Methods to assess the impact of UV irradiation on the surface chemistry and structure of multiwall carbon nanotube epoxy nanocomposites
Introduction
One prominent area of nanotechnology that is expected to dramatically increase in future years is use of nanocomposites in a variety of consumer, industrial, and manufacturing applications. Nanocomposite materials contain a nanofiller, which is defined as any particle with a characteristic dimension between 1 nm and 100 nm, incorporated into a matrix material (e.g., polymer, ceramic, etc.) in order to enhance the useful properties of the original matrix material. For example, incorporating multiwall carbon nanotubes (MWCNTs) into polymer matrices (i.e., MWCNT polymer nanocomposites) yields materials whose properties can readily be engineered for applications in aerospace [1], construction [2], and consumer products [3]. MWCNT nanocomposites offer important novel or substantially improved properties (e.g., mechanical, electrical, and light weight) compared to traditional fiber-reinforced polymer composites [4], [5].
While the incorporation of nanofillers to form a polymer nanocomposite has received significant research interest, far fewer studies have assessed the impact of environmental stresses (e.g., biodegradation, ultraviolet (UV) light, and rain) on nanocomposites and on the potential release of nanomaterials [3], [6], [7], [8], [9], [10]. The release of carbon nanotubes (CNTs) from nanocomposites has been investigated during machining processes (e.g., sanding, solid core drilling, and cutting) [11], [12], [13], [14], [15] and in end use applications during which the nanocomposites may be exposed to abrasion, photo or hydrolytic degradation [14], [16], [17], [18], [19], [20], [21]. CNT release is particularly important given their potential environmental and human health risks [6], [22], [23], [24]. In environmental uptake studies, CNT accumulation at high concentrations has been observed in water fleas (Daphnia magna) [25], [26] but not in soil or sediment organisms [6], [27], [28], [29], [30], [31]. CNTs have also shown the capacity to cause inflammation, oxidative stress, and potential genotoxicity that may cause risks for workers if exposure is not controlled [23], [32].
Studies on MWCNT release from polymer nanocomposites have sought to determine release rates and whether MWCNTs are released as free particles or are encapsulated in a polymer. While two studies have shown the release of free MWCNTs from an epoxy-based nanocomposite after abrasion [17] and sanding (but only for the 4% MWCNT loading) [15], release of individual MWCNTs has generally not been detected [11], [12], [13], [14], [16], [21]. These observations do not, however, preclude the possibility of MWCNT release under certain circumstances. In order to assess the likelihood of free MWCNT generation, it is necessary to understand the mechanisms that may lead to its occurrence. This information is needed for life cycle assessments of polymer nanocomposites. In this study, we focus on the effects of matrix degradation induced by UV radiation (i.e., photodegradation) – the most important degradation process for polymeric materials exposed to weathering environments [33]. While several studies have shown MWCNT surface accumulation after UV exposure [14], [16], [20], [21], the changes in the structure and surface chemistry of the nanocomposite were not fully assessed – principally due to a lack of optimized analytical methods – making the development of accurate mechanistic models, and thus prediction of release scenarios, challenging.
In this study, we have developed and applied a comprehensive suite of analytical methods to investigate dose-dependent effects of UV irradiation on the fate of MWCNTs and surface chemistry and structure of a MWCNT epoxy nanocomposite. Accelerated testing was performed using intense UV irradiation in the same spectral regime as the UV portion of natural sunlight (295 nm– 400 nm) at elevated temperature (50 °C) and humidity (75% relative humidity). Surface and bulk material chemistry and structure were analyzed using gravimetry, scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy in attenuated total reflection mode (FTIR-ATR), and X-ray photoelectron spectroscopy (XPS). Additionally, the sub-surface structure of the nanocomposites was investigated using SEM, bright-field transmission electron microscopy (TEM), and energy-filtered TEM (EFTEM) after preparing cross-sections using cryo-fracturing or a focused ion beam (FIB). This allowed for an investigation of the mechanism of damage to polymer MWCNT nanocomposite as a function of increasing UV exposure. As we show, no single analytical method provided all the necessary information – it was essential to develop and optimize a range of techniques to provide a complete picture. While most prior studies focused almost exclusively on nanoparticle (NP) release, this is the first study to investigate in depth the transformations of both the surface chemistry and surface morphology of a MWCNT polymer nanocomposite during UV degradation processes using a suite of optimized analytical methods. Further, the results obtained are helpful in assessing potential risks during the life cycle of MWCNT polymer nanocomposites, and the methods developed will facilitate a robust assessment of the impact of environmental stresses on polymer nanocomposites in future studies.
Section snippets
Experimental
MWCNT epoxy nanocomposite samples were prepared by mixing a MWCNT-pre-dispersed liquid epoxy resin with an aliphatic amine curing agent, and the mixture was then drawn down on a Mylar sheet to produce free standing films. These samples were subjected to a series of precisely controlled UV doses up to 1089 MJ/m2 using the National Institute of Standards and Technology (NIST) SPHERE (Simulated Photodegradation via High Energy Radiant Exposure) [34] in a 50 °C and 75% relative humidity (RH)
Effects of UV irradiation on bulk material
The mass loss of neat epoxy and 3.5% MWCNT epoxy nanocomposite samples as a function of UV dose in the NIST SPHERE is displayed in Fig. 1. Except for a small increase in mass at very early exposure, the mass loss in both materials increased with increasing UV dose. The early mass increase was probably due to moisture uptake when the samples were transferred from the 45% RH ambient condition to the 75% RH of the exposure chamber. At this early stage, the mass gained by the moisture uptake was
Conclusions
By using a comprehensive suite of analytical methods we have identified two primary trends for MWCNT polymer composites exposed to UV radiation: a photostabilization effect from the presence of the MWCNT nanofiller and increasing accumulation of a dense, entangled MWCNT layer on the surface of the nanocomposite samples with increasing UV dose. The increasing surface coverage by MWCNTs was strongly supported by both spectroscopic and microscopic techniques showing the convergence of the
Acknowledgements
Disclaimer: Certain commercial product or equipment is described in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that it is necessarily the best available for the purpose.
C.J. Long acknowledges support under the Cooperative Research Agreement between the University of Maryland and the National Institute of Standards and
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