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Photo- and thermo-oxidation of poly(p-phenylene-vinylene) and phenylene-vinylene oligomer
Polymer Degradation and Stability 96 (2011) 1149e1158
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Photo- and thermo-oxidation of poly(p-phenylene-vinylene) and phenylene-vinylene oligomer Sylvain Chambon a, b, *, Agnès Rivaton a, b, Jean-Luc Gardette a, b, *, Muriel Firon c a
Clermont Université, Université Blaise Pascal, LPMM, BP 10448, F-63000 Clermont-Ferrand, France CNRS, UMR 6505, LPMM, BP 80026, F-63171 Aubiere, France c CEA-Grenoble DRT/LITEN/DTS/LCS, INES-RDI, Laboratoire des Composants Solaires, 50 avenue du Lac Léman BP 332, 73377 Le Bourget du Lac, France b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2010 Accepted 4 February 2011 Available online 13 February 2011
This paper reports a study of the photo- and thermo-degradation of poly(p-phenylene-vinylene)-type (PPVs) materials, including a five-ring n-octyloxy substituted phenylene-vinylene oligomer (Ooct-OPV5) and poly(p-phenylene-vinylene) (PPV). The modifications in the chemical structure of the thin films submitted to UVevisible light irradiation and thermal oxidation were analysed with infrared spectroscopy, and the oxidation products were identified by derivatisation reactions. Results showed that the photochemical and thermal behaviour of the Ooct-OPV5 oligomer was similar to that of MDMO-PPV (poly[2-methoxy-5-(30 ,70 -dimethyloctyloxy)-1,4-phenylenevinylene), which is a polymer used in organic solar cells. Additionally, the identification of the products resulting from the oxidation of the vinylene bonds was simpler in Ooct-OPV5 and PPV compared to MDMO-PPV In contrast, the oxidation mechanisms of PPV, which has no ether substituent, and MDMO-PPV are not identical; the disappearance of the double bonds in PPV does not involve the formation of aromatic ketones. It was also shown that the absence of ether substituents does not decrease the rate of photo-oxidation of PPV compared to MDMOPPV. Finally, as the same mechanisms proposed for Ooct-OPV5 and PPV occur under both photo- and thermo-oxidative conditions of ageing, this confirms that singlet oxygen does not play a decisive role in the photo-oxidation of PPVs. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: PPV Ooct-OPV5 Conjugated polymers Degradation Infrared spectroscopy Organic photovoltaic
1. Introduction The discovery of conducting polymers in the 1970s resulted in intense research, which was largely motivated by the possible replacement of conventional inorganic or metallic components in a wide range of applications. Organic solar cells, though less efficient than silicon cells, exhibit a unique combination of interesting properties, including low cost, flexibility and large surface processability. The first interpenetrated network tested in solar cells was MDMO-PPV (Poly[2-methoxy-5-(30 ,70 -dimethyloctyloxy)-1,4-Phenylenevinylene) blended with fullerene derivatives [1]. However, similar to numerous other conjugated macromolecules, MDMO-PPV has low photochemical stability, which leads to reduced operating lifetimes of the photovoltaic devices [2e9]. Understanding the degradation processes that result from the interaction of conducting polymers with solar light seems then relevant to improving the stability of devices that integrate such polymers. * Corresponding authors. CNRS, UMR 6505, LPMM, BP 80026, F-63171 Aubiere, France. E-mail address: [email protected] (S. Chambon). 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.02.002
In previous work, we studied in detail the mechanism of photooxidation of MDMO-PPV [10,11]. To understand the changes in the chemical structure of polymers submitted to oxidative degradation, several tools exist that permit the analysis and understanding of the ageing mechanisms. The most useful of these methods is most likely infrared (IR) spectroscopy, which gives information about the modifications of the chemical structure which result from the ageing of the material. In the case of MDMO-PPV it has been shown that the formation of oxidation products lead to the appearance of new absorption bands in the hydroxyl (broad absorption observed in the 3800e3000 cm1 range) and carbonyl regions (envelope observed in the 1900e1500 cm1 range) with two absorption maxima as shown in Fig. 1A. The first maximum is a large band centred at 1735 cm1, which increases in intensity with increasing irradiation, and the second maximum is centred at 1680 cm1. With increasing exposure, the band centred at 1735 cm1 enlarges and progressively overlaps the thin band at 1680 cm1. The thermo-oxidation of MDMO-PPV lead to the formation of the same absorption bands, which means the same oxidation products (Fig. 1B). Therefore, as the production of singlet oxygen is not possible under thermal degradation conditions, this indicates that
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Fig. 1. IR spectra changes in the carbonyl region of MDMO-PPV during (A) UVevisible light irradiation (l > 300 nm) and (B) thermo-oxidation at 60 C.
singlet oxygen is not the principal reactive intermediate involved in the MDMO-PPV photo-oxidation [12], which conflicts previous reports [13,14]. We have shown [12] that the radical cation MDMOPPVþ is generated after excitation. Atmospheric oxygen and the photodegradation products were found to act as electron acceptors. Consequently, it was suggested that the radical cation may react with oxygen, evolve by scission, and generate radicals that can enter the photo-oxidative degradation mechanism of MDMO-PPV by abstraction of the labile hydrogen atoms of the polymer. Chemical derivatisation methods [15,16] were used to simplify the spectrum of the oxidised samples, which was important because the bands overlapped [11]. This approach, which can be described as chemical deconvolution, permits the identification of the various absorbing species that compose the IR spectrum of the oxidised materials. The band at 1680 cm1 was ascribed to aromatic ketones formed after the oxidation of the double bonds. The aromatic ketones were shown to be thermally stable, but under irradiation, scission generated via a Norrish reaction leads to the formation of aromatic carboxylic acids, which participate in the development of the complex carbonyl absorption centred at 1735 cm1. The development of this broad absorption was also ascribed to the formation of photoproducts resulting from the oxidation of the ether substituent, these photoproducts being identified as phenyl formate, aromatic ester and aliphatic acids. Based on the identification of the several formed photoproducts, a two-steps radical mechanism, involving at first the oxidation of the ether substituent and then the double bonds, was shown to account for the modification of the chemical structure of MDMO-PPV induced by photo and thermal ageing [11]. The radicals involved in the ether oxidation directly add on vinylene bonds, which in turn leads to the formation of an unstable hydroperoxide. The dissociation of this hydroperoxide gives a macro-alkoxy radical and hydroxyl radical. As a summary, three different pathways were suggested to occur:
cage reaction (a) is favoured compared to the b-scission (b). As a result, the cage reaction was proposed to be the main route. Moreover, if formed by route (b) the aldehydes are rapidly oxidised, making it difficult to determine whether route (b) is involved in the formation of the aromatic carboxylic acids. ii) Aromatic acids appear to be the key products in the degradation of MDMO-PPV, as their formation is linked to the loss of conjugation of MDMO-PPV, to chain scissions and to a decrease of the visible absorbance, all phenomena which drastically shorten the lifetime of the organic cells. The formation of this key product was indirectly observed by SF4 derivative treatment that selectively converted the carboxylic acids into acid fluoride. iii) It has been suggested that the length of the macromolecular chains and the nature of the substituent are likely to play significant roles in the photodegradation mechanism of PPV polymers, as previously shown for the oligomers of phenylene-vinylene [20,21]. Therefore, to improve the understanding of the degradation mechanism of MDMO-PPV, we focus our attention in the present paper to the photo-ageing of a five-ring n-octyloxy substituted phenylene-vinylene oligomer (Ooct-OPV5) and poly(p-phenylenevinylene) (PPV). In this study, thin films were submitted either to irradiation at long-wavelengths (l > 300 nm, 60 C) or to thermal degradation at 60 C, both in ambient air. The modifications in the chemical structure of the thin films (z100e200 nm) were studied by infrared and UVevisible spectroscopies. IR spectroscopy coupled with derivatisation treatments was used to identify the nature of the photo-oxidation products which were formed on the
- Route (a): The main route is a cage reaction that leads to the formation of an aromatic ketone (1680 cm1). The Norrish I reaction transforms the ketones into aromatic carboxylic acids, which participate in the development of the band at 1735 cm1. - Route (b): The b-scission leads to the formation of aromatic aldehydes, which rapidly oxidise into aromatic carboxylic acids. - Route (c): Hydrogen abstraction leads to the formation of an alcohol (3460 cm1). Three points should be noted concerning the proposal of this mechanism. i) According to the concepts of polymer photochemistry [17e19], when oxidation occurs on a secondary carbon, the
Scheme 1. Chemical structure of MDMO-PPV.
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Scheme 2. Chemical structure of Ooct-OPV5 and PPV.
macromolecular chains throughout the ageing. Due to different ring substitution in Ooct-OPV5 compared to MDMO-PPV and no substituent in PPV, it was therefore possible to more accurately understand the mechanisms of oxidative degradation of the PPVs (Schemes 1 and 2). 2. Material and methods PPV and Ooct-OPV5 were obtained from IMEC, IMOMEC Division. The thin samples (z100e200 nm) of Ooct-OPV5 were prepared by spin-coating (G3P-8 Spincoat from Cookson Electronics Equipment) onto KBr substrates. The PPV was prepared via a processable precursor [22,23]. Thin films of the precursor deposited on the KBr substrates were converted to fully dense PPV by heating to above 200 C under nitrogen flow for 3 h. The conversion was confirmed by IR spectroscopy. The irradiations were performed in a SEPAP 12/24 unit. This apparatus was designed for the study of polymer photodegradation by artificial ageing corresponding to medium-accelerated conditions [24]. The chamber is composed of a square reactor equipped with four medium-pressure mercury lamps (Mazda MA 400) situated in vertical position at each corner of the chamber. Wavelengths below 300 nm are filtered by the glass envelope of the sources. In the centre of the chamber, the samples were fixed on a rotating carousel that was 13 cm in diameter and could hold 24 samples. In this series of experiments, the temperature at the surface of the samples was fixed at 60 C. The low temperature thermo-oxidation experiments were performed in a ventilated oven at 60 C. Photolysis (irradiation in the absence of oxygen) was performed on the samples in Pyrex tubes sealed under a vacuum of 105 mbar, which was obtained using a mercury diffusion vacuum line. Afterwards the tubes were placed in either the SEPAP or the oven. The infrared spectra were recorded in transmission mode with a Nicolet 760-FTIR spectrophotometer with OMNIC software. The spectra were obtained using 32 scan summations and a 4 cm1 resolution. The changes in the UVevisible spectra were determined with a Shimadzu UV-2101PC spectrophotometer equipped with an integrating sphere.
Most of the oxidation products were identified using chemical derivatisation treatments that selectively convert the oxidation products into groups with different IR absorptions [15,16]. In these experimental conditions, treatment with sulphur tetrafluoride (SF4) was shown to react only with carboxylic acids, transforming them into acid fluoride. The reaction with SF4 induces both the complete loss of the acid carbonyl band and the formation of an acid fluoride C]O band, which is largely shifted towards the high frequencies. Saturated and aromatic acid fluorides have a carbonyl absorptions near 1843 and 1811 cm1, respectively [25]. The irradiated samples were exposed to SF4 in an all-Teflon reactor. The SF4 treatments for 24 h were necessary for the complete reaction of the acid groups, which was confirmed by the total loss of the eOH absorption at 3400 cm1. Gaseous ammonia (NH3) reacts with carboxylic acid groups to give absorptions above 1550 cm1 (eC(]O)eO) [15,16,26]. NH3 also reacts with ester groups to produce amide groups, which present two characteristic absorption bands around 1670 cm1 (amide I band) and 1630 cm1 (amide II band). The irradiated samples were exposed to NH3 at room temperature in a polyethylene reactor for 30 min to achieve an invariable state corresponding to the total conversion of the reactive species. It was confirmed that spectra of pristine OoctOPV5 and PPV before and after treatment with SF4 or NH3 presented no differences. 3. Results and discussion 3.1. Photo- and thermo-oxidation of Ooct-OPV5 3.1.1. Evolution of the IR spectra The photo- and thermo-oxidation of Ooct-OPV5 led to notable changes in the IR spectra. The loss of several functions of the polymer was observed, and similar to MDMO-PPV, the ether groups and double bonds disappeared at a higher rate than the aromatic rings and CH2 groups. Two maxima developed concurrently in the carbonyl region during UV-light exposure (Fig. 2A): the first one was centred at 1698 cm1, and the second one was at 1725 cm1 with a shoulder near 1780 cm1. The thermo-oxidation led to the
Fig. 2. The IR spectra changes in the 1900e1500 cm1 region of Ooct-OPV5 caused by (A) long-wavelengths irradiation (l > 300 nm) and (B) thermo-oxidation at 60 C.
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Fig. 3. Photolysis of the Ooct-OPV5 sample that was first thermo-oxidised for 2777 h (solid line) and then submitted to photolysis (dash line).
development of an intense band at 1675 cm1 accompanied by a band at 1725 cm1 with a shoulder at 1735 cm1, which was observed after a long duration of the thermal treatment (Fig. 2B). In both types of ageing, the shift in the quadrant stretch of the phenyl ring band from 1591 cm1 to 1610 cm1 indicates changes in the substitution of the ring due to the oxidative processes [11,27]. The main difference between the two types of ageing for OoctOPV5 is related to the major product formed. Thermal ageing leads to the formation of a product with an absorption band at 1675 cm1, while photochemical ageing induces the formation of a compound with an absorption band centred at 1698 cm1. Considering these frequencies, both species can be ascribed to aromatic structures. It should also be noted that the evolution of the IR spectra of Ooct-OPV5 does not resemble the one observed in the case of MDMO-PPV (compare Figs. 1 and 2). Of course, no precise assignment for the carbonyl regions was possible because the distinct absorption bands overlap. In addition, the hydrogen bonds and chemical environment may provoke significant spectral shifts [27]. Therefore, chemical and physical derivatisation methods were used to identify the two main absorbing species composing the spectra of the oxidised Ooct-OPV5 samples. 3.1.2. Products resulting from the oxidation of vinylene bonds Attention was first focused on the main oxidation product formed in thermo-oxidation and detected at 1675 cm1. It was first confirmed that this IR band was not modified by a treatment with SF4 and NH3. Subsequently, to evaluate the stability under irradiation of the product absorbing at 1675 cm1, the Ooct-OPV5 film was
thermo-oxidised and then submitted to photolysis (irradiation in the absence of oxygen). As a result, the absorption band at 1675 cm1 completely disappeared after exposure (Fig. 3). These results suggest that the absorption at 1675 cm1 is an aromatic ketone. This product is not stable under UVevisible light exposure, as a consequence of a Norrish type I process [18,19,28,29]. Attention was then focused on the main oxidation product observed in photo-oxidation at 1698 cm1. The Ooct-OPV5 sample was exposed to SF4 after 105 min of light irradiation. Fig. 4 shows the direct spectra of the Ooct-OPV5 sample irradiated for 105 min before and after SF4 treatment (A), and the subtraction of these two spectra (B). The results shown in Fig. 4 show the formation of saturated acid fluorides (1839 cm1) and aromatic acid fluorides (1811 cm1) resulting from the conversion of the corresponding acids. It can be also observed that the bands at 1698 and 1725 cm1 reacted with SF4. The subtraction spectra (Fig. 4B) show negative maxima centred at 1720 cm1 and 1698 cm1. These two frequencies can be attributed to the saturated and aromatic acids, respectively. The residual IR bands that did not react with SF4 can be attributed to esters, ketones, formates or aldehydes. It should be noted that a shoulder appeared at 1675 cm1 after treatment, therefore confirming that the aromatic ketones are formed under exposure but that they are readily transformed into carboxylic acids. As a conclusion, the main oxidation product formed by thermal ageing is an aromatic ketone (1675 cm1). This compound should also be formed during photo-oxidation, but due to its photochemical instability it rapidly converts into aromatic carboxylic acid under UVevisible exposure in the presence of oxygen. Indeed, the derivatisation treatments have demonstrated the presence of aromatic carboxylic acids (1698 cm1), which accumulate as the main oxidation products in photo-oxidised materials. Therefore, the oxidation of the vinylene units leads to the formation of the same species in both MDMO-PPV and Ooct-OPV5. The only difference is the frequency of the oxidation products due to different substitutions in the two materials. During photo-oxidation, the rate of conversion from the aromatic ketone to the aromatic carboxylic acid seems also faster in the case of Ooct-OPV5 compared to MDMO-PPV. 3.1.3. Products resulting from the oxidation of the ether substituent The Ooct-OPV5 sample was exposed to NH3 after 120 min of irradiation. Fig. 5 shows the direct spectra of the 120 min irradiation sample before and after NH3 treatment (A) and the subtraction spectra (B). The NH3 treatment provoked a decrease in the intensity of the absorption band of the oxidation products. In parallel, the formation of ammonium carboxylates detected around 1587 and
Fig. 4. SF4 treatment of Ooct-OPV5 samples photo-oxidised for 105 min. The IR spectra at 1900e1650 cm1 before (solid line) and after (dot line) treatment (A) and the subtraction spectra (after e before) (B).
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Fig. 5. NH3 treatment of the Ooct-OPV5 sample photo-oxidised for 120 min. The IR spectra at 1900e1550 cm1 before (solid line) and after (dot line) treatment (A) and the subtraction spectra (after e before) (B).
1550 cm1 indicated the conversion of carboxylic acids (1720 and 1698 cm1), which therefore confirm the results of the SF4 treatments. This could also reflect the conversion of formate, which reacts with the NH3 treatment to yield the carboxylate ion. A band with a maximum at 1677 cm1 was also formed under the NH3 treatment. This band corresponds to the characteristic absorption bands of amide groups, which indicates the reaction of an ester or anhydride with NH3. The reacting band at 1732 cm1 (Fig. 5B) was attributed to esters and formates, whereas the disappearance of the shoulder at 1780 cm1 suggests the presence of anhydrides. For MDMO-PPV, it was concluded that several species resulting from the oxidation of the ether substituent are convoluted in a large band. In the case of Ooct-OPV5, the band is centred at 1725 cm1 (1735 cm1 in MDMO-PPV [11]) and is attributed to the formation of saturated carboxylic acids (1720 cm1), esters (1732 cm1) and anhydrides (1780 cm1). Due to different substitutions and chain lengths, it can be concluded that the experimental results reported in this study of the ageing of the oligomer Ooct-OPV5 were able to better discriminate the different aromatic species resulting from the oxidation of the double bonds as well as the photoproducts resulting from the oxidation of the ether substituent. We therefore obtained confirmation of the conversion of aromatic ketones into aromatic carboxylic acids. Additionally, it appears that the degradation mechanism of Ooct-OPV5 is similar to that of MDMO-PPV. The photo-oxidation process leads to the formation of an aromatic ketone from the oxidation of the double bond. Under irradiation in the presence of oxygen, the aromatic ketone is transformed into aromatic carboxylic acid, and this formation involves chain scission and a loss of conjugation. As observed for MDMO-PPV, the degradation of the ether linkages of the oligomer leads to the formation of ester-, anhydride- and formate-type species, and this degradation is the source of radicals that cause the loss of double bonds. Finally, as the same products are formed in both photo- and thermo-oxidation, this further invalidates the intervention of singlet oxygen as the main intermediate in the photodegradation process. This result confirms the data obtained with MDMO-PPV [11,12].
development of a broad band around 3200 cm1 was observed, which was attributed to the hydrogen-bonded eOH stretching of carboxylic acids [27]; moreover, this result was similar to that observed for MDMO-PPV [11]. However, in contrast to MDMO-PPV, we did not observe the development of a band for PPV at 3460 cm1, which was attributed to the hydrogen-bonded eOH stretching of alcohols and hydroperoxides [27]. In the carbonyl domain, the evolution of the IR spectra of PPV did not resemble the one observed for MDMO-PPV (Fig. 6 compared to Fig. 1). In the case of PPV, a large band centred at 1695 cm1 was observed, and the intensity of this band increased with irradiation time. In addition, two shoulders were formed around 1720 and 1784 cm1. The development of an IR band at 1784 cm1, which is usually observed in the degradation of polymers, can be ascribed to the formation of anhydride functions. Based on the IR frequencies and previous results, the bands at 1695 and 1720 cm1 could be attributed to the formation of aromatic and saturated carboxylic acids, respectively. This result in good accordance with the development of bands at 1275 and 1213 cm1 that could be respectively attributed to the CeO vibration of aromatic and saturated carboxylic acids [27]. Finally, the increase in intensity and the shift of the 1591 cm1 band throughout irradiation indicate that changes are occurring in the substitution of the ring during the photo-oxidation process [11,27].
3.2. Photo- and thermo-oxidation of PPV 3.2.1. Photo-oxidation of PPV IR analysis. The photo-oxidation of PPV leads to rapid and notable changes in the IR spectra of the samples. A rapid loss of ethers and double bonds was observed. In addition, new absorption bands in the carbonyl (Fig. 6) and hydroxyl (not shown) region were concurrently formed. In the hydroxyl absorption region, the
Fig. 6. IR spectra changes in the carbonyl region of PPV irradiated from 0 to 180 min at long-wavelengths (l > 300 nm) under ambient air.
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Fig. 7. UVevisible (A) and fluorescence (B) spectra changes for PPV during long-wavelength irradiation (l > 300 nm) under ambient air from 0 to 180 min.
UVevisible and fluorescence analysis. Photo-oxidation also leads to dramatic changes in the UVevisible spectrum of the polymer (Fig. 7A). Prior to exposure, pristine PPV presents a broad absorption band centred at 419 nm that results from the conjugated structure. During irradiation, a decrease of absorbance accompanied by a blue shift was observed. This suggests a loss of the vinylene-type functions, thereby reducing the conjugation length of the polymer and leading to the photo-bleaching of the polymer (loss of visible absorption). Changes in the fluorescence spectra were also recorded throughout PPV photo-oxidation. The spectra reported in Fig. 7B show that the fluorescence disappeared considerably faster than the UVevisible absorbance. Kinetic analysis. Fig. 8A shows the decrease in the double bonds of PPV (960 cm1), as well as the decrease in the absorbance at 419 nm and in the fluorescence intensity at 554 nm (normalisation was used to facilitate the comparison). The increase in the IR band at 1695 cm1 is also reported in this figure. The formation of the photoproduct at 1695 cm1 can be correlated with the disappearance of the double bonds (960 cm1) and with the decrease in the UVevis band at 419 nm. Fig. 8B illustrates the strong correlation between the photoproduct that absorbs at 1695 cm1 and the double bonds, confirming that the oxidation product at 1695 cm1 results from the degradation of the double bonds. It can also be observed that after 15 min of exposure, the fluorescence intensity (554 nm) decreased by a factor 10, while the decrease in the UVevis band was only 20%. Therefore, the decrease in the UVevisible band was only partially responsible for the disappearance of the fluorescence intensity. One possible explanation is that photo-oxidation of PPV leads to the formation of photoproducts that can quench the PPV singlet state, provoking
a rapid decrease in the PPV fluorescence. As shown for MDMO-PPV [12], a photo-induced electron transfer from the singlet state of the PPV to the photoproducts may be possible. 3.2.2. Thermo-oxidation Considering the modifications of the UVevis spectra (not shown) under thermo-oxidation, two-steps were observed. During the first period (600 h) of heating, an increase of absorbance was observed, suggesting a reorganisation of the polymer with an improvement of the p-stacking. After this phase, a progressive decrease in the absorbance was observed. As reported in the case of photo-oxidation, thermo-oxidation of PPV provoked the decreases in both ether and the double bonds, accompanied by changes in the carbonyl region of the IR spectra (Fig. 9). The two absorption bands that appeared under the photooxidative conditions (1695 cm1 and 1720 cm1) were also observed under the thermo-oxidation. However, they accumulate in different ratios under thermo-oxidation compared to photooxidation. The increased intensity of the band at 1594 cm1 reflects a change in the substitution of the aromatic ring or, more specifically, a dissymmetry. In the CeO region, two IR bands developed at 1270 and 1213 cm1, as was also observed under the photooxidative conditions. Additionally, a decrease in the IR band at 1047 cm1 was observed, which was attributed to residual impurities from the synthesis. The decrease in some specific IR bands and UVevisible absorbance at 419 nm of the PPV sample, as well as the development of a band at 1695 cm1, are reported in Fig. 9B. In addition to the previous observations (disappearance of double bonds, disappearance of the UVevis band and formation of the product at
Fig. 8. Kinetics of the degradation of PPV during photo-oxidation. (A) Increase in the absorbance at 1695 cm1 (-) as a function of irradiation. Normalised decay of the IR and UVevisible absorbance of PPV: double bonds at 960 cm1 (B); UVevis absorbance at 419 nm (6) and fluorescence intensity at 554 nm (7) as a function of irradiation. (B) Evolution of the band at 1695 cm1 as a function of the percent of double bonds (960 cm1).
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Fig. 9. (A) IR spectra changes in the 1900e1550 cm1 region of PPV exposed to thermal degradation from 0 to 2457 h under ambient air (T ¼ 60 C). (B) Kinetics of the degradation of PPV during thermo-oxidation at 60 C. The increase in absorbance at 1695 cm1 (-) as a function of irradiation time; normalised decay of the IR and UVevisible absorbances of PPV: double bonds at 960 cm1 (B), UVevisible absorbance at 419 nm (6) and residual impurities at 1049 cm1 (7).
1695 cm1), we observed that the formation of the oxidation product at 1695 cm1 involves two phases. The rate of formation of the oxidation product and the decrease in the band centred at 1047 cm1 are important in the first 600 h. A second phase in which the rate is much slower then occurs. This two-step process may be linked to the disappearance of the residual impurities. Consequently, it can be suggested that impurities are a source of radicals that initiate the degradation of the polymer. Once these defects are consumed, the oxidation process would become slower. Several major results were highlighted by the analysis of the photo- and thermo-oxidation of PPV: First of all, the same oxidation products are formed in both photo- and thermo-oxidation, which confirms that singlet oxygen does not play a decisive role in the photodegradation of PPVs. In addition, the photodegradation rate of PPV is of the same order of magnitude as that of MDMO-PPV; in the experimental conditions of this study, approximately 150e180 min of exposure provokes the total disappearance of the UVevis absorption and the doubles bonds of both polymers. However, PPV has no ether groups in its structure. Ether groups were previously shown to play an important role in the degradation of MDMO-PPV. These results suggest that the defects formed by synthesis of PPV are a source of radicals that can initiate the degradation. In addition, the excited state of PPV may be able to sensitise a large number of transient species, which induce the degradation of PPV [12]. Moreover, the oxidation products formed in PPV are photochemically and thermally stable. It is worth recalling that the oxidation of MDMO-PPV leads to the formation of an aromatic ketone (1680 cm1) which is thermally stable, but photochemically
unstable. This difference may suggest that the oxidation mechanism of PPV is different from that of MDMO-PPV. The identification of the oxidation products should confirm or refute this hypothesis. 3.2.3. Identification of the oxidation products NH3 treatment. The PPV sample was exposed to NH3 after 75 min of irradiation. Fig. 10 shows the direct spectra of the 75 min irradiated sample before and after NH3 treatment (A) and the subtraction spectra (B). Fig. 10 show that three products formed in the photo-oxidation (1695, 1720 and 1782 cm1) reacted with NH3. In parallel, two bands appeared at 1591 and 1547 cm1, which were characteristic of carboxylates, and one band at 1676 cm1, which indicated the formation of amides. The photoproducts at 1695 and 1720 cm1 correspond to the aromatic and saturated carboxylic acids, respectively. After NH3 treatment, the carboxylic acids are transformed into carboxylates (absorption between 1540 and 1595 cm1). The shoulder at 1784 cm1 can be ascribed to anhydrides that are transformed into amides. SF4 treatment. A PPV sample was exposed to SF4 after 75 min of irradiation. Fig. 11 shows the direct spectra of the PPV samples irradiated for 75 min before and after SF4 treatment (A) and the subtraction of these two spectra (B). The results given in Fig. 11A show the high reactivity of the products with the two bands at 1695 and 1720 cm1; this figure also shows the formation of a main band attributed to aromatic acid fluorides (1809 cm1) accompanied by a band from saturated acid fluorides (1842 cm1), which arise from the conversion of the corresponding acids. These results confirm our previous
Fig. 10. NH3 treatment of the PPV samples photo-oxidised for 75 min. The IR spectra at 1900e1550 cm1 before (solid line) and after (dot line) treatment (A) and the subtraction spectra (after e before) (B).
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Fig. 11. SF4 treatment of the PPV samples photo-oxidised for 75 min. The IR spectra at 1900e1650 cm1 before (solid line) and after (dot line) treatment (A) and the subtraction spectra (after e before) (B).
Scheme 3. Mechanism of formation of the main oxidation photoproduct of PPV.
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assignments: the absorption band centred at 1695 cm1 corresponds to aromatic carboxylic acids, which react with SF4 to give the main derivation product at 1809 cm1; and the band at 1720 cm1 corresponds to saturated carboxylic acids, which react with SF4 and give the weak shoulder at 1842 cm1. Main route of photo- and thermo-oxidation of PPV. It is worth to summarise the main the main oxidation routes of PPV. - The major oxidation product formed in the photo-and thermooxidation of PPV is an aromatic carboxylic acid (1695 cm1 and w3200 cm1). The kinetic data indicate that the formation of these products results from the oxidation of the vinylene functions. - However, the oxidation mechanism of the vinylene functions in PPV differs from the one given for MDMO-PPV. Indeed, the oxidation of PPV does not involve the formation of aromatic ketones, which were observed to be formed in the oxidation of MDMO-PPV according to the cage reaction (route (a), see introduction section). Therefore, the aromatic acids formed in PPV can only result from the oxidation of the benzaldehyde species, and route (a) suggested for MDMO-PPV is not involved in the case of PPV. - Finally, because no alcohol groups were found to be formed in PPV, route (c) suggested for MDMO-PPV does not participate in the oxidation of PPV. Based on these results, a general mechanism is proposed (Scheme 3) for the main route of the photo- and thermo-oxidation of PPV. Similar to MDMO-PPV, the first step (i) corresponds to the formation and decomposition of hydroperoxide which is formed on the double bonds. It is known that radicals can directly add onto unsaturated bonds [30], which leads in turn to the saturation of the double bonds and the formation of a new radical. The thermal and photochemical decomposition of the hydroperoxide yields a hydroxyl radical and a macro-alkoxy radical. The main route of evolution of these species is the b-scission that gives a macro-alkyl radical and benzaldehyde end-groups (b). This reaction corresponds to route (b) proposed in the case of MDMO-PPV (see introduction section). In the presence of oxygen and due to the effect of irradiation and/or temperature, benzaldehydes end-groups are rapidly oxidised into aromatic carboxylic acids end-chains (iii). The macro-alkyl radical formed in (b) also yields benzaldehyde after oxygen fixation and hydrogen abstraction (iv). Benzaldehyde is then oxidised into aromatic carboxylic acid. 4. Conclusions The study of the photodegradation in ambient air of Ooct-OPV5 has improved the understanding of the mechanism of degradation for MDMO-PPV. Indeed, we showed that the behaviour of the OoctOPV5 oligomer under photo- and thermo-oxidation was similar to that of MDMO-PPV. The mechanism of photo-oxidation primarily involved the formation of an aromatic ketone, which was rapidly oxidised into aromatic carboxylic acid according to the Norrish type I process. The ether substituent is responsible for the formation of the anhydride-, ester- and formate-type species identified by the chemical derivatisation treatments. The oxidation mechanisms of PPV and MDMO-PPV are not identical. The disappearance of the double bonds in PPV does not involve the formation of aromatic ketones. After decomposition of the hydroperoxides, which are the primary oxidation products, the main route of evolution is b-scission. This evolution yields benzaldehyde, end-groups which are rapidly oxidised into aromatic carboxylic acid under the effects of temperature and UVevisible
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light. In addition, the absence of ether substituents does not decrease the rate of photo-oxidation of PPV. Structural defects and transient species could be responsible for this phenomenon. Finally, the same mechanisms proposed for Ooct-OPV5 and PPV occur under both photo- and thermo-oxidative conditions of ageing. Consequently, this further invalidates the intervention of singlet oxygen as the main intermediate in the degradation process, thereby confirming the results previously reported in the case of MDMO-PPV. Acknowledgements The authors wish to thank Laurence Lutsen from IMEC/IMOMEC Division for providing Ooct-OPV5 and PPV samples, and for the valuable discussions. References [1] Yu G, Gao J, Hummelen JC, Wudl F, Heeger AJ. Polymer photovoltaic cells: enhanced efficiencies via a network of internal donoreacceptor heterojunctions. Science 1995;270(5243):1789e91. [2] Schaer M, Nüesch F, Berner D, Leo W, Zuppiroli L. Water vapor and oxygen degradation mechanisms in organic light emitting diodes. Adv Funct Mater 2001;11(2):116e21. [3] Cumpston BH, Parker ID, Jensen KF. In situ characterization of the oxidative degradation of a polymeric light emitting device. J Appl Phys 1997;81(8):3716e20. [4] Dennler G, Lungenschmied C, Neugebauer H, Sariciftci NS, Latreche M, Czeremuszkin G, et al. A new encapsulation solution for flexible organic solar cells. Thin Solid Films 2006;511e512:349e53. [5] Janssen FJJ, van IJzendoorn LJ, Schoo HFM, Sturm JM, Andersson GG, van der Gon AWD, et al. Degradation effects in poly para-phenylene vinylene derivatives due to controlled oxygen exposure. Synth Met 2002;131(1e3):167e74. [6] Janssen FJJ, Sturm JM, Denier van der Gon AW, van IJzendoorn LJ, Kemerink M, Schoo HFM, et al. Interface instabilities in polymer light emitting diodes due to annealing. Org Electron 2003;4(4):209e18. [7] Krebs FC, Carle JE, Cruys-Bagger N, Andersen M, Lilliedal MR, Hammond MA, et al. Lifetimes of organic photovoltaics: photochemistry, atmosphere effects and barrier layers in ITO-MEHPPV: PCBM-aluminium devices. Sol Energ Mater Sol Cell 2005;86(4):499e516. [8] Norrman K, Krebs FC. Lifetimes of organic photovoltaics: using TOF-SIMS and 18 O2 isotopic labelling to characterise chemical degradation mechanisms. Sol Energ Mater Sol Cell 2006;90(2):213e27. [9] Scott JC, Kaufman JH, Brock PJ, DiPietro R, Salem J, Goitia JA. Degradation and failure of MEH-PPV light-emitting diodes. J Appl Phys 79(5); 1996:2745e51. [10] Chambon S, Manceau M, Firon M, Cros S, Rivaton A, Gardette J.-L. Photooxidation in an 18O2 atmosphere: a powerful tool to elucidate the mechanism of UV-visible light oxidation of polymers e application to the photodegradation of MDMO-PPV. Polymer 3288;49(15):3288e3294. [11] Chambon S, Rivaton A, Gardette J-L, Firon M, Lutsen L. Aging of a donor conjugated polymer: photochemical studies of the degradation of poly[2methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene]. J Polym Sci Part A Polym Chem 2007;45(2):317e31. [12] Chambon S, Rivaton A, Gardette J-L, Firon M. Reactive intermediates in the initiation step of the photo-oxidation of MDMO-PPV. J Polym Sci Part A Polym Chem 2009;47(22):6044e52. [13] Cumpston BH, Jensen KF. Photo-oxidation of polymers used in electroluminescent devices. Synth Met 1995;73(3):195e9. [14] Scurlock RD, Wang B, Ogilby PR, Sheats JR, Clough RL. Singlet oxygen as a reactive intermediate in the photodegradation of an electroluminescent polymer. J Am Chem Soc 1995;117(41):10194e202. [15] Carlsson DJ, Brousseau R, Zhang C, Wiles DM. ACS Symp Ser 1988;364:376. [16] Wilhelm C, Gardette J- L. Infrared identification of carboxylic acids formed in polymer photooxidation. J Appl Polym Sci 1994;51(8):1411e20. [17] Gardette J.-L., Delprat P. Science and technology of polymers and advanced materials – emerging technologies and business opportunities. New York: Plenum; 1998. p. 587e96. [18] McKellar JF, Allen NS. Photochemistry of man-made polymers. Essex: Applied Science Publishers LTD; 1979. [19] Rabek JF. Photodegradation of polymers. Berlin: Springer; 1996. [20] Dam N, Scurlock RD, Wang B, Ma L, Sundahl M, Ogilby PR. Singlet oxygen as a reactive intermediate in the photodegradation of phenylenevinylene oligomers. Chem Mater 1999;11(5):1302e5. [21] Ma L, Wang X, Wang B, Chen J, Wang J, Huang K, et al. Photooxidative degradation mechanism of model compounds of poly(p-phenylenevinylenes) [PPVs]. Chem Phys 2002;285(1):85e94. [22] Lutsen L, Adriaensens P, Becker H, Van Breemen AJ, Vanderzande D, Gelan J. New synthesis of a soluble high molecular weight poly(arylene vinylene):
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poly[2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylene vinylene]. Polymerization and device properties. Macromolecules 1999;32(20):6517e25. [23] Lutsen L, Van Breemen AJ, Kreuder W, Vanderzande D, Gelan J. Highly selective route to unsymmetrically substituted 1-{2-[(butylsulfanyl)methyl]5-(chloromethyl)-4-methoxyphenoxy}-3,7-dimethyloctane and isomers toward synthesis of conjugated polymer OC1C10 used in LEDs: synthesis and optimization. Helv Chim Acta 2000;83(12):3113e21. [24] Philippart J-L, Sinturel C, Gardette J- L. Influence of light intensity on the photooxidation of polypropylene. Polym Degrad Stab 1997;58(3):261e8. [25] Heacock JF. Determination of carboxyl groups in the presence of carbonyl groups in oxidized polyolefins by using sulfur tetrafluoride. J Appl Polym Sci 1963;7(6):2319e22.
[26] March J. Advanced organic chemistry: reactions, mechanisms and structure. New York: John Wiley and Sons; 1992. [27] Lin-Vien D, Colthup NB, Fateley WG, Grasselli JG. The handbook of infrared and Raman characteristic frequencies of organic molecules. San Diego: Academic Press; 1991. [28] Gardette J-L, Mailhot B, Lemaire J. Photooxidation mechanisms of styrenic polymers. Polym Degrad Stab 1995;48(3):457e70. [29] Gugumus F. Contribution to the role of aldehydes and peracids inpolyolefin oxidation1. Photolysis and photooxidation of aldehydes in polyethylene. Polym Degrad Stab 1999;65(2):259e69. [30] Fossey J, Lefort D, Sorba J. Les radicaux libres en chimie organique. Paris: Masson; 1993.
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Photo- and thermo-oxidation of poly(p-phenylene-vinylene) and phenylene-vinylene oligomer Sylvain Chambon a, b, *, Agnès Rivaton a, b, Jean-Luc Gardette a, b, *, Muriel Firon c a
Clermont Université, Université Blaise Pascal, LPMM, BP 10448, F-63000 Clermont-Ferrand, France CNRS, UMR 6505, LPMM, BP 80026, F-63171 Aubiere, France c CEA-Grenoble DRT/LITEN/DTS/LCS, INES-RDI, Laboratoire des Composants Solaires, 50 avenue du Lac Léman BP 332, 73377 Le Bourget du Lac, France b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 December 2010 Accepted 4 February 2011 Available online 13 February 2011
This paper reports a study of the photo- and thermo-degradation of poly(p-phenylene-vinylene)-type (PPVs) materials, including a five-ring n-octyloxy substituted phenylene-vinylene oligomer (Ooct-OPV5) and poly(p-phenylene-vinylene) (PPV). The modifications in the chemical structure of the thin films submitted to UVevisible light irradiation and thermal oxidation were analysed with infrared spectroscopy, and the oxidation products were identified by derivatisation reactions. Results showed that the photochemical and thermal behaviour of the Ooct-OPV5 oligomer was similar to that of MDMO-PPV (poly[2-methoxy-5-(30 ,70 -dimethyloctyloxy)-1,4-phenylenevinylene), which is a polymer used in organic solar cells. Additionally, the identification of the products resulting from the oxidation of the vinylene bonds was simpler in Ooct-OPV5 and PPV compared to MDMO-PPV In contrast, the oxidation mechanisms of PPV, which has no ether substituent, and MDMO-PPV are not identical; the disappearance of the double bonds in PPV does not involve the formation of aromatic ketones. It was also shown that the absence of ether substituents does not decrease the rate of photo-oxidation of PPV compared to MDMOPPV. Finally, as the same mechanisms proposed for Ooct-OPV5 and PPV occur under both photo- and thermo-oxidative conditions of ageing, this confirms that singlet oxygen does not play a decisive role in the photo-oxidation of PPVs. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: PPV Ooct-OPV5 Conjugated polymers Degradation Infrared spectroscopy Organic photovoltaic
1. Introduction The discovery of conducting polymers in the 1970s resulted in intense research, which was largely motivated by the possible replacement of conventional inorganic or metallic components in a wide range of applications. Organic solar cells, though less efficient than silicon cells, exhibit a unique combination of interesting properties, including low cost, flexibility and large surface processability. The first interpenetrated network tested in solar cells was MDMO-PPV (Poly[2-methoxy-5-(30 ,70 -dimethyloctyloxy)-1,4-Phenylenevinylene) blended with fullerene derivatives [1]. However, similar to numerous other conjugated macromolecules, MDMO-PPV has low photochemical stability, which leads to reduced operating lifetimes of the photovoltaic devices [2e9]. Understanding the degradation processes that result from the interaction of conducting polymers with solar light seems then relevant to improving the stability of devices that integrate such polymers. * Corresponding authors. CNRS, UMR 6505, LPMM, BP 80026, F-63171 Aubiere, France. E-mail address: [email protected] (S. Chambon). 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.02.002
In previous work, we studied in detail the mechanism of photooxidation of MDMO-PPV [10,11]. To understand the changes in the chemical structure of polymers submitted to oxidative degradation, several tools exist that permit the analysis and understanding of the ageing mechanisms. The most useful of these methods is most likely infrared (IR) spectroscopy, which gives information about the modifications of the chemical structure which result from the ageing of the material. In the case of MDMO-PPV it has been shown that the formation of oxidation products lead to the appearance of new absorption bands in the hydroxyl (broad absorption observed in the 3800e3000 cm1 range) and carbonyl regions (envelope observed in the 1900e1500 cm1 range) with two absorption maxima as shown in Fig. 1A. The first maximum is a large band centred at 1735 cm1, which increases in intensity with increasing irradiation, and the second maximum is centred at 1680 cm1. With increasing exposure, the band centred at 1735 cm1 enlarges and progressively overlaps the thin band at 1680 cm1. The thermo-oxidation of MDMO-PPV lead to the formation of the same absorption bands, which means the same oxidation products (Fig. 1B). Therefore, as the production of singlet oxygen is not possible under thermal degradation conditions, this indicates that
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Fig. 1. IR spectra changes in the carbonyl region of MDMO-PPV during (A) UVevisible light irradiation (l > 300 nm) and (B) thermo-oxidation at 60 C.
singlet oxygen is not the principal reactive intermediate involved in the MDMO-PPV photo-oxidation [12], which conflicts previous reports [13,14]. We have shown [12] that the radical cation MDMOPPVþ is generated after excitation. Atmospheric oxygen and the photodegradation products were found to act as electron acceptors. Consequently, it was suggested that the radical cation may react with oxygen, evolve by scission, and generate radicals that can enter the photo-oxidative degradation mechanism of MDMO-PPV by abstraction of the labile hydrogen atoms of the polymer. Chemical derivatisation methods [15,16] were used to simplify the spectrum of the oxidised samples, which was important because the bands overlapped [11]. This approach, which can be described as chemical deconvolution, permits the identification of the various absorbing species that compose the IR spectrum of the oxidised materials. The band at 1680 cm1 was ascribed to aromatic ketones formed after the oxidation of the double bonds. The aromatic ketones were shown to be thermally stable, but under irradiation, scission generated via a Norrish reaction leads to the formation of aromatic carboxylic acids, which participate in the development of the complex carbonyl absorption centred at 1735 cm1. The development of this broad absorption was also ascribed to the formation of photoproducts resulting from the oxidation of the ether substituent, these photoproducts being identified as phenyl formate, aromatic ester and aliphatic acids. Based on the identification of the several formed photoproducts, a two-steps radical mechanism, involving at first the oxidation of the ether substituent and then the double bonds, was shown to account for the modification of the chemical structure of MDMO-PPV induced by photo and thermal ageing [11]. The radicals involved in the ether oxidation directly add on vinylene bonds, which in turn leads to the formation of an unstable hydroperoxide. The dissociation of this hydroperoxide gives a macro-alkoxy radical and hydroxyl radical. As a summary, three different pathways were suggested to occur:
cage reaction (a) is favoured compared to the b-scission (b). As a result, the cage reaction was proposed to be the main route. Moreover, if formed by route (b) the aldehydes are rapidly oxidised, making it difficult to determine whether route (b) is involved in the formation of the aromatic carboxylic acids. ii) Aromatic acids appear to be the key products in the degradation of MDMO-PPV, as their formation is linked to the loss of conjugation of MDMO-PPV, to chain scissions and to a decrease of the visible absorbance, all phenomena which drastically shorten the lifetime of the organic cells. The formation of this key product was indirectly observed by SF4 derivative treatment that selectively converted the carboxylic acids into acid fluoride. iii) It has been suggested that the length of the macromolecular chains and the nature of the substituent are likely to play significant roles in the photodegradation mechanism of PPV polymers, as previously shown for the oligomers of phenylene-vinylene [20,21]. Therefore, to improve the understanding of the degradation mechanism of MDMO-PPV, we focus our attention in the present paper to the photo-ageing of a five-ring n-octyloxy substituted phenylene-vinylene oligomer (Ooct-OPV5) and poly(p-phenylenevinylene) (PPV). In this study, thin films were submitted either to irradiation at long-wavelengths (l > 300 nm, 60 C) or to thermal degradation at 60 C, both in ambient air. The modifications in the chemical structure of the thin films (z100e200 nm) were studied by infrared and UVevisible spectroscopies. IR spectroscopy coupled with derivatisation treatments was used to identify the nature of the photo-oxidation products which were formed on the
- Route (a): The main route is a cage reaction that leads to the formation of an aromatic ketone (1680 cm1). The Norrish I reaction transforms the ketones into aromatic carboxylic acids, which participate in the development of the band at 1735 cm1. - Route (b): The b-scission leads to the formation of aromatic aldehydes, which rapidly oxidise into aromatic carboxylic acids. - Route (c): Hydrogen abstraction leads to the formation of an alcohol (3460 cm1). Three points should be noted concerning the proposal of this mechanism. i) According to the concepts of polymer photochemistry [17e19], when oxidation occurs on a secondary carbon, the
Scheme 1. Chemical structure of MDMO-PPV.
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Scheme 2. Chemical structure of Ooct-OPV5 and PPV.
macromolecular chains throughout the ageing. Due to different ring substitution in Ooct-OPV5 compared to MDMO-PPV and no substituent in PPV, it was therefore possible to more accurately understand the mechanisms of oxidative degradation of the PPVs (Schemes 1 and 2). 2. Material and methods PPV and Ooct-OPV5 were obtained from IMEC, IMOMEC Division. The thin samples (z100e200 nm) of Ooct-OPV5 were prepared by spin-coating (G3P-8 Spincoat from Cookson Electronics Equipment) onto KBr substrates. The PPV was prepared via a processable precursor [22,23]. Thin films of the precursor deposited on the KBr substrates were converted to fully dense PPV by heating to above 200 C under nitrogen flow for 3 h. The conversion was confirmed by IR spectroscopy. The irradiations were performed in a SEPAP 12/24 unit. This apparatus was designed for the study of polymer photodegradation by artificial ageing corresponding to medium-accelerated conditions [24]. The chamber is composed of a square reactor equipped with four medium-pressure mercury lamps (Mazda MA 400) situated in vertical position at each corner of the chamber. Wavelengths below 300 nm are filtered by the glass envelope of the sources. In the centre of the chamber, the samples were fixed on a rotating carousel that was 13 cm in diameter and could hold 24 samples. In this series of experiments, the temperature at the surface of the samples was fixed at 60 C. The low temperature thermo-oxidation experiments were performed in a ventilated oven at 60 C. Photolysis (irradiation in the absence of oxygen) was performed on the samples in Pyrex tubes sealed under a vacuum of 105 mbar, which was obtained using a mercury diffusion vacuum line. Afterwards the tubes were placed in either the SEPAP or the oven. The infrared spectra were recorded in transmission mode with a Nicolet 760-FTIR spectrophotometer with OMNIC software. The spectra were obtained using 32 scan summations and a 4 cm1 resolution. The changes in the UVevisible spectra were determined with a Shimadzu UV-2101PC spectrophotometer equipped with an integrating sphere.
Most of the oxidation products were identified using chemical derivatisation treatments that selectively convert the oxidation products into groups with different IR absorptions [15,16]. In these experimental conditions, treatment with sulphur tetrafluoride (SF4) was shown to react only with carboxylic acids, transforming them into acid fluoride. The reaction with SF4 induces both the complete loss of the acid carbonyl band and the formation of an acid fluoride C]O band, which is largely shifted towards the high frequencies. Saturated and aromatic acid fluorides have a carbonyl absorptions near 1843 and 1811 cm1, respectively [25]. The irradiated samples were exposed to SF4 in an all-Teflon reactor. The SF4 treatments for 24 h were necessary for the complete reaction of the acid groups, which was confirmed by the total loss of the eOH absorption at 3400 cm1. Gaseous ammonia (NH3) reacts with carboxylic acid groups to give absorptions above 1550 cm1 (eC(]O)eO) [15,16,26]. NH3 also reacts with ester groups to produce amide groups, which present two characteristic absorption bands around 1670 cm1 (amide I band) and 1630 cm1 (amide II band). The irradiated samples were exposed to NH3 at room temperature in a polyethylene reactor for 30 min to achieve an invariable state corresponding to the total conversion of the reactive species. It was confirmed that spectra of pristine OoctOPV5 and PPV before and after treatment with SF4 or NH3 presented no differences. 3. Results and discussion 3.1. Photo- and thermo-oxidation of Ooct-OPV5 3.1.1. Evolution of the IR spectra The photo- and thermo-oxidation of Ooct-OPV5 led to notable changes in the IR spectra. The loss of several functions of the polymer was observed, and similar to MDMO-PPV, the ether groups and double bonds disappeared at a higher rate than the aromatic rings and CH2 groups. Two maxima developed concurrently in the carbonyl region during UV-light exposure (Fig. 2A): the first one was centred at 1698 cm1, and the second one was at 1725 cm1 with a shoulder near 1780 cm1. The thermo-oxidation led to the
Fig. 2. The IR spectra changes in the 1900e1500 cm1 region of Ooct-OPV5 caused by (A) long-wavelengths irradiation (l > 300 nm) and (B) thermo-oxidation at 60 C.
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Fig. 3. Photolysis of the Ooct-OPV5 sample that was first thermo-oxidised for 2777 h (solid line) and then submitted to photolysis (dash line).
development of an intense band at 1675 cm1 accompanied by a band at 1725 cm1 with a shoulder at 1735 cm1, which was observed after a long duration of the thermal treatment (Fig. 2B). In both types of ageing, the shift in the quadrant stretch of the phenyl ring band from 1591 cm1 to 1610 cm1 indicates changes in the substitution of the ring due to the oxidative processes [11,27]. The main difference between the two types of ageing for OoctOPV5 is related to the major product formed. Thermal ageing leads to the formation of a product with an absorption band at 1675 cm1, while photochemical ageing induces the formation of a compound with an absorption band centred at 1698 cm1. Considering these frequencies, both species can be ascribed to aromatic structures. It should also be noted that the evolution of the IR spectra of Ooct-OPV5 does not resemble the one observed in the case of MDMO-PPV (compare Figs. 1 and 2). Of course, no precise assignment for the carbonyl regions was possible because the distinct absorption bands overlap. In addition, the hydrogen bonds and chemical environment may provoke significant spectral shifts [27]. Therefore, chemical and physical derivatisation methods were used to identify the two main absorbing species composing the spectra of the oxidised Ooct-OPV5 samples. 3.1.2. Products resulting from the oxidation of vinylene bonds Attention was first focused on the main oxidation product formed in thermo-oxidation and detected at 1675 cm1. It was first confirmed that this IR band was not modified by a treatment with SF4 and NH3. Subsequently, to evaluate the stability under irradiation of the product absorbing at 1675 cm1, the Ooct-OPV5 film was
thermo-oxidised and then submitted to photolysis (irradiation in the absence of oxygen). As a result, the absorption band at 1675 cm1 completely disappeared after exposure (Fig. 3). These results suggest that the absorption at 1675 cm1 is an aromatic ketone. This product is not stable under UVevisible light exposure, as a consequence of a Norrish type I process [18,19,28,29]. Attention was then focused on the main oxidation product observed in photo-oxidation at 1698 cm1. The Ooct-OPV5 sample was exposed to SF4 after 105 min of light irradiation. Fig. 4 shows the direct spectra of the Ooct-OPV5 sample irradiated for 105 min before and after SF4 treatment (A), and the subtraction of these two spectra (B). The results shown in Fig. 4 show the formation of saturated acid fluorides (1839 cm1) and aromatic acid fluorides (1811 cm1) resulting from the conversion of the corresponding acids. It can be also observed that the bands at 1698 and 1725 cm1 reacted with SF4. The subtraction spectra (Fig. 4B) show negative maxima centred at 1720 cm1 and 1698 cm1. These two frequencies can be attributed to the saturated and aromatic acids, respectively. The residual IR bands that did not react with SF4 can be attributed to esters, ketones, formates or aldehydes. It should be noted that a shoulder appeared at 1675 cm1 after treatment, therefore confirming that the aromatic ketones are formed under exposure but that they are readily transformed into carboxylic acids. As a conclusion, the main oxidation product formed by thermal ageing is an aromatic ketone (1675 cm1). This compound should also be formed during photo-oxidation, but due to its photochemical instability it rapidly converts into aromatic carboxylic acid under UVevisible exposure in the presence of oxygen. Indeed, the derivatisation treatments have demonstrated the presence of aromatic carboxylic acids (1698 cm1), which accumulate as the main oxidation products in photo-oxidised materials. Therefore, the oxidation of the vinylene units leads to the formation of the same species in both MDMO-PPV and Ooct-OPV5. The only difference is the frequency of the oxidation products due to different substitutions in the two materials. During photo-oxidation, the rate of conversion from the aromatic ketone to the aromatic carboxylic acid seems also faster in the case of Ooct-OPV5 compared to MDMO-PPV. 3.1.3. Products resulting from the oxidation of the ether substituent The Ooct-OPV5 sample was exposed to NH3 after 120 min of irradiation. Fig. 5 shows the direct spectra of the 120 min irradiation sample before and after NH3 treatment (A) and the subtraction spectra (B). The NH3 treatment provoked a decrease in the intensity of the absorption band of the oxidation products. In parallel, the formation of ammonium carboxylates detected around 1587 and
Fig. 4. SF4 treatment of Ooct-OPV5 samples photo-oxidised for 105 min. The IR spectra at 1900e1650 cm1 before (solid line) and after (dot line) treatment (A) and the subtraction spectra (after e before) (B).
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Fig. 5. NH3 treatment of the Ooct-OPV5 sample photo-oxidised for 120 min. The IR spectra at 1900e1550 cm1 before (solid line) and after (dot line) treatment (A) and the subtraction spectra (after e before) (B).
1550 cm1 indicated the conversion of carboxylic acids (1720 and 1698 cm1), which therefore confirm the results of the SF4 treatments. This could also reflect the conversion of formate, which reacts with the NH3 treatment to yield the carboxylate ion. A band with a maximum at 1677 cm1 was also formed under the NH3 treatment. This band corresponds to the characteristic absorption bands of amide groups, which indicates the reaction of an ester or anhydride with NH3. The reacting band at 1732 cm1 (Fig. 5B) was attributed to esters and formates, whereas the disappearance of the shoulder at 1780 cm1 suggests the presence of anhydrides. For MDMO-PPV, it was concluded that several species resulting from the oxidation of the ether substituent are convoluted in a large band. In the case of Ooct-OPV5, the band is centred at 1725 cm1 (1735 cm1 in MDMO-PPV [11]) and is attributed to the formation of saturated carboxylic acids (1720 cm1), esters (1732 cm1) and anhydrides (1780 cm1). Due to different substitutions and chain lengths, it can be concluded that the experimental results reported in this study of the ageing of the oligomer Ooct-OPV5 were able to better discriminate the different aromatic species resulting from the oxidation of the double bonds as well as the photoproducts resulting from the oxidation of the ether substituent. We therefore obtained confirmation of the conversion of aromatic ketones into aromatic carboxylic acids. Additionally, it appears that the degradation mechanism of Ooct-OPV5 is similar to that of MDMO-PPV. The photo-oxidation process leads to the formation of an aromatic ketone from the oxidation of the double bond. Under irradiation in the presence of oxygen, the aromatic ketone is transformed into aromatic carboxylic acid, and this formation involves chain scission and a loss of conjugation. As observed for MDMO-PPV, the degradation of the ether linkages of the oligomer leads to the formation of ester-, anhydride- and formate-type species, and this degradation is the source of radicals that cause the loss of double bonds. Finally, as the same products are formed in both photo- and thermo-oxidation, this further invalidates the intervention of singlet oxygen as the main intermediate in the photodegradation process. This result confirms the data obtained with MDMO-PPV [11,12].
development of a broad band around 3200 cm1 was observed, which was attributed to the hydrogen-bonded eOH stretching of carboxylic acids [27]; moreover, this result was similar to that observed for MDMO-PPV [11]. However, in contrast to MDMO-PPV, we did not observe the development of a band for PPV at 3460 cm1, which was attributed to the hydrogen-bonded eOH stretching of alcohols and hydroperoxides [27]. In the carbonyl domain, the evolution of the IR spectra of PPV did not resemble the one observed for MDMO-PPV (Fig. 6 compared to Fig. 1). In the case of PPV, a large band centred at 1695 cm1 was observed, and the intensity of this band increased with irradiation time. In addition, two shoulders were formed around 1720 and 1784 cm1. The development of an IR band at 1784 cm1, which is usually observed in the degradation of polymers, can be ascribed to the formation of anhydride functions. Based on the IR frequencies and previous results, the bands at 1695 and 1720 cm1 could be attributed to the formation of aromatic and saturated carboxylic acids, respectively. This result in good accordance with the development of bands at 1275 and 1213 cm1 that could be respectively attributed to the CeO vibration of aromatic and saturated carboxylic acids [27]. Finally, the increase in intensity and the shift of the 1591 cm1 band throughout irradiation indicate that changes are occurring in the substitution of the ring during the photo-oxidation process [11,27].
3.2. Photo- and thermo-oxidation of PPV 3.2.1. Photo-oxidation of PPV IR analysis. The photo-oxidation of PPV leads to rapid and notable changes in the IR spectra of the samples. A rapid loss of ethers and double bonds was observed. In addition, new absorption bands in the carbonyl (Fig. 6) and hydroxyl (not shown) region were concurrently formed. In the hydroxyl absorption region, the
Fig. 6. IR spectra changes in the carbonyl region of PPV irradiated from 0 to 180 min at long-wavelengths (l > 300 nm) under ambient air.
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Fig. 7. UVevisible (A) and fluorescence (B) spectra changes for PPV during long-wavelength irradiation (l > 300 nm) under ambient air from 0 to 180 min.
UVevisible and fluorescence analysis. Photo-oxidation also leads to dramatic changes in the UVevisible spectrum of the polymer (Fig. 7A). Prior to exposure, pristine PPV presents a broad absorption band centred at 419 nm that results from the conjugated structure. During irradiation, a decrease of absorbance accompanied by a blue shift was observed. This suggests a loss of the vinylene-type functions, thereby reducing the conjugation length of the polymer and leading to the photo-bleaching of the polymer (loss of visible absorption). Changes in the fluorescence spectra were also recorded throughout PPV photo-oxidation. The spectra reported in Fig. 7B show that the fluorescence disappeared considerably faster than the UVevisible absorbance. Kinetic analysis. Fig. 8A shows the decrease in the double bonds of PPV (960 cm1), as well as the decrease in the absorbance at 419 nm and in the fluorescence intensity at 554 nm (normalisation was used to facilitate the comparison). The increase in the IR band at 1695 cm1 is also reported in this figure. The formation of the photoproduct at 1695 cm1 can be correlated with the disappearance of the double bonds (960 cm1) and with the decrease in the UVevis band at 419 nm. Fig. 8B illustrates the strong correlation between the photoproduct that absorbs at 1695 cm1 and the double bonds, confirming that the oxidation product at 1695 cm1 results from the degradation of the double bonds. It can also be observed that after 15 min of exposure, the fluorescence intensity (554 nm) decreased by a factor 10, while the decrease in the UVevis band was only 20%. Therefore, the decrease in the UVevisible band was only partially responsible for the disappearance of the fluorescence intensity. One possible explanation is that photo-oxidation of PPV leads to the formation of photoproducts that can quench the PPV singlet state, provoking
a rapid decrease in the PPV fluorescence. As shown for MDMO-PPV [12], a photo-induced electron transfer from the singlet state of the PPV to the photoproducts may be possible. 3.2.2. Thermo-oxidation Considering the modifications of the UVevis spectra (not shown) under thermo-oxidation, two-steps were observed. During the first period (600 h) of heating, an increase of absorbance was observed, suggesting a reorganisation of the polymer with an improvement of the p-stacking. After this phase, a progressive decrease in the absorbance was observed. As reported in the case of photo-oxidation, thermo-oxidation of PPV provoked the decreases in both ether and the double bonds, accompanied by changes in the carbonyl region of the IR spectra (Fig. 9). The two absorption bands that appeared under the photooxidative conditions (1695 cm1 and 1720 cm1) were also observed under the thermo-oxidation. However, they accumulate in different ratios under thermo-oxidation compared to photooxidation. The increased intensity of the band at 1594 cm1 reflects a change in the substitution of the aromatic ring or, more specifically, a dissymmetry. In the CeO region, two IR bands developed at 1270 and 1213 cm1, as was also observed under the photooxidative conditions. Additionally, a decrease in the IR band at 1047 cm1 was observed, which was attributed to residual impurities from the synthesis. The decrease in some specific IR bands and UVevisible absorbance at 419 nm of the PPV sample, as well as the development of a band at 1695 cm1, are reported in Fig. 9B. In addition to the previous observations (disappearance of double bonds, disappearance of the UVevis band and formation of the product at
Fig. 8. Kinetics of the degradation of PPV during photo-oxidation. (A) Increase in the absorbance at 1695 cm1 (-) as a function of irradiation. Normalised decay of the IR and UVevisible absorbance of PPV: double bonds at 960 cm1 (B); UVevis absorbance at 419 nm (6) and fluorescence intensity at 554 nm (7) as a function of irradiation. (B) Evolution of the band at 1695 cm1 as a function of the percent of double bonds (960 cm1).
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Fig. 9. (A) IR spectra changes in the 1900e1550 cm1 region of PPV exposed to thermal degradation from 0 to 2457 h under ambient air (T ¼ 60 C). (B) Kinetics of the degradation of PPV during thermo-oxidation at 60 C. The increase in absorbance at 1695 cm1 (-) as a function of irradiation time; normalised decay of the IR and UVevisible absorbances of PPV: double bonds at 960 cm1 (B), UVevisible absorbance at 419 nm (6) and residual impurities at 1049 cm1 (7).
1695 cm1), we observed that the formation of the oxidation product at 1695 cm1 involves two phases. The rate of formation of the oxidation product and the decrease in the band centred at 1047 cm1 are important in the first 600 h. A second phase in which the rate is much slower then occurs. This two-step process may be linked to the disappearance of the residual impurities. Consequently, it can be suggested that impurities are a source of radicals that initiate the degradation of the polymer. Once these defects are consumed, the oxidation process would become slower. Several major results were highlighted by the analysis of the photo- and thermo-oxidation of PPV: First of all, the same oxidation products are formed in both photo- and thermo-oxidation, which confirms that singlet oxygen does not play a decisive role in the photodegradation of PPVs. In addition, the photodegradation rate of PPV is of the same order of magnitude as that of MDMO-PPV; in the experimental conditions of this study, approximately 150e180 min of exposure provokes the total disappearance of the UVevis absorption and the doubles bonds of both polymers. However, PPV has no ether groups in its structure. Ether groups were previously shown to play an important role in the degradation of MDMO-PPV. These results suggest that the defects formed by synthesis of PPV are a source of radicals that can initiate the degradation. In addition, the excited state of PPV may be able to sensitise a large number of transient species, which induce the degradation of PPV [12]. Moreover, the oxidation products formed in PPV are photochemically and thermally stable. It is worth recalling that the oxidation of MDMO-PPV leads to the formation of an aromatic ketone (1680 cm1) which is thermally stable, but photochemically
unstable. This difference may suggest that the oxidation mechanism of PPV is different from that of MDMO-PPV. The identification of the oxidation products should confirm or refute this hypothesis. 3.2.3. Identification of the oxidation products NH3 treatment. The PPV sample was exposed to NH3 after 75 min of irradiation. Fig. 10 shows the direct spectra of the 75 min irradiated sample before and after NH3 treatment (A) and the subtraction spectra (B). Fig. 10 show that three products formed in the photo-oxidation (1695, 1720 and 1782 cm1) reacted with NH3. In parallel, two bands appeared at 1591 and 1547 cm1, which were characteristic of carboxylates, and one band at 1676 cm1, which indicated the formation of amides. The photoproducts at 1695 and 1720 cm1 correspond to the aromatic and saturated carboxylic acids, respectively. After NH3 treatment, the carboxylic acids are transformed into carboxylates (absorption between 1540 and 1595 cm1). The shoulder at 1784 cm1 can be ascribed to anhydrides that are transformed into amides. SF4 treatment. A PPV sample was exposed to SF4 after 75 min of irradiation. Fig. 11 shows the direct spectra of the PPV samples irradiated for 75 min before and after SF4 treatment (A) and the subtraction of these two spectra (B). The results given in Fig. 11A show the high reactivity of the products with the two bands at 1695 and 1720 cm1; this figure also shows the formation of a main band attributed to aromatic acid fluorides (1809 cm1) accompanied by a band from saturated acid fluorides (1842 cm1), which arise from the conversion of the corresponding acids. These results confirm our previous
Fig. 10. NH3 treatment of the PPV samples photo-oxidised for 75 min. The IR spectra at 1900e1550 cm1 before (solid line) and after (dot line) treatment (A) and the subtraction spectra (after e before) (B).
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Fig. 11. SF4 treatment of the PPV samples photo-oxidised for 75 min. The IR spectra at 1900e1650 cm1 before (solid line) and after (dot line) treatment (A) and the subtraction spectra (after e before) (B).
Scheme 3. Mechanism of formation of the main oxidation photoproduct of PPV.
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assignments: the absorption band centred at 1695 cm1 corresponds to aromatic carboxylic acids, which react with SF4 to give the main derivation product at 1809 cm1; and the band at 1720 cm1 corresponds to saturated carboxylic acids, which react with SF4 and give the weak shoulder at 1842 cm1. Main route of photo- and thermo-oxidation of PPV. It is worth to summarise the main the main oxidation routes of PPV. - The major oxidation product formed in the photo-and thermooxidation of PPV is an aromatic carboxylic acid (1695 cm1 and w3200 cm1). The kinetic data indicate that the formation of these products results from the oxidation of the vinylene functions. - However, the oxidation mechanism of the vinylene functions in PPV differs from the one given for MDMO-PPV. Indeed, the oxidation of PPV does not involve the formation of aromatic ketones, which were observed to be formed in the oxidation of MDMO-PPV according to the cage reaction (route (a), see introduction section). Therefore, the aromatic acids formed in PPV can only result from the oxidation of the benzaldehyde species, and route (a) suggested for MDMO-PPV is not involved in the case of PPV. - Finally, because no alcohol groups were found to be formed in PPV, route (c) suggested for MDMO-PPV does not participate in the oxidation of PPV. Based on these results, a general mechanism is proposed (Scheme 3) for the main route of the photo- and thermo-oxidation of PPV. Similar to MDMO-PPV, the first step (i) corresponds to the formation and decomposition of hydroperoxide which is formed on the double bonds. It is known that radicals can directly add onto unsaturated bonds [30], which leads in turn to the saturation of the double bonds and the formation of a new radical. The thermal and photochemical decomposition of the hydroperoxide yields a hydroxyl radical and a macro-alkoxy radical. The main route of evolution of these species is the b-scission that gives a macro-alkyl radical and benzaldehyde end-groups (b). This reaction corresponds to route (b) proposed in the case of MDMO-PPV (see introduction section). In the presence of oxygen and due to the effect of irradiation and/or temperature, benzaldehydes end-groups are rapidly oxidised into aromatic carboxylic acids end-chains (iii). The macro-alkyl radical formed in (b) also yields benzaldehyde after oxygen fixation and hydrogen abstraction (iv). Benzaldehyde is then oxidised into aromatic carboxylic acid. 4. Conclusions The study of the photodegradation in ambient air of Ooct-OPV5 has improved the understanding of the mechanism of degradation for MDMO-PPV. Indeed, we showed that the behaviour of the OoctOPV5 oligomer under photo- and thermo-oxidation was similar to that of MDMO-PPV. The mechanism of photo-oxidation primarily involved the formation of an aromatic ketone, which was rapidly oxidised into aromatic carboxylic acid according to the Norrish type I process. The ether substituent is responsible for the formation of the anhydride-, ester- and formate-type species identified by the chemical derivatisation treatments. The oxidation mechanisms of PPV and MDMO-PPV are not identical. The disappearance of the double bonds in PPV does not involve the formation of aromatic ketones. After decomposition of the hydroperoxides, which are the primary oxidation products, the main route of evolution is b-scission. This evolution yields benzaldehyde, end-groups which are rapidly oxidised into aromatic carboxylic acid under the effects of temperature and UVevisible
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light. In addition, the absence of ether substituents does not decrease the rate of photo-oxidation of PPV. Structural defects and transient species could be responsible for this phenomenon. Finally, the same mechanisms proposed for Ooct-OPV5 and PPV occur under both photo- and thermo-oxidative conditions of ageing. Consequently, this further invalidates the intervention of singlet oxygen as the main intermediate in the degradation process, thereby confirming the results previously reported in the case of MDMO-PPV. Acknowledgements The authors wish to thank Laurence Lutsen from IMEC/IMOMEC Division for providing Ooct-OPV5 and PPV samples, and for the valuable discussions. References [1] Yu G, Gao J, Hummelen JC, Wudl F, Heeger AJ. Polymer photovoltaic cells: enhanced efficiencies via a network of internal donoreacceptor heterojunctions. Science 1995;270(5243):1789e91. [2] Schaer M, Nüesch F, Berner D, Leo W, Zuppiroli L. Water vapor and oxygen degradation mechanisms in organic light emitting diodes. Adv Funct Mater 2001;11(2):116e21. [3] Cumpston BH, Parker ID, Jensen KF. In situ characterization of the oxidative degradation of a polymeric light emitting device. J Appl Phys 1997;81(8):3716e20. [4] Dennler G, Lungenschmied C, Neugebauer H, Sariciftci NS, Latreche M, Czeremuszkin G, et al. A new encapsulation solution for flexible organic solar cells. Thin Solid Films 2006;511e512:349e53. [5] Janssen FJJ, van IJzendoorn LJ, Schoo HFM, Sturm JM, Andersson GG, van der Gon AWD, et al. Degradation effects in poly para-phenylene vinylene derivatives due to controlled oxygen exposure. Synth Met 2002;131(1e3):167e74. [6] Janssen FJJ, Sturm JM, Denier van der Gon AW, van IJzendoorn LJ, Kemerink M, Schoo HFM, et al. Interface instabilities in polymer light emitting diodes due to annealing. Org Electron 2003;4(4):209e18. [7] Krebs FC, Carle JE, Cruys-Bagger N, Andersen M, Lilliedal MR, Hammond MA, et al. Lifetimes of organic photovoltaics: photochemistry, atmosphere effects and barrier layers in ITO-MEHPPV: PCBM-aluminium devices. Sol Energ Mater Sol Cell 2005;86(4):499e516. [8] Norrman K, Krebs FC. Lifetimes of organic photovoltaics: using TOF-SIMS and 18 O2 isotopic labelling to characterise chemical degradation mechanisms. Sol Energ Mater Sol Cell 2006;90(2):213e27. [9] Scott JC, Kaufman JH, Brock PJ, DiPietro R, Salem J, Goitia JA. Degradation and failure of MEH-PPV light-emitting diodes. J Appl Phys 79(5); 1996:2745e51. [10] Chambon S, Manceau M, Firon M, Cros S, Rivaton A, Gardette J.-L. Photooxidation in an 18O2 atmosphere: a powerful tool to elucidate the mechanism of UV-visible light oxidation of polymers e application to the photodegradation of MDMO-PPV. Polymer 3288;49(15):3288e3294. [11] Chambon S, Rivaton A, Gardette J-L, Firon M, Lutsen L. Aging of a donor conjugated polymer: photochemical studies of the degradation of poly[2methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylenevinylene]. J Polym Sci Part A Polym Chem 2007;45(2):317e31. [12] Chambon S, Rivaton A, Gardette J-L, Firon M. Reactive intermediates in the initiation step of the photo-oxidation of MDMO-PPV. J Polym Sci Part A Polym Chem 2009;47(22):6044e52. [13] Cumpston BH, Jensen KF. Photo-oxidation of polymers used in electroluminescent devices. Synth Met 1995;73(3):195e9. [14] Scurlock RD, Wang B, Ogilby PR, Sheats JR, Clough RL. Singlet oxygen as a reactive intermediate in the photodegradation of an electroluminescent polymer. J Am Chem Soc 1995;117(41):10194e202. [15] Carlsson DJ, Brousseau R, Zhang C, Wiles DM. ACS Symp Ser 1988;364:376. [16] Wilhelm C, Gardette J- L. Infrared identification of carboxylic acids formed in polymer photooxidation. J Appl Polym Sci 1994;51(8):1411e20. [17] Gardette J.-L., Delprat P. Science and technology of polymers and advanced materials – emerging technologies and business opportunities. New York: Plenum; 1998. p. 587e96. [18] McKellar JF, Allen NS. Photochemistry of man-made polymers. Essex: Applied Science Publishers LTD; 1979. [19] Rabek JF. Photodegradation of polymers. Berlin: Springer; 1996. [20] Dam N, Scurlock RD, Wang B, Ma L, Sundahl M, Ogilby PR. Singlet oxygen as a reactive intermediate in the photodegradation of phenylenevinylene oligomers. Chem Mater 1999;11(5):1302e5. [21] Ma L, Wang X, Wang B, Chen J, Wang J, Huang K, et al. Photooxidative degradation mechanism of model compounds of poly(p-phenylenevinylenes) [PPVs]. Chem Phys 2002;285(1):85e94. [22] Lutsen L, Adriaensens P, Becker H, Van Breemen AJ, Vanderzande D, Gelan J. New synthesis of a soluble high molecular weight poly(arylene vinylene):
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