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Thermodynamically-driven exfoliation and self-assembly of pristine graphene sheets is shown to provide thermally and electrically functional polymer composites. The spreading of graphene sheets at a high energy liquid/liquid interface is driven by lowering the overall energy of the system, and provides for the formation of water-in-oil emulsions stabilized by overlapping graphene sheets. Polymerization of the oil phase, followed by removal of the dispersed water phase, produces inexpensive and porous composite foams. Contact between the graphene-stabilized water droplets provides a pathway for electrical and thermal transport through the composite. Unlike other graphene foams, the graphite used to synthesize these composites is natural flake material, with no oxidation, reduction, sonication, high temperature thermal treatment, addition of surfactants, or high shear mixing required. The result is an inexpensive, low-density material that exhibits Joule heating and displays increasing electrical conductivity with decreasing thermal conductivity. [ABSTRACT FROM AUTHOR]

Inexpensive, strong, and electrically conductive pristine graphene composites are synthesized from graphene-stabilized emulsions. These materials do not utilize oxidized graphite (GO) or reduced oxidized graphite (rGO), instead taking advantage of the recently demonstrated surfactant qualities of pristine graphene sheets. Using monomer as the oil phase, water-in-oil emulsions stabilized by graphene are polymerized to form porous composites with a continuous network of hollow graphene lined spheres. The effects of graphite flake size, emulsion settling, graphite concentration, and solvent ratio on composite properties such as electrical conductivity, strength, and density are investigated. We relate changes in the composite morphology to the observed property changes, and discuss the cause and effect of larger sphere sizes in the composite materials. Controlling and understanding properties such as conductivity and strength is a critical requirement for envisioned applications such as batteries, capacitors, and structural materials. [ABSTRACT FROM AUTHOR]

Graphene oxide (GO) is a two-dimensional material with a hydrophobic carbon lattice functionalized with hydrophilic oxygen groups on the edges and basal plane. Its hydrophobic/hydrophilic nature allows it to behave as a surfactant, stabilizing emulsions of oil in water. In the investigation described here, we study GO’s emulsifying ability by using it to template the polymerization of polystyrene while tuning the hydrophobicity and degree of oxidation of the GO sheets. SEM, TGA, optical microscopy, acoustic spectroscopy, and digital image analysis are used to characterize the system. The size of the GO templated PS spheres is found to be a function of the amount of GO in the system, with too little GO resulting in PS spheres with no GO shell. Increasing the hydrophobicity of the sheets is shown to lead to the formation of inverse emulsions, while increasing the graphitic character of the GO results in distorted styrene droplets as the GO sheets become more planar and less able to conform to a smoothly curving interface. [ABSTRACT FROM AUTHOR]

We introduce a "grafting-through" brush polymerization mechanism where monomers are supplied through the surface on which the initiators are attached rather than from solution as in the "grafting-from" technique. This is accomplished by attaching the initiator to the surface of a dialysis membrane and supplying monomers through the membrane to the growing brush. This avoids the growth of very long chains while promoting the growth of shorter chains by reversing the monomer concentration gradient found in the commonly used grafting-from technique, where monomer concentration is lowest at the substrate and highest in the surrounding solution. Reversing this monomer concentration gradient results in shorter chains experiencing a higher local monomer concentration than longer chains, thus speeding up their growth relative to the longer ones. It is shown by AFM that brush layers made by this method are thicker and have lower roughness than brushes made by a grafting-from approach. Coarse-grained molecular dynamics simulations of brush polymerizations with monomers supplied through a permeable substrate provide insight into the mechanism of the grafting-through brush growth process. Simulations show that it is possible to obtain a brush layer with a chain dispersity index approaching unity for sufficiently long chains. FTIR, contact angle measurements, SEM, and kinetic studies are used to characterize and elucidate the growth mechanism of brushes synthesized by the new grafting-through strategy. [ABSTRACT FROM AUTHOR]

The molecular weight and polydispersity of the chains in a polymer brush are critical parameters determining the brush properties. However, the characterization of polymer brushes is hindered by the vanishingly small mass of polymer present in brush layers. In this study, in order to obtain sufficient quantities of polymer for analysis, polymer brushes were grown from high surface area fibrous nylon membranes by ATRP. The brushes were synthesized with varying surface initiator densities, polymerization times, and amounts of sacrificial initiator, then cleaved from the substrate, and analyzed by GPC and NMR. Characterization showed that the surface-grown polymer chains were more polydisperse and had lower average molecular weight compared to solution-grown polymers synthesized concurrently. Furthermore, the molecular weight distribution of the polymer brushes was observed to be bimodal, with a low molecular weight population of chains representing a significant mass fraction of the polymer chains at high surface initiator densities. The origin of this low MW polymer fraction is proposed to be the termination of growing chains by recombination during the early stages of polymerization, a mechanism confirmed by molecular dynamics simulations of brush polymerization. [ABSTRACT FROM AUTHOR]

Block copolymers (BCPs) can self-assemblein selective solvents to form morphologies based on their chemistriesand architectures. In this study, we investigate BCPs having bothbrush and linear segments in what is referred to as “toothbrush”architecture. Using anionic polymerization, we have synthesized alinear poly(styrene-block-isoprene) followed by growingpoly(ethylene oxide) grafts from the isoprene block to form a brushsegment. A combination of nonselective and selective solvents is usedto drive the self-assembly of the toothbrush polymers. Analysis bydynamic light scattering and transmission electron microscopy showthat the self-assembly is highly influenced by the density of graftsin the brush segment, with densely grafted polymers forming only sphericalmicelles and sparsely grafted brushes forming higher order structuresincluding cylindrical micelles and vesicles. Additionally, calculationsof micelle corona graft density and corona thickness indicate thatthe corona of the micelles formed by densely grafted polymers is composedof the entire brush segment while the corona of micelles formed bysparsely grafted polymers is composed solely of grafts. The resultsof this study will inform the design of BCP self-assembled morphologiesthrough architecture control, enabling the design of polymer aggregateswith programmed morphologies and sizes in aqueous solutions. [ABSTRACT FROM AUTHOR]