Christopher W. Macosko

Cytotoxicity of Graphene Oxide and Graphene in Human Erythrocytes and Skin Fibroblasts

Two-dimensional carbon-based nanomaterials, including graphene oxide and graphene, are potential candidates for biomedical applications such as sensors, cell labeling, bacterial inhibition, and drug delivery. Herein, we explore the biocompatibility of graphene-related materials with controlled physical and chemical properties. The size and extent of exfoliation of graphene oxide sheets was varied by sonication intensity and time. Graphene sheets were obtained from graphene oxide by a simple (hydrazine-free) hydrothermal route. The particle size, morphology, exfoliation extent, oxygen content, and surface charge of graphene oxide and graphene were characterized by wide-angle powder X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, dynamic light scattering, and zeta-potential. One method of toxicity assessment was based on measurement of the efflux of hemoglobin from suspended red blood cells. At the smallest size, graphene oxide showed the greatest hemolytic activity, whereas aggregated graphene sheets exhibited the lowest hemolytic activity. Coating graphene oxide with chitosan nearly eliminated hemolytic activity. Together, these results demonstrate that particle size, particulate state, and oxygen content/surface charge of graphene have a strong impact on biological/toxicological responses to red blood cells. In addition, the cytotoxicity of graphene oxide and graphene sheets was investigated by measuring mitochondrial activity in adherent human skin fibroblasts using two assays. The methylthiazolyldiphenyl-tetrazolium bromide (MTT) assay, a typical nanotoxicity assay, fails to predict the toxicity of graphene oxide and graphene toxicity because of the spontaneous reduction of MTT by graphene and graphene oxide, resulting in a false positive signal. However, appropriate alternate assessments, using the water-soluble tetrazolium salt (WST-8), trypan blue exclusion, and reactive oxygen species assay reveal that the compacted graphene sheets are more damaging to mammalian fibroblasts than the less densely packed graphene oxide. Clearly, the toxicity of graphene and graphene oxide depends on the exposure environment (i.e., whether or not aggregation occurs) and mode of interaction with cells (i.e., suspension versus adherent cell types).

An Integral Constitutive Equation for Mixed Flows: Viscoelastic Characterization

A viscoelasticconstitutive equation of the single‐integral form has been designed. Its memory function is factored into a time‐dependent part and a strain‐dependent part. The time function is the usual series of exponential relaxations. Its relaxation times and weighting coefficients are determined by nonlinear regression on linear viscoelastic data: stress relaxation after small‐strain and small‐amplitude sinusoidal oscillations. The relative accuracy of linear and nonlinear regression fitting is compared. The strain‐dependent function is new. It is of a simple sigmoidal form with only two parameters: one determined from shear and the other from extensional data. Its sigmoidal form provides a finite linear viscoelastic region, a steady viscosity in uniaxial extension, and a well‐behaved power‐law shear viscosity at high shear rate. An efficient strategy for collecting sufficient data to determine the parameters of the equation is described. Predictions of the equation are tested against shear and extension data collected on the Rheometrics System Four for two polydimethylsiloxanes and against data for other polymer melts from the literature. Both uniaxial and biaxial extension as well as shear data are described. Transient shear normal stresses are somewhat underpredicted. The constitutive equation has the potential for modeling mixed shear and extensional flows as encountered in processing operations and is simple enough to be attractive for efficient computer‐aided analysis by modern finite element methods.

Morphology development during the initial stages of polymer-polymer blending

Abstract The development of morphology from pellet-sized particles to submicrometre droplets during the polymer blending process is investigated for several polymer blends. In order to determine the morphology at short mixing times, a model experiment is developed that allows the matrix to be dissolved away so that the dispersed phase may be observed directly using scanning electron microscopy. The dispersed phase for the model experiments is an amorphous nylon. The matrix phases investigated include polystyrene, an oxazoline functional polystyrene, a styrene-maleic anhydride copolymer, an amorphous polyester and a polycarbonate. These model experiments dramatically reveal the primary modes of particle deformation and the nature of the morphologies at short mixing times. The major reduction in phase domain size occurs in conjunction with the melting or softening of the components. The initial mechanism of morphology development involves the formation of sheets or ribbons of the dispersed phase. These sheets or ribbons become unstable due to the effects of flow and interfacial tension. Holes develop in the ribbons, which grow in size and concentration until a fragile lace structure is formed. The lace structure breaks into irregularly shaped particles, which are then broken up into nearly spherical particles. This mechanism results in very fast formation of small dispersed-phase particles, which are nearly the same size as those observed at long mixing times. Continued mixing action primarily reduces the size of the largest particles in the size distribution. kw ]blend; morphology development; scanning electron microscopy

Lubricated Squeezing Flow: A New Biaxial Extensional Rheometer

Squeezing flow between two disks with lubricated surfaces was found to generate a homogeneous compression or equal biaxial extension in a high viscosity polydimethylsiloxane sample. The apparatus is extremely simple: two steel disks with a central rod to provide alignment and prevent sample slip, an LVDT to measure displacement, and a silicone oil bath. The mass and area of the upper disk provide for a constant force boundary condition. The biaxial viscosity was found to be approximately six times the shear viscosity over biaxial extension rates er from 0.003 to 1.0 s−1. Lubrication could be achieved up to Hencky strains of about 2.5. Some data were also taken on the same polyisobutylene sample used by Stephenson and Meissner in their biaxial stretching study. Agreement was very good.

Aqueous only route toward graphene from graphite oxide

We report a new, simple, hydrazine-free, high-yield method for producing single-layer graphene sheets. Graphene sheets were formed from graphite oxide by reduction with simple deionized water at 95 °C under atmospheric pressure. Over 65% of the sheets are single graphene layers; the average sheet diameter is 300 nm. We speculate that dehydration of graphene oxide is the main mechanism for oxygen reduction and transformation of C-C bonds from sp(3) to sp(2). The reduction appears to occur in large uniform interconnected oxygen-free patches so that despite the presence of residual oxygen the sp(2) carbon bonds formed on the sheets are sufficient to provide electronic properties comparable to reduced graphene sheets obtained using other methods.

Effect of reinforcing fillers on the rheology of polymer melts

A single fumed silica, consisting of spherical primary particles (∼10 nm diameter) which are fused into aggregates (∼200 nm in diameter after mixing) was subjected to different surface treatments, then mixed with polymethylsiloxanes (PDMS, Mw=2000–325 000) and tested under oscillatory shear. The dynamic moduli showed a strong strain dependence, while the frequency dependence changed dramatically with the silica concentration. Experimental results are explained by agglomeration of silica aggregates through bridging polymer chains. Agglomeration increased with silica concentration, amount of silanols on the silica surface, and polymer molecular weight, and the break down of agglomeration occurred at decreasing strain levels with increasing filler concentration. Suspensions made with low molecular weight PDMS showed a different dynamic behavior due to the absence of entanglements and the small probability of bridging chains.

Quantifying dispersion of layered nanocomposites via melt rheology

Rheological measurements are used to compare clay nanocomposites prepared through melt mixing using two different polypropylene matrices. Steady state and transient nonlinear rheological experiments are employed to separate the contributions of flow induced orientation of the tactoids and particulate network build-up. The conditions under which the rheological properties are dominated by the aggregate network are subsequently identified. Under these conditions, the low frequency linear viscoelastic behavior is analyzed using scaling concepts for fractal networks to determine the degree of network formation by exfoliation. Moreover, the high frequency behavior of the moduli can be used to quickly assess the dispersion quality. The results from the analysis of the linear viscoelastic data are compared to structural features extracted from electron microscopy and small angle X-ray scattering data.

Spin coating: One-dimensional model

The model of the spin‐coating process presented here accounts for variations of concentration, viscosity, and diffusivity across the thickness of the spin‐coated film. The flow of the liquid is governed by a balance between centrifugal driving force and viscous resisting force. Radial variations in film thickness and concentration are neglected. The Galerkin/finite‐element method is employed to solve the equation set. Film thinning slows initially due to decreasing film thickness and ceases finally due to dramatically increasing viscosity of the coating liquid as solvents evaporate. The formation of a region of extremely low solvent concentration and correspondingly high viscosity and low binary diffusivity at the free surface, i.e., a solid ‘‘skin,’’ is predicted. Coating defects can occur if convective flow has not completely ceased when this skin forms. Skin formation can be eliminated or delayed by partially saturating the overlying gas with solvent or by using mixed solvents (having both high and low...

Rheology of Xanthan Gum

Rheology of aqueous solutions of xanthan gum was studied over a wide range of shear rate and concentration. At sufficient dilution and low shear rates xanthan solutions show a region of Newtonian viscosity behavior. More concentrated solutions appear to show a yield stress. These results with preliminary normal stress data and effects of salt concentration are discussed in terms of the structure of this polysaccharide in solution. Intrinsic viscosity data are modeled as a suspension of rods and found to be in agreement with existing theory. We speculate the conformation of xanthan in solution is rodlike, having some flexibility. A hydrodynamic length near 1.5 μm is estimated, assuming a diameter of 19 A.

How Dilute are Dilute Solutions in Extensional Flows

We investigate the concentration dependence of the characteristic relaxation time of dilute polymer solutions in transient uniaxial elongational flow. A series of monodisperse polystyrene solutions of five different molecular weights (1.8×106⩽M⩽8.3×106g∕mol) with concentrations spanning five orders of magnitude were dissolved in two solvents of differing solvent quality (diethylphthalate and oligomeric styrene). Optical measurements with a capillary breakup extensional rheometer of the rate of filament thinning and the time to breakup in each fluid are used to determine the characteristic relaxation time. A criterion for a lower sensitivity limit is introduced, in the form of a minimum concentration cmin necessary for experimental resolution of the effects of polymeric viscoelasticity. This criterion is validated by experiment and comparison to numerical calculations with a multimode bead-spring dumbbell model. These calculations also rationalize previous paradoxical observations of extensional thinning i...