J. Phys. Chem. B 2000, 104, 475-484
475
Radiolysis of Polyethylene Films Containing Arenes: Bromopyrene Dissociation and Pyrene Binding in Polymer Films Michael Biscoglio and J. Kerry Thomas* Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556 ReceiVed: July 29, 1999; In Final Form: NoVember 3, 1999
Radiation induced dehalogenation of haloarenes and binding of arenes have been studied in polyolefin films. Dehalogenation takes place from a highly excited triplet state that is formed by the recombination of arene radical ions, while binding takes place through a polymer aryl radical intermediate. The latter process is dependent on the reactivity of the polymer radical and decreases in low molecular weight and branched polymer films. It is found that energy deposited by high-energy irradiation is transferred efficiently out of the crystalline zones and into the amorphous zones of the polymer.
Introduction
Experimental Section
Photochemical dehalogenation of arenes has received considerable attention because it is important to the degradation of persistent environmental pollutants such as PCBs and DDT.1-4 However, less information exists on other reactions of arenes, such as the binding of arenes to hydrocarbons, which require either biphotonic or far UV irradiation.5-8 Since the latter process may also excite the hydrocarbon solvent, it is of interest to study the reactions of arenes excited by energy deposited directly into the hydrocarbon media. This is accomplished by high-energy radiolysis which gives new insights into the photochemical reactions that occur. Mechanistic pathways involving both the upper excited singlet and triplet states have been shown to cause dehalogenation of aryl halides.1,9 Triplet pathways are more efficient and quantum yields as high as 0.54 have been reported for aryl chlorides, which are of the magnitude of their intersystem crossing.1 It is our first objective to show that, following radiolysis, highly excited singlet and/or triplet states formed by the recombination of aryl radical ions lead to dehalogenation. Covalent attachment of photophysical probes to polymer chains is of great utility in studying polymer environments.10 Unfortunately, attachment processes usually require copolymerization of the probe during synthesis of the polymer. In the case of hydrocarbons, pyrene has been covalently bound to alkanes by photolysis.7,10 This is an inefficient process and dependent on the penetration of irradiation throughout the film. Therefore, a nonhomogeneous distribution of binding occurs when using highly scattering media such as partially crystalline polyethylene. By using high-energy irradiation, probes could in effect be bound either near the surface with low energy beams or throughout the medium by penetration of irradiation into the material. It is our second objective to show that radiolysis of polyethylene films containing arenes provides an efficient means of binding arenes to hydrocarbon chains. This is studied through the production of a polymer pyrenyl radical adduct, formed by the reaction of the radiation induced polymer radical with pyrene.
Materials. Polyethylene Rx (Rigidex 80 linear mw ) 80 K), Pn (BPE PN 220 branched mw ) 100 K),11 B (linear mw ) 300 K), and Low (linear mw ) 1263 K)12 were purified by soxolation with cyclohexane and subsequently analyzed for impurities by fluorescence and UV-vis spectroscopy. PS (polystyrene mw ) 280 K), PMMA (poly(methyl methacrylate) mw ) 28 K), I-PP (isotatic polypropylene, mw ) 280 K), PVB (poly(vinyl butyral) mw ) 12 K), and PVA (poly(vinyl alcohol) 100% hydrolyzed mw ) 14 K) were obtained from Aldrich and used as received. Pyrene (Aldrich, 99%) was purified by three passages down an activated silica gel column using cyclohexane as the eluant. Bromopyrene, chloropyrene, and perylene were recrystallized three times from methanol before final chromatographic separation. Cyclohexane HPLC grade was used as received from Aldrich. Absolute methanol, spectrophotometry grade, from J. T. Baker and HCl obtained from Fischer were used as received. Sulfur hexafloride and prepurified oxygen were used as received from the Mittler Co. Specific amounts of gases were added to the polymers in an evacuated chamber from a reservoir on a vacuum system. The polymer samples were allowed to equilibrate at each oxygen pressure for 15 min before each measurement was taken. Sample Preparation. Preparation of polymer films has been described previously.13 Probe molecules were incorporated into polymer films by two methods, i.e., by solution casting and by dissolving the probe in the polymer melt. Poly(methyl methacrylate) films were prepared by dissolving both the polymer and probe molecules in toluene followed by casting the system onto a glass slide. The usual method for solvent removal was to allow the system to evaporate for 2 days, followed by heating to 80 °C under vacuum (0.1 Pa) for an additional day. For polyolefin samples, known amounts of probe molecules were introduced directly into the melted samples and were then molded into films using a hot press. Instrumentation. Differential Scanning Calorimetry. A Perkin-Elmer DSC-4 calorimeter with a system 4 microcomputer controller and a intracooler 2 chiller was used to collect calorimetric data, and the data were scanned into computer data files with Adobe Photoshop. The calorimeter was calibrated
* To whom correspondence should be addressed.
10.1021/jp9926847 CCC: $19.00 © 2000 American Chemical Society Published on Web 12/28/1999
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using docosane as a standard with an enthalpy of fusion (∆Hf ) 58 cal/g, 242 J/g).14 Crystallinity is defined as Xc ) ∆Hm/∆Hc, where ∆Hm and ∆Hc are the enthalpies of fusion of the polymer melt and purely crystalline samples, respectively.15 GCMS. A JEOL GCMate in the EI mode with an HP-5 30 m × 0.32 mm i.d. column having a 0.25 µm film thickness was used to separate pyrene from bromopyrene in cyclohexane. The injector temperature was set to 280 °C and the oven was ramped from 150 °C to 325 °C at 10 °C per minute. Mass spectra were scanned over the range of 50 to 500 Daltons at a rate of 0.6 seconds per scan. Steady-State Absorption Measurements. Steady-state absorption spectra were obtained on a Varian Cary 3 UV-vis spectrophotometer controlled by an IBM-compatible computer using a spectral band-pass of 0.2 nm. Fluorescence Spectroscopy - Steady State. An SLM/Aminco SPF-500C spectrofluorimeter was used to measure fluorescence spectra. Typically, the band-pass for emission spectra was 7.5 nm for the excitation band and 1 nm for the emission band, and vice versus for excitation spectra. The data were collected by an instrument microprocessor and then transferred to an IBMcompatible computer. Fluorescence Spectroscopy - Time ResolVed. Fluorescence studies were carried out using either a Nitromite LN-100 with a 0.2 ns fwhm and 70 µJ pulse or a LN-1000 laser having a 1.2 ns fwhm and 1.4 mJ pulse. The latter was used in conjunction with an LSI mirror DLM dye laser and 2 mM R-NPO laser dye, obtained from Exciton, in toluene to produce pulses at 400 nm. As in steady-state measurements, the emission of the fluorophore was collected at 90° to the excitation source. A Bausch and Lomb monochromator was used to select the monitoring wavelength, and the emission at this wavelength was detected by a Hamamatsu R1664U microchannel plate photomultiplier. The signal was amplified using a 7A29 amplifier in a Tektronix 7912AD programmable digitizer with a 7B10 time base. The digitized signal was averaged from several traces and stored on an IBM-compatible computer. Time-ResolVed Phosphorescence and Transient Absorption Spectroscopy. A Laser-Photonics model UV-24 with a 10 ns fwhm and 10 mJ pulse and a Hamamatsu IP28 photomultiplier was used for excitation and detection, respectively. For phosphorescence studies, a nine-stage IP28 photomultiplier was used to detect the long-lived emission. This system has been previously described.13 Decay times that were longer than 5 ms were collected on a Tektronix TDS210 and then transferred to a computer using the Wavestar program. Faster decays were obtained on the previously described Tektronix 7912AD using a 7A13 differential comparator. Pulse Radiolysis. Pulses (2 ns) of 0.4 MeV electrons produced by a Febetron 706 were used for pulsed radiolysis experiments. The pulse energy was calculated to be 1.04 kGy/pulse. Transient absorption spectra were performed either as described above for fast time scales or by using a S2000 UV-vis CCD linear array from ocean optics in conjunction with a flash lamp delivering a 10 J, 2 µs pulse for species that live much longer than the flash lamp pulse. γ-Radiolysis. γ-Radiolysis of polymer samples was carried out in sealed tubes by exposure to a Sheppard 109 60Co source producing (3.7 kGy/hr). Data Analysis. Three types of models were used to simulate the decay kinetics presented in this study. For homogeneous environments, such as solution, the single exponential function
It ) I0 exp-kt
(1)
is used to describe the decay of species. Heterogeneous systems may be described by two or more independently decaying species from multiple environments and require a biexponential of the form
It ) I0[A exp-k1t + (1 - A) exp-k2t]
(2)
or even larger multiple exponential functions. In the latter case, the Gaussian distribution model has been developed to describe such systems, where many different conditions give rise to a distribution (γ) of decay rates (kgaus) related to an average site decay (kav).16 This distribution can be written as
kav ) kgaus exp(γx)
(3)
Integration over the distribution exp(-x2) gives
It
∫ exp(-x2) exp[-kavt exp(γx)]dx ) I0 ∫ exp(-x2)dx
(4)
which is fitted to the experimentally determined decay profiles. Though a number of studies report mathematical treatments of radiolytic reactions,17,18 we acknowledge an inability to determine the concentration and size of spurs produced by radiolysis. Therefore, we have found it convenient to compare decays of radical cations and anions by fitting them to the Gaussian equation or to refer to first half-lives as these methods lead to the fewest stipulations. Results and Discussion For the purpose of presentation, methods for the identification of bromopyrene dissociation and pyrene binding are first outlined. Subsequently, studies using a combination of timeresolved and quenching methods indicate mechanisms for these reactions. Bromopyrene dissociation takes place from a highly excited state created by the recombination of its radical ions, while pyrene binding is due to the interaction of free pyrene with a polymer radical. For the latter process, the long-lived polymer substituted pyrenyl radical is observed, which acts as the intermediate in this reaction. Once these mechanisms are established, comparisons between polymer matrices hosting the reactions will be drawn to show the efficiency of charge generation and energy migration within the media and their influence on rates and yields of these reactions. Mechanism of Bromopyrene Dissociation. Figures 1a and 1b show the UV absorption spectra and fluorescence emission decays of pyrene and bromopyrene in polyethylene Rx, respectively. The red shifted absorption spectra of bromopyrene (Figure 1a) and its 2 orders of magnitude faster decay rate relative to pyrene (Figure 1b), make the two probes readily distinguishable by these methods. After radiolysis of a polymer film, initially containing only bromopyrene, the absorption spectra and fluorescence decays are used to show that bromopyrene has been reduced to pyrene. Figure 2 shows the absorption spectra of a bromopyrene sample before and after irradiation. Quantitative yields of pyrene are determined by spectral subtraction of the UV absorption spectra. The rate of bromopyrene dehalogenation is monitored by the development of pyrene fluorescence, after radiolysis of the film. Figure 3 shows the fluorescence produced by a nitrogen laser in a 1 mol/m3 bromopyrene Rx film, before pulse radiolysis, and the growth of pyrene fluorescence 14.5 µs, 500 µs, and 3 s later in Rx. Integration of the pyrene fluorescence curves indicates that 28%, 69%, and 100% of the total pyrene product has formed by 14.5 µs, 500 µs, and 3 s after the pulse,
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J. Phys. Chem. B, Vol. 104, No. 3, 2000 477
Figure 3. Time-resolved fluorescence decay at 395 nm of bromopyrene in Rx before irradiation, 14.5 µs, 500 µs, and 3 s after a 1.04 kGy electron pulse, excitation at 337 nm.
mination. The overall energetics of this process may be expressed as
E ) -I + P+ + EAp-
Figure 1. (a) Steady-state absorption spectra of pyrene and bromopyrene in polyethylene Rx. The sloping baseline is due to the partially crystalline polymer scattering the analyzing light. (b) Time-resolved fluorescence decay at 395 nm of pyrene and bromopyrene in polyethylene Rx following excitation at 337 nm. Scattering from laser pulse is monitored at 337 nm.
Figure 2. Steady-state absorption spectra of bromopyrene in Rx before irradiation, after 10 kGy of irradiation of a γ source, and the absorption due to pyrene formed by spectral subtraction.
respectively. Since all excited states have decayed within a few nanoseconds after the pulse and the triplet state itself (2.6 eV, (0.42 aJ)) is not sufficiently energetic to cause dehalogenation,19 only the radical ions and polymer radicals are available for this process. Analysis of the bromopyrene anion transient decay under the same conditions indicates that its OD has dropped by 24%, 63%, and 100% respectively at 14.5 µs, 500 µs, and 3 s after the pulse. We shall see below that interaction of pyrene with the polymer radicals takes a significantly longer period of time. The above studies suggest that the recombination of the radical anion and cation of bromopyrene gives rise to debro-
where I is the gas phase ionization potential of bromopyrene, P+ is the polarization energy of the radical cation of pyrene, and EAp- is the electron affinity of pyrene. The exact values for the above parameters are not known, but reasonable estimates may be made. The values presented in the literature are I ) 7.41 eV (1.19 aJ),20 P+ ∼ 1.9 eV (0.30 aJ),21 and EAp- ∼ 0.5 eV (0.08 aJ).22,23 This gives E ∼ -5.0 eV (0.81aJ), which is larger than the C-Br and C-Cl bond energies in typical arenes, e.g., benzene, where the C-Br and C-Cl bond energies are 3.1 eV (0.50 aJ) and 3.7 eV (0.59 aJ) respectively, and is similar to the C-H bond energy in benzene E ∼ 4.8 eV (0.77 aJ). This energy corresponds to photolysis with far UV excitation at λ < 250 nm. The above C-Br and C-Cl bond energies indicate that some dehalogenation is expected at wavelengths less than 400 nm for bromopyrene and 335 nm for chloropyrene. However, the C-X bond breakage competes with internal conversion to the first excited state,9 and the rate of C-X bond breakage increases with the energy of the dissociating state.24 Excitation at shorter wavelengths increases the latter process and gives rise to larger yields of debromination. These data agree with the concept that higher energies give rise to greater debromination, and that the rate of debromination should be temperature dependent. This agrees with the fact that deiodination of triplet excited iodonaphthalene is very temperature dependent in liquid toluene.24 Studies9 also show that the major C-X bond breakage occurs from the triplet excited state which is the primary state formed from ion recombination. It is of interest here to point out that no photodissociation of bromo- or chloropyrene is observed by excitation with wavelengths longer than 300 nm. Excitation directly into the S3 state by 254 nm in a Rayonet photoreactor produced a low yield of pyrene from bromopyrene, but no photoproducts of pyrene or chloropyrene, even after long periods of exposure.1 The stronger C-H and C-Cl bonds are broken by intense laser pulses at 266 nm, through biphotonic chemistry or directly by one photon of 185 nm. This is evident by inefficient binding of pyrene to a hydrocarbon solvent7 and by dechlorination of chloropyrene in these solvents. Electron and γ-radiolysis of polyethylene containing arenes produced only binding of chloropyrene and pyrene, but no observable dechlorination of chloropyrene. Table 1 gives the radiolytic Gpy yields obtained from dissociation of bromopyrene in various polymer films. Poly-
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TABLE 1: G Value Yields of Debromination of Bromopyrene by Radiolysis
polymer
Gdissociation at 1 mM (mol/m3) and 10 kGy
Rx (linear) at 100 K w/ 10 kPa O2 B (linear) Pn (branched) polypropylene cyclohexane polystyrene a
.45 .11 .28 .43 .33 .37 .13 0a
mw 80 k 300 k 100 k 250 k 84 280 k
No pyrene was detected by fluorescence emission.
TABLE 2: Oxygen Quenching (Kq) Rates (s-1) of the Bromopyrene Radical Ion and Triplet after Laser Photolysis and Pulse Radiolysis polymer
Kq(anion)rad
Kq (3*)rad
Kq (3*)laser
polystyrene polyethylene (Rx) (Pn)
3.88E + 08 1.56E + 08 3.79E + 08
1.03E + 08 7.50E + 07 1.27E + 08
1.16E + 08 1.74E + 08 2.48E + 08
olefin samples produced G values of pyrene that were within a narrow range (Gpy(1 mol/m3, 10 kGy) ) 0.33-0.45), and this is consistent with the observation that similar yields of pyrene radical ions are also produced in these films. No dissociation is seen in polystyrene even though there are significant yields of the pyrene excited states and radical ions. This is due to rapid energy transfer from the bromopyrene higher excited state formed by the ion neutralization to the phenyl groups in the polymer matrix. This is followed by internal conversion to the first excited state of polystyrene, and subsequent energy transfer (of lower energy) to bromopyrene. Bromopyrene dissociation is also measured through GCMS by determining the amount of pyrene and bromopyrene that is washed out of the film following radiolysis. It shows that the dissociation is reduced by 24% in a 30 mol/m3 bromopyrene sample by the addition of 0.3 mol/ m3 at 1 kPa oxygen in Rx, which interferes with the ion recombination reaction. The O2 reacts with BrPy-• giving BrPy + O2- as seen by the more rapid decay of BrPy-• in the presence of O2. In the presence of O2, the cation also decays at a faster rate, due to BrPy+• + O2-, where the mobility of O2is greater than that of BrPy-• in the system. The reduction in bromopyrene dissociation is in good agreement with the 22% loss of BrPy-• due to the production of O2- and the subsequent loss of excited BrPy-• via the neutralization reaction. Table 2 lists the oxygen quenching rates of the pyrene radical anion and triplet obtained by radiolysis and laser photolysis. Triplet quenching rates determined in radiolysis are slower than those previously reported by photolysis.13 This is due to consumption of the O2 by the large radiation pulse used in the radiolysis experiments. Decreasing the temperature from room temperature to 100 K reduced the Gpy yields from 0.45 to 0.11 in Rx. This again illustrates that the C-Br bond breakage in the excited state is temperature activated. Mechanism of Pyrene Binding. Irradiation (γ rays or fast electrons) of polyethylene films containing pyrene leads to pyrene that is covalently bound to the polymer. Unattached pyrene is readily removed from the films by washing with cyclohexane and leaves the bound pyrene in the film. Figure 4 shows the red shifted UV absorption of bound pyrene (λmax ) 345 nm) from pyrene, which is similar to that of pyrene derivatives. An additional short-lived absorption at 410 nm is observed for a few minutes after radiolysis, which is due to the polyethylene substituted pyrenyl radical (P-Py•). This absorp-
Figure 4. Steady-state absorption spectra of pyrene and bound pyrene in polyethylene Rx.
tion is similar to the hydropyrenyl (H-Py•) radicals formed in irradiated methanol-pyrene glasses or liquids25 and is stabilized by the rigid polymer matrix, but is rapidly quenched by oxygen when the sample is exposed to air. It is instructive to compare this radical to the H-Py• radical produced by the γ irradiation of a pyrene-methanol glass at 77 K. Figures 5a and 5b compare the fluorescence excitation and emission spectra of pyrene, bound pyrene, and P-Py• in Rx and H-Py• in methanol glass at 77 K. Similar fluorescence excitation and emission spectra are obtained for the pyrene radical adducts.25,26 All pyrene adducts exhibit a much faster fluorescence decay than pyrene. Figure 6 shows the different fluorescence decay of P-Py• (k ) 1.69 × 108) and the hydropyrenyl radical (k ) 1.55 × 108), which are much faster than the bound pyrene (k ) 1.26 × 107) or free pyrene (k ) 2.7 × 106) in this system. Pulse radiolysis of polyethylene films containing pyrene gives transient absorption spectra typical of pyrene excited states and radical ion species 1Py*, 3Py*, Py+•, Py-•.17,27 Figure 7 shows the transient absorption spectra of 10 mol/m3 pyrene in Rx immediately after the pulse, 4 ms later, and 3 s after the pulse was given. The bands at 498, 385, and 365 nm are assigned to the pyrene radical anion (Py-•), 455 nm to the pyrene radical cation (Py+•), and 415 nm to the pyrene triplet state (3Py*).28 After 4 ms have passed, the pyrene excited states and radical ions have reacted, so no pyrene transients are observed in the absorption spectra. Polymer radicals, however, are still present in the film,29 and after long periods of time these radicals react with pyrene. Consequently, seconds after the pulse an absorption appears that is due to the formation of P-Py• (Figure 7). It is interesting to note that free radical migration has been observed in solid tetracosane by Clough et al. via isotopic exchange between molecular chains.30 Figure 8 shows the concentration of bound pyrene versus gamma irradiation dosage in polyethylene Rx for 1 and 10 mol/ m3 films. After 10 kGy of irradiation, the film containing only 1mol/m3 pyrene has significantly lost its free pyrene concentration and no longer shows an overall increase in bound pyrene concentration with irradiation. As additional pyrene is binding, the pyrene that is already bound is being burnt out of the film. As shown later, low concentrations of pyrene (millimolar) are not able to provide a high degree of protection against crosslinking in polyethylene films. By monitoring the G value versus dosage (Figure 8 insert), extrapolation back to initial conditions gives G values of binding of 0.49 and 0.74, respectively, for 1 and 10 mol/m3 Rx films. By comparing the solubilities of films irradiated with and without added pyrene, bound arenes were determined to be only
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J. Phys. Chem. B, Vol. 104, No. 3, 2000 479
Figure 7. Transient absorption spectra of 10 mM (mol/m3) pyrene in Rx taken immediately after a 1.04 kGy electron pulse, 4 ms and 3 s later.
TABLE 3: Gbinding Values of 1 mM (mol/m3) Arene Probe into Polymer Films under Vacuum and at Room Temperature unless Otherwise Indicated
Figure 5. (a) Steady-state fluorescence excitation spectra of 1 mM (mol/m3) pyrene and bound pyrene in Rx, with their emission monitored at 395 nm, and the P-Py• intermediate and hydropyrenyl radical, with their emission monitored at 450 nm. (b) Steady-state fluorescence emission spectra of 1 mM (mol/m3) bound pyrene and free pyrene (inset) in Rx excited at 337 nm as well as that of the P-Py• intermediate and hydropyrenyl radical, excited at 385 nm.
Figure 6. Time-resolved fluorescence decay at 450 nm of the P-Py• intermediate and hydropyrenyl radical, excited by a dye laser at ∼400 nm. Scattering from laser pulse is monitored at 400 nm.
singly linked to the polymer chain. The latter films (containing no pyrene) did not dissolve due to polymer cross-linking.31 Further irradiation of a film containing only bound pyrene produced no new UV absorption or fluorescence emission decay. Hence, only freely diffusing pyrene is able to reach the reactive polymer radicals. Table 3 gives the Gbinding yields in a variety of systems. It is found that oxygen and the polymer chosen have a greater influence on the binding reaction than on dehalogenation from bromopyrene; i.e., only 1 kPa of oxygen is needed to cut the binding yield in half for 1 mol/m3 pyrene in Rx, while 10 kPa
matrix
Gbinding at 10 kGy
Rx (linear) at 100 K at 413 K w/ 1 kPa O2 Rx Cl-pyrene Rx Br-pyrene Rx perlyene B (linear) Pn (branched) polypropylene low (linear) cyclohexane at 100 K polystyrene poly(methyl methacrylate)
.31 .32
