Graphene is a distinct two-dimensional material that offers a wide range of opportunities for membrane applications because of ultimate thinness, flexibility, chemical stability, and mechanical strength. We demonstrate that few- and several-layered graphene and graphene oxide (GO) sheets can be engineered to exhibit the desired gas separation characteristics. Selective gas diffusion can be achieved by controlling gas flow channels and pores via different stacking methods. For layered (3- to 10-nanometer) GO membranes, tunable gas transport behavior was strongly dependent on the degree of interlocking within the GO stacking structure. High carbon dioxide/nitrogen selectivity was achieved by well-interlocked GO membranes in high relative humidity, which is most suitable for postcombustion carbon dioxide capture processes, including a humidified feed stream.

The thermomechanical responses of polymers, which provide limitations to their practical use, are favourably altered by the addition of trace amounts of a nanofiller. However, the resulting changes in polymer properties are poorly understood, primarily due to the non-uniform spatial distribution of nanoparticles. Here we show that the thermomechanical properties of 'polymer nanocomposites' are quantitatively equivalent to the well-documented case of planar polymer films. We quantify this equivalence by drawing a direct analogy between film thickness and an appropriate experimental interparticle spacing. We show that the changes in glass-transition temperature with decreasing interparticle spacing for two filler surface treatments are quantitatively equivalent to the corresponding thin-film data with a non-wetting and a wetting polymer-particle interface. Our results offer new insights into the role of confinement on the glass transition, and we conclude that the mere presence of regions of modified mobility in the vicinity of the particle surfaces, that is, a simple two-layer model, is insufficient to explain our results. Rather, we conjecture that the glass-transition process requires that the interphase regions surrounding different particles interact.

Polymer properties, such as their mechanical strength, barrier properties, and dielectric response, can be dramatically improved by the addition of nanoparticles. This improvement is thought to be because the surface area per unit mass of particles increases with decreasing particle size, R, as 1/R. This favorable effect has to be reconciled with the expectation that at small enough R the nanoparticles must behave akin to a solvent and cause a deterioration of properties. How does this transition in behavior from large solutes to the solvent limit occur? We conjecture that for small enough particles the layer of polymer affected by the particles (“bound” polymer layer) must be much smaller than that for large particles: the favorable effect of increasing particle surface area can thus be overcome and lead to the small solvent limit with unfavorable mechanical properties, for example. To substantiate this picture requires that we measure and compare the “bound polymer layer” formed on nanoparticles with those near large particles with equivalent chemistry. We have implemented a novel strategy to obtain uniform nanoparticle dispersion in polymers, a problem for many previous works. Then, by combining theory and a suite of experimental techniques, including differential scanning calorimetry and positron annihilation lifetime spectroscopy, we show that the immobilized poly(2-vinylpyridine) layer near 15 nm diameter silica particles (∼1 nm) is considerably thinner than that at flat silica surfaces (∼4 to 5 nm), which is the limit of an infinitely large particle. We have also determined that the changes in the polymer’s glass-transition temperature due to the presence of this strongly interacting surface are very small in both well-dispersed nanocomposites and thin films (<100 nm). Similarly, the polymer’s fragility, as determined by dielectric spectroscopy, is also found to be little affected in the nanocomposites relative to the pure polymer. While a systematic study of the dependence of the bound polymer layer thickness on particle size remains an outstanding challenge, this first study provides conclusive evidence for the hypothesis that the bound polymer layer can be significantly smaller around nanoparticles than at chemically similar flat surfaces.

Single-wall carbon nanotubes (SWNTs) have been widely touted as attractive candidates for use as fillers in composite materials due to their extremely high Young's modulus, stiffness, and flexibility. 1 Successful applications of such composite systems require well-dispersed nanotubes with good adhesion with the host matrix, which, unfortunately, is not easily realized. Processing is rendered difficult by poor solubility of SWNTs, and the exfoliation of nanotube bundles is a major challenge. Moreover, inherently weak nanotube-polymer interactions result in poor interfacial adhesion, which can lead to nanotube aggregation within the matrix. Although a variety of chemical routes have been investigated to achieve nanotube solubility, 2 most methods either shorten the nanotubes or induce excessive functionalities that disrupt the original structure of the tubes. Polymer grafting, to improve the nanotube-polymer interface, has mainly been achieved on acidtreated nanotubes, 3 which may result in partial destruction of the tubular framework.Here, we report the development of a novel approach to in situ composite synthesis by attachment of polystyrene (PS) chains to full-length pristine SWNTs without disrupting the original structure, based on an established anionic polymerization scheme. 4 The process requires no nanotube pretreatment and works well with asproduced SWNTs. Both debundling of SWNT ropes and polymer attachment were achieved in a single step, and well-defined composites with a homogeneous dispersion of nanotubes were obtained.SWNTs produced by the HiPCO process 5 were used without further purification, as purification procedures might introduce functionalities that hinder carbanion formation. Dried pristine SWNTs were dispersed by sonication in purified cyclohexane. secButyllithium in slight excess of a predetermined amount (to ensure the removal of protic impurities on the SWNT surface) was added to this dispersion and sonicated in a bath for an hour. A homogeneous light yellow solution was obtained to which styrene monomer was added and polymerized at 48°C for 2 h under sonication. Carbanions are introduced on the SWNT surface by treatment with the anionic initiator that serves to exfoliate the bundles and provide initiating sites for the polymerization of styrene ( Figure 1). The negatively charged nanotubes are separated from the bundles and stay in solution due to mutual electrostatic repulsion between individual tubes, which was confirmed by long-term solution homogeneity. When styrene is added, both free secbutyllithium and the nanotube carbanions initiate polymerization, resulting in an intimately mixed composite system. The polymerization was terminated using degassed n-butanol, and the composite was recovered by precipitation with methanol. The composites were soluble in organic solvents such as dimethyl formamide, chloroform, and tetrahydrofuran.Composites with matrix molecular weights ranging from 1600 to 100 000 g mol -1 and polydispersities of ∼1.02 (determined using size exclusion chromatog...