DAVIDC,WATERMAN AND MALCOLM DOLE
3988 in spectra from pyrolysis of I has proved that the dissociation process goes primarily through formation of CeHbHgBr rather than of CeHbHgCl, in agreement with reactions of I in solution and its decomposition in the solid It may be concluded from the study of the pyrolysis of C6HiHgCC12Brthat this reaction is at presept the best source of dichlorocarbene in the gas phase.
Acknowledgments. This work was supported by the U, S. Atomic Energy Commission. Liquid helium was supplied by a grant from the Office of Naval Research. The experimental part of this work was done at the Department of Chemistry, Rice University, while one of us (A. K. M.) was a Visiting Scientist under auspices of the U. S. S. R. and U. S. A. Academies of Science.
The Infrared Spectrum of Polyethylene Irradiated at 4 ° K by David C. Waterman Department of Chemistry and Materials Research Center, Northwestern University, Evanston, Illinois 60201
and Malcolm Dole* Department of Chemistrg, Baylor Univwsity, Waco, Texas 76'708 (Received J u l y 26, 1971) Publication costs assisted by the U . S . Atomic Energy Commission
Both ultraviolet and infrared measurements were made at 4'K on a high-density polyethylene after irradiation with 1-MeV electroiis at 4°K. No significantchanges were observed in the uv spectrum due to the irradiation. On warming to 77°K a small shoulder appeared at 258 nm equivalent to about 4% of the allyl free radical ultimately formed after annealing at room temperature. I n the ir spectra two overlapping absorption bands at about 973 and 966 cm-l were produced at 4'K, but on warming to 77°K the two bands were transformed into the siiigle sharp band characteristic of the vinylene group at 966 em-'. The total area of the double band was almost exactly equal to that of the single band. The new band at 973 cm-l is tentatively assigned to the transient, the -CH=C+Hlon. Irradiations at 77°K with subsequent heating to room temperature demonstrated that there is no decay of the trans-vinylenegroup during low-temperatureirradiations.
Introduction In this paper both the ultraviolet and infrared spectra are reported of linear polyethylene irradiated with 1-MeV electrons at liquid helium temperature and at 77°K. The uv observations will be described, but they were not as interesting as the changes in the ir spectra; hence the emphasis in this report will be on the latter. As far as me are aware, there has hitherto been no observations of the uv or ir spectra at 4°K of polyethylene (PE) irradiated at that temperature. Rexroad and Gordyl irradiated PE at liquid helium temperature and examined its esr spectrum to see if hydrogen atoms could be formed and trapped at 4"K, but none was , ~ the radiothermoluminesfound. Aulov, et ~ l .studied cence on heating to higher temperatures of both lowand high-density polyethylene irradiated at liquid helium temperature.
Experimental Section Marlex 6002 PE which was used in our previous studies3r4was melted onto nickel wire mesh having a reported transparency of 80%. By supporting the PE T h e Journal of Physical Chemistry, Vol. 76, N o . 36, 1971
in this way rapid heat transfer could be achieved between the thin layer of PE, its nickel support, and the copper block to which the nickel mesh was clamped with the nickel mesh side in contact with the fixed half of the block. A liberal application of Apiezon grease N around the edge of the niclrel mesh improved the thermal contact. In the case of the irradiations at 77"K, Marlex-6002 films 0.26 and 0.023 mm in thickness were used. The combined irradiation and spectroscopic cell is illustrated in Figure 1. It was an adaptation of the cell previously used3 at 77°K. A and B are the filling tubes for the liquid nitrogen, C, and helium, D, rcservoirs, respectively. Inasmuch as the cell was held at an angle of about 30" under the electron beam generator so that the stream of electrons could pass through the H. N. Rexroad and W. Gordy, Phys. Res., 125,242 (1962). (2) V. A. Aulov, F. F. Sukhov, I. V. Chernyak, and N. A. Slovokhotova, Khim. V y s . Energ., 2 , 191 (1968); 3,452 (1969). (3) D. C. Waterman and M. Dole, J . P h y s . Chem., 74, 1906 (1970). (4) D . C . Waterman and M. Dole, ibid., 74, 1913 (1970). (1)
INFRARED SPECTRUM OF POLYETHYLENE
w 6
A
C
L
1
Figure 1. Low-temperature combined irradiation and spectroscopic cell. Parts shaded with slant marks were made out of copper while the completely shaded areas were made of brass. Other parts were made of stainless steel.
windows of the cell and the PE sample in a vertical direction, the cell windows on jacket J of Figure 1 should be rotated through 90" about the vertical axis in the figure to conform to actual practice. E is a port containing the electrical connections for a thermocouple vacuum gauge; F is the connection to the vacuum pump. G contains the electrical leads to the chromel-alumel thermocouples on the liquid nitrogen shield and to the carbon resistor (used to measure the temperature) on the sample holder, H contains the leads to the heaters on the nitrogen radiation shield and sample holder, respectively. At point I are the bolts that hold the lower section surrounding the sample holder to the liquid helium reservoir. Evacuation of the gas in the space surrounding the liquid helium reservoir D takes place through holes at the bottom of the radiation shield at 11 and N. The outer section which is attached at J after the irradiation is evacuated through the port 0. When this is accomplished, the titanium windows through which the electron beam passed during the irradiation fall off and can be rolled aside. Atmospheric pressure then holds the outer windows in place for the spectroscopic observations. In
3989 this way the cell windows can be changed after the irradiation and before the spectroscopic observations without raising the temperature of the sample or losing the vacuum about it. K is the sample holder which can be heated by the heating elements at L. Irradiation doses were 20 and 40 Mrads at a dose rate of about 100 Mrads hr-l, After the sample had been irradiated its uv or ir spectrum ivas observed, first at 4°K and again at 4°K after heating the sample t o 77"K, No attempt was made to exclude visible light from the irradiation-spectroscopic cell during any of this work. One experiment was done with a dose of 5 Xrads in order to observe any possible changes in the uv, visible, or near-ir part of the spectrum.
Results and Discussion Irradiations at 4°K. First of all, it can be said that within the accuracy of our experiments no difference in the spectra taken at 77 and 4°K of unirradiated PE could be observed. In the case of the uv, visible, and near-ir study, after the PE sample was irradiated to & dose of 5 Mrads its spectrum was taken at 4°K; it was then heated to 77°K and its spectrum was again taken after recooling to 4"K, There was no change in the intensity of the alkyl radical (-CH2CHCH2-) absorption band at 215 nm during the heating and cooling process, nor was there any change of the intensity of the spectrum in the region of the conjugated diene absorption, about 236 nm. A small shoulder in the spectrum appeared at 258 nm, which is the wavelength attributed by Waterman and Dole3to the allyl free radical This shoulder in irradiated PE, -6HCH=CH--. was definitely not present in the spectrum immediately after the irradiation at 4"K, and it demonstrates the slight formation of the allyl radical on heating from 4 to 77°K. The amount of allyl radical formed was about 4% of the total amount observed after heating to room temperature and after all of the alkyl free radicals had decayed. No other changes were observed in the spectrum from 200 to 2000 nm. Cooling the irradiated sample from 77 to 4°K had little effect on the intensity of the allyl or dienyl absorbance, and in general the uv studies were rather unproductive of significant observations. No indication of absorptions due to trapped electrons was observed; they may have been bleached by visible light before the spectrum was takenS6 Shida and Hamilla studied the absorption spectra of positive olefin ions trapped in 7-irradiated organic glasses and found that vinyl-type positive ions were not detectable and probably could not be trapped as such. Vinylene and vinylidene positive ions exhibited rather broad ab( 5 ) R . M. Keyser, J. Lin, K . Tsuji, and F. Williams, PoZym. P r e p . , Amer. Chem. SOC., Div. Polum. Chem., 9,227 (1968). (6) T. Shida and IT, H. Hamill, J . Amer. Chem. Soc., 88, 5376 (1966).
The Journal of Physical Chemistry, Vol. 76, ;L'o. 86, 2.971
3990
DAVID @. WATERMAN AND MALCQLM DOLE
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I 910
Figure 2 , Infrared spectrum of unirradiated Marlex-6002 polyethylene a t 4"K, dashed line. Solid line is the infrared spectrum of the same sample taken a t 4"K after an irradiation to 40 Mrads at 4'K.
sorption spectra in the range 600-800 nm. In the case of 5 mol % cyclohexene in sec-BuC1, the absorption was quite weak with a broad maximum at 700 nm. The absorption band of the positive olefinic ion in pure hexene-2 was stronger with a smaI1 maximum around 650 nm and a broad more intense maximum around 1600 nm. Perhaps such absorption bands could have been observed in this work if doses larger than 5 Mrads had been used. More interesting were the ir absorbances. In Figure 2 the 4°K spectra of unirradiated PE and PE irradiated to 40 Mrads at 4°K are compared. It can be seen that there was a small amount of vinyl group decay, about 14%j during the 4°K irradiation. However, there was no further decay of the vinyl group, Figure 3, on heating from 4 to 77°K. This is in contrast to the significant amount of vinyl decay that occurs on heating from 77'K to room t e m p e r a t ~ r e . ~Inasmuch as allyl free radicals were probably formed4 by reaction of an alkyl free radical with a double b0nd, the double bond involved in the allyl free radical formation on heating from 4 to 77°K must have been the vinylene group. Shida and Hamilla attribute the lack of absorption spectra characteristic of positive olefinic ions in the case of vinyl type unsaturation such as in hexene-1 to a rapid dimerization type reaction of a vinyl positive ion with a neutral vinyl group as postulated by Chang, Yang, and Wagner.' No diene absorption in the ir at 988 cm-l could be detected, either after the irradiation at 4°K or after heating to 77°K. However, diene was formed on heating from 77°K to room temperature. Partridges proposed that the diene could be produced by the scission of two CH bonds on one carbon atom. The liberated H atoms would then be required to abstract H atoms from adjacent carbon atoms follerwed by a shift of a. The JournaE of Physical Chemistrg, Val. 76,No. $6,2.971
Figure 3. Solid line same as the solid line in Figure 2; dashed line, infrared spectrum of polyethylene taken at 4°K after an irradiation 5t 4'K to 40 Mrads and after heating briefly to 77°K.
hydrogen atom two positions along the chain or to abstract H atoms from one adjacent carbon atom and from one 6 to the free radical followed by a shift of one H atom one position along the chain. As calculated by Dole, Bohm, and W a t ~ r n a n H-atom ,~ migration along a chain is unlikely to happen, even at room temperature unless the -CH2- group exists in an excited state. Fa& gatter and Dolelo suggested that two hydrogen molecules might be liberated in the same act to form bicyclo[l.1.O]butane
Ha
H H
which is unstable and reverts to the diene. At 4°K or 77°K the bicyclobutane might be stable enough not to revert to the diene. In Figure 2 can be seen what are apparently two overlapping ir absorption bands in the neighborhood of 966 em-', the frequency of the absorption due to the vinylene group. This double absorption band merges to a single band on heating from 4" to 77°K (Figure 3) with the position of the peak of the absorption band accurately at the wave number expected for the tran;rvinylene group. As a tentative suggestion for the assignment of the transient absorption band at about 973 cm-1 we wish to suggest that the band could be due to the -CH=C+Hcation. On warming from 4" t5 77°K the cations could reunite with trapped electrons to become the trans-vinylene group. Mitigating against (7) P.C. Chang, N. C.Yang and C. D. Wagner, J . Amer. Chem. 8oc 81,2060(1959). (8) R.H.Partridge, J . Chem. Phys., 52,1277(1970). (9) M.Dole, G.G. A. Bohm, and D. C. Waterman, Bur. Polym. J . Suppl., 93 (1969). (10) IT.E%.Fallgalter and M. Dole, J . Phys. Chem., 68,1988 (1 964). I
INFRARED SPECTRUM OF POLYETHYLENE this hypothesis is the fact that no spectra due t o trapped electrons could be detected, but their absorption may have been too broad or too weak to have been detected in the uv studies. On the other hand the total area of the two overlapping bands at 973 and 966 cm-l of Figure 2 is almost exactly equal to the area of the single band of Figure 3 at 966 om-'. This strongly suggests that the transient species giving rise to the 973 band is converted quantitatively into the trans-vinylene group and that its extinction coefficient must be very nearly equal to that of the trans- vinylene band a t 966 cm-l. Furthermore, in the published report of the radio thermoluminescence studies of Aulov, et aL12on PE irradiated at 4°K a small shoulder on the luminescence curve at about GO to 70°K can be seen. This is consistent with the postulate that the conversion of the 973-cm-l band to the 966-cm-I band is due to the untrapping of electrons below 77°K. Boustead and Charlesby'l studied thermoluminescence curves above 77°K and concluded that a glow peak at 102°K was associated with trapping at a vinyl group, and a second peak at 104°K at a vinylene group, Boustead and Charlesby calculated from the equation E(ev) = 2 X lO-3T("I
