J. Phys. Chem. 1987, 91, 1225-1236 3 1
1225
These data may be fit to expressions of the form kHS/kDS(T)= I exp(C/RT). Figure 4 shows the kHS/kDSdata and fits. Best-fit values of I and C for cyclopentane are 1.16 f 0.10 and 505 f 30 cal mol-' while those for cyclohexane are 1.16 f 0.06 and 471 f 18 cal mol-'. These parameters are close to those found in our studies of the deuterium-isotope effect for abstraction from the methylene sites in propane and n-butane, 1.13 f 0.19 and 52 1 f 155 cal mol-' for propane and 1.31 f 0.12 and 390 f 64 cal mol-l for n-butane. Indeed, as noted previously, the magnitudes of the deuterium-isotope effects found in our studies of OH alkane reactions fall into groups according to the relation kH/kD (primary site) > kH/kD(secondary site) > kH/kD(tertiary site). A theoretical study of this result is in progress and will be reported in a subsequent paper.
-F
+
Summary We have accurately determined rate coefficients as functions of temperature for the reactions of OH with the cycloalkanes c-C5HI0,c-C5DI0,c-C6HI2,and c-C6D12. Reactivities per C H 2 group and deuterium-isotope effects were characterized and compared with our previous determinations of methylene-site chemistry in the reactions of OH with propane and n-butane.
1
2
15
25
3
35
1000/ T(K) Figure 4. Kinetic-isotope effect for abstraction of H- and D-atoms from cycloalkanes by OH: 0, cyclopentane; A, cyclohexane. Error bars
represent f 2 a estimates of the total experimental error. neighboring alkyl groups on methylene-site reactivity; procedures for estimating these effects were recently developed by Atkinson.8 From our results we may also examine the deuterium-isotope effect at the secondary abstraction sites in cyclopentane and cyclohexane. Table 11 lists kHS/kDS(T) values (kl/k2(T) for cyclopentane, k3/k4(T) for cyclohexane) for the two compounds.
Acknowledgment. This research was supported by the Division of Chemical Sciences, the Office of Basic Energy Sciences, the US.Department of Energy. Registry No. H,,1333-74-0;D,,7782-39-0;OH, 3352-57-6;cyclopentane, 287-92-3;cyclohexane, 110-82-7.
Irradiation of Poly(perfluoropropy1ene oxide) by a 25-kV Electron Beam: Electron Beam Induced Chemistry of Poly(perfluoropropylene oxide) in the Absence of Oxygen J. Pacansky,* R. J. Waltman, and M. Maier IBM Almaden Research Laboratory, S a n Jose, California 95120-6099 (Received: April 21, 1986; In Final Form: October 13, 1986)
Studies are reported on the 25-kV electron beam exposure of poly(perfluoropropy1eneoxide), PPFPO, when enclosed in a vacuum system; contrary to electron beam exposures conducted in air, where the viscosity decreased as a function of absorbed dose, the viscous polymeric liquid solidified to a point where a film thickness could be measured by a profilometer. Concomitant with the radiation induced physical change from a liquid to a solid, a rapid mass loss of the material was observed. Specular reflection infrared spectroscopy was used to follow the electron beam induced changes and film thickness loss in PPFPO as a function of incident charge density and absorbed energy. The slope of semilog plots for the normalized thickness loss, h / b , vs. incident charge density decreased as higher molecular weights of PPFFQ were taken for analysis. In order to understand the spectroscopic analysis of the electron beam induced changes, a study was made for the reflectance of the vacuum-PPFPO-gold system. After a classical dispersion analysis was performed on PPFPO to obtain the real and imaginary parts of its refractive index, n2, and K ~ Maxwell's , equations were numerically solved on a computer to obtain the reflectance spectra for the three-layered system. The calculations not only provided insight for the reflectance measurements but enabled the electron beam induced etch rate and G value for polymer-to-monomerdegradation to be determined. A mechanism, proposed to explain the radiation induced etching and gelation within the vacuum system, is based on a vacuum assisted mass loss that depends on the pumping action of the vacuum system. In essence, the removal of low molecular weight material by the vacuum pump relaxes the conditions for gelation and permits the formation of gel even when r, the probability for scission per monomer per unit dose, is 4 times greater than c, the probability that a monomer has an active site for cross-linking per unit dose.
Introduction blood substitute^.^ The poly(perfluoro ethers) that are used Poly(perfluor0 ethers) are viscous liquids with exceptional extensively are prepared either by direct fluorination of polyetherslo chemical and ohvsical orouerties. and as a conseouence. thev are O r by Photooxidation of PerfluOroethYlene" and Perfluoroused extensively in industry as 'lubricants,' dieiectric' f l ~ i h s , ~ . ~ ,~~~ diffusion pump oils$-6 and, due to their high O2 s o l ~ b i l i t yas (7) Lawson. D. D.; Moacanin. J.; Scherer, Jr.. K. V.; Terranova, T. F.; 1
.
' .
(1) Sianesi, D.; Zamboni, V.;Fontanelli, R.; Binaghi, M. Wear 1971, 18,
85.
(2) Devins, J. C. NAS-NRC, Annu. Rep. 1977, 398. (3) Luches, A,; Provenzano, L. J . Phys. D 1977, 10, 339. (4) Holland, L.; Laurenson, L.; Baker, P. N. Vacuum 1972, 22, 315. (5) Hennings, J.; Lotz, H. Vacuum 1977, 27, 171. (6) Laurenson, L.; Dennis, N. T. M.; Newton, J. Vacuum 1979, 29,433.
0022-365418712091-1225$01 SO10
Ingham, J. D. J . Fluorine Chem. 1978, 12, 221. (8) Wessler, E. P.; Iltis, R.; Clark, Jr., L. C. J . Fluorine Chem. 1977, 9, 137. (9) Fed. Proc. 1975, 34, 1428. Fed. Proc. 1970, 29, 1695. Geyer, R. P. In Drug Design; Ariens, E. J., Ed.; Academic: New York, 1976; Vol. VIII, p
1.
(10) Gerhardt, G. E.; Lagow, R. L. J . Chem. SOC.,Chem. Commun. 1977, 259.
0 1987 American Chemical Society
1226 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Pacansky et al.
propylene.12 The structures for the polymeric materials formed during the photooxidation process are poly(perfluoroethy1ene oxide) and poly(perfluoropropy1ene oxide), hereafter called PPFEO and PPFPO, respectively. They differ, for the most part, by the perfluoromethyl group in PPFPO. CF3
I
+CF2CF2-03-
+CFCF,-O+
PPFEO
PPFPO
Due to the widespread use of PPFEO and PPFPO, an understanding of the mechanisms for degradation thermally and by high-energy radiation is required. Sianesi et al.‘ have studied the thermal degradation of PPFPO at high temperatures and noted that decomposition of the polymer commenced at 350 OC under inert conditions and at 300 OC in the presence of metals. The stable products ultimately formed are C3F6, CF3COF, and COF,; the latter two hydrolyze to H F CF3COOH and HF + CO,, respectively. The major product is COF, when the decomposition is performed in an oxygen atmosphere. Eyring and co-workers” studied the thermal decomposition of PPFPO on Ti surfaces using reflectance spectroscopy to follow the reaction; a decrease in reflectance occurred when Ti was converted to TiF3 by the thermal decomposition products of PPFPO. They concluded that the rate of Ti corrosion is enhanced by the presence of 02,and there also is a direct dependence of the rate of corrosion on the amount of TiF3 initially on the titanium surface. In essence, most of the available work on the thermal decomposition of PPFEO and PPFPO deals with final product distributions and, hence, does not directly address the mechanistic details required to understand the polymer degradation. Sianesi and co-workers,’ however, speculate that the degradation is of the free radical type; the initiation reaction, a C-C rupture, is followed by a series of @-scissionreactions to decompose the whole macromolecule with the evolution of gaseous molecules. The initial interest in the radiation chemistry of poly(perfluor0 ethers) was a result of a search for pump oils that did not leave a residue on the walls of a vacuum system when exposed to ionizing radiation. Baker and co-workers,14 followed by Holland et al.,ls performed a series of experiments that indicated PPFPO is resistant to further polymerization by electron impact (incident 1 kV, incident current density 1 mA/cm2) when voltage irradiated in either the liquid or vapor state although the parent molecule is fragmented. The conclusions are based on electron beam exposures of thick films of PPFPO that visually did not appear to gel or leave a remaining residue even after -50% of the film was vaporized by the beam. Exposures at lower but still rather high current densities (-0.1 mA/cm2) gave the same results. Mechanistic details for the effects of y radiation on PPFEO were reported by Barnaba and co-workers.I6 Viscosity analysis of PPFEO irradiated under vacuum showed that the main effect was to induce main chain scission. A value of G = 1.8 was calculated from the viscosity data. COF, was formed during the irradiation with G = 1.7, suggesting that its formation is a result of a p-scission reaction occurring after the main chain was cleaved by the high-energy radiation. The EPR spectrum of the irradiated sample consists of two parts, one of which has a higher thermal stability. This latter component consists of the -CF2-CF2-0and =F-CF,-Oradicals. The first radical decays slowly at room temperature, and the second is unchanged for extended periods.
+
-
-
(1 1) Wamell, J. L. US.Patent 3 125 599, March 17, 1964. Barnaba, P.; Cordishi, D.; Lend,M.; Mele, A. Chim. Ind. (Milan) 1966.47, 1060. Donato, A.; Lenzi, M.; Mele, A. J . Macromol. Sci., Chem. 1967, A I , 429. (12) Sianesi, D.; Pasetti, A.; Corti, C. Makromol. Chem. 1965, 86, 308. Sianesi, D.; Fontanelli, R. Makromol. Chem. 1967, 102, 1 15. (13) Chandler, W. L.; Lloyd, L. B.; Farrow, M. F.; Burnham, R. K.; Eyring, E. M. Corrosion (Houston) 1980, 36, 152. (14) Baker, M. A.; Holland, L.; Laurenson, L. Vacuum 1971, 21, 479. (15) Holland, L.; Laurenson, L.; Hurley, R. E.; Williams, K. Nucl. Insrrum. Methods 1973, I l l , 555. (16) Barnaba, P.; Cordischi, D.; Delle Site, A.; Mele, A. J . Chem. Phys. 1966, 44, 3672.
MS
Figure 1. A schematic of the experimental apparatus used to spectroscopically detect electron beam induced reactions. The upper part of the figure is a sketch of the infrared spectrometer while the lower describes the vacuum chamber and electron beam gun. Legand: NG = Nernst glower, CH = chopper, RR = reflectance reference, RS = reflectance sample, M = monochromator, CP = cryopump, FC = Faraday cup, IG = ionization gauge, SP = sorption pumps, W = window, MD = magnetic deflection section, ML = magnetic lens, AS = alignment section, BS = beam shutter, IP = ion pumps, EBG = electron beam gun.
The thermally unstable spectrum disappears in a few hours and is tentatively assigned to the -0-CF2-CF2 radical. The EPR studies reported by Faucitano et al.” on oligomers of PPFPO with acid fluoride end groups support the work reported by the Bamaba group. The mechanism consistent with their EPR results, which pertains only to the main chain of the oligomer, is cleavage of the C-O bonds and involves further decomposition of the radicals to produce COF2 and CF3CF0. Evidence for the radical decay by loss of CF3 was not observed. In this report we investigate the response of PPFPO to an electron beam in a vacuum system. These studies are conducted because of their relevance to materials research in the space industry; the low pressure of our vacuum system (lo-* mmHg) and the high energy of our incident electron beam (25 keV) simulate the conditions in space. Furthermore, our studies are also applicable to situations encountered under ambient temperature and pressure where a vacuum pumping action is inadvertently simulated; a pertinent example is poly(perflu0rinated ether) lubricants used by the electronics industry for magnetic media like dish.’ Here the high rotational speed of the disk creates a pumping action similar to a molecular turbo pump whereby molecules due to their vapor pressure are transported away by the high relative velocity of the air layer above the disk. Hence, the spinning action alone will tend to remove the low molecular weight components from the lubricant; the effect of any further stimuli on the molecular weight distribution, like thermal transients or radiation, must be considered along with the pumping action of the system. Here, we study the vacuum electron beam exposure of PPFPO; a subsequent report discusses the results obtained for exposures under 1 atm of nitrogen. Experimental Section
The main part of the apparatus consisted of a stainless steel vacuum chamber whose volume was approximately 5 5 L. A drawing of the apparatus is shown in Figure 1. The chamber was sequentially pumped down with a Teflon-vane pump, liquid nitrogen cooled sorption pumps, and an Air Products closed-cycle cryopump with an 8-in. cold surface. Pressure in the chamber (17) Faucitano, A.; Buttafava, A.; Faucitano Martinotti, F.; Caporiccio, G.; Corti, C. J. Chem. SOC.,Perkin Trans. 2 1981, 3, 425.
Irradiation of Poly(perfluoropropy1ene oxide)
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
1227
& Vacuum
Figure 3. An illustration of the three-layer system vacuum-PPFPGgold used for the dispersion analysis and the reflectance calculations. Figure 2. Top view of the double-beam goniometer: NG = Nernst glower, M1, M2 = focusing mirrors, PM = plane mirrors, RR = reflectance reference, RS = reflectance sample, fh = high-frequency chopper, fi = low-frequency chopper, TH = angle of incidence on the
reflectance sample. was typically in the 10-8-10-9-Torr range. Connected to the vacuum chamber is an electron beam gun and a quadrupole mass spectrometer (UTI Model 300C). In these studies the quadrupole mass spectrometer was only used for residual gas analysis and to detect the mass of fragments ejected from PPFPO during the electron beam exposure. A detailed study of the fragments was abandoned because the PPFPO fragments interacted with the walls of the vacuum chamber. Thus, in order to quantitatively conduct an experiment using the quadrupole mass spectromfter, the vacuum system must be reconfigured. The electron beam gun is pumped with two 20 L/s ion pumps and is capable of operating from 0 to 30 kV. The electron beam is focused with a magnetic lens and positioned and raster-scanned by using a Celco dual-ramp generator with a blanking amplifier. The path of the electron beam could be interrupted by an airoperated shutter downstream from the filament or with a set of blanking plates. The current in the electron beam was measured with a Faraday cup connected to a Keithly 480 picoammeter. The current measurement was made either by moving the Faraday cup into the path of the beam or by positioning the beam onto the Faraday cup. Also shown in Figure 1 is the location of the reflectance infrared spectrometer relative to the vacuum chamber. The spectrometer consists of a double-beam goniometer, which accurately controls the angle of incidence on the reflectance sample and reference, and a Spex Industries Inc. Model 1701, 3/4-m monochromator. A schematic of the double-beam goniometer is shown in Figure 2. The three arms on the goniometer are as follows: the first, the refocusing arm, is stationary and attached to the monochromator; the second, the 0 arm, is rotated at an angle 0 from the refocusing arm while the third, the 20 arm, is rotated at an angle 20 from the refocusing arm. Consequently, the angle of incidence may be varied 0 degrees by a synchronous movement of the 0 and 20 arms while maintaining optical alignment and focusing of the infrared source on the entrance slits of the monochromator. The IR source is mounted on the 20 arm with the high-frequency chopper,fh, and the refocusing mirror M1. The 0 arm houses the reflectance sample and reference, R S and RR, respectively, and the butterfly mirror which is silvered on one side to reflect infrared light to RR. The butterfly mirror rotates to alternatively direct the infrared radiation to the sample or reference and also functions as the low-frequency chopper,f;. The electronics for the detection system was described in a previous report.ls (18) Pacansky, J.; Home, D. E.; Gardini, G. P.; Bargon, J. J. Phys. Chem. 1911.81, 2149.
Sample substrates were prepared by vacuum-depositing a layer of chromium, followed by >2 pm of gold onto a 1-in.-diameter copper disk that was 3 mm thick. The chromium served as an adhesion layer for the gold. The substrates were mounted by bolting the disk to an OFHC copper block. Indium wire was placed between the disk and the copper block to ensure thermal and electrical contact with the sample substrate. PPFPO was applied to the gold substrates by spin-coating the neat liquid onto the substrate. Experience showed that adhesion of the sample to gold was optimum immediately after the metal was thermally evaporated; serious coating problems were frequently encountered when the gold substrates were used more than once. Electron beam exposures were performed on four PPFPO samples which differed in molecular weight. The materials were purchased from Montedison USA Inc. and are sold under the trademark Fomblin Fluids. The samples are Fomblin Y25 (viscosity-average molecular weight, M , = 3000) Fomblin Y45 ( M , = 4100), and Fomblin YR ( M , = 6500 and 7200). The sample with M , = 7200 was characterized by Cantow et al.” and found to have a weight-average molecular weight of M , = 9700 and a dispersion of M,/M,, = 1.4, where M,, is the number-average molecular weight. Hereafter, we shall refer to the respective PPFPO samples according to their M , values.
Reflectance Measurements and Calculations A complete account of the method used to determine the refractive index of PPFPO has been reported.20 Therefore, only those details pertinent for this report will be given. As an orientation for the optical parameters required to describe the specular reflection of the vacuum-PPFPO-gold system, consider Figure 3. The three refractive indexes are as follows: for vacuum, nl = 1.0, for PPFPO A2 = n2 + iK2 (1) and for the gold substrate fi3 = n3
+ i~~
In eq 1 and 2, n2 and n3 are the real parts of the refractive indexes for PPFPO and gold while K~ and K 3 are the imaginary parts, respectively. In the calculations that follow the angle of incidence for the infrared light is maintained at 14’, and h is the film thickness. The optical constants for PPFPO were individually related to the wavenumber through a dispersion analysis. Since the dispersion analysis is only performed on PPFPO,then for the sake of simplicity in notation, the subscript 2 will be dropped in the following discussion for the analysis. The refractive index for PPFPO is cast in the form of the dielectric constant as t = fi2 = c, + ic, (3) (19) Cantow, M. J. R.; Larrabee, R. B.; Barrall, E. M.; Butner, R. S.; Ting,T.Y . Makromol. Chem., in press. (20) Pacansky, J.; England, C.; Waltman, R. J. Appl. Opt. 1986, 40, 8.
Cotts, P.; Levy, F.;
1228 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Pacansky et al.
where
24-
e,
= n2 - K~
(4)
22-
and
20-
ti = 2nK (5) Using eq 4 and 5, we obtain the following expressions for n and K:
n2 = (1 /2)((t,Z K2
= (1 /2)((€;
+ ti2)ll2+ e,]
(6)
+ €i2)'/*
(7)
- tr)
18-
"2
The real and imaginary part of the dielectric constant, 6, and ti, were determined by using the classical dispersion analysis reported by Spitzer and Kleinman21and Schatz and Maeda22as given below j=N
t,
=
€0
+ c47rpjv: j=1
y.z
I
AA
16-
lo{ 08 05
+ yj2vj2v2
07
I
I
I
,
08
09
10
11
12
13
14
15
Wavenumber (cm-') x lo3
- y2
(vj2 - v2)2
I
06
(8)
and
Figure 4. The real part of the refractive index, n2, for PPFPO as a function of wavenumber, Y (cm-I), determined by the classical dispersion analysis described in the text.
14
where the summation is over the N oscillators in the system; to is the short-wavelength dielectric constant; vI is the frequency of the j t h oscillator; the damping constant, yl, is related to the bandwidth at half-height by 7) =
Av1/2/v]
K2
(10)
and pl is related to the oscillator strength by pj = FiAj/(47r3v/2)
(11)
where A] = (l/h)J-ln
( T o / T ) dv
(12)
A is the mean refractive index,21 and To/ T is the inverse of the
transmittance. To obtain n and K , it is necessary to have the short-wavelength contribution to the dielectric constant, Le., in this case the electronic contribution, the number of oscillators, and their frequency, width, and strength. The short-wavelength contribution to the dielectric was taken as the value measured at the 0.6328-pm line of a He/Ne laser. The refractive index at this wavelength is 1.3012 while at the 0.5890-pm N a line the refractive index is 1,3040; therefore, the dispersion at this short wavelength changes insignificantly for the purposes of this calculation, and the square of the refractive index at 0.6328 pm was taken as the value for eo. The process of obtaining a dispersion analysis is, in essence, an iterative process where initially the integrated absorption intensities, Aj, are obtained by using transmission spectra. Subsequently, the spectra are fit to eq 6 and 7. The reflectance in the infrared was calculated following the formalism given by Born and W 0 1 f e . ~ ~Thus, a FORTRAN computer code was written to calculate the reflectance as a function of wavenumber by using the complex refractive indexes of gold and PPFPO. In this manner reflectance spectra may be calculated for any thickness and angle of incidence. The results of the dispersion analysis for PPFPO are illustrated in Figures 4 and 5 for nz and K2, as a function of wavenumber V, respectively, for v = 1500-500 cm-l, the spectral region of interest. At frequencies higher than 1500 cm-I the refractive index is essentially the real part, Le., the electronic contribution at the He/Ne 0.6328-pm line. Since the angle of incidence for all of the specular reflection infrared spectra used to monitor the electron beam chemistry was (21) Spitzer, W. G.; Kleinman, D. A. Phys. Reu. 1961, 121, 1324. (22) Schatz, P. N.; Maeda, S . ; Hollenberg, J. J.; Dows, D.A. J . Chem. Phys. 1961, 34, 175. (23) Born, M.; Wolf, E. Principles of Optics; Permagon: New York, 1975.
Wavenumber (cm-') x l o 3
Figure 5. The imaginary part of the refractive index, K ~ for , PPFPO as a function of wavenumber, Y (cm-I), determined by the classical dispersion analysis described in the text.
maintained at 14O, the remainder of the results only pertain to this value. As shown in Figure 6, for the calculated reflectance spectra of films from 0.1 to 0.9 pm, the spectra resemble those recorded in a transmission mode; this trend continues up to film thicknesses of 1.5 pm; however, for film thicknesses greater than this, the reflectance oscillates in a very complex manner. The oscillatory nature of the reflectance is clearly exhibited in Figure 7a-d for film thicknesses from 1.0 to 4.0 pm, respectively. Inspection of the 980-cm-' band in Figure 7 as a function of film thickness indicates that the reflectance at that frequency goes through a minimum at -2.0 pm and is approaching another for h > 4.0 pm; furthermore, the series of spectra in Figure 7a-d also displays the highly damped nature of the oscillation in the reflectance in the 980-cm-' band. The explanation for the spectral changes noted in the discussion above is based on the subtle interplay between wavelength, angle of incidence, and film thickness. The changes in the spectral features with film thickness, for a fixed angle of incidence, are due to oscillations in the electric field. These oscillations are not obvious for the thin films but are quite evident for the thicker films, as shown in Figure 7. There, it is seen that the oscillation of the 980- and 1305-cm-I bands is not damped as heavily as the 1225-cm-' band. The much larger damping of the 1225-cm-' band is a direct consequence of the large value for K~ at the frequency for the band center. In Figures 8 and 9, plots are displayed for the reflectance at the band centers vs. film thickness for PPFPO to emphasize the complex changes occurring in the spectra. Figure 8, the plot for the 740-, 805-, and 980-cm-' band centers, offers a comparison between the relatively weak features at 740 and 805 cm-I and the much stronger one at 980 cm-I while Figure 9 is a comparison for the stronger features at 1120, 1178, 1225, and 1305 cm-I. In general, the reflectance as a function of increasing
-
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 1229 1000
-
0875
-
0750
-
‘”
0625 -
0o j625 oo[
0 500 0375
-
0 375
0125 -
0 250
0 250
0 000
0O 000 lZ5
1 000
1000
0 875
0 075
0 750
E
1
1
1
0 750
m
0625
0625
-Y 0 5 0 0
I
;”
d
0375
0 250
0500 0 375
0 250
0 125
0 125
0 000
0 000
1000 0875
-
0750
-
0625
-
0 500
-
0 375
-
0250
-
0 250 O 375 0 125
i1
1
0 O 000 lZ5 05
06
07
08
09
10
11
12
13
14
15
05
06
07
08
09
10
1 1
12
13
14
15
Wavenumber icm”) x l o 3
Wavenumber (cm” j x103
Figure 6. The calculated specular reflection spectra for films with thicknesses (a) 0.1, (b) 0.3, (c) 0.4, (d) 0.5, (e) 0.7, and (f) 0.9 pm (0 = 1 4 O ) .
film thickness reaches a minimum for a thinner film thickness as the magnitude of K 2 increases; concomitant with this is that further oscillations in the reflectance with film thickness are more heavily damped. The optimum spectral feature for following the electron beam induced changes in PPFPO is the 980-cm-’ band. It is separated from the other bands and involves vibrational modes associated with the CF3 group. If PPFPO does not suffer a loss of film thickness upon exposure to a high-energy electron beam, then it is easily shown that this band may be used to quantitatively record the decrease in the number of CF3groups. The situation is more complex, but tractable, when the optical constants are known for the system. The plot for In (&/R) vs. film thickness (a reflectance optical density plot) shown in Figure 10 is not linear but increases with film thickness, goes through a maximum at h = 1.8 pm, and then oscillates as discussed above. The functional form for the reflectance from h = 0 to 1.8 pm closely approximates an exponential. Thus, an etch rate induced by a high-energy electron beam as measured by reflectance infrared spectroscopy will appear faster than the actual value because of the nonlinear relation between specular reflection and film thickness; this will be discussed below in more detail. Two additional details must be discussed. Both are in reference to the 980-cm-I band and hence are associated with the limitations on the film thickness loss measurements. The first involves the changes PPFPO experiences as a result of the exposure, Le., the change from a viscous liquid to gel, and the extent t o which it effects the refractive index in the infrared. This has a negligible
effect on the refractive index of PPFPO because, as shown below, all of the IR bands decrease as a function of the irradiation, and hence no new oscillators must be introduced into eq 8 and 9; the result is that the refractive index changes very little during the exposure. In an investigation into the problems associated with relating band intensities from specular reflection IR to concentration (or film thickness) for kinetic studies,24it was shown that the analysis must be modeled and corrections made when the band centers of two oscillators are separated by several wavenumbers and one appears while the other decays from the spectrum. The other concern over the 980-cm-’ band center is the uncertainty in the film thickness measurement via specular IR reflection. In Figure 8 the results of measuring the thicknesses of several films are shown on the calculated reflectivity vs. film thickness plot. The agreement is good and within about 4~8%. We expect similar uncertainty in the measurements over the film thickness range studied.
Dosimetry Electron beam exposures within a vacuum system facilitate the measurement of the current density I (A/cm2) of the incident beam. The product of Z and time t gives q, the incident charge density (C/cm2) administered to a sample. Due to the relative ease with which q is measured, many authors report the response of a system as a function of this value. In this report we also use (24) Pacansky, J.; England, C. J . Phys. Chem. 1986, 90, 4499.
Pacansky et al.
1230 The Journal of Physical Chemistry, Vol. 91, No. 5. 1987 1.000
-.
0.875 0.750 -
.$ 0.625 .. d
-
-
-
0.500
0.375 -
0.375 -
0.250 -
0.250' -
c
2
-
0.500 -
0.125 0.000
-
0.125 - (b)
(a)
'
0.000
'
'
1.000I 0.875 1
I
'.O0O
'
I
I
'
'
1
1
I
I
I
0.750 0.625 0.500 0.375 0 250
-
0.125 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5
Wavenumber
x103
(cm")
1
0.000-
i
(d)
'
'
'
Wavenumber
(cm-') x103
Figure 7. Calculated specular reflection infrared spectra for film thicknesses (a) 1.0, (b) 2.0, (c) 3.0, and (d) 4.0 pm (0 = 14').
01251
,
, - , 0 00
0 000 00 04
08
12 16 20 24 28 Film Thickness i p m )
32
36 40
Figure 8. Reflectivityvs. thickness for the bands centered at 740, 805, and 980 cm-' (0 = 14"): calculated (-); experimental (e). 1000 0 075 0 750
8
0625
C
+ L?
0 500 0375 0 250 0 125
_ _ _0 0
n - nnn
04
08
12 1 6 2 0 2 4 28 Film Thickness [ p m )
32
36
40
Figure 9. Reflectivity vs. thickness for the bands centered at 1120, 1178, 1225, and 1305 cm-' (0 = 14").
incident charge density and, in addition, list the current density. The latter value is given so that our results may be compared with others using a different I to avoid confusion that may arise from a dose rate effect. The incident charge density is used to determine Q,the charge density that the thin film experiences from the incident beam and
00
Figure 10. In (I?/&) (0 = 14")
02
04
08 08 10 12 14 Film Thickness (bm)
16
18
20
for the 980-cm-I feature as a function of thickness
that fraction of the beam backscattered from the substrate. The charge density is defined as Q = (1 + v k (13) where 7 is the backscattering coefficient of the substrate. All of our experimental results are reported in terms of Q because it is a more precise measure of the number of electrons traversing a thin film. The incident dose (eV/cm2) delivered to the sample may be obtained by taking the product of q and E,,, where Eo is the incident beam voltage. We prefer to report also our results in terms of absorbed energy (eV) because in this form the results are useful in the radiation chemistry literature. Therefore, the response of PPFPO to a 25-kV electron beam as a function of absorbed energy is E, (in eV), where E, = E,f Eab (14) E,f is the energy absorbed from the beam in the forward direction, and E,b is the energy absorbed from the beam backscattered from the substrate. The energy deposited into a free-standing film traversed by an electron beam was obtained by using the wellknown Grun25 curves for energy dissipation of a beam as it
+
( 2 5 ) Griin, A. E. 2.Naturforsh., A : Astrophys., Phys. Phys. Chem. 1957, IZA, 8 9 .
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987 1231
Irradiation of Poly(perfluoropropy1ene oxide)
25
dWdZ
20
-
15
-
10
-
E(z)
5 -
'
'
01 I I I I 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
I
'
I
0.8 0.9 1.0
zbm) 6
I
I
I
I
01
02
03
04
I
I
I
I
I
I
I
z(m)
Figure 11. The depth-dose plot for the rate of energy dissipation d E / d z and the energy of the electron beam E ( z ) as a function of z for PPFPO ( p = 1.094 g/cm3, Eo = 25 kV,RG = 6.714 pm). See eq 16 for details.
penetrates into a solid. Everhart and Hoff26 obtained a polynomial fit for the normalized depth-dose function
A y ) = 0.6
+ 6.21f
- 12.4f2 + 5.69y
(15) I
Here f = z/RG, A y ) = d(E/Eo)/df, and
00
R, = (0.049/p)Eo'~7s
(16)
where & is the Griin range in pm, Eois the incident electron beam voltage in kV, and p is the density in g/cm3. The rate of energy dissipation in the film as a function of z per incident electron is readily obtained from the normalized depthdose function by using dE/dz = - ( E o / R G ) A ~ )
(17)
Equation 17 is plotted for PPFPO as a function of z in Figure 11 ( p = 1.915 g/cm3, Eo = 25 kV, RG = 6.71 pm). The energy E of the beam a t a depth z in the film is obtained by obtaining the area under the curve shown in Figure 11; thus E(z) = Eo - I ' 0( d E / d z ) dz
(18)
A plot for E(z) as function of z is also given in Figure 11. After a slight rearrangement of eq 18 one obtains the quantity (1.0 E/Eo) which directly gives the fraction of the incident energy absorbed in a film of thickness z. Since films 1 pm thick are used in this study, we show in Figure 12 plots for dE/dz and E as a function of z for a 1.O-pm-thick film. Due to the high density of PPFPO, the energy dissipation is not constant across the film, but as shown in Figure 12 only 15% of the energy is absorbed by the 1.0-pm-thick film. Utilizing the measured incident dose and the Grun relations, we determined the energy absorbed in a film supported by a substrate from the following expression
-
-
where h(t) is the film thickness (in cm) at time t , Eois the incident beam voltage (in V), I is the incident beam current density (in A/cm2), and q is the coefficient for electron backscattering from (26) Everhart, T. E.; Hoff, P. H. J . Appl. Phys. 1911.42, 5837.
05 06 z(ctm)
I
I
I
07
08
09
I
10
Figure 12. The rate of energy dissipation d E / d z and E ( z ) as a function of z for a PPFPO film ( E , = 25 kV, thickness 1.0 pm).
the substrate. As shown in a previous report,27the value for q may be taken as that for the bare substrate as long as the film thickness is not more than -20% of the electron range. For gold q = 0.513. The first term on the right-hand side of eq 19 is the energy absorbed from the beam in the forward direction, and the second term is the energy absorbed from the backscattered beam.
Electron Beam Exposure Thin films of PPFPO were spin-coated onto gold substrates for exposure to a 25-kV electron beam with a current density of 0.17 pA/cm2. The irradiation was followed by specular reflection infrared measurements; a spectrum was recorded before exposure to define the initial reflectance, Ro, and R, the reflectance after a time period t required to administer a charge density Q (see eq 13) to the sample. The infrared spectra recorded for PPFPO as a function of Q all exhibited the same trend, regardless of molecular weight; all of the bands disappeared as Q increased. Eventually, a charge density was reached at which the PPFPO spectrum was barely detectable. Semilog plots for R/Ro against charge density were constructed by following the changes in reflectance of the 980-cm-' band. The results of the exposures for a viscosity-average molecular weight of 6500 are shown in Figure 13; similar plots are obtained for viscosity-average molecular weights of 7200, 4200, and 3000. The initial film thicknesses, or the film thicknesses after any Q, were obtained by matching the observed reflectance of the 980-cm-' band with the calculated value shown in Figure 10. The slope for each plot was taken as the best straight line back to zero charge density. The values thus obtained are 0.037 cm2/pC for M, = 7200,0.043 cm2/& for M , = 6500,0.047 cmZ/pC for M, = 4100, and 0.056 cmZ/pC for M , = 3000. Concomitant with the rapid decrease in the infrared spectrum of PPFPO, a large increase was observed in the vacuum system pressure when the sample was exposed to the 25-kV electron beam. Since these observations are indicative of an electron beam induced (27) Pacansky, J.; Waltman, R. J. J . Radial. Curing 1985, 12, 6
1232 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Pacansky et al.
0.5 l'O
0.5 0.6
~
0
8
16
24
32
40
48
56
64
72
80
Charge Density (pC/cm')
0.1 I
'
'
20
10
30
40
60
50
I
70
Charge Density (pC/cm*)
Figure 13. The semilog plot for R/Ro vs. charge density (pC/cm2) for PPFPO ( M , = 6500, initial film thickness 0.92 pm, incident beam voltage 25 kV, current density 0.17 pA/cm2). The slope of the line is 0.043
cm2/pC. IC
AI
05
A
OE
A
A
07
A
0.f
A A
OE
A
04 h/hi
0.: unexposed
"L:",H'd
Area
PPFPO
~
0.2
Gold Substrate
0.1
10
-
t 0 56pm
20
30
at the surface of the gold substrate and then moves toward the PPFPO film. The initial film thickness, h, = 1.33 pm, is found by comparison of the observed reflectance of the 980-cm-I band to the calculated values in Figure 10. The final film thickness, that is, the thickness after the PPFPO was exposed to a dose of 63 pC/cmZ, may again be determined by comparison of observed and calculated reflectances or taken from the profilometer measurements. The former value is hf = 0.65 pm while the latter is 0.56 Fm, and hence, both values are in excellent agreement. If we assume that the film thickness loss is exponential, which will be justified below, then using h, = 1.33 pn and hf = 0.56 pm we obtain a value for the etch rate using the following expression
hf = h,e-keL?
Film
1
Figure 15. Semilog R / R , vs. charge density Q plots obtained from the computer model for specular reflection described in the text. The reflectance was calculated by starting at the initial film thickness (see Figures 15-18) for each molecular weight and then assuming an exponential decay for the film thickness loss. The process was iterated until each simulated plot matched the experimental one: (a) M y = 7200, k,, = 0.0130 cm2/pC; (b) M , = 6500, k, = 0.0136 cm2/pC; (c) M, = 4100, k , = 0.0160 cm2/pC; (d) M, = 3000, k,, = 0.0182 cm2/pC.
40
50
60
Charge Density (pC/cm')
Figure 14. A semilog plot for the normalized thickness loss h / h , vs. charge density Q for PPFPO ( M , = 6500). The results of a tally step film thickness measurement are also included for PPFPO after it was exposed to a 25-kV electron beam (current density 0.17 pA/cm2). The sample was covered with a copper mask with a 1-cm2opening to define the exposure. The step at the left of the figure defines the edge of the mask and hence divides the unexposed from the exposed area. The initial film thickness was 1.33 pm (calculated), and the final film thickness obtained by the profilometer measurements was 0.56 f 0.04 pm; the final film thickness obtained by the specular reflection IR experiments and calculations was 0.65 f 0.05 pm.
mass loss, a number of experiments were conducted to determine the extent to which the sample was being etched away. Results exemplifying these are shown in Figure 14. PPFPO, of M , = 6500, was exposed to the 25-kV electron beam with a current density of 0.17 pA/cm2. The sample was covered with a copper mask with a 1-cm2 opening to define exposed and unexposed regions of the sample. After the PPFPO was exposed to a charge density of 63 pC/cm2, the sample was removed from the vacuum system and washed with Freon solvent. PPFPO on the areas of the substrate not exposed to the electron beam dissolved in the Freon solvent; PPFPO in the area exposed by the beam was insoluble in Freon leaving behind the open image formed by the mask. In Figure 14 the trace recorded by the tally step profilometer is shown. To the left of the figure the trace commences
(20)
where k,, is the rate constant for the electron beam induced thickness loss in units of cm2/pC and Q is the charge density defined by eq 13. By use of the initial and final film thicknesses noted above k,, = 0.0137 cm2/pC. In order to justify an exponential form for the etch rate, first consider that the charge density plot for the example in Figure 13 contains a contribution to the reflectance due to the film thickness loss, which is different for different h,. As a consequence of this, the R/& vs. charge density plots cannot be fit to an obvious function. Alternatively, the experimental semilog R/Rovs. charge density plots were simulated by using the reflectance calculations discussed above. An exponential function was used in the simulation to reduce the film thickness according to the Q required to etch the films experimentally. Since all of the experiments used the same current density, the charge density was identical for all of the experiments. The etch rate constants were varied until the simulated plots were identical with the observed plots. These results, displayed in Figure 15, reproduce the experimental In ( R / R o )vs. charge density plots when k,, = 0.0182, 0.0160, 0.0136, and 0.0130 cmZ/pCfor M, = 3000,4100,6500, and 7200, respectively. Further support for an exponential film thickness is obtained by the shape of the calculated R/Roplots when a linear, quadratic, or exponential etch rate is used. The results of this calculation are contained in Figure 16 and clearly show that the linear and quadratic functions do not reproduce the experimental results as well as the exponential function. The most compelling evidence for an exponential film thickness loss is contained in Figure 17, for example, for M , = 6500 where the semilog plot for the normalized thickness loss as a function of charge density is shown. The h / h , values were obtained by matching the experimental reflectance for the 980-cm-' band with the calculated reflectance vs. thickness plot in Figure 10. The normalized thickness vs. charge density plots reveal that an exponential function is reasonable for all molecular weights and
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Irradiation of Poly(perfluoropropy1ene oxide)
1233
1.0
0.9 0.8
0.7 h/h,
0.6 0.5
0.4
0.3 0.2 0.1
0.0 0.00
1.00
0.50
1.50
2.00
2.50
Energy Absorbed ( e V x 10”)
Charge Density (pC/cm2)
Figure 16. Calculated semilog R/Rovs. charge density Q plots for (a) an exponential, (b) a linear, and (c) a quadratic film thickness loss. For each case k,, = 0.0128 cm*/gC.
Figure 18. Normalized thickness loss vs. absorbed energy E, plots for PPFPO. E, is in units of eV and was determined by eq 18 in the text. TABLE II: G Values for Monomer Formation and Complete Decomposition of a Polymer Molecule G for G for av deg of monomer polymer M, polymerization formation decomposition 7200 6500 4200 3000
43 39 25 18
23 24 28 32
0.5 0.6 1.1 1.8
molecules decomposed per 25-keV electron. The values contained in Table I are very large and reveal the extensive damage induced by the electron beam exposure of PPFPO in a vacuum system. Due to the rapid loss in film thickness, the efficiency of the electron beam chemistry, in the form of a G value, is determined by 100(hi - h)pNo
G=
01
0
10
20
30
40
50
60
70
Charge Density (pC/cm2)
Figure 17. Normalized h / h i semilog plots for PPFPO as a function of charge density Q. PPFPO molecular weight M, = 6500. The initial film thicknesses are hi = 0.57 ( O ) , 0.93 (A)and 1.33 pm (W). TABLE I: Efficiencies of the Electron Beam Induced Chemistry Based on M . (See Eq 13) no. of monomers no. of monomers/ no. of polymers/ M , for 0 = 10 uC/cm2 incident electron incident electron 7200 6500 4200 3000
8.48 8.83 1.03 1.15
X X X X
10l6 loL6 10” lo1’
1356 1413 1648 1840
31.5 36.2 65.9 102.2
-
initial film thicknesses. In addition, they also uncover a film 1.0 pm. Also for thickness dependence which commences at h films with thicker hi, a shoulder a t charge densities near zero followed by a slight downward curvature is observed. This is consistently observed for films with initial thicknesses greater than 1 pm. Efficiency of the Electron Beam Chemistry The plots for the film thickness vs. charge density Q quantitatively relate film thickness loss to the number of incident plus backscattered electrons. This value is readily converted to the number of monomer molecules (-CF(CF,)CF,O-) produced per 25-keV electron; these are listed in Table I as a function of initial viscosity-average molecular weight. The values were calculated by determining the mass loss induced by 10 wC/cm2. The values in the second column of Table I are the number of monomer molecules produced for Q = 10 pC/cmZ. Those in the third column are the number of monomer molecules produced per electron while the last column lists the number of polymer
EaM
where G is defined as the number of monomeric molecules produced per 100 eV of absorbed energy, hi is the initial film thickness, h is the film thickness remaining after receiving an absorbed energy E,, M is the monomeric molecular weight, No is Avogadro’s number, and p is the PPFPO density. E, was determined via eq 19 by substituting eq 20 for h ( t ) . In Figure 18, we show the normalized thickness loss as a function of E , for each initial viscosity-average molecular weight. The curves are all linear down to 30% of the initial film thickness, after which a slight curvature is found. The slopes for the h/hi vs. E, plots correlate with those for the h/h, vs. Q plots; that is, they all increase with lower initial viscosity-average molecular weight. However, unlike the exponential h/hi vs. Q plots, these are linear over a broad range. We should note that thickness losses for most of the samples reported here were not measured for h/hi C 0.30. For thickness losses less than a few tenths of a micron our infrared experimental apparatus lacked the sensitivity required to detect the small changes is specular reflection. The G values for monomer formation were found by converting mass loss to the number of monomers produced for a given E,. They are listed in Table I1 and appear to be large. If we divide these values by the average degree of polymerization, p , then the G value for complete decomposition of a polymer molecule is obtained. These latter values increase from G 0.5 for M, = 7200 to G 1.8 for the lowest initial viscosity-average molecular weight M, = 3000. The G value for polymer decomposition may also be defined as the G for main chain scission. Hence, this latter G value when defined as a main chain scission process does not appear to be unusual because it is about an order of magnitude less than the G values for main chain scission of poly( 1-butene sulfone) and poly(1-hexene sulfone),28 which are 12.2 and 10.7, respectively. The unusual aspect of the PPFPO system is that
-
-
-
(28) Schnabel, W. Polymer Degradation; Macmillan: New York, 1981.
1234 The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Pacansky et al.
the G values increase as the initial molecular weights decrease. This immediately suggests that another physical or chemical process is aiding the radiation induced events.
Discussion Irradiation of thin films of PPFPO by a 25-kV electron beam while under vacuum simultaneously induces a loss in film thickness and a gradual change from a liquid film to a gel, and finally to a solid. The solid film that is formed during the irradiation has sufficient hardness to have its thickness measured by a stylus profilometer and also is insoluble in the Freon solvent used to initially cast the film. When the irradiation is followed with a quadrupole mass spectrometer attached to the vacuum system, relatively large fragments, up to the mass of two monomer units, are detected; in concert the specular reflection infrared measurements show that the material is being etched away by the electron beam. In addition, the IR measurements also reveal that the loss of material approximates an exponential function, and when the initial film is more than 1 pm, a small shoulder appears in film thickness vs. dose plots. A plausible explanation for the observed mass loss, and perhaps of other chemical and physical changes observed during the irradiation, is heating of the film by the electron beam. In a previous reportz9we addressed this problem by modeling our system using the Fourier heat equation. The results of these studies showed that the temperature rise as a result of energy dissipation from the electron beam was 0.0001 K for a beam with an incident current density equal to 0.1 pA/cmz and an accelerating voltage equal to 25 kV. Beam heating, for the experimental conditions used in these studies, becomes a problem when I 1 mA/cmz. Since all of the PPFPO exposures reported above use I = 0.17 pA/cmZ,we conclude that heating effects are negligible and, hence, the mass loss and other effects reported herein are not due to thermally induced events. The ultimate response of a polymeric system to high-energy radiation usually manifests itself as a change in molecular weight. As elegantly discussed in some monographs, for example, by Charlesby30 and by S h a p i r ~increases ,~~ in molecular weight are associated with a radiation induced chemical event that cross-links polymer chains while scission of polymer chains decreases molecular weight. When a polymer only cross-links, a gel point is reached at which the molecular weight is infinite. If both cross-linking and scission reactions are concurrently induced by high-energy radiation, Charlesby30 and S a i t have ~ ~ ~shown that gel formation will still be observed but only when the fraction of scissioned monomers per unit dose does not exceed the fraction of cross-linked monomers per unit dose by a factor of 4. Hence, from the observations stated above it appears that the mechanism for the vacuum electron beam exposure of PPFPO involves simultaneous cross-linking and main chain scission of the polymer chains. Cross-linking is required because the material gels, solidifies, and becomes insoluble in the Freon solvent. Likewise, main chain scission occurs because of the extensive film thickness loss and fragmentation observed during the exposure. The pertinent details for the vacuum electron beam exposure of PPFPO, such as the G values for cross-linking and scission, may be obtained by studying molecular weight distributions and dose to gel p ~ i n t , ’ ~but , ~due ~ to the penetration depth of the 25-kV electron beam only 1-pm-thick films could be uniformly irradiated. Since a 1.O-cm2 area was exposed, the quantity of material available g, thus rendering analysis very diffor analysis was only ficult. In spite of this the experiments presented in this report provide enough information to outline the details for the crosslinking and scission reactions. There are two possible explanations for the rapid film thickness loss which commences after main chain scission. The first is N
N
N
-
(29) Pacansky, J.; Maier, M.; Fromm, J. Ultramicroscopy 1985, 26, 81. (30) Charlesby, A. Atomic Radiation and Polymers; Pergomon: London, 1960. ( 3 1) Shapiro, A. Radiation Chemistry of Polymeric Systems; Interscience: New York, 1962. ( 3 2 ) Saito, 0. J . Phys. SOC.Jpn. 1958, 13, 198.
Molecular Weight ( x 1000)
Figure 19. The plot of electron beam induced etch rate of PPFPO as a function of molecular weight. The following exponential function fits the data, k,, = 0.023 e-* 0 5 x 1 0 - 5 M v , where M, is the PPFPO number-average
molecular weight. unzipping of the polymer chain to monomers; the second does not require extensive unzipping of the chain but depends only on the vapor pressure of the fragments produced when electron beam impact fractures the main chain. Unzipping of the polymer chain is characteristically observed for vinyl polymeric materials, a condition which PPFPO does not apparently meet; however, it is possible that after the main chain is broken perfluoroformaldehyde and perfluoropropylene are released by a subsequent scission of a bond p to the radical center on the end of the fragmented chain.” An unzipping mechanism, however, is not required to explain the film thickness loss because the vacuum environment has a profound effect on the electron irradiation of PPFP0.33 Therefore, the vapor pressure scheme is favored on the basis of the ensuing discussion. In Figure 19 a semilog plot is shown for the electron beam induced etch rate, k,,, as a function of PPFPO molecular weight. The data accurately fit the exponential function k,, = 0,023@ oX10‘5Yv
(22)
where M, is the viscosity-average molecular weight of PPFPO. One should note that, as described by eq 21, k,, decreases exponentially with increasing molecular weight. Furthermore, the vapor pressure34 of PPFPO is also an exponential function of molecular weight. The molecular weights used in this study ranged from M , = 3000 to 7200. The vapor pressure34for a molecular weight of 7200 is close to 1O-Io mmHg, while for a molecular 10” mmHg, which is very weight of 3000 the vapor pressure is close to the operational vacuum system pressure of our apparatus. In fact, attempts to study the electron beam exposure of M , = 1500 (Fomblin Y04)in our system all failed because the material was pumped away before the sample could be exposed. The vapor mmHg; conpressure for a molecular weight of 1500 is sequently, it is reasonable to conclude that fragmented polymer chains with a molecular weight of 5 1500 are pumped away in our vacuum system. Therefore, in the initial M, = 3000 only one main chain scission, on the average, is needed to reduce the molecular weight to a point where the vapor pressure is too high to remain in the vacuum system. The number of scissions needed to reduce a larger initial molecular weight to smaller fragments that are removed by the pumping action of the system certainly increases with molecular weight. The mechanism for the vacuum pumping assisted electron beam induced etch rate of PPFPO discussed in the above paragraph is consistent with a main chain scission reaction only. However, since PPFPO forms a gel at a relatively low dose, the system must have a finite G value for cross-linking and, as determined from the etch rate studies, also has a high G value for mass loss. Charlesby30 and S a i t ~have ~ ~ shown that if the scission-tocross-linking ratio is greater than four, gel formation will not occur. We therefore concluded that some computer modeling was justified to investigate the effect of the vacuum environment on the electron beam exposure. Due to the length of the calculation, only some
-
N
(33) Pacansky, J.; Waltman, R. J., results to be published. (34) Fomblin data sheet on vapor pressure, Montedison USA Inc. (1 114 Ave. of the Americas, New York, NY 10036).
The Journal of Physical Chemistry, Vol. 91, No. 5, 1987
Irradiation of Poly(perfluoropropy1ene oxide)
10-0'1
0
I
I
10
20
p (degree of polymerization)
240
pertinent results are given here; the details are given in a subsequent report.33 S a i t has ~ ~shown ~ for any initial molecular weight distribution the distribution function after exposure to a charge. density Q, where only main chain scission occurs, is n(p,T) =
= rQ
(24)
Here r is the probability a monomer is scissioned per unit charge density. The main chain scission and cross-linking processes are statistically independent events, and hence the latter process may be accounted for by starting with a distribution n(p,T). Therefore, the number- and weight-average molecular weights as a function of dose after main chain scission and cross-linking are determined by the following expressions
Mn,fo,o M, = fo.0 -
'
60
I
70
I
80
'
90
I
100
P
I
I
L
2101 180
I5O
2 120
A
(23)
where n(p,O)is the number of polymer molecules with degree of polymerization p at Q = 0, and 7
I
50
Figure 21. A semilog plot for N Q / N Ivs. charge density Q for p , = 43, 25, and 18. The values were calculated by removing those molecules with p < 9 formed by the radiation induced scission process.
cm2/gC.
+ T 2 1P m d pl mP n ( p , O )dp1e-P'
'
40
Charge Density (!.fC/cm')
Figure 20. The effect of main chain scission on the initial molecular weight distribution. A generalized Poisson distrib~tion~~ was used for n(p,O) (see eq 22 in the text). The charge density Q (gC/cmz) required to alter the distribution is given in the figure. The initial M , = 7200, the average degree of polymerization is 43, and r = 0.0028 X
(n@,O) + 2rXmn(p,O)dp
'
30
t
and
0
10
20
30
40
50
60
70
80
90
100
Charge Density (pC/cm')
Figure 22. A plot for 1/(2f2,J and t vs. charge density Q for p = 43, r = 0.0028 X cm*/gC, and r / c = 3, 4, and 5, for curves a, b, and c,
respectively. The crossing of a dashed and solid line indicates the dose required to reach the gel point. molecular weight distribution of polymer molecules. Figure 20 contains the calculated results for the changes in a molecular weight distribution as a function of charge density Q; here, a Poisson d i s t r i b ~ t i o nwas ~ ~ used for n(p,O), the average degree of polymerization, p , was 43, the dispersion M , / M , was 1.4, and r was set equal to 0.0028 cm2/FC. This particular value for r was necessary for the computed results to approximate the observed radiation effects. (As discussed in the Experimental Section, the number-average molecular weight for the M , = 7200 sample is M , = 7030. Hence, for the remainder of this section was shall set M, M,,.) As shown in Figure 20, the initial Poisson distribution is rapidly converted to an exponential, random distribution, and as a consequence, the distribution mwes toward a system with a lower p . This calculation was repeated, but in order to simulate the pumping action of a vacuum system, molecules with p < 9 were removed from the distribution as they were formed. These results are contained in Figure 21 in the form of a semilog plot for NQ/NIvs. dose where Nl is the initial number of monomers in the system and NQ is the number of monomers after a charge density, Q. The fraction N Q / N Iis the amount of material remaining in the system as a result of the combined action of the exposure and pumping action of the vacuum and hence is related to the experimental normalized thickness loss. The salient feature of the plots in Figure 21 is that their slopes increase as f~decreases from 43 to 18, which is in accordance with the experimental results. The simulated etch rate studies are accompanied by a relaxation of the Q required for gelation. The Q required to reach the gel point is defined by the denominator in eq 26, Le., as 1 - 2f2,0t approaches zero, M , becomes very large. Thus, a plot of 1/(2f&,) and t against Q when r / c > 4 is a set of parallel lines, whereas for r / c < 4 the lines are not parallel and cross at the Q required
-
The quantities M,,,, M,,,, fo,o, and f2,0 are defined as
Mn, = M/fo,o M W . 1
fo,o =
=
M/f2,0
1
-!@ ! N
dp
and
N is the total number of monomers, and t = cQ
1235
(31)
where c is the probability that a monomer has an active site for cross-linking per unit charge density, M,,' and MW,7are the number- and weight-average molecular weights after scission, and M is the monomer molecular weight. Equation 23-3 1 thus provide a set of formulas to model the combined effects of radiation induced scission and cross-linking on an arbitrarily chosen initial
(35) Inokuti, M.; Katsuura,
K. J . Phys. SOC.Jpn.
1959, 1 4 , 79.
J . Phys. Chem. 1987, 91, 1236-1241
1236
for gelation. The gel dose for the vacuum exposure of PPFPO was simulated by determining the combined effects of scission and cross-linking on an initial Poisson distribution. In addition, molecules with p < 9 were removed from the system as they were formed by the irradiation. The pertinent quantities l/(2fz,o) and t were computed for a range of r / c values defined by the following: r = 0.0028 cmZ/pC and c = r/3, r/4, and r / 5 . The results for this investigation are contained in Figure 22. The line for l/(2fz,o) is independent of the r / c value because it only depends on the scission probability; the line for t , of course, changes with c. For a r l c = 3 and 4, we find a gel dose at -22 and 38 pC/cm2. A gel dose is also found for r / c = 5 at -54 pC/cm2. In contrast to this when the same calculation was repeated for the case where no molecules were removed from the system, the 1/(2f2,,) and t lines did not cross the r / c = 5 and r l c = 4 within the range of Q studied. Thus, the condition for gelation, that is r / c < 4, is relaxed for the vacuum exposure of PPFPO. This is primarily due to the removal of low molecular weight material from the film by the vacuum system, and it is this effect that is responsible for
the rapid etch rate and gelation of the system. Hence, we conclude that removal of molecular fragments from a system under irradiation has a profound effect on the radiation induced physical properties of the system. Concluding Remarks and Summary
The radiation induced mechanisms operative when PPFPO is exposed to a high-energy electron beam within a vacuum are influenced by the pumping action of the vacuum system. Due to main chain scission reactions, lower molecular weight polymers are produced with vapor pressures high enough to be removed by the pumping action of the vacuum system. The net result of the removal of material by the vacuum system is to produce an etch rate that decreases with increasing initial molecular weight. In addition, the removal of lower molecular weight materials from the irradiated PPFPO films inhibits the lowering of the average molecular weight and in effect permits gel formation when the ratio r / c > 4. Registry No. PPFPO, 25038-02-2
35CINQR Study of Molecular Motion of Polycrystalline Insecticide p ,p’-DDT. 1. Modulation Effects in Temperature Dependence of ”CI NQR Spin-Lattice Relaxation Time in p,p’-DDT Boleslaw Nogaj’ Institute of Physics, A. Mickiewicz University, Grunwaldzka 6, 6 0 - 780 Poznafi. Poland (Received: April 28, 1986)
Investigation of 35ClNQR spin-lattice relaxation times (TI) was performed for the polycrystalline insecticide p,p’-DDT (CC1,CH(C6H4C1),) at temperatures ranging from 77 K to the melting point of the compound (380 K). A change of conformationof two phenyl rings (at 165 K) and hindered rotations of phenyl rings and CC13groups were found. The modulation minima in the temperature dependence of T , were ascribed to hindered rotations of two dynamically nonequivalent groups of phenyl rings. The contribution of individual relaxation mechanisms in TI was discussed. The activation energies for individual hindered rotations were determined from temperature dependences of the relaxation time TI as well as from the shift of the modulation minima.
I. Introduction The activity of insecticides has been found to depend on the shape of the molecules and on the charge distribution within it. DDT-type insecticides have molecules shaped like wedges. The top of the wedge is made of the groups -CC13 in DDT, -CHCl2 in DDD, or =CC12 in DDE, while its base is phenyl rings. Charge distribution in a wedgelike molecule is essential. Holan and Spurling’ have shown that with increasing total negative charge at the top of the wedge the lethal dose of the compound (LD,,) decreases. Chlorine atoms that occur at characteristic sites at both the top and the base of the wedge allow NQR investigation on the 3sCl isotope to be performed. Thus, the measurement of 35ClNQR frequency provides information on charge distribution in a molecule of the insecticide. As far as one could find, the only literature data on 3sClNQR studies are the resonance frequencies for p,p’-DDT measured at 77 K given in Bray’s work.z In our paper3 the 3sCl NQR studies of insecticides were developed on p,p’-DDMU, p,p’-DDE, and p,p’-DDD. This paper is a continuation of these studies. We have concentrated our interest on p,p’-DDT, the insecticide that has been known for 11 1 years and has become a standard. This compound has won a distinguished place in the history of world agriculture. Present address: Institut d’Electronique Fondamentale, Labratoire associt au CNRS, BBtiment 220, UniversitC Paris XI, 91405 Orsay Cedex, France.
0022-365418712091-1236$01 .50/0
The application of DDT in the period 1955-1972 in 117 countries resulted in complete suppression of malaria and averted the danger of this disease from more than 1 billion p e ~ p l e . ~Despite the fact that this compound has been replaced by other insecticides, it still remains a standard for a large group of insecticides, especially DDT-like. Though many attempts have been undertaken to explain the insecticidal activity of p,p’-DDT, no definite answer has been obtained. According to some of the t h e ~ r i e s the , ~ insecticidal activity of this compound is associated with the probable rotations of phenyl rings. The aim of this paper was to study the possible reorientations of atomic groups. A molecule of p,p’-DDT alone is interesting for a physicist who can expect the possibility of reorientations of phenyl rings as well as CC13groups on the basis of its structure. Studies of nuclear quadrupole spin-lattice relaxation times measured on chlorines at phenyl rings as well as on chlorines of CCl, groups should prove interesting, and it is the subject of this paper. (1) Holan, G.; Spurling, T.H. Experienrin 1974, 30, 480. (2) Bray, P. J. J. Chem. Phys. 1955, 23, 703. (3) Nogaj, B.; Pietrzak, J.; Wielopolska, E.; Schroeder, G.; Jarczewski, A. J . Mol. Strucr. 1982, 83, 265. (4) Metcalf, R. L. J. Agric. Food Chem. 1973, 21, 511. (5) Riemschneider, R.; Otto, H. D. 2. Naturforsch., B: Anorg. Chem., Org. Chem., Biochem., Biophys., Biol. 1954, 9, 95.
0 1987 American Chemical Society
