J. Phys. Chem. B 2002, 106, 3375-3382
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Selectivity and Efficiency of Pyrene Attachment to Polyethylene Films by Bombardment with MeV-Range Protons Gerald O. Brown,† Noel A. Guardala,‡ Jack L. Price,‡ and Richard G. Weiss*,† Department of Chemistry, Georgetown UniVersity, Washington, D.C. 20057-1227, and NaVal Surface Warfare Center, Carderock DiVision, 9500 MacArthur BouleVard, West Bethesda, Maryland 20817 ReceiVed: August 17, 2001; In Final Form: December 23, 2001
We demonstrate the attachment of pyrene molecules to chains of polyethylene (PE) films using 1-4.5 MeVrange protons (H+). Despite the enormous energy available for reaction, the attachment is exceedingly selective; MeV-range protons lead to photochemistry as selective as that initiated by eV-range photons (>300 nm). The effects of proton kinetic energy, proton dose, polymer crystallinity, and initial pyrene concentration within the films on the selectivity of the attachment process are analyzed on the basis of UV/vis absorption spectroscopy, steady state and dynamic fluorescence measurements, and differential scanning calorimetry. Attachment selectivity is independent of proton kinetic energy but increases with decreasing proton dose. The efficiency of pyrenyl attachment increases with decreasing polymer crystallinity. In polyethylene of 42 % crystallinity, the efficiency of attachment is only slightly dependent on pyrene dopant concentration (in the range 10-4-10-2 mol/kg) at low doses but increases significantly with concentration at higher doses. Time-resolved fluorescence studies on films subjected to high doses indicate either a drastic change in local environments around attached 1-pyrenyl groups or that the attached groups have suffered secondary transformations. Extensive cross-linking of proton-bombarded PE samples, indicative of C-H and C-C bond scission, was observed in both the absence and presence of pyrene. At all proton energies, films containing 10-2 mol/kg pyrene and exposed to 1013 H+/cm2, the highest fluence employed, exhibited a pronounced, broad emission band centered at ∼470 nm, attributed to excimer. Dissolution of these samples led to loss of the excimer band, indicating that the aggregated pyrenyl groups are attached to neighboring polyethylene chains. A mechanism for attachment, based on the addition of preformed carbon-centered polymer radicals or cations to ground-state pyrene molecules, is proposed. It is contrasted with the mechanism for attachment initiated by eV-range photons.
Introduction Lamotte and co-workers have demonstrated that ultraviolet (eV range) irradiation of pyrene in solid and liquid alkane solutions yields 1-alkylpyrenes in addition to other photoproducts whose number and type depend on photon wavelength.1 Photoreactions of pyrene in very polar, protic media are known to produce at least two products.2,3 We have also shown that eV-range photons can lead to attachment of pyrene to chains of polyethylene films.4 In n-alkane matrixes, the mode of attachment is very dependent on solvent phase, pyrene concentration, radiation wavelength, and alkane chain length; pyrene molecules reside and react preferentially at interfaces between lamellae in solid phases of long-chained n-alkanes.5 Recently we have observed that pyrene can be attached both selectively and efficiently to chains of polyethylene films by bombardment with MeV-range protons.6 Relatively few studies have compared the chemistries induced by eV and MeV-range radiation,7-11 and most have employed fast electrons and γ-rays as the high-energy source. Those involving polymers have * Corresponding author. Phone: (202) 687-6013. Fax: (202) 687-6209. E-mail:
[email protected]. † Georgetown University. ‡ Naval Surface Warfare Center.
focused on changes in mechanical properties,12 degrees of chain scission13 and cross-linking,14 and production of molecular hydrogen.15 Here, we examine several important factors, including proton energy, proton dose, polymer crystallinity, and initial pyrene dopant concentration, influencing the efficiency and selectivity of pyrene attachment to polyethylene chains initiated by MeVrange protons and compare mechanistic aspects of attachment by UV-irradiation and proton bombardment. Experimental Section Materials. PE42 (Sclairfilm 300 LT-1, Dupont of Canada), PE73 (type ES-300, Polialden, Petroquimica), and PE32 (EXACT 3132, Exxon Chemical Co., Baytown Polymers CenterPolyethylene Technology Division, Baytown, TX) films (Table 1) were immersed in several aliquots of chloroform for more than 1 week to remove antioxidants and plasticizers before being used. Pyrene (Aldrich, 99%) was recrystallized from benzene, passed through an alumina column using benzene as eluant, and recrystallized twice from ethanol to yield pale yellow crystals, mp 148.6-149.1 °C (lit.16 mp 149-150 °C). 1-Ethylpyrene, mp 95-96 °C (lit.17 mp 94-95 °C) was obtained from Molecular Probes and used as received. Xylene (EM Science reagent grade) was dried over CaH2 and distilled under a
10.1021/jp013200l CCC: $22.00 © 2002 American Chemical Society Published on Web 03/05/2002
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Brown et al.
TABLE 1: Properties of Polyethylene Films PE32 PE42 PE73 a
3
thickness (µm)
density (g/cm )
% crystallinity
32 76 20
0.900 0.91822 0.94522
32 (12) 42 (24) 73 (50)
a
By X-ray diffraction. Values in parentheses by DSC analyses.
nitrogen atmosphere. Methanol (EM Science, HPLC grade) and chloroform (Fisher, HPLC grade) were used as received. Pyrene-Doped PE Films. Pieces of polyethylene film were immersed in chloroform solutions of pyrene for 1 day and dried under a stream of nitrogen, and their surfaces were washed with methanol (a nonswelling solvent) followed by drying under a stream of nitrogen. Immediately prior to being bombarded, residual chloroform and molecular oxygen were removed from the films by placing them in a chamber at 10-7 Torr pressure (see below). The concentrations of pyrene within the films were determined from the average optical densities of three different spots (Perkin-Elmer Lambda-6 UV/Vis spectrophotometer) and Beers law using the molar extinction coefficient in petroleum ether at 335 nm, 55 000 M-1 cm-1.18 They are precise to within (10%. Bombardment of Films and Analyses. Uniform beams of 4.5, 2.2, or 1 MeV protons produced by a 3 MV NEC Pelletron tandem accelerator operated by the Naval Surface Warfare Center (NSWC; Carderock Division, West Bethesda, MD) were passed through a 1 mm aperture in a tantalum foil and projected onto ca. 1 cm × 1 cm film surfaces.19 Exposure times ranged from 4 s (109 H+/cm2) to 300 s (1013 H+/cm2). The proton generator and compartment holding the films were maintained at ca. 10-7 Torr prior to and during bombardments. The dose in gray (1 Gy ) 1 joule/kg) delivered to each film was determined from eq 1,20 where F is the particle fluence in cm-2, E is the particle energy in MeV, R is the particle range (i.e., maximum depth of particle penetration) in cm calculated from the TRIM (transport of ions in materials) code,21 and F is the density of the material in g/cm3.
dose (Gy) ) (1.60 × 10-10)FE/RF
(1)
Following bombardment, films were soaked in several aliquots of chloroform until UV absorption and fluorescence spectra of the last chloroform wash showed no pyrene or other unattached aromatic species (usually 2 days) and were then dried under a stream of nitrogen. Unless indicated otherwise, all reported spectroscopic data were acquired after this extraction process to remove noncovalently attached lumophores. Fluorescence excitation (corrected for detector response) and emission spectra (uncorrected) under vacuum ( PE73. However, even at comparable doses, the efficiency of pyrenyl attachment is greater in PE32 and PE42 than in PE73. The G values of doped PE32, PE42, and PE73 are 0.6, 0.5, and 0.09 µmol/J, respectively, when the doses are in the ∼1-4 kGy range. The lower efficiency and higher selectivity of pyrenyl attachment in the high crystallinity polyethylene may also be related to morphological differences. The content of amorphous and interfacial regions in polyethylenes of higher crystallinity is lower than in polyethylenes of lower crystallinity.46 Pyrene molecules reside in the amorphous and (amorphous/crystallite) interfacial regions of polyethylene and are excluded from its crystalline domains.46 For this reason, pyrene molecules are distributed in a smaller volume fraction of PE73, and their average separation is smaller than in the lower crystallinity polyethylenes. Additionally, future experiments will be required to determine whether energy from proton tracks is deposited preferentially into amorphous and interfacial regions (as some of our current data suggests). Comparison of eV-Range Photon and MeV-Range Protons as Energy Sources for Pyrenyl Attachment. The photoattachment of pyrene to polyethylene using UV radiation (>300 nm) can be very selective also when doses are low and fluxes are high.47 Attachment of comparable selectivity is attained using low doses of MeV-range protons. Excitation and emission spectra from lumophores attached by photons and MeV-range protons are almost indistinguishable from those of 1-ethylpyrene doped in the same polymer (Figure 1). However, at higher proton (Figure 5B) or photon (see Figure 2 of the Supporting Information) doses, the selectivity of both photoattachments decreases. The spectral changes with increasing photon dose (N.B., the loss of 377 nm peak) are very similar to those observed with proton bombardments. The lowest and highest photon doses responsible for the film changes were estimated using both a thermopile surface absorber head attached to a power meter and a fulgide actinometer,23 assuming an average absorption wavelength of 313 nm from the 450-W mediumpressure lamp source. Although dose determinations by the two methods differ by a factor of 4, they indicate that the lowest photon dose is ∼(2-7) × 103 kGy and the highest is ∼(2-9) × 104 kGy. The magnitude of even the lower estimate of the lowest photon dose (for which 1-pyrenyl emission is dominant) is much greater than the highest proton dose (that resulted in significant amounts of secondary attachment products and excimer emissions). Attachment initiated by eV-range photons must be much less efficient than by MeV-range protons. Clearly, the vast majority of singlet and triplet excited states of pyrene (steps 3 and 4 of Scheme 1) return radiatively or radiationlessly to their ground state without reacting when the excitation source is a UV lamp like ours.
J. Phys. Chem. B, Vol. 106, No. 13, 2002 3381 Conclusions Attachment of pyrenyl groups to polyethylene chains, when initiated by MeV-range proton bombardments, can be both selective and efficient. At constant dose, the selectivity of attachment is independent of proton energy in the range of 1-4.5 MeV and increases with decreasing dose. At low doses (0.344 kGy), attachment efficiency is only slightly dependent on initial pyrene dopant concentrations (10-4-10-2 mol/kg). The concentration of pyrene molecules must be much greater than the number of polymer-based radicals and ions capable of reacting with them. At 100-fold higher doses (344 kGy), the efficiency of attachment, as measured by G, is very dependent on initial pyrene concentration and increases by >100 between 10-4 and 10-2 mol/kg. At the higher dose, the number of polyethylene-based radicals and ions must exceed the number of nearby pyrene molecules. At comparable doses, G is higher in polyethylenes of lower crystallinity. We conjecture that energy from proton tracks may be deposited preferentially in the amorphous and interfacial regions where dopant molecules reside. The fact that the selectivity of pyrene attachment from bombardment with MeV-range protons and eV-range photons (>300 nm) is comparable at low doses is somewhat surprising, because they do not occur via the same mechanism, and the much greater energy per proton should allow less discriminate reactions. Apparently, the less discriminate processes occur predominantly within the polymer matrix, leading to proton-induced C-H and C-C bond scissions and some cross-linking of polyethylene chains. Polyethylene films doped with 10-2 mol/kg pyrene and exposed to the highest proton dose (irrespective of proton energy or polymer crystallinity) exhibit a prominent, excimer-like band centered at ∼470 nm that arises primarily from “interchain” interactions between pyrenyl groups attached to neighboring polyethylene chains. These results, in combination with those from eV-range photon-induced attachments, are consistent with a mechanism in which polyethylene-based radicals and ions add selectively to the 1-position of pyrene in the product-determining step. Acknowledgment. We thank Mr. Mario Lutterotti of Dupont of Canada, Mississauga, Ontario, Prof. Teresa Atvars of the State University of Campinas, Brazil, and Ms. Nancy Richter of the Exxon Chemical Co., Baytown, TX, for supplying the films employed in this work. R.G.W. is grateful to the National Science Foundation for financial support. This article is dedicated to Prof. Silvia Braslavsky on the occasion of her 60th birthday. Supporting Information Available: Two figures, a fluorescence decay histogram of a 2.2 MeV proton-bombarded 10-2 mol/kg pyrene in PE42 film at low dose and emission spectra of a 10-2 mol/kg pyrene in PE42 film irradiated at >300 nm for different periods. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Lamotte, M.; Joussot-Dubien, J.; Lapouyade, R.; Pereyre, J. In Photophysics and Photochemistry aboVe 6 eV; Lahmani, F. Ed.; Elsevier: Amsterdam, 1985; p 577. (b) Lamotte, M.; Pereyre, J.; Lapouyade, R.; Joussot-Dubien, J. J. Photochem. Photobiol. A 1991, 58, 225. (2) Sun, Y. P.; Ma, B.; Lawson, G. E.; Bunker, C. E.; Rollins, H. W. Anal. Chim. Acta 1996, 319, 379. (3) (a) Pandey, S.; Acree, W. E., Jr. Anal. Chim. Acta 1997, 343, 155. (b) Sun, Y. P.; Ma, B.; Lawson, G. E.; Bunker, C. E.; Rollins, H. W. Anal. Chim. Acta 1997, 343, 159. (4) Naciri, J; Weiss, R. G. Macromolecules 1992, 25, 1568.
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