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Langmuir 2008, 24, 9056-9065
“Inside” vs “Outside” Photooxygenation Reactions: Singlet-Oxygen-Mediated Surface Passivation of Polymer Films Elsa Silva Gonc¸alves and Peter R. Ogilby* Center for Oxygen Microscopy and Imaging, Department of Chemistry, UniVersity of Aarhus, DK-8000 Århus, Denmark ReceiVed April 30, 2008. ReVised Manuscript ReceiVed May 15, 2008 Films of poly(acrylonitrile-co-2,3-dimethyl-1,3-butadiene) were exposed to singlet oxygen. The extent of polymer oxygenation was monitored for singlet oxygen generated (1) within the polymer film and (2) at the polymer surface in an aqueous medium. When singlet oxygen is generated within the film, oxygenation of the polymer is pronounced and extensive. When singlet oxygen is generated at the polymer surface, oxygenation reactions are limited to the surface. The data suggest that the initial oxygenation reactions at the film surface passivate the polymer against further reaction with singlet oxygen and, hence, also minimize the progressively detrimental effects of secondary reactions. These results indicate that one should exercise restraint when implicating singlet oxygen as a reactive species in some processes of polymer oxygenation.
Introduction One principal tenet of chemistry is that the extent and rate of a given reaction can depend significantly on the local environment. The ramifications of this phenomenon are often clearly manifested in phase-separated heterogeneous systems.1 Hydrophobic polymers exposed to aqueous media provide one example of a system where such differential reactivity can be important, and it is here that we focus our current attention. We have recently examined events that occur upon exposure of hydrophobic polymer films to chlorinated water.2–4 In the case of poly(1,4-phenylene sulfide), for example, reaction of the polymer with water-borne Cl2 and HOCl results in the production of functional groups that increasingly render the polymer hydrophilic.4 These reactions that initially occur at the polymer surface facilitate progressively deeper penetration of the waterborne reagents into the bulk material. Ultimately, these reactions result in macromolecular chain scission and substantial material loss. The first excited electronic state of molecular oxygen, singlet oxygen, O2(a1∆g), is also known to react with a host of polymeric materials.5,6 Much effort has been devoted to characterizing features of this metastable state that distinguish it from groundstate oxygen, O2(X3Σg-). It is well established that these two states of oxygen have different reactivities that derive from their respective electronic structures.7 Ground state oxygen is a triplet spin state and undergoes chemistry characteristic of radicals. On the other hand, singlet oxygen adds to organic molecules in different ways. Furthermore, singlet oxygen can be deactivated to the ground triplet state upon collision with specific functional * To whom correspondence should be addressed. E-mail: progilby@ chem.au.dk. (1) Starks, C. M.; Liotta, C. L.; Halpern, M. Phase-Transfer Catalysis: Fundamentals, Applications, and Industrial PerspectiVes.; Springer: New York, 1994. (2) Dam, N.; Ogilby, P. R. HelV. Chim. Acta 2001, 84, 2540–2549. (3) Zebger, I.; Goikoetxea, A. B.; Jensen, S.; Ogilby, P. R. Polym. Degrad. Stab. 2003, 80, 293–304. (4) Zebger, I.; Elorza, A. L.; Salado, J.; Alcala, A. G.; Gonc¸alves, E. S.; Ogilby, P. R. Polym. Degrad. Stab. 2005, 90, 67–77. (5) Rabek, J. F.; Rånby, B. Photochem. Photobiol. 1978, 28, 557–570. (6) Golub, M. A. Pure Appl. Chem. 1980, 52, 305–323. (7) Foote, C. S., Valentine, J. S., Greenberg, A., Liebman, J. F., Eds. ActiVe Oxygen in Chemistry; Chapman and Hall: London, 1995.
groups and moieties. The presence of these latter groups can thus modulate the effective reactivity of singlet oxygen in given circumstances. The possibility of deactivating singlet oxygen through collisions is an important feature that sets this reactive species apart from the key components in, for example, chlorinated water (i.e., Cl2 and HOCl). For the present study, we wanted to exploit this feature of singlet oxygen to explore environment-dependent aspects of the reactivity of bulk polymer samples. More specifically, we wanted to elaborate on the so-called “inside vs outside” problem, differentiating the relative reactivity of singlet oxygen with a given polymer when singlet oxygen is generated (1) inside the bulk polymer and (2) outside the bulk polymer. This work complements an earlier study from our group in which we examined the effects of solvent cages and diffusion on the reactivity of singlet oxygen in glassy polymers.8 To appreciate the problem, it is useful to provide a brief introduction to pertinent aspects of singlet oxygen behavior. Indeed, with respect to singlet oxygen, interpretation of the “inside vs outside” problem has only really become tractable because of results obtained over the last 10-15 years. Singlet Oxygen. 1. Photosensitized Production. Singlet oxygen can be generated by a number of methods.9 The photosensitized production of singlet oxygen is commonly encountered, and it was used for the present experiments. In this method, singlet oxygen is produced by energy transfer from a photoexcited molecule (the so-called sensitizer) to groundstate oxygen (Figure 1). Upon irradiation of the sensitizer ground state, S0, one populates a singlet excited state, S1. Although quenching of this singlet state by O2(X3Σg-) can produce singlet oxygen,9 efficient sensitizers generally intersystem cross to produce a longer-lived triplet state, T1. For a wide range of molecules, energy transfer from T1 to O2(X3Σg-) generates singlet oxygen in high yield. A feature of the photosensitized process is the ability to spatially localize the production of singlet oxygen, with the latter being dependent on the location of the sensitizer. As such, singlet oxygen can be produced either inside or outside bulk polymer samples (8) Scurlock, R. D.; Kristiansen, M.; Ogilby, P. R.; Taylor, V. L.; Clough, R. L. Polym. Degrad. Stab. 1998, 60, 145–159. (9) Schweitzer, C.; Schmidt, R. Chem. ReV. 2003, 103, 1685–1757.
10.1021/la801353n CCC: $40.75 2008 American Chemical Society Published on Web 07/10/2008
“Inside” Vs “Outside” Photooxygenation Reactions
Figure 1. Diagram showing the photosensitized production of singlet oxygen. IC and ISC denote processes of internal conversion and intersystem crossing, respectively. Scheme 1. Representation of the Singlet Oxygen Ene Reaction with a Tetra-Substituted Alkene
depending on whether the sensitizer is contained in the polymer matrix or dissolved in a solution surrounding the sample. 2. EnVironment-Dependent Lifetimes. Interaction with solvent molecules significantly influences radiative and nonradiative singlet oxygen deactivation.9,10 Solvent-dependent singlet oxygen lifetimes are invariably determined by the nonradiative deactivation channel and vary from a few microseconds in H2O to tens of milliseconds in halogenated hydrocarbons. The singlet oxygen lifetime also depends on the solvent isotopic composition where the substitution of hydrogen by deuterium results in a significant lifetime increase (e.g., the lifetime in H2O is 3.5 µs,11 whereas it is 68 µs in D2O12). This is a consequence of the mechanism of solvent-mediated nonradiative singlet oxygen deactivation: electronic-to-vibrational energy transfer.9 Deactivation of singlet oxygen can also occur upon interaction with solutes that contain specific functional groups (e.g., amines, alcohols).9 These processes of physical deactivation kinetically compete with the chemical reactions of singlet oxygen (Figure 1), and this competition is a critical component of our present study. 3. Singlet Oxygen Chemistry. The chemistry of singlet oxygen has been extensively studied.13 For our present application, we have designed our system such that we are initially limited to the so-called “ene” reaction (Scheme 1). Much effort has been put into exploring the regio- and stereoselectivity of this reaction, and how this selectivity depends on the olefin involved.13–15 The immediate product of the ene reaction is a hydroperoxide which can cleave to yield the hydroxyl radical and an alkoxide radical that will undergo further reactions (Vide infra). (10) Ogilby, P. R. Acc. Chem. Res. 1999, 32, 512–519. (11) Egorov, S. Y.; Kamalov, V. F.; Koroteev, N. I.; Krasnovsky, A. A.; Toleutaev, B. N.; Zinukov, S. V. Chem. Phys. Lett. 1989, 163, 421–424. (12) Ogilby, P. R.; Foote, C. S. J. Am. Chem. Soc. 1983, 105, 3423–3430. (13) Clennan, E. L.; Pace, A. Tetrahedron 2005, 61, 6665–6691. (14) Stratakis, M.; Orfanopoulos, M. Tetrahedron 2000, 56, 1595–1615. (15) Griesbeck, A. G. In CRC Handbook of Organic Photochemistry and Photobiology; Horspool, W. M., Song, P.-S., Eds.; CRC Press: Boca Raton, FL, 1995; pp 301-310.
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4. Singlet Oxygen and Bulk Polymers. Key aspects of the photosensitized production and subsequent decay of singlet oxygen in bulk polymers have been examined. For a given photosensitizer, the quantum yield of singlet oxygen production is independent of whether the experiment is performed in a glassy polymer or liquid solution.16 Of course, one must be aware that, in a polymer glass, solute diffusion coefficients are much smaller than those in a corresponding liquid. As such, the rate of sensitizer deactivation by oxygen will be slower in the polymer and higher oxygen concentrations will be required to ensure complete removal of the sensitizer excited state by oxygen.17 Similarly, the singlet oxygen lifetime is independent of whether the surrounding medium is a glassy polymer or an analogous liquid solution (e.g., polystyrene vs ethyl benzene).17,18 Rather, the lifetime depends only on the presence of suitable functional groups that can accept the excitation energy of singlet oxygen in a process of electronic-to-vibrational energy transfer (Vide supra). On the other hand, when comparing glassy polymers with liquid solvents, appreciable differences can be observed in the rate constant for interaction between singlet oxygen and a dissolved solute.8,19 This reflects differences in solute diffusion coefficients and is manifested both in physical as well as chemical channels of singlet oxygen removal. Processes that occur with a large rate constant (i.e., at or near the diffusion controlled limit) are adversely affected in the change from a liquid solution to a glassy polymer. In contrast, the change from a liquid to a glass will result in an increase of comparatively small bimolecular rate constants due to an increased number of collisions within the solvent-defined encounter cage. 5. Singlet Oxygen Reactions with Bulk Polymers: The Inside Vs Outside Problem. Over the years, the reactions of singlet oxygen with bulk polymers have been examined from a range of perspectives.5,6,20 Most of the more convincing studies have focused on the reactions with olefin-containing polymers where the ene reaction plays a key role. In some of these studies, singlet oxygen was generated outside the polymer. For example, singlet was generated by a microwave discharge and the polymer film was then exposed to a stream of gas containing ∼5-15% singlet oxygen.5,21,22 Under these conditions, the formation of small amounts of polymer hydroperoxides was observed. Moreover, evidence was provided to indicate that these hydroperoxides were apparently localized near the polymer surface.22 On the other hand, exposure of polymer films to liquid solutions in which singlet oxygen had been created by photosensitization have, thus far, revealed no conclusive evidence of singlet-oxygen-mediated oxygenation reactions.3 In contrast, the photosensitized production of singlet oxygen inside bulk polymer samples, and in samples of dissolved polymers, clearly results in polymer oxygenation.6,20,23,24 Furthermore, there is appreciable evidence to indicate that the initial (16) Scurlock, R. D.; Ma´rtire, D. O.; Ogilby, P. R.; Taylor, V. L.; Clough, R. L. Macromolecules 1994, 27, 4787–4794. (17) Clough, R. L.; Dillon, M. P.; Iu, K.-K.; Ogilby, P. R. Macromolecules 1989, 22, 3620–3628. (18) Ogilby, P. R.; Dillon, M. P.; Gao, Y.; Iu, K.-K.; Kristiansen, M.; Taylor, V. L.; Clough, R. L. AdV. Chem. Ser. 1993, 236, 573–598. (19) Ogilby, P. R.; Dillon, M. P.; Kristiansen, M.; Clough, R. L. Macromolecules 1992, 25, 3399–3405. (20) Rabek, J. F. In Singlet Oxygen. Volume 4. Polymers and Biomolecules; Frimer, A. A., Ed.; CRC Press: Boca Raton, 1985; pp 1-90. (21) Kaplan, M. L.; Kelleher, P. G. Science 1970, 169, 1206–1207. (22) Breck, A. K.; Taylor, C. L.; Russell, K. E.; Wan, J. K. S. J. Polym. Sci., Part A: Polym. Chem. 1974, 12, 1505–1513. (23) Ng, H. C.; Guillet, J. E. Macromolecules 1978, 11, 929–937. (24) Scurlock, R. D.; Wang, B.; Ogilby, P. R.; Sheats, J. R.; Clough, R. L. J. Am. Chem. Soc. 1995, 117, 10194–10202.
9058 Langmuir, Vol. 24, No. 16, 2008 Chart 1. Chemical Structure of Poly(AN-co-DMB)
singlet-oxygen-mediated oxygenation products spawn subsequent reactions that ultimately lead to macromolecular chain scission.24 At present, one can only speculate about the possible origins for these differences in “inside” vs “outside” photooxygenation reactions. To our knowledge, experiments have not been performed that would allow one to compare data recorded under these respective conditions. To this end, we set out to examine the reaction of an olefin-containing polymer with singlet oxygen that had been generated inside and, independently, outside the bulk material.
Results and Discussion We chose to work with a polymer that would react with singlet oxygen via the ene reaction. Singlet oxygen is an electrophile, and the rates of its reactions with olefins increase with the number of alkyl substituents on the olefin.25,26 As such, we set out to work with variations of poly(2,3-dimethyl-1,3-butadiene) whose double bond bears four alkyl groups. We decided to work with a copolymer of poly(2,3-dimethyl1,3-butadiene) because the homopolymer is a rubber that is difficult to handle. We chose to copolymerize 2,3-dimethyl1,3-butadiene (DMB) with acrylonitrile (AN). AN was chosen for a number of reasons. First, it supplies stiffness to the copolymer, thereby making it easier to handle and suitable for experiments with free-standing films. Second, the polymerized AN moiety contains no functional group that would react with singlet oxygen (e.g., singlet oxygen has a comparatively long lifetime in CH3CN, and the mechanism for deactivation in this case does not involve a chemical reaction). Third, we use IR spectroscopy to monitor chemical changes in the polymer, and polymerized AN does not absorb in the regions where oxygenation products are usually observed (∼3400 cm-1 for hydroxyl or hydroperoxide groups and ∼1700 cm-1 for carbonyl groups). The copolymer, poly(acrylonitrile-co-2,3-dimethyl-1,3-butadiene) (poly(AN-co-DMB)) (Chart 1), was prepared with a 1:1 AN/DMB ratio (see the Supporting Information). 1. Formation of Singlet Oxygen Inside the Polymer Matrix. In these experiments, poly(AN-co-DMB) films were exposed to singlet oxygen generated inside the polymer film by a photosensitized process. The films were not immersed in a solvent. The sensitizer chosen was meso-tetraphenylporphine, TPP. Upon irradiation into either the Soret band at 421 nm or a Q-band at 516 nm, TPP has a high quantum yield for singlet oxygen production (Φ∆ ) 0.68).27 At these wavelengths, poly(AN-coDMB) does not absorb light. TPP was incorporated into the polymer films by dissolving it with the polymer in the solution used to cast the film (see the Experimental Section). Previous experiments have shown that the molar absorption coefficient of a porphyrin dissolved in a polymer film is not appreciably different from that in an analogous liquid solvent.28 Thus, on the basis of the absorption spectrum (25) Foote, C. S.; Denny, R. W. J. Am. Chem. Soc. 1971, 93, 5162–5167. (26) Monroe, B. M. J. Phys. Chem. 1978, 82, 15–18. (27) Wilkinson, F.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1993, 22, 113–262. (28) Gao, Y.; Ogilby, P. R. Macromolecules 1992, 25, 4962–4966.
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of the polymer in a film whose thickness has been independently measured, we can ascertain the concentration of TPP in the film with reasonable accuracy. For the present study, although the sensitizer concentration varied slightly from one experiment to the next, we invariably worked with samples where [TPP] ) ∼10-3 M. At this concentration, the presence of the sensitizer and any products of sensitizer oxidation do not influence the IR spectra recorded to monitor polymer oxygenation (see the Supporting Information for control experiments). The IR spectra of our samples were normalized using an internal reference to account for thickness variations across the sample and among different samples. The internal reference chosen was the band at 2240 cm-1, which corresponds to the C-N stretching mode in the AN group. We have noted that singlet oxygen will not react with a nitrile. Moreover, the nitrile group is stable under thermal oxidative conditions.29 1.1. Infrared Changes. Upon TPP irradiation for a defined period of time under an ambient air atmosphere, the IR spectrum of the polymer film was recorded. Irradiation of the film was then resumed. The IR spectra were observed to evolve with increasing exposure time to singlet oxygen (Figure 2). For the sake of accuracy, we chose to only monitor increases in the intensity of selected bands in isolated regions of the IR spectra. IR bands with a high absorbance (A > 1) were not used. The most significant change observed in the IR spectra is the appearance of a broad band at 3380 cm-1 (Figure 2a). This band must correspond to the O-H stretching mode of hydroxyl and/ or hydroperoxide groups. Note that although this 3380 cm-1 band is broad, the band maximum does not change over time. Thus, the data imply that the OH groups probed, and their respective environments, do not change appreciably over the time window examined (irradiation was stopped after 10 h). Figure 3a shows how the intensity of this band increases with the elapsed irradiation time of TPP. After such a long period of irradiation, TPP bleaching was considerable (Supporting Information). Accordingly, the decrease in the rate at which this band appeared with elapsed irradiation time is partly a consequence of a decrease in the sensitizer concentration. The appearance of bands at ∼1630 and ∼1720 cm-1 was also observed (Figure 2b). The band at 1630 cm-1 can be assigned to R,β-unsaturated carbonyl groups by analogy to data recorded from poly(butadiene) and poly(isoprene).30,31 The apparent shift of this band to a higher wavenumber upon prolonged irradiation may reflect a change in the nature of the conjugated carbonyls. Nevertheless, the formation of new olefin groups cannot be discarded. The band at 1720 cm-1 most likely corresponds to the formation of nonconjugated carbonyl groups. Several changes can be observed in the IR spectrum between 1500 and 1000 cm-1 (Figure 2c). The more pronounced changes are an increase in the intensity of the bands at 1380 and 1060 cm-1. The band at ∼1380 cm-1 has been assigned to C-H symmetric deformations in the -CH3 group of the DMB unit.32 However, it has also been reported that the C-O stretching mode absorbs at ∼1380 cm-1,33 and the data could thus reflect the appearance of a broad band onto which the sharper 1380 cm-1 peak is superimposed. The band at 1060 cm-1 cannot be unequivocally assigned, but it is likely due to C-O vibrations.30,33 (29) Delor-Jestin, F.; Barrois-Oudin, N.; Cardinet, C.; Lacoste, J.; Lamaire, J. Polym. Degrad. Stab. 2000, 70, 1–4. (30) Piton, M.; Rivaton, A. Polym. Degrad. Stab. 1996, 53, 343–359. (31) Dos Santos, K. A. M.; Suarez, P. A. Z.; Rubim, J. C. Polym. Degrad. Stab. 2005, 90, 34–43. (32) Gaylord, N. G.; Kossler, I. J. Polym. Sci., Part C: Polym. Symp. 1968, 16, 3097–3108. (33) Makino, D.; Kobayashi, M.; Tadokoro, H. Spectrochim. Acta, Part A 1975, 31, 1481–1495.
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Figure 2. Normalized IR spectra for poly(AN-co-DMB) films containing TPP that had been irradiated at 421 nm under an air atmosphere. (a) With an increase in elapsed irradiation time, the intensity of a band assigned to O-H stretching modes increases. Data are shown for elapsed irradiation times of 0, 1, 2, 3, 4, 5, 6, 7, and 10 h. (b) With an increase of the irradiation time, the intensity of bands assigned to double bonds and/or conjugated carbonyls (1630 cm-1) as well as unconjugated carbonyls (1720 cm-1) increases. (c) With an increase of the irradiation time, the intensity of a band at 1380 cm-1 that corresponds either to the formation of C-H groups or to C-O groups increases. The band at 1060 cm-1 has been assigned to C-O vibrations. (d) With an increase of the irradiation time, the intensity of a band at ∼905 cm-1 assigned to either exomethylene double bonds or epoxide rings increases. The band at ∼850 cm-1 corresponds to the formation of trisusbtituted double bonds. For the data shown in panels b-d, elapsed irradiation times are 0, 1, 2, 4, 8, and 10 h.
Figure 3. Changes in normalized integrated absorbance as a function of elapsed irradiation time, in hours, of poly(AN-co-DMB) films containing TPP irradiated under an air atmosphere at 421 nm (9), 476 nm (2), and 516 nm (b). The changes recorded for poly(AN-co-DMB) films containing no TPP irradiated at 421 nm are also shown (O). Data recorded upon 421 nm irradiation for 10 h of TPP-containing samples was used to establish the 100% limit. The changes in normalized integrated absorbance were quantified using the (a) 3380 cm-1, (b) 1720 cm-1, (c) 1630 cm-1, and (d) 905 cm-1 bands. The error bars shown derive from multiple recordings of spectra at different positions on the given film. As discussed in the text, these data also reflect the effects of sensitizer bleaching over the irradiation period.
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In the part of the spectrum below 1000 cm-1, we see an increase in the intensity of bands at ∼905 and ∼850 cm-1 (Figure 2d). The appearance of these bands has been attributed to the formation of exomethylene (R2C(dCH2)) and trisubstituted double bonds (R2CdCHR) upon the exposure of poly-DMB to singlet oxygen.6 However, we note that epoxide groups (ring vibration) would also absorb at ∼905 cm-1.6 The apparent shift in the peak maximum of the band at ∼905 cm-1 upon prolonged irradiation may reflect, among other things, a change in the relative amounts of these functional groups over time. Of course, apparent shifts in the maximum of a given band could also reflect oxygenation-derived effects on the polymer morphology which, through changes in segmental motions of the macromolecule and concomitant changes in local environment, could alter the position of a given band. 1.2. Control Experiments. Several experiments were performed to establish that the irradiation-dependent changes observed in poly(AN-co-DMB) were initiated by reaction with singlet oxygen. In one experiment, poly(AN-co-DMB) films containing TPP were irradiated at 516 nm. At 516 nm, TPP has an absorption band that is significantly less intense than the Soret band at 421 nm (see the Supporting Information). Although TPP produces singlet oxygen with the same quantum yield upon irradiation at 516 nm, less singlet oxygen will nevertheless be formed compared to the 421 nm experiment due to the relative intensities of the respective absorption bands (assuming identical irradiation fluxes). In a second experiment, a poly(AN-co-DMB) film containing TPP was irradiated at 476 nm. TPP does not absorb at this wavelength, and thus, no singlet oxygen will be produced in this experiment. In the last control experiment, a poly(AN-co-DMB) film lacking TPP was irradiated at 421 nm under the same conditions as the experiments in which TPP was used. Data recorded in these control experiments are shown in Figure 3. To facilitate data comparison, the results have been normalized; the extent of reaction recorded for the samples containing TPP irradiated at 421 nm for 10 h has been set to 100%. On the basis of data recorded using four separate IR bands (Figure 3), we make the following conclusions: (1) Polymer oxygenation is pronounced under conditions in which singlet oxygen is produced. (2) No significant changes were recorded from samples in which singlet oxygen was not produced. (3) 421 nm irradiation of TPP led to more oxygenation than 516 nm irradiation which, as expected, reflects the differences in absorbance at the different wavelengths and the corresponding amount of singlet oxygen produced. For the experiments performed upon 516 nm irradiation, attempts were made to increase the extent of singlet-oxygenmediated oxygenation by increasing the concentration of sensitizer in the film. Although the sample absorbance at 516 nm indeed increased (A516 ) 0.5), the extent of oxygenation actually decreased relative to the experiments performed using a lower TPP concentration. This observation likely reflects the fact that, when present in high concentrations, TPP itself will quench singlet oxygen.12,34 These experiments performed at a higher TPP concentration also indicate that our data are indeed not likely to be influenced by artifacts associated with the bleaching of TPP over prolonged periods of irradiation. Thus, the main problem associated with TPP bleaching over time is that one must simply exercise caution in interpreting the rate at which oxygenation occurs. (34) Wilkinson, F.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1995, 24, 663–1021.
Gonc¸alVes and Ogilby Scheme 2. Singlet Oxygen Ene Reaction with Poly(AN-co-DMB)
The data in Figure 3 also reveal a subtle and interesting phenomenon. When monitoring the IR bands at 3380, 1630, and 905 cm-1, one observes very little change upon prolonged irradiation at 421 nm of a TPP-free sample. On the other hand, within the first 3 h of 421 nm irradiation of a TPP-free sample, we observe a noticeable increase in the intensity of the 1720 cm-1 band assigned to the CdO stretch of a carbonyl group. This increase in the intensity of the 1720 cm-1 band was not observed upon irradiation of the TPP-free sample at wavelengths longer than 421 nm. These data indicate that the carbonyl giving rise to the 1720 cm-1 band appears to be a product of a selfsensitized and/or photoinduced autoxidation reaction in addition to being one of the products of a TPP-sensitized singlet oxygen reaction. The data indicate that the inherent chromophore responsible for this reaction does not absorb appreciably at wavelengths longer than ∼420 nm. 1.3. Degradation Mechanism. We now attempt to correlate the IR observations with a possible mechanism for the oxygenation of poly(AN-co-DMB) upon exposure to singlet oxygen. Given the structure of poly(AN-co-DMB), we expect that the ene reaction will play an important role and the newly formed double bond will be formed as illustrated in Scheme 2.35 The resultant gem-dialkyl olefin will react less readily with singlet oxygen than the tetra-alkylated olefin in poly(AN-coDMB).36–38 The appearance of the IR band at 3380 cm-1, which could be assigned to the O-H stretch in a hydroperoxide, is consistent with the suggestion that DMB reacts with singlet oxygen according to the ene reaction. Of course, the 3380 cm-1 band could also derive from the O-H stretch of an alcohol. Nevertheless, this too is consistent with the ene reaction, since the hydroperoxide is labile and readily forms alcohols upon cleavage and hydrogen abstraction (Scheme 3). This reaction to produce the dCH2 group is also consistent with the appearance of the characteristic band at 905 cm-1 (Figure 2d).6 However, the associated appearance of the band at 850 cm-1 suggests that the initial reaction is not 100% selective and that other kinds of double bonds are also formed during the reaction of poly(AN-co-DMB) with singlet oxygen. One option is the result of the ene reaction to form the more highly substituted double bond in the macromolecular backbone (Scheme 4). As mentioned, the absorbance at 905 cm-1 and the progressive shift of this band to larger wavenumbers might also reflect the formation of an epoxide. The formation of the latter can readily be accommodated, and it is even expected upon the decomposition of the hydroperoxide formed initially through the ene reaction. The nascent alkoxy radical can cyclize (Scheme 5, path A) to form the epoxide, generating a carbon-centered radical which can either abstract hydrogen from the polymer matrix or combine with oxygen prior to hydrogen abstraction. In support of this mechanism, it has been reported that hydroperoxidized 1,4poly(2,3-dimethyl-1,3-butadiene) is quite unstable and that epoxide groups are formed after a short time.6 (35) Orfanopoulos, M.; Stratakis, M.; Elemes, Y. J. Am. Chem. Soc. 1990, 112, 6417–6419. (36) Clennan, E. L. Tetrahedron 1991, 47, 1343–1382. (37) Koch, E. Tetrahedron 1968, 24, 6295–6318. (38) Rabek, J. F.; Rånby, B. J. Appl. Polym. Sci. 1979, 23, 2481–2491.
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Scheme 3. Hydroperoxide Cleavage and the Formation of an Alcohol
Scheme 4. Alternative Singlet Oxygen Ene Reaction with Poly(AN-co-DMB)
On the other hand, the nascent alkoxy radical can also lead to R,β-unsaturated carbonyl compounds via carbon-carbon bond cleavage (Scheme 5, path B).23 Such compounds could eventually evolve to form saturated carboxylic acids and lactones.39 Carbonyl-containing compounds can also be formed by the so-called “Hock-cleavage”.40 Although this cleavage is generally acid catalyzed, it can also occur in the absence of added acid.40 Unsaturated and saturated carbonyl groups indeed appear in the IR spectra of our irradiated films at 1630 and 1720 cm-1, respectively. 2. Formation of Singlet Oxygen Inside the Polymer for a Sample Immersed in D2O. In a second series of experiments, poly(AN-co-DMB) films containing TPP inside the polymer matrix were immersed in D2O and irradiated. The aim here, in part, was to bridge the gap between the experiments on TPPcontaining samples exposed to air (Vide supra) and experiments in which singlet oxygen is generated in the solvent surrounding the polymer sample (Vide infra). These experiments could potentially help elucidate the effect of the surrounding medium on the reaction of singlet oxygen with the polymer. Because of our desire to work with a solvent that does not dissolve or swell the polymer sample, we chose to use water as the surrounding medium. Moreover, to be consistent with our experiments described in the next section wherein singlet oxygen is generated in the surrounding solvent, we opted to work with D2O instead of H2O. Under conditions in which the TPP-containing polymer was immersed in air-saturated D2O and irradiated at 421 nm, IR spectra indicated that polymer oxygenation occurred. Changes in the IR spectra were similar to those observed in the experiments on samples exposed to air, and only the magnitude of the changes varied (Figure 4). The data in Figure 4 clearly show that, in comparison to samples exposed to dry air, oxygenation is less pronounced for samples immersed in air-saturated D2O. This observation could reflect a number of phenomena. The most likely explanation is that the amount of oxygen in air-saturated water is less than that in the ambient atmosphere of air (i.e., [O2] ) ∼2 × 10-4 M in airsaturated water at 760 Torr41). Thus, as oxygen is consumed in the polymer over the irradiation period, exposure to an ambient atmosphere of air provides a better source of additional oxygen than does exposure to air-saturated water. (39) Adam, C.; Lacoste, J.; Lemaire, J. Polym. Degrad. Stab. 1989, 24, 185– 200. (40) Frimer, A. A.; Stephenson, L. M. In Singlet Oxygen; Frimer, A. A., Ed.; CRC Press: Boca Raton, FL, 1985; Vol. 2, pp 67-91. (41) Battino, R., Ed. IUPAC Solubility Data Series. Volume 7: Oxygen and Ozone.; Pergamon Press: Oxford, 1981.
In another experiment, poly(AN-co-DMB) samples containing TPP were immersed in oxygen-saturated D2O and irradiated at 421 nm. In this study, oxygen was continuously bubbled into the solution over the period of the experiment (5 h). Under these conditions, the extent of both polymer oxygenation and TPP bleaching was significantly more pronounced than that observed from samples exposed to dry air and air-saturated D2O. Data recorded from these O2-equilibrated samples clearly showed that, as the oxygenation reactions proceed and hydrophilic functional groups are produced, water penetrates into the polymer matrix (D2O IR band at ∼2500 cm-1). This water was readily removed upon placing the sample in a desiccator for 12 h. Thus, it appears that, for these O2-saturated experiments, incorporated D2O swells the polymer, causing an increase in the oxygen diffusion coefficient in the matrix. Moreover, it is also likely that the increased D2O content in the matrix facilitates singlet oxygen reactions with the polymer by decreasing the extent to which polymer CH-bond-induced nonradiative deactivation channels play a role. Under our conditions, we found no evidence of water incorporation for the samples irradiated in air-saturated D2O. In any event, it is clear that, in an absolute sense, immersion of a TPP-containing polymer sample into air-saturated water results in a decrease in the rate of the photoinduced, singletoxygen-mediated oxygenation compared to data recorded from a sample exposed to the ambient atmosphere. Of course, in making this comparison, we are assuming that the intensities of irradiating light in the two experiments are similar (i.e., changes in refractive index due to the presence of the water do not appreciably influence the incident intensity at the sample). 3. Formation of Singlet Oxygen Outside the Polymer Matrix. In these experiments, poly(AN-co-DMB) films were immersed in solutions of D2O containing a water-soluble singlet oxygen sensitizer, 5,10,15,20-tetrakis(N-methyl-4-pyridyl)21H,23H-porphine, TMPyP. This sensitizer produces singlet oxygen with approximately the same quantum yield (Φ∆ ) 0.77 ( 0.04)42 as TPP (Φ∆ ) 0.68) which was used in the experiments where singlet oxygen was generated within the polymer matrix. Water was chosen as the solvent in which to immerse the polymer sample because poly(AN-co-DMB) is hydrophobic. Thus, the expectation is that, at least initially, water will not perturb the polymer morphology, and results obtained from these experiments can be readily compared to those obtained from samples containing the sensitizer (Vide supra). Indeed, IR experiments indicate the absence of sorbed water for films that had been immersed in the dark for 40 h. Of course, upon oxygenation of the polymer surface by externally generated singlet oxygen, hydrophilic functional groups will be produced and this could facilitate water sorption into the bulk matrix. The lifetime of singlet oxygen in D2O (68 µs)12 is significantly longer than that in H2O (3.5 µs).11 As such, it is to our benefit to first perform these experiments using films immersed in D2O rather than H2O simply because we will optimize conditions for singlet-oxygen-mediated reactions at the polymer surface. The experiment was performed such that singlet oxygen would be (42) Frederiksen, P. K.; McIlroy, S. P.; Nielsen, C. B.; Nikolajsen, L.; Skovsen, E.; Jørgensen, M.; Mikkelsen, K. V.; Ogilby, P. R. J. Am. Chem. Soc. 2005, 127, 255–269.
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Scheme 5. Selected Products Formed upon Homolytic Cleavage of the Allylic Hydroperoxide
generated right at the surface of the immersed polymer (Figure 5). 3.1. Transmission-Based IR Measurements. Upon prolonged irradiation into the Soret band of TMPyP at 421 nm, changes were observed in the transmission-based IR spectra of the poly(AN-co-DMB) film (Figure 6). Specifically, we were able to see irradiation-dependent increases in the intensities of the 3380 and 1720 cm-1 bands. However, these changes do not appear to derive from the production of singlet oxygen in the surrounding water bath. Rather, the data are more consistent with oxidation reactions initiated upon light absorption by the polymer itself. This is perhaps best exemplified by the data in
Figure 4. Change in the normalized integrated absorbance of selected IR bands plotted against the elapsed 421 nm photolysis time for poly(ANco-DMB) films. Data from TPP-containing samples exposed to air (9) and shown in Figure 3 are reproduced here. The abscissa scale was established using these data as a standard where the IR absorption signal recorded after 10 h of irradiation was set to be 100%. Changes recorded upon 421 nm irradiation of poly(AN-co-DMB) films immersed in airsaturated D2O are also shown. Films containing TPP (O) and without TPP (2) were examined. Percent oxygenation was quantified using the (a) 3380 cm-1 and (b) 1630 cm-1 IR bands. The error bars shown derive from multiple recordings of spectra at different positions on the given film.
Figure 6b where the appearance of the 1720 cm-1 band is most pronounced for films exposed to water lacking TMPyP (i.e., it appears that the TMPyP solution simply acts as a light filter to reduce the intensity of light at the polymer sample). Note that these 1720 cm-1 results are consistent with those presented in Figure 3b. 3.2. ATR-IR Measurements. Given the results obtained from the transmission-based IR measurements on our polymer films, we thought it prudent to look more closely at events occurring at the polymer surface exposed to the sensitizer-containing water. To this end, we employed attenuated total reflection infrared (ATR-IR) spectroscopy to monitor irradiation-induced changes in the samples. ATR-IR spectroscopy is a surface analysis technique, and data thus obtained yield information about functional groups present within the first ∼300 nm of the polymer surface43,44 (see the Supporting Information). Unfortunately, absorption bands associated with ambient water vapor and carbon dioxide appear in the ATR-IR spectra obtained. The CO2 band at ∼2350 cm-1 affects the intensity of the CN band. Thus, to compare the different ATR spectra with each other, we normalized the intensity of the pertinent bands using the 3000-2800 cm-1 band as an internal reference. Of course, we assume that the intensity of this band associated with aliphatic C-H stretching modes is not excessively affected by the oxygenation reactions. Due to interference from atmospheric water bands, we are also hesitant to ascribe meaning to observed changes in the intensity of the OH band at ∼3400 cm-1. Nevertheless, we were able to systematically record reproducible ATR-IR data in the 1800-1600 cm-1 range (Figure 7) and, through this, obtain useful information about singlet-oxygenmediated reactions. The ATR-IR spectra show a number of interesting features, particularly when considered in light of the corresponding transmission-based IR spectra. First, even in the absence of light, we observe the appearance of an IR band at ∼1740 cm-1 that can be assigned to a carbonyl. Thus, autoxidation reactions readily occur. These reactions may be initiated by defects (e.g., radicals) in the polymer that were generated during the polymerization and that, over time, are subsequently trapped by ground-state oxygen.
Figure 5. Diagram of the experiment to produce singlet oxygen at the surface of the immersed polymer film. The absorbance of the water solution containing the sensitizer was sufficiently low that light propagated through the 1 mm cuvette.
“Inside” Vs “Outside” Photooxygenation Reactions
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Figure 7. Normalized ATR-IR spectra for poly(AN-co-DMB) films. Spectrum A was recorded from a sample that had been immersed in a D2O/TMPyP solution and irradiated for 40 h at 421 nm. Spectrum B was recorded from a sample immersed in D2O that lacked TMPyP and that had been irradiated for 40 h at 421 nm. Spectrum C was recorded from a sample immersed in D2O but had been kept in the dark for 40 h, and spectrum D was recorded from a “fresh” sample that had just been prepared.
Figure 6. Change in the normalized integrated absorbance of selected transmission-based IR bands plotted against the elapsed 421 nm photolysis time for poly(AN-co-DMB) films immersed in D2O. The polymer film contains no sensitizer. Rather, when used, the sensitizer, TMPyP, was dissolved in the surrounding water. The abscissa scale was established using data from air-exposed samples as a standard (See Figures 3 and 4) where the IR absorption signal recorded after 10 h of irradiation was set to be 100%. Data were recorded from samples that had been immersed in D2O containing TMPyP (0) and in D2O without the sensitzer (2). Data recorded for a poly(AN-co-DMB) film kept in the dark in D2O are also shown (b). Percent change was quantified using the (a) 3380 cm-1 and (b) 1720 cm-1 IR bands. The error bars shown derive from multiple recordings of spectra at different positions on the given film.
Second, 421 nm irradiation of the film in the absence of the sensitizer results in an increase in the intensity of bands at ∼1630 and ∼1660 cm-1 which can likewise be assigned to carbonyl absorptions. The initial chromophores are most likely defect sites in the polymer that result from autoxidation reactions (Vide supra). Third, 421 nm irradiation of the system in which the surrounding water bath contains a singlet oxygen sensitizer results in a noticeable increase in the intensity of all bands over the 1800-1600 cm-1 spectral range. Thus, these data clearly indicate that externally generated singlet oxygen can indeed contribute to the oxygenation of polymer films immersed in aqueous solutions. Figure 8 shows the change in the normalized integrated absorbance over the range 1800-1600 cm-1 as a function of the elapsed exposure time recorded for the different experiments. Finally, the data obtained were independent of whether D2O or H2O was used as the surrounding medium. Thus, the extent of oxygenation does not respond to a change in the singlet oxygen lifetime in the ambient environment. Rather, features of the film and/or the film surface determine the extent of oxygenation. In light of our results discussed in section 2 (Vide supra), these data also indicate that solvent penetration into the film does not appear to play a significant role. Moreover, even though changes are occurring at the polymer surface that likely increase the
hydrophilicity of the polymer (i.e., formation of hydroperoxides, carbonyls), absorption spectra of the films indicate that the watersoluble sensitizer does not migrate into the sample. Therefore, the photosensitized production of singlet oxygen occurs outside the polymer film throughout the time period of the investigation (40 h). 3.3. Surface PassiVation Model. The combination of our transmission-based IR spectra that characterize the bulk polymer and the ATR-IR data that characterize the polymer surface provide an interesting and important perspective on the role of singletoxygen-mediated reactions in bulk polymer samples. Over the elapsed time period of these studies (40 h), it appears that, when generated at the polymer surface, singlet oxygen does not penetrate deeply into the polymer and that secondary events derived from an initial singlet-oxygen-mediated reaction likewise do not propagate into the bulk polymer. Rather, the action of singlet oxygen is limited to the surface of the polymer. It is first significant to note that this observation contrasts with that observed, for example, upon exposure of bulk poly(phenylene sulfide), PPS, samples to chlorinated water.4 In this latter case, although PPS is not soluble in water, the initial reaction products formed at the polymer surface render the material more hydrophilic which, in turn, allows the pertinent water-borne reactive species to progressively penetrate deeper into the polymer bulk. Our present data indicate that polymer oxidation initiated by the external photosensitized production of singlet oxygen behaves differently, at least in the early stages of the overall process. This unique behavior of the photosensitized singlet oxygen system, certainly in comparison to the chlorinated water system, can be ascribed to a special phenomenon. Singlet oxygen is a species with a finite lifetime. It can decay to the triplet ground state upon collision with other molecules (i.e., it does not react with 100% efficiency). Thus, for singlet oxygen produced outside the polymer film, there will be a finite penetration depth into the polymer defined by the competition between nonreactive and reactive deactivation channels. This latter point leads us to the most plausible, and arguably most interesting, explanation for the lack of a progressive “outsideinitiated” singlet-oxygen-mediated process of polymer oxygenation.
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arrested upon chemical reaction which, in turn, contributes to the progressive degradation of the polymer.
Conclusions
Figure 8. Changes in the normalized integrated ATR-IR absorbance over the 1800-1600 cm-1 range as a function of the elapsed exposure time, in hours, for poly(AN-co-DMB) films. The plot represents the ratio of the normalized integrated absorbance recorded from the exposed sample (A) to the normalized integrated absorbance recorded prior to the start of the experiment (A0). Data were recorded from poly(ANco-DMB) films irradiated at 421 nm that were immersed in either D2O containing TMPyP (0) or D2O that lacked TMPyP (2). Data are also shown for a poly(AN-co-DMB) film kept in the dark in D2O (b).
First, even with a nonoxidized fresh film, singlet oxygen will not penetrate very far into the polymer matrix. If we assume a singlet oxygen lifetime, τ, of 10 µs and an oxygen diffusion coefficient, D, of 1 × 10-6 cm2/s in the rubbery hydrocarbon film,45 the root-mean-square linear displacement of oxygen, d, in this 10 µs period will be ∼45 nm [i.e., d ) (2τD)1/2]. For reference, note that the oxygen diffusion coefficient in 25 °C glassy polystyrene is ∼3 × 10-7 cm2/s46 and is ∼2 × 10-5 cm2/s in water.47 Second, as we have outlined, products of the reaction between singlet oxygen and poly(AN-co-DMB) are hydroperoxides and alcohols. The OH functional group is effective at promoting the nonreactive deactivation of singlet oxygen back to ground-state oxygen through the mechanism of electronicto-vibrational energy transfer (Vide supra). Hydroperoxides may also promote singlet oxygen deactivation via an interaction wherein charge is donated from the hydroperoxide to oxygen (i.e., CT-mediated deactivation9). As such, we can envision a scenario where the singlet oxygen lifetime in the polymer film only becomes shorter. Finally, it is possible that peroxides produced as a consequence of initial reactions of singlet oxygen at the polymer surface could, through the formation of alkoxyl radicals (Scheme 3), result in some cross-linking of the polymer. In turn, such cross-linking could make water and sensitizer penetration into the polymer more difficult. (Note that the rate of hydroperoxide cleavage in the “outside” reaction could be substantially different from that in the “inside” reaction due, for example, to metals in the water that could act as a catalyst.) Thus, we suggest that the initial reaction of singlet oxygen with the poly(AN-co-DMB) surface leads to the passivation of the latter and the creation of a coating that protects against further reaction of the polymer with singlet oxygen. A similar passivation cannot occur when the reactive species is HOCl, for example. Here, penetration of the reagent into the polymer will only be (43) Popov, V. Y.; Lavrent’ev, V. V. J. Appl. Spectrosc. 1980, 32, 193–198. (44) Gedam, P. H.; Sampathkumaran, P. S. Prog. Org. Coat. 1983, 11, 313– 338. (45) Pauly, S. In Polymer Handbook; 4th ed.; Brandrup, J., Immergut, E. H., Grulke, E. A., Eds.; John Wiley and Sons: Hoboken, 1999; Vol. 2, pp 543569. (46) Poulsen, L.; Ogilby, P. R. J. Phys. Chem. A 2000, 104, 2573–2580. (47) Tsushima, M.; Tokuda, K.; Ohsaka, T. Anal. Chem. 1994, 66, 4551– 4556.
Oxygenation of poly(AN-co-DMB) films upon exposure to singlet oxygen has been investigated. Two ways of producing singlet oxygen were chosen: the photosensitized formation of singlet oxygen inside and outside the polymer matrix. Pronounced oxygenation of the polymer was observed when a homogeneous distribution of singlet oxygen was created inside the polymer matrix. Oxygenation most likely starts with the ene reaction between singlet oxygen and the polymer. After the initial formation of hydroperoxides, subsequent reactions lead to the formation of carbonyl groups in the polymer. In contrast, the extent of polymer oxygenation was relatively minor when singlet oxygen was generated in the solvent surrounding the polymer films. Moreover, the singlet-oxygenmediated changes were restricted to the surface of the polymer. The limited action of singlet oxygen under the latter conditions likely reflects processes that result in the physical deactivation of singlet oxygen at the polymer surface and, with a finite lifetime, a short penetration distance into the polymer. We suggest that the initial reaction of singlet oxygen with the polymer results in a self-protective coating that retards further singlet-oxygenmediated oxygenation. With the formation of such a self-protective coating on the bulk polymer, we must reconsider the role played by singlet oxygen in many processes of polymer degradation. Indeed, many studies of singlet-oxygen-mediated polymer degradation have been performed on polymers dissolved in a solution where oxygenation products will not have the same protective effect as those on the surface of bulk polymer samples. With data obtained from such solution-phase studies in hand, it is often implied that the results are equally valid for all materials made from that given polymer. Our present data indicate, rather, that the actual impact of singlet oxygen on the degradation of polymeric materials may, in some cases, be overestimated. We have shown that singlet oxygen will significantly affect the degradation of a suitably reactive bulk polymer sample if it is homogeneously generated within that bulk material. For the photoinitiated production of singlet oxygen, this can be achieved in a number of ways: (1) A singlet oxygen sensitizer is dissolved in the bulk material. This chromophore can be an additive or an inherent part of the macromolecule. (2) Singlet oxygen can be generated upon irradiation of the oxygen-polymer charge-transfer absorption band.48,49 (3) Compounds present in the surrounding solution can diffuse into the polymer bulk and then act as an “internal” singlet oxygen sensitizer. Although the generation of singlet oxygen outside a bulk polymer sample can, indeed, result in oxygenation of the polymer surface, the ramifications of such oxygenation with respect to the overall degradation of the polymer sample should be carefully considered.
Experimental Section Sample Preparation and Characterization. Poly(AN-co-DMB) was prepared using a free-radical polymerization. The details of this approach and the subsequent characterization of the polymer are provided in the Supporting Information. For experiments in which the singlet oxygen sensitizer was incorporated into the polymer, the sensitizer used was meso(48) Scurlock, R. D.; Ogilby, P. R. J. Phys. Chem. 1989, 93, 5493–5500. (49) Ogilby, P. R.; Kristiansen, M.; Clough, R. L. Macromolecules 1990, 23, 2698–2704.
“Inside” Vs “Outside” Photooxygenation Reactions tetraphenyl porphine, TPP (Porphyrin Products Inc., product no. T614). The polymer and the sensitizer were dissolved in an arbitrary amount of chloroform. The polymer/sensitizer ratio in this solution was adjusted such that, when cast, the dried film would have an absorbance of ∼0.5 at the 421 nm maximum of the TPP Soret band (see the Supporting Information for the spectrum). The films were prepared by casting the homogeneous chloroform solution on a NaCl substrate and were dried in the dark for 2 days under vacuum. The choice of NaCl was guided by the ease of peeling the film away from the substrate (using a drop of water if necessary), thus preventing the mechanical creation of stress in the film. The resulting films were sensitive to light and were kept in a light-tight desiccator until use. Polymer films prepared without a sensitizer were likewise cast from chloroform solutions onto a NaCl substrate. Photosensitized Singlet Oxygen Experiments. Steady-state irradiation was achieved using a 200 W Xe(Hg) lamp (Ushio UXM200H) as the source. The lamp output was passed successively through a water filter and Schott KG filters to remove a large portion of the infrared components. A monochromator (Oriel model 77250) was then used to select the appropriate excitation wavelength. (Under our conditions, both TMPyP and TPP have a Soret band λmax of 421 nm and, with our monochromator slit widths, we have an excitation fwhm of 20 nm.) The resultant beam was focused to a 5 mm × 5 mm spot on the surface of the polymer sample. In the “inside” experiments, where the polymer film contained TPP, some bleaching of TPP was observed upon irradiation of our samples at either 421 or 516 nm. Despite this bleaching, we established that irradiation-dependent changes observed in the IR spectra of our poly(AN-co-DMB) samples solely reflect the effect of singlet oxygen produced by TPP and do not result from a byproduct of the bleaching reactions. The pertinent data and discussion are provided in the Supporting Information. In the “outside” experiments, sensitizer-free polymer films were used. The films were immersed in D2O (99.9% D, Deutero GmbH) containing 5,10,15,20-tetrakis(N-methyl-4-pyridyl)-21H,23H-porphine, TMPyP (Aldrich, product no. 323497), as the singlet oxygen sensitizer. The concentration of the sensitizer was adjusted to yield an absorbance of ∼0.5 at 421 nm (i.e., TMPyP Soret band maximum) in a 1 mm path length cell. Over the course of the irradiation, the sensitizer solution was changed every 3 h to counteract the effects
Langmuir, Vol. 24, No. 16, 2008 9065 of sensitizer bleaching and thereby ensure that the polymer was exposed to a relatively constant concentration of singlet oxygen. IR Measurements. Transmission-based Fourier transform infrared (FTIR) spectra were recorded using a Bruker IFS-66v/S spectrometer operated in the continuous scan mode. Polymer films were mounted on the stage of a microscope attached to the spectrometer (Bruker IRscopeII), and spectra were recorded from small spatial domains in the films (∼70 µm in diameter). The size of the domain probed was determined by the iris aperture at the image plane of the microscope.50,51 Samples were moved using a calibrated x-y translation stage (MCL-2, Lang GmbH). Data were recorded with a spectral resolution of 4 cm-1, and 20 scans were used for signal averaging. Attenuated total reflection infrared (ATR-IR) spectra were recorded on a FTS-65A spectrometer from Bio-Rad using a single-reflection Ge crystal (GATR unit from Harrick Scientific). For the present experiments, the spectra recorded in the 1800-1600 cm-1 region reveal information about functional groups present within the first 270-300 nm of the polymer surface. We elaborate on this point in the Supporting Information.
Acknowledgment. This work was supported by the Danish National Research Foundation through a block grant for the Center for Oxygen Microscopy and Imaging. E.S.G. thanks the Danish Graduate School of Polymer Science and Grundfos Management for her Ph.D. stipend. We thank (1) Sokol Ndoni and Lotte Nielsen of Risø National Laboratories for the characterization of poly(ANco-DMB) and (2) Steen Uttrup Pedersen and Kim Daasbjerg of Aarhus University for the use of their ATR-IR spectrometer. Supporting Information Available: Polymer synthesis and characterization; instrumentation, experimental techniques, and control experiments. This material is available free of charge via the Internet at http://pubs.acs.org. LA801353N (50) Andersen, L. K.; Ogilby, P. R. Photochem. Photobiol. 2001, 73, 489–492. (51) Snyder, J. W.; Zebger, I.; Gao, Z.; Poulsen, L.; Frederiksen, P. K.; Skovsen, E.; McIlroy, S. P.; Klinger, M.; Andersen, L. K.; Ogilby, P. R. Acc. Chem. Res. 2004, 37, 894–901.
