526
B. J. L Y O NAND ~ MALCOLM DOLE
Radiation Chemistry of Polyethylene.
VI.
Radiolysis in the Presence of Nitric and Nitrous Oxides
by B. J. Lyons] and Malcolm Dole Department of Chemistry and Materials Research Center, Northwestern University, Evanston, Illinois (Receiued September 21, 1963)
The radiation of polyethylene and polypropylene in the presence of nitric and nitrous oxides showed that the consumption of NO was independent of pressure, that the hydrogen evolution was unaffected by the presence of NO, that the ratio of NO consumed to Nz produced was about 3.4, that some water was produced, that NzO reacted a t about onetenth the rate of NO, that the reaction of XO with isotactic polypropylene during the irradiation probably followed the same mechanism as in the case of polyethylene, but with somewhat lower yields, and that the yield of free radicals, G(R.), was the same order as found by e.s.r. measurements. There was no evidence of the existence of a chain reaction. A mechanism is suggested for the enhancement of cross linking by nitrous oxide.
Introduction Xitric oxide has frequently been used in the study of reaction mechanisms as a free radical scavenger, especially in the case of gaseous reactions. Recent examples are the papers of Blackmore and Hinshelwood2 and Iletcalfe and Trotman-Dicken~on.~Yang and JIanno4 and Yangb found in the radiolysis of hydrocarbons that nitric oxide was “one of the few radical scavengers which is satisfactory to estimate the energy yield of radicals, as well as the contribution of free radical reactions to the over-all radiolysis of gaseous hydrocarbons.” We concluded, therefore, that it would be interesting to study radiation effects in polyethylene in the presence of nitric oxide as well as other nitrogen oxides. Our plans for this research were also stimulated by the interesting observations of Okada and Arnemiyab,’ that nitrogen dioxide completely suppressed gel formation in a low density polyethylene film up to a dose of 50 Jh., that nitric oxides considerably reduced gel formation, but that nitrous oxide slightly enhanced gel formation. After our research was initiated, X l h a u d and Durupg published the results of their extensive investigation of the radiolysis of liquid cyclohexane and cumene in the presence of nitric oxide. For the most part they used the same technique as in this research and one that Matsuo arid Dolelo previously used in the study of the radiolyThe Journal of Physical Chemistry
sis of polyethylene in the presence of oxygen. I n the case of cyclohexane, Milhaud and Durup’s results in several respects were very similar to those given below; their major conclusions may be summarized as follows. Uptake of NO was linear with time and independent of pressure over the pressure range studied; the mole ratio of NO consumed to N2 evolved was 3 : 1; for several reasons they believed that YO did not interfere with the radiological production of free radicals but only reacted with them; the mechanism of the initial and subsequent reactions is probably that postulated by Brown“; G-values for NO uptake were so high, about 50, that a chain reaction must have existed. I n the case of cumene, G(-?;O) rose to 16,000 molecules ~~~~~
~~~
~~~~
~~~
~~~~
(1) Raychem Corgoration, Redwood City, Calif. (2) D . R. Blackmore and C. Hinshelwood, Proc. Roy. SOC.(London); A268,21 (1962). (3) E. L. Metcalfe and A. F. Trotman-Dickenson, J . Chem. SOC.; 3833 (1962). (4) K.Yang and 1’. J. Manno, J . A m . Chsm. Soc., 81, 3507 (1959). (5) K.Yang, J . Phys. Chem., 65, 42 (1961). (6) Y. Okada and A. Amemiya, J. Polymer Sci., 50, 522 (1961). (7) Y. Okada, J . Appl. Polymer Sci., 7 , 695,703 (1963). (8) Y.Okada, ihid., 7, 1153 (1963). (9) J. Milhaud and J . Durup, J . Chim. Phys., 59, 309, 315 (1962). (10) H.Matsuo and M . Dole, J . Phys. Chem., 63, 837 (1959). (11) J. F. Brown, Jr., Abstracts 126th National Meeting of the American Chemical Society, New York, N. Y., Sept., 1954, pp. 42-0 and 43-0. See also ref. 21.
RADIATION CHEMISTRY OF POLYETHYLENE
of NO consumed per 100 e.v. of energy absorbed when the liquid was well stirred. Ogihara has studied the thermal oxidation of a low density polyethylene by nitrogen dioxide a t looo, especially from the standpoint of changes in the infrared spectra and assignment of the observed absorption bands. l 2 I n the work described below, one experiment was carried out using isotactic polypropylene powder, one experiment with NzO gas instead of NO, and one with low density polyethylene. The remaining experiments were all on a linear high density polyethylene in the presence of NO. l 3
Experimental Materials. Film samples of high and low density polyethylene were used. Antioxidant-free linear polyethylene, 2 mils thick, the same material used by Matsuo and Dole,Iowas the high density polyethylene. The low density material was the same as that used by Dole, Milner, and Williams14 and the isotactic polypropylene powder was the same as that used by Schnabel and Dole.'5 The NO gas was purified by repeated distillations between traps at solid C02 temperature and liquid nitrogen temperature. After distillation, the C 0 2 trap was isolated and warmed to room temperature. When discoloration due to NO2 no longer appeared, the NO was pumped out at liquid nitrogen temperature by repeated expansion of its vapor into a large vacuum. (The vapor pressure of NO at liquid nitrogen temperature is too high for continuous pumping.) Finally, the first fraction of NO to boil off when the liquid nitrogen trap was removed was used to fill the radiation cells. NzO was purified by pumping at liquid nitrogen temperature to separate N2and NO. The first fraction to boil off when the liquid nitrogen trap was removed was discarded. Radiation Cells and Source. Cobalt-60 y-rays a t an intensity of about 0.1 Mrad/hr. constituted the source of high-energy radiation. The cells were similar to those used in the Matsuo-Dole10 work, and the temperature of the irradiation was in all cases room temperature, 25-28', Pressure Measurements. The total gaseous pressure in the cell was measured manometrically a t about 2-hr. intervals. This pressure was the sum of the partial pressures of NO, of H2 evolved by direct radiolysis of the polyethylene or polypropylene, of N2 evolved by the reactions of NO with the free radicals, and of a small amount of NO2, NzO, and Hz0 vapors. By cooling the top of the cell a t the break-off seal with liquid nitrogen, it was possible to condense out all gases except Nz and Hz (assuming no CO was pro-
527
duced by the radiolysis). Blank experiments with NP alone in the cell gave a correction factor that enabled the room temperature pressure to be calculated from the liquid nitrogen temperature pressure measurements. There was no evidence of a reaction of NO with the polyethylene in the absence of the irradiation. The temperature a t room temperature was controlled by a constant temperature bath to 25.8'. The NO pressure at liquid nitrogen temperature was estimatedl6 to be about 7 X low3mm. and was neglected in comparison with the N2 and H2 pressures. The pressure of NzO a t liquid nitrogen temperature is even smaller, 2.2 X lo-' rnm.l6 In some experiments, the pressures were read with solid CO2-acetone mixture replacing the liquid nitrogen. At this temperature the total pressure should be the sum of the N2,H2, N20, and NO pressures. Assuming the Dry Ice-acetone mixtures to ha!ve a temperature of 196'K., the vapor pressure16 of solid NOz is 0.043 mm., and of ice, less than 1 f i , The pressures of those solids can be neglected, therefore, at the Dr'y Ice temperature. Methods of Analysis. Peak heights on a Consolidated analytical mass spectrometer, Model 21-130, were measured on gas samples obtained in four experiments but results on two of the experiments had to be rejected because of leakage of air into the samples. Gas concentrations were calculated from the peak heights using empirical correction factors determined from an analysis of a known mixture. Infrared absorption spectra of the polyethylene film both before and after irradiation were also recorded and compared with spectra taken on films irradiated in vacuo.
Results and Discussion Generalizations from Pressure Measurements. The general run of the pressure measurements is illustrated in Fig. 1 for NO plus polyethylene and in Fig. 2 for NO plus polypropylene. The curves are roughly comparable, so it can be seen that the NO consumption, open squares, occurs about twice as fast in the polyethylene as in the polypropylene. Also shown a t the top of Fig. 1 are the pressure curves for the total pressure and the pressure of the liquid nitrogen temperature-condensable gases in the case of the irradia(12) T. Ogihara, Bull. Chem. SOC.Japan, 36, 58 (1963). (13) Unfortunately, this research was abruptly terminated on November 1, 1962,after 6 months of study. As there seemed to be no prospects of continuing the work, we give here the results obtained
to date. (14) M.Dole, D. C. Milner, and T. F. Williams, J . Am. Chem. SOC., 80, 1580 (1958). (15) W. Schnabel and M. Dole, J. Phys. Chem., 67, 295 (1963). (16) "Handbook of Chemistry and Physics," 40th Ed., Chemical Rubber Publishing Co., Cleveland, Ohio, p. 2343.
Volume 68,Numher 9
March, 196J
528
IP
B. J. LYONSA N D MAT~CCXM DOLE
cms. Hg
2
4
Figure 1. Pressure measurements in irradiation cells as a Function of dose during the irradiation of polyethylene at room temperature in the presence of NO or N10 gas. N20 curves: top curve (open circles), total pressure; lower curve (squares), pressure of liquid nitrogen temperature-condensable gases. NO curves: open circles, total pressure; squares, pressure of liquid nitrogen temperature-condensable gases; and closed circles, pressure of gases uncondensable a t liquid nitrogen temperature.
tion of polyethylene in the presence of N20 gas. In this experiment, the evolution of hydrogen preponderated over the consumption of N20 whose rate of reaction was about one-tenth that of YO, and the experiment had to be terminated before the 1JzO was entirely consumed because the pressure of the hydrogen became too great to measure in our type of radiation cells. In the case of an experiment using initially NOz gas, the reaction of the NO, with the polyethylene occurred so rapidly even before the irradiation could be started that this experiment had to be abandoned. Okada8 found that KO2 causes chain scission to occur in the absence of radiation. There was little postThe Journal of Physical Chemistry
irradiation decrease in the total gas pressure. For example, in one experiment after a dose, r, of 1.0 X lozoe.v.g.-l a t an NO pressure of 16.8 cm., the total pressure fell from 20.97 to 20.65 cm. on standing 11 hr. This lack of a post-irradiation effect was probably the result of the fact that thin films of polyethylene were used. Another generalization from our pressure measurements is that the initial rate of NO uptake was independent of the initial NO pressure. Only when the YO pressure became low did the rate fall off significantly. This agrees with the observations of Milhaud and D ~ r u p . ~ The slopes of the straight line sections of the curves have been calculated by the least-squares method and the results along with other pertinent data are given in Table I. The value of APNo/Ar was corrected for the estimated initial growth rate of water and other liquid nitrogen temperature condensable gases. There is some uncertainty in this correction for the condensable gases inasmuch as there were indications that the evolution of water had an induction period. Furthermore, some NOz was evolved as demonstrated by a tarnishing of the mercury surface in the manomter, but the partial pressure of NOz must have been small because it reacts with polyethylene even in the absence of the irradiation. The mass spectrometric analysis indicated only traces of NO2 in the product gas; the peaks at mass 44 could have been due to CO, or NzO, or to a mixture of both. A small peak a t mass 30, about one-tenth of the peak a t mass 44,may have been due to some residual NO, but undoubtedly not because the mass spectral pattern of NtO predicts a mass 30 peak about one-third of the parent peak. This fact indicates that the 44 peak probably contained mostly carbon dioxide in the case of experiments 7 and 8. G-Values calculated from the slopes and other Gvalues calculated as explained below are given in Table 11. Before discussing the latter, it is interesting to point out that the ratios - A P N o / A P ( H+, N ~ are ) more constant than the individual G-values. These ratios should be independent of the calculated dose, initial pressure of NO, etc. as long as the reaction mechanisms are the same in each case. It is seen that in the case of five experiments including experiment 15 on isotactic polypropylene that these ratios, Table I, are all equal to 1.51 f 0.006. Calculations of G-Values. G(H2). G-Values for hydrogen evolution can be calculated by two independent methods. Assuming that only Hz was generated in the last stages of the radiolysis, where both the total pressure and pressure with the liquid nitrogen temperature-condensable vapors frozen out increased linearly
RADIATION CHEMISTRY OF POLYETHYLENE
529
Table I : Radiolysis Data for Polyethylene and Polypropylene In the Presence of NO or N20 a t Room Temperature
- APNo/AT, pol ymerb
Gas and polymer'
Expt. no.
2 3 7 8 10 12
NO NO NO NO.
13 15
N10 NO PP
N10
NO low density P
wt.,
Cell6 volume,
Initial pressure, cm.
cy. (e.v.)-l X
8.
ml.
3.987 3.340 4.135 3.585 3.567
26.0 29.2 27.4 29.4 28.7
6.67 13.85 11.27 22.59 16.51 14,06
2.02 4.84 5.72 5.45 0.40 5.00
1.29 3.23 3.69 3.74 1.61 2.15
3.585 3.170
29.4 27.0
7.48 14.79
0.16 2.50
1.32 1.68
g.
g.
APBI+ N,/AT. cm. (e.v.)-1 X 10'0
Initial
1020
Af'total/Ar,c g . crn. [e.v.)-'
x
10'0
- APh.o/ AP(e,t N ~ )
Final
Final
0.97 1.49 1.46
1,14 1.52 1.52
1.18
1.19
1.57 1.50 1.55 1.46 0.25 2.72
0.84
0.83
0.12 1.49
P represents polyethylene and PP, polypropylene. 0 Unless otherwise indicated, the polymer was Marlex-50. the table signify no data taken. A P t o t n i / A i " should equal A P ( H p + N 2 ) / A T f i n a l .
* Blank
spaces in
Table 11: G-Values
Expt. no.
3 7 8 -. 10 12 13 15 Av. of 3, 7, 8, 12 Av. of 3, 7, 8 0
Gas
NO NO NO NzO NO NZO NO ( P P )
G(-NO) or G(-NzO), molecules/ 100 e.v.
-------G(€IzP----Initial slope
,---------a( Final slope
Initial slope
Nz)----Final intercepts
C(-NO)/ G(Ndav
11.8 18.8 14.0 1.8 14.5 1.0 7.9 14.8
3.29 5.05 3.88
2.22 4.25 3.19
3.50 5.38 4.14
5.39 4.49
3.37 3.49 3.16
2.70
3.07
2.88
2.89
5.03
2.24" 3.73
2.29 3.18
2.39 3.97
2.21 4.26
3.43 3.36*
14.8
4.07
3.22
4.34
4.94
3.34
Includes any CH, that might have been evolved. Includes the value for expt. 15.
I,
Includes the value of expt. 15 and excludes that for expt,. 12.
G(HzO)/ G(-NO)
0.14 0.13 0.14 O.4lc 0.11 0. 57c 0.13 0. 13d
c
G(H20)/
G(-N20).
with dose a t very closely the same rate, and also assuming that HP was generated in the earlier stages of radiolysis a t the same rate as in the later stages, it is possible to extrapolate the straight lines back to zero dose and to obtain as intercepts on the ordinate the total pressure of N P plus liquid nitrogen temperature condensates, upper intercept, and the total pressure of Nz alone, lower intercept. By subtracting the total Rressure of N P from the final total pressure of N P Hz, the moles of hydrogen liberated during the radiolysis can be calculated, and from this result the G(Hz)value. An equivalent G(H9) can also be calHz curve; culated from the final slope of the Nz both calculations should yield the same result and the average values of these calculations labeled G(H2)sinal are collected in Table 11. From previous
+
+
on Marlex-50 polyethylene, it is known that G(H2) decreases as hydrogen gas accumulates in the radiation cell. From this standpoint one might expect the initial G(H2) to be greater than G ( H 2 ) r i n a l . However, in the presence of NO, the free radical steady state concentration may be much less than in the absence of KO and the molecular hydrogen back reaction, therefore, much less, or perhaps completely eliminated. Another method of estimating G(HP) consists in determining the fraction of gas giving the initial increase with dose in pressure of the liquid nitrogen temperature-uncondensable gas due to hydrogen alone, and then multiplying G(H2 N 2 ) i n i t i a l by this fraction.
+
(17) M. Dole, T. F. Williams, and A . J. A r v i a , Proc. &id Intern. Conf. Peaceful Uses At. Energv, Geneva, 29, 171 (1958).
Volume 68, Numher 3 March, 1964
B. J. LYONSANT) MALCOLM DOLE
530
16
I
I
I
I
I
12
0
4
Figure 2. Curve designations the same as for Wg. 1, NO case, but for isotactic polypropylene as the solid.
Two gas analyses were successfully carried out on samples from experiments 2 and 4. These samples were taken a t the end of the radiation; in the case of experiment 4, the pressure of NO a t the end was still high because the initial pressure had been atmospheric. In the case of experiment 2, about 0.2 of the initial NO remained a t the end, but the total rate of production of ?1T2 plus HZ was constant over the whole period Q, was 0.94 of the irradiation. The H z ~ I Nratio, ~ in experiment 2, but only 0.42 in experiment 4. These facts demonstrate that a t the high NO pressure, a reaction between KO and hydrogen atoms to form HNO probably occurred as has previously been observed by Hengleinls and others. Inasmuch as two molecules of H S O react to form water and Nq0, a reaction of atomic hydrogen with NO should also yield Nz0 in relatively large proportions. Mass spectrometric analyses revealed that the N20/N2 ratio in experiment 4 was 0.124 compared to 0.090 in experiment 2. These results also suggest that G(Nz0) was a t least 0.5, but a back reaction of NzO with the polyethylene during the irradiation prevented an accurate estimate of G(N2O) from being made. A low HJNz ratio could also be partly explained on the basis of an abnormally high The Journal of Physical Chemistry
yield of nitrogen gas. Such a high yield of NP would require a high rate of NO consumption for which we have no evidence. Assuming that the Q-value of experiment 2 was valid for the initial liquid nitrogen temperatureuncondensable gas evolved in experiments 3, 7, 8, 12, and 15, initial G(Hz) values given in Table I1 were H2)initisl by Q/ calculated by multiplying G(N2 (1 Q). G(HJ values obtained by the two methods are not in very good agreement except in the case of experiment 15 on isotactic polypropylene. However, in three cases out of four the initial G(H2) was higher than G( H z ) f i n a l , (as might be expected if NO scavenges free radicals with which Hz could back react). This indicates that a t the rather low initial NO pressures used in this research, reaction between atomic hydrogen and NO was negligible This result may be explained as due to the fact suggested by the hydrogen isotope exchange experiments of Dole and Cracco,I9 that hydrogen atoms on formation probably abstract nearest
+
+
(18) A. Henglein, Intern. J. A p p l . Radiation Isotopes, 8 , 149 (1960). (19) M. Dole and F. Cracco, J. Phys. Chem., 66, 193 (1982).
531
RADIATION CHEMISTRY OF POLYETHYLENE
hydrogen neighbors or close to nearest neighbors to form moleculalr hydrogen. Unless a NO molecule happened to be on a nearest neighbor site, reaction with atomic hydrogen would not occur. Furthermore, if the XO dissolved only in the amorphous zones of the polyethylene, most of the hydrogen atoms released would not come into contact with NO molecules. The average of all four results on polyethylene for G(Hz)tinalis 3.17, whereas about 3.2 to 3.4 would be expected a t the average Hz pressure of the radiations. I" G(H2) for polypropylene was smaller than expected, 2.2 as compared to 2.8. The polypropylene was much less crystalline than the Marlex-50 polyethylene ; the KO may have been more soluble in it and may have reacted to some extent with atomic hydrogen. There is no evidence, however, that the reaction mechanisms in polypropylene were any different from those in polyethylene. G(-NO). The rate of consumption of NO is given in Table I. From these rates G(-NO) is readily calculated in Table 11. Again the absolute values are erratic, but the ratios G(-NO)/G(Ht) are closer by comparison in the case of experiments 3, 7, 8, and 12 done on polyethylene than are the individual values. The average value of G(-.NO) for these four experiments is 14.8. G(Nz). Just as G(Hz) can be calculated by two separate methods, so can G(N2). Knowing G(-YO) and assuming that G(Hz) is constant over the whole dose, G(KJ can be calculated from the estimated final pressure of Ii2. Thus, G(Nz) ix equal to G(-NO) multiplied by the ratio of the final pressure of N2 to the initial pressure of NO in the case of the experiments where sill the NO was consumed. The second N,) calculated method is to subtract from G(Hz from the initial slopes of the pressure-dose curves G(H2)ln,tla~ estimated using the mass spectral data,. The two sets of results agree quite well, Table 11. I n the eighth column of Table I1 ratios of G(-NO)/G(PJS). are given. Except for experiment 12 done using a low density polyethylene, the ratios lie between 3.16 and 3.49 with an average value of 3.36 f 0.23 (including the ratio for isotactic polypropylene), G(HpO). The G(H2O) values are very uncertain and include contributions from all products uncondensable at liquid nitrogen temperature after all the NO had been consumed. Nevertheless, the data yield an average of 1.9 f 0.5. Some of this may have been due to I f 2 0 inasmuch as the mass spectrometric results indicated that the NzO yield was about 1% of the nitrogen yield.
+
Interpretations and Conclusions We consider first the ratio APNo/AP(H,+ N ~ )which can be seen from Table I to be remarkably constant and equal to about 1.5 for all experiments except those for NzO and those done on the low density polyethylene. This value of 1.5 can be understood if we assume : (1) that a chain reaction involving several initial free radicals does not exist; (2) that G(R.) in polyethylene is equal to 3 a8 estimated by Lawton, Balwit, and Powell20 from e.s.r. studies; ( 3 ) that G(HI) is equal to 4.0; and (4) that 3.5 molecules of KO are consumed and 1 nitrogen molecule liberated per initial free radical. Thus
APNO
3.5G(R-) A P H ~ + N ~ G(H2) G(R.1
+
-=
= 1.5
I n the case of polypropylene, the ratio 1 . 5 can also
be obtained if G(R.) and G(H2) are reduced in the same proportion. The above calculation requires that G(-XO)/G(N2) be equal to 3.5; from Table I1 it is secn to be 3.34. Conversely, one can use the measured G(-NO)/G(N,) and APNo/AP,,,+N*)ratios along with G(H,) to calculate G(R.). In the case of JIarlex50 polyethylene, G(R.) was calculated to be 3.3 and in the case of polypropylene, 1.87. The low value for polypropylene is in line with the low G(-KO) and low G(H,). The one experiment done on the low density polyethylene yielded data a t variance with those obtained using the high density material. To be sure of such results, the experiment should be repeated; furthermore, for reliable G-value calculations, an analysis of the product gases should be made in order to see whether the G(He)/G(S2) ratio was actually the same in the low density as in the high density polyethylene (this assumption was made in calculating G(H2)from the initial slope). The observed ratios of G(-NO) to G(NJ equal 1,c; 3.34 and of G(H,O) to G(-NO) equal to 0.13 are similar to the ratios found by Brown2l for the "heterolytic" decomposition reaction between KO and isobutylene, ViZ.
CAH,
+ 3.5NO C.iH?NOz
+ 0.5H20 + 0.5S02 + X2
(1)
In this reaction the ratios G(-XO)/G(N,) and G(H,O)/ G(-NO) are 3.5 and 0.14, respectively, and are to be compared with 3.34 and 0.13 obtained in this work. (20) E. J. Lawton, J. 9. Aalwit, and R. S. Powell, J . Chem. Phys., 33,395,405 (1960). (21) J. F. Brown, Jr., J. Am. Chem. SOC.,79, 2480 (1957).
Volume 68,Ifumber 3
March, 1964
532
B. J. LYONSAND MALCOLM DOLE
An infrared study of a film of Marlex-50 polyethylene irradiated in the presence of 1 atm. pressure of NO showed that the vinyl groups had decayed more rapidly than for an irradiation in vacuo. NO will not of itself react with double bonds, but the reaction might he initiated by the irradiation. The 502 generated by reaction 1 must have back-reacted with the polyethylene. Another possibility is the reaction sequence adopted by Milhaud and Durup following the chemical studies of Brown"
R.
+ 3'0
RNO 0
RXO
+ 2x0 + [R-X=X-O-K=O]I + K2 + ROE02 (2)
This sequence yields a ratio of G(-rVO)jG(Sg) equal to 3 but does not explain the formation of water. At any rate, we conclude that very probably each free radical initiated one reaction sequence such as (1) or (a), but did not initiate a chain. It is interesting to note that the relative yield of water was much greater in the case of the N20 experiments than in the case of thc nitric oxide-polyethylene irradiations. A possihlc rcaction scquence is
+ Xz0 Xz + OH OH. + RH * 1320 + Rb H*
+
(3) (4)
According to Dainton and Peterson,22 reaction 3 is slow, a t lcast in aqueous systems, but we have to consider the relative rate of (3) with (6) H.
+ RH
---f
HZ
+R
(5)
Using the reaction rate constant of reaction 3 equal to 1 2 5 x io4 1. rnoIc-* set.-' auoted by Dainton and Peterson, Okada's v a l ~ cfor~ ~ tha solubility of XZ0 In polyethylene, and an estimated reaction rate constant for reaction 6 taken from Trotman-Dickcnson,2a we calculate that reaction 5 is faster than reaction 3 by a factor of 2 X lo5. The enhanced relative yield of water in the KZO experiment cannot be explained, therefore, on the basis of reaction 3. Dainton and Petersonz2 point out that N20 is an efficient electron scavenger. We suggest, therefore, that the reaction
I
CHz+
I
+ N20-
I + Nz + OH 1
--+ * C H
(6)
is a possible source of OH and, correspondingly, water. Reaction 6 coupled with reactjon 4 would The Journal of Physical Chemistry
promote cross linking by forming two free radicals which could then unite to form a cross link. Another possibility is that through the stabilization of the positive ion by formation of YzO-, the following reaction would be promoted and could occur.
I I
CHi+
+ S20
-I
CHI-
I
+ Nz -+ OH
(7)
Reaction 7 is very slightly exothcrmic, AE equal to -0.07 e.v. molecule-1 as calculated from gas phase data. Both the positive ion of (7) and the OH through (4) could enter into subsequent exothermic cross linking reactions. Thus this reaction scheme explains Okada's observation? of thc cnhancernent of cross linking due to N z ~ .Tf reactions 6 and 7 represent the only reactions that n - 2 0 enters into during the irradiation of polyethylene, then G(-K&) should equal G(X2) and G(Hp0) The G(H2O) was roughly estimated to be about one-half of G(-NzO). This means that ?T20 was disappearing by other mechanisms or that not all the water produced was measured. From the limited data obtained in this research it was impossible to calculate G ( S 2 ) or G(Hz), Two determinations of G(H2 t X2),which could be calculated, yielded 3.9, a very small value, This suggcsts that G(hTz) was probably equal to G(-N,O) (no nitro-absorbing bands could be seen in the infrared spectra); inasmuch as G(-NzO) averagcd 1.4) G(H2) can then be calculated to be 2.5. This value is lower than expected, but the dosimetry may he a t fault. Another possibility for thc reaction mechanism of NzO is that the NzO decomposes24 under radiolysis 0 (80%) and K NO in the gas phase to give ?;* (20Lji). The oxygen atoms liberated conld react with the polyethylene to abstract hydrogen atoms to form OH and then water, or they could add to free radicals directly. The former probably occurs, because the latter reaction would decrease the cross linking yield whereas the former would enhancr it. The G-valuG4 for the gas phase decomposition of NzO is 12. However, considering the relative weights and electron fractions of the polyethylene and NZO at the gas pressure of the latter, the consumption of KzO in the gas phase should have been only 2y0of the decomposition in the polyethylene and therefore negligfhle. If dissolved NzO undergoes radiolysis in the same way that gaseous K70 does, then SOTo of the
+
+
(22) F. S.Dainton and D. B. Peterson, Proc. Roy. Soc. (London), A267, 443 (1962). (2.7) A. F. Trotmnn-Dickenson, "Gas Kinetics," Academic Press, New York, N. Y . , 1955, pp. 177 and 255. (24) P. Harteck and 5.Dondes, Nucleonics, 14, 66 (1966).
533
RADIATION CHEMISTRY OF POLYETHYLENE
+
radiolyzed NzO would form N2 0 or eventually N2 H?O and 20% N XO. The nitrogen atoms would probably abstract hydrogen atoms to form NH and eventually XH3. The nitric oxide would react as described above, giving nitrogen and other products. The observed two to one ratio of ?1T2 to H20 could be explained if less than ;SO% of the dissolved N20 decomposed to ?;* 0. An infrared examination of the polyethylene irradiated in NzO was made after almost a year’s standing in air. The vinylene band was normal, the vinyl decay appeared to be somewhat greater than normal, and the presence of an absorption band a t 5.8 p demonstrated some oxidation either during the irradiations or subsequent to it. To study the effect of T O on the infrared absorption bands, an experiment was done with KO a t atmospheric pressure in the radiation cell and the infrared spectrum of the film taken. From the infrared assignments given by Ogiharax2 and Silverstein arid Rassler, 25 it is clear that the nitrogen-containing groups listed in Table I11 were present. In addition, thc usual
+
+
+
Table 111: Infrared Absorption Rands Present in Polyethylene Irradiated in Presence of Nitric Oxide Wave
nurrrbrr, C I I I . - ~
Group
sssivntnrnt
Nitrogen-containing groups
629
NO*bonding
675 778
NOz bending in nit,rate ester (weak) trans 9-0 st,ret,rhingin nitrit,e ester (weak) NOz sym. stretching in nitrate ester SO4nit,ro-group sym. st,rctlrhing ( o t ) s c m d at 1340) KOz nitro-group nayin. ptrct c hing SO?mym. st,retrhing in nit,ritt,e e&r “ 0 , or OH stretching
1280 1:lNi
1554 1OX6
1713 9011 and !I!M 966
!I45 -!)50
Olefinir groups Vinyl Vinylenc I’ossihly allyl free radicals
vinylene band at 966 cm - I appeared, the viiiyl bands a t 909 and 990 cm.-’ decreased, and a new band a t about 945-950 cm.-l appeared. A hand at 944 cm.-’ has been attrihitrd by Icoritskii, et ~ 1 . , ~to6 the allyl frcc radical, -eHCH=CHbecause it disappears on heating. Thr 944 cm.-’ band is only seen in polyethylene a t doses much higher than those used in this resrarc11.~~Hencc, if the 944 cm.-’ band is correctly interpreted as due to the allyl group, its unexpectedly
high concentration must be the result of the presence of the nitric oxide.2s Reactions which might have yielded the allyl free radical are
-CH=CHCH2-
(in excited state)
HNO
+ KO +
+ -CHCH=CH-
(8)
or
NO NO-
+
+ XO-
e-
+ --CH=C+I>CH2-
+
HNO
+ --CH=CHCH--
(9)
Rudolph, Rielton. and Reg11nZ9have demonstrated the existence of the NO- ion in the mass spectrum of NO although a t very much smaller intensities than the Oion. A comparison of the vinyl group decay as compared to the vinylene band growth revealed that a t a dose of 14 RIrads in the presence of X O a t atmospheric pressure, the vinyl group concentration had been reduced t o 1.9 X mole g.-’ as compared to a normal vacuum irradiated value of 4.0 X while the vinylene growth was only slightly greatrr than normal, 2.8 X as compared to 2.5 X 10-6 mole g - l . Thus, if the vinylene growth was promoted by t,he presence of the YO according to thc reaction YO -CI%CHHSO -Cl-I=CHas suggested by Yang and ;\fa11110,~~~ then t,he XO milst also have caused its more rapid decay. It should tw pointed out that whcreas the vinyl groups probably wcrc concentrated in the amorphoiis rcgions and so readily accessible to the dissolvcd nitric oxide, t,hr vinylcne groups arc bclicvcd to be formed uniformly i l l the crystdlina as wrll as t,hc amorphous zones, m t l thus were lcss readily accessible, if a t all, t,o t.lie dissolved nhrlc oxidr. The enhancement of the vinyl decay by the NO may he the result of t,he presence of ?\TO2produced by reactions such as (1). NO2 reach with dorihle bonds.*l As mentioned ahoxre, the vinyl groups were probably located in the amorphous regions of the polyethylene
-
+
+
(25) It. 11. Silverstein and G . C. Rrtssler, “Spc.trornetrir Iderttifirntion of Organic Compounds,” John Wiley and Sons. I w . , New York, N. Y.,1962, p. 69. (26) N. A. Slovokhotova, A. T. Koritskii. and N. Y. Ruben. nokl. Akad. Naitk, S S S R , 129, 1347 (1959). (27) M .R. 1:allg~tterand AT. Dole, to he published. (28) Slovokhotova.Koritskii. and Ruhen also report that R hand at 938 cni. - 1 is to he attrilnited to a cis-trans coniugated diene group.
Tr. Tashkcntsk. I c o n f . PO Mimomit I s p o l r . A t . Enrrgii, Akad. .Vatrk UZ.SSR. 2 , 430 (1960). We hare not detected such n Iiand ourselves. (29) P. S. II
--1
exp
-~[T/T,
- 11
where D,is the depth of the potential well, b is a constant, and r e is the equilibrium internuclear distance, has been used t u describe the singlet bound state of two potassium atoms apparently with good successs, (1) J. 0. Hirschfelder, C. F. Curtiss, and R. B. Bird, “Molecular Theory of Gases and Liquids,” John Wiley and Sons, New York, N. Y . , 1954, p. 247. (2) See, for example, K. S. Pitzer, J . Am. Chcm. Soc., 77, 3427
(1955). (3) R. J. Thorn and G. H. Winslow, J . Phyr.Chem., 65, 1297 (1961). (4) J. F. Walling, ibkd., 67, 1380 (1963).
