1988
M. B. FALLGATTER AXD RTALcoLru DOLE
The Radiation Chemistry of Polyethylene.
VII.
Polyene Formation’
by M. B. Fallgatter and Malcolm Dole Department of Chemistry and Materials Research Center, Xorthwestern University, Evanston, Illinois (Receined March 5 , 1564)
Linear polyethylene irradiated with Co-60 y-rays up to doses of 300 blrads exhibits a number of ultraviolet absorption bands characteristic of polyenes of the structure CH,(CH= CH),CH, where n has values from 2 to 5. Other ultraviolet peaks might be due to polyenyl free radicals, but these assignments have yet to be confirmed. It is demonstrated that diene production occurs a t a much greater rate than expected statistically and that it is not proportional to vinylene concentration. Diene is formed a t a considerably reduced rate in low density polyethylene, in linear polyethylene swollen with benzene, and apparently not at all in the liquid state. Vinylene growth is approximately the same under all these conditions. Coilsideration of diene formation brings about material balance for room temperature irradiation of linear Marlex-50 polyethylene but not for irradiations in the liquid state. A suggested mechanism is given.
Introduction Conjugated double bond systems, such as linear dienes, trienes, etc., can be detected in semicrystalline polyethylene by means of infrared or ultraviolet absorption spectra. The first ultraviolet absorption spectra of irradiated polyethylene were observed rather crudely in this laboratory in 1948 by Rose2 using a Beckinan ilZodel D-318 spcctrophotometer, and published3 in 1950. A low density polyethylene was the polymer sample, and while the ultraviolet absorption showed a large increase in intensity with decreasing wave length beginning a t about 350 .nipqno structure in the absorption bands was detected. Although the ultraviolet absorption spectra have been taken on a number of other types of irradiated polymers, as for example, 011 irradiated polyvinyl chloride, there have been no systematic studies on polyethylene. Lawton, Balwit, and Powells stated that a sample of :\larlex-SO polyethylene irradiated to 2000 Jlrads at room temperature and then annealed for 5 min. a t 250” in nitrogen to destroy trapped radicals “showed weak absorption bands in the ultraviolet at 356, 322, 307, 285, and 275 mp” which they attributed to conjugated polyenes. Ohnishi, Sugimoto, and S i t t a 6 very recently deiiionstrated by means of e.s.r. observ%tionsthat an irradiation by ultraviolet light a t 2537 A. and liquid nitrogen temperature of a sample of previously electron-irT h e Journal of Physical Chemistry
radiated Marlex-50 polyethylene caused the transformation of allyl free radicals into alkyl. They noted that the ultraviolet absorption band at 236 mp decreased in intensity during the ultraviolet irradiation ; hence, they attributed this band to the allyl free radical. Other bands a t longer wave lengths a t 274, 286, 310, 323, and 359 nip increased slightly in intensity and these were qualitatively explained by Ohnishi, el al., on the basis of conjugated polyenes and polyenyl free radicals. I n 1957 Dole, Mlner, and 1VilIia1-n~~~~ observed the growth of an infrared absorption band a t about 985 cni. - on irradiation of the high density polyethylene, Marlex-50. They attributed this band to cyclic groups such as tetrasubstituted cyclopentanes or cyclohexanes which could be the result of the formation ~~~
~~~~~
~~~~~~
~
(1) The previous publication of this series was B. J. Lyons and M. Dole, J . Phya. Chem., 6 8 , 526 (1964). (2) D. G. Rose, M.S. Thesis, Xorthwestern University, 1949. (3) .M.Dole in “Report of Symposium I V . Chemistry and Physics of Radiation Dosimetry.” Army Chemical Center, Edgewood, Md., 1950. (4) G. J. Atchinson. J . A p p l . Polymer Sci., 7, 1471 (1963). (5) E. J. Lawt,on, J. S.Balwit, and R. S. Powell, J . Chem. Phys., 33, 405 (1960). (6) S. Ohnishi, S. Sugimoto, and I. Nitta, ibid., 39, 2647 (1963). (7) NI. Dole, D . C. Milner, and T . B. Williams, J . Am. Chem. SOC., 79, 4809 (1957). (8) M . Dole, D. C. Milner, and T . F. Williams, ibid., 80,1580 (1958).
RADIATION CHEMISTRY OF POLYETHYLENE
of two cross links close together. trans-Disubstituted cyclohexanes are known to absorb in the infrared a t about this f r e q ~ e n c y . ~ToddlO has also suggested the formation of "ringlinks" because of an infrared band at 1016 an.-l observed by him in irradiated polyethylene. However, Polak, Topchiev, Cherniak, and Kachkurova'l demonstrated by means of ultraviolet absorption studies that conjugated dienes mere produced by irradiation in normal saturated paraffinic hydrocarbons and Slovokhotova, Koritskii, and Buben12 suggested that the 985 cm.-l infrared band which they observed in irradiated polyethylene was due to the conjugated diene groups. This band was produced in a low density polyethylene on irradiation at - 100". The infrared absorption band was much less sharp when observed a t 25". Ahlers, Brett, and McTaggartl3 found a strong infrared absorption band at about 985 cm.-' in trans,trans-9,ll-linoleicacid which contains a conjugated diene group. The band was shifted to about 950 em.-' in 9,12-linoleic acid in which the diene is not conjugated. Conjugated trienes and tetraenes were found to absorb a t atiout 990-1000 ~ i - n - ~ . This research was undertaken partly to see if ultraviolet absorption studies could resolve the problem of the origin of the infrared band a t 985 cin: and partly to investigate the possibility of the formation of conjugated double bond systems in irradiated polyethylene. A knowledge of the polyene yields and polyene spectra is important not only from the standpoint of understanding the fundamental radiation chemistry of polyethylene, but also for achieving material balance, for comparing ultraviolet absorption maxima of polyenes in polyethylene with polyene spectra in other compounds both in the liquid (solution) and solid states, and for observing and identifying polyenyl free radicals whose absorption spectra are unknown to date.
Experimental A . Materials. The properties of the three different samples of polyethylene used in this research are collected in Table I. For the infrared measurements two thicknesses of 0.009-cm. thick film were used in the case of the low density B-3125 polyethylene, but in the case of the Marlex-50 polyethylene, seven layers of 0.004-em. thick film were stacked between two polished aluminuim plates and the sandwich bolted together using 0.25mm. spacers. The ,sandwich was first heated at 80" under vacuum for several hours to remove dissolved gases, then several hours a t 100-llOo, and then 0.5 hr. a t 145-155". Under this treatment the
1989
Table I : Properties of Polyethylene Samples
--
Manufacturer's designation--MarlexB-3125' Marlex-50b 5003b
Unsaturation in moles g.-1 x 104 Vinyl Vinylene Vinylidene ,Density, g. r n k 1 Branchesper 100 carbon atoms
M, M
W
a
D u Pont.
0.087 0.061 0.414 0.909 2.5 17,500 266,000
0.86 0,029 ,956
0 12,300 156,000
0.3 10,100 194,000
Phillips Petroleum Co.
films fused into a single thick film. After a slow cooling over several hours to rooin temperature, the sandwich vias inserted into a boiling dilute detergent solution for several minutes to break the bond between the film and the aluminum. The Marlex-50 film was too thick for the ultraviolet studies, so thin films of Marlexcm. thick 5003 polyethylene between 7-18 X were obtained from the Visking Co. of Chicago. Information received from J. A. Reid of the Phillips Petroleum Co. states that Marlex-5003 is a copolymer of ethylene and 1-butene containing about three ethyl branches per 1000 carbon atoms and 0.02% of the antioxidant, 2,6-di-t-butyl-4-niethylphenol, some of which may have evaporated from the film during prcparation. Infrared spectra of the film indicated that vinyl unsaturation and crystallinity were comparable to those in Rlarlex-50. Because of the lower crystallinity of the low density B-3125 film and consequently lower light scattering, thicker films of B-3125 polyethylene could be used in the ultraviolet light studies thereby coiisiderably increasing the sensitivity of the measurements. B . Radiation Source and Cells. Our Co60 y-ray source, originally of 234 c. in May, 1956, was removed and replaced with 1288 e. of Co60 in November, 1962. The new radiation intensity was 1.0 RIrad hr.-', and the heating effect of the radiation was such that the temperature of the polyethylene films during the irradiation was about 38". The intensity of the source (9) R. T. O'Connor and L. A. Goldblatt, Anal. Chem., 26, 1726 (1954). (10) M. N. Todd, Jr., Nucl. Sci. Abstr., 15, No. 4346 (1961). (11) L. S. Polak, A. V. Topchiev, N. I. Cherniak, and 1. I. Kachkurova, Dokl. Akad. N a u k S S S R , 119, 117 (1958). (12) N. A. Slovokhotova, A . T. Koritskii, and N. Y. Buben. ibid., 129, 1347 (1959). (13) K. H. E. Ahlers. R. A. Brett, and N. G. McTaggart, J . A p p l . Chem., 3, 433 (1953).
Volume 6R, A'umber 7
J u l y , 1964
4990
was determined both by ineans of the Fricke dosimeter as previously described7,*and by measuring the rate of vinyl decay in the polyethylene films. Knowing the first-order vinyl decay constant7r8 and the time, the dose rate could be calculated. The two methods agreed within 3%. The radiation cells m r e similar to those used in other investigations in this laboratory. All polymer samples in the cells were evacuated a t 10-3-10-5 nzin. for 48 hr. or inore before being sealed off for the irradiation. Some experiments were done in which the polyethylene samples were equilibrated with benzene at its equilibrium vapor pressure a t 36", liquid benzene being contained in a side arni of the radiation cell. The usual freeze-pump-thaw cycle was carried out several times to remove all dissolved air from the benzene after which the polyethylene samples were evacuated with the benzene frozen for 48 hr. or more in the case of the thick samples and for about 8 hr. in the case of the thin. Several experiments were carried out a t liquid nitrogen temperature in which helium a t 1 atm. pressure was added to the cell a t rooin temperature to improve heat transfer a t the low temperatures. Before obtaining the new Co60 several of the irradiations were performed in the ?-irradiation facility of the Argonne A-ational Laboratory a t a dose rate of about 1 RIrad hr.-l. C. Hydrogen Evolution, Gel, and Density Measurements. The total yield of hydrogen was measured by means of a Toepler pump, and in the case of four samples irradiated a t the Argonne Xational Laboratory over the dose range 12-113 Nrads the average G value a t room temperature was 3.7 f 0.1 molecules of hydrogen liberated per 100 e.v. of energy absorbed as compared to 3.8 in the previous work. The insoluble material (gel) produced by the irradiations was determined in the usual way by weighing the film after extraction of a soluble fraction with boiling toluene containing 0.05-0.1% N, X'-di-P-naphthyl-p-phenylenediamine as an antioxidant. The density of irradiated and extracted films was determined by a flotation method using a water-ethanol mixture as the flotation medium. D . Infrared arld Ultraviolet Techniques. The infrared measurements were made mostly as before7,* using a Baird double, beam recording spectrophotometer with sodium chloride optics. The slit width was set a t twice its normal value to improve quantitative accuracy (but with somewliat reduced resolution). A control sample of Marlex-50 polyethylene was used in each study to set the shutter openings so that thc background of the control sample was near 80yo transinission over the 10.6-11.4 p region. All bands of The Journal of Physical Chemistry
h'f. B.
FALLGATTER A N D RfALCOLM
DOLE
interest in the unknown samples were corrected by a factor Av,O/Av, where A v , was the absorbance of the vinyl group in the control, measured a t the time the spectrum of the saniple under study was taken, and Av,O was an arbitrarily chosen standard, the average of a nuniber of values of A v , . Table I1 lists the molar extinction coefficients used in calculating the concentrations in the infrared studies.
Table I1 : Infrared Molar Extinction Coefficients Emax,
Xmax
Structure
Group
Ir
1. mole-' om
-1
Ref.
Infrared
R
H
\
Vinyl
H
/
/"=".,
11 0 p
153
8
11 2
159
8
10 34
139
8
360
13
R
\
Vinylidene
R
/C=CHz
R trans-Vinylene
trans,transConjugated diene
/"
\
/c=c\ H
R
R
H
/
\
/"
/C=C\ H
/c=C\ H
R
The ultraviolet absorption spectra of the thin ;\Iarlex-5003 films were taken with a Beckman DK-2 ratio recording spectrometer with quartz optics, soinetinies against air as a reference and sonietiines against an unirradiated filin as a standard. The most significant ultraviolet band observed at 236 nip was that due either to the conjugated diene double bond group or to the allyl free radical or perhaps to both. The band a t 236 nip can be seen from Fig. 1 to be very broad, any determination of group concentrations from its peak height can be only approximately correct a t best. The concentration, c, of the diene and other polyenes in units of moles of conjugated groups per gram was calculated froin the equation c =
( A - Ao),,J1000~dp
(1)
where E is the extinction coefficient at the wave length of the peak maxiinurn in units of 1. mole-1 cin-', d is the thickness of the film in cm., p is the density of the
RADIATIOX CHEMISTRY OF POLYETHYLEXE
1991
-
~ ~ _ _ _ _ _
Table I11 : Ultraviolet Molar Extinction Coefficients %l&X3
Group
Xmhx,
1. mole-'
mfi
ern.-'
Structure
Ref.
H
\
/
H
\ /
C-R \H
2o
L 230
240
L
250
260
270
280
290
Triene
-CHn( CH=CH)aCHn-
274 41,800
Tetraene
-CHZ( CH=CH)dCHz-
310
300
m/i
Figure 1. Ultraviolet absorption spectrum measured against air of Marlex-5003 polyethylene film. Curve 1, unirradiated film. Curve 3, film irradiated t o 113 Mrads, maintained in N n atmosphere until 15 min. before taking spectrum. Curve 2 , aame as curve 3 but after storage in air for 24 hr.
film in g. and A and A. are the peak heights of the irradiated and uiiirradiated film, respectively. The thickness of the thin films was calculated froni the weight of a measured area and the density. The ultraviolet extinction coefficients used in the calculations of this paper are collected m Table 111. The value listed for the diene was obtained from nieasurements on hexane solutions of A2,4-l~exadiene.Other dienes or other solvents give somewhat different A,, and €-values. Inasmuch as A,, and E vary both with the nature of the solvent and type of diene, the assignment of the 236 nip peak to the diene must be considered tentative. However, diene concentrations calculated from the peak height, at 236 mk agreed with those calculated from the infrared absorbance a t 10.1 k within about 60% although direct comparison of the two 011 the same film mas not possible because of the necessity of using films of different thickness in the two experiments. I n the case of one experiment on the benzene vapor saturatcd film, G(die1ie) was calculated to be 0.087 and 0.080 from the ultraviolet and infrared absorbance measurements, respectively. I n the benzene saturated film there were probably few allyl free radicals to interfere with the diene estimates. The assignment of the ultraviolet absorption peaks a t longer wave lengths than that of the diene will be discussed below. E . Irradiation of Benzene Saturated Fzlms. Several experiments were performed on films saturated with benzene a t about 316". The technique consisted in maintaining the radi,stioii cell containing the Marlex5003 films in the thermostat a t 38" for 10 days while
58,900
a H. Booker, L. K. Evans, and A. E. Gillam, J . Chem. Soc., 1453 (1940.). F. Bohlmann, Ber., 86, 63 (1953). E. A. Brande and C. J. Timmons, J . Chem. Soc., 2000 (1950); E. A. Brande and J. A . Coles, zbid., 1425 (1952). e R. T. O'Connor and L. A. Goldblatt, Anal. Chem., 26, 1726 (1954).
the side arm containing the liquid benzene was kept at 36". At the end of this period the cells were transferred directly to the y-ray source and the irradiations carried out. During the irradiation the side arm of the cell was outside the zone of direct y-ray irradiation where the radiation intensity must have been at least a factor of 10 lower than that a t the location of the polyethylene samples. Coupled with low hydrogen yields from liquid benzene, the over-all contribution of hydrogen from benzene to the total observed hydrogen must have been negligible. By weighing the bensenesaturated film in air as a function of time and extrapolating the weights back to zero time it was estimated that the films contained 8.570 by weight of benzene. Some years ago14 we irradiated Marlex-50 immersed in liquid benzene a t about 25" with a yield of hydrogen equal to that of this paper.
Results and G Values A . General Type of Cltraviolet Spectra Observed. Figure 1 illustrates the ultraviolet absorption spectra of irradiated Marlex-5003 film over the spectral range 225-300 nib. Curve 1 for the uiiirradiated film demonstrates that the ultraviolet spectrum has no structure in the absence of the irradiation. The spectrum illustrated by curve 3 was taken several days after the irradiation on Marlex-5003 irradiated in the y-ray facility of the Argonne rational Laboratory to 113 Mrads. The ultraviolet spectrum mas taken in air within 15 min. after the removal of the film from the ~
(14) Experiment conducted by Dr. A. J. Arvia.
V o l u m e 68, S u m b e r 7
J u l y , 1864
h1. B.
1992
irradiation cell. 011 standing in air for 24 hr. curve 2 was obtained. The ultraviolet spectrum over the 300400 nip range is shown in Fig. 2. This is for a much higher dose, 290 Mrads, than that of Fig. 1. The film was interniittently removed from the source and its ultraviolet spectrum taken 15 tiines during the irradiation. Kote the existence of five well defined peaks and possibly a sixth a t 375 mp. The ultraviolet absorption spectrum of a low density polyethylene, B-3125, irradiated to 49.8 Slrads is illustrated in Fig. 3. Kote that no absorption peaks in addition to the diene peak at 236 m p could be detected. Absorption peaks at higher wave lengths were also
'\
'.'. '..
40
IOOA 30
FALLGATTER A&D ;\IALCOLI\I
DOLE
undetectable in the case of Marlex-5003 films irradiated in the presence of benzene vapor. In the case of an irradiation of Marlex-5003 a t -196" to a dose of 50 X lozoe.v. g.-l with about 1 atni. pressure of helium in the radiation cell to improve heat transfer, no absorption peaks a t wave lengths greater than 285 inp could be detccted. A band at about 259 mp existed and was relatively more pronounced than this band in the case of the room temperature irradiation, possibly because of great care taken to prevent access of the film to oxygen by purging the spectrophotometer cell compartment with a stream of nitrogen during the mcasurenient of the ultraviolet spectrum. I n Fig. 1 there is an indication of a band a t 259 m p which decreased rather sharply in intensity on storage of the film in air for 24 hr. B . Dzene Concentrations and G Values. Using the infrared and ultraviolet extinction coefficients given in Tables 11 and 111 diene concentrations were calculated and are plotted in Fig. 4. As mentioned above diene estimates from the infrared measurenients were about 60% of the estimates from the ultraviolet studies, except in the case of the irradiation in the presence of benzene vapor. The higher ultraviolet
20
10
-
I
300
310
320
330
340
350
360
370
380
390 400
mll
Figure 2. Ultraviolet absorption spectrum of Marlex-5003 film irradiated intermittently (16 increments) t o 290 hlrads with ultraviolet measurements between each dose; SI-5003 film as reference.
ao
'
50 A
0
r e.v.g-' x w Figure 3. Ultraviolet spectrum of low density B-3126 polyethylene film: lower curve, unirradiated film; upper curve, irradiated to 49.8 Mrads.
T h e Journal of Physical Chemistry
IO-''
Figure 4. Estimated diene Concentrations in Marlex-5003 polyethylene as a functlon of dose: 0, continuously irradiated, e, intermittent irradiation; -0-, irradiated In benzene vapor; A, low density polyethylene; G ,infrared estimates in i'v1arlex-50. Dotted line statistical expectations.
RADIATIOS CHEMISTRY OF POLYETHYLEXE
1993
Table IV : Material Balance D a t a : G Values in Units of Groups Produced per 100 E.v. of Energy Absorbed Polymer
Marlex-50 B-3125 Marlex-50 Marlex-50 a
+ CSHs
Temp.
G(X)
G(V1)
2G(di)
3G(tri)
Sum
G(I3z)
r.t. r.t. 142" 38"
0.8 0.8 1.8 0.59
2.4 1 7 3.1 2.4
0.4 0.1 0 0.17
0.07 0 0
2.67 2.5 4.9 3.2
3.7 4.1" 6.0 2.4
Diff.
-0
1.6 1.1 -0.8
T. F. Williams, Ph.D. Thesis, University of London, 1960, p. 51. Work performed a t Northwestern University.
estimates may be the result of the allyl free radicals absorbing also in the same 'ultraviolet wave length region as the conjugated diene group, or it may be that the extinction coefficients were in error. The latter were not checked by studying the ultraviolet absorption spectrum of polyethylene containing known concentrations of dienes. However, the fairlg good agreement of G(diene) values calculated from the infrared and ultraviolet studies in the case of the films irradiated in the presence of benzene vapor where less trapping of allyl free radicals would be expected suggests that the extinction coefficients cannot be greatly in error. G(diene) was calcula1,ed from the initial linear portion of the diene growth curves of Fig. 4. Results obtained are collected in Table 1V. C. Material Balance Calculatiom. Over the long history of the radiation chemistry of polyethylene many attempts have been made to obtain material balance by equating the hydrogen yield to cross links plus single double bond formation, or
G(HJ
=
G(V1)
+ G(X)
(2)
where G(V1) and G(X) are the G values for vjnylene formation and cross linking, respectively. Chapirolb has shown that material balance can be attained a t room temperature froni ( 2 ) if G(X) is computed from modulus of elasticity nieasurements. Williams and DoleI6 mere unsuccessful in their attenipts a t material balance when they used G(X) values computed from gel measurements and theory. The disagreement was worse for radiations in the molten state a t 142" than a t room temperature. Dole, Milner, and Williams7 introduced the concept of intrainolecular cross linking or "ringlinks" to acclount for the discrepancy. However, taking into consideration diene and higher polyene formation and neglecting, for the moment, ring linking, the material balance eq. 2 becomes G(H2) = G(X)
+ G(TY1) + 2G(diene) + 3G(triene)
+.
.
(3)
Data for the testing of eq. 3 are collected in Table I V where G(di) and G(tri) stand for G(diene) and G(triene), respectively.
It will be seen that good material balance is obtained in the case of Marlex-50 a t room temperature (r.t,), but not in the case of the low density polyethylene. B-3125, nor in the case of Marlex-50 in the molten state where infrared studies indicated that the absorption band a t 988 em.-' did not exist. It should be noted that the G(X) values of Table IV are uncorrected for chain degradation, or for decay of unsaturation initially present in the polymer. Correcting for vinyl decay would tend to lower17 the G(X) values while correcting for degradation mould raise them. l8 The differences between G(H2) and the sum of the G- values for cross linking and unsaturation of Table IV can be accounted for' either by assuming the formation of intramolecular bonds, or by assuming that the G(X) values as calculated from gel measurements are incorrect. Inasmuch as there is no reason for accepting the G(X) values for the room temperature irradiations and rejecting them for the irradiations a t 142", we conclude that the material balance discrepancy probably arises from intramolecular cross linking which is not measured by the gel measurements.
Mechanism of Diene Formation A . Statistical Expectation. If we assume that diene groups are formed by the simple process of hydrogen elimination at the same rate as for vinylene production but with a probability of being located on either side of a vinyleiie group along the chain equal to the number of such sites divided by the total number of possible sites, then the following differential equation for diene growth with dose may be written d(diene)ldr
=
2p[Tr1]/ [-CH,-]
(4)
(15) A. Chapiro, "Radiation Chemistry of Polymeric Systems," Interscience Publishers, New York, N. Y., 1962, p. 439. (16) T. F. Williams and M. Dole, J . Am. Chem. Soc., 81, 2919 (1959). (17) M . Dole, T. J. Stolki, and T. F. Williams, J . Polymer Sci., 48, 61 (1961). (18) M . Dole in "Crystalline Olefin Polymers," Val. I, R. Raff and K . W. Doak, Ed., to be published by Interscience Publishers, New 1-ork, N. Y., Chapter 16.
Volume 68, Num.ber 7
J d y 4 1964
M. B. FALLGATTER AKD M A L C O L M DOLE
1994
In eq. 4 [Vl] represents the concentration of vinylene groups in moles g.-l; Y, the dose in e.v. g.-l; 9,the initial rate of production of vinylene groups in moles (e.v.)-I; and [-CH,-] the total moles of -CHzCH2groups per gram of polymer. Dole, Milner, and W i l l i a n i ~found ~ ~ ~ that a t relatively low doses the vinylene concentration could be expressed by the equation
where IC, is the first-order constant for disappearance of vinylene groups and [Vl]a represents the initial vinylene concentration. Introducing (5) into (4) and integrating, we obtain [diene] =
2cp[~1,]r [-CHz- I
+
mediate in the sequence of events leading to conjugated diene reacts in some other way than to form diene in polyethylene samples of low or zero crystallinity. To give any detailed mechanism for the formation of diene groups a t this time would be pure speculation. A suggestion which has occurred to us is that the first stage after excitation or ionization is elimination of niolecular hydrogen from hydrogen atoms in closest position to each from two neighboring locations, namely
H'
~ [ v ~ ] , [vL ~ l ~ ] [ e - "" 11 [-CHz-
I (7)
(6) where [VI,] equals p/kz. Using the values for cp and IC2 determined by Dole, Nilner, and Williams, it is possible to calculate the expected diene concentration a t each value of r. The dotted line of Fig. 4 repre-ents these calculated values. It is immediately apparent that there is absolutely no correspondence betwcen fact and expectation based on this statistical picture. Equation 6 requires that G(diene) be the low value of 0.0002 a t aero dose whereas in fact there is no induction period for diene formation, the diene concentration increases linearly with the dose a t the lowest doses and the initial G is about 0.2, some 1000-fold greater. At higher doses the rate of production of diene falls off while the statistical theory predicts an increase in the rate a t the high doses. It is interesting that the diene growth remains linear up to the rather high dose of about 50 X lozoe.v. g. before secondary processes cause a reduction in the rate. B . Postulated Mechanism The above calculation demonstrates that diene growth does not depend upon the pre-existence of vinylene double bonds. One might have predicted that the reaction sequence would have been vinylene allyl free radical diene or a stepwise elimination of hydrogen atoms to forin the diene. Any correct mechanism of diene formation niust explain the following facts : the initially linear growth of the diene with dose, the lower diene yields in low density polyethylene, the absence of diene formation for irradiations in the liquid state, and the reduction in diene growth in the benzene saturated films. Inasmuch as vinyleuc growth is even greater in the liquid than in the solid seniicrystalline state, it is obvious that the niechanisnis for vinylene and diene formation must be distinctly different. Probably some inter-+
The Journal o,f Physical Chemistry
-+
I
H
I
H
The elimination of the two hydrogen molecules could form either the diene or a disubstituted bicyclobutane ring. Lemal, Menger, and Clarkl9 have shown that the photolysis of allyl diazoinethane at - 78" yielded butadiene and bicycloll.l.O]butane in the ratio 5: 1. The concentration of the bicyclo compound dropped to zero after a few days even at temperatures me11 below 0" because of polynierization or of autoxidation. It is assumed that only in the case of the linear chains held in good alignment in the crystal lattice would diene formation occur. This assumption would account for the reduced or zero yield of diene in the liquid state, in lorn density polyethylene, and in polyethylene swollen with benzene. On this model it is possible to calculate mathematically the expected diene G value. Imagine a sphere of radius 4 A. about a selected vinylene group. I n the act of forming this particular double bond much energy will be deposited and if we assume that the second vinylene group will be formed a t a rate equal to cp of eq. 4, but with the ratio 2[Vl]/[-CH,-] calculated for the sphere rather than for the whole system, then G(diene) mill be given by the equation G(diene)
=
100NAcp(2/12)
(8)
where N A is Avogadro's number and 12 is, the number of -CHz- groups iii the sphere of radius 4 A. This calculation yielded G(diene) equal to 0.37 which is greater than the observed values; hence, this inechanism s e e m possible. (19) D. M. Lemal, F. Menger, and G. W. Clark, J . Am. Chem. Sot., 8 5 , 2529 (1963).
RADIATIOX CHEMISTRY OF POLYEVHYLEXE
1995
Another suggestion is that after the formation of one vinylene group by the slightly exo1,hermicl8 reaction -CHzCHZ+-
+H,
+ -CH=CH+(AB
=
(9)
-0.15 e.v.)
the vinylene ion is neutralized by the process -CH=CH+-
+
E
+-CH=CH--
(excited)
(10) which leaves the vinylene group in an highly excited state equal to the energy of electron-ion recombination (-+9 e.v.). A certain fraction of the vinylene groups might then become thermalized by aid of elimination of molecular hydrogen according to the reaction -CHzCHzCH==CH-
Hz
(excited) -+
+ --CH=CH-CH=CH-
(11)
If this mechanism is solely responsible for diene formation, then the fraction of excited vinyliene groups yielding diene would be equal a t low doses to the ratio G (diene)/G(Vl) or about 0.08. This fraction is not large enough to interfere significantly with vinylene production. We assume that if the polyniethylene chain is not in the appropriate trans-trans configuration for hydrogen elimination, then deactivation of excited vinylene groups by reaction 11 would not occur. This would be the situation in the liquid state or partially so in the benzene-swollen polyethylene and in low density polyethylene. There is also the possibility that diene formation partially occurs by formation of an allyl radical followed by elimination of hydrogen to yield the diene. This mechanism doef not seem verg likely in light of the recent observations of Ohnishi16 et al., who believe that ultraviolet radiation transformed allyl free radicals into alkyl rather than into stable diene groups. It is interesting to note that jn the work of Evans, Higgins, and Turner20 on the ultraviolet absorption spectra of irradiated rubber G(diene) found by thein averaged 0.08, but G(triene) calculated from an absorption band a t 290 my was much lless, about 0.002. Considering the high initial concentration of single unconjugated double bonds in the rubber, this low yield of triene groups (if the band a t 290 my really represented triene groups) demonstrates again that polyene formation is not dependent, a t least to a first approximation, on the prior presence of single double bonds.
Higher Polyenes A . Identi5cation of Peaks at Longer Wave Lengths. I n Table V we have listed the A, values of all of the
ultraviolet absorption bands observed by us along with the experimental values of polyenes of the type CH3(CH=CH),CH3 tabulated by Bohlniann and Mannhardt21according to their n value. I n the third of the and fourth columns of 'Table V are given A,, longest wave length peak, Az, and the next to longest peak, XI, as found experinientally by n'ayler and Whiting.22 I n some of these polyenes three or four welldefined bands were observed, but XI and Az are the A,, values of the two most intense bands. I n the fifth values calculated according to the column are the, , ,A free electron model of Bar, Huber, Handschig, Martin, and K ~ h n .I n~ the ~ sixth column are the ultraviolet band assignments for irradiated rubber adopted by Evans, Higgins, and TurnerlZ0and in the last coluinn are the A,, values observed in this work classified according to order of appearance along the ascending wave length scale.
Table V : Ultraviolet Absorption Maxima
--Dimethyl
polyenes in solutionX,,xc
n
Amax"
2 3 4 5 6
227 263 299 326 352
7 8
Xib
heb
264 298 326
275 310 342
396 413
9
calcd.
238 286 323 352 377 396 413 428
Irradiated rubber Xmaxd
245 290
Irradiated polyethylene, --this paperBand number ,,,A
1 2 3 4 5 6 7
8 9 SO
a
Ref. 21.
* Ref.
22.
Ref. 23.
Ref. 20.
e
230 236 245e 274 285e 310 323" 340 35ge 396"
These peaks
belong to group A.
Our observed ultraviolet absorption bands may be grouped into two classes, strong bands, group A, and less intense bands, group B, as illustrated in Fig. 5. The first three bands in Table V may be simply three branches of the diene band, or they might include a contribution from the allyl free radical inasmuch as
(20) M. B. Evans, G. M . C. Higgins, and D. T. Turner, J . A p p l . Polymer Sci., 2, 340 (1959). (21) F. Bohlmann and H. J. Mannhardt, Ber., 89, 1307 (1956). (22) P. Nnyler and M. C. Whiting, J. Chem. Soc., 3037 (1955). (23) F. Biir, W. Huber, G. Handschig, H. Martin, and H. Kuhn, J. Chem. Phys., 32, 470 (1960).
Volume 68, Number 7
J u l y , 1964
M. B. FALLGATTER ASD MALCOLM DOLE
1996
LOG
A
340 -
mtL 320 -
300
-
280
-
260
-
240
-
220
-
0 0 THIS PAPER 0 CH,(CH=CH),CH,
HOURS Figure 6. First-order plot of the decrease in the ultraviolet absorption a t 285 mg of a Marlex-5003 film on exposure t o air after a n irradiation t o a dose of 290 Mrads.
the free radicals. Second, some of the peaks assigned to group A might be branches of those of group B. As demonstrated by the A1 and Xz values of Table V and as noted by othersz5ultraviolet absorption bands usually contain two or three strong branches. However, in the case of the dimethyl polyenes in solution the branch a t the shorter wave length usuallyzZ is the more intense band in contrast to the pairs of bands illustrated in Fig. 1 and 2, namely the 274 and 285, 310 and 323, and 340 and 359 nip pairs where the band a t the longest wave length is more intense than the band at the next lower wave length. Obviously, more work needs to be done before the assignment of all of the bands of Fig. 1 and 2 can be made with certainty. B. Decay of Absoyption Bands on Standing in Air. I n Fig. 6 is illustrated a first-order plot of the relative decrease of the absorption band at 285 nip with time. This plot is also similar to that for the bands a t 323 and 359 nip. A significant observation here is that even after many days in air, a residue of ultraviolet absorption still remains. This was true for all of the ultraviolet, peaks. This result may be interpreted by assuming that certain of the reactive groups are trapped in the crystalline lattice in such a way as to prevent reaction.
b 2
4
8
6
IO
I2
14
16
2n+l Figure 5. Wave length of the absorption maxima of polyenes plotted as a function of 2n 1 where n is the number of conjugated double bonds per group. I n the case of group ,4, the band at 246 mg was arbitrarily assumed to be due to the allyl free radical, z.e., to contain one double bond.
+
Longuet-Higgins and Poplez4 have calculated on the basis of a wave mechanical treatment that the ultraviolet absorption of the allyl free radical should exhibit a strong band a t 234 nip, If we assign the band a t 245 mp' to the group of strong bands and then plot their A, values as a function of 2n 1, the straight line labeled A in Fig. 5 is obtained. The weak bands, i.e., bands 2,4,6, and 8 of Table V, constitute another group and when plotted siiiiilarly to the first group give the sinooth curve labeled B in Fig. 5. This curve parallels quite nicely the observed curvez1 for the dimethyl polyenes and also the curve of the calculated values of Kuhn, et aLZ3 It is tempting to identify the, , ,A values of group A with the absorption bands of polyenyl free radicals, but the following objections may be raised against this assignment. First, some of these bands were observed by Lawton, Balwit, and Powell5 after heating their heavily irradiated polyethylene to 250" for 5 min. in nitrogen. E.s.r. measurements demonstrated that niost of the free radicals had been destroyed by a 10min. heat treatment; however, the 5 min. of heating may not have been quite enough to eliminate completely
+
The Journal of Physical Chemistry
Acknowledgments. This research was supported by the U. S. Atomic Energy Comniission and by the Advanced Research Projects Agency of the Department (24) H. C. Longuet-Higgins and J. A. Pople, Proc. P h y s . SOC.(London), A48, 591 (1955). (25) F. Sondheimer, D. A. Ben-Efraim, and R. Wolovsky, J . A m . Chem. Soc., 83, 1675 (1961).
NOTES
of Defense through the Northwestern University Materials Research Center. We are grateful for the gift of the Narlex-50 from the Phillips Petroleum Co. and of the Marlex-5003 film from the Visking Co. of
1997
Chicago. The cooperation of the staff of the Argonne Yational Laboratory in carrying out irradiations for us and in effecting the transfer of the new Co60 to our radiation source is hereby acknowledged.
NOTES
A Complicating Factor in the Photochemical Reaction of Aliphatic Ketones with Oxygen-18 by R. Srinivasan I B M W a t s o n Research Center, Yorktown Heights, New York (Received February 84, 1964.)
It has been reported that on exposure of a mixture of acetone vapor and oxygen-18 gas to 3130 b. radiation, acetone-0I8 is produced.' A similar reaction was also observed in the system diethyl ketone:0182. The quantum yields for these reactions are high, being of the order of 0.,5, A surprising fact about these processes is that while the oxygen-16 atom in the ketone molecule is replaced by an oxygen-18 atom, ithe isotopic composition of the oxygen gas does not undergo any change at all. This fact has been confirmed by Pearson2 as well as by Gal and K u t ~ c h k e who1 , ~ used 0'8-labeled ketone and light oxygen. The prlesent study was undertaken in a n attempt to understand the mechanism of this reaction and examine its applicability as a probe for the tripllet states of carbonyl compounds. It soon became apparent that the conditions under which the labeling reaction occurred had not been fully established. As a result, much of this work was devoted to the development of the conditions for bringing about the reaction. Experimental Diethyl ketone and 5-hexen-2-one (both Eastman Kodak White Label products) were distilled at atmospheric pressure, were admitted to a vacuum line by bulb-to-bulb distillation, and were degassed before use. The sample of oxygen-18 gats was obtained from the Weizniann Institute of Science. 1t)s com-
position, which was given as 018,97.56; O", 0.615 was confirmed by analysis. The apparatus that was used was similar to the one described before.' In addition to nixing before irradiation, thc 1 eactants were stirred a t intervals during irradiation. The light source was a General Electric AH-6 high pressure mercury arc lamp. Since its intensity decreased steadily with time, it was calibrated with a chemical actinometer (diethyl ketone) after every two runs. Only the ketone fraction of the photolysate was analyzed by mass spectrometry. A Consolidated Electrodynamics 21-620A mass spectrometer was used. In the analysis of 5-hexen-2-one-0 l8 some difficulty was experienced due to interference from the doubly ionized niercury peak at 100. However, a coniparison of the ratio of mass 45 (CH3C018)to mass 43 (CHaCO16) with the ratio of mass 100 (hexenone-018)to 98 (hexenone-0I6) served to correct for the contribution from the mercury peak.
Results The rates of formation of diethyl ketone-018 in the system diethyl ketone-0182 are given in Table I. The results are arranged in the order in which the runs were carried out. Initially, the cell had been used in three experiments on the photolysis of 5-hexen-2-one in the presence of oxygen. The "conditioning" of the cell consisted of photolysis cf diethyl ketone ( p 30 mm.) in the presence of oxygen-18 ( p 20 mm.) with the radiation from an unfiltered medium pressure mercury arc (100 w.). Extensive oxidation of the ketone was
- -
(2) G. S. Pearson, J Phys. Chem., 67, 1686 (1963). (3) D. Gal and K . 0. Kutschke, Abstracts, 145th National Meeting of the American Chemical Society, New York, N. Y . , September,
1963, p. 12W.
Volume 68, Number 7
July, 1964
