Microscale polymer-nanotube composites

Effects of particle stiffness on the extensional rheology of model rod-like nanoparticle suspensions

The linear and nonlinear rheological behavior of two rod-like particle suspensions as a function of concentration is studied using small amplitude oscillatory shear, steady shear and capillary breakup extensional rheometry. The rod-like suspensions are composed of fd virus and its mutant fdY21M, which are perfectly monodisperse, with a length on the order of 900 nm. The particles are semiflexible yet differ in their persistence length. The effect of stiffness on the rheological behavior in both, shear and extensional flow, is investigated experimentally. The linear viscoelastic shear data is compared in detail with theoretical predictions for worm-like chains. The extensional properties are compared to Batchelor's theory, generalized for the shear thinning nature of the suspensions. Theoretical predictions agree well with the measured complex moduli at low concentrations as well as the nonlinear shear and elongational viscosities at high flow rates. The results in this work provide guidelines for enhancing the elongational viscosity based on purely frictional effects in the absence of strong normal forces which are characteristic for high molecular weight polymers.

Nanoaramid Dressed Latex Particles: The Direct Synthesis via Pickering Emulsion Polymerization

The direct synthesis of polymer microspheres modified by aramid nanofibers (ANFs) is an interesting challenge. This work describes a simple aqueous process to prepare polystyrene (PS)/ANF composite microspheres, where the specific ANF network was "dressed" on PS. ANF was derived from the copolymerization of terephthaloyl chloride, p-phenylene diamine, and methoxypolyethylene glycol and could be dispersed in water stably. We applied the as-synthesized ANF as a Pickering emulsifier in the o/w emulsion of styrene monomer. Radical polymerization was subsequently initiated in the Pickering emulsion system. The combination of ANF with polymer spheres was revealed by scanning electron microscopy (SEM) and thermal gravity analysis. The role of ANF in the monomer emulsion as well as in the polymerization was studied through SEM, optical microscopy, optical stability analyzer, and pulse nuclear magnetic resonance combined with the polymerization kinetic analysis. Moreover, we investigated the effects of other synthesis parameters, such as monomer type, monomer content, pH value, and salt concentration.

Coiling ultrathin tellurium nanowires into nanorings by Pickering emulsion

Well-defined hydrophilic ultrathin tellurium nanowires (TeNWs) can be coiled into nanorings by Pickering emulsion at room temperature.

Self-assembly of nanomaterials at fluid interfaces

Recent developments in the field of the self-assembly of nanoscale materials such as nanoparticles, nanorods and nanosheets at liquid/liquid interfaces are reviewed. Self-assembly behavior of both biological and synthetic particles is discussed. For biological nanoparticles, the nanoparticle assembly at fluid interfaces provides a simple route for directing nanoparticles into 2D or 3D constructs with hierarchical ordering. The interfacial assembly of single-walled carbon nanotubes (SWCNTs) at liquid interfaces would play a key role in applications such as nanotube fractionation, flexible electronic thin-film fabrication and synthesis of porous SWCNT/polymer composites foams. Liquids can be structured by the jamming of nanoparticle surfactants at fluid interfaces. By controlling the interfacial packing of nanoparticle surfactants using external triggers, a new class of materials can be generated that combines the desirable characteristics of fluids such as rapid transport of energy carriers with the structural stability of a solid.

Enveloping self-assembly of carbon nanotubes at copolymer micelle cores

Carbon nanotubes (CNTs) and their polymer nanocomposites are interesting materials for future applications, for example in optics or electronics. Research faces two major challenges with these outstanding nanofillers: control over dispersion and spatial arrangement within the nanocomposite, both required to achieve optimal structure and properties of CNT-based nanocomposites. We report on novel self-assembled multiwall CNT (MWCNT)/block copolymer (BCP) nanostructures realized by patterning MWCNTs with amphilphilic diblock copolymer micelles. A high molecular weight poly(styrene)-b-poly(2-vinylpyridine) BCP which forms large micelles (250 nm) was chosen to facilitate the templating by reducing the bending energy induced in the MWCNTs. We tested the hypothesis that it is possible to use an amphiphilic BCP as a dispersing agent and its spherical micelles as a template at the same time without modification of the CNTs. In thin films of the MWCNT/BCP micelles, highly separated MWCNTs were repeatedly observed which enveloped the core of the BCP micelles, i.e., the unfunctionalized MWCNTs segregated to the interface between the two BCP phases. Depending on the size of the MWCNTs, ring-like (split-ring) or network forming structures were obtained. The MWCNT templating mechanism, i.e., the segregation to the interface, is explained by the interfacial tension within the BCP interface and the chain entropy. The reported new complex nanocomposite has potential to be applied for example as cost-effective split-ring resonators for metamaterials or for conductive polymer films with an extremely low percolation threshold.

Purifying colloidal nanoparticles through ultracentrifugation with implications for interfaces and materials

Liquid-phase processing and colloidal self-assembly will be critical to the successful implementation of nanotechnology in the next generation of materials and devices. A key hurdle to realizing this will be the development of efficient methods to purify nanomaterials composed of a variety of shapes, including nanocrystals, nanotubes, and nanoplates. Although density-gradient ultracentrifugation (DGU) has long been appreciated as a valuable tool for separating biological macromolecules and components, the method has recently emerged as an effective way to purify colloidal nanoparticles by size and optical and electronic properties. In this feature article, we review our recent contributions to this growing field, with an emphasis on some of the implications that our results have for interfaces and materials. Through transient or isopycnic DGU performed in both aqueous and organic environments, we demonstrate some explicit examples of how the mechanical, electronic, and optical properties of thin films assembled from two specific colloidal nanomaterials--single-walled carbon nanotubes and silicon nanocrystals--can be modified in response to fractionation.

Strings of polymer microspheres stabilized by oxidized carbon nanotubes

Oxidized carbon nanotubes (CNTOs) with hydrophilic oxygen-containing functional groups and hydrophobic conjugated structure are prepared by the oxidation of carbon nanotubes (CNTs). After the polymerization of styrene with CNTOs dispersed in aqueous phase, polystyrene (PS) microspheres with string-like structure are obtained. Thermogravimetic analysis (TGA), differential scanning calorimeter (DSC) and Raman results indicate the strong interaction between the separated PS chains from the oil phase and CNTOs during the initial stage of the polymerization. These adsorbed PS chains on the surface of CNTOs are quickly swollen by the monomer and they grow in size during the further polymerization. The pH value and the ion strength of aqueous phase obviously affect the stability of PS microspheres. The particle size of microspheres is also determined by the pH. We demonstrate that the one-dimensional structure of CNTOs is responsible for the formation of polymer microspheres with special architecture.

Assembly of acid-functionalized single-walled carbon nanotubes at oil/water interfaces

The efficient segregation of water-soluble, acid-functionalized, single-walled carbon nanotubes (SWCNTs) at the oil/water interface was induced by dissolving low-molecular-weight amine-terminated polystyrene (PS-NH2) in the oil phase. Salt-bridge interactions between carboxylic acid groups of SWCNTs and amine groups of PS drove the assembly of SWCNTs at the interface, monitored by pendant drop tensiometry and laser scanning confocal microscopy. The impact of PS end-group functionality, PS and SWCNT concentrations, and the degree of SWCNT acid modification on the interfacial activity was assessed, and a sharp drop in interfacial tension was observed above a critical SWCNT concentration. Interfacial tensions were low enough to support stable oil/water emulsions. Further experiments, including potentiometric titrations and the replacement of SWCNTs by other carboxyl-containing species, demonstrated that the interfacial tension drop reflects the loss of SWCNT charge as the pH falls near/below the intrinsic carboxyl dissociation constant; species lacking multivalent carboxylic acid groups are inactive. The trapped SWCNTs appear to be neither ordered nor oriented.

Dispersions of attractive semiflexible fiberlike colloidal particles from bacterial cellulose microfibrils

We prepared dispersions from bacterial cellulose microfibrils (CMF) of a commercial Nata de Coco source. We used an ultra-high-energy mechanical deagglomeration process that is able to disperse the CMFs from the pellicle in which they are organized in an irregular network. Because of the strong attractions between the CMFs, the dispersion remained highly heterogeneous, consisting of fiber bundles, flocs, and voids spanning tens to hundreds of micrometers depending on concentration. The size of these flocs increased with CMF concentration, the size of the bundles stayed constant, and the size of the voids decreased. The observed percolation threshold in MFC dispersions is lower than the theoretical prediction, which is accounted for by the attractive interactions in the system. Because bacterial cellulose is chemically very pure, it can be used to study the interaction of attractive and highly shape-anisotropic, semiflexible fiberlike colloidal particles.

Electronic durability of flexible transparent films from type-specific single-wall carbon nanotubes

The coupling between mechanical flexibility and electronic performance is evaluated for thin films of metallic and semiconducting single-wall carbon nanotubes (SWCNTs) deposited on compliant supports. Percolated networks of type-purified SWCNTs are assembled as thin conducting coatings on elastic polymer substrates, and the sheet resistance is measured as a function of compression and cyclic strain through impedance spectroscopy. The wrinkling topography, microstructure and transparency of the films are independently characterized using optical microscopy, electron microscopy, and optical absorption spectroscopy. Thin films made from metallic SWCNTs show better durability as flexible transparent conductive coatings, which we attribute to a combination of superior mechanical performance and higher interfacial conductivity.

Separation of empty and water-filled single-wall carbon nanotubes

The separation of empty and water-filled laser ablation and electric arc synthesized nanotubes is reported. Centrifugation of these large-diameter nanotubes dispersed with sodium deoxycholate using specific conditions produces isolated bands of empty and water-filled nanotubes without significant diameter selection. This separation is shown to be consistent across multiple nanotube populations dispersed from different source soots. Detailed spectroscopic characterization of the resulting empty and filled fractions reveals that water filling leads to systematic changes to the optical and vibrational properties. Furthermore, sequential separation of the resolved fractions using cosurfactants and density gradient ultracentrifugation reveals that water filling strongly influences the optimal conditions for metallic and semiconducting separation.

Carbon nanotubes: measuring dispersion and length

Advanced technological uses of single-walled carbon nanotubes (SWCNTs) rely on the production of single length and chirality populations that are currently only available through liquid-phase post processing. The foundation of all of these processing steps is the attainment of individualized nanotube dispersions in solution. An understanding of the colloidal properties of the dispersed SWCNTs can then be used to design appropriate conditions for separations. In many instances nanotube size, particularly length, is especially active in determining the properties achievable in a given population, and, thus, there is a critical need for measurement technologies for both length distribution and effective separation techniques. In this Progress Report, the current state of the art for measuring dispersion and length populations, including separations, is documented, and examples are used to demonstrate the desirability of addressing these parameters.

Sequential coating of magnetic carbonyliron particles with polystyrene and multiwalled carbon nanotubes and its effect on their magnetorheology

A two-step process for the sequential coating of magnetic carbonyliron (CI) particles with polystyrene (PS) and multiwalled carbon nanotubes (MWCNTs) was used to improve the sedimentation stability of micrometer-sized magnetic CI particles for magnetorheological (MR) applications under an applied magnetic field. The CI particles were initially coated with nanosized PS beads using an in situ dispersion polymerization method and then wrapped with a dense MWCNT nest through a solvent-casting method in a water/oil emulsion system. The morphology, MR performance, and sedimentation stability of the synthesized magnetic composite particles were examined. The composite particles showed enhanced MR characteristics and dispersion stability.