Graphene oxide/waterborne polyurethane nanocoatings: effects of graphene oxide content on performance properties.

Graphene oxide (GO) is a good nanofiller candidate for waterborne coatings because of its outstanding physical and mechanical properties, good dispersibility in water, and low cost relative to graphene. Here, we report on the performance of a one-part, waterborne polyurethane (WPU) nanocoating formulated with four different GO loadings ([0.4% to 2.0%] by mass). The degree of GO dispersion/adhesion was evaluated using scanning electron microscopy, laser scanning confocal microscopy, and Raman microscopy. Nanocoating performance was evaluated using a dynamic mechanical thermal analyzer for mechanical properties, a customized coulometric permeation apparatus for oxygen barrier properties, a combustion microcalorimeter for flammability, a hot disk analyzer for thermal conductivity, thermogravimetric analysis for thermal stability, and a moisture sorption analyzer for water uptake. The results show that GO sheets were well dispersed in, and have good adhesion to, WPU. At the higher mass loadings ([1.2% or 2%] by mass), GO increased the modulus and yield strength of WPU by 300% and 200%, respectively, increased the thermal conductivity by 38%, reduced the burning heat release rate (flammability) by 43%, and reduced the oxygen permeability by up to sevenfold. The presence of GO, however, increased water vapor uptake at high humidity; the moisture content of 2% mass loading GO/WPU nanocoatings at 90% RH was almost twice that of the moisture content for unfilled WPU. Overall, with the exception of water uptake at very high humidity (> 70% RH), the observed improvements in physical and mechanical properties combined with the ease of processing suggest that GO is a viable nanofiller for WPU coatings.

Microstructure of a granular amorphous silica ceramic synthesized by spark plasma sintering.

We study the microstructure of a granular amorphous silica ceramic material synthesized by spark plasma sintering. Using monodisperse spherical silica particles as precursor, spark plasma sintering yields a dense granular material with distinct granule boundaries. We use selective etching to obtain nanoscopic pores along the granule borders. We interrogate this highly interesting material structure by combining scanning electron microscopy, X-ray computed nanotomography and simulations based on random close packed spherical particles. We determine the degree of anisotropy caused by the uni-axial force applied during sintering, and our analysis shows that our synthesis method provides a means to avoid significant granule growth and to fabricate a material with well-controlled microstructure.

Estimation of mass thickness response of embedded aggregated silica nanospheres from high angle annular dark-field scanning transmission electron micrographs.

In this study, we investigate the functional behaviour of the intensity in high-angle annular dark field scanning transmission electron micrograph images. The model material is a silica particle (20 nm) gel at 5 wt%. By assuming that the intensity response is monotonically increasing with increasing mass thickness of silica, an estimate of the functional form is calculated using a maximum likelihood approach. We conclude that a linear functional form of the intensity provides a fair estimate but that a power function is significantly better for estimating the amount of silica in the z-direction. The work adds to the development of quantifying material properties from electron micrographs, especially in the field of tomography methods and three-dimensional quantitative structural characterization from a scanning transmission electron micrograph. It also provides means for direct three-dimensional quantitative structural characterization from a scanning transmission electron micrograph.

Hydrodynamic dispersion in ß-lactoglobulin gels measured by PGSE NMR.

The displacement scale dependent molecular dynamics of solvent water molecules flowing through [Formula: see text] -lactoglobulin gels are measured by pulse gradient spin echo (PGSE) nuclear magnetic resonance (NMR). Gels formed under different p H conditions generate structures which are characterized by magnetic resonance imaging (MRI) and PGSE NMR measured dynamics as homogeneous and heterogeneous. The data presented clearly demonstrate the applicability of the theoretical framework for modeling hydrodynamic dispersion to the analysis of protein gels.

Erratum to: Hydrodynamic dispersion in ß-lactoglobulin gels measured by PGSE NMR.

The displacement scale dependent molecular dynamics of solvent water molecules flowing through ß-lactoglobulin gels are measured by pulse gradient spin echo (PGSE) nuclear magnetic resonance (NMR). Gels formed under different p H conditions generate structures which are characterized by magnetic resonance imaging (MRI) and PGSE NMR measured dynamics as homogeneous and heterogeneous. The data presented clearly demonstrate the applicability of the theoretical framework for modeling hydrodynamic dispersion to the analysis of protein gels.

Pixel-based analysis of FRAP data with a general initial bleaching profile.

In Jonasson et al. (2008), we presented a new pixel-based maximum likelihood framework for the estimation of diffusion coefficients from data on fluorescence recovery after photobleaching (FRAP) with confocal laser scanning microscopy (CLSM). The main method there, called the Gaussian profile method below, is based on the assumption that the initial intensity profile after photobleaching is approximately Gaussian. In the present paper, we introduce a method, called the Monotone profile method, where the maximum likelihood framework is extended to a general initial bleaching profile only assuming that the profile is a non-decreasing function of the distance to the bleaching centre. The statistical distribution of the image noise is further assumed to be Poisson instead of normal, which should be a more realistic description of the noise in the detector. The new Monotone profile method and the Gaussian profile method are applied to FRAP data on swelling of super absorbent polymers (SAP) in water with a Fluorescein probe. The initial bleaching profile is close to a step function at low degrees of swelling and close to a Gaussian profile at high degrees of swelling. The results obtained from the analysis of the FRAP data are corroborated with NMR diffusometry analysis of SAP with a polyethylene glycol probe having size similar to the Fluorescein. The comparison of the Gaussian and Monotone profile methods is also performed by use of simulated data. It is found that the new Monotone profile method is accurate for all types of initial profiles studied, but it suffers from being computationally slow. The fast Gaussian profile method is sufficiently accurate for most of the profiles studied, but underestimates the diffusion coefficient for profiles close to a step function. We also provide a diagnostic plot, which indicates whether the Gaussian profile method is acceptable or not.

A pixel-based likelihood framework for analysis of fluorescence recovery after photobleaching data.

A new framework for the estimation of diffusion coefficients from data on fluorescence recovery after photobleaching (FRAP) with confocal laser scanning microscopy (CLSM) is presented. It is a pixel-based statistical methodology that efficiently utilizes all information about the diffusion process in the available set of images. The likelihood function for a series of images is maximized which gives both an estimate of the diffusion coefficient and a corresponding error. This framework opens up possibilities (1) to obtain localized diffusion coefficient estimates in both homogeneous and heterogeneous materials, (2) to account for time differences between the registrations at the pixels within each image, and (3) to plan experiments optimized with respect to the number of replications, the number of bleached regions for each replicate, pixel size, the number of pixels, the number of images in each series etc. To demonstrate the use of the new framework, we have applied it to a simple system with polyethylene glycol (PEG) and water where we find good agreement with diffusion coefficient estimates from NMR diffusometry. In this experiment, it is also shown that the effect of the point spread function is negligible, and we find fluorochrome-concentration levels that give a linear response function for the fluorescence intensity.

Transport properties and aggregation phenomena of polyoxyethylene sorbitane monooleate (polysorbate 80) in pig gastrointestinal mucin and mucus.

The aqueous environment in the gastrointestinal tract frequently requires solubilization of hydrophobic drug molecules in appropriate drug delivery vehicles. An effective uptake/absorption and systemic exposure of a drug molecule entails many processes, one being transport properties of the vehicles through the mucus layer. The mucus layer is a complex mixture of biological molecules. Among them, mucin is responsible of the gel properties of this layer. In this study, we have investigated the diffusion of polyoxyethylene sorbitane monooleate (polysorbate 80), a commonly used nonionic surfactant, in aqueous solution, in mucin solutions at 0.25 and 5 wt %, and in mucus. These measurements were done by using the pulsed field gradient spin echo nuclear magnetic resonance (PGSE-NMR) technique. We conclude that polysorbate 80 is a mixture of non-surface-active molecules that can diffuse freely through all the systems investigated and of surface-active molecules that form micellar structures with transport properties strongly dependent on the environment. Polysorbate 80 micelles do not interact with mucin even though their diffusion is hindered by obstruction of the large mucin molecules. On the other hand, the transport is slowed down in mucus due to interactions with other components such as lipids depots. In the last part of this study, a hydrophobic NMR probe molecule has been included in the systems to mimic a hydrophobic drug molecule. The measurements done in aqueous solution revealed that the probe molecules were transported in a closely similar way as the polysorbate 80 micelles, indicating that they were dissolved in the micellar core. The situation was more complex in mucus. The probe molecules seem to dissolve in the lipid depots at low concentrations of polysorbate 80, which slows down their transport. At increasing concentration of polysorbate 80, the diffusion of the probe molecules increases indicating a continuous dissolution of hexamethyldisilane in the core of polysorbate 80 micelles.

Water mobility in heterogeneous emulsions determined by a new combination of confocal laser scanning microscopy, image analysis, nuclear magnetic resonance diffusometry, and finite element method simulation.

Nuclear magnetic resonance (NMR) diffusometry and confocal laser scanning microscopy (CLSM) were combined in a quantitative way in finite element calculations of water propagation in CLSM images obtained from a very heterogeneous emulsion. The propagators calculated on the basis of microstructure were Fourier transformed and subsequently compared with the echo decays obtained by the NMR diffusometry method. The results showed very good agreement between microstructure based calculations and experiments, indicating that the short gradient pulse approximation in the NMR diffusometry experiment holds for a certain q range. Furthermore, the CLSM was able to achieve a relevant two-dimensional microstructure although some discrepancy at low q values was noted. This effect is attributed to the inherent three-dimensional connectivity between the water domains in this type of structures, making the calculations slightly underestimate the water diffusion at longer distances.

Power laws in polymer solution dynamics.

The dynamical screening length xi(h) in semidilute to highly concentrated polymer solutions of poly(methyl methacrylate) in propylene carbonate has been examined using photon correlation spectroscopy and pulsed field gradient nuclear magnetic resonance. A crossover between different concentration dependent regimes, xi(h) approximately phi(-alpha), where alpha is found to be approximately 0.5, approximately 1, and approximately 2, is observed when the local viscosity is taken into account. Here phi is the volume fraction of polymer in the solution. Well-defined crossovers between alpha=0.5 and alpha=1 corresponding to a transition from a marginal solvent to a theta solvent behavior have been predicted to occur due to the reduction of excluded-volume effects between the spatially correlated polymer segments with increasing polymer volume fraction. However, a clear experimental validation of the crossover has never been presented before. The third regime (alpha approximately 2) is observed in the highly concentrated region where the static screening length is comparable to the persistence length of the polymer. The observation indicates that the rigid rod model previously used to describe concentrated solutions is an oversimplification valid only in the very high concentration limit. The obtained results at high concentrations are discussed in the frame of a simple physical model where segments at the persistence length scale are treated as flexible rodlike segments.

Flocculation behavior of asphaltenes in solvent/nonsolvent systems.

Diffusion coefficients of asphaltenes dissolved in two aromatic solvents, toluene-d(8) or ethylbenzene-d(10), were measured with the pulsed-field gradient spin echo nuclear magnetic resonance (PFG-SE NMR) technique upon addition of flocculant (pentane-d(12) or heptane-d(16)). It was observed that the change in the diffusion coefficients, as a function of amount of added flocculant, was small in the concentration interval studied (up to 30 wt% alkane). Complementary kinetic flocculation studies were made at alkane additions above 55 wt%. The initial change in turbidity upon the addition of alkane was measured with an UV-VIS spectrophotometer. The obtained stability ratio, W, showed that asphaltenes were least stable in the ethylbenzene-pentane system and most stable in the toluene-heptane system. These findings were in agreement with the PFG-SE NMR. When combining the results from the two different techniques it appeared as if there was a dramatic increase in flocculation above a certain "threshold concentration" of added alkane. Furthermore, the flocculation appeared to be reaction controlled until as much as 63 wt% of n-pentane or, alternatively, 68 wt% of n-heptane had been added to the systems, after which the flocculation became primarily diffusion controlled. Finally, careful relaxation measurements showed that the asphaltenes displayed two distinctly different transverse (T(2)) relaxation times (most probably averages), one at 0.6 ms and the other at 7 ms.

Interactions between a lipase and charged surfactants--a comparison between bulk and interfaces.

The interaction between a charged surfactant and a lipase has been investigated by several methods. Interactions in aqueous bulk phase was studied by NMR and by microcalorimetry. Surface tension and neutron reflectivity were used for studies at the air-water interface. Interactions at the interface between a hydrophobic solid surface and water was investigated by ellipsometry. The results obtained are as follows. The cationic surfactant, tetradecyltrimethylammonium bromide (iodide in the NMR experiments), showed strong interaction at the air-water and the hydrophobic solid-water interfaces but no clear indication of an interaction in bulk phase was seen. The anionic surfactant showed no interaction with the lipase neither at the interfaces, nor in bulk. The difference in behavior of the system cationic surfactant-lipase in bulk and at the interfaces may be due to the change in enzyme conformation that is known to occur at interfaces between water and an apolar phase.

An NMR Self-Diffusion Investigation of Aggregation Phenomena in Solutions of Ethyl(hydroxyethyl)cellulose.

Water solutions of ethyl(hydroxyethyl)cellulose have been investigated with the PFG NMR technique and by means of 1H NMR relaxation measurements. The echo decays in PFG experiments have been recorded for EHEC with concentrations ranging from the dilute (c < c) to the semidilute (c > c). In addition, the temperature dependence of the echo decay was investigated. It is shown that the echo decays deviate substantially from Gaussian diffusion behavior. We discuss this in terms of polydispersity, aggregation, and scaling effects. It is argued that the observed deviations are too large to be caused by polydispersity effects alone. We base this conclusion on the observation that the molar mass scaling of the polymer diffusion coefficient goes roughly as M-4. We focus on using a log-normal distribution of diffusion coefficients in the evaluation procedure of the PFG data. It is found that by comparing the diffusion coefficients extracted from this type of procedure with "Dmean" as extracted from a "stretched exponential" approach, there is a difference in the evaluated diffusion constant by up to 1 order of magnitude. The difference depends on the degree of deviation from linear behavior in the echo decay when represented in a logarithmic plot vs the relevant experimental parameter.

High magnetic field gradient PGSE NMR in the presence of a large polarizing field

A description is given of pulsed gradient spin echo (PGSE) NMR experiments in which large pulsed magnetic field gradients may be required. The design contraints are discussed and, in particular, the problem of the use of large pulsed magnetic field gradients in conjunction with large polarizing fields is considered. Issues addressed concern probe mechanical assembly, current supply requirements, and pulse shape design. We describe a quadrupolar coil with a gradient amplitude of 1.65 T m-1 A-1 which has been used successfully up to a maximum gradient of around 40 T m-1. A diffusion coefficent of 7.5 x 10(-16) m2 s-1 has been measured using this system, the lowest yet achieved by PGSE NMR methods. Copyright 1998 Academic Press.