Tinh Nguyen

Potential release pathways, environmental fate, and ecological risks of carbon nanotubes

Carbon nanotubes (CNTs) are currently incorporated into various consumer products, and numerous new applications and products containing CNTs are expected in the future. The potential for negative effects caused by CNT release into the environment is a prominent concern and numerous research projects have investigated possible environmental release pathways, fate, and toxicity. However, this expanding body of literature has not yet been systematically reviewed. Our objective is to critically review this literature to identify emerging trends as well as persistent knowledge gaps on these topics. Specifically, we examine the release of CNTs from polymeric products, removal in wastewater treatment systems, transport through surface and subsurface media, aggregation behaviors, interactions with soil and sediment particles, potential transformations and degradation, and their potential ecotoxicity in soil, sediment, and aquatic ecosystems. One major limitation in the current literature is quantifying CNT masses in relevant media (polymers, tissues, soils, and sediments). Important new directions include developing mechanistic models for CNT release from composites and understanding CNT transport in more complex and environmentally realistic systems such as heteroaggregation with natural colloids and transport of nanoparticles in a range of soils.

Sorption and Diffusion of Water, Salt Water and Concrete Pore Solution in Composite Matrices

In recent years, the use of fiber-reinforced polymer composites in civil infrastructure has been promoted as a solution to the deterioration of bridges, buildings, and other structures composed of traditional materials, such as steel, concrete, and wood. Any application of a polymer composite in an outdoor environment invariably involves exposure to moisture. There is also potential for exposure to saline conditions in waterfront or offshore structures, and alkaline environments, as would be encountered by a reinforcing bar in a cementitious material. This study characterizes the sorption and transport of distilled water, salt solution, and a simulated concrete pore solution in free films of vinyl ester, isophthalic polyester (isopolyester) and epoxy resins, all commercially important materials for use in structural composites. Diffusion of all three liquids in each of the three materials was observed to follow a Fickian process. Mass loss was observed for the isopolyester in salt water and concrete pore solution at 60°C, suggesting hydrolysis that was accelerated by the high temperature exposure. Both the rate of uptake, as well as the equilibrium uptake, were greater at 60°C, compared with ambient conditions. Diffusion coefficients calculated from the mass uptake data revealed that, although the epoxy resin had the highest equilibrium uptake, it had the lowest diffusion coefficient.

In situ measurement of water at the organic coating/substrate interface

In situ and quantitative information on the water layer at the organic coating/substrate interface is crucial for understanding and preventing the failure of organic coating systems. A technique, based on a two-layer model derived rigorously from internal reflection theory, has been developed for measuring in situ the thickness and amount of the water layer at the organic coating/substrate interface. The technique gives new insight into the processes by which water degrades the coating/substrate bonds. In this technique, a transparent or an opaque organic coating of sufficient thickness is applied to an internal reflection element (IRE) with or without a thin metallic film, which is used as the substrate. A water chamber is attached to the organic-coated specimen. After adding water to the chamber, Fourier transform infrared-multiple internal reflection (FTIR-MIR) spectra are taken automatically at specified time intervals without disturbing the specimens or the instrument. Water uptake in the coating and FTIR-MIR spectra of water on the coating-free substrate are also used for the analysis. Examples of clear and pigmented coatings on untreated and treated substrate surfaces are given to demonstrate the technique. Results of water accumulation at the coating/iron interface with and without applied electrical potentials are given. In addition to measuring water at the coating/substrate interface, the technique provides a means for studying the transport of water through a coating adhered to a substrate. Information on water at the interface and its transport properties through coatings applied to a substrate is valuable for interpreting corrosion, blistering and delamination of organic coating systems, and for developing models for use in predicting the serivce lives of protective coatings.

Characterization of polyester degradation using tapping mode atomic force microscopy: exposure to alkaline solution at room temperature

Abstract Tapping mode atomic force microscopy (AFM) was used to examine the microstructure of polyester films before and after exposure to an alkaline solution. Phase imaging and force curves showed differences in properties between the degraded and undegraded regions. Additionally, chemical analyses of the degraded films and the immersion solutions were carried out using attenuated total reflection Fourier transform infrared spectroscopy, total carbon analysis and liquid chromatography–mass spectrometry to aid in the interpretation of AFM data. The results showed that the base-catalyzed hydrolysis of polyester was a heterogeneous process, involving the formation of pits that increase in number and size with exposure time. Information provided by this study can be used to better understand the degradation mode and mechanism of polyester coatings in alkaline media.

Relating laboratory and outdoor exposure of coatings: II. Effects of relative humidity on photodegradation and the apparent quantum yield of acrylic-melamine coatings

The effect of relative humidity (RH) from ≪1% to 90% on the photodegradation and quantum efficiency for a partially-methylated melamine acrylic coating exposed to UV/50°C condition has been investigated. The UV source is supplied by two 1000 W Xenon arc solar simulators and the relative humidities are provided by specially designed humidity generators, which control relative humidity in the 0 to 90% range to within <3% of the measured values. Radiation absorbed in the coating and degradation of the films are measured by UV-visible and Fourier transform infrared spectroscopies, respectively. The degradation at a particular RH/UV condition consists of four different modes: reactions taken place during post curing, hydrolysis due to water in the film at a particular RH, photodegradation, and moisture-enhanced photodegradation. Total degradation, hydrolysis, and moisture-enhanced photodegradation increase with increasing RH. At low relative humidities, photodegradation is an important degradation mode but hydrolysis dominates the degradation at high RH levels. Moisture in the film is found to increase the quantum efficiency of acrylic melamine coating photodegradation.

Subsurface characterization of carbon nanotubes in polymer composites via quantitative electric force microscopy

Subsurface characterization of carbon nanotubes (CNTs) dispersed in free-standing polymer composite films was achieved via quantitative electric force microscopy (EFM). The effects of relative humidity, EFM probe geometry, tip-sample distance and bias voltage on the EFM contrast were studied. Non-parabolic voltage dependence of the EFM signal of subsurface CNTs in polymer composites was observed and a new mechanism was proposed taking consideration of capacitive coupling as well as coulombic coupling. We anticipate that this quantitative EFM technique will be a useful tool for non-destructive subsurface characterization of high dielectric constant nanostructures in low dielectric constant matrices.

Relating laboratory and outdoor exposure of coatings. IV: Mode and mechanism for hydrolytic degradation of acrylic-melamine coatings exposed to water vapor in the absence of UV light

Acrylic-melamine coatings are known to be susceptible to hydrolysis when exposed to water or humid environments. The mode and specific pathways for hydrolytic degradation of acrylic-melamine coatings exposed to water vapor in the absence of ultraviolet light are presented. Samples of a partially methylated melamine-acrylic coating applied to CaF2 substrates were subjected to five different relative humidity levels ranging from approximately 0 to 90% at 50°C. Coating degradation was measured with transmission Fourier transform infrared spectroscopy (FTIR) and tapping mode atomic force microscopy (AFM). In humid environments, partially methylated melamine-acrylic coatings undergo hydrolysis readily, causing considerable material loss and formation of mainly primary amines and carboxylic acids. The rate of hydrolysis increases with increasing RH. Hydrolytic degradation of acrylic-melamine coatings is an inhomogeneous process in which pits form, deepen, and enlarge with exposure. Such localized degradation mode suggests that hydrolysis of this material is an autocatalytic progression where acidic degradation products formed in the pits catalyze and accelerate the hydrolysis reactions.

Development of a conceptual framework for evaluation of nanomaterials release from nanocomposites: Environmental and toxicological implications

Despite the fact that nanomaterials are considered potentially hazardous in a freely dispersed form, they are often considered safe when encapsulated into a polymer matrix. However, systematic research to confirm the abovementioned paradigm is lacking. Our data indicates that there are possible mechanisms of nanomaterial release from nanocomposites due to exposure to environmental conditions, especially UV radiation. The degradation of the polymer matrix and potential release of nanomaterials depend on the nature of the nanofillers and the polymer matrix, as well as on the nature of environmental exposure, such as the combination of UV, moisture, mechanical stress and other factors. To the best of our knowledge there is no systematic study that addresses all these effects. We present here an initial study of the stability of nanocomposites exposed to environmental conditions, where carbon nanotube (CNT) containing polymer composites were evaluated with various spectroscopic and microscopic techniques. This work discusses various degradation mechanisms of CNT polymer nanocomposites, including such factors as UV, moisture and mechanical damage. An in vivo ingestion study with Drosophila showed reduced survivorship at each dose tested with free amine-functionalized CNTs, while there was no toxicity when these CNTs were embedded in epoxy. In addition to developing new paradigms in terms of safety of nanocomposites, the outcomes of this research can lead to recommendations on safer design strategies for the next generation of CNT-containing products.

Relating laboratory and outdoor exposures of acrylic melamine coatings. I. Cumulative damage model and laboratory exposure apparatus

A cumulative damage model and a laboratory apparatus are described for linking field and laboratory photodegradation results and for predicting the service life of polymeric materials exposed in the laboratory and field. The apparatus was designed to independently and precisely monitor and control in both space and time the three primary weathering factors causing polymeric materials to degrade when exposed in the field. These factors include temperature, relative humidity, and spectral ultraviolet radiation. A model acrylic melamine coating was exposed in the laboratory apparatus to each of 12 different spectral wavebands and four temperature and four relative humidity environments. The spectral dosage and material damage were measured for each exposure treatment and this data input into the cumulative damage model from which estimates of the spectral quantum yield were made. Variables affecting the accuracy of the measurements are discussed.

Load-displacement relations for nanoindentation of viscoelastic materials

A model based on the Burgers viscoelastic concept has been developed to describe the nanoindentation behaviors of polymeric materials. An analytical solution of displacement at the indenter tip has been derived based on the analog of the governing equation of elasticity in the time coordinate system to the governing equation of the viscoelastic model in Laplace transform coordinate system. The solution consists of the elastic, viscous, and plastic displacements during loading and unloading. Nanoindentation experiments have been conducted for poly(methyl methacrylate), polycarbonate, hydroxyethyl methacrylate copolymer, amorphous syndiotactic polystyrene, and fast-cure acrylic polymers to provide data for validating the model. The results show excellent agreement between experimental load-displacement data and model prediction for both the loading and unloading before the contact area decreases for all five polymers. The viscosity decreases but the hardness increases with increasing loading rate. Young’s m...