a model for clay platelets) in a dynamic matrix of polymer chains is investigated by a computer simulation model. How the interplay between the thermodynamics (interaction-driven) and conformational (structural constraints) entropy affects the exfoliation of sheets is the subject of this study. A stack of four sheets with a small initial interlayer distance constitutes the layer on a discrete lattice. The layered platelets are immersed in a matrix represented by the mobile polymer chains which occupy a fraction (concentration) of the lattice sites. Both sheets and chains are modeled by the bond-fluctuation mechanism and execute their stochastic motion via Metropolis algorithm. An attractive and a repulsive interaction between the polymer matrix and platelets are considered. Exfoliation of the sheets is examined by varying the molecular weight of the polymer chains forming a dynamic network matrix with various degrees of entanglements. At low-molecular weight of the polymer, exfoliation is achieved with repulsive interaction and the exfoliation is suppressed with attractive matrix as sheets stick together via polymer mediated interaction introduced by intercalated polymer chains. Increasing the molecular weight of the polymer matrix suppresses the exfoliation of sheets primarily due to enhanced entanglement—at high-molecular weight (with the radius of gyration of polymer chains larger than the characteristic linear dimension of the platelets), the stacked (layered) morphology is arrested via entropic trapping and exfoliation ceases to occur. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2696–2710, 2008     

Monitoring Diffusion of Reptating Polymer Chains by a Direct Energy Transfer Method: A Monte Carlo Simulation

A kinetic Monte Carlo method was used to simulate the diffusion of reptating polymer chains across an interface. A time‐resolved fluorescence technique in conjunction with a direct energy transfer method was used to measure the extent of diffusion of dye‐labeled reptating polymer chains. The diffusion of donor‐ and acceptor‐labeled polymer chains between adjacent compartments was randomly generated. The fluorescence decay profiles of donor molecules were simulated at several diffusion steps to produce mixing of the polymer chains. Mixing ratios of donor‐ and acceptor‐labeled polymer chains in compartments were measured at various stages (snapshots) of diffusion. It was observed that for a given molecular weight, the average interpenetration contour length was found to be proportional to the mixing ratio. Monte Carlo analysis showed that the curvilinear diffusion coefficient is inversely proportional to the weight of polymer chains during diffusion.     

Confinement induced conformational changes in n-alkanes sequestered within a narrow carbon nanotube

While alkanes in solution exhibit predominantly extended conformations, nanoscale confinement of these chains within protein binding sites and synthetic receptors can significantly alter the conformer distribution. As a simple model for the effect of confinement on the conformation, we report molecular simulations of n-alkanes absorbed from a bulk solvent into narrow carbon nanotubes. We observe that confinement of butane, hexane, and tetracosane induces a trans to gauche conformational redistribution. Moreover, confined hexane and tetracosane exhibit cooperative interactions between neighboring dihedral angles, which promote a helical gauche conformation for the portions of the chain within the nanotube. Hexane absorbed into the nanotube from water or benzene exhibits essentially the same conformation regardless of the bulk solvent. The PMF between the nanotube and hexane along the central nanotube axis finds that nanotube absorption is favorable from aqueous solution but neutral from benzene. The interaction between hexane and the nanotube in water is dominated by the direct interaction between the alkane and the nanotube and weakly opposed by indirect water-mediated forces. In benzene, however, the direct alkane/nanotube interaction is effectively balanced by the indirect benzene-mediated interaction. Our simulations in water stand in difference to standard interpretations of the hydrophobic effect, which posit that the attraction between non-polar species in water is driven by their mutual insolubility.     

NMR observation of the conformations and motions of polymers confined to the narrow channels of their inclusion compounds

When polymers are guests in crystalline inclusion compounds (ICs) formed with small-molecule hosts, they occupy a unique environment. In a cocrystallization process the small-molecule host forms a crystalline lattice containing long narrow channels where the guest polymer chains are included. Because of the narrow channel diameter and because neighboring channels are separated by walls formed exclusively from the small-molecule host lattice, the included polymer chains are highly extended and separated from polymer chains in other IC channels. As a consequence, polymer-IC crystals provide a unique solid state environment for the included polymer chains and serve as models useful for assessing the contributions made by the inherent behavior of individual polymer chains to the properties of ordered, bulk polymers, which can be obscured by pervasive interactions between their tightly packed polymer chains. In this paper we describe the conformations and motions of polymer chains confined to the narrow channels of the following polymer-ICs: i. polyethylene and trans-1, 4-polybutadiene in their ICs with perhydrotriphenylene, ii. polyepsilon caprolactone and its diblock and triblock copolymers with polybutadiene and poly (ethylene oxide) in their ICs with urea, and iii. nylon-6 in its ICs with alpha-, beta-, and gamma-cyclodextrins. High resolution, solid state NMR serves as both the conformational (C-13 chemical shifts) and motional (relaxation times and line shapes) probe. Comparison with identical NMR measurements performed on the bulk homo- and copolymer samples permits us to draw several conclusions regarding the relationships between the conformations and motions of polymers and their dependence on their ordered solid state environments.