Mechanisms of Aging and Release from Weathered Nanocomposites - Safety of Nanomaterials along Their Lifecycle - page 317

mass fraction to 0.5% mass fraction. This study quantified the nanocomposite degra-
dation by measuring the etching depth with an AFM (see Figure 14.1 for all methods
and their abbreviations). The enhanced UV resistance of the nanocomposite was
attributed to the electron ring of the MWCNT network, which can disperse and filter
radiation energy, and the strong interaction between free radicals (generated during
irradiation) and MWCNTs. Similarly, the photo-oxidation of vinyl acetate (EVA)/
MWCNT nanocomposites was observed to be lower than that of neat EVA, and the
effect was attributed to MWCNTs acting as filters as well as an antioxidant (Morlat-
Therias 2007). This study was conducted using UV radiation >300 nm at 60 o C in
the presence of oxygen, and photo-oxidation was characterized by FTIR. Similar
MWCNT photostabilization effect was also observed for high density polyethylene
(HDPE) (Grigoriadouet al. 2011).
However, chemical analysis by Wohlleben et al. (2011) suggested that an order
of magnitude higher MWCNT content in polyoxymethylene accelerated the deg-
radation of this photolabile polymer after irradiating it with an UV radiation dose
equivalent to nine-month outdoor exposure. These authors also observed that a
MWCNTs-containing layer having a thickness of approximately 3.0 µm was formed
on the sample surface, and the surface-exposed MWCNTs were devoid of poly-
mer matrix. Kumar et al. (2009) reported that, in the presence of singlet oxygen,
MWCNT also catalyze the photodegradation of ethylene propylene diene mono-
mer (EPDM). The enhanced degradation was attributed to the photocycloaddition
reaction between singlet oxygen and double bonds on the composites, followed by
cleavage. A study on graphene oxide (GO)/polyurethane (PU) nanocomposite under
295-400 nm UV radiation revealed that this carbon nanofiller did not seem to affect
the rate of photodegradation or mass loss as compared to neat polymer (Bernard
et al. 2011). However, both AFM and SEM images of this nanocomposite clearly
showed the presence of GO particles accumulated on the composite surface with
irradiation time.
Probably the most extensive study on the aging of polymer/MWCNT nanocom-
posites was performed by Nguyen and coworkers (Nguyen et al. 2008, 2009, 2011;
Petersen et al. 2013) of the National Institute of Standards and Technology (NIST).
Using the NIST Simulated Photodegradation via High Energy Radiant Exposure
(SPHERE) and a variety of analytical techniques, they investigated the photodegrada-
tion of epoxy and PU nanocomposites containing between 0.72% and 3.5% mass frac-
tion of pristine and isocyanate functionalized MWCNTs. This UV chamber provides
a highly uniform UV radiation of 290-400 nm (Chin et al. 2004). Their spectroscopic
(XPS and FTIR) results demonstrated that the degradation rates of the MWCNT nano-
composites were lower than those of the corresponding neat polymers. One example
of their results is displayed in Figure 14.2 for an epoxy/0.72% MWCNT nanocom-
posite. Figures 14.2a and b show changes in the transmission FTIR intensity with UV
irradiation time at 50 o C and 75% RH for the C-O band at 1245 cm −1 of the epoxy
chains and the C=O band at 1714 cm −1 formed during irradiation. (It is worth noting
that the degradation behavior and degradation mechanism of this epoxy at 75% RH
are similar to those exposed to outdoor during the summer in the state of Maryland.)
These two FTIR bands represent chain scission (a) and oxidation (b), respectively.
Figures 14.2a and b also include FTIR intensity changes of an epoxy/5% nanosilica