R.S. Lankone, J. Wang, J.F. Ranville, D.H. Fairbrother
Johns Hopkins University,
Keywords: CNT, sp-ICP-MS, photodegradation, release
Summary:As the development, production, and use of polymer nanocomposites continues to rise, so too does the likelihood for exposure of their embedded nanoparticles to humans and the environment following nanoparticle release. The release of carbon nanotubes (CNTs) from carbon nanotube polymer nanocomposites (CNT-PNCs) is of particular interest due to both CNTs’ potential hazard posed to organisms, as well as the variety of applications in which PNCs may be utilized, ranging from sporting equipment to the wingtip fairing of Lockheed Martin’s F-35 Joint Strike Fighter. Weathering of CNT-PNCs, specifically photodegradation of the polymer matrix, is a pathway to CNT release. However, while many studies have examined the release of CNTs from CNT-PNCs6, detailed information is still lacking on the effect CNTs have on the mechanism and extent of polymer photodegradation, as well as how the quantity, kinetics, and form of CNTs released evolve as a function of both initial CNT loading (% w/w) and the extent of irradiation. To address these knowledge gaps, we have focused our effort on developing a detailed, mechanistic understanding of CNT-PNC photodegradation, with a key experimental component being the use of single particle inductively coupled plasma mass spectrometry (sp-ICP-MS) to simultaneously quantify the concentration and characterize the form (individual CNTs vs. aggregates of CNTs embedded in polymer fragments) of released CNTs. This is accomplished through the use of metal nanoparticles, residual from CNT synthesis, as proxies for CNT detection. CNT-PNCs of a variety of CNT loadings (0 – 5% w/w) all experienced rapid mass loss and photodegradation during the initial period of irradiation; however the absolute magnitude of degradation is strongly inhibited with increasing CNT loading (Figure 1a). As irradiation continues and the presence of available chromophores initially present in the polymer matrix is depleted, the rate of mass loss experienced by the PNCs slows and eventually plateaus. The attenuation of mass loss with increasing CNT loading is attributed to the CNTs’ ability to shade pristine polymer beneath, thereby restricting the photolysis depth and preserving a greater extent of polymer as loading increases (Figure 1b,c). For CNT-PNCs with a high initial CNT mass loading (5% w/w), during the initial stages of photodegradation when mass loss is most rapid, CNTs are released as aggregates embedded within photolyzed polymer fragments (Figure 2). As irradiation continues, a dense CNT mat forms on the surface as the polymer matrix is photodegraded, and the extent of CNT release is greatly diminished. For the remaining period of irradiation, the CNT mat formed at the surface remains stable and well anchored to the PNC beneath, evinced by the minimal detection of CNTs with sp-ICP-MS. CNT-PNCs of lower initial CNT loadings also undergo CNT release, however the low overall CNT concentration leads to the release of smaller aggregates and individual CNTs. These CNT-PNCs are also not found to form visible CNT mats at the surface. These findings highlight the dynamic nature of CNT release and its dependence on initial CNT loading, as shown in Scheme 1.