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Polymer Degradation and Stability 98 (2013) 2054e2062

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate/polydegstab

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

Paul D. Sylvester a,b, Helen E. Ryan c, Craig D. Smith c, Aaron S. Micallef d,Carl H. Schiesser a,b, Uta Wille a,b,*

aARC Centre of Excellence for Free Radical Chemistry and Biotechnology, Australiab School of Chemistry, Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne, 30 Flemington Road, Victoria 3010, Australiac PPG Industries Australia Pty Ltd, 14 McNaughton Road, Clayton, Victoria 3168, AustraliadAustralia Institute for Bioengineering and Nanotechnology and Centre for Advanced Imaging, University of Queensland, Brisbane 4072, Queensland,Australia

a r t i c l e i n f o

Article history:Received 6 December 2012Received in revised form4 July 2013Accepted 5 July 2013Available online 13 July 2013

Keywords:Profluorescent nitroxideFluorescenceMelamine-formaldehyde crosslinkedpolyesterRadical degradationAccelerated weathering

* Corresponding author. School of Chemistry, BiBiotechnology Institute, The University of Melbourne,3010, Australia. Tel.: þ61 3 8344 2425; fax: þ61 3 93

E-mail address: uwille@unimelb.edu.au (U. Wille)

0141-3910/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymdegradstab.2013.07.0

a b s t r a c t

A profluorescent nitroxide possessing an isoindoline nitroxide moiety linked to a perylene fluorophorewas developed to monitor radical mediated degradation of melamine-formaldehyde crosslinked poly-ester coil coatings in an industry standard accelerated weathering tester. Trapping of polyester-derivedradicals (most likely C-radicals) that are generated during polymer degradation leads to fluorescentclosed-shell alkoxy amines, which was used to obtain time-dependent degradation profiles to assess therelative stability of different polyesters towards weathering. The nitroxide probe couples excellentthermal stability and satisfactory photostability with high sensitivity and enables detection of free radicaldamage in polyesters under conditions that mimic exposure to the environment on a time scale of hoursrather than months or years required by other testing methods. There are indications that the pro-fluorescent nitroxide undergoes partial photo-degradation in the absence of polymer-derived radicals.Unexpectedly, it was also found that UV-induced fragmentation of the NOeC bond in closed-shell alkoxyamines leads to regeneration of the profluorescent nitroxide and the respective C-radical. The maximumfluorescence intensity that could be achieved with a given probe concentration is therefore not onlydetermined by the amount of polyester radicals formed during accelerated weathering, but also by thelight-driven side reactions of the profluorescent nitroxide and the corresponding alkoxy amine radicaltrapping products. Studies to determine the optimum probe concentration in the polymer matrixrevealed that aggregation and re-absorption effects lowered the fluorescence intensity at higher con-centrations of the profluorescent nitroxide, but too low probe concentrations, where these effects wouldbe avoided, were not sufficient to trap the amount of polyester radicals formed upon weathering. Theoptimized experimental conditions were used to assess the impact of temperature and UV irradiance onpolymer degradation during accelerated weathering.

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1. Introduction

Profluorescent nitroxides are comprised of a sterically hinderedpersistent nitroxide radical, which is covalently linked to a suitablefluorophore. Due to the unpaired electron in the nitroxide moiety,enhanced intersystem crossing leads to quenching of the inherentfluorescence of the fluorophore [1]. However, when the nitroxideradical moiety reacts with other chemical species to form a

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diamagnetic (closed-shell) compound through either redox orradical trapping reactions, this intramolecular quenching process isinhibited and fluorescence is restored. In general, nitroxides reactrapidly 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, throughformation of zwitterionic oxyaminoethers >Nþ(O�)-OR [2]. Pro-fluorescent nitroxides have, therefore, been successfully applied asdiagnostic probes to detect both radical and redox activity in bio-logical 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

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industry [5] has been studied with profluorescent nitroxides, and itwas shown that this technology was superior to other analyticalmonitoring techniques, such as chemiluminescence and Fouriertransform infrared spectroscopy (FTIR), particularly with regard tosensitivity and time required to detect the earliest onset of degra-dation [5]. Most importantly, profluorescent nitroxides enabledetection of free radical oxidative damage already during the so-called “induction period” of the degradation process [5], whereconventional techniques lack sensitivity.

Polymer coatings exposed to the outdoor environment, how-ever, are not only threatened by extreme surface temperatures,which can reach up to 95 �C in certain latitudes, but also by highlydamaging solar UV radiation. In fact, profluorescent nitroxides havebeen used to study photo-oxidative degradation of polypropylene[5], but these particular nitroxide probes were not suitable tomonitor radical degradation in melamine-formaldehyde cross-linked polyesters, because their absorption spectra in the UV didoverlap with those of the polyesters at wavelength of l < 295 nm.Studies reported in the literature, where profluorescent nitroxideswith absorption maxima of l > 300 nmwere used and which couldbe selectively excited in the presence of polymers possessing aro-matic building blocks, have, unfortunately, been hampered by theinsufficient photostability of these probes [5].

In order to overcome these problems, we have designed andsynthesised profluorescent nitroxide 3, which possesses a perylenefluorophore (Scheme 1). Perylenes, such as the diimide 1a, aregenerally known for their chemical resistance and stability towardshigher temperatures and UV irradiation [6].

The perylene chromophore absorbs and emits in the visibleregion of the spectrum (lmax z 500 nm for 1a), which does notoverlap with the absorptions of polyesters, with fluorescencequantum yields near unity [7]. We chose an isoindoline nitroxide 2as the fluorophore quenching/radical trap component of the probe,which is mounted onto the perylene through a covalent imidebond. The rigid isoindoline nitroxides are more stable than nitro-xides possessing a piperidine framework, which are known toundergo photo-degradation via a-cleavage that leads to formationof an alkene with loss of nitric oxide [8e10]. Profluorescent nitro-xide 3, which possesses a branched alkyl substituent at theopposing imide moiety to ensure sufficient solubility in organicsolvents, was therefore expected to be a superior probe for moni-toring the thermo- and photo-oxidative degradation of polymersand other advanced manufactured materials, in particular those

Scheme 1. Synthesis of the perylene-substituted nitroxide 3 and alkoxy amine 4. ReactionImidazole, ZnOAc$2H2O, 130 �C, 39%. [d] AIBN, C6H6, 60 �C, 70%. Fluorescence quantum yie

with strong absorptions in the UV. It should be noted that radicalprocesses in biological systems have previously been studied usingperylene substituted nitroxides, for example for the indirectdetection of reactive oxygen species [11], and a perylene linkedisoindoline nitroxide has been developed for potential two-photonexcitation fluorescence measurements [12].

In this paper we assess the performance of the profluorescentnitroxide 3 as a probe to detect and monitor free radical degrada-tion of melamine-formaldehyde crosslinked polyesters (PE) inaccelerated weathering testers, which are widely used in thecoatings industry to simulate the effects of years of outdoor expo-sure on a very short time scale. We used two known commercialmelamine-formaldehyde crosslinked polyesters, e.g. PE-A and PE-B,with different stability towards weathering to investigate thethermal- and photo-stability of the nitroxide probe and to evaluateits sensitivity for monitoring radical degradation under differentconditions of accelerated weathering.

2. Experimental

2.1. Materials

Melamine-formaldehyde crosslinked polyesters were used asclearcoats and contained no pigments, extenders or photo-stabilizers. Each clearcoat was formulated using industry standardmethods to contain the same amount of crosslinker, based on thecontent of non-volatile material.

Profluorescent nitroxide 3 was synthesised through coupling ofthe nitroxide moiety 1,1,3,3-tetramethyl-5-nitroisoindolin-2-yloxyl2b [5] with the fluorophore N-(1-hexylheptyl)perylene-3,4,9,10-tetracarboxyl-3,4-anhydride-9,10-imide 1b (Scheme 1) [6]. Wealso prepared the cyano isopropyl adduct 4, which was used in thiswork as model for closed-shell alkoxy amines that result fromtrapping of 3 by C-centred radicals (which are the likely primaryproducts during polyester degradation), by reacting 3 with azobi-sisobutyronitrile (AIBN) in degassed benzene under an argon at-mosphere at 60 �C for 3e4 h, followed by purification bypreparative TLC. Details are given in the SupplementaryInformation. It should be noted that OeC bond cleavage in 4 pro-duces a stabilized cyano isopropyl radical. Although the chemicalnature of the C-radicals formed upon polyester degradation is notknown, it is reasonable to assume that damage will preferentiallyoccur at the most “vulnerable” positions in the polyester. Thus, the

conditions: [a] H2SO4, HNO3/AcOH, 40 �C, 95%. [b] H2, Pd/C, PbO2, MeOH, r.t., 97%. [c]lds F: 1a: F ¼ 1.00 � 0.02 [6]); 3: F ¼ 0.05 � 0.02, 4: F ¼ 0.95 � 0.02.

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majority of polyester radicals will benefit from some stabilizationby neighbouring substituents, for example an oxygen atom or aC]O group in the ester moiety.

Since terminal substitution in perylene diimides has little effecton the optical properties, the absorption spectra of the perylenes1a, 3 and 4 in chloroform are practically identical, showing strongmaxima at l ¼ 490 and 530 nm (see Fig. S1 in Supporting Infor-mation) [7,13].

2.2. Doping polyester clearcoats with nitroxide 3

Nitroxide 3was dissolved in a small amount of solvent from thesolvated industry clearcoat formulation of the respective polyestersPE-A e C, and added with stirring. Approximately 5e10 mL of thisclearcoat/nitroxide mixture was drawn over an aluminium panelwith a wire wound drawdown bar. The panel was then placed for40 s in a fan-forced oven set at 270 �C during which the aluminiumpanel reached a peak temperature of 232 �C and the polyester resinreacted with the crosslinker to form a cured coating of 18e20 mmthickness, as determined using a customised high precisiondrill [14]. The coated panel was cut into square samples(12 mm � 12 mm). Unless stated otherwise, in the experiments inthis work the concentration of nitroxide 3was 0.200% (w/w) of thecured clearcoat, which showed a light red colour due to the per-ylene chromophore.

Because of the aluminium backing of the clearcoat films stan-dard absorption measurements could not be performed, and ab-sorption data were therefore obtained using reflectancespectrometry (see Section 2.5). The exemplary reflectance spec-trum of the clearcoat film of PE-A doped with 0.200% (w/w) ofnitroxide 3 in Fig. 1 shows the clear separation between the ab-sorption of the polyester in the UV region (polyesters of this typestrongly absorb at l < 320 nm) and the absorption of the perylenemoiety in the visible region of the electromagnetic spectrum. Theabsorption spectra of PE-A and nitroxide 3 in isolation are given inFigs. S2 and S3 in the Supporting Information.

2.3. Degradation of polyester clearcoats in an acceleratedweathering tester

Accelerated weathering was conducted in a Q-LAB QUV-seAccelerated Weathering Tester, which uses UVA-340 fluorescentlamps to emit UV radiation at wavelengths that simulate naturalsunlight. Unless otherwise stated, the samples were exposed toweathering cycles comprising of 0.89 W m�2 UV irradiance for 8 hat a temperature of 60 �C, followed by 4 h of condensing humidity

Fig. 1. Reflectance spectrum of polyester PE-A doped with 0.200% (w/w) nitroxide 3before degradation.

at 50 �C (ASTM International Standard G154 Cycle 1). Fluorescencemeasurements of the samples were taken in triplicates at differentexposure times by pausing the cycle and removing the samples.

Reflectance spectra of the polyester samples (in the absence of3) after 140 h of exposure to accelerated weathering did not revealformation of degradation products that absorb in the visible rangeof the electromagnetic spectrum. An exemplary reflectance spec-trum of un-doped PE-B before and after weathering is shown inFig. S4 in the Supplementary Information. It should be noted that,because of the strong absorption of these polymers in the UV re-gion, changes of the absorption properties of the polymer materialthat could result from structural alterations of the various UVchromophores upon weathering could not be determined. On theother hand, the reflectance spectra clearly revealed that the opticalclarity of the polyester films was not adversely affected byweathering.

2.4. Fluorescence measurements

The fluorescence quantum yields, F, for 3 and 4 were deter-mined on a Horiba Jobin Yvon Fluorolog 3 equippedwith a standard450 W xenon lamp and a Horiba Jobin Yvon FluoroHub singlephoton counting controller. The collected data were processed us-ing the Horiba Jobin Yvon FluorEssence software package (version2.5.2.0). Excitationwas performed at l¼ 494 nmwith a slit width of1.5 nm. Emissionwas detected in the range of l¼ 520e700 nmwitha slit width of 1.5 nm and an increment time of 0.5 s. All mea-surements were performed in UVevis grade dichloromethane us-ing a comparative method [15]. The fluorescence quantum yield ofnitroxide 3 is very low with F ¼ 0.05 � 0.02, but rises to nearlyunity upon reactionwith C-centred radicals (F¼ 0.95� 0.02 for themodel system 4). This confirms the excellent profluorescent prop-erties of nitroxide 3.

Fluorescence measurements of the clearcoat films doped withnitroxide 3were performed on a similar instrument equipped witha Horiba Jobin Yvon F-3018 integrating sphere, which was inter-nally coated with a reflective Spectralon� resin with >95% reflec-tance across the visible spectrum. The data were recordedaccording to the method described by Porres et al. [16]. Since thepolyester samples were used as films on aluminium backing (freefilms without solid support were too fragile), only one side wasexposed to irradiation. The recorded data, therefore, do not repre-sent the actual fluorescence quantum yields but are relative fluo-rescence values, which henceforth will be referred to asfluorescence intensity.

2.5. Reflectance measurements

UVevis absorptions of the clearcoat films on aluminium back-ings that were either blank or doped with perylenes 1a, 3 and4 were recorded through reflectance spectrometry using a ThermoScientific Evolution 220 UVeVisible Spectrophotometer equippedwith an integrating sphere, from l ¼ 800e190 nm at a scan rate of2 nm s�1 and an integration time of 0.5 s.

2.6. FTIR measurements

FTIR spectrawere recorded for free clearcoat films (dopedwith 3)on a PerkineElmer Spectrum One FT-IT spectrometer (universalATR), at a scan rate of 20 cm�1 s�1 and an integration time of 0.5 sfrom 3730 to 2390 cm�1, to determine the oxidation index accordingto the methodology established by Nichols [17] and Gerlock [18],which has been successfully applied to coil coatings by Bottle et al. todifferentiate polyester surface coatings with different performancelevels [5]. Details are given in the Supporting Information.

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2.7. Analytical HPLC

Analytical HPLC was performed on an Agilent 1100 LC systemequipped with a variable wavelength UV detector (l ¼ 254, 350and 520 nm), using a Phenomenex Luna 5m PFP (2) 100A(150� 4.60mm) column and a gradient of 50:50water/THF to 100%THF over 30 min at a flow rate of 1 mL min�1. Rt (3) ¼ 12.1 min, Rt(4) ¼ 14.6 min. The eluents were collected and analysed by HRMS(ESI) using a Thermo-Finnigan LTQ FT-ICR hybrid massspectrometer.

3. Results and discussion

3.1. Monitoring polyester degradation

The capability of nitroxide 3 to detect and monitor polymerdegradation through radical pathways was evaluated using twohigh 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 moreresistant to weathering than PE-B (see Figs. S5 and S6 in the Sup-porting Information). Our initial experiments were therefore aimedat exploring, whether and on which time scale nitroxide 3 clearlyrevealed the relative stability of these two polyester coatings to freeradical degradation upon exposure to accelerated weathering.

Fig. 2 shows the results of the degradation experiments of bothpolyesters doped with nitroxide 3 in the accelerating weatheringtester. Fluorescence measurements during the first 140 h of expo-sure were performed every 20 h. In order to facilitate comparisonbetween the two polyesters, the fluorescence data are given rela-tive to the initial fluorescence intensity. The error bars representthe second standard deviation (2s) of the triplicate samples.

The data clearly reveal an increase of the fluorescence emissionof each polyester sample with increasing duration of exposure toaccelerated weathering. This corresponds to formation of fluores-cent alkoxy amines of type 4 through trapping of radicals that resultfrom degradation of the polyesters (since the primary radical spe-cies formed upon polyester degradation are most likely C-centred,we will focus on these in this work). Remarkably, a clear differencebetween the performance of PE-A and PE-B was already evidentafter only 60e80 h of exposure to accelerated weathering. Thistrend was further consolidated when the exposure time waselongated to up to 140 h. The more rapid increase of the fluores-cence intensity in PE-B suggests that C-centred radicals aregenerated at a faster rate in this polyester than in PE-A. This is a

Fig. 2. Degradation profiles of PE-A (B) and PE-B (,) doped with 0.200% (w/w) ofnitroxide 3 (relative fluorescence intensities in arbitrary units, degraded at 60 �C and0.89 W m�2 irradiance; error bars are 2s).

qualitative indication that PE-B is less stable toweathering than PE-A, which is in excellent agreement with the results from the real-time outdoor exposure studies. Thus, although both PE-A and PE-B are advanced, high quality melamine crosslinked polyesters,nitroxide 3 is sufficiently sensitive to detect differences in relativeperformance already after an exceedingly short period of exposure(less than a week) to accelerated weathering. In contrast to this,conventional industry techniques that are based on gloss loss andcolour fade (in the case of pigmented samples) require months todistinguish between the performances of similar high-performingpolyesters. It is worth noting that control measurements of thefluorescence intensities for various polyester samples after severalmonths fully reproduced the data obtained directly after theaccelerated weathering experiments.

After prolonged exposure (395 h) the fluorescence intensitiesfor both PE-A and PE-B converged to the same value. This suggeststhat, while the rate at which this final fluorescence intensity isreached depends on the rate of radical generation (which is basedon the stability of thematerial), the actual value of this fluorescenceintensity is determined by the concentration of the nitroxide probe3 in the polyester. However, there are strong indications that thisexplanation is too simplified and that the observed plateau of thefluorescence intensity after extended exposure to acceleratedweathering is caused by a light-driven steady state that is estab-lished between the nitroxide 3 and closed-shell alkoxy amines oftype 4. This will be further outlined in Section 3.2.

We have considered whether differences in mobility of thenitroxide probe 3 in the two polymer matrices could explain thedifferent rate of fluorescence increase, because a higher mobility of3 would enable a more rapid migration to radical sites in thepolyester. To our knowledge, the mobility of profluorescent nitro-xides embedded in a cured cross-linked polymer has never beenexamined in previous applications of this technology [5]. Accordingto a recent model, the mobility of additives in polymers depends onthe ratio between the volume needed for the mobile moiety tojump and the free volume in the polymer [19]. For a given polymer,the latter is dependent on its state, e.g. whether the polymer isglassy or rubbery. We have therefore measured the thermal soft-ening temperatures, Ts, of the polymer films (Ts is not the same asthe glass transition temperature, Tg, but usually close to it) usingthermo-mechanical analysis (TMA), which revealed Ts for PE-A ¼ 47.0 �C and Ts for PE-B ¼ 19.0 �C. Since the acceleratedweathering experiments were performed at 60 �C, which is wellabove Ts of either polymer, it can be assumed that nitroxide 3should have adequate mobility in both polymers. Thus, thedifferent rate of fluorescence increase in PE-A and PE-B shouldprimarily result from different rates of radical production in thesepolyesters during weathering.

However, it needs to be pointed out that reduced mobilitywould not only affect the nitroxide probe but very likely also thepolyester degradation products. In a polyester matrix with higherdensity (as reflected by the Ts value), degradation could thereforebe slowed down as a result of the lower mobility of the reactivedegradation intermediates that propagate the degradation process.

To explore this further, we used polyester PE-C to produce twoclearcoat films with different crosslinker ratios, which were dopedwith 0.200% (w/w) of nitroxide 3 and subsequently exposed toaccelerated weathering. Indeed, the polyester with the higher levelof crosslinking (and consequently higher density) developedconsiderably lower fluorescence intensities over the time of expo-sure than the polyester with the lower degree of crosslinking (Fig. 3).

In fact, it has been shown that the degree of crosslinking has asignificant effect on the rate of free radical generation and stabilityof clearcoats subjected to photo-oxidation [20]. Thus, the lowerfluorescence intensity observed in the case of PE-C with higher

Fig. 3. Degradation profiles of PE-C with high crosslinker level (�) and low crosslinkerlevel (#), doped with 0.200% (w/w) of nitroxide 3 (relative fluorescence intensities inarbitrary units, degraded at 60 �C and 0.89 W m�2 irradiance; error bars are 2s).

P.D. Sylvester et al. / Polymer Degradation and Stability 98 (2013) 2054e20622058

degree of crosslinking could be attributed to the intrinsically higherstability of the polymer itself and/or the reduced mobility of bothprobe and polymer radicals, which indicates that profluorescentnitroxides as stand-alone technology are not suitable to distinguishbetween these possibilities. It should also be noted that in highlycross-linked polyesters further complications could arise from areduced mobility of oxygen, which could react with C-centredpolymer radicals to form peroxyl radicals. Peroxyl radicals do notreact with nitroxides and can therefore not be detected by thismethod. It is not possible without analysis of polyester morphologyand mobility of additives (for example through microtome analysisor confocal fluorescence microscopy) to assess the impact of thesefactors on the observed fluorescence intensity. This needs to beconsidered when profluorescent nitroxides are employed to pro-vide more than qualitative information about radical degradationin polymers.

From the manufacturer’s point of view it would be highlydesirable to be able to state the amount of radicals formed duringpolyester degradation. We therefore explored whether alkoxyamines of type 4 would represent a suitable standard to determinethe maximum fluorescence intensity that could be achieved inthese polymer films if trapping of nitroxide 3 by polymer-derivedradicals would be quantitative. Unfortunately, during curing of thepolyester films of PE-A and PE-B, which were both doped withalkoxy amine 4, the latter underwent thermal degradation to aconsiderable extent to form 3. This was clearly revealed fromsubsequent polyester degradation experiments, where exposure ofcured samples of PE-A and PE-B containing 0.200% (w/w) of 4(before curing) to standard accelerated weathering for 140 h led toan increase in fluorescence intensity similar to that observed forPE-A and PE-B doped with profluorescent nitroxide 3 uponaccelerated weathering (data not shown). Thermal homolyticscission of the labile NOeC bond that links the nitroxide moietywith the alkyl group is, in fact, a highly common process in alkoxyamines derived from nitroxides and C-radicals [21]. On the otherhand, dissolving 4 into the polyester matrix after curing is not aviable alternative for determining the maximum fluorescence in-tensity, because it is, without elaborate analytical techniques, notclear whether and how uniformly the probe would be distributedin the polymer.

Fig. 4. Reflectance spectra of samples of PE-A (d) and PE-B (—) containing 0.200% (w/w) 3 prior to accelerated weathering versus the corresponding PE-A ($$$) and PE-B($$-$$-$$-) after exposure.

3.2. Photostability of nitroxide 3

After prolonged exposure to weathering (>200 h) it wasobserved that the light red colouration of the nitroxide-doped

polyester samples became visibly faded, which suggests thatnitroxide 3 and/or the closed-shell alkoxy amines of type 4resulting from radical trapping underwent degradation in thepolyester matrix under the conditions of accelerated weathering.

The photostability of the perylene derivatives 3 and 4within thepolymer matrix was qualitatively assessed with the help of reflec-tance spectrometry by monitoring changes in the absorptionspectra of the polymer films in the visible region of the electro-magnetic spectrum in response to accelerated weathering.

We first explored the photostability of the perylene chromo-phore itself, without the nitroxide moiety, by exposing samples ofPE-A and PE-B containing 0.200% (w/w) of the perylene diimide 1ato accelerated weathering for 140 h. Analysis of the reflectancespectra through integration of the area in the range l ¼ 400e600 nm revealed that about 99% of the initial reflectance wasretained in both PE-A and PE-B films, compared with the respectiveundegraded samples (see Fig. S7 in Supporting Information).However, we also noticed that the fluorescence intensity of 1a inboth PE-A and PE-B dropped by about 20% over this time (data notshown). This indicates that 1a undergoes photo-degradation tosome extent under these conditions, which appears to be inde-pendent of the nature of the polymer matrix, despite the differentstability of these polyesters to accelerated weathering. However,this photo-degradation obviously did not alter the structuralintegrity of the perylene moiety sufficiently to result in a measur-able change of the absorption characteristics.

In contrast to this, the reflectance spectra of PE-A (solid line) andPE-B (dashed line) containing 0.200% (w/w) of nitroxide 3 revealedthat only about 51% of reflectance in the wavelength region be-tween l ¼ 400e600 nmwas retained in both of the polymers PE-Aand PE-B after 140 h of exposure to accelerated weathering (Fig. 4).

This indicates that photo-excitation of the nitroxide moiety in 3within the polyester matrix obviously leads to partial degradationof the perylene fluorophore. However, we also noticed that a slightincrease of the UV irradiance to 1.04 W m�2 (which is 10% higherthan the standard weathering conditions) had nomeasurable effecton the rate of nitroxide degradation. Additional support for aphoto-decomposition pathway in 3 was obtained from weatheringexperiments of polysiloxane polymers that were doped with0.200% (w/w) of nitroxide 3. Polysiloxanes are highly resistant tofree radical degradation, and it was found from the reflectancespectra that 3 underwent complete photo-degradation within 20 hof exposure of this polymer to accelerated weathering (data notshown).

Because of these findings, the photo-stability of nitroxide 3 wasfurther studied in the absence of the polymer matrix, where a

P.D. Sylvester et al. / Polymer Degradation and Stability 98 (2013) 2054e2062 2059

solution of 3 (1 mM) in degassed benzene in a sealed quartz tubewas exposed to UV light in the QUV for 6 h. Although HPLC analysisrevealed 3 as major constituent of the reaction mixture after UVirradiation, a number of new products were formed, but theamounts were too small to enable further analysis. Interestingly,when 3 was irradiated under similar conditions in the presence ofstoichiometric amounts of AIBN, rapid formation of 4 occurred andphoto-degradation of 3 was not observed. On the other hand,exposure of solutions of alkoxy amine 4 in benzene to UV lightproduced a mixture containing both 3 and 4, with no measureablephoto-decomposition products of 3. This clearly shows that the UVlight emitted by the QUV induces homolytic scission of the NOeCbond in 4 to release 3. After 24 h of UV exposure the reactionmixture contained equal amounts of 3 and 4 (according to HPLCanalysis), which indicates a light-driven steady state, where pro-duction and degradation of fluorescent species occurs at constantrate.

These findings are summarized in Scheme 2. Thus, photo-degradation of 3, at least in solution, occurs readily in theabsence C-radicals to produce non-fluorescent products (eq. 1).When C-radicals are present, reversible recombination with 3 andlight-induced fragmentation of 4 become the dominant processes(eq. 2), and photo-degradation of 3 is suppressed.

These outcomes can be used to tentatively assess the fate of 3within the polymer matrix upon exposure to accelerated weath-ering. If the polymer material is not particularly instable to degra-dation, it is not very likely that accelerated weathering will rapidlyproduce a sufficient amount of C-centred polyester radicals to trapnitroxide 3 as closed-shell compound of type 4 and to preventphoto-degradation of 3 through eq. 1. The extent of photo-degradation of 3 depends, therefore, on the amount of C-radicalsformed during weathering of the polymer. The higher the stabilityof the polymer the lower the concentration of ‘active’ nitroxide 3 inthe polymer film becomes, and in most cases [3] will therefore belower than the amount added to the polymer mixture prior tocuring. On the other hand, the photochemical cleavage of 4 (eq. 2,reverse reaction) produces 3 and the respective polyester radical inclose proximity in the polymer. Since 3 does not undergo photo-degradation under these conditions, we believe that separation of3 and the corresponding C-radical through diffusion across thepolymer matrix cannot compete with their rapid and barrier-lessrecombination that regenerates 4.

The outcomes from this and the previous section clearly showthat application of the nitroxide technology to study UV-inducedradical degradation in polymers has some serious pitfalls, and itis essential to carefully assess the stability of both the nitroxideprobe and the alkoxy amine trapping products under differentconditions. We would like to point out that nitroxide 3 appears tobe a highly suitable probe for the qualitative assessment of poly-ester performance on an exceptionally short time scale. However,as a result of the competing reactions outlined in Scheme 2, careshould be taken with regards to quantitative conclusions on abso-lute concentration of radicals produced in polyesters upon expo-sure to accelerated weathering. Thus, even if the resistance of thepolyester to accelerated weathering would be so poor that everymolecule of 3would be trapped as alkoxy amine of type 4 at a fasterrate than undergoing photo-degradation according to eq.1, because

Scheme 2. Reaction of nitroxide 3 under UV-irradiation.

of the light-driven reverse reaction in eq. 2 the expected maximumfluorescence intensity cannot be reached. In addition to this,nitroxides are generally known to undergo cycles of forming alkoxyamines and re-forming the active nitroxide moiety, in particular inthe presence of oxygen, according to the Denisov cycle [2,22].Therefore, even if a sufficient amount of radicals is present, only aproportion of the active nitroxide will find itself trapped as closed-shell (and fluorescent) species after a given exposure period.

3.3. Thermal stability of nitroxide 3

In order to assess the thermal stability of the nitroxide probe,solid samples of 3 were placed in a convection oven at 150 �C for48 h. Although this temperature considerably exceeded themaximum temperature that polyester coatings could face underextreme environmental conditions (see above) [5], through expo-sure to 150 �C we intended to ensure that nitroxide 3 is thermallystable over a large temperature range. The solid samples were thenimmersed in dichloromethane to extract and dissolve any decom-position products. Since subsequent analysis through TLC, HPLC,HRMS (ESI), and IR did not reveal any significant differencescompared to a dichloromethane extract of 3 prior to exposure tohigh temperatures, a high thermal stability of nitroxide 3 could beconfirmed.

3.4. Optimizing the probe concentration

The influence of probe concentration on the degradation pro-files of polyesters was evaluated by doping PE-Bwith 0.200% (w/w),0.100% (w/w) and 0.025% (w/w) of 3. Fig. 5 shows relative fluo-rescence emission spectra of the polyester films prior to exposureto accelerated weathering (as mentioned before, because thesamples were films on an aluminium backing, absolute emissionspectra could not be obtained). It was found that the polyestersample with 0.100% of 3 had the highest maximum fluorescenceintensity, while the samples with both the highest and lowestconcentration of 3 gave significantly lower fluorescence intensities.

The considerable concentration dependence of the fluorescenceintensity can be rationalised by two effects. The change in therelative intensity of the emission maxima at 0.100% (w/w) of 3,compared to the sample with 0.025% (w/w) of 3, suggests re-absorption effects, while the slight red-shift in the peak positioncan be explained by aggregation of the nitroxide 3 through inter-molecular pep stacking. Aggregation is known to decrease the

Fig. 5. Relative fluorescence emission spectra (in arbitrary units) of PE-B containing0.200% (w/w) (d), 0.100% (w/w) (—), and 0.025% (w/w) ($$$) of 3, prior to exposure toaccelerated weathering (relative fluorescence intensities in arbitrary units, degraded at60 �C and 0.89 W m�2 irradiance; error bars are 2s).

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fluorescence quantum yield in both solution and films [23e25].These two effects are further exacerbated at 0.200% (w/w) of 3,where they cause a significant decrease of the fluorescence emis-sion. Support for this hypothesis was obtained from independentexperiments in solution, where fluorescence emission spectra ofperylene diimide 1a (whose fluorophore and emission spectrum isidentical with nitroxide 3) were recorded (i) at different concen-trations of 1a, and (ii) at a fixed concentration of 1a, to which 3wasadded in increments. Similar red-shifts and changes in intensity ofthe absorption maxima with increasing concentration of 1a or 3,respectively, were observed in both sets of experiments (seeFigs. S8 and S9 in Supporting Information), which confirms thatconcentration-dependent aggregation and re-absorption areindeed important factors that determine the fluorescence emissionof the nitroxide probe. These experiments further revealed thatpolyester films doped with only 0.025% (w/w) of nitroxide 3 did notshow noticeable aggregation and re-absorption effects.

The polyester films containing the different concentrations of 3were subsequently exposed to accelerated weathering for a total of140 h. The time-dependent fluorescence intensity profiles in Fig. 6clearly show that a probe concentration of only 0.025% (w/w) of 3resulted in a lower maximum fluorescence intensity, whichappeared to remain constant (within error) after exposure timesbeyond 20 h. This suggests that by that time all nitroxide 3 wasconsumed by the reactions outlined in Scheme 2 and that a steadystate between 3 and the alkoxy amine of type 4 was reached.

In the case of the polyester doped with 0.100% (w/w) of nitro-xide 3, virtually no change in fluorescence intensity could bemeasured after 70e80 h of exposure to accelerated weathering.Because of the findings outlined in Section 3.1, which show that atleast 80 h of exposure to accelerated weathering are required toclearly evaluate the performance of a polyester, this suggests thatthese probe concentrations are too low for the amount of radicalsformed during polyester degradation.

On the other hand, in the PE-B sample with the highest con-centration of nitroxide 3 (0.200% (w/w)), the fluorescence in-tensity continued to increase beyond 80 h of exposure toaccelerated weathering, which shows that under these conditionsthe probe was in excess over the amount of radicals formed uponpolyester degradation. Thus, despite the considerable concentra-tion and re-absorption effects that affect the fluorescence intensityunder these conditions, a minimum concentration of 0.200% (w/w) of 3 appears to be required for a reliable, qualitative assessmentof the performance of high quality polyesters to acceleratedweathering.

Fig. 6. Effect of nitroxide 3 concentration on degradation profiles of PE-B: 0.200% (w/w) (,), 0.100% (w/w) (D) and 0.025% (w/w) (þ) (relative fluorescence intensities inarbitrary units, degraded at 60 �C and 0.89 W m�2 irradiance; error bars are 2s).

3.5. Sensitivity of nitroxide 3

The optimized experimental conditions were used to explorethe sensitivity of nitroxide 3 to report the impact of variations tothe accelerated weathering conditions, e.g. small changes in tem-perature and UV irradiance of the QUV cycles and their role inpolyester degradation.

3.5.1. Effect of temperaturePE-A and PE-B doped with nitroxide 3 (0.200% (w/w)) were

subjected to a modified weathering cycle, where the internalchamber temperature during the irradiance cycle was increased by10 �Ce70 �C. The data in Fig. 7, which include the degradationprofiles for both polyesters at 60 �C for comparison, show that thistemperature increase resulted in a considerable increase in fluo-rescence intensity for both polyesters. In essence, at 70 �C the morestable PE-A produced a degradation profile that closely resembledthe profile of the less stable PE-B at 60 �C. This clearly shows thatnitroxide 3 is a highly sensitive probe, which enables it to monitorincreased radical damage in polyesters that is caused by amoderatetemperature increase (about 10%) during the irradiance cycle ofaccelerated weathering.

In order to explore the role of temperature on the degradationprofiles in isolation, samples of PE-A and PE-B doped with 0.200%(w/w) of nitroxide 3 were placed in a convection oven at 95 �C,which is, as mentioned before, the maximum surface temperaturethat manufactured coating materials are expected to experienceduring service life for roofing applications [5]. Interestingly, neitherPE-A nor PE-B showed any significant increase in fluorescence in-tensity even after 140 h of exposure (see Fig. S10a in SupportingInformation). This finding could be interpreted in two ways such asthat under these conditions (i) either both polyesters were stable,or (ii) both underwent degradation through non-radical pathways.Since independent FTIR measurements to determine the oxidationindex [5,17,18] for both PE-A and PE-B (doped with 0.200% (w/w) of3) did not reveal any change in the spectrum after exposure to 95 �Cfor 140 h (see Fig. S11 in Supporting Information), and 1H NMRstudies further showed that alkoxy amines of type 4 do not undergothermal degradation at this temperature, which would lead to lossof fluorescence intensity (data not shown), it is suggested that bothpolyesters are stable under these conditions. However, it cannot beexcluded that at this elevated temperature trapping of polyesterradicals by oxygen may be faster than by nitroxides. In conclusion,these findings clearly show that it is not suitable to assess the

Fig. 7. Effect of temperature on the degradation profiles of PE-A (B) and PE-B (,)doped with 0.200% (w/w) nitroxide 3 at 60 �C versus PE-A (5) and PE-B (@) 70 �C(relative fluorescence intensities in arbitrary units, degraded at 0.89 Wm�2 irradiance;error bars are 2s).

P.D. Sylvester et al. / Polymer Degradation and Stability 98 (2013) 2054e2062 2061

performance of polyesters, such as PE-A and PE-B, under environ-mental conditions on this time scale solely through exposure tohigher temperatures.

In this regard, it is worth to note that control experiments whereboth polyester samples were kept in the convection oven at 150 �Cfor 10 h, revealed an increase in fluorescence intensity at a rate thatwas the same for both PE-A and PE-B. The fluorescence profiles aregiven in Fig. S10b in the Supporting Information. Although theprobe 3 in isolation is stable under these conditions (see Section3.3), it is possible that the observed fluorescence could arise from athermolytic reaction of the nitroxide. At this high temperaturenitroxide 3 could abstract a hydrogen atom from the polyester togive a fluorescent hydroxylamine (not shown) as well as productsarising from the C-centred radicals, which are formed during thisprocess. Radical degradation could also occur in both polyesterswithout involvement of the nitroxide. We did not further explorethese reactions, which obviously occurred at a similar rate andindependent of the structure of the polyester, since this tempera-ture was far too high to be of environmental relevance.

3.5.2. Effect of UV irradianceThe influence of UV irradiance on polyester degradation was

determined by subjecting PE-A and PE-B, doped with 0.200% (w/w)3, to a modified weathering cycle in the QUV, where the irradiancewas increased by about 10% from 0.89 W m�2 to 1.04 W m�2 at60 �C. The results are shown in Fig. 8, which include the data ob-tained at the lower UV irradiance for comparison.

It is obvious that an increase in UV irradiance had no discernibleeffect on the degradation profile of either PE-A or PE-B, even after140 h of exposure. Thus, if natural weathering would be induced byUV radiation alone, the fluorescence intensity would be expected toincrease at a faster rate at higher UV irradiance. That this is not thecase indicates that UV radiation in isolation is not responsible forpolyester weathering under environmental conditions.

In summary, these experiments show that nitroxide 3 has therequired sensitivity to qualitatively reveal that polyester degrada-tion upon weathering results from the interplay between temper-ature, UV irradiation and humidity. The observed accelerating effectof temperature is in accordance with mechanistic considerationsfor the degradation of melamine-formaldehyde crosslinked poly-esters, which suggest that initiation of auto-oxidation is likely toinvolve a thermally controlled process, such as hydrolysis oroxidation [26,27]. Our findings demonstrate the importance ofprecise control over weathering parameters, e.g. temperature, UVirradiance and humidity, in the evaluation of polyester materials.

Fig. 8. Effect of UV irradiance intensity on the degradation profiles of PE-A (B) and PE-B (,) doped with 0.200% (w/w) of nitroxide 3 at 0.89 W m�2 irradiance versus PE-A(5) and PE-B (@) at 1.04 W m�2 irradiance (relative fluorescence intensities in arbi-trary units, degraded at 60 �C; error bars are 2s).

4. Conclusions

We have designed and synthesised profluorescent nitroxide 3 asprobe to detect and monitor radical mediated degradation of highquality melamine-formaldehyde crosslinked polyesters underaccelerated weathering conditions. The probe concentration andexposure times were optimized to obtain polyester degradationprofiles that allow rapid and qualitative assessment of high per-forming polyesters. Nitroxide 3 can be selectively excited withvisible light in the presence of polyesters possessing aromaticbuilding blocks, and its excellent thermal stability and satisfactoryphotostability makes it possible to study polymer degradationunder harsh environmental conditions. With this probe a qualita-tive assessment of polyester performance upon acceleratedweathering can be achieved on a time scale of less than aweek. Thisis a significant advantage to conventional industrial testing pro-cedures, which involve monitoring changes in specular gloss andcolour-fading (in the case of pigmented materials), and oftenrequire months of accelerated weathering to establish reliabletrends of relative performance.

While we have so far used profluorescent nitroxide 3 only toqualitatively investigate the performance of polyesters with Tsvalues below the temperature of the accelerating weathering ex-periments, it is not possible at this stage to assess whether thenitroxide technology can also be applied to investigate free radicaldegradation in polyesters with higher Ts values, where mobility ofboth probe and polyester-derived radicals might be constricted.Detailed studies of the morphology of different melamine-crosslinked polyesters and the mobility of additives, in particularprofluorescent nitroxides of type 3 and model systems of polymerradicals, within these polymer matrices are urgently required.

This work revealed important insight into the stability of bothnitroxide 3 and the alkoxy amine radical adducts of type 4 underUV irradiation. Model studies involving these two species in solu-tion showed that 3 undergoes photo-degradation to some extent inthe absence of radicals. On the other hand, when radicals are pre-sent, for example through polymer degradation, trapping bynitroxide 3 and formation of alkoxy amines of type 4 becomes thedominant reaction. Surprisingly, 4 is also not stable under UV-irradiation and readily undergoes photo-induced scission of theNOeC bond that regenerates 3 and the respective polymer-derivedradical. Our polyester degradation studies suggest that a similarphoto-degradation pathway of 3 and the light-driven steady statebetween the non-fluorescent 3 and the fluorescent 4 also existswhen these species are embedded in the polyester matrix. There-fore, the maximum fluorescence intensity, which would be pre-dicted if all molecules of 3 would be converted into alkoxy aminesof type 4 through reaction with polymer radicals, cannot beexperimentally achieved in accelerated weathering experiments.Because of the unexpected complexity of the reaction system underUV irradiation, it is not possible with this technology to determinethe concentration of radicals produced in polyesters upon weath-ering. Whether the observed UV-induced dissociation of the NOeCbond is a general reaction of alkoxy amines that are derived fromprofluorescent nitroxides or unique to this particular isoindoline-perylene chromophore will be explored in future work.

Given the general stability of the perylene chromophore itself,the use of perylene-based nitroxides for monitoring radicaldegradation in polyesters under accelerated weathering conditionsappears to be a promising concept at first sight. Our unexpectedobservation that incorporation of a nitroxide moiety lowers thephoto-stability of the perylene considerably clearly show that thebehaviour of both nitroxide and alkoxy amine under UV irradiationneed to be carefully evaluated, if these probes are intended to beused for assessing weathering resistance in materials.

P.D. Sylvester et al. / Polymer Degradation and Stability 98 (2013) 2054e20622062

Acknowledgements

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

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.polymdegradstab.2013.07.006.

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