Oct 24, 1989 - the formation of free radicals, electron paramagnetic resonance ..... in Figure IO, and the resulting relaxation times are T2 = 9.2 X s. The nearly ...
J . Phys. Chem. 1990, 94, 5 159-5 164
5159
Electron Paramagnetic Resonance Investigation of Free Radicals in Polyimide Films Michael A. George, B. L. Ramakrishna, and William S. Glaunsinger* Department of Chemistry, Arizona State University, Tempe, Arizona 85287- 1604 (Received: October 24, 1989)
Both unirradiated and far-ultraviolet irradiated Kapton H polyimide films have been investigated by electron paramagnetic resonance spectroscopy as a function of temperature, microwave power, irradiation time, and chemical treatment. Free radicals are always observed in the bulk. Irradiation of Kapton H films in air produces relatively stable oxidized and unoxidized aromatic-type radicals in close vicinity of the surface, whereas film irradiation in the absence of oxygen yields only the latter radicals. The g values, line widths, spin concentrations, and relaxation times of both the bulk radicals in unirradiated films and the surface radicals in films irradiated in air have been determined. It was found that irradiation of Kapton H enhanced the adhesion of gold films to the polyimide surface. On the basis of the results of this study, a qualitative model for the radical-formation mechanism is proposed.
Introduction Polymers are playing an increasingly important role in the microelectronics industry for integrated-circuit bonding and the interconnection of integrated circuits on printed wiring boards, flexible substrates, and high-density multilayer packages.] The utility of polymers for such high-technology applications has increased the need to better understand their physical properties and to develop both improved and new polymers having desirable properties. In this regard, polyimide polymers are well suited for a wide variety of applications because they are light, inexpensive, mechanically tough, chemically resistant, electrically insulating, and thermally stable to relatively high temperatures ( ~ 4 0 0C).2-4 0 These polymers also have the advantage of being able to form a good planar surface for high-density mutlilevel metal-insulator struct~res.~,~ Polyimide surfaces have been exposed to various treatments, including chemical,6-'0 thermal,]'9l2 and r a d i a t i ~ n , ' ~ ,with ~ ~ - the '~ goal of modifying their surface chemistry in a controlled fashion and enhancing adhesion. Photochemistry is a particularly promising approach to the controlled modification of the polyimide surface.I*l7 Far-ultraviolet (far-UV) radiation (200-1 50 nm) is especially attractive for this purpose for the following reasons:'* (1) it is quite practical since wavelengths in the 200-180-nm range can be achieved easily with a mercury lamp and lasers and silica, nitrogen, and water do not adsorb appreciably at these wavelengths; (2) nearly all aromatic polymers absorb intensely in this wavelength region: (3) for strong UV adsorbers, the penetration depth of this radiation is only about 10 nm before approximately 95% of its intensity is absorbed;19 and (4) the quantum yield for q'~ bond breaking is high (0.1-1 .0).19-21 Previous s t ~ d i e s l ~ of polyimide films by X-ray photoelectron spectroscopy (XPS) have indicated that continuous-wave (CW) far-UV irradiation in air leads to rapid oxidation of the surface region, whereas ablation of the film with a pulsed laser results in a surface that is depleted in oxygen. In both methods, new functionalities are created on the polymer surface that have different chemical properties than the unirradiated polymer. Furthermore, C W far-UV radiation appears to increase the number of nucleating sites for the deposition of silver fiIms.22 Since the photooxidation of polymers can be accompanied by the formation of free radicals, electron paramagnetic resonance (EPR) should be a powerful method for probing the paramagnetic products of the photooxidation process. EPR has been used extensively to investigate the effects of polymer irradiation, including the nature of the free radicals as well as their mechanisms of formation and decay."*23 In the present study, we have employed EPR to study the photooxidation of poly[N,N'-(p,p'-oxydiphenylene)pyromellitimide] (Kapton H) films (Figure 1). The results of this work demonstrate that C W far-UV irradiation of Kapton H in air produces relatively stable free radicals which are located at, or very near, the polyimide surface. *To whom correspondence should be addressed
0022-3654/90/2094-5159$02.50/0
Experimental Section Kapton H films were obtained from E. I. du Pont de Nemours and Co., Inc. This all-purpose polyimide polymer was prepared by a polycondensation reaction between pyromellitic dianhydride and 4,4-diaminodiphenyl ether. The Kapton H was prepared with no additives and has a narrow molecular weight distribution, so it is believed to be very pure. The polymer was characterized by XPS; infrared, ultraviolet-visible, and X-ray spectroscopy; and thermogravimetric analysis (TGA). The X P S spectra revealed strong carbon and oxygen and relatively weak nitrogen peaks, which are consistent with the chemical formula in Figure 1 as well as previous ~ o r k . ~ Infrared ~ , ~ ' spectra exhibited characteristic adsorptions in the carbonyl, tertiary nitrogen, aromatic ether, and benzene regions. Ultraviolet-visible spectra showed a strong absorption in the range 600-450 nm to essentially zero transmittance below 450 nm, which accounts for the yellow-orange color of the polymer, with the excited states most likely being extended over at least part of the highly conjugated polymeric chains. X-ray pole-figure studies indicated that these polymeric films are amorphous. Finally, TGA runs in dry argon up to 1000 "C yielded a black carbon residue and a mass loss (37%) in good agreement with the loss of all 0 and N atoms and half of the nonaromatic cations from the polymide (Figure I ) . Kapton H films were irradiated with an unfiltered Oriel 200-W mercury vapor light source providing a strong 185-nm line for
( I ) Lai, J . H.; Jenekhe, S. A.; Jensen, R. J.; Roger, M. Solid Sfate Technol. 1984, 27:9-12, 165-171. (2) Sato, K.; Harada. S.; Saiki, A.; Kimura, T.; Okuba, T.; Muakai. K. IEEE Trans. Parts, Hybrids, Packag. 1973, 9, 176. (3) Zielinski, L. B. US. Patent 3,985,597, 1973. (4) Rothman, L. B. J . Electrochem. Soc. 1980, 127, 2216. (5) Wilson, A. M. Thin Solid Films 1981, 83, 145. (6) Hermes, J . US. Patent 3,770,528, 1973. (7) Yuan, E. L. US. Patent 3,822,175, 1974. (8) Rasmussen, J. R.; Stedronsky, E. R.; Whitesides, G. M . J . Am. Chem. Soc. 1977. 99. 4782. (9) Rasmussen, J. R.; Bergbreiter, D. E.; Whitesides, G.M. J . Am. Chem. Soc. 1977, 99, 4740. (10) Srinivasan, R.; Lazare, S. Polymer 1984, 26, 1297. (11) Bruck, S. D. Polymer 1965, 6, 319. (12) Hu, C. Z.; Andrade, J. D. J . Appl. Polym. Sci. 1985, 30, 4409. ( 1 3) Owens, D. K. J . Appl. Polym. Sci. 1975, 19, 33 15. (14) Tazuke, S.; Kimura, H. Makromol. Chem. 1978, 179, 2603. (15) Peeling, J.; Clark, D. T. J . Appl. Polym. Sci. 1981, 26, 3761. (16) Peeling, J.; Clark, D. T. J . Appl. Polym. Sci. 1984, 22, 419. (17) Lazare, S.; Hoh, P. D.; Baker, J . M.; Srinivasan, R. J . Am. Chem. Soc. 1984, 106, 4288. (18) Srinivasan, R.; Leigh, W. J. J . Am. Chem. Soc. 1982, 104, 6784. (19) von Sonntag, C.; Schuchmann, H. D. Ado. Phofochem. 1977, IO, 59. (20) Srinivasan, R.; Ors, J. A. J . Am. Chem. Soc. 1979, 101, 3411. (21) Srinivasan, R.; Baum, T.; Ors, J. A. Tetrahedron Left. 1981, 22,4795. (22) Srinivasan, R.; Jipsan, V. B.; Poirier, M. J. Surf. Sci. Lett. 1983, 130, L344.
(23) Ranby, B.; Rabek, J. F. ESR Spectroscopy in Polymer Research; Springer-Verlag: New York, 1977.
G 1990 American Chemical Society
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The Journal of Physical Chemistry. Vol. 94, No. 12, 1990
i
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Cieorge et ai.
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Figure 1 The repeat unit of Kapton H
.
0
2
4
6 [POWER
8
13
I?
19
(Mu] 'I2
Figure 3. EPR integrated intensity as a function of microwave power at 295 K for Kapton H films.
4LJr" Figure 2. EPR spectra of a 4-pm Kapton H film at 295 (a) and 77 K
(b).
the EPR studies. The lamp-to-film distance was approximately 2 cm, and all films were irradiated ex situ. A Bruker-IBM ER-200 D EPR spectrometer operating a t X-band microwave frequencies and using a 100-kHz magnetic field modulation frequency was used throughout this work. Three different film thicknesses (4, 8, and 20 pm) were studied by EPR, and rectangular samples (2 mm x 5 mm) were cut to fit conveniently into the EPR resonator. Films were placed along the central axis of the resonator, and there was no angular dependence of either the spectra or resonator Q. Modulation amplitudes up to 1 G did not cause any measurable line broadening. EPR spectra were usually obtained at a microwave power level of 0.16 m W to avoid saturation. The standard magnetic field sweep width was 100 G . The microwave frequency for all experiments was 9.24 GHz. A standard TE 102 resonator was used to record spectra at ambient and liquid nitrogen temperatures. A high-temperature Bruker-IBM ER-4114 H T resonator was used to acquire spectra at temperatures ranging from 300 to 665 K. Several spectra were recorded at each temperature during a 15-min period to verify signal stability. Spin concentrations and g values were determined using both NBS-ruby and weak-pitch standards. The integrated intensity of the EPR absorption signal was determined by double integration of the first-derivative EPR signal. Spin-trapping experiments were carried out by submerging the polymer in a solution containing 3,5-dibromonitrosobenzenesulfonatein an acetone-water solution ( 1 :SO by volume). Microwave magnetic fields were determined from saturation data on a DPPH standard
Results and Discussion Unmodified Kapton H. At ambient temperature and above in air all Kapton H samples exhibited a weak, symmetrical orientation-independent EPR signal, as shown in Figure 2, having a g value of 2.005 and a peak-to-peak line width of 8 G. This signal probably originates from free radicals produced in the film-formation process, since it is observed in all Kapton H films, Both the g value and line width are somewhat larger than those typical of carbonaceous organic free radicals (2.002-2.003 and =3 G,24respectively) The slightly larger g value in Kapton H (24) Noda. S.; Hoiki, T. Carbon 1984, 22, 3 5 9
may be due to unpaired electrons that are delocalized in the aromatic rings containing nitrogen and oxygen (Figure I ) , since the interaction of an unpaired electron with nitrogen and oxygen generally causes g values to increase due to their larger spin-orbit coupling constant.25 The larger line width in Kapton H possibly originates from the unresolved hyperfine interaction of the unpaired electron with the nitrogen nucleus.2s The absence of any I3C satellites in Kapton H at the highest spectrometer sensitivities, including signal averaging, is consistent with the above picture of the delocalization of the unpaired electrons over the aromatic rings, since for such radicals the electronic wave function has a node at the carbon nucleus. The orientation independence of the EPR signal is evidently due to the random orientation of the aromatic rings within the polymer matrix. Also, at liquid nitrogen temperature the EPR signal becomes noticeably asymmetric (asymmetry parameter = I .6) and there is some indication that the EPR line may become more complex (Figure 2), which is possibly a result of more restricted molecular motions,23 i.e.. incomplete motional averaging, at 77 K. The g value, line width, and signal intensity are independent of sample thickness of equal masses (2.2 mg) of Kapton H, which demonstrates that the radicals are located in the bulk rather than at the surface of these films. The bulk spin concentration of Kapton H was determined to be 8.3 X 10l6 spins/g, which translates into an average of about one radical per 2.3 X lo4 monomeric units, so that the radical concentration is in the magnetically dilute range. Moreover, the integrated intensity of the EPR signal, which is proportional to the spin concentration, is independent of temperature in the range 300-665 K, which is the temperature region where Kapton H is known to be stable. The EPR signal of Kapton H could be saturated at microwave powers above about 0.2 mW, which provides a convenient method for determining the spin-spin and spin-lattice relaxation times. The integrated intensity as a function of microwave power is displayed in Figure 3. For homogeneously broadened lines, the spinjpin relaxation time ( T 2 )can be estimated from the line width below saturation by using the expression
where AHp.pis the peak-to-peak line width.26 The spin-lattice relaxation time ( T I )can be determined in the saturation region from the equation T , = (0.49
X
10-7)AHp.p/gH,2
(2)
where H I is the microwave magnetic field at which the peak-topeak amplitude of the EPR signal is maximum. Although ( 1 ) and (2) are strictly valid for homogeneously broadened lines, these simple expressions are still useful for obtaining the relative values ( 2 5 ) Lewis, I . C.; Singer, L. S . Chemisfry and Physics qf Carbon: Marcel Dekker: New York. 1981; Vol. 17. p I . ( 2 6 ) P d e , C. P., Jr. Electron Spin Resonance: A Comprehensice Treatise on Experimental Techniques; W h y : N e w York, 1983; p 592. ( 2 7 ) Blais. P.: Day. M.:Wiles, D. M. J . A p p / . Po/ym. Sci. 1973. 17. 1895.
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 5161
EPR of Free Radicals in Polyimide Films
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1
1
0
1
1
1
1
1
1
,
5
,
,
10
,
,
,
1
15
TIRE (hrs)
Figure 5. Change in integrated intensity as a function of time for UVirradiated Kapton H at 295 K. The UV radiation was stopped after 2.25 h (arrow).
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-I/""
Figure 4. EPR spectra of Kapton H films at 295 K: (a) unirradiated, (b) UV-irradiated for 1 h in air, and (c) UV-irradiated for 1 h under vacuum.
of T , and T z for inhomogeneously broadened lines if the line-width and line-shape parameters are the same. The data in Figure 3 demonstrate that the line is inhomogeneously broadened, so that only the relative values of Ti and T z are significant. In this case, s and T I = 2.7 the resulting relaxation times are T2 = 9.3 X X s. The calculated value for T 2 may be somewhat shorter than the actual value due to possible inhomogeneous broadening from unresolved hyperfine interactions of the unpaired electron with nitrogen. These relaxation times are compared to those for UV-irradiated Kapton H in the next section. UV-Irradiated Kapton H . The free-radical concentration for Kapton H changes substantially upon UV irradiation. A comparison of the EPR spectra of films both before and after irradiation for 1 h at ambient temperature in air is shown in Figure 4a and Figure 4b, respectively. Although the g value, line width, and isotropy of the EPR signals are unchanged after irradiation, which suggests that the electronic nature of the radicals before and after irradiation are similar, the integrated intensity increases by a factor of 2.7. If the films are irradiated under vacuum ( 5 X Torr) for 1 h, then the integrated EPR signal intensity increases by about a factor of 1.9 relative to unirradiated samples in air, as shown in Figure 4c. Therefore, there is about a 2-fold increase in radical concentration for films irradiated in air relative to those irradiated under vacuum. Moreover, the EPR line shapes for irradiated samples under vacuum are slightly asymmetrical. Therefore, it appears that oxygen must play an important role in the radical-formation process, yet radicals are formed even in the absence of oxygen. Neglecting any combination reactions, a simplified free-radical chain mechanism for the oxidation of polymers26~z8 can be written as follows: polymer R'
-
+ hu
+0 2
+ polymer ROOH + hu
ROO'
R'
+ R'
ROO' ROOH
RO'
+ R"'
+ 'OH
(3) where ROO' is a peroxy radical and similar reactions can be written for R" and R"', which can be formed by direct reactions either on the aromatic rings or on single bonds along the polymer chain. Field-ion and electron impact mass spectrometry invest i g a t i o n ~ *have ~ indicated that bridges in aliphatic polymides are -+
(28) Reference 23, Chapter 7 .
most vulnerable. Such studies at 600 OC of the polymer in Figure 1 have implicated scissions at several single bonds along the polymer chain, with the oxygen bridge between the biphenyl rings weakening the structure and being very susceptible to scission. Radical-radical reactions in the above reaction sequence will generally terminate the chain. Such a reaction sequence is consistent with XPS studies of polyimide films in both this and p r e v i o ~ s ' ~work, ~ ' ~ where substantial superficial oxidation and new functional groups were observed after 10 min of far-UV irradiation. In particular, the O/C ratio increased by about a factor of 2, and the C Is peaks corresponding to single and double bonding to oxygen increase relative to the unoxidized C 1s peak, with approximately equal amounts of carboxylic and hydroxyl groups being formed. However, films irradiated similarly under vacuum contained primarily carboxylic groups and very few hydroxyl groups.1° It is also possible that water may be involved in these near-surface reactions, since recent ultraviolet photoemission spectroscopic studies30 have shown that water adsorbs strongly to the polyimide surface and does not completely desorb until the temperature exceeds about 300 "C. Although the mechanism of these complex surface reactions is unresolved at present, it is clear from the EPR spectra in Figure 4 that radicals are formed in the absence of oxygen and that a substantially higher concentration of radicals is created in the presence of oxygen. In this regard, it is of interest to compare these EPR results to previous studies of peroxy-type radicals, even though the observed signals may not originate from simple radicals of this type.28 Most polymers exhibit an asymmetrical single-line spectrum attributed to ROO' radicals formed after introduction of oxygen to samples containing R'-type radicals. If motional freedom is restricted, then in some cases it is possible to resolve the anisotropy in the EPR spectrum of ROO'. However, sufficient motional freedom can effectively average out the g-factor anisotropy and yield a symmetrical single-line spectrum, as shown in Figure 4. In agreement with this expected thermal behavior, EPR spectra at 77 K for Kapton H films irradiated in air are quite asymmetrical (asymmetry parameter = 2.4). The more pronounced asymmetry a t 77 K for this signal relative to that for unirradiated Kapton H shown in Figure 2 reflects the chemical differences between the two types of radical. Since the electronic natures of the radicals detected in the absence and presence of oxygen by conventional EPR are similar (Figure 4), attention will be focused exclusively on further characterization by EPR of Kapton H films that were irradiated in air. In order to determine the rate of formation and stability of the free radicals found after UV irradiation at ambient temperature, the integrated intensity was measured as a function of both irradiation time and time elapsed after irradiation was terminated. (29) Dussel, H. J.; Rosen, H.; Hummel, D. 0. Makromal. Chem. 1976, 177, 2343. (30) Reference 23, pp 188-189 and references therein.
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1100
1
1
500
"33
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Figure 6. Temperature dependence of the integrated intensity-temperature product for unirradiated ( 0 )and UV-irradiated (1 h ) (0)Kapton H films.
The results are shown in Figure 5. It can be seen that the intensity rapidly reaches a plateau after about 1 h of irradiation, which correlates with the moderate increase in O / C peak area ratio as a function of irradiation time in XPS experiments.I0 After the radiation is stopped, the intensity decays slowly and linearly over an extended period of time. Three hours after irradiation, only 6% of the radicals have decayed, and after 12 h only 26% of the radicals have been destroyed. Extrapolation of the linear decay curve indicates that approximately 50 h are required for complete decay, which has been confirmed experimentally. The slow decay rate and intrinsic stability of the radiation-induced paramagnetic species are characteristic of aromatic radicakZ8 The high degree of aromaticity of the wave function of the unpaired electron can account for its stability as well as the similar electronic nature of radiation-induced radicals both in the absence and in the presence of oxygen. The thermal stability of the radiation-induced radicals was investigated by measuring the temperature dependence of the integrated intensity. A comparison of the intensity-temperature product, which is proportional to the number of spins if the Curie law is obeyed, vs temperature for unirradiated and irradiated films is depicted in Figure 6. In contrast to unirradiated Kapton H, where the spin concentration is independent of temperature, the radical concentration in irradiated films decays linearly up to 360 K. more rapidly between about 360 and 390 K, and then more slowly above 390 K to essentially the value of unirradiated films near 665 K. The more rapid decay between 360 and 390 K may be due to thermally activated molecular motions of the polyimide, as observed for trapped radicals in other polymers,30 and/or to compositional differences between the polymer surface and bulk. Three different experiments were performed to demonstrate that the radicals formed by UV irradiation of Kapton H in air are located at (or very near) the surface. The radiation-induced radicals are quite stable on the time scale of these experiments. The first method was to subtract the unirradiated bulk-radical EPR signal from that obtained after UV irradiation to obtain the signal from radiation-induced radicals only (Figure 7), after which it was found that the integrated intensity of the EPR signal was identical for different film thicknesses having the same area and increased linearly with the surface area of the film. These results show that the radical concentration is proportional to the film area rather than volume, but no molecular information is provided concerning the location of the radicals. The second method for determining radical location was to treat the films chemically. EPR spectra were obtained at ambient temperature for different film thicknesses before and after UV irradation as well as after soaking the films for 1 h in distilled water, 5% nitric acid (by volume), and acetone. In these experiments the quality factor of the resonator was monitored and was found to remain unchanged. The integrated intensity of the resulting EPR signals, which includes the contribution from the bulk radicals in Kapton H, as a function of film thickness is displayed in Figure 8 for treatment with distilled water. Distilled water decreases the signal intensity of the UV irradiated films by about 60-70%. For a
Figure 7. EPR spectra at 295 K for unirradiated (a) and UV-irradiated (1 h) (b) Kapton H films ( 1 wm). The difference spectrum (c) originates from radiation-induced radicals. 20 18
-
unirradiated film
(e)
UV-irradiated film
(0)
water-treated
(A)
film
16 14
12 c >
2
10
c w
z a 6 ii
1
2
3
4
5
THICKNESS b" Figure 8. Integrated intensity as a function of film thickness at 295 K for Kapton H before ( 0 )and after (0)UV irradiation and after soaking the film for 1 h in distilled water (A).
polyimide film exposed to UV radiation for 10 min and then soaked for I h in water, XPS showed a decrease in the O / C ratio to essentially the value for the untreated polymide, indicating that the highly photooxidized species are effectively removed by wetting." In this case, the EPR results show that about 30-40% of the radicals are not of the highly photooxidized type, which is in agreement with the finding that a substantial concentration of radicals are created by UV radiation of films under vacuum. Similar EPR behavior was observed for 5% nitric acid. In contrast, acetone treatment resulted in a decrease in signal intensity to a level near that before UV irradiation for the 20-ym film and produced signal intensities slightly smaller than the unirradiated-film signal for the 4- and 8-ym samples. These experiments suggest that most of the UV-induced radicals are near the surface and that acetone apparently attacks surface photooxidized and unphotooxidized radicals as well as subsurface (bulk) radicals. The third, more definitive, method was to evaporate a thin film of gold (=500 A) at ambient temperature in an ultra-high-vacuum evaporator onto a UV-irradiation film. As shown in Figure 9, the EPR spectrum of the film taken within 20 min after the film was irradiated and covered with gold was identical with that observed before irradiation. Since gold is not expected to penetrate into the film, it follows that the radiation-induced radicals are located almost entirely in close vicinity of the surface. This result
EPR of Free Radicals in Polyimide Films
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The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 5163 + 5
4
E r
v)
-
E 3
A
2
0
2
4
6
[POWER
8
1
0
1
2
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Figure 10. Integrated intensity as a function of microwave power at 295 K for UV-irradiated (1 h ) Kapton H films. kV
KAPTON H
Figure 9. EPR spectra of Kapton H at 295 K before U V irradiation (a), after U V irradiation for 1 h (b), and after depositing a thin film (=500 A) of gold (c). Spectrum (c) was taken within 20 min after the film was irradiated and covered with gold.
is consistent with the short penetration depth of the UV radiation (110 mm). Furthermore, it has also been found that the chemical bonds formed between the radicals on irradiated films and the interfacial gold atoms increase substantially the adhesion of gold films to Kapton H. The spin concentration of a Kapton H film after 1 hr of UV irradiation was measured to be 1.85 X 10'' spins/g, which is about 2.5 times the value for an unirradiated film having the same area. Assuming that all radicals resulting from irradiation are a t the surface, this increase in radical concentration upon irradiation corresponds to 1 radical per 68 A2, which is approximately the 4 Therefore, it appears that the area of one monomer ( ~ 6 A2). radical concentration a t (or near) the surface is relatively high. These results further demonstrate that the radicals are intrinsic to the polymer and are not due to impurities. Again, the isotropy of the EPR signals probably results from the random orientation of the polymer chains near the surface, with the radiation-induced radicals being present in both photooxidized and unphotooxidized forms. In an effort to obtain structural information on the radicals in unirradiated and irradiated Kapton H, spin-trapping experiments were conducted using 3,5-dibromonitrosobenzenesulfonate. However, no radicals could be trapped, presumably because the radicals in Kapton H are too stable. Finally, the spin-spin and spin-lattice relaxation times for UV-irradiated Kapton H were determined at ambient temperature according to the same procedure used for the unirradiated polymer. The integrated intensity as a function of microwave power is shown in Figure I O , and the resulting relaxation times are T2 = 9.2 X s and T , = 7.8 X s. The nearly identical T2values and line widths for unirradiated and irradiated films indicates that even after irradiation the spin concentration is still too low for any appreciable spin-spin interactions. However, T , is about 3 times longer for irradiated than for unirradiated films. This result is consistent with the finding that the radiation-induced radicals reside primarily a t the surfce, since in this case coupling to the motional degrees of freedom of the polymer will be less efficient, which should result in longer TI values. Conclusions A summary of the EPR parameters for both unirradiated and
UV-irradiated Kapton H films is given in Table I . In this study
Figure 11. Schematic representation of the effect of far-UV radiation on Kapton H. Irradiation of the polymer (a) results in initial bond breaking to a depth of about 300 nm (b). Rapid recombination of radiation-induced radicals in the bulk leaves radicals near the surface (c), and the reaction of radicals with oxygen yields photooxidized radicals (d). TABLE I: EPR Parameters for Unirradiated and UV-Irradiated Kapton H Films at 295 K
unirradiated 2.005 UV-irradiatedd 2.005
8 8
8.3
X 10l6
1.8 X IO"
2.7 7.8
X X
9.3 X 9.2 X
Measured using a weak-pitch standard. Measured using weakpitch and NBS-ruby standards.
