VACUUM-ULTRAVIOLET PHOTOLYSIS OF POLYDIMETHYLSILOXANE
823
Vacuum-'UltravioletPhotolysis of Polydimethylsiloxane. Gas Yields and Energy Transfer 1 by Seymour Siege1 and Thomas Stewart Aerospace Corporation, El Segundo, California
(Received July 1 0 , 1 9 6 8 )
A polydimethylsiloxane polymer was irradiated with 1470- and 1236-19 radiation] and the quantum yields
of the gas products were determined. Analysis of the data indicates that the breaking of the Si-CH3 bond is the most probable reaction occurring at both wavelengths. For surface irradiations, C&bI CH4, and HZ are primary products, while for immersion irradiations, only CHI and H2 are produced. The differences in the two types of experiments are ascribed to the competition between bimolecular CH3 combination and radical abstraction reactions; the surface irradiation favors the former mechanism. The quantum yields for 1470 were 0.033, 0.020, and 0 009 for C2Ha1 CHI, and H2, respectively, with surface irradiation. Quantum yields for the immersion irradiation were 0.087 and 0.021 for CHI and H2. The precision of these numbers is f0.002 for all values. For 1236-19 surface irradiations] secondary photolytic reacti2ns were complicating factors and quantum yields were not obtained. In the immersion irradiations] 1236-A radiation gave quantum yields of 0.087 f 0.016 and 0.049 i 0.005 for CH4 and H2, respectively. A ornixed methyl-phenyl polymer [(CH8) (Ph)zSi-O-Si( CH3)z-O-Si( CHB) (Ph)2 1 was also irradiated at 1470 A, and quantum yields of 1.O f 0.5 X and 6 4 X 1 O W were found for CH4 and Hz, respectively, in both surface and immersion irradiations. No Cz& was found for this polymer. Also solutions of the phenyl-containing polymer in the polydimethylsiloxane were irradiated. It was found that the gas yields were reduced by both radical addition to the phenyl groups as well as intermolecular energy transfer between the two polymers; however, there is no evidence for the presence of long-range transfer involving energy transfer along the chain. Analysis of the data indicates that the rate constant for H atom addition to a phenyl group is 10 times larger than the corresponding rate constant for CH3 radicals.
-
Introduction The vacuum-ultraviolet (vuv) region of the electromagnetic spectrum is commonly considered to start at about 2000 A (6.2 eV) and extend to shorter wavelengths where it merges with the X-ray region. Many studies have been published2 dealing with the vuv photochemistry of low molecular weight molecules with the major emphasis on gas-phase reactions. Two of the more popular vuv photolytic wavelengths have been 1470 19 (8.4 eV) and 1236 19 (10.0 eV) because of the development of easily constructed resonance lamps. These studies have shown very interesting reactions occurring as a function of wavelength, perhaps the most striking example being the wavelength dependence of molecular hydrogen elimination mechanisms. While the early work concentrated on gasphase reactions, the vuv photochemistry of the solid and liquid phase has been receiving more attention However, the vuv photochemistry of polymeric materials has not been examined a t all. This paper describes the results of a study which examined the evolved gas products produced by the irradiation of polydimethylsiloxane (DMS) with 1470and 1236-19 radiation. The inter- and intramolecular effects of phenyl groups on the efficiency of production of these products were also studied. One complicating factor in this type of investigation is the very small (-500 %.) penetration depth of the radiation. This small penetration depth results in emphasizing the importance of gas-phase reactions or surface reactions. This point will be discussed in more detail later.
The high-energy ionizing radiation chemistry of DMS at room temperature has been shown to be predominantly controlled by free-radical mechanisms with ionic reactions having little, if any, contributionas However, the excitation by X-rays or fast electrons is random in character with the chemistry occurring after considerable intramolecular and/or intermolecular migration of energy, With vuv radiation, the excitation is much more specific with well-characterized initial transitions occurring such as u - + u * , n + T * , and n -+ u* transitions. Also, jt has been commonly assumed that the particulate radiation chemistry occurring via excited electronic states involves the same electronic states excited by uv light. Therefore, it is of interest to compare the results of the two types of radiation in order to observe the effects of differences in the nature of the deposition of energy and in the excitation of the molecules present.
(1) This paper was prepared under U.S. Air Force Contract F0469567-0-0158. (2) J. R. McNesby and H. Okabe, "Advances in Photochemistry," Vol. 11, Interscience Publishers, Kew York, N. Y.,1963, p 157. (3) (a) P. Ausloos, R . E. Rebbert, and S. G. Lias, J. Chem. Phys., 4 2 , 540 (1965): (b) W. M. Jackson, J. Faris, and N. J. Buccos, i b i d . , 4 5 , 4145 (1966). (4) R. E. Rebbert and P. Ausloos, i b i d . , 46, 4333 (1967). (5) J. Y. Yang, F. M. Servedio, and R. A. Holroyd, i b i d . , 48, 1331 (1968). (6) R . A. Holroyd, J. Y. Yang, and F. M. Servedio. i b i d . , 4 6 , 4540 (1967). (7) H.R. Ward and J. S. Wishnok, J. Amer. Chem. Soc.. 90, 1085 (1968). (8) A. A. Miller, i b i d . , 8 2 , 3519 (1960);83, 31 (1961). Volume YS, Number 4 April 1969
824
Experimental Section Materials. The primary DMS used in this study was a Dow-Corning 360 dimethylsilicone medical fluid with a viscosity of 350 CP and a molecular weight of lo4. A higher viscosity DMS fluid with a viscosity of 12,500 CPand a molecular weight of 5 X lo4 was also used in several experiments. The methylphenylsiloxane used was also a Dow-Corning fluid \vi th a molecular weight of 408. The fluids were purified by vacuum distillation; the medium-boiling fraction of the distillate was used. The gases used in making the rare gas resonance lamps were research grade xenon and krypton obtained from J. T. Baker Co. Photolysis Apparatus. The xenon resonance lamp was constructed by attaching a piece of 12-mm glass tubing with a fritted disk to a 34/45 greaseless inner joint. A 1-in. djameter sapphire-to-Pyrex graded seal window was attached to the joint. Barium alloy getters were placed in the lamp to remove any impurities that might alter the spectral content of the lamp. After the lamp was thoroughly outgassed at 10-j Torr, il mas filled with 1 mm of xenon. The lamp was sealed and the getters flashed. The krypton lamp was constructed by attaching a U-tube to a 34/45 inner joint. A magnesium fluoride window was put on the lamp with an epoxy adhesive. The lamp was filled with 0.5 mm of krypton. This gas pressure resulted in the best lamp performance. Barium getters were not used because the krypton Seems to be gettered by the barium alloy; therefore, a liquid nitrogen trap was used to trap any impurities present. The spectral purity of the lamps was examined with a McPherson (Model 218) monochromator. With the xenon lamp, only the 1470-A resonance line in the vuv was observed. However, with the krypton lamp both the 1236- and 1165-A lines mere observed; the 1165-A line contributed -10 to 15% of the total output of the lamp. In this paper, the output of the krypton lamp will be considered to be all at 1236 A; any uncertainties arising from the small amount of 1165-A radiation present is probably less than the errors inherentJin the various measurement techniques. The cells were fabricated from the 34/45 outer O-ring seal joint. Holders were placed 1 in. above the bottom of the cells so that the calibrated screens used in the relative light intensity experiments could be fastened. The lamps inated into the cells. Power was supplied to the lamps by a Raytheon PGlO microwave power generator through an Evenson cavity. Photolysis Procedure. For the surface mode experiments, 1 mm of silicone fluid was placed in the cell and thoroughly outgassed at Torr. The irradiations at 1470 were usually 45 min in duration with readings of gas yields taken at 5-min intervals. The duration of 1236-A experiments varied from 5 min for The Journal of Physical Chemistry
SEYMOUR SIEGELAND THOMAS STEWART 100% lamp intensity to 12 min for 25% intensity; product readings were taken at 1-min intervals. Samples were stirred constantly during the irradiations. Immersion mode experiments at 1470 A were made by immersing the window of the xenon lamp 6 mm into the polymer fluid. Despite the fact that the samples were vigorously stirred, effects of bulk diffusion of the product gases were observed. When the gas evolution data were analyzed, a straight line was obtained, but the line intersected the time axis at t > 0. To correct for the effects of diffusion, the samples were irradiated for 10 min and the gas yield was measured. The lamp was then turned off and the sample stirred very vigorously for an additional 10 min; then a second set of gas measurements was made. The gas yields went up by a factor of 5 upon standing without irradiation. For example, the CH, yield went from 8 to 39 p . The gas yields obtained from the stirring in the dark method of analysis were considered to be the more representative of the true yields and are the values given in this paper. The immersion mode experiments with the krypton lamp mere performed somewhat differently than with the xenon lamp. Because of the geometry of the krypton lamp and cell, the immersion experiments at this wavelength were made by putting a 6-mm layer of liquid on the surface of the lamp window and stirring the liquid. Under these conditions, the liquid volume was small and the product gas diffusion problem was minimized; the gas evolution curves intersected the time axis at t = 0, and the slopes of the lines were used to obtain gas yields. With the immersion mode irradiations, polymer films were formed on the windows of both lamps; however, the problem was considerably more pronounced with the krypton lamp. A similar problem occurred in the surface mode irradiations using the krypton lamp, but not with the xenon lamp. The polymer film was removed by polishing the windows after each irradiation experiment. Actinometry. The photolysis of C02 was used as an actinometer in calibrating the photon flux of the lamps. The quantum efficiency for production of CO from CO, was taken to be ~ n i t y . ~ 'Seventy-six '~ millimeters of COz was placed in a cell designed for actinometry with a cold finger attached to the cell that trapped the C02. The cold trap was isolated from the cell by a stopcock. The lamps were started and allowed to run for 5 min, at which time C02 was expanded into the cell from the cold finger. The photolysis period was exactly 5 min. The photon flux at 1470 A was 3.3 f 0.2 X 1015photons/sec, and at 1236 8 it was 3.3 f 0.7 X l O I 5 photons/sec. At (9) B. H. Mahan, J. Chem. Phys., 33, 969 (1950). This paper gives @(GO)= 1.1 f 0.1 and 1.2 i 0.1 a t 1470 and 1236 A, respectively, where both values are upper limits. (10) J. Y.Yang and F. M. Servedio, Can. J. Chem., 46, 338 (196s).
VACUUM-ULTRAVIOLET PHOTOLYSIS OF POLYDIMETHYLSILOXANE 1236 fi, a decrease in production rate of CO was observed after 10 min of exposure. A similar effect was observed by Yang and Servedio,lo which they attributed to polymer formation on the window. The window was cleaned by operating the lamp in air for 15 min. The polymer formation was not observed with the 1470 fi lamp. Analysis. The photolysis cells were connected directly to an Electronic Associates Series 200 residual gas analyzer (a quadrupole mass spectrometer). The gases were introduced into the mass spectrometer chamber by a manually controlled variable leak. The mass spectrometer was calibrated for determining absolute product yields. Total gas pressures were measured with a McLeod gauge. The mass spectral peak heights were read out on a dual-trace oscilloscope. Polaroid photos were taken of the peak responses on the oscilloscope, and quantitative measurements were made from them. Peak height deflection was directly proportional to the pressure of gas introduced into the mass spectrometer chamber. For the 1470-fi experiments, reproducibility errors were 10% or less, whereas with the 1236-A experiments, errors were 25% or less. The uncertainties given in this paper are reproducibility errors,
Results 1470-A Irradiation. When DMS was irradiated in the surface mode, the gas products consisted of H B ,
525
CHI, and %He. For stirred samples the yields for all three products were linear with irradiation time for a constant incident radiation intensity ( I ); typical gas evolution vs. time plots are shown in Figure 1. Within experimental error the quantum yields (@) are independent of I over a factor of 10 in the magnitude of I , as illustrated in Figure 2. The values of @ are given in Table I.
Table I: Quantum Yields (@) of Gas Products from 1470-A Irradiation of DMS Product
9"
a35
(surface)
(immersion)
0.033 0.020 0.009 0.086
N.d. 0.087 0.021 0.087
CzHe CH4 Hz Total CHsb
a The precision of all the quantum yields is 3=0.002with the exception of @(CH3),which has an estimated precision of lt0.006. The quantum yield for total CH, was taken to be equal to 2@(C*He) Q(CH4).
+
The values of @ for the immersion mode experiments are also given in Table I. As can be seen from the values of CP for the two irradiation modes, the relative quantum yields for CHI and C2H6 are strongly dependent upon the experimental irradiation conditions. This point is further illustrated by the data given in Table 11, which lists the slopes (12) of the gas evolution curves for several experimental conditions. The important point to be noted is that while the yield of total CHI is constant, the fraction of CH, and Hz produced increases with conditions which promote
Table 11: Variation of k" with Experimental Conditions (1470-A Irradiation)
-__________ TIME, min.
Product
Figure 1. Gas evolution vs. irradiation time curves for 1470-A irradiation. l
'
l
'
l
'
l
'
l
RELATIVE INTENSITY
Figure 2. Relative gas yields vs. light intensity.
'
l
'
l
CzHe CH4 Hz Total CHa
Surface (stirred)
1.50 0.90 0.40 3.90
surfaceb (not stirred)
1.36 1.44 0.60 4.10
Mode_l________l, surfacec Surfaced (not (high Immerstirred) viscosity) sion
1.20 1.88 0.82 4.20
1.10 1.80 1.02 4.00
N.d. 3.90 0.95 3.90
" k = slope of experimental gas evolution curves with units of r/min. The values of k can be converted into quantum yields by multiplying by 0.022. The reproducibility is +lo%. b These values of k were obtained by taking the slopes during the first 45 min of irradiation, which was the normal irradiation time in these experiments. These values of k were obtained by irradiating to 120 min and then taking the slopes of the gas evolution curves evolved during the subsequent 45 min of irradiation. d The high-viscosity material has a mol wt of 5 X lo4and a viscosity of 12,500 cP; the low-viscosity material used in the rest of the experiments has a mol wt of 1 X 104 and a viscosity of 350 cP. The sample was stirred but the efficiency of stirring was low because of the high viscosity of the material. Volume 73,Number 4 April 1969
826
SEYMOURSIEGELAND THOMAS STEWART
homogeneous reactions and minimize diffusion to the surface. In the extreme of the immersion irradiation mode, no C2H6 is formed. 1236-d Irradiation. Analysis of the data from the 1236-A irradiations in the surface mode was complicated by the secondary photolysis of the CZH6 formed. The primary products were still CJh, CH4, and Hz, but C2H4,C2H2, and H2 were also formed as secondary p r o d u c t ~ . ~ The J ~ gas evolution curves were nonlinear with irradiation time, and the quantum yields were a function of incident light intensity. Formation of the polymer on the window during the irradiation which reduced the photon flux at the sample was an added complication. Because of these complications, meaningful quantum yields could not be derived from the data obtained from the surface mode irradiations. The secondary products CzH2 and C2H4 were not observed when the 1236-A irradiation was performed in the immersion mode. Despite the absence of secondary photolytic reactions, the immersion mode experiments still gave nonlinear gas-time evolution curves. However, in this case the nonlinearity arises only from the rapid formation of the polymer film on the lamp window. The initial slopes of the evolution curves were used to approximate the quantum yields. As in the case of the 1470-%1irradiations, no CZH6 is found in the immersion experiments; the only products observed were CH4 and H2. The quantum yields were @(CH4)= 0.087 f 0.016 and @(H2)= 0.049 f 0.005. Energy Transfer. In order t o study the protective effect of aromatic substituents on the vuv photochemistry of silicone polymers, a mixed methyl phenyl silicone (MPS) was examined. The polymer has the struc ture'l C6H.5
I
CH3
I
CH3-Si-O-Si-O-Si-CH3
I
CaH5
I
CHs
CeHs
I
I CsHs
The gas yields from MPS irradiated with 1470-A radiation were much less than those found for DMS. was In the surface mode a value of 1.0 0.5 X observed for @(CHi). No CzHGwas observed, and (The background pro(a(H2)was less than 4 X duction of Hz was too high to obtain a more accurate value for the latter quantum yield.) In the immersion mode the corresponding yields were essentially the same, i.e., (P(CH4) = 1.0 f 0.5 X and These results demonstrate @(H2) 5 3 f 1 X the strong protective effect produced by aromatic substituents on the vuv photochemistry of these types of polymers. However, it should be pointed out that these quantum yields are significantly higher than those found in the 2537-A photolysis of RIPS," where and @(CH4) = 0.6 X a(H2) = 2.6 X The Journal of Phgsical Chemietry
Solutions of MPS in DMS were also examined. These experiments were performed in the immersion mode. Concentrations of RIPS ranged from 0.11 (saturated solution) to 0.01 M ;the concentrations oi phenyl groups are four times larger. The results of these experiments are given in Table I11 in terms of
Table 111: Variation of ka with MPS Concentration in Immersion Mode [MPS], M
k(CHJb
0 0.01 0.025 0.05 0.08 0.11
3.9 3.7 3.6 3.2 2.3 1.8
~(HZ)"
0.95 0.80 0.51 0.30 0.20 0.20
a k is the slope of the experimental gas evolution curves with units of p/min. The values of IC can be converted into quantum yields by multiplying by 0.022. *The precision is 1 0 . 1 M/min. 6 The precision is f0.05 plmin.
the values of k . Measurements of the absorption of 1470-A radiation as a function of RIPS concentration show that the radiation is being absorbed by the DMS even at the highest concentrations of MPS ~ s e d . Therefore, ~ ~ J ~ the variations in gas yields arc! not due to competition between solvent and solute for the available radiation. The Hz yield is more sensitive to the presence of NIPS than is the CH4 yield, and the effect on the H2 yield is significant, even in the 0.01 M solution. This difference in sensitivity is probably due, at least in part, to the relative efficiencies of H and CH3 additions to phenyl groups. The leveling off of the H Zyield at high MPS concentrations is probably due to insufficient corrections for background Hz production from the lamp walls and other sources. It should be pointed out that the value of Hz production obtained at 0.11 M MPS is essentially that found for the irradiation of pure MPS. Considering the known efficiency of energy conversion in phenyl groups as well as their large radical-scavenging power, it seems likely that a significant fraction of the value of @(H2)obtained for pure MPS may be due to background H2 evolution. (11) 8 . Siegel, R. J. Champetier, and A. R . Calloway, J. Polurn. Sci., Part A , 4, 2107 (1966). (12) S. Siegel and T. Stewart, Aerospace Oorp., unpublished data. (13) This conclusion is also supported by published measurements on other systems. For example, using the absorption coefficients for benzene a t 1470 8 of -5000 cm-1 [A. Quemarais, M . Morlais, and S. Robin, C.R. A c a d . Sei. Paris, Ser. B , 265, 649 (1967)l.we calculate that for a 0.4 M solution only 10 %oaf 1470-A radiation would be absorbed in a path length of 5500 A. The data on the absorption of polystyrene [R. H. Partridge, J . Chem. Phys., 47, 4223 (1967)l also indicate that the phenyl groups do not make a larger contribution to the absorption than the CHI backbone. The penetration depth for polyethylene [R. H. Partridge, i b i d . , 45, 1685 (1966)l a t 1470 A is approximately 500-1000 8.
VACUUM-ULTRAVIOLET PHOTOLYSIS OF POLYDIMETHYLSILOXANE
Discussion The primary reactions occurring during the 1236and 1470-%1irradiations of DMS are CHZ -Si-0-
I + hv
I
4
+ H.
-Si-0-
(1)
I
I
CHs All the data obtained in this study are consistent with the postulate that CzHC is produced only in the gas phase, or on the cell walls, by the combination of methyl radicals. The penetration depth of vuv radiation is very shallow; therefore, when the irradiation is performed in the surface mode, some of the CH3 radicals can diffuse to the surface before interacting with the matrix, The rest of the CH3 radicals produced by reaction 2 abstract a hydrogen atom from the solvent, Le.
[:I -Si-0-
1.
CH2
+ CH3-+ CHI + -Si-0-I I
(3)
CHs*
As the viscosity of the material is increased, diffusion is hindered and reaction 3 plays a more important role. Since no CzH6 is produced in the immersion mode irradiations, CHa radicals are consumed exclusively by the abstraction reactions. The quantum yield for reaction 2 is independent of the experimental conditions; only the fate of the methyl radicals is variable. Similar considerations pertain to the fate of the H atoms formed by reaction 1. The H atoms formed in the immersion mode are most probably consumed by hydrogen abstraction to form H2. When surface mode irradiations are considered, the observed Hz comes almost entirely from H H combination reactions after H atoms escape from the liquid, since @(H2) drops by about a factor of 2. Also, some of the H atoms may be involved in CHs H combination processes. Unfortunately, it is not possible to definitely determine from the available data what fractions of CH4 and Hz formed in the surface mode irradiations arise from the combination reactions. The quantitative assessment of the relative importance of molecular elimination reactions such as14
+
+
[ E: 1. -Si-0-
CH
+ hv + -Si-0-I I
+ HZ
(4)
CH3
also can not be made. However, consideration of the large increase in 9 ( H z ) in going from the surface to
827
immersion modes would indicate that reaction 4 is not a large contributor to the observed H2yield. Some preliminary experiments, using NO as a radical scavenger, and the results in Table I11 support the latter conclu~ion.~~ The results from the immersion experiments in Table I, together with the assumption that the radicals are totally consumed by abstraction reactions, indicate that for 1470 1 reaction 2 is four times more probable than reaction 1 after the absorption of a photon. When 1236-1 radiation is used, the observed quantum yields indicate that reaction 2 is only twice as probable as reaction 1. The significant increase in the probability of the H-atom elimination at shorter wavelengths may be due to the differences in the nature of the electrons being excited.16 With 1236-1 radiation, the electrons in the C-H bonds are being excited directly, while for 1470-1 irradiations it is likely that the polymer backbone and the Si-C electrons are being primarily excited. It is of interest to compare the results of the present study with the published results obtained from particulate ionizing radiation experiments. Gases are evolved under ionizing irradiation'' with G values (i.e.7 molecules produced per 100 eV of absorbed energy) as follows: G(H2) = 0.8, G(CH4) = 1.2, G(CzH6) = 0.3, and G(CH3) = 1.8. The value of G(H) can be calculated from these data only if the mechanism of Hz and CH4 production is assumed; the radical combination processes give G(H) = 2.8 and the abstraction mechanism G(H) = 0.8. Analysis of the vuv immersion mode irradiation results in the same units and, using the abstraction assumption, gives (a) 1470 A: G(CH3) = 1.0, G(H) = 0.25 and (b) 1236 1: G(CH8) = 0.87, G(H) = 0.49. Comparison of the relative gas yields shows that for the immersion vuv experiments no CZH6 was found, while approximately one-third of the total CH3 yield appeared as C2H6 in the ionizing radiation studies. Part of the CzHs in the latter studies may arise from spur reactions where the local concentration of CH3 is large. However, the variations in ratios of the various products with type of radiation may also indicate that different excited (14) 0.P. Strauss, K . Obi, and W. K. nuholke, J. Amer. Chenz. SOL,90, 1359 (1968). (15) Some preliminary experiments using various pressures of KO as a radical scavenger and the surface irradiation mode were also performed. A t the highest concentration of NO used ( L e . , 200 p ) , we found that the CzHs yield was decreased by a factor of 2 while the H Zyield was reduced by 25%. The CHI yield was essentially unaffected by the presence of N O . These data conflrm the conclusion that CzHe is formed in the gas phase. They also indicate that the OH, from the surface mode is mainly formed in the bulk material, while an appreciable fraction of the H Zarises from H atoms that have escaped. The result that O(Hz) from the immersion mode is twice the value found from the surface mode also indicates that most of the H atoms formed in the surface irradiations escape from the liquid. (16) J. W. Raymonda and W. T. Simpson, J. Chem. Phys., 47, 430 (1967). (17) M.G. Ormerod and A. Charlesby. Polymer, 4, 459 (1963).
Volume YS, Number 4 April 1968
SEYMOUR SIEGELAND THOMAS STEWART
828 states are involved. Also, it is possible that ionic species play a more important role in the particulate radiation chemistry of DRIS than has been previously thought.'8 It is interesting to note that the uv radiation is less efficient than the ionizing radiation in producing gas products. Yang, Servedio, and €Iolroydb*Ghave shown in recent papers that benzene reduces the product yields from the 1470-A irradiation of cyclohexane by two mechanisms. One mechanism that is effective a t low concentrations is the attachment of H atoms to benzene. This process is 300 times more effective than H-atom abstraction from cyclohexane. The second mechanism that predominates a t high concentrations is energy transfer from excited cyclohexane to benzene. For our system ( L e , , MPS dissolved in DMS) these two mechanisms would yield the following expression for the RIPS concentration dependence of the observed CH, and H2 quantum yields in the immersion mode
Po C P P
-a0= -
(5)
where CPo is the quantum yield at [RIPS] = 0, CP is the quantum yield a t [MPS] # 0, Pa is the radical production rate at [RlPS] = 0, P is the radical production rate at [MPS] # 0, k , is the rate constant for radical addition to MPS, and k, is the rate constant for radical abstraction from DNS. The value of Po/P is a function of the probability of energy transfer. Analysis of the data in Table I11 in terms of eq 5 can be performed only with some approximations. First, since the CHI yield decreases by only 20% at
The Journal of Physical Chemistry
[MPS] = 0.05 M , while the Hz yield decreases by a factor of 3, it is apparent that at low concentratioris of RIPS the H2 yield is affected mainly by the radical addition reaction. Therefore, we assumed that up to 0.05 M MPS, Po/P = 1. With this assumption, we find that the slope of the straight line obtained yields k./k, M 1100. Considering the fact that each MPS molecule contains four phenyl groups, the latter ratio is in good agreement with the results of Yang, et al. Making the same assumption for CHI yields as for Hz yields, at [MPS] = 0.05 M , k,/k, M 100. This supports the above conclusion that the phenyl groups are less efficient scavengers of CH, radicals than of H atoms. The latter estimate is probably too high, since energy transfer was completely ignored. However, using this value of lc./km, we find that for CHI, Po/P at 0.11 M RIPS = 1.5, whereas, if radical addition is ignored, Po/P = 2.2 at the same concentration. If diffusion is neglected and an active volume c ~ n c e p t ' ~ is adopted, these values for P o / P correspond to an interaction radius for energy transfer of 11.4 to 14.0 8. Obviously, the interaction requires very close overlap of the electron distribution of the two molecules for transfer to occur. There does not, seem t o be any migration of energy involved along the polymer chain. The data for Hz at the high concentrations of MPS are too uncertain to perform a similar calculation. (18) While it is not known with any certainty if ionization occurs in the 1236- and 1470-kirradiation of DMS, the results of Partridge (ref 13) on polyethylene and polystyrene indicate that, a t least for 1470-A radiation, ionization is not an important process. It is most probable that the results reported here are due to primary cxcitation into nonionized states. (19)S . Siege1 and H. Judeikis, J. Chern. Phys., 48, 1613 (1968).
