Xiaohong Gu

Relationship between chemical degradation and thickness loss of an amine-cured epoxy coating exposed to different UV environments

The relationship between chemical degradation and thickness loss of an unpigmented, non UV-stabilized, crosslinked amine-cured epoxy coating exposed to three UV conditions was investigated. Spin-coated samples having a thickness of approximately 7 μm on an Si substrate were prepared from a stochiometric mixture of a bisphenol A epoxy resin and a tetra-functional amine curing agent. Samples were exposed outdoors and to two accelerated laboratory UV environments. Chemical degradation and thickness loss were measured by transmission Fourier transform infrared spectroscopy (FTIRS) and laser scanning confocal microscopy (LSCM), respectively. In addition, surface roughness and morphological changes were measured by atomic forcemicrosocopy (AFM) and LSCM. Substantial chemical degradation, thickness loss, and morpholocal changes occurred in the exposed films, and the rate of chemical degradation was greater than that due to the thickness loss. This additional chemical loss was attributed to an inhomogeneous degradation process in which nanoscale localized depressions initiate at certain sites on the surface, which then enlarge and deepen with exposure time. The results of this study provide a better understanding of the degradation mechanism and should lead to the development of scientific-based models for predicting the service life of crosslinked amine-cured epoxy coatings.

A quantitative study of nanoparticle release from nanocoatings exposed to UV radiation

Nanoparticles are increasingly used in polymer coatings (i.e., nanocoatings) to improve multiple properties including the mechanical, electrical, gas barrier, and ultraviolet (UV) resistance of traditional coatings. These high performance nanocoatings are often used in outdoor environments. However, because polymers are susceptible to degradation by weathering elements, nanoparticles in a nanocoating may be released into the environment during its life cycle, which potentially poses an environmental health and safety concern and may hinder application of these advanced coatings. This study presents protocols and experimental technique to quantify the release of nanosilica from epoxy nanocoating as a function of UV exposure. Specimens of an epoxy coating containing 5% untreated nanosilica in specially designed holders were exposed to UV radiation (295–400xa0nm) in a well-controlled high-intensity UV chamber. Exposed specimens were removed at specified UV dose intervals for measurements of coating chemical degradation, mass loss, nanosilica accumulation on specimen surface, and nanosilica release as a function of UV dose. Measurement of nanosilica release was accomplished by (a) periodically spraying UV-exposed specimens with water, (b) collecting runoff water/released particles, and (c) analyzing collected solutions by inductively coupled plasma-optical emission spectrometry using a National Institute of Standards and Technology (NIST)-developed protocol. Results demonstrated that the amount of nanosilica release was substantial and increased rapidly with UV dose. Mass loss, chemical degradation, and silica accumulation on specimen surface also increased with UV dose.

Near-Field Infrared Imaging and Spectroscopy of a Thin Film Polystyrene/Poly(ethyl acrylate) Blend

The application of broadband, near-field infrared microscopy to the characterization of the mesoscale structure of a thin film polymer blend is described. Key features of this instrument, which couples the nanoscale spatial resolution of scanning probe microscopy with the chemical specificity of vibrational spectroscopy, include broad tunability and bandwidth, parallel spectral detection for high image acquisition rates, and infrared-transparent aperture probes. Near-field spectral transmission images of a thin film of polystyrene/poly(ethyl acrylate) acquired in the C–H stretching region are reported. An assessment of the relative importance of transmission image contrast mechanisms is a significant aim of this work. Analysis of the near-field infrared spectra indicates that the image contrast in the C–H stretching region is largely due to near-field coupling and/or scattering effects. Identification and differentiation of the operative contrast mechanisms on the basis of their relative dependence on wavelength is discussed. Analysis of the contrast attributed to absorption is consistent with the chemical morphology of this sample derived from previous chemical modification/atomic force microscopy studies.

New insights into subsurface imaging of carbon nanotubes in polymer composites via scanning electron microscopy

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 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. 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.

Degradation modes of crosslinked coatings exposed to photolytic environment

The objective of this study is to assess the degradation modes of crosslinked coatings exposed to photolytic environments. Three model crosslinked coatings were exposed in various ultraviolet environments. Atomic force microscopy and Fourier transform infrared spectroscopy were used in following nanoscale physical and chemical degradation during exposures. Results indicated that photodegradation of crosslinked coatings is a spatially localized (inhomogeneous) process in which nanometer-sized pits are initially formed; these pits deepen and enlarge with exposure. A conceptual model is proposed to explain the inhomogeneous degradation mode. The model proposes that nanosize “hydrophilic” domains are dispersed randomly with the highly crosslinked units. These hydrophilic domains, which are energetically preferred, comprise polar, unreacted and partially polymerized molecules, chromophores, and other additives. Photodegradation initiates at degradation-susceptible hydrophilic domains spreading to surrounding areas contiguous with the initiation site.

Effect of Pigment Dispersion on Durability of a TiO 2 Pigmented Epoxy Coating During Outdoor Exposure

The effect of pigment dispersion on durability of a TiO 2 pigmented epoxy coating during outdoor exposure has been investigated. Well-dispersed and poorly dispersed coating samples were prepared through the addition or absence of a dispersant in the coating formulation. Ultra small angle neutron scattering (USANS) and scanning electron microscopy (SEM) showed that pigment aggregation occurs in the absence of dispersant. A thin, clear layer of epoxy was observed at the air/exposed surface interface in both the dispersed and non-dispersed samples. Chemical degradation and physical changes during UV exposure were measured by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), atomic force microscopy (AFM), and laser scanning confocal microscopy (LSCM). Results showed that the degree of pigment dispersion and the thickness of the clear layer contributed to weathering. Changes in surface topography and gloss loss during UV degradation were correlated with degree of pigment dispersion. Ripples and bumps on the top surface of the poorly dispersed coating greatly affected gloss. Bulk and surface mechanical properties were investigated using dynamic mechanical thermal analysis (DMTA) and instrumented indentation, respectively. Relative to the neat epoxy coatings, the addition of TiO 2 particles into the epoxy coatings increased elastic modulus but decreased the glass transition temperatures (T g ) of both of the pigmented coatings. Relationships between surface and bulk mechanical property changes and chemical degradation are discussed.

EFFECT OF COMPOSITION AND PROCESSING CONDITION ON MICROSTRUCTURAL PROPERTIES AND DURABILITY OF FLUOROPOLYMER/ACRYLIC BLENDS

Fluoropolymer blends have been widely used as binders for exterior coatings because of their excellent resistance to ultra-violet (UV) radiation as well as to many corrosive chemical agents. It is known that the fluorinated component usually has a lower glass transition temperature and easily crystallizes in the final structure depending upon the blend composition and sample annealing condition. We investigated the effect of blend composition and annealing process (slow and fast cooling) on the surface morphology and microstructure a poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) blend before and after UV exposure. Surface and subsurface microstructures were studied by atomic force microscopy (AFM) and laser scanning confocal microscopy (LSCM). Bulk microstructure of PVDF-coatings before and after UV exposure were characterized using small angle neutron and light scattering. Higher PVDF content and a slow cooling process result in larger spherulite crystallite structure and rougher surface morphology. Significant ordering in the spherulite crystallite structure has been observed on the surface and the bulk films after UV exposure.

Critical role of particle/polymer interface in photostability of nano-filled polymeric coatings

Nanoparticle-filled polymeric coatings have attracted great interest in recent years because the incorporation of nanofillers can significantly enhance the mechanical, electrical, optical, thermal, and antimicrobial properties of coatings. Due to the small size of the fillers, the volume fraction of the nanoparticle/polymer interfacial area in nano-filled systems is drastically increased, and the interfacial region becomes important in the performance of the nano-filled system. However, techniques used for characterizing nanoparticle/polymer interfaces are limited, and thus, the mechanism by which interfacial properties affect the photostability and the long-term performance of nano-filled polymeric coatings is not well understood. In this study, the role of the nanoparticle/polymer interface on the ultraviolet (UV) stability of a nano-ZnO-filled polyurethane (PU) coating system was investigated. The effects of parameters influencing the particle/polymer interfacial properties, such as size, loading, surface modification of the nanoparticles, on photodegradation of ZnO/PU films were evaluated. The nature of the interfacial regions before and after UV exposures were characterized by atomic force microscopy (AFM)-based techniques. Results have shown that the interfacial properties strongly affect chemical, thermo-mechanical, and morphological properties of the UV-exposed ZnO/PU films. By combining tapping mode AFM and novel electric force microscopy (EFM), the particle/polymer interfacial regions have been successfully detected directly from the surface of the ZnO/PU films. Further, our results indicate that ZnO nanoparticles can function as a photocatalyst or a photostabilizer, depending on the UV exposure conditions. A hypothesis is proposed that the polymers in the vicinity of the ZnO/PU interface are preferentially degraded or protected, depending on whether ZnO nanoparticles act as a photocatalyst or a photostabilizer in the polymers. This study clearly demonstrates that the particle/polymer interface plays a critical role in the photostability of nano-filled polymeric coatings.

Linking accelerated laboratory and outdoor exposure results for PV polymeric materials: a mechanistic study of EVA

Linking accelerated laboratory test to field performance for predicting the service life of polymeric materials are being investigated at NIST using the reliability-based methodology. Based on this methodology, a successful linkage between the laboratory and field exposure data for a model polymeric material has been made. Recently, this methodology, for the first time, was introduced to the lifetime assessment of PV polymeric materials. In this paper, a mechanistic study of the degradation of three unstabilized model ethylene vinyl acetate (EVA) systems---uncured EVA, cured EVA and laminated EVA---was carried out under accelerated laboratory exposure and outdoor exposure. The NIST SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) was used for the accelerated laboratory tests, and the outdoor exposure was conducted in Gaithersburg, Maryland. Simultaneous multiple stresses, including temperature, relative humidity and UV radiation, were applied individually or in combination during SPHERE exposure. The effects of the environmental factors on the main degradation mechanisms of different EVA systems were investigated. The results showed that the UV radiation was the most important factor for the degradation of EVA and a synergistic effect occurred between UV radiation and relative humidity. A slower degradation rate was observed for the laminated system as a result of limited diffusion of O2 and H2O into EVA. It was also found that the substantial chemical changes of the uncured EVA system did not yield yellowing, which was dramatically different from the peroxide cured EVA system. Additionally, the chemical degradation modes of the three EVA systems exposed outdoors appeared to be similar to those exposed to the SPHERE. The implication of this work to the current test standards was discussed.

More durable or more vulnerable? – Effect of nanoparticles on long-term performance of polymeric nanocomposites during UV exposure

ZnO nanoparticle is being used as an inorganic UV absorber for polymers. However, the mechanism of how ZnO nanoparticles influence the photo degradation of polymersis is not well understood. This study has investigated the role of ZnO nanoparticles in the long-term performance of a polyurethane (PU) nanocomposite subject to UV radiation. PU samples containing different levels of ZnO nanoparticles were exposed to the NIST Simulated Photodegradation via High Energy Radiant Exposure (SPHERE) UV chamber under both dry (0% RH) and moist (75% RH) conditions at 45°C. Chemical and physical properties with exposure times were characterised using multiple spectroscopic and microscopic techniques. The results indicated that the studied ZnO nanoparticles acted as a catalyst and accelerated the photodegradation of PU. The photo-catalytic effect was dependent on ZnO concentration and RH. It is suggested that systematical long-term performance study under different exposure environments is important for correctly evaluating the role of nanoparticles on durability and sustainability of polymer nanocomposites.