Perylene-based profluorescent nitroxides for the rapid monitoring of polyester degradation upon weathering: An assessment

https://doi.org/10.1016/j.polymdegradstab.2013.07.006Get rights and content

Abstract

A profluorescent nitroxide possessing an isoindoline nitroxide moiety linked to a perylene fluorophore was developed to monitor radical mediated degradation of melamine-formaldehyde crosslinked polyester coil coatings in an industry standard accelerated weathering tester. Trapping of polyester-derived radicals (most likely C-radicals) that are generated during polymer degradation leads to fluorescent closed-shell alkoxy amines, which was used to obtain time-dependent degradation profiles to assess the relative stability of different polyesters towards weathering. The nitroxide probe couples excellent thermal stability and satisfactory photostability with high sensitivity and enables detection of free radical damage in polyesters under conditions that mimic exposure to the environment on a time scale of hours rather than months or years required by other testing methods. There are indications that the profluorescent nitroxide undergoes partial photo-degradation in the absence of polymer-derived radicals. Unexpectedly, it was also found that UV-induced fragmentation of the NO–C bond in closed-shell alkoxy amines leads to regeneration of the profluorescent nitroxide and the respective C-radical. The maximum fluorescence intensity that could be achieved with a given probe concentration is therefore not only determined by the amount of polyester radicals formed during accelerated weathering, but also by the light-driven side reactions of the profluorescent nitroxide and the corresponding alkoxy amine radical trapping products. Studies to determine the optimum probe concentration in the polymer matrix revealed that aggregation and re-absorption effects lowered the fluorescence intensity at higher concentrations of the profluorescent nitroxide, but too low probe concentrations, where these effects would be avoided, were not sufficient to trap the amount of polyester radicals formed upon weathering. The optimized experimental conditions were used to assess the impact of temperature and UV irradiance on polymer degradation during accelerated weathering.

Introduction

Profluorescent nitroxides are comprised of a sterically hindered persistent nitroxide radical, which is covalently linked to a suitable fluorophore. Due to the unpaired electron in the nitroxide moiety, enhanced intersystem crossing leads to quenching of the inherent fluorescence of the fluorophore [1]. However, when the nitroxide radical moiety reacts with other chemical species to form a diamagnetic (closed-shell) compound through either redox or radical trapping reactions, this intramolecular quenching process is inhibited and fluorescence is restored. In general, nitroxides react rapidly with carbon-centred radicals R⋅ to form alkoxy amines >N-OR. Nitroxides also react with oxygen-centred alkoxyl radicals RO⋅, albeit in an energetically slightly less favourable reaction, through formation of zwitterionic oxyaminoethers >N+(O)-OR [2]. Profluorescent nitroxides have, therefore, been successfully applied as diagnostic probes to detect both radical and redox activity in biological and manufactured materials [1]. For example, the thermo-oxidative degradation of polymers, including stabilized [3] and non-stabilized [4] polypropylene, as well as melamine-formaldehyde crosslinked polyesters used in the coil coatings industry [5] has been studied with profluorescent nitroxides, and it was shown that this technology was superior to other analytical monitoring techniques, such as chemiluminescence and Fourier transform infrared spectroscopy (FTIR), particularly with regard to sensitivity and time required to detect the earliest onset of degradation [5]. Most importantly, profluorescent nitroxides enable detection of free radical oxidative damage already during the so-called “induction period” of the degradation process [5], where conventional techniques lack sensitivity.

Polymer coatings exposed to the outdoor environment, however, are not only threatened by extreme surface temperatures, which can reach up to 95 °C in certain latitudes, but also by highly damaging solar UV radiation. In fact, profluorescent nitroxides have been used to study photo-oxidative degradation of polypropylene [5], but these particular nitroxide probes were not suitable to monitor radical degradation in melamine-formaldehyde crosslinked polyesters, because their absorption spectra in the UV did overlap with those of the polyesters at wavelength of λ < 295 nm. Studies reported in the literature, where profluorescent nitroxides with absorption maxima of λ > 300 nm were used and which could be selectively excited in the presence of polymers possessing aromatic building blocks, have, unfortunately, been hampered by the insufficient photostability of these probes [5].

In order to overcome these problems, we have designed and synthesised profluorescent nitroxide 3, which possesses a perylene fluorophore (Scheme 1). Perylenes, such as the diimide 1a, are generally known for their chemical resistance and stability towards higher temperatures and UV irradiation [6].

The perylene chromophore absorbs and emits in the visible region of the spectrum (λmax ≈ 500 nm for 1a), which does not overlap with the absorptions of polyesters, with fluorescence quantum yields near unity [7]. We chose an isoindoline nitroxide 2 as the fluorophore quenching/radical trap component of the probe, which is mounted onto the perylene through a covalent imide bond. The rigid isoindoline nitroxides are more stable than nitroxides possessing a piperidine framework, which are known to undergo photo-degradation via α-cleavage that leads to formation of an alkene with loss of nitric oxide [8], [9], [10]. Profluorescent nitroxide 3, which possesses a branched alkyl substituent at the opposing imide moiety to ensure sufficient solubility in organic solvents, was therefore expected to be a superior probe for monitoring the thermo- and photo-oxidative degradation of polymers and other advanced manufactured materials, in particular those with strong absorptions in the UV. It should be noted that radical processes in biological systems have previously been studied using perylene substituted nitroxides, for example for the indirect detection of reactive oxygen species [11], and a perylene linked isoindoline nitroxide has been developed for potential two-photon excitation fluorescence measurements [12].

In this paper we assess the performance of the profluorescent nitroxide 3 as a probe to detect and monitor free radical degradation of melamine-formaldehyde crosslinked polyesters (PE) in accelerated weathering testers, which are widely used in the coatings industry to simulate the effects of years of outdoor exposure on a very short time scale. We used two known commercial melamine-formaldehyde crosslinked polyesters, e.g. PE-A and PE-B, with different stability towards weathering to investigate the thermal- and photo-stability of the nitroxide probe and to evaluate its sensitivity for monitoring radical degradation under different conditions of accelerated weathering.

Section snippets

Materials

Melamine-formaldehyde crosslinked polyesters were used as clearcoats and contained no pigments, extenders or photostabilizers. Each clearcoat was formulated using industry standard methods to contain the same amount of crosslinker, based on the content of non-volatile material.

Profluorescent nitroxide 3 was synthesised through coupling of the nitroxide moiety 1,1,3,3-tetramethyl-5-nitroisoindolin-2-yloxyl 2b [5] with the fluorophore N

Monitoring polyester degradation

The capability of nitroxide 3 to detect and monitor polymer degradation through radical pathways was evaluated using two high quality melamine-formaldehyde crosslinked polyesters (i.e. PE-A and PE-B), which were designed for different applications. Real-time outdoor exposure studies showed that PE-A is more resistant to weathering than PE-B (see Figs. S5 and S6 in the Supporting Information). Our initial experiments were therefore aimed at exploring, whether and on which time scale nitroxide 3

Conclusions

We have designed and synthesised profluorescent nitroxide 3 as probe to detect and monitor radical mediated degradation of high quality melamine-formaldehyde crosslinked polyesters under accelerated weathering conditions. The probe concentration and exposure times were optimized to obtain polyester degradation profiles that allow rapid and qualitative assessment of high performing polyesters. Nitroxide 3 can be selectively excited with visible light in the presence of polyesters possessing

Acknowledgements

This work was supported by the Australian Research Council and PPG Industries Australia Pty Ltd. We thank Professor Ken Ghiggino, Dr Philip Barker and Mena Param for helpful discussions.

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