Impact of UV irradiation on multiwall carbon nanotubes in nanocomposites: formation of entangled surface layer and mechanisms of release resistance
Tinh Nguyen, Elijah J. Petersen, Bastien T. Pellegrin, Justin M. Gorham, Thomas F. Lam, Minhua Zhao, Li Piin Sung
Carbon nanotubes (CNTs) are increasingly used in consumer and structural polymeric products to enhance a variety of properties. Under weathering, the polymer matrix will degrade and the nanofillers will potentially be released from the products, which would have negative effects on the environment, health and safety. This study has investigated the degradation of a 0.72 % MWCNT/amine-cured epoxy nanocomposite irradiated with high intensity ultraviolet (UV) light at various doses and its effect on the surface accumulation and release of MWCNTs. Specially-designed cells containing nanocomposite specimens were exposed to high intensity UV radiation at 50 oC and 75 % RH up to a 4865 MJ/m2 dose. 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, the nanocomposite matrix underwent photodegradation, resulting in accumulation of a large concentration of MWCNTs on the surface, which aggregated and formed a dense, entangled network structure. Such MWCNT surface structure has shielded the epoxy matrix underneath from further degradation and curtailed its thickness increase with increasing UV dose. Despite a thick MWCNT layer formed on the composite surface, no MWCNT release was detected, even at very high UV doses, suggesting that the MWCNT surface layer formed by UV irradiation of polymer nanocomposites has a strong resistance to release from the surface. Four possible release resistance mechanisms of the UV-induced MWCNT surface layer are presented and discussed.