A chemical sensor for the detection of cocaine has been developed, based on a molecularly imprint... more A chemical sensor for the detection of cocaine has been developed, based on a molecularly imprinted polymer (MIP) containing a fluorescein moiety as the signalling group. The fluorescent MIP was formed and covalently attached to the distal end of an optical fibre. The sensor exhibited an increase in fluorescence intensity in response to cocaine in an aqueous acetonitrile mixture. Selectivity for cocaine over codeine has been demonstrated.
Proceedings of the 41st SICE Annual Conference. SICE 2002.
Presented and discussed in this paper are mathematical model used for expressing ship motions, ap... more Presented and discussed in this paper are mathematical model used for expressing ship motions, application of decoupling control methodology to construct the automatic control system and corresponding designing issues. Simulation results for a very large crude carriage in several harbor maneuvers under various strong environmental disturbances are given to illustrate the application and show excellent effectiveness of the system.
A regulatory protein, PpaA, involved in photosystem formation in the anoxygenic phototrophic prot... more A regulatory protein, PpaA, involved in photosystem formation in the anoxygenic phototrophic proteobacterium Rhodobacter sphaeroides has been identified and characterized in vivo. Based on the phenotypes of cells expressing the ppaA gene in extra copy and on the phenotype of the ppaA null mutant, it was concluded that PpaA activates photopigment production and puc operon expression under aerobic conditions. This is in contrast to the function of the PpaA homologue from Rhodobacter capsulatus, AerR, which acts as a repressor under aerobic conditions [Dong, C.,
Journal of the Mechanics and Physics of Solids, 2005
Single-walled carbon nanotubes (SWNTs) in crystalline bundles may exhibit a transition in which t... more Single-walled carbon nanotubes (SWNTs) in crystalline bundles may exhibit a transition in which the cross-sections of tubes turn from perfectly circular to hexagonal, depending upon the tube diameter and externally applied pressure, and this structural instability leads to an abrupt change in the bulk elastic properties of SWNT bundles. This paper presents a hybrid atom/continuum model to study the bulk elastic properties of SWNT bundles, and the predicted characteristics of this structural instability agree well with the experimental observations available in the literature. Linearized bulk elastic properties of SWNT bundles with respect to a stable configuration are transversely isotropic and hence can be characterized by five independent elastic moduli. A complete set of these five moduli is predicted for the first time. It is found that the deformability of tube cross-sections play a dominant role in characterizing the transverse moduli.
Theoretical and experimental studies were carried out on the transport of cations in the channel ... more Theoretical and experimental studies were carried out on the transport of cations in the channel between a polymer coating and a metal substrate from a defect in the absence of an applied electrical potential. The model consists of two stages: an initial period during which ions diffuse in the coating/metal interracial 'channel' and adsorb on the coating surface, and a propagation period during which ions atso diffuse into the coating. The mathematical models were solved to predict the cation concentration and flux under the coating and the relative rate of diffusion between the initial and propagation periods. Model parameter values were derived from the results of an experiment conducted in a specially designed diffusion cell. The experiment measured the depletion of Na+ ions in a cylindrical, central reservoir, which was placed within the perimeter of a defect through the coating of an epoxy-coated steel panel. Model predictions of concentration versus time agreed well with the experimental results, which showed that most of Na+ ions were removed by lateral diffusion from the reservoir during the initial period. Further, the transport during the initial period was much faster than that during the propagation period. The results also indicated that during the propagation period, the rate-limiting step was the lateral diffusion along the coating/metal interface rather than dhlusion through the coating.
The substrate specificities of two incorrectly annotated enzymes belonging to cog3964 from the am... more The substrate specificities of two incorrectly annotated enzymes belonging to cog3964 from the amidohydrolase superfamily (AHS) were determined. This group of enzymes is currently misannotated as either dihydroorotase or adenine deaminase. Atu3266 from Agrobacterium tumefaciens C58 and Oant2987 from Ochrobactrum anthropi ATCC 49188 were determined to catalyze the hydrolysis of acetyl-R-mandelate and similar esters with values of k cat /K m that exceed 10 5 M −1 s −1. These enzymes do not catalyze the deamination of adenine or the hydrolysis of dihydroorotate. Atu3266 was crystallized and the structure determined to a resolution of 2.62 Å. The protein folds as a distorted (β/α) 8-barrel and binds two zincs in the active site. The substrate profile was determined via a combination of computational docking to the three-dimensional structure of Atu3266 and screening of a highly focused library of potential substrates. The initial weak hit was the hydrolysis of N-acetyl-D-serine (k cat /K m = 4 M −1 s −1). This was followed by the progressive identification of acetyl-R-glycerate (4 × 10 2 M −1 s −1), acetyl glycolate (k cat /K m = 1.3 × 10 4 M −1 s −1) and ultimately acetyl-R-mandelate (k cat /K m =2.8 × 10 5 M −1 s −1). The development of a comprehensive strategy for the functional annotation of proteins and enzymes whose sequences have been deposited in public databases has proven to be a difficult and persistent challenge. Utilization of homology-based sequence comparisons for functional annotation of newly sequenced genes can often lead to the annotation of the wrong function when unreasonable threshold values are used (1-3). The end result is often a misrepresentation of the potential metabolic transformations contained within a given organism. In addition to the misannotation of enzyme function there is a significant fraction
Effects of Spectral UV on Degradation of Acrylic-Urethane Coatings ... Tinh Nguyen, Jonathan W. M... more Effects of Spectral UV on Degradation of Acrylic-Urethane Coatings ... Tinh Nguyen, Jonathan W. Martin, Eric Byrd and Edward Embree Building and Fire Research Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899 USA
Fourier transform infrared (FTIR) spectroscopy has being emphasised as a widespread technique in ... more Fourier transform infrared (FTIR) spectroscopy has being emphasised as a widespread technique in the quick assess of food components. In this work, procyanidins were extracted with methanol and acetone/water from the seeds of white and red grape varieties. A fractionation by graded methanol/chloroform precipitations allowed to obtain 26 samples that were characterised using thiolysis as pre-treatment followed by HPLC-UV and MS detection. The average degree of polymerisation (DPn) of the procyanidins in the samples ranged from 2 to 11 flavan-3-ol residues. FTIR spectroscopy within the wavenumbers region of 1800-700 cm −1 allowed to build a partial least squares (PLS1) regression model with 8 latent variables (LVs) for the estimation of the DPn, giving a RMSECV of 11.7%, with a R 2 of 0.91 and a RMSEP of 2.58. The application of orthogonal projection to latent structures (O-PLS1) clarifies the interpretation of the regression model vectors. Moreover, the O-PLS procedure has removed 88% of non-correlated variations with the DPn, allowing to relate the increase of the absorbance peaks at 1203 and 1099 cm −1 with the increase of the DPn due to the higher proportion of substitutions in the aromatic ring of the polymerised procyanidin molecules.
Polymer nanocoatings are increasingly used outdoors and in harsh environments. However, because m... more Polymer nanocoatings are increasingly used outdoors and in harsh environments. However, because most common polymers degrade by the weathering elements, nanoparticles in polymer nanocoatings may be released into the environments. Such nanoparticle release potentially poses an environmental health and safety risk. This study investigated the effects of temperature on the surface accumulation and release of nanosilica for an epoxy nanocoating exposed to ultravioiet (UV) radiation. Specimens of an amine-cured epoxy containing 5 mass % nanosilica were exposed to 295 nm to 400 nm UV radiation at three temperatures (40 C, 50 C, and 60 C). Surface accumulation and release of nanosilica as a function of UV dose were measured by atomic force microscopy and inductively-coupled plasma optical emission spectrometry, respectively. Nanosilica accumulated rapidly on the specimen surface at low UV doses but the rate of accumulation slowed down at high UV doses. Further, the amount of surface accumu...
Nanocoatings have numerous potential applications in the indoor environment, such as flooring fin... more Nanocoatings have numerous potential applications in the indoor environment, such as flooring finishes with increased scratch- and wear-resistance. However, given concerns about the potential environmental and human health effects of nanomaterials, it is necessary to develop standardized methods to quantify nanomaterial release during use of these products. One key choice for mechanical wear studies is the abrasion wheel. Potential limitations of different wheels include the release of fragments from the wheel during abrasion, wearing of the wheel from the abrasion process, or not releasing a sufficient number of particles for accurate quantitative analysis. In this study, we evaluated five different wheels, including a typically used silicon oxide-based commercial wheel and four wheels fabricated at the National Institute of Standards and Technology (NIST), for their application in nanocoating abrasion studies. A rapid, nondestructive laser scanning confocal microscopy method was d...
Multiwall carbon nanotubes (MWCNTs) are nanofillers used in consumer and structural polymeric pro... more Multiwall carbon nanotubes (MWCNTs) are nanofillers used in consumer and structural polymeric products to enhance a variety of properties. Under weathering, the polymer matrix will degrade and the nanofillers may be released from the products potentially impacting ecological or human health. In this study, we investigated the degradation of a 0.72 % (by mass) MWCNT/amine-cured epoxy nanocomposite irradiated with high intensity ultraviolet (UV) light at various doses, the effects of UV exposure on the surface accumulation and potential release of MWCNTs, and possible mechanisms for the release resistance of the MWCNT surface layer formed on nanocomposites by UV irradiation. Irradiated samples were characterized for chemical degradation, mass loss, surface morphological changes, and MWCNT release using a variety of analytical techniques. Under 295 nm to 400 nm UV radiation up to a dose of 4865 MJ/m(2), the nanocomposite matrix underwent photodegradation, resulting in formation of a de...
Carbon nanotube composites are lightweight, multifunctional materials with readily adjustable mec... more Carbon nanotube composites are lightweight, multifunctional materials with readily adjustable mechanical and electrical propertiesrelevant to the aerospace, automotive, and sporting goods industries as high-performance structural materials. Here, we combine well-established and newly developed characterization techniques to demonstrate that ultraviolet (UV) light exposure provides a controllable means to enhance the electrical conductivity of the surface of a commercial carbon nanotube−epoxy composite by over 5 orders of magnitude. Our observations, combined with theory and simulations, reveal that the increase in conductivity is due to the formation of a concentrated layer of nanotubes on the composite surface. Our model implies that contacts between nanotube-rich microdomains dominate the conductivity of this layer at low UV dose, while tube− tube transport dominates at high UV dose. Further, we use this model to predictably pattern conductive traces with a UV laser, providing a facile approach for direct integration of lightweight conductors on nanocomposite surfaces.
Many coatings properties such as mechanical, electrical, and ultra violet (UV) resistance are gre... more Many coatings properties such as mechanical, electrical, and ultra violet (UV) resistance are greatly enhanced by the addition of nanoparticles, which can potentially increase the use of nanocoatings for many outdoor applications. However, because polymers used in all coatings are susceptible to degradation by weathering, nanoparticles in a coating may be brought to the surface and released into the environment during the life cycle of a nanocoating. Therefore, the goal of this study is to investigate the process and mechanism of surface degradation and potential particle release from a commercial nanosilica/polyurethane coating under accelerated UV exposure. Recent research at the National Institute of Standards and Technology (NIST) has shown that the matrix in an epoxy nanocomposite undergoes photodegradation during exposure to UV radiation, resulting in surface accumulation of nanoparticles and subsequent release from the composite. In this study, specimens of a commercial polyurethane (PU) coating, to which a 5 mass % surface treated silica nanoparticles solution was added, were exposed to well-controlled, accelerated UV environments. The nanocoating surface morphological changes and surface accumulation of nanoparticles as a function of UV exposure were measured, along with chemical change and mass loss using a variety of techniques. Particles from the surface of the coating were collected using a simulated rain process developed at NIST, and the collected runoff specimens were measured using inductively coupled plasma-optical emission spectroscopy (ICP-OES) to determine the amount of silicon released from the nanocoatings. The results demonstrated that the added silica nanoparticle solution decreased the photodegradation rate (i.e., stabilization) of the commercial PU nanocoating. Although the degradation was slower than the previous nanosilica epoxy model system, the degradation of the PU matrix resulted in accumulation of silica nanoparticles on the nanocoating surface and release to the environment by simulated rain. These experimental data are valuable for developing models to predict the long-term release of nanosilica from commercial PU nanocoatings used outdoors and, therefore, are essential for assessing the health and environmental risks during the service life of exterior PU nanocoatings.
Despite many studies of subsurface imaging of carbon nanotube (CNT)-polymer composites via scanni... more Despite many studies of subsurface imaging of carbon nanotube (CNT)-polymer composites via scanning electron microscopy (SEM), significant controversy exists concerning the imaging depth and contrast mechanisms. We studied CNT-polyimide composites and, by three dimensional reconstructions of captured stereo-pair images, determined that the maximum SEM imaging depth was typically hundreds of nanometers. The contrast mechanisms were investigated over a broad range of beam accelerating voltages from 0.3 kV to 30 kV, and ascribed to modulation by embedded CNTs of the effective secondary electron (SE) emission yield at the polymer surface. This modulation of the SE yield is due to non-uniform surface potential distribution resulting from current flows due to leakage and electron beam induced current (EBIC). The importance of an external electric field on SEM subsurface imaging was also demonstrated. The insights gained from this study can be generally applied to SEM nondestructive subsurface imaging of conducting nanostructures embedded in dielectric matrices such as graphene-polymer composites, silicon-based single electron transistors, high resolution SEM overlay metrology or e-beam lithography, and have significant implications in nanotechnology.
Any link(s) to website(s) in this document have been provided because they may have information o... more Any link(s) to website(s) in this document have been provided because they may have information of interest to our readers. NIST does not necessarily endorse the views expressed or the facts presented on these sites. Further, NIST does not endorse any commercial products that may be advertised or available on these sites.
Accelerating the photodegradation of polymeric materials is of great practical interest in weathe... more Accelerating the photodegradation of polymeric materials is of great practical interest in weathering research. Acceleration can be achieved by exposing polymeric materials to a high radiant flux; however, questions have arisen within the weathering community as to whether high radiant flux results can be extrapolated to in-service flux levels. Experiments designed to test this premise are called reciprocity law experiments. An extensive review has been conducted to assess the state-of-the-art of reciprocity law experiments in the photography, photoconductivity, photo-medicine, photobiology, and polymer photodegradation literatures. From this review, the Schwarzschild law (a power law generalization of the reciprocity law) appears to model adequately photoresponse vs. radiant flux for most materials and systems. A band theory model has been presented to explain variations in the Schwarzschild law coefficients and other experimental phenomena commonly associated with reciprocity experiments. Obstacles to the general acceptance of high radiant flux, laboratory-based experiments are discussed.
One of the obstacles hindering the acceptance of polymer composites in civil engineering applicat... more One of the obstacles hindering the acceptance of polymer composites in civil engineering applications is the susceptibility of the polymeric matrix to degradation that is initiated by moisture, temperature, and corrosive chemical environments. The objective of this study was to characterize chemical and physical changes in polymer matrix resins following exposure to these environments. Resin systems studied were vinyl ester and isophthalic polyester, both of which are proposed for use in construction applications. Unreinforced free films were exposed to water, alkaline and saline environments at ambient and elevated temperatures for extended periods of time. Changes in strength and thermophysical properties were evaluated through tensile testing, dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). Chemical degradation of the polymers was characterized using Fourier transform infrared (FTIR) spectroscopy. Energy dispersive X‐ray (EDX) analysis of specimen...
A chemical sensor for the detection of cocaine has been developed, based on a molecularly imprint... more A chemical sensor for the detection of cocaine has been developed, based on a molecularly imprinted polymer (MIP) containing a fluorescein moiety as the signalling group. The fluorescent MIP was formed and covalently attached to the distal end of an optical fibre. The sensor exhibited an increase in fluorescence intensity in response to cocaine in an aqueous acetonitrile mixture. Selectivity for cocaine over codeine has been demonstrated.
Proceedings of the 41st SICE Annual Conference. SICE 2002.
Presented and discussed in this paper are mathematical model used for expressing ship motions, ap... more Presented and discussed in this paper are mathematical model used for expressing ship motions, application of decoupling control methodology to construct the automatic control system and corresponding designing issues. Simulation results for a very large crude carriage in several harbor maneuvers under various strong environmental disturbances are given to illustrate the application and show excellent effectiveness of the system.
A regulatory protein, PpaA, involved in photosystem formation in the anoxygenic phototrophic prot... more A regulatory protein, PpaA, involved in photosystem formation in the anoxygenic phototrophic proteobacterium Rhodobacter sphaeroides has been identified and characterized in vivo. Based on the phenotypes of cells expressing the ppaA gene in extra copy and on the phenotype of the ppaA null mutant, it was concluded that PpaA activates photopigment production and puc operon expression under aerobic conditions. This is in contrast to the function of the PpaA homologue from Rhodobacter capsulatus, AerR, which acts as a repressor under aerobic conditions [Dong, C.,
Journal of the Mechanics and Physics of Solids, 2005
Single-walled carbon nanotubes (SWNTs) in crystalline bundles may exhibit a transition in which t... more Single-walled carbon nanotubes (SWNTs) in crystalline bundles may exhibit a transition in which the cross-sections of tubes turn from perfectly circular to hexagonal, depending upon the tube diameter and externally applied pressure, and this structural instability leads to an abrupt change in the bulk elastic properties of SWNT bundles. This paper presents a hybrid atom/continuum model to study the bulk elastic properties of SWNT bundles, and the predicted characteristics of this structural instability agree well with the experimental observations available in the literature. Linearized bulk elastic properties of SWNT bundles with respect to a stable configuration are transversely isotropic and hence can be characterized by five independent elastic moduli. A complete set of these five moduli is predicted for the first time. It is found that the deformability of tube cross-sections play a dominant role in characterizing the transverse moduli.
Theoretical and experimental studies were carried out on the transport of cations in the channel ... more Theoretical and experimental studies were carried out on the transport of cations in the channel between a polymer coating and a metal substrate from a defect in the absence of an applied electrical potential. The model consists of two stages: an initial period during which ions diffuse in the coating/metal interracial 'channel' and adsorb on the coating surface, and a propagation period during which ions atso diffuse into the coating. The mathematical models were solved to predict the cation concentration and flux under the coating and the relative rate of diffusion between the initial and propagation periods. Model parameter values were derived from the results of an experiment conducted in a specially designed diffusion cell. The experiment measured the depletion of Na+ ions in a cylindrical, central reservoir, which was placed within the perimeter of a defect through the coating of an epoxy-coated steel panel. Model predictions of concentration versus time agreed well with the experimental results, which showed that most of Na+ ions were removed by lateral diffusion from the reservoir during the initial period. Further, the transport during the initial period was much faster than that during the propagation period. The results also indicated that during the propagation period, the rate-limiting step was the lateral diffusion along the coating/metal interface rather than dhlusion through the coating.
The substrate specificities of two incorrectly annotated enzymes belonging to cog3964 from the am... more The substrate specificities of two incorrectly annotated enzymes belonging to cog3964 from the amidohydrolase superfamily (AHS) were determined. This group of enzymes is currently misannotated as either dihydroorotase or adenine deaminase. Atu3266 from Agrobacterium tumefaciens C58 and Oant2987 from Ochrobactrum anthropi ATCC 49188 were determined to catalyze the hydrolysis of acetyl-R-mandelate and similar esters with values of k cat /K m that exceed 10 5 M −1 s −1. These enzymes do not catalyze the deamination of adenine or the hydrolysis of dihydroorotate. Atu3266 was crystallized and the structure determined to a resolution of 2.62 Å. The protein folds as a distorted (β/α) 8-barrel and binds two zincs in the active site. The substrate profile was determined via a combination of computational docking to the three-dimensional structure of Atu3266 and screening of a highly focused library of potential substrates. The initial weak hit was the hydrolysis of N-acetyl-D-serine (k cat /K m = 4 M −1 s −1). This was followed by the progressive identification of acetyl-R-glycerate (4 × 10 2 M −1 s −1), acetyl glycolate (k cat /K m = 1.3 × 10 4 M −1 s −1) and ultimately acetyl-R-mandelate (k cat /K m =2.8 × 10 5 M −1 s −1). The development of a comprehensive strategy for the functional annotation of proteins and enzymes whose sequences have been deposited in public databases has proven to be a difficult and persistent challenge. Utilization of homology-based sequence comparisons for functional annotation of newly sequenced genes can often lead to the annotation of the wrong function when unreasonable threshold values are used (1-3). The end result is often a misrepresentation of the potential metabolic transformations contained within a given organism. In addition to the misannotation of enzyme function there is a significant fraction
Effects of Spectral UV on Degradation of Acrylic-Urethane Coatings ... Tinh Nguyen, Jonathan W. M... more Effects of Spectral UV on Degradation of Acrylic-Urethane Coatings ... Tinh Nguyen, Jonathan W. Martin, Eric Byrd and Edward Embree Building and Fire Research Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899 USA
Fourier transform infrared (FTIR) spectroscopy has being emphasised as a widespread technique in ... more Fourier transform infrared (FTIR) spectroscopy has being emphasised as a widespread technique in the quick assess of food components. In this work, procyanidins were extracted with methanol and acetone/water from the seeds of white and red grape varieties. A fractionation by graded methanol/chloroform precipitations allowed to obtain 26 samples that were characterised using thiolysis as pre-treatment followed by HPLC-UV and MS detection. The average degree of polymerisation (DPn) of the procyanidins in the samples ranged from 2 to 11 flavan-3-ol residues. FTIR spectroscopy within the wavenumbers region of 1800-700 cm −1 allowed to build a partial least squares (PLS1) regression model with 8 latent variables (LVs) for the estimation of the DPn, giving a RMSECV of 11.7%, with a R 2 of 0.91 and a RMSEP of 2.58. The application of orthogonal projection to latent structures (O-PLS1) clarifies the interpretation of the regression model vectors. Moreover, the O-PLS procedure has removed 88% of non-correlated variations with the DPn, allowing to relate the increase of the absorbance peaks at 1203 and 1099 cm −1 with the increase of the DPn due to the higher proportion of substitutions in the aromatic ring of the polymerised procyanidin molecules.
Polymer nanocoatings are increasingly used outdoors and in harsh environments. However, because m... more Polymer nanocoatings are increasingly used outdoors and in harsh environments. However, because most common polymers degrade by the weathering elements, nanoparticles in polymer nanocoatings may be released into the environments. Such nanoparticle release potentially poses an environmental health and safety risk. This study investigated the effects of temperature on the surface accumulation and release of nanosilica for an epoxy nanocoating exposed to ultravioiet (UV) radiation. Specimens of an amine-cured epoxy containing 5 mass % nanosilica were exposed to 295 nm to 400 nm UV radiation at three temperatures (40 C, 50 C, and 60 C). Surface accumulation and release of nanosilica as a function of UV dose were measured by atomic force microscopy and inductively-coupled plasma optical emission spectrometry, respectively. Nanosilica accumulated rapidly on the specimen surface at low UV doses but the rate of accumulation slowed down at high UV doses. Further, the amount of surface accumu...
Nanocoatings have numerous potential applications in the indoor environment, such as flooring fin... more Nanocoatings have numerous potential applications in the indoor environment, such as flooring finishes with increased scratch- and wear-resistance. However, given concerns about the potential environmental and human health effects of nanomaterials, it is necessary to develop standardized methods to quantify nanomaterial release during use of these products. One key choice for mechanical wear studies is the abrasion wheel. Potential limitations of different wheels include the release of fragments from the wheel during abrasion, wearing of the wheel from the abrasion process, or not releasing a sufficient number of particles for accurate quantitative analysis. In this study, we evaluated five different wheels, including a typically used silicon oxide-based commercial wheel and four wheels fabricated at the National Institute of Standards and Technology (NIST), for their application in nanocoating abrasion studies. A rapid, nondestructive laser scanning confocal microscopy method was d...
Multiwall carbon nanotubes (MWCNTs) are nanofillers used in consumer and structural polymeric pro... more Multiwall carbon nanotubes (MWCNTs) are nanofillers used in consumer and structural polymeric products to enhance a variety of properties. Under weathering, the polymer matrix will degrade and the nanofillers may be released from the products potentially impacting ecological or human health. In this study, we investigated the degradation of a 0.72 % (by mass) MWCNT/amine-cured epoxy nanocomposite irradiated with high intensity ultraviolet (UV) light at various doses, the effects of UV exposure on the surface accumulation and potential release of MWCNTs, and possible mechanisms for the release resistance of the MWCNT surface layer formed on nanocomposites by UV irradiation. Irradiated samples were characterized for chemical degradation, mass loss, surface morphological changes, and MWCNT release using a variety of analytical techniques. Under 295 nm to 400 nm UV radiation up to a dose of 4865 MJ/m(2), the nanocomposite matrix underwent photodegradation, resulting in formation of a de...
Carbon nanotube composites are lightweight, multifunctional materials with readily adjustable mec... more Carbon nanotube composites are lightweight, multifunctional materials with readily adjustable mechanical and electrical propertiesrelevant to the aerospace, automotive, and sporting goods industries as high-performance structural materials. Here, we combine well-established and newly developed characterization techniques to demonstrate that ultraviolet (UV) light exposure provides a controllable means to enhance the electrical conductivity of the surface of a commercial carbon nanotube−epoxy composite by over 5 orders of magnitude. Our observations, combined with theory and simulations, reveal that the increase in conductivity is due to the formation of a concentrated layer of nanotubes on the composite surface. Our model implies that contacts between nanotube-rich microdomains dominate the conductivity of this layer at low UV dose, while tube− tube transport dominates at high UV dose. Further, we use this model to predictably pattern conductive traces with a UV laser, providing a facile approach for direct integration of lightweight conductors on nanocomposite surfaces.
Many coatings properties such as mechanical, electrical, and ultra violet (UV) resistance are gre... more Many coatings properties such as mechanical, electrical, and ultra violet (UV) resistance are greatly enhanced by the addition of nanoparticles, which can potentially increase the use of nanocoatings for many outdoor applications. However, because polymers used in all coatings are susceptible to degradation by weathering, nanoparticles in a coating may be brought to the surface and released into the environment during the life cycle of a nanocoating. Therefore, the goal of this study is to investigate the process and mechanism of surface degradation and potential particle release from a commercial nanosilica/polyurethane coating under accelerated UV exposure. Recent research at the National Institute of Standards and Technology (NIST) has shown that the matrix in an epoxy nanocomposite undergoes photodegradation during exposure to UV radiation, resulting in surface accumulation of nanoparticles and subsequent release from the composite. In this study, specimens of a commercial polyurethane (PU) coating, to which a 5 mass % surface treated silica nanoparticles solution was added, were exposed to well-controlled, accelerated UV environments. The nanocoating surface morphological changes and surface accumulation of nanoparticles as a function of UV exposure were measured, along with chemical change and mass loss using a variety of techniques. Particles from the surface of the coating were collected using a simulated rain process developed at NIST, and the collected runoff specimens were measured using inductively coupled plasma-optical emission spectroscopy (ICP-OES) to determine the amount of silicon released from the nanocoatings. The results demonstrated that the added silica nanoparticle solution decreased the photodegradation rate (i.e., stabilization) of the commercial PU nanocoating. Although the degradation was slower than the previous nanosilica epoxy model system, the degradation of the PU matrix resulted in accumulation of silica nanoparticles on the nanocoating surface and release to the environment by simulated rain. These experimental data are valuable for developing models to predict the long-term release of nanosilica from commercial PU nanocoatings used outdoors and, therefore, are essential for assessing the health and environmental risks during the service life of exterior PU nanocoatings.
Despite many studies of subsurface imaging of carbon nanotube (CNT)-polymer composites via scanni... more Despite many studies of subsurface imaging of carbon nanotube (CNT)-polymer composites via scanning electron microscopy (SEM), significant controversy exists concerning the imaging depth and contrast mechanisms. We studied CNT-polyimide composites and, by three dimensional reconstructions of captured stereo-pair images, determined that the maximum SEM imaging depth was typically hundreds of nanometers. The contrast mechanisms were investigated over a broad range of beam accelerating voltages from 0.3 kV to 30 kV, and ascribed to modulation by embedded CNTs of the effective secondary electron (SE) emission yield at the polymer surface. This modulation of the SE yield is due to non-uniform surface potential distribution resulting from current flows due to leakage and electron beam induced current (EBIC). The importance of an external electric field on SEM subsurface imaging was also demonstrated. The insights gained from this study can be generally applied to SEM nondestructive subsurface imaging of conducting nanostructures embedded in dielectric matrices such as graphene-polymer composites, silicon-based single electron transistors, high resolution SEM overlay metrology or e-beam lithography, and have significant implications in nanotechnology.
Any link(s) to website(s) in this document have been provided because they may have information o... more Any link(s) to website(s) in this document have been provided because they may have information of interest to our readers. NIST does not necessarily endorse the views expressed or the facts presented on these sites. Further, NIST does not endorse any commercial products that may be advertised or available on these sites.
Accelerating the photodegradation of polymeric materials is of great practical interest in weathe... more Accelerating the photodegradation of polymeric materials is of great practical interest in weathering research. Acceleration can be achieved by exposing polymeric materials to a high radiant flux; however, questions have arisen within the weathering community as to whether high radiant flux results can be extrapolated to in-service flux levels. Experiments designed to test this premise are called reciprocity law experiments. An extensive review has been conducted to assess the state-of-the-art of reciprocity law experiments in the photography, photoconductivity, photo-medicine, photobiology, and polymer photodegradation literatures. From this review, the Schwarzschild law (a power law generalization of the reciprocity law) appears to model adequately photoresponse vs. radiant flux for most materials and systems. A band theory model has been presented to explain variations in the Schwarzschild law coefficients and other experimental phenomena commonly associated with reciprocity experiments. Obstacles to the general acceptance of high radiant flux, laboratory-based experiments are discussed.
One of the obstacles hindering the acceptance of polymer composites in civil engineering applicat... more One of the obstacles hindering the acceptance of polymer composites in civil engineering applications is the susceptibility of the polymeric matrix to degradation that is initiated by moisture, temperature, and corrosive chemical environments. The objective of this study was to characterize chemical and physical changes in polymer matrix resins following exposure to these environments. Resin systems studied were vinyl ester and isophthalic polyester, both of which are proposed for use in construction applications. Unreinforced free films were exposed to water, alkaline and saline environments at ambient and elevated temperatures for extended periods of time. Changes in strength and thermophysical properties were evaluated through tensile testing, dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). Chemical degradation of the polymers was characterized using Fourier transform infrared (FTIR) spectroscopy. Energy dispersive X‐ray (EDX) analysis of specimen...
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