Fullerene-Enhanced Photodegradation in Polymers
Authors: Kalkan Kaan , Zhang Linqi , Karumuri Sriharsha 
Published: February 2020

Polymer and carbon nanostructure blends have attracted significant attention in recent years for structural, thermal, and optoelectronic applications. Therefore, understanding of degradation mechanisms in polymer/carbon nanocomposites is of interest. As such, photodegradation (PD) is a major concern. Interestingly, PD in conjugated polymers is substantially mitigated in presence of carbon nanostructures owing to ultrafast quenching of photoexcited polymer states by electron transfer to the carbon nanostructures. On the other hand, when the carbon nanostructures are photoexcited, they can efficiently generate reactive oxygen species by energy transfer, such as singlet oxygen, 1O2, which may subsequently oxidize and degrade the polymer. The present work investigates photodegradation mechanisms in a thermoplastic (poly-styrene) as well as a thermoset (epoxy) in presence of C60 particles and under ultraviolet radiation (UVA and UVB). Yellowing of polymers under sunlight is a common indication of photodegradation, but it is difficult to investigate quantitatively due to heterogeneity of PD reactions along the polymer thickness as a result of sharp decay of radiation intensity. Here, we overcome this limitation using thin films (0 to 5% C60 content) coated on quartz substrates (~650 nm thick). Subsequently, we monitor the PD kinetics from the photoproduct quantified from differential optical absorption spectroscopy. PD in polymers is propelled by a series of photochemical reactions: oxidation, dissociation, crosslinking and chain scission. To reveal the mechanistic steps of degradation, we have formulated an ab-initio kinetic model that encompasses these 4 photochemical reactions in the form of coupled first order differential equations. Our model exhibits an excellent agreement with the data systematically recorded by optical absorption (Figure 1a) as well as FTIR spectroscopies when the photoproduct is adopted as the carbonyl (C═O) sites. In the absence of C60, the polymer PD is found to be significantly slower, but autocatalytic. Our analysis shows new C═O sites are generated by UVA-excited C═O moieties through energy transfer to ground state triplet oxygen and subsequent reaction of active singlet oxygen with the −CH2− sites (Figure 1b). These electronically excited C═O moieties also undergo dissociation (if they do not transfer their energy to oxygen) creating free radical sites, which undergo either crosslinking or chain scission (Figure 1b). Additionally, the kinetics model is employed to elucidate the propagation of PD reactions along the thickness that are difficult to monitor experimentally. The presence of C60 is easily incorporated in our model by adopting C60 as a singlet oxygen sensitizer of constant density (with time). In other words, our work identifies the role of C60 as a permanent singlet oxygen sensitizer (Figure 1b). As such, PD is significantly accelerated in C60-blended polymer samples with a dramatically enhanced oxidation rate at the beginning of the UV exposure (Figure 1a). With increased population of C60 beyond a certain concentration, the PD rate is observed to be reduced, that we attribute to self-quenching.

Figure 1