Continuous Wave and Time-Resolved Electron Paramagnetic

Jun 24, 2016 - Fluoroacrylic polymers with inherent micro/nanoporosity are promising media for incorporation of fluorescent molecules and following ...
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Continuous Wave and Time-Resolved Electron Paramagnetic Resonance Study of Photoinduced Radicals in Fluoroacrylic Porous Polymer Films A. M. Sheveleva,†,§,∥ M. Yu. Ivanov,†,§,∥ I. K. Shundrina,‡,§ A. D. Bukhtoyarova,‡ E. G. Bagryanskaya,‡,§ and M. V. Fedin*,†,§ †

Laboratory of Magnetic Resonance, International Tomography Center SB RAS, Institutskaya Street 3a, Novosibirsk, 630090, Russia N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, Lavrentiev Avenue 9, Novosibirsk, 630090, Russia § Novosibirsk State University, Pirogova Street 2, Novosibirsk, 630090, Russia ‡

S Supporting Information *

ABSTRACT: Fluoroacrylic polymers with inherent micro/nanoporosity are promising media for incorporation of fluorescent molecules and following application as pressure-sensitive paints (PSPs), and UV photostability of PSPs is critically important for their long-term performance. Although photodegradation mechanisms of fluoroacrylic polymers have been studied previously in solutions, they have never been addressed in practically relevant for PSPs solid-state porous films. In this work we combined continuous wave (CW) and time-resolved (TR) electron paramagnetic resonance (EPR) to study UV photodegradation of thin porous films of a few representative fluoroacrylic polymers. Different types of spectra were detected using CW and TR EPR and assigned to the species formed on the inner surface of the pores and in the bulk of the polymer, respectively. The radical pairs formed in the bulk are short-lived, as is evidenced by TR EPR, and most likely recombine back to the initial polymer. On the contrary, the radicals formed on the surface of the pores are metastable in the absence of oxygen; they can be studied by CW EPR and clearly assigned to the radicals of type ·C(CH3)CH2− (so-called propagating radicals) formed via the cleavage of the C−C bond of the ester side chains and consecutive β-scission. Remarkably, their CW EPR spectra closely resemble solution-state spectra, indicating that these radicals are localized in the pores where the mobility of methyl and methylene protons is not suppressed. Thus, based on complementary results of CW and TR EPR, we conclude that UV photodegradation of porous fluoroacrylic polymer films mainly occurs on the inner surface of the pores, which needs to be considered in future development of this type PSPs. the fluorescence quenching by atmospheric oxygen. Another aspect of practical importance is the adhesive ability of such polymers, and this property of the most widely used copolymer FIB was recently improved by development of advanced copolymers NS4 and NS5 (Scheme 1).16,17 Recently, we have demonstrated that inherent porosity of fluoroacrylic polymers FIB, NS4, and NS5 allows incorporation of small molecules, namely, nitroxide radicals, into the pores and investigation of pore sizes using continuous wave electron paramagnetic resonance (CW EPR) of slow-motion nitroxides.17 Depending on the size of the pore, nitroxide TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl) exhibited different rotational correlation time, which thus can be used as a simple estimate of the pore size. Moreover, we demonstrated that nitroxides adsorbed into the pores of polymeric films allow

1. INTRODUCTION Porous media of organic, inorganic, and metal−organic origin draw enormous attention due to the multitudes of possible applications as catalysts, molecular containers, and other functional materials of various kinds.1−5 Among these, polymers with inherent porosity are of practical interest, being easily produced and applicable for many particular tasks. One of such examples is the development of “smart” polymerbased materials, whose properties are contributed by host polymeric matrixes and guest molecules deliberately embedded into the pores. For instance, the so-called pressure- (or temperature)-sensitive paints (PSP/TSP)6−12 composed of oxygen-sensitive fluorescent molecules and an oxygen-permeable binder (usually a porous polymer) are effectively used in investigation of flow fields on the wings of aircraft during the flight.13−15 Along with the porosity itself, the key property of porous polymers for PSP/TSP applications is the UV stability of polymer matrix, since UV light is routinely used for monitoring © 2016 American Chemical Society

Received: May 18, 2016 Revised: June 23, 2016 Published: June 24, 2016 14767

DOI: 10.1021/acs.jpcc.6b05016 J. Phys. Chem. C 2016, 120, 14767−14773

Article

The Journal of Physical Chemistry C

described in patent in ref 30. The ratio of monomers was 50:50 (1,1,1,3,3,3-hexafluoroisopropyl methacrylate/2,2,3,3,4,4,4-heptafluorobutyl methacrylate). Syntheses of NS4 and NS5 have been described previously in ref 16. The corresponding ratios of monomers were 44:48:8 (1,1,1,3,3,3-hexafluoroisopropyl methacrylate/2,2,3,3,4,4,4-heptafluorobutyl methacrylate/1-(4-(4chloro-2,3,5,6-tetrafluorophenyl)piperazin-1-yl)prop-2-en-1one) and 42:53:5 (1,1,1,3,3,3-hexafluoroisopropyl methacrylate/2,2,3,3,4,4,4-heptafluorobutyl methacrylate/1-(4-(4-tertbutylphenylsulfonyl)piperazin-1-yl)prop-2-en-1-one), respectively. Polymers were dissolved in a α,α,α-trifluorotoluene or acetone (15% solution g/g). The solutions were filtered through a 0.5 mm Teflon membrane filter and placed into quartz EPR sample tubes (o.d., 4 mm; i.d., 3 mm). Then solvent was evaporated using first membrane and then turbomolecular pump (8−12 and 24 h, 1.5 and 1 × 10−6 Torr pressure, respectively). As a result, thin (∼300 μm) polymer film was formed on the inner surface of the sample tube, which was consequently sealed off. The use of evacuated (oxygen-free) samples was advantageous for EPR and TR EPR studies, enhancing the lifetimes of photogenerated radicals and increasing their relaxation times. EPR Measurements. CW X-band (9 GHz) EPR spectra were measured using the commercial spectrometer Bruker Elexsys E580 equipped with Oxford Instruments temperature control system (ER 4112HV with helium cryostat ER 4118CFO). Theoretical modeling of EPR spectra was performed using EasySpin toolbox (version 4.5.1) for Matlab.31,32 TR EPR measurements were done using a homemade TR EPR setup based on an X-band Bruker EMX spectrometer (9 GHz). A Nd:YaG laser LOTIS-TII with the excitation wavelength of 266 nm (fourth harmonic) was used. Photoirradiation. For CW EPR, UV irradiation was carried out using a high-pressure mercury lamp (500 W) positioned at a distance of ∼1 m from the polymer placed in the EPR sample tube. The intensity of UV irradiation was about 5 mW/cm2. IR light was cut off using distilled water filter in front of the sample in order to maintain polymer at room temperature during irradiation. No other filters have been used. After UV exposure, samples were additionally investigated using sol−gel analysis to determine the ratio of soluble/insoluble fractions, and characterized using NMR. 19F NMR (300 MHz) spectra were recorded using Bruker AV-300 spectrometer (282.37 MHz) and C6F6 (δ = −163 ppm relative to CCl3F) as internal standard.

Scheme 1. Structure of Porous Fluoroacrylic Polymers FIB, NS4, and NS5 Studied in This Work

monitoring the UV degradation processes. It was observed that upon UV irradiation signals of adsorbed nitroxides gradually vanished due to the reaction with photoinduced polymer radicals. Because of that, more detailed studies of radical intermediates formed upon photodegradation of fluoroacrylic polymer films became an interesting and topical task. The photodegradation mechanisms of polymers have been intensively studied over the last decades.18 However, most often mechanisms of photochemical reactions are studied in liquids, e.g., using laser flash photolysis (LFP) or time-resolved (TR) EPR, because it is more convenient from experimental point of view. For instance, TR EPR is known to be a valuable tool for identification of paramagnetic intermediates formed during photochemical reactions in liquids,19−25 and similar application to the photolysis of polymers in solutions proved to be fruitful as well.19,26−29 In particular, it was shown that photolysis of acrylic polymers in solutions results in formation of the main-chain and oxo-acyl radicals, with subsequent transformation of the former one to the propagating radical via the β-scission. Although in general one would expect similar initial steps of the photolysis in solid and liquid states, clearly the molecular dynamics in the two cases must be drastically different. In this work we apply CW and TR EPR for investigation of the peculiarities of UV degradation in solid-state porous fluoroacrylic polymer films, i.e., in conditions close to practical ones. As expected, significant differences in kinetic behavior of photoinduced radicals compared to the solution-state photolysis have been found. Moreover, a combined CW/TR EPR approach allowed conclusions on radical intermediates formed in the pores versus in the bulk of the polymer, as we describe in detail in the following sections.

3. RESULTS AND DISCUSSION If photolysis of polymers proceeds with a formation of transient radical intermediates, the most suitable methods to investigate these mechanisms are LFP and TR EPR. However, in both cases such experiments are usually carried out in solutions using the flow system to remove the reaction products and refill the photochemical cell with fresh reactants. In addition, such experimental settings allow optimum irradiation conditions to be adjusted. Even more important, in TR EPR measurements one can use elevated temperatures to speed up the rotational motion of radical intermediates and in this way to obtain better resolved and easier assignable EPR spectra. Unfortunately, all these advantages are not effective for solid-state photolysis in polymer films; at the same time, the peculiarities of photodegradation in the solid state are not accessible in liquid-state experiments.

2. EXPERIMENTAL SECTION Synthesis and Preparation of Polymer Films. FIB is a fluorinated copolymer synthesized according to procedure 14768

DOI: 10.1021/acs.jpcc.6b05016 J. Phys. Chem. C 2016, 120, 14767−14773

Article

The Journal of Physical Chemistry C

shape is that the spectra belong to radicals localized in the pores of the polymers, where the mobility is less restricted compared to the bulk polymer. Note that our previous study addressed stable nitroxide radical (TEMPO) embedded in the same fluoroacrylic polymer films,17 and high mobility of TEMPO inside the pores was clearly evidenced; it is also established for nitroxides in other porous media.33−35 Spectral lines of FIB and NS5 are well-resolved, and the superposition of narrow-line quintet and broader quartet is clearly seen. Such spectrum is one-to-one similar to the previously obtained spectra of the so-called “propagating” radicals (Scheme 2) ·C(CH3)CH2− generated during the

One would expect that in general the mechanism of photolysis of fluoroacrylic polymers should be similar in liquid and solid state, but with drastically different kinetic behaviors of the photogenerated radical intermediates. This occurs because the mobility of photogenerated radicals is strongly suppressed in the solid state, and thus, the recombination processes might occur on the different time scale. Depending on spatial arrangement of two radicals formed in the solid state and on the efficiency of singlet−triplet conversion, the lifetime of radical pairs (RPs) formed in the solid state might be either much longer or much shorter compared to that in solutions. In crystals or rigid glasses photoinduced radicals can even be metastable; however, this is not anticipated for such relatively soft media as polymers. Another interesting difference in morphology compared to solutions is heterogeneity of fluoroacrylic polymer films, i.e., their inherent porosity. In general, one might expect different kinetics of radical intermediates formed inside or outside the porous regions. First we verified whether or not long-lived radicals are formed during steady-state photolysis of polymeric films. We prepared a series of samples sealed off in evacuated quartz EPR tubes, and then irradiated them for 4−8 h using a mercury UV lamp to mimic some aging via UV degradation. Surprisingly, CW EPR spectra were readily detected at room temperature; however, the signals were quite weak and the lifetime of radicals was found to be a few hours only (after switching the UV light off), making long accumulations problematic. Therefore, in order to prolong radicals lifetime we have studied freshly irradiated samples at 80 K. Figure 1 shows CW EPR spectra of all three studied polymers after UV irradiation.

Scheme 2. Principal Scheme of the Photolysis of Fluoroacrylic Polymers

solution-state polymerization of methacrylates (see Figures 4 and 5 of ref 36). Initially, photoexcitation leads to the cleavage of C−C bond of the ester side chains and formation of primary radical pair consisting of main-chain radical of type ·C(CH3)(CH2)2 and oxo-acyl radical (Scheme 2). However, it is known that next the main-chain radical rapidly rearranges to the propagating radical via the β-scission (Scheme 2). The observed CW EPR spectra of UV-irradiated FIB and NS5 can be simulated using reasonable values of isotropic HFI constants for the propagating radical [a(CH3) = 2.20 mT, a(Hβ) = 0.89 mT, a(Hβ′) = 1.47 mT], which agree well with those for similar radicals.36 Thus, the assignment of the observed CW EPR spectra (Figure 1) to the propagating radicals is undoubted. There are no any signatures of other radicals, namely, partner oxo-acyl radicals, which are expected according to the Scheme 2. We speculate that acyl radicals are known to be more reactive compared to alkyl radicals; therefore, they decay faster than the propagating radicals. More importantly, after the cleavage of C−C bond on the inner surface of the pores, the oxo-acyl radicals are allowed to diffuse into the free volume of polymer (sketched in Figure 2). This increases the probability of decay of oxo-acyl radicals in side

Figure 1. CW EPR spectra of UV-irradiated films of FIB, NS4, and NS5 (indicated on the plot) at 80 K. Red curve on top (sim) represents simulation of propagating radical formed using the parameters given in the text. The bottom trace (monomer NS4) shows the CW EPR spectrum of UV-irradiated monomer of NS4 containing Cl/F-substituted phenyl group [1-(4-(4-chloro-2,3,5,6tetrafluorophenyl)piperazin-1-yl)prop-2-en-1-one)].

First of all, CW EPR spectra of UV-irradiated FIB and NS5 shown in Figure 1 are isotropic and can be simulated using isotropic hyperfine interaction (HFI) constants (vide infra), which is not typical of the radicals immobilized in the solid matrixes; instead, such spectra are more typical of free radicals in liquids. The reasonable explanation of the observed spectral 14769

DOI: 10.1021/acs.jpcc.6b05016 J. Phys. Chem. C 2016, 120, 14767−14773

Article

The Journal of Physical Chemistry C

Figure 2. Schematic structure of porous polymers with radicals created by photoirradiation in the bulk of the polymer and in the inner volume of the pores.

reactions and, at the same time, strongly decreases the probability of their reverse recombination with geminate main-chain radicals, which transform to the propagating radicals in the meantime. CW EPR spectrum of UV-irradiated NS4 noticeably differs from those of FIB and NS5. One observes that the spectrum is dominated by a broad isotropic line; however, small peaks similar to FIB and NS5 and assigned to the propagating radicals are also visible on top of a broad spectrum. Therefore, it is reasonable to assume that the same propagating radicals are formed upon UV irradiation of FIB, NS4, and NS5, but in case of NS4 an additional mechanism of radical formation coexists that should be assigned to the photochemistry of the specific monomer present only in NS4 [1-(4-(4-chloro-2,3,5,6tetrafluorophenyl)piperazin-1-yl)prop-2-en-1-one)]. The latter was convincingly verified by UV irradiation of this pure monomer. The resulting EPR spectrum (Figure 1, bottom trace) closely corresponds to the broad component of the photoinduced spectrum of NS4. Most probably, UV light leads to the abstraction of Cl or F atoms from the phenyl group and formation of corresponding phenyl-type radicals (see details in Supporting Information). Interestingly, using CW EPR we could not detect either longlived main-chain/propagating or oxo-acyl radicals immobilized in the bulk of the polymer (outside the pores). The main-chain or propagating radicals trapped in the bulk of the polymer, if present, would be essentially immobile and lead to the solidstate type of spectrum with broadened lines due to anisotropy of proton HFIs. However, no indication of such radicals is observed, nor are any signals assignable to oxo-acyl radicals found. It is plausible that such radicals generated in the bulk of the polymer have much shorter lifetime compared to those formed in the pores, and thus cannot be detected by CW EPR. Therefore, in order to investigate formation and decay of shortlived radicals or radical pairs on the nanosecond time scale we applied TR EPR. Figure 3 shows TR EPR spectra measured during the photolysis of FIB, NS4, and NS5 at 298 and 90 K (again, all samples were preliminary evacuated and sealed off in the EPR sample tubes). The signals were very weak and required several hours of accumulation. We used relatively small laser intensities (6−15 mJ per pulse, 10 Hz) to avoid damaging of the sample. Noticeably, no stable radicals were detected by CW EPR for the same samples after TR EPR experiment was completed,

Figure 3. (a−c) TR EPR spectra measured during the photolysis of NS4, NS5, and FIB at 295 and 90 K (as indicated in the legends). (d) 1, experimental TR EPR spectrum of NS5 at 90 K; 2, simulated zeroharmonic CW EPR spectrum of oxo-acyl radical using a(CH2) = 0.32 mT and a(CF2) = 0.08 mT; 3, integrated CW EPR spectrum of propagating radical formed upon photolysis of NS5 (obtained from trace shown in Figure 1); 4, simulated zero-harmonic CW EPR spectrum of main-chain radical with halved HFI constants (SCRP model) a(CH3) = 1.15 mT, a(Hβ) = 0.84 mT, a(Hβ′) = 0.56 mT. Typical HFI constants of oxo-acyl and main-chain radicals were taken from ref 19. Spectra 2−4 are inverted for comparison with experimental TR EPR spectrum.

meaning that overall degradation of the polymer remained negligible (see also Supporting Information). Contrary to CW EPR, the spectra of TR EPR are represented by a single emissive line of ca. 2 mT width (fwhh) with g ∼ 2.004. No indication of the main-chain or propagating radicals with several HFI constants (as those shown in Figure 1) is found. The observed TR EPR signal is noticeably narrower than the integrated (zero-harmonic) experimental spectrum of propagating radical (Figure 3d). At the same time, it is much broader compared to expected spectrum of oxo-acyl radicals, which are known to have small HFI constants on CH2 protons, ∼0.32 mT, and even smaller HFI constants on CF2 groups, ∼0.08 mT (Figure 3d).19 Thus, one cannot assign the observed TR EPR signal to either main-chain/propagating or oxo-acyl radicals. Because of that, it is reasonable to assume that the observed signal refers to the spin-correlated radical pair (SCRP),37,38 not to the individual radicals formed. As a rule, SCRP mechanism is effective for radicals that cannot separate from each other, e.g., in the solid state or in confined media, and manifests itself in absorption/emission (A/E) or emission/absorption (E/A) patterns due to the interplay of exchange, dipolar, and hyperfine interactions. Such A/E and E/A patterns are clearly not observed in our TR EPR experiment. Note, however, that A/E or E/A SCRP patterns refer to the rather long inter-radical distances and, as a result, rather small mean exchange values (e.g., comparable to hyperfine interactions). Contrary, if separation of radicals is restricted and they reside close to each other, e.g., diffuse or tumble at radical−radical distances