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Photostability of acrylate photopolymers used as components in recording materials
Polymer Degradation and Stability 119 (2015) 208e216
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Photostability of acrylate photopolymers used as components in recording materials Georgia G. Goourey a, b, Pascal Wong-Wah-Chung c, d, Florence Delor-Jestin b, e, €l Israe €li a, b, * geret f, g, h, Lavinia Balan i, Yae Bertrand Le Clermont Universit e, Universit e Blaise Pascal, BP 10448, 63000 Clermont-Ferrand, France Institut de Chimie de Clermont-Ferrand (ICCF), UMR CNRS 6296, BP 80026, 63171 Aubi ere, France c ^le de l'Arbois, BP 80, 13545 Aix-en-Provence Cedex 4, France Aix-Marseille Universit e, Laboratoire Chimie Environnement, Europo d CNRS, FRE 3416, 13545 Aix-en-Provence Cedex 4, France e Clermont Universit e, Ecole Nationale Sup erieure de Chimie de Clermont-Ferrand, BP 10187, 63000 Clermont-Ferrand, France f CEA Cadarache, IBEB, Lab Bioenerget Biotechnol Bacteries & Microalgues, Saint-Paul-lez-Durance F-13108, France g CNRS, UMR Biol Veget & Microbiol Environ, 13108 Saint-Paul-lez-Durance, France h Aix-Marseille Universit e, 13108 Saint-Paul-lez-Durance, France i Institut de Science des Mat eriaux de Mulhouse 15, Rue Jean Starcky, BP 2488, 68057 Mulhouse Cedex, France a
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a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 February 2015 Received in revised form 20 May 2015 Accepted 22 May 2015 Available online 23 May 2015
The present work was devoted to the photostability of two homopolymers, resulting from the polymerization of acrylate monomers used as components of recording materials. This study was undertaken in conditions simulating solar light. The objective was to identify the key elements specific of the degradation of each polymer. This approach passed through the understanding of the chemical modifications of the polymer matrix and therefore through the elucidation of the mechanism of degradation. Photo-IR experiments and GCeMS analysis were performed to identify the organic volatile compounds stemming from the photo-oxidation of each homopolymers. A new methodology based on the photopurge-trap-GCeMS was implemented to have a better insight of the elucidation of the pathways of degradation. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Degradation GCeMS Mass spectrometry Photopolymerization Thermosets
1. Introduction The emerging applications of holography in optical data storage [1e4], optical elements [5,6], holographic sensor [7], artistic display [6,8] and security integrated devices [6,9] require to used highperformance recording materials with high diffraction efficiency, good spatial-frequency response and long shelf-life stability. Their ease of feasibility, self-developing character, shape flexibility and low cost make photopolymers attractive and versatile materials for holographic recording [10e16]. In order to achieve improved optical performance that was driven by large diffraction efficiency and, to enable high storage capacity, many studies were focussed in the development of the composition of the recording photopolymer materials since the last decades. Recent reviews described
* Corresponding author. Institut de Chimie de Clermont-Ferrand (ICCF), UMR re, France. CNRS 6296, BP 80026, 63171 Aubie €li). E-mail address: [email protected] (Y. Israe http://dx.doi.org/10.1016/j.polymdegradstab.2015.05.016 0141-3910/© 2015 Elsevier Ltd. All rights reserved.
the material requirements for holographic data storage [2,3]. They also showed the chemically diverse and interesting photopolymer media [2,3]. A number of photopolymerizable systems was proposed such as for example a liquid crystalline monomer mixed with a multifunctional acrylate [17], phenanthrenequinone-doped polymethacrylate (PQ) [3,5], PQ-polymethylmethacrylate (PMMA)polyvinyl butyral [14]. Since the photopolymerization reaction is based on the formation of the grating, some additional compound could be introduced in the recording media to modify the kinetics of the reaction. Thus, to record reflection holograms with high spatial frequency in polyvinyl alcohol/acrylamide (PVA/AA), a cross linker added in the formulation contributed to speed up the polymerization reaction of AA monomers [1,15]. In the same way, in the presence of a chain transfer agent, the obtained low molecular weight polymer allowed to improve the optical performance [18]. Unfortunately, from an environmental viewpoint, acrylamide was known for its carcinogenic property [19]. The diacetone acrylamide monomer which exhibited a less cytotoxicity was also used to
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replace AA [7]. Recently, sodium acrylate, a “green” photopolymer, was developed as an alternative to acrylamide derivatives [20,21]. It is worthy to remind that the photoinitiation reaction is the key step of the polymerization process. Nevertheless, the composition of the photoinitiating system is often disregarded in the development of high-performance recording materials. In the literature, recent studies showed that the implementation of a three component system or a photocyclic initiating systems afforded high reactivity in the photopolymerization systems that make them good candidates for grating recording [22]. For commercial applications, among the various requirements for recording material, the long-term stability (mainly meaning dark storage) is one of the most essential criteria. The main drawback during the dark storage is the formation of a certain radicals [23]. One solution consists in introducing monomer stabilizer to prevent radical polymerization [23]. When the diffusion process occurs in the dark and induces the loss of the physical property, the temperature is a parameter which could play a role. Indeed, in the case of PQ/PMMA after the photopatterning process, the material undergoes two continuous phenomena: 1) a diffusion of PQ molecules that leads to an enhancement of the physical property and, 2) a diffusion of the corresponding photoproducts of PQ that gives rise to a decay of the diffraction efficiency [5,24]. The dark storage at low temperature contributes to optimize the stability of the grating [24]. Depending on the applications, the photo-patterned polymer could be exposed to UV light. Nevertheless, it is well known that, the polymers could undergo a photodegradation under the combined effect of solar light and oxygen [25e29]. This gives rise to chemical modifications (formation of photo-oxidation products) and/or architectural changes (chain scission and crosslinking) that modify the physical properties of the polymer and decrease the performance of the material. In this context, we have recently introduced an acrylate photopolymerizable formulation doped with ZnO quantum dots [30]. The monomer mixture consisted of pentaerythritol tetraacrylate (PETTA) and 2-(2-ethoxyethoxy)ethyl acrylate (2EEEA). The resulting copolymer was a crosslinked polymer (a thermoset) due to the tetra-functionality of PETTA monomers. Due to the rather complexity of its whole structure, the expertise of the stability of the matrix in the absence and in the presence of oxygen required in a first approach to evaluate the aging resistance of the two homopolymers synthesized from the photopolymerization of the corresponding acrylate PETTA and 2EEEA monomers, respectively. They will be referred below as homo-PETTA (carrier of ester groups) and homo-2EEEA (carrier of ester and ether groups), respectively. This approach was all the more important that previous studies performed on linear poly(ester-ether) [31,32] pointed out that the ether groups were the main site of the oxidation upon irradiation at wavelengths longer than 300 nm. From the investigation of the photo-oxidative degradation of poly(ethylene oxide) in solid state [33,34], the following mechanism was postulated (Fig. 1): the first step is the hydrogen atom abstraction in the a position with respect to oxygen atom followed by the formation of alkoxy and hydroxyl radicals through the generation of hydroperoxide derivative. These latter species could further undergo b scission, cage reaction or hydrogen abstraction reaction and would cause the chemical modifications of the polymer matrix. Besides, MALDI analysis performed on poly(butylene succinate) highlighted that, upon irradiation, this polymer undergoes oxidation of the hydroxyl end groups, a-hydrogen abstraction and Norrish I cleavage reactions [35]. Thus, the present work was devoted to the photostability of the two homopolymers, homo-PETTA and homo-2EEEA, respectively, under irradiation conditions simulating solar light. The photochemical modifications were monitored by infrared spectroscopy
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Fig. 1. Photo-oxidation mechanism for poly(ethylene oxide) in solid state [33,34].
and characterized mainly by gas chromatography coupled with mass spectroscopy (GCeMS). Accounting for the chemical modifications of the polymer matrix generally passed through the proposal of the mechanism of degradation. So, in this work, the approach based on the identification of the volatile organic compounds was used to draw up a mechanism of degradation. 2. Experimental 2.1. Materials Pentaerythritol tetraacrylate (PETTA) and 2-(2-ethoxyethoxy) ethyl acrylate (2EEEA) (Fig. 2) were purchased from Aldrich and used as received without further purification. The typical photopolymerizable formulations were prepared in low-light conditions by mixing PETTA or 2EEEA or the two acrylate monomers (20:80 PETTA:2EEEA or 80:20 PETTA:2EEEA) with a photoinitiator (DL-camphorquinone [CQ] from EGA Chemie, 1.5 wt%) and a co-initiator (ethyl 4-dimethylaminobenzoate [DABE] from Aldrich, purum > 99%, 0.6 wt%). The polymer samples were prepared by coating the photopolymerizable mixture onto a glass slide or on a small aluminum rectangular crucible, followed by irradiation performed in a deoxygenated atmosphere using a conventional irradiation device (monochromator) at 488 nm with an intensity of 0.6 mW cm 2, in the same conditions used for recording. 2.2. Experimental set-up and procedure For photoaging investigation, two artificial aging devices were used. The first one was a SEPAP 14/24 unit equipped with a medium pressure mercury lamp (Novalamp RVC 400 W). The source was
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Fig. 2. Monomers (a) Pentaerythritol tetraacrylate (PETTA) and (b) 2-(2-ethoxyethoxy)ethyl acrylate (2EEEA).
located along one of the focal axis of a cylinder with an elliptical base. The samples were arranged on a rotating carousel that could receive 15 samples, located at the other focal axis. Experiments were performed at 40 C. The second aging device was a SEPAP 12/ 24 unit from Atlas, equipped with four medium pressure mercury lamps (Novalamp RVC 400 W) located in a vertical position at each corner of the chamber. Wavelengths less than 295 nm were filtered by the glass envelope of the sources. A rotating carousel, on which the samples were fixed, was placed in the center of the chamber. The temperature at the surface of the sample was fixed at 60 C. Differential scanning calorimetry (DSC) measurements were performed on a Mettler Toledo DSC822e under a nitrogen flow (30 mL min 1), in the temperature range from 60 to 300 C with a heating rate of 15 C min 1. Infrared spectra were recorded on a Nicolet 5SXC FTIR spectrometer with Omnic software. Spectra were obtained with a 4 cm 1 resolution and 32 scan summation. UVevisible spectra were recorded on a Shimadzu UV-2101 PC spectrophotometer. The photo-IR combines light irradiation and infrared analysis using a Hamamatsu light generator equipped with a medium pressure Xe/Hg source (Lightningcure LC6 1W) and Thermo Optek Nexus FTIR spectrometer. The source was filtered by a sapphire disk to deliver wavelengths longer than 300 nm, which was representative of outdoor aging. The films (deposited in rectangle aluminum crucible) were beforehand placed in a sealed gas cell that was located in the infrared analysis cavity equipped with an oven [36]. The temperature in the oven was maintained constant at 40 C. The infrared spectra of the released gas were collected every two hours during 60 h with a 4 cm 1 resolution and 32 scan summation. For solid-phase microextraction (SPME) experiments, the photopolymer films (deposited on an aluminum rectangle crucible) were irradiated in the SEPAP 12/24 unit, in sealed vials to collect the volatile organic compounds (VOCs) stemming from the photodegradation. CarboxenePDMS fiber (75 mm) purchased from Supelco (Bellefonte, PA, USA) was used to extract the volatile organic compounds (VOCs) in the headspace (HS) [37,38]. VOCs were analyzed by gas chromatography/mass spectrometry (GCeMS) with a 6890N Agilent GC coupled to a 5973 Agilent mass detector. The GC was equipped with a SupelcowaxTM 10 column (30 m 0.25 mm 0.25 mm) from Supelco. The analysis conditions were previously described in details in Ref. [37]. Mass spectra and reconstructed chromatograms (total ion current, TIC) were acquired under the electron ionization mode (EI) at 70 eV and recorded from 20 to 400 m/z. The compounds were identified by comparison with the mass spectra of the spectral library. VOCs stemming from the in situ irradiation of the photopolymer sample were also analyzed by using a purge and trap dynamic headspace extraction coupled with polychromatic irradiation and GCeMS system. A modified Tekmar LCS 2000 concentrator (Tekmar-Dohrmann, Cincinnati, USA) was used connected to a homemade hermitical water-jacketed Pyrex reactor and a GCeMS apparatus through a transfer line. The polymer was directly irradiated in a hermetically sealed photoreactor by using a LOT xenon
lamp system (300 W) whose emission was filtrated by a Pyrex filter to deliver 290e800 nm wavelengths. The photo-purge-trapGCeMS instrument allowed to purge the reactor headspace with the carrier gas held (helium at a constant pressure of 1.5 bar) before and during irradiation. Sample and reactor were placed at 60 C by a water flow of a cryostat. VOCs were trapped on a third-stage trap Vocarb 3000 placed at 35 C during a selected period of time. After this period, the light source was switched off. After a dynamic thermal desorption (220 C during 0.5 min), all VOCs were carried away through transfer line and analyzed by GCeMS after a cryo focalization step.
3. Results and discussion 3.1. Photostability of homo-PETTA The photopolymerization of the tetrafunctional monomer PETTA gave rise to the formation of a network with high density. Regarding to the literature [39], a high glass transition temperature, Tg was expected due to the fact that the crosslinking prevented the chain mobility. Indeed, Tg was not shown under our experimental conditions by DSC analysis. The exposure of the obtained polymer film under accelerated photoaging conditions led to chemical modifications followed by infrared spectroscopy. As an example, Fig. 3a displays the changes in the spectra of a thin film in the 3800e1420 cm 1 domain. The studied thin film allowed permeability to oxygen within its thickness. The evolution of the infrared spectra revealed two distinct and simultaneous phenomena. Light exposure induced the postpolymerization of the residual vinyl groups of the acrylate units. Indeed, the bands at 3037, 1633 and 1410 cm 1 attributed to CH] CH2 groups [40] were observed to decrease while the bands at 2935 and 1454 cm 1 assigned to methylene groups arising from the polymerization of the vinylic functions, increased. As expected, absorptions at 1454 cm 1 (generation of methylene groups) and 1633 cm 1 (consumption of the CH]CH2 groups) were linearly correlated (not shown). The second phenomenon was ascribed to the photodegradation of the polymer matrix. According to the literature [31,35,41e44], the development of a broad band in the hydroxyl domain (3600e3200 cm 1, Fig. 3a), the broadening of the initial CO band at 1724 cm 1 and the release of carbon dioxide (band at 2337 cm 1) [45] revealed that homo-PETTA underwent an oxidative reaction upon irradiation at wavelengths longer than 300 nm. It is worthy to remind that in general, the proposed degradation mechanism is based on bimolecular processes. These processes depend on the ability of radicals or molecules to diffuse through the polymeric matrix. However, in a glassy material as homo-PETTA, even if the movement of the reacting species is lower, this does not prevent the degradation of the polymer [46]. The same experiments were carried out on a film sandwiched between two glass slides in order to simulate an irradiation in deoxygenated atmosphere. Such conditions allowed the post-polymerization process to be confirmed. They also clearly evidenced the
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from chain scission reactions as in the case of carbon dioxide [35,47]. Homo-PETTA films were also analyzed by UVevisible spectrophotometry. As shown in Fig. 3b, the band at 310 nm significantly decreased. We remind that a two component photosensitizer system (photoinitiator/tertiary amine) was used to synthesize the homopolymer. Despite the fact that the residual tertiary amide (DABE) absorbed at 310 nm, the complex medium could not allow to assign with accuracy the observed absorption band. However, it could be noted that the decrease of the absorbance at 310 nm was linearly correlated with the consumption of the residual acrylate vinylic groups (band at 1633 cm 1) (Fig. 3c). This correlation might suggest that these two phenomena occurred simultaneously. HSeSPME/GCeMS analysis of a homo-PETTA film irradiated 150 h revealed the main formation of ethanoic acid corresponding to a retention time of 29.1 min. This species was the result of chain scission reactions mentioned above. Indeed, the decarboxylation led to a macroradical as shown by the proposed mechanism displayed in Fig. 4. This latter in the presence of oxygen and the polymer (noted PH) yielded hydroperoxide, which then evolved in aldehyde. The release of carbon dioxide or carbon monoxide from the formed species would give rise to the formation of ethanal, which could subsequently oxidize in ethanoic acid [48]. 3.2. Photostability of homo-2EEEA
Fig. 3. Changes in (a) the infrared spectra in the 3800e1420 cme1 domain and (b) the UVevisible spectra of a homo-PETTA film upon irradiation in a SEPAP 14/24 accelerated aging device (l 300 nm, 40 C). (c) Linear correlation between the absorbance at 1633 cm 1 (consumption of the residual acrylate vinylic groups and the absorbance at 310 nm.
formation of carbon dioxide trapped in the polymer matrix. According to the literature [35,47], Norrish I cleavage and ester elimination reactions (chain scission reactions) accounted for the formation of this gas. To get more information about the photochemical aging of the polymer, photo-IR experiments were performed. In situ infrared spectra not only confirm the release of CO2 but also show the presence of carbon monoxide. This latter resulted
The photopolymerization of the monofunctional monomer 2EEEA yielded a crumbly polymer film. Its brittleness did not allow the study of its durability to be undertaken. To circumvent this problem, a copolymer PETTA/2EEEA (20/80) was synthesized. This copolymer (named “homo-2EEEA”), was further used to identify the photoproducts resulting from the photo-oxidative degradation of 2EEEA. It was a well known fact that the ether groups are the main sites of the oxidation [31e34]. This latter took place on the carbon atom in the a position to the oxygen atom (Fig. 1). Thus, by analogy with the photo-oxidative degradation displayed in Fig. 1, a mechanism with three possible sites of attack could be proposed for homo2EEEA (Fig. 5). The hydrogen abstraction from CH2 groups might give rise to the formation of ethanal, ethyl formate and/or 2ethoxyethanol with subsequent radical chain reactions of the polymer. To verify this assumption, the volatile organic compounds stemming from the irradiation of a homo-2EEEA sample were monitored by in situ infrared spectroscopy. The release of CO and CO2 observed in Fig. 6(a and b) were the results of chain scission reactions (Norrish I cleavage and ester elimination reactions) that took place both on PETTA and 2EEEA units. These gases, as a consequence of chain scission processes, were also detected when the experiment was performed in deoxygenated atmosphere (film sandwiched between two glass slides). Fig. 6b evidences also the formation of CeH (2933 cm 1), C]O (1757 cm 1) and CeO (1120 and 1180 cm 1) bonds. The comparison with the spectra of ethyl formate and 2-ethoxyethanol confirmed the formation of these products from the mechanism proposed in Fig. 5. Concerning ethanal, according to the literature [49], the stretching vibration bands of the C]O groups are expected to be detected at 1761, 1743 and 1732 cm 1. The observed experimental wavenumbers correspond to these theoretical values within experimental error limits (Fig. 6b). In the literature, some studies demonstrated the efficiency of using SPME/GCeMS to monitor the degradation of the polymer and to elucidate the mechanism of degradation [50e55]. Therefore, to gain better understanding in the behavior of homo-2EEEA and to build up a mechanism of degradation, an approach based on the
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Fig. 4. Proposed mechanism of formation of ethanoic acid from photo-oxidative degradation of homo-PETTA initiated by chain scission reactions.
identification of volatile organic species by HSeSPME/GCeMS was used. Fig. 7a displays the chromatogram of a film irradiated for 150 h. It revealed that various low molecular weight compounds have migrated in the gas phase. With the exception of compound F, all the volatile products labeled from A to J, were identified (Table 1). It is worthy of notice that all the assignations were performed with a high confidence index. The lowest value (83%) was for 2-ethoxyethyl acetate (G) and ethyleneglycol diformate (I). To follow the kinetics of formation of the gases at earlier exposure times, a purge and trap dynamic headspace extraction coupled with a polychromatic in situ irradiation and GCeMS systems was used. From exposures exceeding 30 min, the detected volatile compounds were ethanal (1.8 min), ethyl formate (2.7 min),
ethanol (4.3 min), 2-ethoxyethanol (16.9 min) and 2-(2ethoxyethoxy)ethanol (24.1 min). Fig. 7b shows the kinetic evolution of the main photoproducts. The photo-purge-trap-GCeMS instrument revealed the formation of ethanal at shorter exposures. This product could subsequently oxidize to ethanoic acid [48]. This feature could explain the non detection of ethanal by HSeSMPE/GCeMS after a 150 h accelerated aging. To account for the formation of low molecular weight compounds, some mechanistic hypotheses for the degradation of homo-2EEEA are ventured in Figs. 5 and 8 on the basis of H abstraction. Macroalkoxy radicals generated by the oxidation of the polymer by route 1 (Fig. 5) could give rise either 1) ethanal by b
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Fig. 5. Proposed degradation pathways of homo-2EEEA initiated by H abstraction at positions 1, 2 and 4.
Fig. 6. (a) Infrared spectra waterfall and (b) changes in the infrared spectra of a homo2EEEA film upon in situ irradiation at l 300 nm and 40 C.
scission reaction (Fig. 5) which could in turn oxidize to ethanoic acid [48] or 2) some formate end groups carried by side polymer chains (Fig. 8a). These formates would afterwards undergo H abstraction and oxidation reaction that generated alkoxy radicals. As shown in Fig. 8a, these macroradicals would yield ethyleneglycol diformate (compound I) by b scission reaction. Macroalkoxy radicals stemming from route 4 (Fig. 5) evolved either in 1) 2-ethoxyethanol by b scission reaction or 2) 2ethoxyethyl acetate by cage reaction (Fig. 8b). Indeed, the carbonyl photoproduct, as the result of cage reaction, would undergo Norrish
Fig. 7. (a) HSeSPME/GCeMS chromatogram of a homo-2EEEA film irradiated 150 h in SEPAP 12/24 (l 300 nm, 60 C). (b) Kinetic evolution of the volatile organic compounds stemming from the in situ irradiation of a homo-2EEEA film.
I cleavage reaction followed by the decarboxylation process. The resulting CH2COOCH2CH2OC2H5 radicals would be converted into 2-ethoxyethyl acetate (compound G) by abstraction of labile
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G.G. Goourey et al. / Polymer Degradation and Stability 119 (2015) 208e216 Table 1 Volatile organic compounds released from irradiated homo-2EEEA film as identified by HSeSPME/GCeMS. Retention time (min)
Confidence index (%)
Products
2.7
91
O C O CH2CH3 H Ethyl formate (A)
4.3
87
4.6
91
CH3CH2OH Ethanol (B)
O CH3 O 2-methyl-1,3-dioxolane (C)
7.1
91
16.9
94
19.0
83
CH3CH2eOeCH2CH3 Ethoxyethane (D) HOeCH2CH2eOeCH2CH3 2-ethoxyethanol (E)
O C O CH2CH2 O CH2CH3 CH3 2-ethoxyethyl acetate (G)
21.8
O
91
C CH3 HO Ethanoic acid (H) 22.6
83
O C O CH2CH2 O C H
O H
Ethyleneglycol diformate (I) 24.1
90
hydrogen on the surrounding polymer chains (Fig. 8b). The abstraction of hydrogen at position 2 yielded alkoxy radicals. These latter would give rise to the formation of ethyl formate (route 2 in Fig. 5) or, some formate end groups carried by side polymer chain and OCH2CH3 radicals (not shown). In the presence of surrounding polymer chains, these radicals would lead to ethanol formation. In the same way, the macroradicals resulting from hydrogen abstraction at the position 3 (not shown) would give rise both to formate end group chains and CH2OCH2CH3 compounds by subsequent reactions. A mechanism similar to that proposed in Fig. 8a allowed the formation of these species to be explained. The formed radicals could combine with CH3 stemming from the degradation pathway 1 (Fig. 8a) to form ethoxyethane. A mechanism via a hydrogen abstraction from the methylene group adjacent to the ester linkage was mentioned in the literature [35]. If we apply this mechanism to homo-2EEEA, the hydrogen abstraction would yield the formation of 2-(2-ethoxyethoxy)acetaldehyde which could be subsequently oxidized in its corresponding acid, 2-(2-ethoxyethoxy)acetic acid. Unfortunately, in our study, no experimental data were available to confirm or deny the degradation pathway via the oxidation of the methylene group in a position of the ester groups. Indeed, all the attempts to detect the presence of both compound failed even if specific mass extractions at various m/z (45, 59, 72, 102 and 103, mass fragments selected according to the literature [56]) were simultaneously realized on TIC chromatograms recording all along the experiments from the first time (34 min) to several hours. To account for the formation of 2-(2-ethoxyethoxy)ethanol (compound J), a mechanism of photo-oxidation initiated by Norrish
HOeCH2CH2eOeCH2CH2eOeCH2CH3 2-(2-ethoxyethoxy)ethanol (J)
I cleavage and ester elimination reactions, as in the case of homo PETTA, could be proposed (not shown). The O(CH2CH2O)C2H5 and CH2CH2OCH2CH2OC2H5 species, arising from the two chain scission processes, would react with oxygen and yield alkoxy radicals through the formation of unstable hydroperoxides. 2-(2ethoxyethoxy)ethanol was as the result of a labile hydrogen abstraction on the polymer chain by the obtained alkoxy radicals. It could be mentioned that CH2CH2OCH2CH2OC2H5 species could also arise from Norrish I reaction followed by decarboxylation reaction. It was worthy to mention that the mechanism initiated by chain scission reactions also allowed to explain the formation of carbon monoxide and carbon dioxide upon the irradiation of homo-2EEEA. These two gases were evidenced by photo-IR. So, the investigation of the photochemical aging of homo-2EEEA pointed out that light and oxygen exposure entailed the photooxidation of the pendant chains of the polymer evidenced by the release of various volatile organic compounds. However, this study did not enable us to have information about the chemical modifications of the polymeric backbone. At earlier irradiation times, GCeMS experiments in dynamic mode also showed the formation of ethyl benzoate (23.7 min). The detection of this compound might suggest a degradation of ethyl 4dimethylaminobenzoate (DABE, absorbing at 310 nm) one of the components of the photosensitizer system (Fig. 9). This process could explain among others the decrease of the band at 310 nm observed previously by UVevisible spectroscopy. The degradation seemed to occur concurrently to H abstraction in the alpha position with respect to oxygen since the formation of ethyl benzoate linearly correlated with that of ethanal, ethanol, ethyl formate and 2-
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Fig. 8. Other proposed degradation routes of alkoxy radicals derived from (a) degradation pathway 1 and (b) degradation pathway 4 of homo-2EEEA (see Fig. 5).
ethoxyethanol was observed. As an example, the linear correlation between the cumulated area of ethanol and that of ethyl benzoate is shown (Inset of Fig. 9).
4. Conclusion The performance of a PETTA/2EEEA acrylate photopolymer used as recording material might be influenced by changes undergone by the polymer matrix under UV light exposure. However, the assessment of the photostability of such thermoset that presents a
Fig. 9. Kinetic evolution of ethyl benzoate stemming from the in situ irradiation of a homo-2EEEA film. Inset: Linear correlation between the formation of ethyl benzoate and ethanol.
rather complex structure could not be considered without having a deep knowledge of the key elements of the degradation of each of the two homopolymers synthesized from the photopolymerization of the corresponding acrylate monomers (called homo-PETTA and homo-2EEEA). The key elements are linked to the identification of the attack site and the resulting photoproducts and, to the elucidation of some mechanisms of degradation. This was the focus of the present work. For this investigation, HSeSPME/GCeMS and photo-purge-trap-GCeMS (a new methodology) were powerful tools that allowed to propose some mechanisms on the basis of the identification of organic volatile compounds. The implementation of photo-IR also contributed to the elucidation of the degradation mechanisms. The combination of light and oxygen exposure of homo-PETTA (carrier of ester functions) caused a postpolymerization together with a degradation of the polymer. The degradation entailed some chemical modifications of the polymeric backbone (formation of hydroxyl and carbonyl photoproducts). Norrish I and ester elimination reactions (chain scissions), that occurred both in the presence and in the absence of oxygen, accounted for the observed release of carbon monoxide and carbon dioxide gases. Then, we were interested in the photostability of homo-2EEEA (carrier of ester and ether functions). It was demonstrated that the ether groups present in the pendant chains mainly contributed to the instability of the polymer. The abstraction of the hydrogen in the a position with respect to the oxygen atom (attack site) and the chain scission reactions were some pathways of degradation of the material. Indeed, they allowed from the earlier irradiation times to account for the formation of organic volatile compounds evidenced both by photo-IR and GCeMS analysis. The key elements of the degradation of each homopolymer upon irradiation being well identified, one could ask about their impact on the photostability of the corresponding copolymer. The
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Photostability of acrylate photopolymers used as components in recording materials Georgia G. Goourey a, b, Pascal Wong-Wah-Chung c, d, Florence Delor-Jestin b, e, €l Israe €li a, b, * geret f, g, h, Lavinia Balan i, Yae Bertrand Le Clermont Universit e, Universit e Blaise Pascal, BP 10448, 63000 Clermont-Ferrand, France Institut de Chimie de Clermont-Ferrand (ICCF), UMR CNRS 6296, BP 80026, 63171 Aubi ere, France c ^le de l'Arbois, BP 80, 13545 Aix-en-Provence Cedex 4, France Aix-Marseille Universit e, Laboratoire Chimie Environnement, Europo d CNRS, FRE 3416, 13545 Aix-en-Provence Cedex 4, France e Clermont Universit e, Ecole Nationale Sup erieure de Chimie de Clermont-Ferrand, BP 10187, 63000 Clermont-Ferrand, France f CEA Cadarache, IBEB, Lab Bioenerget Biotechnol Bacteries & Microalgues, Saint-Paul-lez-Durance F-13108, France g CNRS, UMR Biol Veget & Microbiol Environ, 13108 Saint-Paul-lez-Durance, France h Aix-Marseille Universit e, 13108 Saint-Paul-lez-Durance, France i Institut de Science des Mat eriaux de Mulhouse 15, Rue Jean Starcky, BP 2488, 68057 Mulhouse Cedex, France a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 February 2015 Received in revised form 20 May 2015 Accepted 22 May 2015 Available online 23 May 2015
The present work was devoted to the photostability of two homopolymers, resulting from the polymerization of acrylate monomers used as components of recording materials. This study was undertaken in conditions simulating solar light. The objective was to identify the key elements specific of the degradation of each polymer. This approach passed through the understanding of the chemical modifications of the polymer matrix and therefore through the elucidation of the mechanism of degradation. Photo-IR experiments and GCeMS analysis were performed to identify the organic volatile compounds stemming from the photo-oxidation of each homopolymers. A new methodology based on the photopurge-trap-GCeMS was implemented to have a better insight of the elucidation of the pathways of degradation. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Degradation GCeMS Mass spectrometry Photopolymerization Thermosets
1. Introduction The emerging applications of holography in optical data storage [1e4], optical elements [5,6], holographic sensor [7], artistic display [6,8] and security integrated devices [6,9] require to used highperformance recording materials with high diffraction efficiency, good spatial-frequency response and long shelf-life stability. Their ease of feasibility, self-developing character, shape flexibility and low cost make photopolymers attractive and versatile materials for holographic recording [10e16]. In order to achieve improved optical performance that was driven by large diffraction efficiency and, to enable high storage capacity, many studies were focussed in the development of the composition of the recording photopolymer materials since the last decades. Recent reviews described
* Corresponding author. Institut de Chimie de Clermont-Ferrand (ICCF), UMR re, France. CNRS 6296, BP 80026, 63171 Aubie €li). E-mail address: [email protected] (Y. Israe http://dx.doi.org/10.1016/j.polymdegradstab.2015.05.016 0141-3910/© 2015 Elsevier Ltd. All rights reserved.
the material requirements for holographic data storage [2,3]. They also showed the chemically diverse and interesting photopolymer media [2,3]. A number of photopolymerizable systems was proposed such as for example a liquid crystalline monomer mixed with a multifunctional acrylate [17], phenanthrenequinone-doped polymethacrylate (PQ) [3,5], PQ-polymethylmethacrylate (PMMA)polyvinyl butyral [14]. Since the photopolymerization reaction is based on the formation of the grating, some additional compound could be introduced in the recording media to modify the kinetics of the reaction. Thus, to record reflection holograms with high spatial frequency in polyvinyl alcohol/acrylamide (PVA/AA), a cross linker added in the formulation contributed to speed up the polymerization reaction of AA monomers [1,15]. In the same way, in the presence of a chain transfer agent, the obtained low molecular weight polymer allowed to improve the optical performance [18]. Unfortunately, from an environmental viewpoint, acrylamide was known for its carcinogenic property [19]. The diacetone acrylamide monomer which exhibited a less cytotoxicity was also used to
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replace AA [7]. Recently, sodium acrylate, a “green” photopolymer, was developed as an alternative to acrylamide derivatives [20,21]. It is worthy to remind that the photoinitiation reaction is the key step of the polymerization process. Nevertheless, the composition of the photoinitiating system is often disregarded in the development of high-performance recording materials. In the literature, recent studies showed that the implementation of a three component system or a photocyclic initiating systems afforded high reactivity in the photopolymerization systems that make them good candidates for grating recording [22]. For commercial applications, among the various requirements for recording material, the long-term stability (mainly meaning dark storage) is one of the most essential criteria. The main drawback during the dark storage is the formation of a certain radicals [23]. One solution consists in introducing monomer stabilizer to prevent radical polymerization [23]. When the diffusion process occurs in the dark and induces the loss of the physical property, the temperature is a parameter which could play a role. Indeed, in the case of PQ/PMMA after the photopatterning process, the material undergoes two continuous phenomena: 1) a diffusion of PQ molecules that leads to an enhancement of the physical property and, 2) a diffusion of the corresponding photoproducts of PQ that gives rise to a decay of the diffraction efficiency [5,24]. The dark storage at low temperature contributes to optimize the stability of the grating [24]. Depending on the applications, the photo-patterned polymer could be exposed to UV light. Nevertheless, it is well known that, the polymers could undergo a photodegradation under the combined effect of solar light and oxygen [25e29]. This gives rise to chemical modifications (formation of photo-oxidation products) and/or architectural changes (chain scission and crosslinking) that modify the physical properties of the polymer and decrease the performance of the material. In this context, we have recently introduced an acrylate photopolymerizable formulation doped with ZnO quantum dots [30]. The monomer mixture consisted of pentaerythritol tetraacrylate (PETTA) and 2-(2-ethoxyethoxy)ethyl acrylate (2EEEA). The resulting copolymer was a crosslinked polymer (a thermoset) due to the tetra-functionality of PETTA monomers. Due to the rather complexity of its whole structure, the expertise of the stability of the matrix in the absence and in the presence of oxygen required in a first approach to evaluate the aging resistance of the two homopolymers synthesized from the photopolymerization of the corresponding acrylate PETTA and 2EEEA monomers, respectively. They will be referred below as homo-PETTA (carrier of ester groups) and homo-2EEEA (carrier of ester and ether groups), respectively. This approach was all the more important that previous studies performed on linear poly(ester-ether) [31,32] pointed out that the ether groups were the main site of the oxidation upon irradiation at wavelengths longer than 300 nm. From the investigation of the photo-oxidative degradation of poly(ethylene oxide) in solid state [33,34], the following mechanism was postulated (Fig. 1): the first step is the hydrogen atom abstraction in the a position with respect to oxygen atom followed by the formation of alkoxy and hydroxyl radicals through the generation of hydroperoxide derivative. These latter species could further undergo b scission, cage reaction or hydrogen abstraction reaction and would cause the chemical modifications of the polymer matrix. Besides, MALDI analysis performed on poly(butylene succinate) highlighted that, upon irradiation, this polymer undergoes oxidation of the hydroxyl end groups, a-hydrogen abstraction and Norrish I cleavage reactions [35]. Thus, the present work was devoted to the photostability of the two homopolymers, homo-PETTA and homo-2EEEA, respectively, under irradiation conditions simulating solar light. The photochemical modifications were monitored by infrared spectroscopy
209
Fig. 1. Photo-oxidation mechanism for poly(ethylene oxide) in solid state [33,34].
and characterized mainly by gas chromatography coupled with mass spectroscopy (GCeMS). Accounting for the chemical modifications of the polymer matrix generally passed through the proposal of the mechanism of degradation. So, in this work, the approach based on the identification of the volatile organic compounds was used to draw up a mechanism of degradation. 2. Experimental 2.1. Materials Pentaerythritol tetraacrylate (PETTA) and 2-(2-ethoxyethoxy) ethyl acrylate (2EEEA) (Fig. 2) were purchased from Aldrich and used as received without further purification. The typical photopolymerizable formulations were prepared in low-light conditions by mixing PETTA or 2EEEA or the two acrylate monomers (20:80 PETTA:2EEEA or 80:20 PETTA:2EEEA) with a photoinitiator (DL-camphorquinone [CQ] from EGA Chemie, 1.5 wt%) and a co-initiator (ethyl 4-dimethylaminobenzoate [DABE] from Aldrich, purum > 99%, 0.6 wt%). The polymer samples were prepared by coating the photopolymerizable mixture onto a glass slide or on a small aluminum rectangular crucible, followed by irradiation performed in a deoxygenated atmosphere using a conventional irradiation device (monochromator) at 488 nm with an intensity of 0.6 mW cm 2, in the same conditions used for recording. 2.2. Experimental set-up and procedure For photoaging investigation, two artificial aging devices were used. The first one was a SEPAP 14/24 unit equipped with a medium pressure mercury lamp (Novalamp RVC 400 W). The source was
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Fig. 2. Monomers (a) Pentaerythritol tetraacrylate (PETTA) and (b) 2-(2-ethoxyethoxy)ethyl acrylate (2EEEA).
located along one of the focal axis of a cylinder with an elliptical base. The samples were arranged on a rotating carousel that could receive 15 samples, located at the other focal axis. Experiments were performed at 40 C. The second aging device was a SEPAP 12/ 24 unit from Atlas, equipped with four medium pressure mercury lamps (Novalamp RVC 400 W) located in a vertical position at each corner of the chamber. Wavelengths less than 295 nm were filtered by the glass envelope of the sources. A rotating carousel, on which the samples were fixed, was placed in the center of the chamber. The temperature at the surface of the sample was fixed at 60 C. Differential scanning calorimetry (DSC) measurements were performed on a Mettler Toledo DSC822e under a nitrogen flow (30 mL min 1), in the temperature range from 60 to 300 C with a heating rate of 15 C min 1. Infrared spectra were recorded on a Nicolet 5SXC FTIR spectrometer with Omnic software. Spectra were obtained with a 4 cm 1 resolution and 32 scan summation. UVevisible spectra were recorded on a Shimadzu UV-2101 PC spectrophotometer. The photo-IR combines light irradiation and infrared analysis using a Hamamatsu light generator equipped with a medium pressure Xe/Hg source (Lightningcure LC6 1W) and Thermo Optek Nexus FTIR spectrometer. The source was filtered by a sapphire disk to deliver wavelengths longer than 300 nm, which was representative of outdoor aging. The films (deposited in rectangle aluminum crucible) were beforehand placed in a sealed gas cell that was located in the infrared analysis cavity equipped with an oven [36]. The temperature in the oven was maintained constant at 40 C. The infrared spectra of the released gas were collected every two hours during 60 h with a 4 cm 1 resolution and 32 scan summation. For solid-phase microextraction (SPME) experiments, the photopolymer films (deposited on an aluminum rectangle crucible) were irradiated in the SEPAP 12/24 unit, in sealed vials to collect the volatile organic compounds (VOCs) stemming from the photodegradation. CarboxenePDMS fiber (75 mm) purchased from Supelco (Bellefonte, PA, USA) was used to extract the volatile organic compounds (VOCs) in the headspace (HS) [37,38]. VOCs were analyzed by gas chromatography/mass spectrometry (GCeMS) with a 6890N Agilent GC coupled to a 5973 Agilent mass detector. The GC was equipped with a SupelcowaxTM 10 column (30 m 0.25 mm 0.25 mm) from Supelco. The analysis conditions were previously described in details in Ref. [37]. Mass spectra and reconstructed chromatograms (total ion current, TIC) were acquired under the electron ionization mode (EI) at 70 eV and recorded from 20 to 400 m/z. The compounds were identified by comparison with the mass spectra of the spectral library. VOCs stemming from the in situ irradiation of the photopolymer sample were also analyzed by using a purge and trap dynamic headspace extraction coupled with polychromatic irradiation and GCeMS system. A modified Tekmar LCS 2000 concentrator (Tekmar-Dohrmann, Cincinnati, USA) was used connected to a homemade hermitical water-jacketed Pyrex reactor and a GCeMS apparatus through a transfer line. The polymer was directly irradiated in a hermetically sealed photoreactor by using a LOT xenon
lamp system (300 W) whose emission was filtrated by a Pyrex filter to deliver 290e800 nm wavelengths. The photo-purge-trapGCeMS instrument allowed to purge the reactor headspace with the carrier gas held (helium at a constant pressure of 1.5 bar) before and during irradiation. Sample and reactor were placed at 60 C by a water flow of a cryostat. VOCs were trapped on a third-stage trap Vocarb 3000 placed at 35 C during a selected period of time. After this period, the light source was switched off. After a dynamic thermal desorption (220 C during 0.5 min), all VOCs were carried away through transfer line and analyzed by GCeMS after a cryo focalization step.
3. Results and discussion 3.1. Photostability of homo-PETTA The photopolymerization of the tetrafunctional monomer PETTA gave rise to the formation of a network with high density. Regarding to the literature [39], a high glass transition temperature, Tg was expected due to the fact that the crosslinking prevented the chain mobility. Indeed, Tg was not shown under our experimental conditions by DSC analysis. The exposure of the obtained polymer film under accelerated photoaging conditions led to chemical modifications followed by infrared spectroscopy. As an example, Fig. 3a displays the changes in the spectra of a thin film in the 3800e1420 cm 1 domain. The studied thin film allowed permeability to oxygen within its thickness. The evolution of the infrared spectra revealed two distinct and simultaneous phenomena. Light exposure induced the postpolymerization of the residual vinyl groups of the acrylate units. Indeed, the bands at 3037, 1633 and 1410 cm 1 attributed to CH] CH2 groups [40] were observed to decrease while the bands at 2935 and 1454 cm 1 assigned to methylene groups arising from the polymerization of the vinylic functions, increased. As expected, absorptions at 1454 cm 1 (generation of methylene groups) and 1633 cm 1 (consumption of the CH]CH2 groups) were linearly correlated (not shown). The second phenomenon was ascribed to the photodegradation of the polymer matrix. According to the literature [31,35,41e44], the development of a broad band in the hydroxyl domain (3600e3200 cm 1, Fig. 3a), the broadening of the initial CO band at 1724 cm 1 and the release of carbon dioxide (band at 2337 cm 1) [45] revealed that homo-PETTA underwent an oxidative reaction upon irradiation at wavelengths longer than 300 nm. It is worthy to remind that in general, the proposed degradation mechanism is based on bimolecular processes. These processes depend on the ability of radicals or molecules to diffuse through the polymeric matrix. However, in a glassy material as homo-PETTA, even if the movement of the reacting species is lower, this does not prevent the degradation of the polymer [46]. The same experiments were carried out on a film sandwiched between two glass slides in order to simulate an irradiation in deoxygenated atmosphere. Such conditions allowed the post-polymerization process to be confirmed. They also clearly evidenced the
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211
from chain scission reactions as in the case of carbon dioxide [35,47]. Homo-PETTA films were also analyzed by UVevisible spectrophotometry. As shown in Fig. 3b, the band at 310 nm significantly decreased. We remind that a two component photosensitizer system (photoinitiator/tertiary amine) was used to synthesize the homopolymer. Despite the fact that the residual tertiary amide (DABE) absorbed at 310 nm, the complex medium could not allow to assign with accuracy the observed absorption band. However, it could be noted that the decrease of the absorbance at 310 nm was linearly correlated with the consumption of the residual acrylate vinylic groups (band at 1633 cm 1) (Fig. 3c). This correlation might suggest that these two phenomena occurred simultaneously. HSeSPME/GCeMS analysis of a homo-PETTA film irradiated 150 h revealed the main formation of ethanoic acid corresponding to a retention time of 29.1 min. This species was the result of chain scission reactions mentioned above. Indeed, the decarboxylation led to a macroradical as shown by the proposed mechanism displayed in Fig. 4. This latter in the presence of oxygen and the polymer (noted PH) yielded hydroperoxide, which then evolved in aldehyde. The release of carbon dioxide or carbon monoxide from the formed species would give rise to the formation of ethanal, which could subsequently oxidize in ethanoic acid [48]. 3.2. Photostability of homo-2EEEA
Fig. 3. Changes in (a) the infrared spectra in the 3800e1420 cme1 domain and (b) the UVevisible spectra of a homo-PETTA film upon irradiation in a SEPAP 14/24 accelerated aging device (l 300 nm, 40 C). (c) Linear correlation between the absorbance at 1633 cm 1 (consumption of the residual acrylate vinylic groups and the absorbance at 310 nm.
formation of carbon dioxide trapped in the polymer matrix. According to the literature [35,47], Norrish I cleavage and ester elimination reactions (chain scission reactions) accounted for the formation of this gas. To get more information about the photochemical aging of the polymer, photo-IR experiments were performed. In situ infrared spectra not only confirm the release of CO2 but also show the presence of carbon monoxide. This latter resulted
The photopolymerization of the monofunctional monomer 2EEEA yielded a crumbly polymer film. Its brittleness did not allow the study of its durability to be undertaken. To circumvent this problem, a copolymer PETTA/2EEEA (20/80) was synthesized. This copolymer (named “homo-2EEEA”), was further used to identify the photoproducts resulting from the photo-oxidative degradation of 2EEEA. It was a well known fact that the ether groups are the main sites of the oxidation [31e34]. This latter took place on the carbon atom in the a position to the oxygen atom (Fig. 1). Thus, by analogy with the photo-oxidative degradation displayed in Fig. 1, a mechanism with three possible sites of attack could be proposed for homo2EEEA (Fig. 5). The hydrogen abstraction from CH2 groups might give rise to the formation of ethanal, ethyl formate and/or 2ethoxyethanol with subsequent radical chain reactions of the polymer. To verify this assumption, the volatile organic compounds stemming from the irradiation of a homo-2EEEA sample were monitored by in situ infrared spectroscopy. The release of CO and CO2 observed in Fig. 6(a and b) were the results of chain scission reactions (Norrish I cleavage and ester elimination reactions) that took place both on PETTA and 2EEEA units. These gases, as a consequence of chain scission processes, were also detected when the experiment was performed in deoxygenated atmosphere (film sandwiched between two glass slides). Fig. 6b evidences also the formation of CeH (2933 cm 1), C]O (1757 cm 1) and CeO (1120 and 1180 cm 1) bonds. The comparison with the spectra of ethyl formate and 2-ethoxyethanol confirmed the formation of these products from the mechanism proposed in Fig. 5. Concerning ethanal, according to the literature [49], the stretching vibration bands of the C]O groups are expected to be detected at 1761, 1743 and 1732 cm 1. The observed experimental wavenumbers correspond to these theoretical values within experimental error limits (Fig. 6b). In the literature, some studies demonstrated the efficiency of using SPME/GCeMS to monitor the degradation of the polymer and to elucidate the mechanism of degradation [50e55]. Therefore, to gain better understanding in the behavior of homo-2EEEA and to build up a mechanism of degradation, an approach based on the
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Fig. 4. Proposed mechanism of formation of ethanoic acid from photo-oxidative degradation of homo-PETTA initiated by chain scission reactions.
identification of volatile organic species by HSeSPME/GCeMS was used. Fig. 7a displays the chromatogram of a film irradiated for 150 h. It revealed that various low molecular weight compounds have migrated in the gas phase. With the exception of compound F, all the volatile products labeled from A to J, were identified (Table 1). It is worthy of notice that all the assignations were performed with a high confidence index. The lowest value (83%) was for 2-ethoxyethyl acetate (G) and ethyleneglycol diformate (I). To follow the kinetics of formation of the gases at earlier exposure times, a purge and trap dynamic headspace extraction coupled with a polychromatic in situ irradiation and GCeMS systems was used. From exposures exceeding 30 min, the detected volatile compounds were ethanal (1.8 min), ethyl formate (2.7 min),
ethanol (4.3 min), 2-ethoxyethanol (16.9 min) and 2-(2ethoxyethoxy)ethanol (24.1 min). Fig. 7b shows the kinetic evolution of the main photoproducts. The photo-purge-trap-GCeMS instrument revealed the formation of ethanal at shorter exposures. This product could subsequently oxidize to ethanoic acid [48]. This feature could explain the non detection of ethanal by HSeSMPE/GCeMS after a 150 h accelerated aging. To account for the formation of low molecular weight compounds, some mechanistic hypotheses for the degradation of homo-2EEEA are ventured in Figs. 5 and 8 on the basis of H abstraction. Macroalkoxy radicals generated by the oxidation of the polymer by route 1 (Fig. 5) could give rise either 1) ethanal by b
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213
Fig. 5. Proposed degradation pathways of homo-2EEEA initiated by H abstraction at positions 1, 2 and 4.
Fig. 6. (a) Infrared spectra waterfall and (b) changes in the infrared spectra of a homo2EEEA film upon in situ irradiation at l 300 nm and 40 C.
scission reaction (Fig. 5) which could in turn oxidize to ethanoic acid [48] or 2) some formate end groups carried by side polymer chains (Fig. 8a). These formates would afterwards undergo H abstraction and oxidation reaction that generated alkoxy radicals. As shown in Fig. 8a, these macroradicals would yield ethyleneglycol diformate (compound I) by b scission reaction. Macroalkoxy radicals stemming from route 4 (Fig. 5) evolved either in 1) 2-ethoxyethanol by b scission reaction or 2) 2ethoxyethyl acetate by cage reaction (Fig. 8b). Indeed, the carbonyl photoproduct, as the result of cage reaction, would undergo Norrish
Fig. 7. (a) HSeSPME/GCeMS chromatogram of a homo-2EEEA film irradiated 150 h in SEPAP 12/24 (l 300 nm, 60 C). (b) Kinetic evolution of the volatile organic compounds stemming from the in situ irradiation of a homo-2EEEA film.
I cleavage reaction followed by the decarboxylation process. The resulting CH2COOCH2CH2OC2H5 radicals would be converted into 2-ethoxyethyl acetate (compound G) by abstraction of labile
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G.G. Goourey et al. / Polymer Degradation and Stability 119 (2015) 208e216 Table 1 Volatile organic compounds released from irradiated homo-2EEEA film as identified by HSeSPME/GCeMS. Retention time (min)
Confidence index (%)
Products
2.7
91
O C O CH2CH3 H Ethyl formate (A)
4.3
87
4.6
91
CH3CH2OH Ethanol (B)
O CH3 O 2-methyl-1,3-dioxolane (C)
7.1
91
16.9
94
19.0
83
CH3CH2eOeCH2CH3 Ethoxyethane (D) HOeCH2CH2eOeCH2CH3 2-ethoxyethanol (E)
O C O CH2CH2 O CH2CH3 CH3 2-ethoxyethyl acetate (G)
21.8
O
91
C CH3 HO Ethanoic acid (H) 22.6
83
O C O CH2CH2 O C H
O H
Ethyleneglycol diformate (I) 24.1
90
hydrogen on the surrounding polymer chains (Fig. 8b). The abstraction of hydrogen at position 2 yielded alkoxy radicals. These latter would give rise to the formation of ethyl formate (route 2 in Fig. 5) or, some formate end groups carried by side polymer chain and OCH2CH3 radicals (not shown). In the presence of surrounding polymer chains, these radicals would lead to ethanol formation. In the same way, the macroradicals resulting from hydrogen abstraction at the position 3 (not shown) would give rise both to formate end group chains and CH2OCH2CH3 compounds by subsequent reactions. A mechanism similar to that proposed in Fig. 8a allowed the formation of these species to be explained. The formed radicals could combine with CH3 stemming from the degradation pathway 1 (Fig. 8a) to form ethoxyethane. A mechanism via a hydrogen abstraction from the methylene group adjacent to the ester linkage was mentioned in the literature [35]. If we apply this mechanism to homo-2EEEA, the hydrogen abstraction would yield the formation of 2-(2-ethoxyethoxy)acetaldehyde which could be subsequently oxidized in its corresponding acid, 2-(2-ethoxyethoxy)acetic acid. Unfortunately, in our study, no experimental data were available to confirm or deny the degradation pathway via the oxidation of the methylene group in a position of the ester groups. Indeed, all the attempts to detect the presence of both compound failed even if specific mass extractions at various m/z (45, 59, 72, 102 and 103, mass fragments selected according to the literature [56]) were simultaneously realized on TIC chromatograms recording all along the experiments from the first time (34 min) to several hours. To account for the formation of 2-(2-ethoxyethoxy)ethanol (compound J), a mechanism of photo-oxidation initiated by Norrish
HOeCH2CH2eOeCH2CH2eOeCH2CH3 2-(2-ethoxyethoxy)ethanol (J)
I cleavage and ester elimination reactions, as in the case of homo PETTA, could be proposed (not shown). The O(CH2CH2O)C2H5 and CH2CH2OCH2CH2OC2H5 species, arising from the two chain scission processes, would react with oxygen and yield alkoxy radicals through the formation of unstable hydroperoxides. 2-(2ethoxyethoxy)ethanol was as the result of a labile hydrogen abstraction on the polymer chain by the obtained alkoxy radicals. It could be mentioned that CH2CH2OCH2CH2OC2H5 species could also arise from Norrish I reaction followed by decarboxylation reaction. It was worthy to mention that the mechanism initiated by chain scission reactions also allowed to explain the formation of carbon monoxide and carbon dioxide upon the irradiation of homo-2EEEA. These two gases were evidenced by photo-IR. So, the investigation of the photochemical aging of homo-2EEEA pointed out that light and oxygen exposure entailed the photooxidation of the pendant chains of the polymer evidenced by the release of various volatile organic compounds. However, this study did not enable us to have information about the chemical modifications of the polymeric backbone. At earlier irradiation times, GCeMS experiments in dynamic mode also showed the formation of ethyl benzoate (23.7 min). The detection of this compound might suggest a degradation of ethyl 4dimethylaminobenzoate (DABE, absorbing at 310 nm) one of the components of the photosensitizer system (Fig. 9). This process could explain among others the decrease of the band at 310 nm observed previously by UVevisible spectroscopy. The degradation seemed to occur concurrently to H abstraction in the alpha position with respect to oxygen since the formation of ethyl benzoate linearly correlated with that of ethanal, ethanol, ethyl formate and 2-
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215
Fig. 8. Other proposed degradation routes of alkoxy radicals derived from (a) degradation pathway 1 and (b) degradation pathway 4 of homo-2EEEA (see Fig. 5).
ethoxyethanol was observed. As an example, the linear correlation between the cumulated area of ethanol and that of ethyl benzoate is shown (Inset of Fig. 9).
4. Conclusion The performance of a PETTA/2EEEA acrylate photopolymer used as recording material might be influenced by changes undergone by the polymer matrix under UV light exposure. However, the assessment of the photostability of such thermoset that presents a
Fig. 9. Kinetic evolution of ethyl benzoate stemming from the in situ irradiation of a homo-2EEEA film. Inset: Linear correlation between the formation of ethyl benzoate and ethanol.
rather complex structure could not be considered without having a deep knowledge of the key elements of the degradation of each of the two homopolymers synthesized from the photopolymerization of the corresponding acrylate monomers (called homo-PETTA and homo-2EEEA). The key elements are linked to the identification of the attack site and the resulting photoproducts and, to the elucidation of some mechanisms of degradation. This was the focus of the present work. For this investigation, HSeSPME/GCeMS and photo-purge-trap-GCeMS (a new methodology) were powerful tools that allowed to propose some mechanisms on the basis of the identification of organic volatile compounds. The implementation of photo-IR also contributed to the elucidation of the degradation mechanisms. The combination of light and oxygen exposure of homo-PETTA (carrier of ester functions) caused a postpolymerization together with a degradation of the polymer. The degradation entailed some chemical modifications of the polymeric backbone (formation of hydroxyl and carbonyl photoproducts). Norrish I and ester elimination reactions (chain scissions), that occurred both in the presence and in the absence of oxygen, accounted for the observed release of carbon monoxide and carbon dioxide gases. Then, we were interested in the photostability of homo-2EEEA (carrier of ester and ether functions). It was demonstrated that the ether groups present in the pendant chains mainly contributed to the instability of the polymer. The abstraction of the hydrogen in the a position with respect to the oxygen atom (attack site) and the chain scission reactions were some pathways of degradation of the material. Indeed, they allowed from the earlier irradiation times to account for the formation of organic volatile compounds evidenced both by photo-IR and GCeMS analysis. The key elements of the degradation of each homopolymer upon irradiation being well identified, one could ask about their impact on the photostability of the corresponding copolymer. The
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